HvDoxia in the Northern Gulf of Mexico
An Update by the EPA Science Advisory Board
-------
United States EPA Science Advisory Board EPA-SAB-08-003
Environmental Protection 1400F December 2007
Agency Washington, D.C. www.epa.gov/sab
The EPA Science Advisory Board (SAB) of the U.S. Environmental Protection Agency is a body
of external scientists who provide independent advice to the EPA Administrator on scientific and
technical issues. The SAB was established in its present form by the Congress in 1978. The
SAB's principal mission is to review the quality and relevance of the scientific information being
used to support Agency decisions, programs and strategies and provide broad strategic advice
on scientific and technological issues. In addition, the SAB occasionally conducts self-initiated
studies or special studies at the request of the Administrator to address current and future
environmental issues. The SAB meets in public sessions, and its committees and review panels
are designed to include a diverse and technically balanced range of views, as required by the
Federal Advisory Committee Act.
Cover photo: Courtesy of Louisiana State University's Earth Scan Laboratory. Satellite image of
the Louisiana coast and Northern Gulf of Mexico captured by Oceansat-1 using Ocean Color
Monitor, April 12, 2007. This image is a true color enhancement using the red, green and blue
channels of the electromagnetic spectrum. The image shows high levels of suspended
sediments delivered to the Gulf via the Mississippi and Atchafalaya Rivers.
-------
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON B.C. 20460
OFFICE OF THE ADMINISTRATOR
EPA SCIENCE ADVISORY BOARD
December 21,2007
EPA-SAB-08-003
Honorable Stephen L. Johnson
Administrator
U.S. Environmental Protection Agency
1200 Pennsylvania Avenue, N.W.
Washington, D.C. 20460
Subj ect: Hypoxia in the Northern Gulf of Mexico: An Update
by the EPA Science Advisory Board
Dear Administrator Johnson:
Over a year ago, the EPA Office of Water (OW) asked the Science Advisory
Board (SAB) to evaluate the most recent science on the hypoxic zone in the Gulf of
Mexico as well as potential options for reducing the size of the zone. The hypoxic zone,
an area of low dissolved oxygen that cannot support most marine life, has been
documented in the Gulf of Mexico since 1985 and was most recently measured at 20,500
km2 The SAB was asked to address the science that has emerged since the 2000
publication of An Integrated Assessment: Hypoxia in the Northern Gulf of Mexico
(IntegratedAssessment), the seminal study by the Committee on Environment and
Natural Resources that served as the basis for activities coordinated by the Mississippi
River/Gulf of Mexico Watershed Nutrient Task Force. The SAB was also asked to
address the most recent science on water quality in the Mississippi Atchafalaya River
Basin, an area of 31 States and Tribes that drains approximately 40% of the contiguous
United States. Further, the SAB was asked to discuss options for reducing hypoxia in
terms of cost, feasibility and social welfare. To address this question, the SAB found it
necessary to discuss recent research on water quality as well as research on policy
options, in particular, those policies that create economic incentives.
Following OW's request, the Science Advisory Board Staff Office convened an
expert panel under the auspices of the chartered SAB. This SAB Panel consists of 21
distinguished scientists from academia, industry and government agencies with expertise
in the fields of oceanography, ecology, agronomy, agricultural engineering, economics
-------
and other fields. Over the past year, the SAB Panel held numerous public meetings and
considered information from invited speakers as well as over 60 sets of public comments
in the development of this report.
In issuing the attached report, the SAB reaffirms the major finding of the
Integrated Assessment, namely that contemporary changes in the hypoxic area in the
northern Gulf of Mexico are primarily related to nutrient loads from the Mississippi
Atchafalaya River basin. If the size of the hypoxic zone is to be reduced, the SAB finds
that a dual nutrient strategy is needed that achieves at least a 45% reduction in both
riverine total nitrogen flux and riverine total phosphorus flux. The SAB offers these as
initial targets while stressing the importance of moving in a direct!onally correct fashion
and adjusting policy adaptively on the basis of future data, changing conditions and
lessons learned. Climate change will likely contribute to changing conditions. A number
of studies have suggested that climate change will create conditions where larger nutrient
reductions, e.g., 50 - 60% for nitrogen, would be required to reduce the size of the
hypoxic zone. An adaptive management approach, coupling nutrient reductions with
continuous monitoring and evaluation, can provide valuable lessons to improve future
decisions.
The SAB was asked to comment on the Task Force's goal of reducing the size of
the hypoxic zone to 5,000 km2 by 2015. With respect to the time frame, the SAB finds
that such a significant reduction is not likely to be achievable over the next eight years.
We conclude this for two reasons. First, there is limited current movement to implement
policies, programs and strategies that reduce nutrients. Second, there are time lags
between reductions in nutrient inputs and the response of the ecological system. Hence,
while the 5,000 km2 target remains a reasonable objective in an adaptive management
context; it may no longer be possible to achieve this goal by 2015. This makes it even
more important to proceed in a directionally correct fashion to manage factors affecting
hypoxia than to wait for greater precision in setting the goal for the size of the zone.
The SAB underscores that in considering management strategies to reduce Gulf
hypoxia, EPA should consider the many benefits of nutrient reduction in the Mississippi
Atchafalaya River basin. Such "co-benefits" include improved groundwater and surface
water quality, wildlife and biodiversity, recreation, soil quality, greenhouse gas reduction
and carbon sequestration. In many cases, co-benefits may exceed the benefits of hypoxia
reduction.
Finally, to reduce hypoxia in the Gulf, a systems view, looking at all sources and
effects, is needed. The SAB urges the Agency to consider its options with respect to both
non-point and point sources. Non-point sources have long been acknowledged as the
primary source of nutrient loadings, however the SAB finds point sources are a more
significant contributor than previously thought. Atmospheric deposition of nitrogen is
also playing a role in hypoxia. In addition, it may be necessary to confront the conflicts
between hypoxia reduction as a goal on the one hand and incentives provided by current
agricultural and energy policy on the other. Some aspects of current agricultural and
energy policies are providing incentives that contribute to greater nutrient loads now and
11
-------
in the future. The SAB recognizes that if agricultural, environmental, and energy policies
are to be aligned to support hypoxia reduction, cooperation across a broad spectrum of
interests, including the highest levels of government, would be required. We note that
regulatory options under the Clean Water Act, an area within EPA's purview, are
addressed by the National Academy of Sciences (NAS) in its recent study, the
"Mississippi River and the Clean Water Act." As pointed out by the NAS, EPA has
regulatory authority under the Clean Water Act to address watershed wide issues.
The Executive Summary in the attached Advisory highlights the SAB's findings
and recommendations with more detailed science presented in the main body of the
report. We appreciate the opportunity to provide advice on this important and timely
topic and look forward to receiving your response.
Sincerely,
/Signed/ /Signed/
Dr. M. Granger Morgan Dr. Virginia Dale
Chair, Science Advisory Board Chair, SAB Hypoxia Advisory Panel
in
-------
NOTICE
This report has been written as part of the activities of the EPA Science Advisory
Board, a public advisory committee providing extramural scientific information and
advice to the Administrator and other officials of the Environmental Protection Agency.
The SAB is structured to provide balanced, expert assessment of scientific matters related
to problems facing the Agency. This report has not been reviewed for approval by the
Agency and, hence, the contents of this report do not necessarily represent the views and
policies of the Environmental Protection Agency, nor of other agencies in the Executive
Branch of the federal government. Mention of trade names or commercial products do
not constitute a recommendation for use. Reports of the EPA SAB are posted at:
http://www.epa.gov/sab
IV
-------
ACKNOWLEDGEMENTS
EPA's Science Advisory Board Hypoxia Advisory Panel would like to
acknowledge many individuals who provided their scientific perspectives for the Panel's
consideration in the development of this report.
Invited Speakers:
James Ammerman, Rutgers University, Nutrient Dynamics
Rich Alexander, U.S. Geological Survey, SPARROW Model
Jeff Arnold, U.S. Department of Agriculture, SWAT Model
* James Baker and Dean Lemke, UMRSUNC, Upper Mississippi Symposia
Summary
Robert Dean, University of Florida, Drawing Louisiana's New Map
Steven DiMarco, Texas A&M University, Physical Oceanography in the
Gulf
* Katie Flahive, U.S. Environmental Protection Agency, Status of the
Management Actions Reassessment Team (MART) Report
Rick Greene (EPA) and Alan Lewitus (National Oceanic and Atmospheric
Administration), Gulf Science Symposia Summary
* Dan Jaynes, U. S. Department of Agriculture, Agricultural N & P
Management Approaches
Bob Kellogg, U.S. Department of Agriculture, Status of the Conservation
Effectiveness Assessment Program (CEAP)
Tim Miller, U.S. Geological Survey, Monitoring Activities in the
Mississippi River basin
Marc Ribaudo, U.S. Department of Agriculture, Costs and Benefits of
Methods to Reduce Nutrient Loads
* Don Scavia, University of Michigan, 1) Science and Policy Context & 2)
Hypoxia Forecast Models
Janice Ward, U.S. Geological Survey, Fate and Transport Symposia
Summary
Invited Technical Reviewers:
Mark Alley, Virginia Tech
Walter Dodds, Kansas State University
Madhu Khanna, University of Illinois
William Wiseman, Jr., National Science Foundation
Public Commenters:
James Baker, Iowa Department of Agriculture and Land Stewardship
Victor Bierman, Donald Boesch, John Day, Robert Diaz, Dubravko Justic,
Dennis Keeney, William Mitsch, Nancy Rabalais, Gyles Randall, Donald
Scavia, and Eugene Turner, Contributors to the Integrated Assessment
-------
Donald Boesch, University of Maryland Center for Environmental Science
Darrell Brown, EPA Office of Water
Daniel Coleman, O'Brien & Gere
Richard Cruse, Iowa State University
Doug Daigle, Lower Mississippi River Sub-basin Committee on Hypoxia
Bob Diaz, Virginia Institute of Marine Sciences
Michael Duffy, Iowa State University
Nancy Erickson, Illinois Farm Bureau
Jason Flickner, Kentucky Waterways Alliance
Norman Fousey, U.S. Department of Agriculture
James Fouss, U.S. Department of Agriculture
Doug Gronau, Iowa Farm Bureau Federation
Ben Grumbles, Assistant Administrator for EPA's Office of Water
Stephen Harper, O'Brien & Gere
Chuck Hartke, Illinois Department of Agriculture
Susan Heathcote, Iowa Environmental Council
Matthew Helmers, Iowa State University
Ed Hopkins, Sierra Club
Chris Hornback, National Association of Clean Water Agencies
Illinois Department of Agriculture
Thomas Isenhart, Iowa State University
Dan Jaynes, U.S. Department of Agriculture
Doug Karl en, U.S. Department of Agriculture
Dennis Keeney, Institute for Agriculture and Trade Policy
Louis Kollias, Metropolitan Water Reclamation District of Greater
Chicago
Dean Lemke, Iowa Department of Agriculture and Land Stewardship
Alan Lewitus and David Kidwell, National Oceanic and Atmospheric
Administration
Antonio Mallarino, Iowa State University
Mark Maslyn, American Farm Bureau Federation
Dennis McKenna, Illinois Department of Agriculture
Mississippi River Water Quality Cooperative (MSWQC)
Bill Northey, Iowa Secretary of Agriculture
Don Parrish, American Farm Bureau
Paul Patterson, City of Memphis
Jean Payne, Illinois Fertilizer and Chemical Association
Michelle Perez, Environmental Working Group
Bob and Kristen Perry, Missouri Clean Water Commission
Nancy Rabalais, Louisiana Universities Marine Consortium
Russell Rasmussen, Wisconsin Department of Natural Resources
Jack Riessen, Iowa Department of Natural Resources
Rick Robinson, Iowa Farm Bureau
Matt Rota, Gulf Restoration Network
VI
-------
John Sawyer, Iowa State University
Al Schafbuch, Affiliation not identified
Theodore Slawecki, Limno-Tech, Inc.
Tim Strickland, U.S. Department of Agriculture
Richard Swenson, U.S. Department of Agriculture
Michael Tate, Kansas Department of Health and Environment
Steve Taylor, Environmental Resource Coalition
Mark Tomer, U.S. Department of Agriculture
Eugene Turner, Louisiana State University
Ford B. West, The Fertilizer Institute
Wendy Wintersteen, Iowa State University
vn
-------
Science Advisory Board
Hypoxia Advisory Panel
U.S. Environmental Protection Agency
CHAIR
Dr. Virginia Dale, Corporate Fellow, Environmental Sciences Division, Oak Ridge
National Laboratory, Oak Ridge, TN
MEMBERS
Dr. Thomas Bianchi, Professor, Oceanography, Geosciences, Texas A&M University,
College Station, TX
Dr. Alan Blumberg, Professor, Civil, Environmental and Ocean Engineering, Stevens
Institute of Technology, Hoboken, NJ
Dr. Walter Boynton, Professor, Chesapeake Biological Laboratory, Center for
Environmental Science, University of Maryland, Solomons, MD
Dr. Daniel Joseph Conley, Professor, Marie Curie Chair, GeoBiosphere
Centre, Department of Geology, Lund University, Lund, Sweden
Dr. William Crumpton, Associate Professor & Coordinator of Environmental Programs,
Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA
Dr. Mark David, Professor, Natural Resources & Environmental Sciences, University of
Illinois, Urbana, IL
Dr. Denis Gilbert, Research Scientist, Ocean and Environment Science Branch,
Maurice-Lamontagne Institute, Department of Fisheries and Oceans Canada, Mont-Joli,
Quebec, Canada
Dr. Robert W. Howarth, David R. Atkinson Professor, Department of Ecology and
Evolutionary Biology, Cornell University, Ithaca, NY
Dr. Catherine Kling, Professor, Department of Economics, Iowa State University,
Ames, IA
Dr. Richard Lowrance, Research Ecologist, Southeast Watershed, Agricultural
Research Service, USD A, Tifton, GA
Dr. Kyle Mankin, Associate Professor, Biological and Agricultural Engineering, Kansas
State University, Manhattan, KS
Vlll
-------
Dr. Judith L. Meyer, Distinguished Research Professor Emeritus, Institute of Ecology,
University of Georgia, Athens, GA
Dr. James Opaluch, Professor, Department of Environmental and Natural Resource
Economics, College of the Environment and Life Sciences, University of Rhode Island,
Kingston, RI
Dr. Hans Paerl, Professor of Marine and Environmental Sciences, Institute of Marine
Sciences, University of North Carolina, Chapel Hill, Morehead City, NC
Dr. Kenneth Reckhow, Professor and Chair, Environmental Science & Policy, Nicholas
School, Duke University, Durham, NC
Dr. James Sanders, Director, Skidaway Institute of Oceanography, Savannah, GA
Dr. Andrew N. Sharpley, Research Soil Scientist, Department of Crop, Soil and
Environmental Sciences, University of Arkansas, Fayetteville, AR
Dr. Thomas W. Simpson, Professor and Coordinator, Chesapeake Bay Programs,
College of Agriculture and Natural Resources, University of Maryland, College Park,
MD
Dr. Clifford Snyder, Nitrogen Program Director, International Plant Nutrition Institute,
Conway, AR
Dr. Donelson Wright, Chancellor Professor Emeritus, School of Marine Science,
Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA
SCIENCE ADVISORY BOARD STAFF
Dr. Holly Stallworth, Designated Federal Officer, EPA Science Advisory Board Staff
Office, Washington, D.C.
Dr. Thomas Armitage, Designated Federal Officer, EPA Science Advisory Board Staff
Office, Washington, D.C.
Mr. David Wangsness, Designated Federal Officer, Senior Scientist on detail to SAB,
U.S. Geological Survey, Atlanta, GA
IX
-------
U.S. Environmental Protection Agency
Science Advisory Board
CHAIR
Dr. M. Granger Morgan, Lord Chair Professor in Engineering; Professor and
Department Head, Department of Engineering and Public Policy, Carnegie Mellon
University, Pittsburgh, PA
SAB MEMBERS
Dr. Gregory Biddinger, Coordinator, Natural Land Management Programs,
Toxicicology and Environmental Sciences, ExxonMobil Biomedical Sciences, Houston,
TX
Dr. Thomas Burke, Professor and Co-Director Risk Sciences and Public Policy Institute,
Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD
Dr. James Bus, Director of External Technology, Toxicology and Environmental
Research and Consulting, Dow Chemical Company, Midland, MI
Dr. Deborah Cory-Slechta, J. Lowell Orbison Distinguished Alumni Professor of
Environmental Medicine, Department of Environmental Medicine, School of Medicine
and Dentistry, University of Rochester, Rochester, NY
Dr. Maureen L. Cropper, Professor, Department of Economics, University of
Maryland, College Park, MD
Dr. Virginia Dale, Corporate Fellow, Environmental Sciences Division, Oak Ridge
National Laboratory, Oak Ridge, TN
Dr. David Dzombak, Professor, Department of Civil and Environmental Engineering,
Carnegie Mellon University, Pittsburgh, PA
Dr. Baruch Fischhoff, Howard Heinz University Professor, Department of Social and
Decision Sciences, Department of Engineering and Public Policy, Carnegie Mellon
University, Pittsburgh, PA
Dr. James Galloway, Professor, Department of Environmental Sciences, University of
Virginia, Charlottesville, VA
Dr. James K. Hammitt, Professor of Economics and Decision Sciences, Harvard Center
for Risk Analysis, Harvard University, Boston, MA
Dr. Rogene Henderson, Scientist Emeritus, Lovelace Respiratory Research Institute,
Albuquerque, NM
-------
Dr. James H. Johnson, Professor and Dean, College of Engineering, Architecture &
Computer Sciences, Howard University, Washington, DC
Dr. Bernd Kahn, Professor Emeritus and Director, Environmental Resources Center,
School of Nuclear Engineering and Health Physics, Georgia Institute of Technology,
Atlanta, GA
Dr. Agnes Kane, Professor and Chair, Department of Pathology and Laboratory
Medicine, Brown University, Providence, RI
Dr. Meryl Karol, Professor Emerita, Graduate School of Public Health, University of
Pittsburgh, Pittsburgh, PA
Dr. Catherine Kling, Professor, Department of Economics, Iowa State University,
Ames, IA
Dr. George Lambert, Associate Professor of Pediatrics, Director, Center for Childhood
Neurotoxicology, Robert Wood Johnson Medical School-UMDNJ, Belle Mead, NJ
Dr. Jill Lipoti, Director, Division of Environmental Safety and Health, New Jersey
Department of Environmental Protection, Trenton, NJ
Dr. Michael J. McFarland, Associate Professor, Department of Civil and
Environmental Engineering, Utah State University, Logan, UT
Dr. Judith L. Meyer, Distinguished Research Professor Emeritus, Institute of Ecology,
University of Georgia, Lopez Island, WA
Dr. Jana Milford, Associate Professor, Department of Mechanical Engineering,
University of Colorado, Boulder, CO
Dr. Rebecca Parkin, Professor and Associate Dean, Environmental and Occupational
Health, School of Public Health and Health Services, The George Washington University
Medical Center, Washington, DC
Mr. David Rejeski, Director, Foresight and Governance Project, Woodrow Wilson
International Center for Scholars, Washington, DC
Dr. Stephen M. Roberts, Professor, Department of Physiological Sciences, Director,
Center for Environmental and Human Toxicology, University of Florida, Gainesville, FL
Dr. Joan B. Rose, Professor and Homer Nowlin Chair for Water Research, Department
of Fisheries and Wildlife, Michigan State University
XI
-------
Dr. James Sanders, Director, Skidaway Institute of Oceanography, University of
Georgia, Savannah, GA
Dr. Jerald Schnoor, Allen S. Henry Chair Professor, Department of Civil and
Environmental Engineering, Co-Director, Center for Global and Regional Environmental
Research, University of Iowa, Iowa City, IA
Dr. Kathleen Segerson, Professor, Department of Economics, University of
Connecticut, Storrs, CT
Dr. Kristin Shrader-Frechette, O'Neil Professor of Philosophy, Department of
Biological Sciences and Philosophy Department, University of Notre Dame, Notre Dame,
IN
Dr. Philip Singer, Professor, Department of Environmental Sciences and Engineering,
School of Public Health, University of North Carolina, Chapel Hill, NC
Dr. Kerry Smith, W.P. Carey Professor of Economics, Dept. of Economics, Carey
School of Business, Arizona State University, Tempe, AZ
Dr. Deborah Swackhamer, Interim Director and Professor, Institute on the
Environment, University of Minnesota, St. Paul, MN
Dr. Thomas L. Theis, Director, Institute for Environmental Science and Policy,
University of Illinois at Chicago, Chicago, IL
Dr. Valerie Thomas, Anderson Interface Associate Professor, School of Industrial and
Systems Engineering, Georgia Institute of Technology, Atlanta, GA
Dr. Barton H. (Buzz) Thompson, Jr., Robert E. Paradise Professor of Natural
Resources Law at the Stanford Law School and Director, Woods Institute for the
Environment Director, Stanford University, Stanford, CA
Dr. Robert Twiss, Professor Emeritus, University of California-Berkeley, Ross, C A
Dr. Lauren Zeise, Chief, Reproductive and Cancer Hazard Assessment Branch, Office
of Environmental Health Hazard Assessment, California Environmental Protection
Agency, Oakland, CA
SCIENCE ADVISORY BOARD STAFF
Mr. Thomas Miller, EPA Science Advisory Board Staff Office, Washington, DC
xn
-------
Table of Contents
Table of Figures xv
List of Tables xix
Glossary of Terms xxi
List of Acronyms xxvi
Conversion Factors and Abbreviations xxx
Executive Summary 1
1. Introduction 10
1.1. Hypoxia and the Northern Gulf of Mexico- A Brief Overview 10
1.2. Science and Management Goals for Reducing Hypoxia 12
1.3. EPA Science Advisory Board (SAB) Hypoxia Advisory Panel 14
1.4. The SAB Panel's Approach 16
2. Characterization of Hypoxia 18
2.1. Processes in the Formation of Hypoxia in the Gulf of Mexico 18
2.1.1. Historical Patterns and Evidence for Hypoxia on the Shelf 18
2.1.2. The Physical Context 21
2.1.3. Role of N and P in Controlling Primary Production 32
2.1.4. Other Limiting Factors and the Role of Si 38
2.1.5. Sources of Organic Matter to the Hypoxic Zone 40
2.1.6. Denitrification, P Burial, and Nutrient Recycling 46
2.1.7. Possible Regime Shift in the Gulf of Mexico 49
2.1.8. Single Versus Dual Nutrient Removal Strategies 52
2.1.9. Current State of Forecasting 54
3. Nutrient Fate, Transport, and Sources 59
3.1. Temporal Characteristics of Streamflow and Nutrient Flux 59
3.1.1. MARB Annual and Seasonal Fluxes 65
3.1.2. Subbasin Annual and Seasonal Flux 73
3.1.3. Key Findings and Recommendations on Temporal Characteristics 83
3.2. Mass Balance of Nutrients 85
3.3. Nutrient Transport Processes 98
3.4. Ability to Route and Predict Nutrient Delivery to the Gulf 107
4. Scientific Basis for Goals and Management Options 120
4.1. Adaptive Management 120
4.2. Setting Targets for Nitrogen and Phosphorus Reduction 125
4.3. Protecting Water Quality and Social Welfare in the Basin 131
4.4. Cost-Effective Approaches for Non-point Source Control 143
4.4.1. Voluntary programs -without economic incentives 144
4.4.2. Existing Agricultural Conservation Programs 145
4.4.3. Emissions and Water Quality Trading Programs 147
4.4.4. Agricultural Subsidies and Conservation Compliance Provisions 148
4.4.5. Taxes 151
4.4.6. Eco-labeling and Consumer Driven Demand 151
4.4.7. Key Findings and Recommendations on Cost Effective Approaches .... 152
Xlll
-------
4.5. Options for Managing Nutrients, Co-benefits, and Consequences 154
4.5.1. Agricultural drainage 154
4.5.2. Freshwater Wetlands 156
4.5.3. Conservation Buffers 161
4.5.4. Cropping systems 165
4.5.5. Animal Production Systems 168
4.5.6. In-field Nutrient Management 175
4.5.7. Effective Actions for Other Non-Point Sources 195
4.5.8. Most Effective Actions for Industrial and Municipal Sources 198
4.5.9. Ethanol and Water Quality in the MARB 202
4.5.10. Integrating Conservation Options 208
5. Summary of Findings and Recommendations 217
5.1. Charge Questions on Characterization of Hypoxia 217
5.2. Charge Questions on Nutrient Fate, Transport and Sources 219
5.3. Charge Questions on Goals and Management Options 221
5.4. Conclusion 223
References 226
A. Appendix A: Studies on the Effects of Hypoxia on Living Resources A-l
B. Appendix B: Flow Diagrams and Mass Balance of Nutrients B-8
_C. Appendix C: EPA's Guidance on Nutrient Criteria C-15
_D. Appendix D: Calculation of Point Source Inputs ofNandP D-19
E. Appendix E: Animal Production Systems E-21
xiv
-------
Table of Figures
Figure 1: Map of the frequency of hypoxia in the northern Gulf of Mexico, 1985-2005. Taken from N.N.
Rabalais, LUMCON, 2006 10
Figure 2: Map showing the extent of the Mississippi-Atchafalaya River basin 11
Figure 3: Plots of the PEB index (%PEB) in sediment cores from the Louisiana shelf. Higher values of the
PEB index indicate lower dissolved oxygen contents in bottom waters. Taken from Osterman et al. (2005).
20
Figure 4: Change in the relative importance of the Atchafalaya flow to the combined flows from the
Mississippi and Atchafalaya Rivers over the 20th Century. Reprinted from Bratkovich et al. (1994) 24
Figure 5: Modelled surface salinity showing the freshwater plumes from the Atchafalaya and Mississippi
Rivers during upwelling favorable winds (top panel) and during downwelling favorable winds 8 days later
(bottom panel). Adapted from Hetland and DiMarco (2007) 26
Figure 6: Proposed diversions of Mississippi effluents for coastal protection. From Coastal Protection and
Restoration Authority (CPRA) of Louisiana, 2007 Integrated Ecosystem Restoration and Hurricane
Protection: Louisiana's Comprehensive Master Plan for a Sustainable Coast. CPRA, Office of the Governor
(La) 117 pp 27
Figure 7: An illustration depicting different zones (Zones 1-4, numbered above) in the NGOM during the
period when hypoxia can occur. These zones are controlled by differing physical, chemical, and biological
processes, are variable in size, and move temporally and spatially. Diagram created by D. Gilbert 29
Figure 8: Response of natural phytoplankton assemblages from coastal NGOM stations to nutrient
additions, March through September. All experiments, except those done in September, indicate a strong
response to P additions. Taken from SyIvan etal., 2006 34
Figure 9: NASA-SeaWiFS image of the Northern Gulf of Mexico recorded in April, 2000. This image
shows the distributions and relative concentrations of chlorophyll a, an indicator of phytoplankton biomass
in this region. Note the very high concentrations (orange to red) present in the inshore regions of the
mouths of the Mississippi and Atchafalaya Rivers 35
Figure 10: Estimated extent of agricultural drainage based on the distribution of row crops, largely corn
and soybean, and poorly drained soils (perD. Jaynes, National Soil Tilth Lab, Ames, IA) 60
Figure 11: Land cover based on Landsat data (adapted from Crumpton etal., 2006) 61
Figure 12: Flow weighted average nitrate concentrations estimated from STORET data selected to exclude
point source influences (adapted from Crumpton et al., 2006) 61
Figure 13: Flow-weighted average nitrate and reduced N versus percent cropland (adapted from Crumpton
etal., 2006) 62
Figure 14: MARB nitrate-N fluxes for 1955 through 2005 water years comparing estimates from various
methods for 1979 to 2005. Based on USGS data from Battaglin (2006) and Aulenbach et al. (2007) 64
Figure 15: Comparison (percent and absolute basis) of MARB nitrate-N fluxes to LOADEST 5 yr method
for 1979 through 2005 water years. Based on USGS data from Battaglin (2006) and Aulenbach et al.
(2007) 65
XV
-------
Figure 16: Schematic showing locations of MARB monitoring sites (Aulenbach et al., 2007) 66
Figure 17: Flow and available nitrogen monitoring data for the MARB for 1955 through 2005 water years.
(LOWESS, Locally Weighted Scatterplot Smooth, curves shown in red). LOWESS describes the
relationship between Y and X without assuming linearity or normality of residuals, and is a robust
description of the data pattern (Helsel and Hirsch, 2002) 67
Figure 18: Flow, available phosphorus, and available silicate monitoring data for the MARB for 1955
through 2005 water years. (LOWESS curves shown in red). Based on USGS data from Battaglin (2006)
and Aulenbach et al. (2007) 68
Figure 19: Ratio of total N to total P and dissolved silicate to dissolved inorganic N for MARB for the
1980 through 2005 water years. Based on USGS data from Battaglin (2006) and Aulenbach et al. (2007).
69
Figure 20: Flow and nitrogen flux for the MARB during spring (April, May, and June) for the period 1979-
2005. (LOWESS curve shown in red). Based on USGS data from Battaglin (2006) and Aulenbach et al.
(2007) 70
Figure 21: Flow, phosphorus, and silicate flux for the MARB during spring (April, May, and June) for the
period 1979-2006. (LOWESS curve shown in red). Based on USGS data from Battaglin (2006) and
Aulenbach etal. (2007) 71
Figure 22: Sum of April, May and June fluxes as a percent of annual (water year basis) for combined
Mississippi mainstem and Atchafalaya River. Box plots show median (line in center of box), 25th and 75th
percentiles (bottom and top of box, respectively), 10th and 90th percentiles (bottom and top error bars,
respectively) and values < 10th percentile and > 90th percentile (solid circles below and above error bars,
respectively). Based on USGS data from Battaglin (2006) and Aulenbach et al. (2007) 72
Figure 23: Ratio of total N to total P and silicate to dissolved inorganic N for the MARB during spring
(April, May, and June) for the period 1980-2006. Based on USGS data from Battaglin (2006) and
Aulenbach et al. (2007) 73
Figure 24: Location of nine large subbasins comprising the MARB that are used for estimating nutrient
fluxes (from Aulenbach et al., 2007) 74
Figure 25: Net N inputs and annual nitrate-N fluxes and yields for the Ohio River subbasin. (LOWESS
curves for riverine nitrate-N shown in red.) Based on USGS data from Battaglin (2006) and Aulenbach et
al. (2007) 78
Figure 26: Net N inputs and annual nitrate-N fluxes and yields for the upper Mississippi River subbasin.
(LOWESS curves for riverine nitrate-N shown in red.) Shown in green is a recalculated net N input for the
upper Mississippi River basin, increasing soybean N2 fixation from 50 to 70% of above ground N, and a
soil net N mineralization rate from 0 to 10 kg N/ha/yr. Based on USGS data from Battaglin (2006) and
Aulenbach et al. (2007) 79
Figure 27: Total P and particulate/organic P fluxes for the Ohio River near Grand Chain, Illinois.
(LOWESS curves shown in black and red). Based on USGS data from Battaglin (2006) and Aulenbach et
al. (2007) 81
Figure 28: Spring water flux and nitrate-N flux for the Mississippi River at Grafton and the Ohio River at
Grand Chain, IL for water years 1975-2005. (LOWESS curves shown in red.) Based on USGS data from
Battaglin (2006) and Aulenbach et al. (2007) 82
xvi
-------
Figure 29: Spring nitrate-N flux (sum of April, May, and June) for the Mississippi River at Grafton plus
Ohio River at Grand Chain subbasins compared to the combined Mississippi and Atchafalaya River for
1979 through 2005. Based on USGS data from Battaglin (2006) and Aulenbach et al. (2007) 83
Figure 30: Area of major crops planted in the MARB from 1941 through 2007. Adapted from Mclsaac,
2006 86
Figure 31: Nitrogen mass balance components and net N inputs for the MARB, as calculated by Mclsaac
et al. (2002) and updated through 2005 by Mclsaac (2006) 88
Figure 32: Net N inputs for the four major regions of the MARB through 2005. Adapted from Mclsaac,
2006 89
Figure 33: Nitrogen mass balance components and net N inputs for the upper Mississippi River basin, as
calculated by Mclsaac et al. (2002) and updated through 2005 by Mclsaac (2006) 90
Figure 34: Phosphorus mass balance components and net P inputs for the MARB. Adapted from Mclsaac,
2006 92
Figure 35: Net P inputs for the four major subbasins of the MARB through 2005. Adaptive from Mclsaac,
2006 93
Figure 36: Phosphorus mass balance components and net N inputs for the upper Mississippi River basin.
Adapted from Mclsaac, 2006 94
Figure 37: Total phosphorus point source fluxes as a percent of total flux for the MARB for 2004 by
hydrologic region 96
Figure 38: Percentage of nutrient inputs to streams that are removed by instream and reservoir processes as
predicted by the SPARROW model (Alexander et al., in press) 98
Figure 39: N removed in aquatic ecosystems (as a % of inputs) as a function of ecosystem depth/water
travel time (modified from David et al., 2006). Values shown are for 23 years in an Illinois reservoir (David
et al., 2006), French reservoirs (Gamier et al., 1999), Illinois streams (an average from Royer et al., 2004),
agricultural streams (Opdyke et al., 2006), and rivers (Seitzinger et al., 2002). The curve from Seitzinger et
al. (2002) is not as steep as the curve that includes information from reservoirs in an agricultural region. 100
Figure 40: A Conceptual Framework for Hypoxia in the Northern Gulf of Mexico 121
Figure 41: Percent mass nitrate removal in wetlands as a function of hydraulic loading rate. Best fit for
percent mass loss = 103*(hydraulic loading rate)-0.33. R2 = 0.69. Adapted from Crumpton et al. (2006, in
press) 158
Figure 42: Observed NO3 mass removal (blue points) versus predicted NO3 mass removal (blue surface)
based on the function [mass NO3 removed = 10.3*(HLR) 0.67 * FWA] for which R2 = 0.94. Blue lines are
isopleths of predicted mass removal at intervals of 250 kg ha/yr. The dashed, red line represents the
isopleth for mass removal rate of 290 kg ha/yr suggested by Mitsch et al. (2005a). The green plane
intersecting function surface represents organic N export. Adapted from Crumpton et al. (2006, in press).
159
Figure 43: Recoverable manure N, assuming no export of manure from the farm, using 1997 census data.
Adapted from USDA (2003) with the author's permission 169
Figure 44: Recoverable manure P, assuming no export of manure from the farm, using 1997 census data.
Adapted from USDA (2003) with the author's permission 170
xvil
-------
Figure 45: Fertilizer N consumption as anhydrous ammonia in leading corn-producing states for years
ending June 30 176
Figure 46: Changes in the consumption of principal fertilizer N sources used in the six leading corn-
producing states (IA, IL, IN, MN, NE, and OH) for years ending June 30 177
Figure 47: Percentage of N fertilized corn acreage which received some amount of N in the fall 178
Figure 48: USD A ARMS data for the three states with highest fall N application, showing total amount of
fall applied N for that crop. Also shown are Illinois sales data for the same period 179
Figure 49: Fraction of annual fertilizer N tonnage in Illinois sold in the fall 180
Figure 50: Average corn yields in six leading corn-producing states (IA, IL, IN, MN, NE, and OH), 1990-
2006 (Source:USDA National Agricultural Statistics Service) 183
Figure 51: Variability in soil test P levels in typical farmer fields in Minnesota (2007 personal
communication with Dr. Gary Malzer, University of Minnesota) 190
Figure 52: Effect of variable-rate versus uniform rate application of liquid swine manure on changes in soil
test phosphorus in Iowa fields [2007 personal communication with Dr. Antonio Mallarino, Iowa State
University and Wittry and Mallarino (2002)] 191
Figure 53: Effect of variable rate versus uniform rate application of fertilizer P on soil test P in multiple
Iowa fields across multiple years (2007 personal communication with A. Mallarino, Iowa State University).
192
Figure 54: Nitrogen Cycle Flow Diagram. Taken from Encyclopedia of Earth (2007) at
http:www.eoearth.org/global_material_cycles B-9
Figure 55: Phosphorus Cycle Flow Diagram. Taken from Encyclopedia of Earth (2007) at
http:www.eoearth.org/global_material_cycles B-10
Figure 56: Silicon Cycle Flow Diagram. Taken from Encyclopedia of Earth (2007) at
http:www.eoearth.org/global_material_cycles B-ll
Figure 57: Annual average deposition of NOy across the United States (kg N /hectare-year) based on beta-
testing runs of the CMAQ model B-14
xvill
-------
List of Tables
Table 1: A partial summary of papers published following the Integrated Assessment related to sources of
organic matter to the Gulf of Mexico 45
Table 2: Site name and corresponding map number for sites discussed in the following section 75
Table 3: Average annual nutrient fluxes for the five large subbasins in the MARB for the 2001-2005 water
years. (Percent of total basin flux shown in parentheses.) 75
Table 4: Average annual nutrient fluxes for ten subbasins in the MARB for the 2001 -2005 water years..
Some subbasin fluxes are calculated as the difference between the upstream and downstream monitoring
station. (Percent of total basin flux shown in parentheses.) 76
Table 5: Average annual nutrient yields for the five large subbasins in the MARB for water years 2001-
2005 77
Table 6: Average annual nutrient yields for nine subbasins in the MARB for the 2001 - 2005 water years.
Some subbasin yields are calculated as the difference between the upstream and downstream monitoring
stations 77
Table 7: Acres of wetlands created, restored or enhanced in major subbasins of the Mississippi River from
2000-2006 under the Wetland Reserve Program (WRP), Conservation Reserve Program (CRP),
Conservation Reserve Enhancement Program (CREP), Environmental Quality Incentive Program (EQIP),
and Conservation Technical Assistance (CTA). (Personal communication, Mike Sullivan, USD A) 104
Table 8. Attributes of models used to estimate sources, transport and/or delivery of nutrients to the Gulf of
Mexico 108
Table 9: Annual and spring (sum of April, May, June) average flow and N and P fluxes for the MARB for
the 1980 to 1996 reference period compared to the most recent five year period (2001 to 2005). Load
reductions in mass of Nor P also shown 127
Table 10: Summary of Study features of Basin wide Integrated Economic-Biophysical Models 138
Table 11: Summary of Policies and Findings from Integrated Economic-Biophysical Models 139
Table 12: Areas (ha) of conservation buffers installed in the six sub-basins of the MARB for FY 2000 -
FY2006 164
Table 13: Status of implementation of permits under the 2003 CAFO rule for states within the MARB.
Data provided by EPA Office of Wastewater Management, 2007 171
Table 14: Estimates of manure production and N and P loss to water and air from Animal Feeding
Operations within the Mississippi River basin, on information from the 2002 U.S. Census of Agriculture
(adapted from Aillery etal., 2005) 173
Table 15: Partial N balance for 4-year rate study by Jaynes et al. (2001). The last two columns added here
and were not part of original table 186
Table 16: Estimated changes in N losses from cropping changes predicted by FAPRI from 2006-2013. 205
Table 17: Potential total nitrogen (TN) and phosphorus (TP) reduction efficiencies (percent change) in
surface runoff, subsurface flow, and tile drainage. Estimates are average values for a multiple year basis,
and some of the numbers in this table are based on a very small amount of field information 210
xix
-------
Table 18: Anticipated benefits associated with different agricultural management options 213
Table 19: Anticipated benefits associated with other management options 214
Table 20: Comparison of MART estimated sewage treatment plant annual effluent loads of total N and P
and values from measurements at each plant for 2004 D-20
Table 21: Farming System and Nutrient Budget E-22
Table 22: Number of animals and amount of manure produced and N and P excreted within the MARB
states based on information from the 1997U.S. Census of Agriculture (data obtained from USDA-ERS,
http://ers.usda.gov/data/MANURE/) E-23
xx
-------
Glossary of Terms
Algae: A group of chiefly aquatic plants (e.g., seaweed, pond scum, stonewort,
phytoplankton) that contain chlorophyll and may passively drift, weakly swim, grow on a
substrate, or establish root-like anchors (steadfasts) in a water body.
Anaerobic digestion: Decomposition of biological wastes by micro-organisms, usually
under wet conditions, in the absence of air (oxygen), to produce a gas comprising mostly
methane and carbon dioxide.
Animal feeding operation (AFO): Agricultural enterprises where animals are kept and
raised in confined situations. AFOs congregate animals, feed, manure and urine, dead
animals, and production operations on a small land area. Feed is brought to the animals
rather than the animals grazing or otherwise seeking feed in pastures, fields, or on
rangeland. Winter feeding of animals on pasture or rangeland is not normally considered
an AFO.
Anoxia: The absence of dissolved oxygen.
Bacterioplankton: The bacterial component of the plankton that drifts in the water
column.
Benthic organisms: Organisms living in association with the bottom of aquatic
environments (e.g., polychaetes, clams, snails).
Best Management Practices (BMPs): BMPs are effective, practical, structural or
nonstructural methods that are designed to prevent or reduce the movement of sediment,
nutrients, pesticides and other chemical contaminants from the land to surface or ground
water, or which otherwise protect water quality from potential adverse effects of
agricultural activities. These practices are developed to achieve a cost-effective balance
between water quality protection and the agricultural production (e.g., crop, forage,
animal, forest).
Bioenergy: Useful, renewable energy produced from organic matter - the conversion of
the complex carbohydrates in organic matter to energy. Organic matter may either be
used directly as a fuel, processed into liquids and gasses, or be a residual of processing
and conversion.
Biogas: A combustible gas derived from decomposing biological waste under anaerobic
conditions. Biogas normally consists of 50 to 60 percent methane. See also landfill gas.
Biomass: Any organic matter that is available on a renewable or recurring basis,
including agricultural crops and trees, wood and wood residues, plants (including aquatic
plants), grasses, animal residues, municipal residues, and other residue materials.
Biomass is generally produced in a sustainable manner from water and carbon dioxide by
xxi
-------
photosynthesis. There are three main categories of biomass - primary, secondary, and
tertiary.
Bioreactor: A container in which a biological reaction takes place. As used in this report
a bioreactor is a container or a trench filled with a biodegradeable carbon source used to
enhance biological denitrification for removal of nitrate from drainage water.
Biosolids: Nutrient-rich soil-like materials resulting from the treatment of domestic
sewage in a treatment facility. During treatment, bacteria and other tiny organisms break
sewage down into organic matter, sometimes used as fertilizer.
Cellulosic ethanol: Ethanol that is produced from cellulose material; a long chain of
simple sugar molecules and the principal chemical constituent of cell walls of plants.
Chlorophyll: Pigment found in plant cells that are active in harnessing energy during
photosynthesis.
Conservation Reserve Program (CRP): CRP provides farm owners or operators with an
annual per-acre rental payment and half the cost of establishing a permanent land cover,
in exchange for retiring environmentally sensitive cropland from production for 10- to
15-years. In 1996, Congress reauthorized CRP for an additional round of contracts,
limiting enrollment to 36.4 million acres at any time. The 2002 Farm Act increased the
enrollment limit to 39 million acres. Producers can offer land for competitive bidding
based on an Environmental Benefits Index (EBI) during periodic signups, or can
automatically enroll more limited acreages in practices such as riparian buffers, field
windbreaks, and grass strips on a continuous basis. CRP is funded through the
Commodity Credit Corporation (CCC).
Conservation practices (CPs): Any action taken to produce environmental
improvements, particularly with respect to agricultural non-point source emissions. The
term is used broadly to refer to structural practices, such as buffers, as well as
nonstructural preactices, such as in-field nutrient management planning and application.
Conservation Practice standards have been developed by NRCS and are available at:
http://www.nrcs.usda.gov/Technical/Standards/nhcp.html.
Corn stover: Corn stocks that remain after the corn is harvested. Such stocks are low in
water content and very bulky.
Cyanobacteria: A phylum (or "division") of bacteria that obtain their energy through
photosynthesis. They are often referred to as blue-green algae, although they are in fact
prokaryotes, not algae. The description is primarily used to reflect their appearance and
ecological role rather than their evolutionary lineage. The name "cyanobacteria" comes
from the color of the bacteria, cyan.
Demersal organisms: Organisms that are, at times, associated with the bottom of aquatic
environments, but capable of moving away from it (e.g., blue crabs, shrimp, red drum).
xxn
-------
Denitrification: Nitrogen transformations in water and soil that make nitrogen effectively
unavailable for plant uptake, usually returning it to the atmosphere as nitrogen gas.
Diatom: A major phytoplankton group characterized by cells enclosed in silicon
frustules, or shells.
Dinoflagellates: Mostly single-celled photosynthetic algae that bear flagella (long cell
extensions that function in swimming) and live in fresh or marine waters.
Edge-of-fieldnitrogen loss: A term that refers to the nitrogen that is lost or exported
from fields in agricultural production.
Effluent: The liquid or gas discharged from a process or chemical reactor, usually
containing residues from that process.
Emissions: Waste substances released into the air or water. See also Effluent.
Eutrophic: Waters, soils, or habitats that are high in nutrients; in aquatic systems,
associated with wide swings in dissolved oxygen concentrations and frequent algal
blooms.
Eutrophication: An increase in the rate of supply of organic matter to an ecosystem.
Greenhouse gases: Gases that trap the heat of the sun in the Earth's atmosphere,
producing the greenhouse effect. The two major greenhouse gases are water vapor and
carbon dioxide. Other greenhouse gases include methane, ozone, chlorofluorocarbons,
and nitrous oxide.
Hydrogen sulfide: A chemical, toxic to oxygen-dependent organisms, that diffuses into
the water as the oxygen levels above the seabed sediments become zero.
Hypoxia: Very low dissolved oxygen concentrations, generally less than 2 milligrams
per liter.
Lignocellulose: A combination of lignin and cellulose that strengthens woody plant cells.
Nitrate: An inorganic form of nitrogen; chemically NOs.
Nitrogen fixation: The transformation of atmospheric nitrogen into nitrogen compounds
that can be used by growing plants.
Non-point source: A diffuse source of chemical and/or nutrient inputs not attributable to
any single discharge (e.g., agricultural runoff, urban runoff, atmospheric deposition).
xxin
-------
Nutrients: Inorganic chemicals (particularly nitrogen, phosphorus, and silicon) required
for the growth of plants, including crops and phytoplankton.
Phytoplcmkton: Plant life (e.g., algae), usually containing chlorophyll, that passively
drifts in a water body.
Plankton: Organisms living suspended in the water column, incapable of moving against
currents.
Point source: Readily identifiable inputs where treated wastes are discharged from
municipal, industrial, and agricultural facilities to the receiving waters through a pipe or
drain.
Pre-sidedress-nitrate test (PSNT): A soil nitrate-N test determined in surface soil
samples (usually 0 to 30 cm or 0 to 12 in deep), collected between corn rows when the
corn is about 15 cm (6 in) tall. Adjustments in the rate of side-dressed N can be made if
the soil test indicates elevated nitrate-N levels, based upon calibrations that vary among
growing regions. When successfully calibrated, the test results can be used as an index of
the amount of N that may be released during the course of the growing season by organic
sources such as soil organic matter, manure, and crop residues.
Productivity: The conversion of light energy and carbon dioxide into living organic
material.
Pycnocline: The region of the water column characterized by the strongest vertical
gradient in density, attributable to temperature, salinity, or both.
Recoverable manure: The portion of manure as excreted that could be collected from
buildings and lots where livestock are held, and thus would be available for land
application.
Recoverable manure nutrients: The amounts of nitrogen and phosphorus in manure that
would be expected to be available for land application. They are estimated by adjusting
the quantity of recoverable manure for nutrient loss during collection, transfer, storage,
and treatment; but are not adjusted for losses of nutrients at the time of land application.
Respiration: The consumption of oxygen during energy utilization by cells and
organisms.
Riparian Jloodplain: Area adjacent to a river or other body of water subject to frequent
flooding.
Soil tilth: The physical condition of the soil as related to its ease of tillage, fitness as a
seedbed, and impedance to seedling emergence and root penetration. A soil with good
"tilth" has large pore spaces for adequate air infiltration and water movement, and holds a
reasonable supply of water and nutrients. Soil tilth is a factor of soil texture, soil
xxiv
-------
structure, and the interplay with organic content and the living organisms that help make
up the soil ecosystem.
Stratification: A multilayered water column, delineated by pycnoclines.
Sustainable: An ecosystem condition in which biodiversity, renewability, and resource
productivity are maintained over time.
Urease and nitrification inhibitors: Urease is a ubiquitous soil microbial enzyme that
facilitates the hydrolysis of urine and urea to form ammonia. In the soil, ammonia
readily hydrolyzes to ammonium. Soil ammonium also is formed by the mineralization
of soil organic matter and manures. Ammonium is then oxidized or "nitrified" first to
nitrite (NO2) and then to nitrate (NOs), which is highly soluble and subject to movement
in the soil with the moisture front, or leaching under certain conditions. Under anaerobic
conditions, NOs can be "denitrified" to the gases nitrous oxide (N2O) and nitrogen (TSk),
and released to the atmosphere. Urease inhibitors are chemicals applied to fertilizers or
manures to reduce urease activity. Under certain environmental conditions urease
inhibitors can temporarily inhibit or reduce ammonia loss (volatilization) to the
atmosphere from urea-containing fertilizers or manures. Nitrification inhibitors are
chemicals which can temporaritly inhibit or reduce nitrification of anhydrous ammonia,
ammonium-containing or urea-containing fertilizers applied to the soil; which may
indirectly help to reduce denitrification losses of N. Under certain environmental
conditions, urease and nitrification inhibitors help improve soil retention and crop
recovery of applied N, which may reduce potential environmental N losses.
Voluntary programs: Voluntary conservation programs that have no significant financial
incentive (positive or negative) to encourage the adoption of conservation practices.
Watershed: The drainage basin contributing water, organic matter, dissolved nutrients,
and sediments to a stream or lake.
Zooplankton: Animal life that drifts or weakly swims in a water body, often feeding on
phytoplankton.
xxv
-------
List of Acronyms
ADCPs - Acoustic Doppler Current Profilers
AFO - Animal Feeding Operation
AMLE - Adjusted Maximum Likelihood Estimate
ANNAMOX - Anaerobic Ammonia Oxidation
A/P ratio - Agglutinated to Porcelaneous ratio (based on the relative abundance of three
low-oxygen tolerant species of benthic foraminifers; Pseudononin altlanticum,
Epistominella vitrea, and Buliminella morganf)
ARS - Agricultural Research Service (USDA)
AUs - Animal Units
BBL - Benthic Boundary Layer
BMPs - Best Management Practices
BNR - Biological Nutrient Removal
BOD - Biochemical Oxygen Demand
Bu/A - Bushels per acre
C - Carbon
CAFO - Concentrated Animal Feeding Operation
CASTnet - Clean Air Status and Trends Network
CC or Ccc - Continuous Corn
CCC - Commodity Credit Corporation
CCOA - Corn-Corn-Oat-Alfalfa (crop rotation)
CDOM - Colored Dissolved Organic Matter
CEAP - Conservation Effectiveness Assessment Program
CENR - Committee on Environment and Natural Resources
Cm - Corn-meadow (crop rotation)
CMAQ - Community Multiscale Air Quality model
COAA - Corn-Oat-Alfalfa-Alfalfa (crop rotation)
CO2 - Carbon Dioxide
cph - cycles per hour
CPRA - Coastal Protection and Restoration Authority
CREP - Conservation Reserve Enhancement Program
CRN - Controlled - and slow Release N fertilizers
CRP - Conservation Reserve Program
CRPA - Coastal Protection and Restoration Authority
CS or CSb - Corn Soybean rotation
CSP - Conservation Security Program
CTA - Conservation Technical Assistance
CTDs - Conductivity, Temperature, and Depth instrumentation
CVs - Coefficients of Variations
DDGs - Dried Distillers Grain
DIN:DIP - Dissolved Inorganic Nitrogen:Dissolved Inorganic Phosphorus
DO - Dissolved Oxygen
DOC - Dissolved Organic Carbon
DOE - Department of Energy
xxvi
-------
DOM - Dissolved Organic Matter
DON - Dissolved Organic Nitrogen
DRP - Dissolved Reactive Phosphorus
EBI - Environmental Benefits Index
ECa - Electrical Conductivity
ENR - Enhanced Nutrient Removal
EPCo - Equilibrium P Concentration
EPIC - Environment Productivity Impact Calculator model
EQIP Environmental Quality Incentives Program
ERS - Economic Research Service (USDA)
Fe+2 - Ferrous Iron
FR - Federal Register
FWA - Flow Weighted Average
GAO - General Accounting Office
GCOOS - Gulf of Mexico Coastal Ocean Observing System
GCTM - Global Chemistry Transport Model
GHG - Green House Gases
GIS - Geographic Information System
GLWQA - Great Lakes Water Quality Agreement
GOM-Gulf of Mexico
GPS - Global Positioning System
GWW - Grass Waterways
HAB - Harmful Algal Bloom
HAP - Hypoxia Advisory Panel or SAB Panel
HEL - Highly Erodable Land
HLR - Hydraulic Loading Rate
HRUs - Hydraulic Response Units
HUC - Hydrologic Unit Code
HYDRA - Hydrological Routing Algorithm
IATP - Institute of Agricultural and Trade Policy
IBIS - Integrated Biosphere Simulator model
IJC - International Joint Commission
IPCC - Intergovernmental Panel on Climate Change
ISNT - Illinois Soil Nitrogen Test
LOADEST - Load Estimator model
LOWESS - Locally Weighted Scatterplot Smooth curves
LSNT - Late Spring Nitrate Test
LUMCON - Louisiana Universities Marine Consortium
M - Million
MGD - Million gallons per day
MARB - Mississippi-Atchafalaya River basin
MART Management Action Reassessment Team
Mn+2 - Manganese (oxidation state common in aquatic-biological systems)
MRB - Mississippi River basin
MR/GMWNTF - Mississippi River/Gulf of Mexico Watershed Nutrient Task Force
MSEA - Management System Evaluation Area
xxvn
-------
N Nitrogen
N2 - Nitrogen gas (colorless, odorless, and tasteless gas that makes up 78.09% of air)
N2O - Nitrous Oxide
NADP - National Air Deposition Program
NANI Net Anthropogenic Nitrogen Inputs
NAS - National Academy of Sciences
NASA - National Aeronautics and Space Administration
NASA-SeaWiFS - NASA Sea-viewing Wide Field-of-view Sensor (project providing
qualitative data on global ocean bio-optical properties)
NASQAN - National Stream Quality Accounting Network (USGS water-quality
monitoring program)
NECOP - Nutrient Enhanced Coastal Ocean Productivity
NGOM - Northern Gulf of Mexico
NH3 Ammonia
NH4+ Ammonium
NHx - The total atmospheric concentration of ammonia (NH3) and ammonium (NH4+)
NOAA - National Oceanic and Atmospheric Administration
NO2 - Nitrite Nitrogen (MV) if in water and Nitrogen Dioxide (NO2) if in air
NO3 - Nitrate nitrogen
NOx - Mono-nitrogen oxides, or the total concentration of nitric oxide (NO) plus
nitrogen dioxide (NOz)
NOy - Reactive odd nitrogen or the sum of NOx plus compounds produced from the
oxidation of NOX, which includes nitric acid, peroxyacetyl nitrate, and other compounds
NPDES - National Pollutant Discharge Elimination System
NPSs - Non-Point Sources
NRC - National Research Council
NRCS - Natural Resource Conservation Service
NRI - National Resources Inventory
NSTC - National Science and Technology Council
O2 - Diatomic Oxygen (makes up 20.95% of air)
OM - Organic Matter
P - Phosphorus
PEB index - An index based on the relative abundance of three low-oxygen tolerant
species of benthic foraminifers; Pseudononin altlanticum, Epistominella vitrea, and
Buliminella morgani
POC - Particulate Organic Carbon
ppmv - Parts per million by volume
ppt - Parts per thousand
PS - Point Source
PSNT - Pre-Sidedress Nitrate Test
RivR-N A regression model that predicts the proportion of N removed from streams
and reservoirs as an inverse function of the water displacement time of the water body
(ratio of water body depth to water time of travel)
SAB - Science Advisory Board
SCOPE - Science Committee on Problems of the Environment
SD - Standard Deviation
xxvin
-------
Si - Silicon
SOC - Soil Organic Carbon
SOM - Soil Organic Matter
SON - Soil Organic Nitrogen
SPARROW - Spatially Referenced Regression on Watershed attributes model
SRP or DRP or ortho P - Soluble Reactive Phosphorus, Dissolved Reactive Phosphorus,
Orthophosphate
STATSGO - State Soil Geographic database
STORET - STOrage and RETrieval data system (EPA's largest computerized
environmental data system)
STPs - Sewage Treatment Plants
SWAT - Soil and Water Assessment Tool model
THMB - Terrestrial Hydrology Model with Biogeochemistry
TKN - Total Kjeldahl Nitrogen
TM3 - Tracer Model version 3 (a global atmospheric chemistry/transport model)
TN - Total Nitrogen
TP - Total Phosphorus
TPCs - Typical Pollutant Concentrations
TSS - Total Suspended Solids
UAN - Urea Ammonium Nitrate
UMRB - Upper Mississipppi River basin
UMRSHNC - Upper Mississippi River Sub-basin Hypoxia Nutrient Committee
USMP - U.S. Agriculture Sector Mathematical Programming model
USAGE - United States Army Corps of Engineers
USDA - United States Department of Agriculture
USEPA or EPA - United States Environmental Protection Agency
USGS - United States Geological Survey
WRP - Wetlands Reserve Program
xxix
-------
Conversion Factors and Abbreviations
MULTIPLY BY
centimeter (cm) 0.3937
millimeter (mm) 0.03 94
meter (m) 3.281
kilometer (km) 0.6214
square kilometer (km2) 0.3861
hectare (ha) 2.471
hectare (ha) 0.01
liter (L) 1.057
liter (L) 0.0284
gram (g) 0.0353
gram per cubic meter (g/m3) 0.00169
kilogram (kg) 2.205
metric tonne (tonne) 2,205.0
metric tonne (tonne) 1.1023
cubic meter per second (m3/s) 35.31
kilogram per hectare (kg/ha) 0.893
TO OBTAIN
inch (in)
inch (in)
foot (ft)
mile (mi)
square mile (mi2)
acre (ac)
square kilometer (km2)
quart (qt)
bushel (bu) US, dry
ounce(oz)
pound per cubic yard (lb/yd3)
pound (lb), avoirdupois
pound (lb), avoirdupois
U.S. short ton (ton)
cubic foot per second (cfs)
pound per acre (Ib/ac)
CONCENTRATION UNIT
milligram per liter (mg/L)
APPROXIMATELY EQUALS
part per million (ppm)
The following equation was used to compute flux of chemicals:
-2
concentration (mg/L) x flow (m /s) x 8.64 x 10" = metric tonne per day (tonne/d)
XXX
-------
Executive Summary
Since 1985, scientists have been documenting a hypoxic zone in the Gulf of
Mexico each year. The hypoxic zone, an area of low dissolved oxygen that cannot
support marine life, generally manifests itself in the spring. Since marine species either
die or flee the hypoxic zone, the spread of hypoxia reduces the available habitat for
marine species, which are important for the ecosystem as well as commercial and
recreational fishing in the Gulf. Since 2001, the hypoxic zone has averaged 16,500 km2
during its peak summer months1 and ranged from a low of 8,500 km2 to a high of 22,000
km2. To address the hypoxia problem, the Mississippi River/Gulf of Mexico Watershed
Nutrient Task Force (or Task Force) was formed to bring together representatives from
federal agencies, states and tribes to consider options for responding to hypoxia. The
Task Force asked the White House Office of Science and Technology Policy to conduct a
scientific assessment of the causes and consequences of Gulf hypoxia through its
Committee on Environment and Natural Resources (CENR). In 2000 the CENR
completed An Integrated Assessment: Hypoxia in the Northern Gulf of Mexico
(IntegratedAssessment), which formed the scientific basis for the Task Force's Action
Plan for Reducing, Mitigating and Controlling Hypoxia in the Northern Gulf of Mexico
(Action Plan., 2001). In its Action Plan, the Task Force pledged to implement ten
management actions and to assess progress every five years. This reassessment would
address the nutrient load reductions achieved, the responses of the hypoxic zone and
associated water quality and habitat conditions, and economic and social effects. The
Task Force began its reassessment in 2005.
In 2006 as part of the reassessment, EPA's Office of Water, on behalf of the Task
Force, requested that the Environmental Protection Agency (EPA) Science Advisory
Board (SAB) convene an independent panel to evaluate the state of the science regarding
hypoxia in the Northern Gulf of Mexico and potential nutrient mitigation and control
options in the Mississippi-Atchafalaya River basin (MARB). The Task Force was
particularly interested in scientific advances since the Integrated Assessment and issued
charge questions in three areas: characterization of hypoxia; nutrient fate, transport and
sources; and the scientific basis for goals and management options. The SAB Hypoxia
Advisory Panel (SAB Panel) began its deliberations in September of 2006 and completed
its report in August of 2007 while operating under the "sunshine" requirements of the
Federal Advisory Committee Act, which include providing public access to advisory
meetings and opportunities for public comment. This Executive Summary summarizes
the SAB Panel's major findings and recommendations.
1 The areal extent of the full hypoxic region has not been mapped with sufficient frequency to completely
understand its temporal variability. The limited number of observations that have been taken more than
once per year suggest that the hypoxic region reaches its maximum extent in late summer. There are
physical and biological reasons to expect such a pattern of temporal variation but available data provide a
conservative estimate of the maximum extent of hypoxia. The actual areal extent may be larger than
estimated.
-------
Findings
Since publication of the Integrated Assessment, scientific understanding of the
causes of hypoxia has grown while actions to control hypoxia have lagged. Recent
science has affirmed the basic conclusion that contemporary changes in the hypoxic area
in the northern Gulf of Mexico (NGOM) are primarily related to nutrient fluxes from the
MARB. Moreover, new research provides early warnings about the deleterious long-
term effects of hypoxia on living resources in the Gulf.
The SAB Panel was asked to comment on the Action Plan's goal to reduce the
rt n
hypoxic zone to a five-year running average of 5,000 km by 2015. The 5,000 km target
remains a reasonable endpoint for continued use in an adaptive management context;
however, it may no longer be possible to achieve this goal by 2015 for two reasons. First,
there is limited current movement to implement policies, programs and strategies that
reduce nutrients. Second, there are time lags between reductions in nutrients and the
response of the ecological system. In August of 2007, the hypoxic zone was measured to
be 20,500 km2 (LUMCON, 2007), the third largest hypoxic zone since measurements
began in 1985. Accordingly, it is even more important to proceed in a direct!onally
correct fashion to manage factors affecting hypoxia than to wait for greater precision in
setting the goal for the size of the zone. Much can be learned by implementing
management plans, documenting practices, and measuring their effects with appropriate
monitoring programs.
To reduce the size of the hypoxic zone and improve water quality in the MARB,
the SAB Panel recommends a dual nutrient strategy targeting at least a 45% reduction in
riverine total nitrogen flux (to approximately 870,000 metric tonne/yr or 960,000 ton/yr)
and at least a 45% reduction in riverine total phosphorus flux (to approximately 75,000
metric tonne/yr or 83,000 ton/yr). Both of these reductions refer to changes measured
against average flux over the 1980 - 1996 time period. For both nutrients, incremental
annual reductions will be needed to achieve the 45% reduction goals over the long run.
For nitrogen, the greatest emphasis should be placed on reducing spring flux, the time
period most correlated with the size of the hypoxic zone. While the state of predictive
and process models of NGOM hypoxia has continued to develop since 2000, models
similar to those in place at that time are still the best tools for producing dose response
estimates for nitrogen (N) reductions, with most recent model runs showing a 45 - 55%
required reduction for N in order to reduce the size of the hypoxic zone. A number of
studies have suggested that climate change will create conditions for which larger
nutrient reductions, e.g., 50 - 60% for nitrogen, would be required to reduce the size of
the hypoxic zone.
New information has emerged that more precisely demonstrates the role of
phosphorus (P) in determining the size of the hypoxic zone. Contrary to conventional
wisdom that N typically limits phytoplankton production in near-coastal waters, the
NGOM exhibits an unusual phenomenon whereby P is an important limiting constituent
-------
during the spring and summer in the lower salinity, near-shore regions. Phosphorus
limitation is now occurring because over the past 50 years excessive N loadings have
dramatically altered nitrogen to phosphorus ratios. Taken together, N and P both
contribute to excess phytoplankton production and the hypoxia associated with such
production, and they will need to be reduced concurrently to make progress in reducing
the size of the hypoxic zone. The SAB Panel's best professional judgment is that
phosphorus reductions will need to be comparable (in percentage terms) to nitrogen
reductions to reduce the size of the hypoxic zone.
Scientific advances have improved our understanding of the physical factors that
contribute to hypoxia. One physical factor that has changed substantially over the past
century is river hydrology. The hydrologic regime of the Mississippi and Atchafalaya
Rivers and the timing of freshwater inputs to the continental shelf are critical to mixing
and hypoxia development. The most important hydrological change over the past century
has been the diversion of a large amount of freshwater from the Mississippi River
through the Atchafalaya River to the Atchafalaya Bay, and maintenance of this diversion
by the U. S. Army Corps of Engineers. The major injection of freshwater into
Atchafalaya Bay, some 200 kilometers to the west of the Mississippi River Delta, has
profoundly modified the spatial distribution of freshwater inputs, nutrient loadings and
stratification on the Louisiana-Texas continental shelf.
Methods used by the U.S. Geological Survey (USGS) to calculate nutrient fluxes
in the MARB have changed since the Integrated Assessment. The latest USGS estimates
show that total N flux averaged 1.24 million metric tonne/yr (1.37 million ton/yr) from
2001 - 2005 (65% of the flux is nitrate), and the total P flux averaged 154,000 metric
tonne/yr (170,000 ton/yr). This change represents a 21% decline in total N flux and a
12% increase in total P flux when compared to the averages from the 1980 - 1996 time
period. The spring (April - June) flux of nutrients appears to be an important
determinant of hypoxia, for that is when the river is disproportionately enriched with both
N (especially nitrate) and P. Spring total N flux has declined since the 1980s; whereas
total P flux shows a 9.5% increase (when average total P flux for 2001-2005 is compared
to the 1980 - 1996 average). USGS data also show that during the last 5 years, the upper
Mississippi and Ohio-Tennessee River subbasins contributed about 82% of nitrate-N
flux, 69% of the TKN flux, and 58% of total P flux, although these sub-basins represent
only 31% of the entire MARB drainage area.
The SAB Panel's estimates of point source discharge show that point sources
represented 22% of total annual average N flux and 34% of total annual average P flux
discharged to the NGOM during the last five years. New methods also have been used to
calculate nutrient mass balances (net anthropogenic N inputs, NANI). NANI for the
MARB has declined in the past decade because of increased crop yields, reduced or
redistributed livestock populations, and little change in N fertilizer inputs. From 1999-
2005, NANI calculations show 54% of non-point N inputs in the MARB were from
fertilizer, 37% from nitrogen fixation, and 9% from atmospheric deposition.
-------
The SAB Panel finds that the Gulf of Mexico ecosystem appears to have gone
through a regime shift with hypoxia such that today the system is more sensitive to inputs
of nutrients than in the past, with nutrient inputs inducing a larger response in hypoxia as
shown for other coastal marine ecosystems such as the Chesapeake Bay and Danish
coastal waters. Changes in benthic and fish communities with the change in frequency
of hypoxia are cause for concern. The recovery of hypoxic ecosystems may occur only
after long time periods or with further reductions in nutrient inputs. If actions to control
hypoxia are not taken, further ecosystem impacts could occur within the Gulf, as has been
observed in other ecosystems.
Certain aspects of the nation's current agricultural and energy policies are at odds
with the goals of hypoxia reduction and improving water quality. Since the Integrated
Assessment, an emerging national strategy on renewable fuels has granted economic
incentives to corn-based ethanol production. The projected increase in corn production
from this strategy has profound implications for water quality in the MARB, as well as
hypoxia in the NGOM. Recent energy policies, combined with pre-existing crop
subsidies, tax policies, global market conditions and trade barriers all provide economic
incentives for conversion of retired and other cropland to corn production for use in
ethanol production. Such conversions are projected to lead to corn production on an
additional 6.5 million ha (16 million ac) in coming years with the majority of this
increase occurring in the MARB. Without some change to the current structure of
economic incentives favoring corn-based ethanol, N loadings to the MARB from
increased corn production could increase dramatically in coming years, rather than
decreasing, as needed for the NGOM.
Recommendations for Monitoring and Research
Most of the research and monitoring needs identified in the Integrated Assessment
have not been met, and fewer rivers and streams are monitored today than in 2000. The
majority of monitoring recommendations in the Integrated Assessment remain relevant
and should be heeded. The SAB Panel affirms and reiterates the CENR's call to improve
and expand monitoring of the temporal and spatial extent of hypoxia and the processes
controlling its formation; the flux of nutrients, carbon, and other constituents from non-
point sources throughout the MARB and to the NGOM; and measured (rather than
estimated) nitrogen and phosphorus fluxes from municipal and industrial point sources.
The SAB Panel affirms the need for research in the following areas identified in
the Integrated Assessment: ecological effects of hypoxia; watershed nutrient dynamics;
effects of different agricultural practices on nutrient losses from land, particularly at the
small watershed scale; nutrient cycling and carbon dynamics; long-term changes in
hydrology and climate; and economic and social impacts of hypoxia.
A suite of models is needed to simulate the processes and linkages that regulate
the onset, duration and extent of hypoxia. Emerging coastal ocean observation and
prediction systems should be encouraged to monitor dissolved oxygen and other physical
and biogeochemical parameters needed to continue improving hypoxia models.
-------
To advance the science characterizing hypoxia and its causes, the SAB Panel
finds that research is also needed to:
collect and analyze additional sediment core data needed to develop a
better understanding of spatial and temporal trends in hypoxia;
investigate freshwater plume dispersal, vertical mixing processes and
stratification over the Louisiana-Texas continental shelf and Mississippi
Sound, and use three-dimensional hydrodynamic models to study the
consequences of past and future flow diversions to NGOM distributaries;
advance the understanding of biogeochemical and transport processes
affecting the load of biologically available nutrients and organic matter to
the Gulf of Mexico, and develop a suite of models that integrate physics
and biogeochemistry;
elucidate the role of P relative to N in regulating phytoplankton production
in various zones and seasons, and investigate the linkages between inshore
primary production, offshore production, and the fate of carbon produced
in each zone;
improve models that characterize the onset, volume, extent, and duration
of the hypoxic zone, and develop modeling capability to capture the
importance of P, N, and P-N interactions in hypoxia formation;
To advancing the science on sources, fate and transport of nutrients, the SAB
Panel recommends research to:
develop models to simulate fluvial processes and estimate N and P transfer
to stream channels under different management scenarios;
improve the understanding of temporal and seasonal nutrient fluxes and
develop nutrient, sediment, and organic matter budgets within the MARB;
To enhance the scientific basis for implementation of management options, the
SAB Panel finds that research is needed to:
examine the efficacy of dual nutrient control practices;
determine the extent, pattern, and intensity of agricultural drainage as well
as opportunities to reduce nutrient discharge by improving drainage
management;
integrate monitoring, modeling, experimental results, and ongoing
management into an improved conceptual understanding of how the forces
-------
at key management scales influence the formation of the hypoxia zone;
and
develop integrated economic and watershed models to fully assess costs
and benefits, including co-benefits, of various management options.
Developments in the biofuels industry have created new questions for researchers
to address. More research is needed on biofuel life cycles in order to identify system
efficiency with respect to environmental effects, economics, and resource availability of
biofuel alternatives. That is, research needs to evaluate the environmental effects of
different biofuel production processes on soil, water quality and climate under realistic
strategies of deploying production facilities and moving the biofuels to the market.
Current incentives favor corn-based ethanol production, although research has thus far
shown fewer environmental consequences with other feedstocks, e.g., cellulosic
feedstocks such as switchgrass. Yet the technology for conversion of cellulosic
feedstocks to biofuel is not yet commercially viable. Policies of all kinds (taxes,
subsidies, trade) could be used to support research and technological developments for
those biofuels that balance high energy yields with the lowest environmental impacts.
Recommendations for Adaptive Management
Adaptive management provides a framework for ongoing management in the face
of uncertainty. It requires that conceptual models be developed to guide management and
that management actions be treated like well-monitored experiments that answer
questions for improving decisions with each successive cycle of learning. The most
urgent need is to decrease nutrient discharge. In fact, nutrients should be decreased as
soon as possible before the system requires even larger nutrient reductions to reduce the
area of hypoxia. Already many taxa are lost during the peak of hypoxia, and there has
been a shift in the relative abundance offish species. Increases in certain pelagic species
can disrupt food web structure, and the new system may respond in a quite different way
to changes in nutrient level. The SAB Panel thus agrees with the CENR's emphasis on
decreasing nutrient discharge in the context of adaptive management.
These adaptive management actions must be interpreted in view of both field
measures and models of their effects. Conceptual models are needed for nutrient
management at several spatial resolutions from small catchments, to large watersheds, to
the entire MARB in order to guide research and ongoing adaptive management at each of
the relevant scales. To the greatest extent possible, feedbacks should be incorporated into
the models so that management is accompanied by learning about the full systems of
linkages between human activities and hypoxia as well as the full range of co-benefits of
N and P reductions.
-------
Management Options
Large N and P reductions, on the order of 45% or more, are needed to reduce the
size of the hypoxic zone. To do this, the SAB Panel found the most significant
opportunities for N and P reductions occur in five areas:
promotion, via research and economic incentives, of environmentally
sustainable approaches to biofuel production and associated cropping systems
(e.g., perennials).
improved management of nutrients by emphasizing infield nutrient
management efficiency and effectiveness to reduce losses;
construction and restoration of wetlands, as well as criteria for targeting those
wetlands that may have a higher priority for reducing nutrient losses;
introduction of tighter N and P limits on municipal and private industrial point
sources; and
improved targeting of conservation buffers, including riparian buffers, filter
strips and grassed waterways, to control surface-borne nutrients.
Importantly, not all approaches will be cost-effective in all locations; the optimal
combination and location of these practices will vary across and within watersheds.
In terms of cropping systems, research comparing nutrient discharge between
alternative cropping systems (including row crops and non-row crops such as perennials)
and a corn-soybean rotation shows that significant nutrient loss reductions could be
achieved by converting current corn-soybean rotations to alternative crops or alternative
rotations. Moreover, since corn crops require more nitrogen input, cellulosic sources
(e.g., perennial grasses, fast-growing woody species, etc.) could, by comparison, provide
alternative energy while protecting water quality. However, the technology for
converting cellulosic sources to biofuel is not yet commercially viable. Significant
reductions in nutrient runoff could also be achieved if nutrients are managed more
efficiently on farms, for example by moving to spring fertilization rather than fall. More
wetlands are needed, especially in those areas that promise the greatest N and P
reductions. Since the greatest N and P runoff is coming from upper Mississippi and
Ohio-Tennessee River subbasins, where the highest proportion of tile drainage occurs,
measures to improve drainage water management are urgently needed. In fact, improved
targeting of almost all agricultural conservation practices in the region [e.g., conservation
buffers, wetlands, land set aside in the Conservation Reserve Program (CRP), drainage
water management, etc.] could achieve greater local water quality benefits and
simultaneously contribute to hypoxia reduction. Nearly all of these opportunities were
recognized in the Integrated Assessment.
-------
The CENR did not emphasize tighter limits on municipal point sources; however
new calculations from the SAB Panel indicate that 22% of annual average total N flux
and 34% of annual average total P flux to the Gulf comes from permitted point-source
dischargers. The SAB Panel's calculations further demonstrate that tighter limits on N
and P in effluent (3 mg N/L and 0.3 mg P/L) from sewage treatment plants could realize
an estimated 11% reduction in annual average total N flux and a 21% reduction in total
annual average P flux to the Gulf. Although the exact N and P limit could be debated,
clearly there are regulatory opportunities to significantly reduce N and P fluxes to the
Gulf. The cost associated with such regulations could be reduced if trading programs for
point and non-point sources are properly developed and implemented concurrently with
regulations.
Protecting and Enhancing Social Welfare in the Basin
Implementing the management options needed to reduce nutrients will clearly
affect the social welfare of many who live in the basin. On the positive side, N and P
reductions will improve environmental quality within the basin and, as the Integrated
Assessment documented, these co-benefits can be highly valuable. Second, if the costs of
implementing these management options are borne largely by residents in the region,
then preserving/enhancing social welfare will require implementing policies that target
the most cost-effective sources and locations for nutrient reductions.
Subsidies, not regulation, have been the government's primary tool for managing
agricultural production and income support in the U.S., as well as conservation in
agriculture. Hence re-structuring subsidies and conservation programs represents an
important tool for reducing nutrient runoff from agricultural production. The Integrated
Assessment recognized numerous agricultural management practices that improve water
quality but did not discuss the efficiency of the tools for their implementation. A large
body of economics literature exists regarding the relative merits and cost-effectiveness of
taxes, regulations, voluntary approaches, permit trading, subsidies, and other instruments
that could apply to reducing nutrient losses. This research indicates that if significant
behavioral changes are to be realized, incentives are needed across a wide range of
sectors. Such incentives can be positive (e.g., subsidies) or negative (e.g., taxes or
direction regulation with enforcement actions), but they must be strong enough to change
behavior. A thorough and quantitative comparison of all possible incentives for all
sectors was beyond the SAB Panel's scope; however, research indicates that the
following approaches are cost-effective.
First, the establishment (and continuation where appropriate) of targeting and
competitive bidding mechanisms results in lands enrolled in conservation programs (e.g.,
the Conservation Reserve Program, the Environmental Quality Incentives Program, and
the Conservation Security Program) that achieve maximum environmental benefits.
Moreover, conservation compliance requirements extended to nutrient management, if
adequately monitored and enforced, could be cost-effective. Targeting conservation
practices to the locations within a watershed where they produce the most N and P
-------
reductions (and co-benefits) and targeting entire watersheds that have relatively high N
and/or high P contributions are both cost-effective targeting approaches.
Second, economic incentives are needed for the full range of conservation
options. Incentives for development of technologies to convert cellulosic perennials to
biofuels would be needed to greatly reduce N and P losses from agricultural systems. Re-
structuring eligibility requirements for existing subsidies to reward conservation in all its
forms (in-field nutrient management, cover crops, conservation buffers, wetlands,
alternative drainage, manure management) could help mitigate the unintended
consequences of agricultural production.
Conclusion
In sum, environmental decisions and improvements require a balance between
research, monitoring and action. In the Gulf of Mexico, the action component lags
behind the growing body of science. Moreover, certain aspects of current agricultural
and energy policies conflict with measures needed for hypoxia reduction. Although
uncertainty remains, there is an abundance of information on how to reduce hypoxia in
the Gulf of Mexico and to improve water quality in the MARB, much of it highlighted in
the Integrated Assessment. To utilize that information, it may be necessary to confront
the conflicts between certain aspects of current agricultural and energy policies on the
one hand and the goals of hypoxia reduction and improving water quality on the other.
This dilemma is particularly relevant with respect to those policies that create economic
incentives. The SAB Panel's recommendation to address the structure of economic
incentives stems from sound science.
Basing management decisions on sound science means taking action at several
different scales, addressing conflicts between policies, and acting in the face of
uncertainties. Lessons learned from current actions can inform and improve future
decisions. While actions must come first, they must also be coupled with monitoring and
modeling of management activities within a conceptual framework to improve
understanding of the system. Done well, this process of adaptive management means
that, over time, society will benefit from cost-effective environmental decisions that
reduce hypoxia in the Gulf and improve water quality in the MARB.
-------
1. Introduction
1.1. Hypoxia and the Northern Gulf of Mexico - A Brief Overview
Nutrient over enrichment from anthropogenic sources is a major stressor of
aquatic, estuarine, and marine ecosystems. Nutrients enter ecosystems through off-target
migration of fertilizer from agricultural fields, golf courses, and lawns; disposal of animal
manure; atmospheric deposition of nitrogen; erosion of soil containing nutrients; sewage
treatment plant discharges; and other industrial discharges. Excessive nutrients promote
nuisance blooms (excessive growth) of opportunistic bacteria, cyanobacteria, and algae.
When the available nutrients in the water column have been sequestered in plant biomass,
the nuisance blooms die, decompose, and deplete dissolved oxygen in the water column
and at the sediment water interface. This oxygen depletion, known as hypoxia, occurs
when normal dissolved oxygen concentrations in shallow coastal and estuarine systems
decrease below the level required to support many estuarine and marine organisms (< 2
mg/L).
Hypoxia can occur naturally in deep basins, fjords, and oxygen minimal coastal
zones associated with upwelling. However, nutrient induced hypoxia in shallow coastal
and estuarine systems is increasing worldwide. A large hypoxic area, averaging about
16,500 km2 (10, 250 mi2) and ranging from 8,500 to 22,000 km2 (3,100 to 7,700 mi2)
forms annually between May and September in the northern Gulf of Mexico. Shown in
Figure 1, the northern Gulf hypoxic zone is the largest in the United States and the
second largest worldwide. Hypoxic conditions result from complex interactions between
climate, weather, basin morphology, circulation patterns, water retention times,
freshwater inflows, stratification, mixing, and nutrient loadings. Nutrient fluxes from the
Mississippi-Atchafalaya River basin (MARB), coupled with temperature and density
induced stratification have been implicated as the primary cause of hypoxia in the
northern Gulf of Mexico (NGOM) (CENR, 2000).
30.0-
29.5-
29.0-
23.5^
rJ t Calcasieu
-93.5
-92.5
-915
-90.5
-69.5
Figure 1: Map of the frequency of hypoxia in the northern Gulf of Mexico, 1985-2005. Taken from N.N.
Rabalais, LUMCON, 2006.
10
-------
The MARB is one of the largest river systems in the world (Figure 2), draining
approximately 40% of the contiguous United States, and is the largest contributor of
freshwater and nutrients to the NGOM. About two thirds of the total Mississippi River
flow enters the northern Gulf via the Mississippi River delta. The remaining third is
diverted to the Atchafalaya River and eventually enters the northern Gulf about 200 km
west of the main Mississippi River delta. Prevailing east-to-west currents in the Gulf
move much of the freshwater, suspended sediments, and dissolved and particulate
nutrients onto the Louisiana-Texas continental shelf.
Figure 2: Map showing the extent of the Mississippi-Atchafalaya River basin.
Land-use activities in the MARB influence water quality in the entire watershed
as well as in the NGOM. Low oxygen events on the Louisiana-Texas continental shelf
have been reconstructed over the past 180 years using the relative abundance of low-
oxygen-tolerant benthic foraminifera in sediment cores (Osterman et al., 2005). These
data show that the prevalence of low oxygen events has increased over the past 50 years.
Several hypoxic events from 1870 and 1910 (prior to widespread fertilizer use) were
attributed to natural variation in river flow that enhanced freshwater and nutrient
transport. The increased prevalence over the past several decades is clearly related to
increased nutrient loads. However, there is substantial variation in year-to-year inputs of
both freshwater and nutrients from the MARB. Since these are correlated, it is not
possible to tease apart the relative importance of increased eutrophication versus
increased stratification in any given year over the recent past. Clearly, land-use practices
in the MARB affect watershed dynamics and water quality within the Basin as well as the
11
-------
northern Gulf. Land-use practices in the Basin are also influenced by various, and
conflicting, national environmental, conservation and agricultural policies.
1.2. Science and Management Goals for Reducing Hypoxia
In 1997, the U. S. Environmental Protection Agency (EPA) established the
Mississippi River/Gulf of Mexico Watershed Nutrient Task Force (or Task Force). The
Task Force brought together federal agencies, states and tribes to consider options for
reducing, mitigating, and controlling hypoxia in the NGOM. The Task Force requested
that the White House National Science and Technology Council (NSTC) conduct a
scientific assessment of the causes and consequences of Gulf hypoxia. The NSTC
Committee on Environment and Natural Resources (CENR) formed a federal intra-
agency Hypoxia Working Group to plan and conduct the assessment. The need for the
assessment was given additional impetus by passage of the Harmful Algal Bloom and
Hypoxia Research and Control Act of 1998. The Act specifically called for an integrated
scientific assessment of causes and consequences of hypoxia in the Gulf of Mexico and a
plan of action to reduce, mitigate, and control hypoxia.
The scientific assessment was led by the National Oceanic and Atmospheric
Administration (NOAA) with oversight among several federal agencies. As a first step,
six reports (available at http.//www.nos.noaa.gov/products/pub_hypox.html) covering
key topics were developed. These include characterization of hypoxia (Rabalais et al.,
1999a); ecological and economic consequences of hypoxia (Diaz and Solow, 1999); flux
and sources of nutrients in the Mississippi-Atchafalaya River basin (Goolsby et al.,
1999); effects of reducing nutrient loads to surface waters within the Mississippi River
basin and Gulf of Mexico (Brezonik et al., 1999); reducing nutrient fluxes, especially
nitrate-nitrogen, to surface water, ground water, and the Gulf of Mexico (Mitsch et al.,
1999); and evaluation of the economic costs and benefits of the methods for reducing
nutrient fluxes to the Gulf of Mexico (Doering et al., 1999).
The six NOAA reports provided the scientific foundation for the Integrated
Assessment of Hypoxia in the Northern Gulf of Mexico (CENR, 2000) {or Integrated
Assessment, available at http://oceanservice.noaa.gov/products/pubs_hypox.html). The
Integrated Assessment concluded that hypoxia in the northern Gulf was caused by excess
nitrogen from the MARB, in combination with stratification of Gulf waters. Informed by
the Integrated Assessment, in 2001 the Task Force completed its Action Plan for
Reducing, Mitigating and Controlling Hypoxia in the Northern Gulf of Mexico
(MR/GMWNTF, 2001) (or Action Plan, available at
http://www.epa.gov/msbasin/taskforce/actionplan.htm). The Action Plan described three
primary hypoxia management goals.
1. Coastal Goal: By the year 2015, subj ect to the availability of additional
resources, reduce the five-year running average of the areal extent of the
Gulf of Mexico hypoxic zone to less than 5,000 km2 (1,930 mi2) through
implementation of specific, practical, and cost-effective voluntary actions
by all states, tribes, and all categories of sources and removals within the
12
-------
Mississippi-Atchafalaya River basin to reduce the annual discharge of
nitrogen into the Gulf.
2. Within Basin Goal: To restore and protect the waters of the 31 states and
tribes within the MARB through implementation of nutrient- and
sediment- reduction actions to protect public health and aquatic life as
well as reduce negative impacts of water pollution on the Gulf of Mexico.
3. Quality of Life Goal: To improve the communities and economic
conditions across the Mississippi-Atchafalaya River basin, in particular
the agriculture, fisheries, and recreation sectors, through improved public
and private land management and a cooperative incentive based approach.
In 2005, the Task Force recognized a need to update the Integrated Assessment
and Action Plan with more recent science. Accordingly, the Task Force sponsored four
symposia on the upper Mississippi River basin; Gulf Hypoxia; the lower Mississippi
River basin, and Nutrient Sources, Fate and Transport. Each of the symposia focused on
scientific developments since 1999. In conjunction with the symposia, the Task Force
also developed a bibliography of recent literature on hypoxia causes, effects, and control
options since the year 2000 (available at
http://www.epa.gov/msbasin/taskforce/reassess2005.htm). In addition to science
activities, the Task Force also compiled information necessary for nutrient management
and control in the MARB in two reports. The Management Action Review Team Report
(MART, 2006a) summarized federal programs that encouraged watershed planning and
land-use practices to reduce nutrient loadings. The Reassessment of Point Source
Nutrient Mass Loadings to the Mississippi River Basin report (MART, 2006b) updated
annual mass loading estimates for total nitrogen (TN), total phosphorus (TP), and
biochemical oxygen demand (BOD) (Task Force documents are available at
http://www.epa.gov/msbasin/taskforce/reassess2005.htm.) The Task Force is also
working with the U. S. Department of Agriculture's (USDA) Conservation Effects
Assessment Program (CEAP) to encourage the quantification and documentation of
environmental effects and benefits of conservation practices on agricultural lands to
control nutrients in the MARB. CEAP documents are available at
http://www.nrcs.usda.gov/Technical/nri/ceap/.
13
-------
1.3. EPA Science Advisory Board (SAB) Hypoxia Advisory Panel
On behalf of the Task Force, EPA's Office of Water requested that the Science
Advisory Board (SAB) evaluate the state-of-the-science regarding hypoxia in the Gulf of
Mexico and potential nutrient mitigation and control options in the Mississippi-
Atchafalaya River basin. In response to this request, the SAB established the SAB
Hypoxia Advisory Panel (SAB Panel). The Office of Water asked the SAB Panel to
focus its evaluation on the following issues and questions.
1. Characterization of Hypoxia - The development, persistence and areal extent
of hypoxia is thought to result from interactions in physical, chemical and
biological oceanographic processes along the northern Gulf continental shelf;
and changes in the Mississippi River basin that affect nutrient loads and fresh
water flow.
A. Address the state-of-the-science and the importance of various processes
in the formation of hypoxia in the Gulf of Mexico. These issues include:
i. increased volume orfunneling of fresh water discharges from the
Mississippi River;
ii. changes in hydrologic or geomorphic processes in the Gulf of Mexico
and the Mississippi River basin;
Hi. increased nutrient loads due to coastal wetlands losses, upwelling or
increased loadings from the Mississippi River basin;
iv. increased stratification, and seasonal changes in magnitude and
spatial distribution of stratification and nutrient concentrations in the
Gulf;
v. temporal and spatial changes in nutrient limitation or co-limitation, for
nitrogen or phosphorus, as significant factors in the development of the
hypoxic zone;and
vi. the implications of reduction of phosphorus or nitrogen without
concomitant reduction of the other.
B. Comment on the state of the science for characterizing the onset, volume,
extent and duration of the hypoxic zone.
2. Characterization of Nutrient Fate, Transport and Sources Nutrient loads,
concentrations, speciation, seasonally and biogeochemical recycling processes
have been suggested as important causal factors in the development and
persistence of hypoxia in the Gulf. The Integrated Assessment (CENR 2000)
presented information on the geographic locations of nutrient loads to the Gulf
and the human and natural activities that contribute nutrient loadings.
A. Given the available literature and information (especially since 2000),
data and models on the loads, fate and transport and effects of nutrients,
evaluate the importance of various processes in nutrient delivery and effects.
These may include:
14
-------
/'. the pertinent temporal (annual and seasonal) characteristics of nutrient
loads/fluxes throughout the Mississippi River basin and, ultimately, to the
Gulf of Mexico;
ii. the ability to determine an accurate mass balance of the nutrient loads
throughout the basin;and
in. nutrient transport processes (fate/transport, sources/sinks,
transformations, etc.) through the basin, the deltaic zone, and into the
Gulf.
B. Given the available literature and information (especially since 2000) on
nutrient sources and delivery within and from the basin, evaluate capabilities
to:
i. predict nutrient delivery to the Gulf, using currently available scientific
tools and models; and
ii. route nutrients from their various sources and account for the transport
processes throughout the basin and deltaic zone, using currently available
scientific tools and models.
3. Scientific Basis for Goals and Management Options The Task Force has
stated goals of reducing the 5-year running average areal extent of the Gulf of
Mexico hypoxic zone to less than 5,000 square kilometers by the year 2015,
improving water quality within the basin and protecting the communities and
economic conditions within the basin. Additionally, nutrient loads from various
sources in the Mississippi River basin have been suggested as the major driver for
the formation, extent and duration of the Gulf hypoxic zone.
A. Are these goals supported by present scientific knowledge and
understanding of the hypoxic zone, nutrient loads, fate and transport, sources
and control options?
i. Based on the current state-of- the-science, should the reduction goal for
the size of the hypoxia zone be revised?
ii. Based on the current state-of-the-science, can the areal extent of Gulf
hypoxia be reduced while also protecting water quality and social welfare
in the basin?
B. Based on the current state-of- the-science, what level of reduction in
causal agents (nutrients/discharge) will be needed to achieve the current
reduction goal for the size of the hypoxic zone?
C. Given the available literature and information (especially since 2000) on
technologies and practices to reduce nutrient loss from agriculture, runoff
from other non-point sources and point source discharges, discuss options
(and combinations of options) for reducing nutrient flux in terms of cost,
feasibility and any other social welfare considerations. These options may
include:
15
-------
/'. the most effective agricultural practices, considering maintenance of
soil sustainability and avoiding unintended negative environmental
consequences;
ii. the most effective actions for other non-point sources; and
Hi. the most effective technologies for industrial and municipal point
sources.
In all three areas, please address research and information gaps (expanded
monitoring, documentation of sources and management practices, effects of
practices, further model development and validation, etc.) that should be
addressed prior to the next 5-year review.
1.4. The SAB Panel's Approach
The NOAA, CENR, and Task Force documents (see Section 1.2 above) provide a
comprehensive scientific review of hypoxia causes, and potential mitigation and control
actions through about 1999 to 2000. Further, more recent science and management
information on the Gulf and MARB has been captured in the Task Force sponsored
symposia, literature search, MART reports, and CEAP activities. Accordingly, the SAB
Panel initiated its deliberations by reviewing these documents. The SAB Panel invited
the chairs of the four symposia to present summaries of key findings, and also invited
selected researchers (see acknowledgements) currently working on hypoxia issues to
present their recent work. The SAB Panel also relied on the individual and collective
experience and expertise of its members to provide additional relevant publications and
information to assist its deliberations. The SAB Panel convened four public face-to-face
meetings and 15 public teleconferences to deliberate and develop this state-of-the-science
report (background and other materials for the meetings may be found at:
http://www.epa.gov/sab/panels/hypoxia_adv_panel.htm).
The SAB Panel recognized the inherent complexity and connectivity between the
Mississippi -Atchafalaya River basin and Gulf of Mexico and agreed that a systems
perspective within an adaptive management framework was needed. The systems
approach allowed understanding of feedback loops so that perturbations in one part of a
system affect the interrelationships and stability of the system as a whole. Adaptive
management seeks to maximize flexibility in management so that learning and
adjustments can occur. Adaptive management employs six basic operating principles: 1)
resources of concern are clearly defined; 2) conceptual models are developed during
planning and assessment; 3) management questions are formulated as testable hypotheses
to guide inquiry; 4) management actions are treated like experiments that test hypotheses
to answer questions and provide future management guidance; 5) ongoing monitoring
and evaluation is necessary to improve accuracy and completeness of knowledge; and 6)
management actions are revised with new cycles of learning.
This report considers models as essential for understanding the inherent
complexities of the MARB and the NGOM. Additionally, the collection of critical data
at appropriate spatial and temporal scales is absolutely necessary to optimize future
16
-------
research and management actions. Data collection should be based on a well-defined
conceptual model of the overall system. Monitoring programs will often provide data for
existing models and assist with broader interpretations of data and information. In
summary, a systems perspective combined with an adaptive management approach will
greatly enhance scientific understanding and management of hypoxia in the MARB and
the NGOM.
This report deals largely with the review of research and findings since the
Integrated Assessment. Background material and findings prior to 2000 are used when
appropriate or when instrumental to understanding the relative importance of more recent
work. However, those interested in the details of the Integrated Assessment and the six
topical reports that provided the scientific basis for the assessment are referred directly to
those documents.
17
-------
2. Characterization of Hypoxia
2.1. Processes in the Formation of Hypoxia in the Gulf of Mexico.
The hypoxic region along the northern Gulf of Mexico (NGOM) extends up to
125 km offshore and to 60 m water depth, has substantial variability with an average mid-
summer areal extent of 16,500 km2 (2001-2007), and extends in some years from the
Mississippi River mouth westward to Texas coastal waters (Rabalais et al., 2007). This
hypoxic region (Figure 1) occurs along a relatively shallow, open coastline with complex
circulation and water column structure typical of many coastal regions and includes
massive inputs of freshwater, weak tidal energies, seasonally varying stratification
strength, generally high water temperature, wind effects from both frontal weather
systems and hurricanes, and mixing of river plumes from the Atchafalaya and Mississippi
Rivers and other smaller sources (DiMarco et al., 2006; Hetland and DiMarco, 2007).
The plumes of the Mississippi and Atchafalya Rivers can be observed as areas of highly
turbid low salinity surface water. The limits of these plumes have been defined in
different ways, but in satellite imagery their boundaries can be clearly observed as sharp
color discontinuities. Since the release of the Integrated Assessment and the Action Plan
in 2001, the measured areal extent of the hypoxic region has averaged 16,500 km2, with a
range of 8,500 to 22,000 km2. Many reports from both the Integrated Assessment and
post-Integrated Assessment periods concluded that physical and morphological
characteristics such as these make the NGOM prone to hypoxic conditions.
2.1.1. Historical Patterns and Evidence for Hypoxia on the Shelf
An important question regarding hypoxia on the Mississippi River shelf is how far
back in time has hypoxia been observed? Is it a recent phenomenon or has hypoxia been
a regular natural feature of a productive shelf region? Unfortunately the monitoring data
are not entirely sufficient to address this question, for only a limited number of
measurements are available prior to the time when wide-spread hypoxia was first
observed on the Louisiana shelf in the mid-1980s (Rabalais et al., 1999a). However, a
limited number of additional paleoecological studies have been carried out on the
Mississippi River shelf since the Integrated Assessment. All studies from dated sediment
cores show recent increases in low oxygen concentrations with time, although the precise
timing and response varies depending upon the proxy studied and the dating of cores.
The accumulated body of evidence shows that the pattern of change is concomitant with
recent (since the 1960s) increases in nutrient loading from the Mississippi River causing
increasingly severe hypoxia on the shelf. The spatial distribution of reliably dated
sediment cores, with most cores taken on the southeastern Louisiana shelf just west of the
Mississippi River delta, is not sufficient to determine the increases in the spatial extent of
hypoxia with time.
A limiting factor in all paleoecological studies is the availability of undisturbed
sediment cores to provide an accurate picture of changes through time. This is a
particular challenge in a hydrologically dynamic, relatively shallow environment as
found on the Mississippi River shelf with resuspension processes, movement of fluid
18
-------
muds, mixing by benthic organisms, and more recently sediment disturbance of upper
sediment layers through bottom trawling. Despite these challenges, a number of
reasonably dated sediment cores, primarily within the Louisiana bight, have provided a
coherent picture of changes in hypoxia with time.
Bacterial pigments measured in sediments at one location on the Louisiana shelf
were characteristic of anoxygenic phototrophic sulfur bacteria and have their highest
concentrations between 1960 and the present (Chen et al., 2001). These bacteriopigments
were not present prior to 1900. Further evidence of increased hypoxia is provided by
Chen et al. (2001) using algal pigments, which show increases in the 1960s. The increase
in these pigments reflects enhanced preservation with hypoxia as well as nutrient-driven
increases in production. Rabalais et al. (2004, 2007) also report increases in algal
pigment concentrations over time from a number of sediment cores, with gradual changes
from 1955 to 1970, followed by a steady increase to the late 1990s. However, the
patterns observed by Rabalais et al. (2004, 2007) are confounded by the rapid
degradation of carbon and algal pigments in upper surface sediments with most studies of
sediment pigments correcting for diagenesis by normalizing pigments with organic
carbon (Leavitt and Hodson, 2001). In addition, there is some evidence for spatial
increases in hypoxic extent through time: increases in pigment concentrations from one
sediment core from west of the Atchafalaya River outflow suggests that nutrient-driven
increases in production occurred later at this location than in the Mississippi River Bight
(Rabalais et al., 2004). There has been an increased accumulation of total organic carbon
and biogenic silica in recent sediments near the mouth of the Mississippi River (Turner
and Rabalais, 1994; Turner et al., 2004), although the spatial and temporal variations
observed between dated sediment cores are large.
Several studies have examined changes in the benthic foraminiferal community in
dated sediment cores (Platon and Sen Gupta, 2001; Osterman et al., 2005; Platon et al.,
2005). Different species of bottom living benthic foraminifera are particularly sensitive
to changes in bottom water oxygen concentrations, and the abundance of these species is
a widely used indicator of hypoxia. Significant changes in the composition of the benthic
foraminiferal community have occurred in the past century. Several indicators, e.g., the
PEB index (the relative abundance of three low-oxygen tolerant species of benthic
foraminifers; Pseudononin altlanticum, Epistominella vitrea, and Buliminella morganf)
(Osterman et al., 2005) and the A/P ratio (agglutinated to porcelaneous orders) (Platon et
al., 2005) indicate that increases in the occurrence of low oxygen events have occurred
over the past 50 years (Figure 3). In addition, the porcelaneous genus Quinqueloculina,
an organism that occurs where dissolved oxygen concentrations are higher than 2 mg/1,
was present but has disappeared from the foraminiferal community since 1900, indicating
that prior to this time there was sufficient oxygen at the sediment-water interface to
enable survival of such species (Rabalais et al., 2007). Osterman et al. (2005) have
shown that several probable low oxygen events that occurred in the past 180 years are
associated with high Mississippi River discharge rates, although the recent changes in
foraminiferal communities are more extreme than any that occurred in the past. The data
support the interpretation that hypoxia is a recent phenomenon and has been amplified
from an otherwise naturally occurring process.
19
-------
Core BC1
Core GC1
Core 91
Cora SO
ISM
0 S 101520253035
% PEB
10 20
% PEB
20 40 n n 100
PEB
5 10
%PEi
Figure 3: Plots of the PEB index (%PEB) in sediment cores from the Louisiana shelf. Higher values of the
PEB index indicate lower dissolved oxygen contents in bottom waters. Taken from Osterman et al. (2005).
Key Findings and Recommendations
The SAB Panel finds that the paleoecological data are consistent with increased
prevalence of hypoxic conditions in recent decades. However, the spatial distribution of
sediment cores is not sufficient to determine the increases in the spatial extent of hypoxia
with time. Although given the complex nature of disturbance, there may be limited
opportunities to determine temporal changes in the extent of hypoxia. To advance the
understanding of spatial and temporal trends in hypoxia in the NGOM, the SAB Panel
offers the following recommendations.
In future research on the Mississippi River shelf, more attention should be
focused on establishing reliable chronologies in additional sediment cores.
In order to establish spatial changes in hypoxia over time, where possible
additional sediment cores should be collected over a broader area of the
Mississippi River shelf.
20
-------
2.1.2. The Physical Context
Oxygen budget: general considerations
The oxygen budget on the NGOM shelf is influenced by several sink and source
terms. Oxygen (02) concentration in the bottom layer will decrease and possibly become
hypoxic or even anoxic when the export and consumption of oxygen by respiration
exceed the import or production of "new" oxygenated water by photosynthesis.
Mathematically, this relationship can be expressed in its simplest form by the following
oxygen balance equation:
eo2 eo2 eo2 eo2 v s2o2 v fs2o2 d2o2] s u + +u .
2- = ^^-v^-w^ + Kz12- + KH12- + 12-+Fas-Resp. +photosynthesis
dt dx dy dz dz { dx dy )
Change (1) (2) (3) (4) (5) (6) (7) (8)
in which the left-hand term represents the change of oxygen concentration with time;
term (1) on the right represents the horizontal advection by across-shelf currents, u; term
(2) represents the horizontal advection by along-shelf currents, v; term (3) represents
vertical transport by upwelling or downwelling; term (4) represents vertical mixing and
Kz (x,y,z) is the vertical eddy diffusivity; term (5) represents horizontal diffusion and
KH (x,y,z) is the horizontal eddy diffusivity; term (6) is oxygen flux across the air-sea
interface; term (7) is the non-conservative sink (i.e., oxygen consumption); and term (8)
refers to in situ production of oxygen by photosynthesis. The horizontal advection terms
may reflect contributions from tides, wind stress, buoyancy, and momentum input from
rivers, large-scale and mesoscale eddies, or topographically trapped shelf waves. Three-
dimensional hydrodynamic models are required to adequately account for these
contributions (Morey et al., 2003a, 2003b; Hetland and DiMarco, 2007). The respiration
term (7) relates directly to organic matter mineralization and must be understood in the
context of water column and sediment biogeochemical processes described in later
sections. As depicted in equation 1, the change in oxygen concentration with time at any
point in the water column is affected by sources and sinks of oxygen at and below the
surface. Term 6 (oxygen flux across the air-sea interface) represents a surface source and
sink, while term 8 (photosynthesis) is a source of oxygen in waters beneath the air-sea
interface. Although equation 1 above suggests that alongshore and cross-shore dispersion
coefficients are of equal magnitude, the Panel notes that this has not been demonstrated.
The effects of cross-shore dispersion processes must be parameterized and additional
research on lateral mixing processes must be completed before such parameterization can
be performed with confidence.
Vertical mixing as a function of stratification and vertical shear
Over the Louisiana-Texas shelf, the vertical mixing term (4) plays a key role in
the local oxygen balance. Its magnitude depends on the value of vertical eddy diffusivity
Kz, which is highly variable in both space and time and depends on the gradient
Richardson number Ri (MacKinnon and Gregg, 2005), defined by
21
-------
dt/V (dv\ (dV
where N is an index of stratification strength known as the buoyancy frequency, p is the
water density, g is the gravitational acceleration (9.8 m/s2), and dV/dz is the vertical shear
of horizontal current. The gradient Richardson number, Ri, expresses the ratio of
turbulence suppression by stratification (numerator) relative to vertical shear production
of turbulence (denominator). When Ri > 1A, turbulence is suppressed, and vertical
transport of oxygen from surface to bottom layers by turbulent mixing is unlikely to
occur. Thus, strong vertical density gradients (for example, when freshwater sits on top
of salty water) and/or weak current shears can suppress vertical mixing and be favorable
to hypoxia. Key physical factors that produce stronger vertical density gradients (dp/dz)
and thus reduce vertical mixing include freshwater inputs from rivers or precipitation,
warmer surface temperatures from absorption of solar radiation or sensible heat input,
and near-bed suspended sediment (which causes benthic stratification). Factors
responsible for producing enhanced vertical shear (dV/dz) and enhanced vertical mixing
include tidal and wind-driven currents, inertial waves, internal tides, surface waves and
Langmuir cells (Kantha and Clayson, 2000). Although no field studies of vertical mixing
by microstructure measurements of the turbulent dissipation rates of velocity, salinity and
temperature fluctuations have been reported for the NGOM, many of the physical
mechanisms described on the New England shelf (MacKinnon and Gregg, 2005) and in
Monterrey Bay (Carter et al., 2005) are at play on the NGOM as well.
While the tributaries within the Mississippi River basin are the sources of nutrient
loading to the river trunk, the distributaries within the Mississippi Delta are critical to the
final dispersal of nutrients, buoyancy and sediment into the Gulf of Mexico. The
multiple distributary mouths of the Mississippi and Atchafalaya Rivers are, for the most
part, highly stratified "salt wedge" estuaries, and their combined effluent debouches onto
the shelf as a discrete layer of fresh water that is spread into the surface layer. Exceptions
occur where smaller distributaries enter shallow bays where salinity is nearly uniform
from top to bottom. Total buoyancy fluxes are, of course, proportional to river discharge
and cause the turbulence suppressing stratification of the upper water column that is
strongly implicated in hypoxia. In most inner shelf environments, tidal currents are the
major source of mixing, and the position of temperature fronts (sharp horizontal
temperature gradients) can often be accurately predicted from the h/Ut3 criterion of
Simpson and Hunter (1974), where h is the local depth and Ut represents the depth-
averaged tidal velocity. Unfortunately, the Simpson-Hunter criterion of tidal mixing has
not yet been mapped for the northern Gulf of Mexico. Nevertheless, it is generally
agreed that tidal mixing over the Louisiana-Texas shelf is very weak because the tidal
range is only about 40 cm and tidal currents typically do not exceed 10 cm/s (Kantha,
2005). So the contribution of tidal mixing to the vertical exchange of oxygen is minimal
over the shelf, particularly off the mouths of the larger distributaries, such as Southwest
22
-------
and South Passes, which debouch into deep water. Wind-driven currents are stronger
than tidal currents but occur episodically (Ohlmann and Niiler, 2005). Winds also cause
breaking and white capping waves as well as vertical circulation (Langmuir) cells
(Thorpe, 2004) that contribute to mixing in the upper water column.
The hydrologic regime of the Mississippi River and the spatial distribution and
timing of freshwater inputs to the shelf relative to the occurrence of energetic currents
and waves are critical to vertical mixing intensity, stratification, and hypoxia. These
influences were recognized in the CENR report (Rabalais et al., 1999). Using oxygen
measurements within 2 m of the bottom and vertical profiles of temperature and salinity
collected during the 1992-1994 LaTex experiment on the Louisiana-Texas shelf and
during the 1996-1998 NEC OP (Northe36astern Gulf of Mexico Chemical Oceanography
Program) in the region east of the Mississippi delta and north of Tampa Bay, Belabassi
(2006) performed an evaluation of the empirical relationships between the maximum
value of the buoyancy frequency Nmax in the water column, bottom silicate concentration
as a proxy of phytoplankton remineralization, and the occurrence of hypoxic waters (< 2
mg/L) or low-oxygen waters (< 3.4 mg/L). She found that low-oxygen and hypoxic
bottom waters only occurred when Nmax, evaluated at a vertical resolution of 0.5 m was
greater than 40 cycles per hour (cph), which corresponds to a buoyancy period shorter
than 1.5 minutes. This result confirms that strong density stratification is a prerequisite
for hypoxia occurrence on the northern Gulf of Mexico shelf. She also found that low-
salinity water from the Mississippi and Atchalafaya Rivers was generally the main
contributor to stratification in spring and summer, although temperature was more
important than salinity in determining stratification during summer at all depths west of
Galveston Bay and at depths greater than 20 m between Galveston Bay and Terrebonne
Bay. Interestingly, stations with strong stratification (Nmax greater than 40 cph) but low
bottom silicate concentrations (less than 18 mmol m"3) did not have low-oxygen or
hypoxic bottom waters. The analyses of Belabassi (2006) thus indicate that strong
stratification (Nmax greater than 40 cph) is a necessary but not sufficient condition for
bottom layer hypoxia; a second necessary condition for hypoxia occurrence is high
bottom water remineralisation as indicated by the proxy of high concentrations of bottom
water silicates (greater than 18 mmol m"3). Simply put, there cannot be hypoxia without
both density stratification and degradation of labile organic matter.
Stow et al. (2005) attempted to disentangle the relative contributions of
eutrophication and stratification as drivers of hypoxia in the NGOM. Their analysis
indicates that the probability of observing bottom hypoxia increases rapidly when the top
to bottom salinity difference reaches a threshold of 4.1. Stow et al. (2005) also showed
that this salinity threshold decreased from 1982 to 2002. Concurrently, they highlighted
that surface temperature had increased, while surface dissolved oxygen decreased,
suggesting that changes in surface mixed layer properties may be partly responsible for
oxygen decrease in the bottom layer.
23
-------
Changes in Mississippi River hydrology and their effects on vertical mixing
By far the most important change in local hydrology has been the increased flow
of the Atchafalaya River during the 20l century. Available data show that in the early
1900's the discharge from the Atchafalaya River accounted for less than 15% of the
combined Atchafalaya-Mississippi River discharge (Figure 4). This proportion
progressively increased to reach about 30% in 1960, peaked at 35% in 1975 and since
then was reduced to 30% by means of regulatory measures (Bratkovich et al., 1994). To
understand the significance of this change on circulation patterns and on the strength of
stratification on the Louisiana-Texas shelf, it must be kept in mind that the Mississippi
River plume enters the shelf near the shelf edge and typically does not extend to the
bottom, even near the river mouth. On the other hand, the Atchafalaya River plume
enters a broader shelf, is more diffuse, and extends to the bottom over a larger distance
from the river mouth.
Atchafalaya; Total
40
' 20-
0-1 1 r
1900 1910 1920 1930 1940 1960 1960 1970 1980 1990
Time (years)
Figure 4: Change in the relative importance of the Atchafalaya flow to the combined flows from the
Mississippi and Atchafalaya Rivers over the 20th Century. Reprinted from Bratkovich et al. (1994).
The short distances (10 to 30 km) separating Mississippi River delta passes from
the shelf break facilitate the export of plume waters offshore and to the east by sporadic
wind events or by eddies present on the upper continental slope, some of which may have
been spun off by the Loop Current (Ohlmann and Niiler, 2005; Oey et al., 2005a, 2005b).
The modeling study of Morey et al. (2003a) shows that a prime export pathway for river
freshwater during the summer months is to the east, and offshore of the Mississippi River
delta. During non-summer months, the main freshwater export pathway consists of a
coastal jet flowing westward to Texas and then southward. Etter et al. (2004) estimate
that 43% ± 10% of the Mississippi River discharge is carried westward to the Louisiana-
Texas continental shelf, the remainder being carried offshore and/or eastward. While this
proportion is slightly lower than the earlier estimate of 53% ± 10% from Dinnel and
Wiseman (1986), both studies indicate that roughly half of the freshwater from the
Mississippi River goes westward, toward the Louisiana-Texas continental shelf.
In contrast, 100% of the Atchafalaya River discharge of freshwater, nutrients and
sediments is delivered to the Louisiana-Texas continental shelf. Moreover, the very
broad shelf near Atchafalaya Bay implies longer residence times of this freshwater source
on the shelf compared with freshwater from the Mississippi River delta. A "back-of-the-
envelope" calculation helps capture the full significance of the increased Atchafalaya
24
-------
River flow. In the early 1900's, for every 100 m3 of water discharged, 85 m3 took the
Mississippi River delta route. Of these, roughly 42.5 m3 went westward and 42.5 m3
went offshore or eastward. The 42.5 m3 that went westward were added to the 15m3 that
took the Atchafalaya River route to give a grand total of 57.5 m3 of freshwater on the
Louisiana-Texas continental shelf. By contrast, in the post-1970's, for every 100 m3 of
combined Atchafalaya and Mississippi River outflows, 70 m3 took the Mississippi River
route. Of these, roughly 35m3 went westward, and 35m3 went offshore or eastward.
The 35m3 that went westward were added to the 30m3 that took the Atchafalaya River
route to give a grand total of 65 m3 of freshwater on the Louisiana-Texas continental
shelf. This simple calculation reveals two things. First, it suggests that even in the
absence of a temporal trend in combined Atchafalaya-Mississippi River freshwater
discharge, the amount of freshwater delivered to the Louisiana-Texas continental shelf
would have increased by 13% (65/57.5 = 1.13). Second and more importantly, it reveals
that in the 1920s, the Atchafalaya River contributed about one quarter (15/57.5 = 0.26) of
the freshwater discharge to the Louisiana-Texas continental shelf. Between 1920 and
about 1960, the Atchafalaya River's contribution markedly increased to about one half
(30/65 = 0.46) of the freshwater discharge to the Louisiana-Texas continental shelf.
While this probably made the Louisiana-Texas continental shelf more prone to hypoxia,
the timing of this change occurred 15 to 20 years earlier than the onset of regular summer
hypoxia (Section 2.1.1).
Future physical modeling studies are needed to investigate the effects of past and
proposed future changes in the distribution of freshwater flows, including inputs to
Atchafalaya Bay some 200 km to the west of the Mississippi River delta, on changes in
the spatial distribution of surface salinity, temperature, and stratification on the
Louisiana-Texas continental shelf and on the Mississippi Sound to the east of the birdfoot
delta. Physical oceanographic models that can adequately answer such questions about
the impacts of flow diversions already exist but have only been run using the post-1970s
flow conditions (30% Atchalafaya River, 70% Mississippi River). One such modeling
study by Hetland and DiMarco (2007) suggests that the freshwater plumes from the
Atchafalaya and Mississippi Rivers are often distinct from one another (Figure 5) and
that both contribute significantly to the development of hypoxia (Figure 1) on the shelf
through their influence on stratification and nutrient delivery (Rabalais et al., 2002a). In
addition, maps of observed surface salinity and satellite images of chlorophyll (e.g.,
figure 9), show the same result. It thus appears likely that increases in freshwater
discharge from the Atchafalaya River and resulting increased stratification from the early
1900's to the mid-1970's have increased the area of the Louisiana-Texas continental shelf
that is prone to bottom layer hypoxia.
25
-------
30°N
29°N
28°N
95°W 94°W 93°W 92°W 91°W 90°W 89°W 88°W
30°N
29°N
28°N
13 August 1993
32
.
24 *
4->
20 |
16 S
HI
12
8
0
95°W 94°W 93°W 92°W 91°W 90°W 89°W 88°W
Figure 5: Modelled surface salinity showing the freshwater plumes from the Atchafalaya and Mississippi
Rivers during upwelling favorable winds (top panel) and during downwelling favorable winds 8 days later
(bottom panel). Adapted from Hetland and DiMarco (2007).
Recently evolved plans for protecting coastal Louisiana (CPRA, 2007) propose
significant diversions of the water, nutrients, and sediment outflow from the Mississippi
River into the Gulf. Figure 6 illustrates a diversion scenario that involves redirecting a
large part of the outflow into shallow bays upstream of the present day "bird's foot"
delta. This scenario could alter the shelf hydrodynamics, particularly if more of the
buoyancy is directed into shallow water instead of the deep water off the active river
mouths, which are near the shelf edge. It is important that three-dimensional numerical
circulation models be applied to these scenarios. Future management strategies may be
able to utilize engineered modulations of the timing of freshwater releases to coincide
more closely with more energetic waves and current conditions, thereby reducing the
strength of stratification (i.e., Ri). This approach will, of course, rely on engineering
innovations and effective diversion management. The opportunity exists for EPA and
other federal and management agencies to urge flow diversion strategies that also
consider the goal of reducing the volume and bottom area of hypoxic waters on the
NGOM shelf without endangering other estuarine and coastal waters. The CPRA/U.S.
26
-------
Army Corp of Engineers proposals also highlight the need for interagency coordination
and for an integrated approach to management strategies for jointly addressing multiple
issues including hypoxia, coastal protection, and coastal inundation.
Figure 6: Proposed diversions of Mississippi effluents for coastal protection. From Coastal Protection and
Restoration Authority (CPRA) of Louisiana, 2007 Integrated Ecosystem Restoration and Hurricane
Protection: Louisiana's Comprehensive Master Plan for a Sustainable Coast. CPRA, Office of the Governor
(La) 117 pp.
Zones of hypoxia controls:
The resulting stratified region influenced by the Mississippi and Atchafalaya
River plumes exerts strong control on the extent and spatial distribution of hypoxia and is
an important factor in determining where hypoxia may occur (Rabalais and Turner,
2006). The buoyancy fluxes from the rivers also contribute to regional circulation in the
form of baroclinic flows (Morey et al., 2003a, 2003b). Following a similar line of
reasoning used in earlier work by Rhoads et al. (1985) off the mouth of the Changjiang
(Yangtze) River, Rowe and Chapman (2002) defined three zones of hypoxia control in
the NGOM. The boundaries between these three zones are admittedly fuzzy, and change
through time; however Figure 7 illustrates the SAB Panel's view of these concepts as
represented by 4 zones. In zone 1, which is most proximal to river mouth sources,
strongly stratified and light- as well as nutrient-limited, respiration of organic carbon
coming both directly from the river efflux and from nutrient-dominated eutrophication
dominates. The relative importance of these organic carbon sources as the cause of
hypoxia remains somewhat uncertain, although the model of Green et al. (2006b)
27
-------
indicates a major dominance by in situ phytoplankton production even in the immediate
plume of the Mississippi River. In the intermediate zone 2, stratification is also strong;
light limitation is less than in zone 1; very high rates of phytoplankton production occur;
and water column respiration fuels bottom layer hypoxia. Farther along the coast from
the river mouths but within the low-salinity coastal plume (zone 3), local phytoplankton
production is less, but labile organic matter may have been imported from zone 2 and
deposited on the bottom. In zone 3, stratification remains strong, and oxygen
consumption in the sediment is more important than water column respiration in driving
hypoxia. Zone 4 depicts the highly productive, coastal current, as suggested by Boesch
(2003).
Boesch (2003) strongly criticized the physical, biological and chemical reasoning
behind the delineation of the Louisiana-Texas continental shelf into these three distinct
zones of hypoxia control. He also argued that these zones did not capture well the
physics and biology of the Louisiana coastal current, which is characterized by low
salinities and high nutrient and chlorophyll levels (Wiseman et al., 2004). Nevertheless,
Rowe and Chapman (2002) stimulated new research into the role that stratification plays
in the reduction of vertical mixing rates and the flux of oxygen through the pycnocline in
the regions of the Louisiana-Texas continental shelf under the influence of the
Mississippi and Atchafalaya River plumes. Using realistic three-dimensional physics
(equation 1) with simple representations of water column and benthic respiration for the
zones A, B and C of Rowe and Chapman (2002), Hetland and DiMarco (2007) were able
to represent the bottom area, thickness, and volume of hypoxic waters over the NGOM
fairly well.
28
-------
98°W
96°W
94°W
92°W
90°W
88 °W
32°N
30°N
28°N
26°N
_L
_L
4000 3000 2500 2000 1500 1000 500 300
Depth (meters)
200 100
50
Figure 7: An illustration depicting different zones (Zones 1-4, numbered above) in the NGOM during the
period when hypoxia can occur. These zones are controlled by differing physical, chemical, and biological
processes, are variable in size, and move temporally and spatially. Diagram created by D. Gilbert.
So far as we are aware, time series measurements of physical oceanographic
parameters are inadequate to support or refute hypotheses regarding changes in shelf
circulation, stratification, and vertical mixing during the 20th century. Initial planning for
a Gulf of Mexico Coastal Ocean Observing System (GCOOS) has begun (for additional
information see: http://www.gcoos.org). As these GCOOS plans continue to evolve and
implementation begins over the next few years, it is important that physical parameters
relevant to oxygen dynamics be included among the measurements. Empirical
parameterizations of vertical eddy diffusivity Kz as a function of vertical shear and
density stratification are available for shallow continental shelf environments
(MacKinnon and Gregg, 2005). These parameterizations enable quantification of vertical
mixing (term 4 in equation 1) with vertical shear measurements from moored Acoustic
Doppler Current Profilers (ADCPs) and vertically profiling conductivity, temperature,
and depth instrumentation (CTDs) tethered on a cable. Ship-based microstructure
measurements of the turbulent rates of dissipation of velocity, salinity, and temperature
fluctuations (Gregg, 1999) should also be conducted occasionally to complement the
moored ADCP and profiling CTD measurements. Physics-based models of ocean mixing
and turbulence exist today and are part of 3-D circulation models (Mellor and Yamada,
29
-------
1982). These models need to be rigorously tested using ADCP, CTD, and microstructure
data because vertical mixing is the most important physical process to model correctly
when hypoxia is under consideration.
Shelf circulation: local versus regional
Circulation in the NGOM can be considered on two scales: Gulf-wide deep-sea
circulation and shelf circulation near the coast. Among the most prominent features of
the large-scale Gulf-wide circulation are the Loop Current and the Loop Current Eddy
System (Oey et al., 2005a, 2005b). Although these features impinge on and affect the
outer shelf, Rabalais et al. (1999) conclude that local wind forcing and buoyancy are
more important to shelf circulation inshore of the 50 meter isobath. Direct ship-board
observations by Jarosz and Murray (2005) during five separate cruises led those authors
to conclude that the momentum balance on the inner and mid shelf to the west of the
active birdfoot delta is indeed dominated by wind stress. During summer, alongshore
sea-surface slope caused by buoyancy forcing was also important in forcing currents. On
the 20 m isobath off Terrebonne Bay, ADCP measurements (Wiseman et al., 2004) show
periods of several days with negligible vertical shear followed by other periods of a few
days with much more elevated vertical shear and reduced density gradients, suggestive of
more intense vertical mixing.
Several physical oceanographic models taking into account the crucial baroclinic
effects that typify the Louisiana-Texas continental shelf are now available (e.g., Morey et
al., 2003a, 2003b; Zavala-Hidalgo et al., 2003). The model results of Hetland and
DiMarco (2007) show that the plume from the Mississippi River, which enters the shelf
near the shelf edge, forms a recirculating gyre in Louisiana Bight and does not interact
with the seabed, whereas the Atchafalaya River plume interacts with the shallow coastal
topography (Hetland and DiMarco, 2007). Both plumes respond directly to local winds
and are advected seaward during upwelling-favorable winds (Figure 5). The distinct
plumes from the Mississippi and Atchafalaya Rivers influence the spatial pattern of
bottom hypoxia on the Louisiana-Texas continental shelf. This influence is clearly seen
on the 1985-2005 map of hypoxia frequency of occurrence (Figure 1) and is even more
obvious in certain years (e.g., 1986, Rabalais and Turner 2006). Given this interaction,
planned diversions of Mississippi River and Atchafalaya River flow may alter shelf
circulation and the spatial pattern of bottom hypoxia. Applications of 3-D baroclinic
models to future scenarios such as that portrayed in Figure 6 are thus important to
planning for future strategies for coastal restoration (CPRA, 2007).
In their analysis of low-frequency (occurring over a time scale greater than 24
hours) currents over the shelf, Nowlin et al. (2005) distinguished between currents that
respond within the "weather band" of 2-10 days and those within the mesoscale band of
10-100 days corresponding to large-scale eddies off the shelf. Inshore of the 50 m
isobath, the local winds within the weather band dominated and drove currents from east
to west during non-summer months influenced by the passage of frontal systems. Current
fluctuations seaward of the 50 m isobath were primarily within the mesoscale band and
predominantly oriented from west to east but with high variability. Along-shelf and
30
-------
across-shelf currents in the upper layer over the inner shelf, as reported by Nowlin et al.
(2005), averaged about 10 cm/s and 1 cm/s, respectively. Over the outer shelf and near
the seabed, flows were weaker.
Key Findings and Recommendations
The SAB Panel finds that 20th century changes in the hydrologic regime of the
Mississippi and Atchalafaya Rivers and the timing of freshwater inputs to the Louisiana-
Texas continental shelf have likely increased the shelf area with potential for hypoxia,
although these changes occurred mostly from the 1920s to the 1960s, before the
measured onset of hypoxia in the mid-1970s. Additional work is needed to advance the
understanding of the relative importance of physical factors in the formation of hypoxia
in the NGOM. The SAB Panel therefore provides the following recommendations.
The development of a new suite of models that integrate physics and
biogeochemistry should be encouraged and supported. This suite should include
multiple types of models [i.e., relatively simple models such as those developed
by Scavia et al. (2003) as well as more complex three-dimensional types such as
Hetland and DiMarco (2007)].
A comparative impact study of past, present, and future river flow diversions and
scenarios of altered nutrient supply to the river mouths should be encouraged and
supported. Three-dimensional hydrodynamic modeling studies are needed to
compare the spatial distribution of salinity and stratification with 15% (early
1900's) and 30% (post-1970's) Atchafalaya River contributions to the combined
Atchafalaya-Mississippi River outflow. Coupling of this three-dimensional
hydrodynamic model with a biogeochemical model would allow quantification of
the impacts of past river flow diversions on the spatiotemporal extent of hypoxia.
In addition, to anticipate the possible effects of proposed future effluent diversion
plans via rerouted deltaic distributaries (CPRA, 2007), these three-dimensional
biogeochemical and baroclinic shelf circulation models need to be applied to
scenarios such as that shown in Figure 6 while also considering the effects of
nutrient-rich Mississippi River waters discharged into local bays and estuaries.
Emerging coastal ocean observing and predicting systems in the Gulf of Mexico
(http://www.gcoos.org) should be encouraged to measure and disseminate
information needed by hypoxia modelers and those charged with adaptive
management. Direct measurements of physical and biogeochemical parameters as
well as direct time series measurement of dissolved oxygen in the bottom
boundary layer should be routinely provided by the next generation of shelf
moorings.
Studies of turbulent mixing processes involving the effects of stratification over
the Louisiana-Texas shelf with instruments and techniques capable of quantifying
turbulent dissipation rates of velocity, salinity, and temperature fluctuations
should also be encouraged. Studies of the importance of lateral mixing processes
should be encouraged.
-------
2.1.3. Role of N and P in Controlling Primary Production
Nitrogen and phosphorus fluxes to the NGOMbackground
Excessive nutrient loading, dominated by discharge from the MARB, enhances
planktonic primary production in the shallow near-shore receiving waters of the NGOM
(Lohrenz et al., 1990, 1992; Turner and Rabalais, 1994; Rabalais et al., 1999a). The
nutrients of concern are nitrogen (N), phosphorus (P), and silicon (Si) in the form of
silicate. Appendix B provides general information on the nitrogen, phosphorus and silica
cycles. Both primary productivity and phytoplankton biomass are stimulated by these
nutrient sources (Lohrenz et al., 1990, 1992; Ammerman and Sylvan, 2004; Sylvan et al.,
2006). The spatial and temporal extent and magnitudes of this stimulation vary
significantly, and their patterns and size appear to be related to 1) amounts of freshwater
discharge and their nutrient loads; 2) the nature and frequencies of discharge (i.e., acute,
storm- and flood-based versus more gradual, chronic, seasonal discharge); and 3) the
direction and spatial patterns of discharge plumes as they enter and disperse in the
NGOM (Justic et al., 1993; Lohrenz et al., 1994; Rabalais et al., 1999b). The Integrated
Assessment concluded that N loading from the MARB was the primary driver for hypoxia
in the NGOM. Since the Integrated Assessment., however, considerable knowledge has
been gained concerning the processes that influence primary production and the relative
importance of elements other than N as is discussed below.
A proportion of the freshwater discharge transits via freshwater and coastal
wetlands and coastal groundwater aquifers, which modify the concentrations and total
loads of nutrients entering the NGOM (Day et al., 2003; Turner, 2005). The extent to
which wetlands alter nutrient loads and the effects wetland losses have had on changes in
nutrient processing and loading are subjects of considerable debate (Mitsch et al., 2001;
Day et al., 2003; Turner, 2005). Nutrients can also enter this region from deeper offshore
sources, by advective transport over the shelf, a modified form of "upwelling" (Chen et
al., 2000; Cai and Lohrenz et al., 2005), although this input is estimated to be only 7% of
the nitrogen coming down the Mississippi River (Howarth, 1998). Lastly, nutrients can
be derived from atmospheric deposition directly onto nutrient-sensitive NGOM waters
(deposition onto the MARB and subsequent downstream export to the Gulf is considered
in later sections). For nitrogen, this direct deposition is estimated to be 13% of the
amount of nitrogen that flows down the river (Howarth 1998).
Historic analyses indicate a great deal of variability in seasonal, interannual and
decadal-scale patterns and amounts of freshwater and nutrient discharge to the NGOM
(Turner and Rabalais, 1991; Rabalais et al., 2002a). As a result, primary productivity and
phytoplankton biomass response can vary dramatically on similar time scales, which
poses a significant challenge to interpreting trends in nutrient-driven eutrophication in the
NGOM as in other systems (Harding, 1994; Boynton and Kemp, 2000; Paerl et al.,
2006b). Furthermore, in the turbid and highly colored waters (containing colored
dissolved organic matter or CDOM) of the river plumes entering the NGOM, nutrient and
light availability strongly interact as controls of primary production and biomass. These
32
-------
interactive controls modulate the relationships between nutrient inputs and phytoplankton
growth responses in this region (Justic et al., 2003a, 2003b; Lohrenz et al., 1994).
Ultimately these interactions affect the formation and fate of autochthonously-produced
organic carbon that provides an important source of the "fuel" for bottom water hypoxia
in this region.
NandP limitation in different shelf zones and linkages between high primary production
inshore and the hypoxic regions further offshore
Physically, chemically and biologically, the NGOM region is highly complex, and
nutrient limitation reflects this complexity. Along the freshwater to full-salinity
hydrologic continuum representing the coastal NGOM influenced by river discharge,
ratios of nutrient concentrations vary significantly, both in time and space. For example,
depending on the season, specific hydrologic events and conditions (storms, floods,
droughts), molar ratios of total N to P (N:P) supplied to these waters can vary from over
300 to less than 5 (Turner et al., 1999; Ammerman and Sylvan, 2004; Sylvan et al., 2006;
Turner et al., in press). Furthermore, additional environmental factors, such as flushing
rate (residence time), turbidity and water color (light limitation), internal nutrient
recycling, and vertical mixing strongly interact to determine which nutrient(s) may be
controlling primary production (Lohrenz et al., 1999b). Compounding this complexity is
the frequent spatial separation between high nutrient loads, the zones of maximum
productivity and hypoxia (e.g., Figure 7). Conceivably, primary production and algal
biomass accumulation limited by a specific nutrient in the river plume region near shore
may constitute the "fuel" for hypoxia further offshore in the next zone, where
productivity in the overlying water column may be limited by another nutrient.
Limitation by different nutrients in different areas appears to be the case during the spring
to summer transitional period, when primary production in the river plume region near
shore is P limited (Lohrenz et al., 1992, 1997; Ammerman and Sylvan, 2004 Sylvan et
al., 2006), but offshore productivity is largely N limited (Lohrenz 1992, 1997; Dortch and
Whitledge, 1992). The relevant questions concerning causes of hypoxia are what are the
relative amounts of inshore river plume (largely P-limited) versus offshore (largely N-
limited) productivity and what roles do these different sources of productivity play in
"fueling" hypoxia.
Early work on NGOM nutrient limitation tended to focus on the waters overlying
the hypoxic zone; typically, these waters are over the shelf but farther offshore than the
river plume waters. Stoichiometric N:P ratios indicated that, during summer months
when hypoxia was most pronounced, N should be the most limiting nutrient (Justic et al.,
1995; Rabalais et al., 2002a). This work has been the basis for the general conclusion
that N is most limiting, and that reductions in N loading would be most effective in
reducing "new" carbon (C) fixation and resultant phytoplankton biomass supporting
hypoxia (Rabalais et al., 2002a, 2004). This conclusion, coupled with the nutrient
loading trend data over the past 40-50 years, which showed N loading increasing more
rapidly than P loading, has formed the basis for arguing that N input reductions would be
most effective in reducing the eutrophication potential and hence formation of "new" C
supporting hypoxic conditions. The 2000 report from the National Academy of Sciences'
33
-------
Committee on Causes and Management of Coastal Eutrophication (National Research
Council, 2000) concluded that nitrogen is the primary cause of eutrophication in most
coastal marine systems in the U.S. at salinities greater than 5-10 parts per thousand
(ppt), including the NGOM.
While it is likely that N limitation characterizes coastal shelf and offshore waters,
more recent nutrient addition bioassays (Ammerman and Sylvan, 2004; Sylvan et al.,
2006) and examinations of nutrient stoichiometric ratios have shown that river plume-
influenced inshore productivity appears to be more P limited, especially during periods of
highest productivity and phytoplankton biomass formation (Feb-May) (Figure 8) when
freshwater discharge and total nutrient loading are also highest (Lohrenz et al., 1999a,
1999b; Sylvan etal., 2006).
March
Contrail N[a| N.Si | P | P, N |P, Si |P,N,Si
Control I N | Si ! N,Si| P | P,N | P, Si |P, N.Si
Nutrient Addition
D 12 Hr Yield
"Max Yield
10 -
Nutrient Addition
2 -
0
Controi| N | Si | N.Si | P | P, N | P, Si |P, N,Si
Nutrient Addition
run
September
-"-in
SI I N,Si I P | P, N | P, Si |P, N.Si
Nutrient Addition
Figure 8: Response of natural phytoplankton assemblages from coastal NGOM stations to nutrient
additions, March through September. All experiments, except those done in September, indicate a strong
response to P additions. Taken from Sylvan et al., 2006.
The strong P limitation during this period appears to be a result of the very high
rates of N loading that have increased more rapidly than P loading over recent history
(the past 50 years) (Turner and Rabalais, 1991; Turner et al., 1999). This situation is
exacerbated during periods of high freshwater runoff, which typically contain very high
N:P ratios. Primary productivity in the river plume region near shore tends to shift into a
more N limited mode once freshwater discharge decreases during the drier summer-fall
period (June-October). However, total primary production and phytoplankton biomass
34
-------
accumulation are far lower during this more N-limited period than during the earlier P-
limited period. Overall, maximum "new" organic C formation in recent years tends to
coincide with periods of highest N:P, which are P limited (Lohrenz et al., 1992, 1997,
1999a; Ammerman and Sylvan, 2004; Sylvan et al., 2006).
Field data and remote sensing imagery indicate that in situ phytoplankton biomass
(as chlorophyll a) concentrations can be quite high in river plume-influenced inshore
waters that have been shown to be P limited. This pattern is evident in Figure 9, an
image provided by the National Oceanic and Atmospheric Administration Sea-viewing
Wide Field-of-view Sensor Project (NASA-SeaWiFS, 2007). Therefore, the following
question emerges. What is the spatiotemporal linkage of this P-limited high primary
production and phytoplankton biomass accumulation to hypoxic bottom waters located
further offshore? Furthermore, what are the relationships between N-limited production
later in the summer and hypoxic conditions, which typically are most extensive during
this period? These potential "relationships" are complicated by the fact that there are
strong, co-occurring physical drivers of hypoxia, including vertical density stratification
and respiration rates, which tend to be maximal during periods of maximum development
of hypoxia (c.f. Rowe and Chapman, 2002;Wiseman et al., 2004; Hetland and DiMarco,
2007; DiMarco et al., submitted).
'Mississippi Plume N. Gulf of Mexico Cldorophyll a from space I SeaWIFS)
Figure 9: NASA-SeaWiFS image of the Northern Gulf of Mexico
recorded in April, 2000. This image shows the distributions and
relative concentrations of chlorophyll a, an indicator of phytoplankton
biomass in this region. Note the very high concentrations (orange to
red) present in the inshore regions of the mouths of the Mississippi and
Atchafalaya Rivers.
35
-------
There are likely to be periods when both P and N are supplied at very low levels
and co-limit phytoplankton production. These periods occur during the transition from
spring to summer. A similar condition is observed in large estuarine systems with a
history of eutrophication, such as Chesapeake Bay (Fisher et al., 1992). Spatially, the
upstream, freshwater segments of Chesapeake Bay tend to be most P limited, especially
during spring runoff conditions, while the more saline down-estuarine waters tend to be
most N limited. In Chesapeake Bay, the more turbid upstream freshwater component
tends to exhibit interactive light and P limitation or N+P co-limitation (Fisher et al.,
1992; Harding et al., 2002). Farther downstream, light limitation plays a less important
role. This scenario could prove similar to the riverine-coastal continuum in the NGOM,
where the most turbid upstream river plume waters are likely to exhibit the highest
probability for light-nutrient interactive limitation of primary production (Lohrenz et al.,
1999a, b).
While bioassay data tend to indicate P limitation during springtime in the lower
salinity portions of this continuum and N and P co-limitation and N limitation in the more
saline offshore waters during summer months, the bioassays do not account for sediment-
water column exchange because sediments are excluded during the course of incubation.
It is possible, although unlikely because of short incubation times, that sediment-water
column P cycling in the shallow NGOM water column may minimize P limitation in situ.
In order for this scenario to be operative, parallel N recycling would have to be far less
efficient than P cycling, which numerous studies suggest is the case (Gardner et al., 1994;
Bode and Dortch, 1996; Pakulski et al., 2000; Wawrik et al., 2004; Jochem et al., 2004;
Cai and Lohrenz, 2005). Bioassay-based N limitation results might also be influenced by
the elimination of "internal" sediment-water column N recycling, although this situation
seems unlikely as well, especially if denitrification is operative (Childs et al., 2002).
Sediment-based denitrification would lead to N "losses" from the system, thereby
exacerbating N limitation. This influence would not be captured in bioassays, which
isolate the sediments from the water column during incubation. The relatively short
incubation times of bioassays probably preclude these potential artifacts. They offer a
"snapshot" of nutrient limitation to complement longer-term, ecosystem-scale
assessments.
The degree of N and P limitation can be calculated from bioassays, and the data
can be used to create ratios of N and P limitation (Dodds et al., 2004). Interestingly, N
and P limitation inferred from stoichiometric ratios of soluble (and hence biologically-
available) inorganic or total N or P concentrations and inputs (loads) tends to confirm
bioassay-based conclusions concerning specific nutrient limitations. For example,
inshore, river-influenced waters exhibit quite high molar N:P ratios, often exceeding 50
[Nutrient Enhanced Coastal Ocean Productivity (NECOP) Reports, NOAA, 2007].
Nutrient addition bioassays initially conducted in these waters by Lohrenz et al. (1999a)
and more recently by Sylvan et al. (2006), consistently revealed P limitation, especially
during spring periods of maximum primary production and phytoplankton biomass
accumulation. These same studies also indicated a tendency towards N and P co-
limitation and exclusive N limitation during later summer months, when soluble and total
N:P values dipped below 15. It should also be noted however that rates of primary
36
-------
production and phytoplankton biomass during this more N-limited period are at least
five-fold lower than spring values, according the Gulf of Mexico NEC OP data (Lohrenz
et al., 1999a, 1999b). Sylvan et al. (2006) point out that P-limited spring production of
"new" C may play a proportionately greater role than N-limited summer production as a
source of "fuel" supporting hypoxia in the NGOM. The degree and extent to which C
from this nutrient-enhanced elevated spring production is transported and accounts for
summer hypoxia need to be quantified. Developing an understanding of processes that
link zones and periods of high primary production and phytoplankton biomass to zones
exhibiting bottom water hypoxia is a fundamentally important and challenging area of
research. Such research is necessary to improve understanding of the linkage between
nutrient-enhanced production and bottom water hypoxia in the NGOM. Extrapolation of
C production to hypoxia data along the entire riverine-coastal shelf continuum, where
zones and periods of maximum productivity and bottom water hypoxia do not necessarily
coincide or overlap, depends on knowing C transport and storage (including burial),
internal nutrient, and C cycling and C consumption (heterotrophic metabolism and
respiration) processes along this continuum (Redalje et al., 1992; Cai and Lohrenz, 2005).
Quantifying the links between locations and periods of specific nutrient limitation (or
stimulation) of production and the fate of this production relative to hypoxia will
contribute to long-term, effective nutrient management strategies for this region.
Key Findings and Recommendations
The SAB Panel finds that there is compelling evidence that the near shore
Mississippi/Atchafalaya River plume-influenced waters are P limited and P-N co-limited
during the spring periods of highest primary production. Nitrogen limitation of primary
production prevails during summer periods. Recent research results indicate that the
spring period of maximum primary production is P-limited in at least the plumes of the
rivers, largely due to excessive N input. As a result of this man-made imbalance in
nutrient loading during this crucial period, P availability plays an important role in
contributing to the production of "new" organic carbon in the spring time and quite likely
contributing in a major way to the "fueling" of summer hypoxia in the NGOM.
However, as stressed elsewhere in this report, there is great uncertainty over the coupling
in space or time of phytoplankton production and its decomposition leading to hypoxia.
Therefore, a better understanding of the spatial extent and temporal patterns of these
nutrient limitations is needed. The SAB Panel recommends that the following work be
undertaken to advance knowledge of the importance of nutrient limitation and co-
limitation as factors in the formation of Gulf hypoxia.
Research should be conducted to develop a more complete understanding of the
spatial and temporal linkages between river plume-influenced inshore P (in
spring) and/or N limited (in summer) primary production, and offshore coastal
shelf, more N-limited production, as well athe fate of C produced in each zone
throughout the year.
Research should be conducted to link near inshore river plume-influenced
37
-------
production in time and space to 62 depletion farther offshore. Green et al.
(2006b) suggest that the small region that the central Mississippi River plume
could supply is responsible for about 25% of the C necessary to fuel hypoxia.
The role of the Atchafalaya plume and other riverine influenced, inshore high
productivity regions in offshore hypoxia needs to be clarified.
Research should be conducted to address the following questions. How closely
linked are the periods of high productivity and hypoxic events throughout the
regions in which they occur? What is the lag between C production and its
ultimate degradation?
2.1.4. Other Limiting Factors and the Role of Si
While excessive N and P loading are implicated in eutrophication of the NGOM,
these nutrients also play a role in the balance, availability and ecological manifestations
of other potentially-limiting nutrients, most notably Si. In the Mississippi River plume
region, N is supplied in excess of the stoichiometric nutrient ratios needed to support
phytoplankton and higher plant growth (i.e., Redfield ratio, Redfield, 1958). If N over-
enrichment persists for days to weeks, other nutrient limitations may, at times, result and
seasonally dominate; the most obvious and important is P limitation, which has recently
been demonstrated in bioassays (Ammerman and Sylvan, 2004; Sylvan et al., 2006). In
addition to P limitation, N and P co-limitation and Si limitation (of diatom growth) have
been observed in the fresh and brackish water components of riverine plumes that can
extend more than 100 km into the receiving waters (Dortch and Whitledge, 1992;
Lohrenz et al., 1999a; Dortch et al., 2001). A similar scenario is evident in the
Chesapeake Bay, where elevated N loading accompanying the spring maximal freshwater
runoff period increases the potential for P limitation (Fisher and Gustafson, 2004). The
biogeochemical and trophic ramifications of such shifts are discussed below.
Can increased N: Si and P: Si fuel an increased microbial loop and exacerbate hypoxia?
With regard to nutrient primary production interactions, it is important to know
who the dominant primary producers are, where they reside, what their contributions to
new production are, and what their fate is. In NGOM waters downstream of the rivers,
wetlands and intertidal regions, microalgae are by far the dominant primary producers
(Lohrenz et al., 1992, 1997; Redalje et al., 1992; Rabalais et al., 1999a). The microalgal
communities are dominated by phytoplankton (Redalje et al., 1994a, 1994b; Chen et al.,
2000) although benthic microalgal communities can also be important sites of primary
production and nutrient cycling, especially in near-shore regions (Jochem et al., 2004).
As nutrient loads and limitations change over time and space, the proportions of
planktonic versus benthic microalgae may also change; i.e., as nutrient inputs are reduced
and planktonic primary production is reduced, the microalgal community may shift to a
more benthic dominated one. This process could yield significant implications for
38
-------
biogeochemical (nutrients, carbon and oxygen) cycling and trophodynamics (Rizzo et al.,
1992; Darrowetal., 2003).
Historic and contemporary evidence supports the contention that
anthropogenically and climatically-induced changes in N and P loading have increased
NGOM primary productivity and phytoplankton biomass and altered phytoplankton
community composition. There are several reasons why phytoplankton community
composition may have been altered by changes in nutrient loading: 1) competitive
interactions among phytoplankton taxa based on varying nutrient supply rates and
differing affinities for nutrient uptake and assimilation (i.e., varying nutrient uptake
affinities and kinetics); 2) competitive interactions based on the relationships between
nutrient supply rates and photosynthetically available light (i.e., low versus high light
adapted taxa); 3) competitive interactions based on changes in N versus P supply rates
(e.g., differential N versus P uptake capabilities and selection for nitrogen fixing
cyanobacteria); 4) competition based on the ratios of N and P versus Si (silicious versus
non-silicious taxa and heavily- versus lightly-silicified diatoms); 5) differential grazing
on phytoplankton taxa (top-down controls); and 6) nutrient-salinity controls (interactive
effects of changes in freshwater discharge on NGOM salinity and nutrient regimes due to
climatic and watershed hydrologic control changes). Each set of controls can influence
the amounts and composition of primary producers. These controls can also interact in
time and space, greatly compounding and confounding the interpretation of their
combined effects.
One important aspect of differential nutrient loading is the well-documented
increase in N and P relative to Si loading. While N and P loads tend to reflect human
activities in and alterations of the watershed, Si loads tend to reflect the mineral (bedrock
and soil) composition of the watershed; a geochemical aspect that is less influenced by
human watershed perturbations. Agricultural, urban and industrial development and
hydrologic alterations in the MARB have led to dramatic increases in N and P relative to
Si loading. In addition, the construction of reservoirs on tributaries of these river systems
has further exacerbated this situation by trapping Si relative to N and P. This
anthropogenic biogeochemical change has been shown to alter phytoplankton community
structure (i.e., away from diatom dominance), with subsequent impacts on nutrient and
carbon cycling and food web dynamics (Humborg et al., 2000; Ragueneau et al., 2006a,
2006b). The overall result has been an increase in N:Si and P:Si ratios that can influence
both the amounts and composition of phytoplankton; including potential shifts from
diatoms to flagellates and dinoflagellates (Turner et al., 1998; Rabalais and Turner, 2001;
Justic et al., 1995). Diatoms are a highly desired food item for a variety of planktonic
and benthic grazers, including key zooplankton species serving an intermediate role in
the NGOM food web (Dagg, 1995). The dinoflagellates, cyanobacteria and even a few
diatom species, while serving important roles in the food web, also contain species that
may be toxic and/or inedible (Anderson and Garrison, 1997; Paerl and Fulton, 2006).
Some of these species can rapidly proliferate or "bloom" under nutrient sufficient and
enriched conditions, and thus constitute harmful algal bloom (HAB) species. Toxicity
may directly and negatively impact consumers of phytoplankton as well as higher-ranked
consumers, including finfish, shellfish and mammals (including humans). If non-toxic
39
-------
but inedible (due to size, shape, coloniality) phytoplankton taxa increase in dominance,
trophic transfer may be impaired. Planktonic invertebrates, shellfish, and finfish
consumers (whose diets are highly dependent on the composition and abundance of
specific phytoplankton food species and groups) may then be affected (Turner et al.,
1998). This could have consequences for C flux, with a relatively higher fraction of C
being processed through microbial pathways (i.e., the "microbial loop") or sedimented to
the bottom. In either case, a greater fraction of the primary production would remain in
the system, as opposed to being exported out of the system by transfer to higher trophic
level and fisheries. The net result would be more C metabolized within the system,
leading to enhanced oxygen consumption and increased hypoxia potentials.
Key Findings and Recommendations
Research has shown the potential importance of silicate in structuring
phytoplankton communities. Based on this finding, the SAB Panel offers the following
recommendation.
The potential for silicate limitation and its effects on phytoplankton production
and composition on the Louisiana-Texas continental shelf should be explored
when carrying out experiments on the importance of N and P as limiting factors
and when considering nutrient management scenarios.
2.1.5. Sources of Organic Matter to the Hypoxic Zone
As noted earlier, the physical and geomorphological conditions found along the
Louisiana coast make the NGOM prone to hypoxic conditions if there is an organic
matter supply sufficient to consume deep water dissolved oxygen (DO) at rates exceeding
DO replenishment rates. Ecosystems such as the NGOM shelf have available to them an
array of organic matter sources, including those transported from the basin by rivers and
those produced in-situ. These include particulate and dissolved organic carbon/colored
dissolved organic matter (POC and DOC/CDOM) from terrestrial sources in the basin,
POC and DOC from coastal wetland losses, and in-situ production by phytoplankton,
macrophytes, and benthic microalgae.
The Integrated Assessment largely supported the argument that hypoxia in the
NGOM was driven by increased N loading to the Gulf of Mexico, which, in turn,
stimulated increased in-situ phytoplanktonic production of labile (i.e., readily
decomposed) organic matter. A portion of this organic matter sinks to deeper, sub-
pycnoclinal waters and is used by the heterotrophic community at rates sufficient to
deplete DO concentrations to hypoxic levels. Emphasis at that time focused on N but
more recent work has indicated that P also plays a role in regulating organic matter (OM)
supply from phytoplankton (see Section 2.1.3). In addition, a number of investigators
have noted that changes in the relative supply rates of N, P and Si lead to changes in
40
-------
species composition of phytoplankton communities, and this would likely modify some
aspects of deposition of OM to deep waters. Substantial rates of primary production have
been measured along the NGOM shelf, and these rates are comparable to those observed
in other eutrophic coastal systems (e.g., Lohrenz et al., 1990; Lohrenz et al., 1997; Nixon,
1992).
In Rabalais et al. (1999a) and the Integrated Assessment, organic matter from the
major rivers was discounted as a major source because 1) there have not been changes in
river OM loads since the beginning of the hypoxic period that account for the current
hypoxic zone size and expansion; 2) dissolved organic matter (DOM) sources from
rivers, while large, would need to be converted into particulate forms, with attendant
losses from this microbial transformation, and hence would be much reduced; 3) much,
but not all, of this terrestrially derived material is far less labile than phytoplanktonic
debris and hence is not readily respired at time scales associated with shelf hypoxia
(weeks to months). Using an estimated annual load of river OM (~ 2.6 x 1012 g C/ yr)
delivered to an average hypoxic area (15,000 km2), and assuming that even as much as
30% of this material were labile, suggests a small impact on DO conditions (-0.3 g O2/m2
/day). Additionally, while there is substantial POC and DOC coming down the
Mississippi River, there was undoubtedly far more 100 - 130 years ago when the
Mississippi River basin was first cleared for agriculture and before the dams in the basin
were built. While this process apparently has not been modeled in the Mississippi River
basin, modeling in other basins strongly suggests a huge increase in organic carbon fluxes
at the time of land-use conversion to agriculture, followed by decreasing fluxes as
agricultural practices improve (Swaney et al., 1996), and globally the flux of carbon in
rivers is tied to agricultural land use (Schlesinger and Melack, 1981). This historical
land-use change may well have contributed to the paucity of low oxygen conditions seen
in the paleoecological record in the late 1800s (Osterman et al., 2005). Given this
historical pattern, Mississippi River derived OM is unlikely to be the trigger for the level
of hypoxia that developed in the NGOM during the past 35 years. This period does
coincide well with the time N loads increased, due mainly to the use of synthetic N
fertilizer in the Mississippi River basin. Given experience in many other coastal and
estuarine regions (e.g., National Research Council, 2000), there are strong reasons to
believe that in situ NGOM primary productivity exploded in response to increased N
inputs over this time scale.
The influence of organic matter losses from coastal wetlands on coastal hypoxia is
still debated but seems unlikely to be a primary factor. Whether or not wetlands lose
more organic C as they degrade is not well known, but at present this also seems unlikely.
While the timing of wetland loss does not coincide with the onset of hypoxia in the 1970s
(marsh loss has been occurring since the 1940s), stable isotope and lignin analyses of OM
over much of the shelf indicates that terrestrially-derived OM is dispersed along and
across the shelf (Goni et al., 1998; Gordon et al., 2001). However, marsh particulate
organic material is refractory (i.e., resistant to decay) and does not contribute much to
hypoxia creation on time scales of weeks to months. Thus, while the conclusion that the
main OM source fueling hypoxia is in-situ production of marine phytoplankton and that
this production increased in response to enhanced nutrient loads from the MARB remains
41
-------
sound, a better understanding of the possible role of other sources would further refine
understanding of hypoxia.
Sources of organic matter to NGOM: post 2000 Integrated Assessment
Since the Integrated Assessment, there has been substantial research activity in the
NGOM regarding organic matter sources, characterization of organic matter, and related
issues. Some of this new work has utilized advanced analytical methods and improved
field techniques. However, as with the advent of sophisticated imaging devices in
medicine, where small and interesting structures in the human body can now be readily
observed but not necessarily interpreted in terms of health threats, in marine waters we
now have an emerging and more detailed description of the complex mix of organic
compounds, which has in the past simply been called organic matter. But it is not yet
clear how important some of this material is with respect to hypoxia issues. This
elaboration of understanding of OM adds interesting and useful dimensions to this story
but does not change the basic theme, which is that enhanced phytoplanktonic production,
based on much increased nutrient loading, is the main biological trigger of NGOM
hypoxia.
In addition, there have been at least two varieties of what can be called synthesis
studies. Studies of the first variety tend to be "review-like" wherein the growing time-
series of observations and new data have been revisited and/or re-analyzed. Several other
efforts of this type have also developed revised conceptual models of the role of OM in
hypoxia, and these will prove especially useful in time. Studies of the second variety,
and these are rarer, involve development of quantitative budgets or models of various
sorts. These efforts indicate that the information base regarding many aspects of OM and
hypoxia is rich enough to begin these more rigorous examinations. But, in virtually all
these efforts, authors conclude that results are preliminary and that more process-based
information is critically needed.
Advances in organic matter understanding: characterization and processes
A detailed review of these diverse studies is beyond the scope of this effort.
However, Table 1 summarizes a selection of those works to provide an indication of the
diversity of information that is becoming available. Some findings of particular
relevance to OM sources are provided below:
POC associated with sand transport in bottom waters in the lower Mississippi
River is similar in magnitude to loading of suspended POC (Bianchi et al., 2007).
The vertical flux of terrestrially-derived particles in the Mississippi River plume
was typically very high and mainly deposits locally (Corbett et al., 2004).
Recent analyses suggested that woody angiosperm material (13C-depleted)
preferentially settled within the lower Mississippi River and in the river plume
(Bianchi et al., 2002). Other work has demonstrated that erosion of relict peat in
42
-------
transgressional fades of the lower Mississippi River provide a source of "old"
vascular plant detritus to the river plume (Galler et al., 2003).
High sedimentation rates in the river plume result in the formation of mobile mud,
commonly observed in other large river-ocean interfaces (McKee et al., 2004). It
is estimated that about 50% of the sediments (and associated OM) delivered to
this region are temporarily stored near the delta - with a large fraction transported
along/across the shelf in the benthic boundary layer (Corbett et al., 2004, 2006).
Diatom signals in surface sediments suggested possible inputs of riverine diatom
phytodetritus to the inner shelf (Wysocki et al., 2006). Previous work showed
higher phytoplankton biomass, mostly as diatoms, than expected in the lower
river (Dagg et al., in press; Duan and Bianchi, 2006) with conversion, via lysis, to
DOC. Hence, river nutrients were converted to river phytoplankton biomass and
then ultimately to DOC, providing a labile food resource for bactedoplankton.
An analysis of OM production to the west of the plume found phytoplankton at
the outer edge of this region declined due to nutrient limitation, microzooplankton
followed trends in phytoplankton, most particle sinking was associated with
mesoplankton fecal pellets, phytoplankton-derived DOM reached a peak and was
correlated with bactedoplankton, and water column recycling was most intense in
this region (Dagg and Breed, 2003).
Estimates suggested 10% to 52% of the DOM in the region west of the plume is
quite labile (Benner and Opsahl, 2001). More recent data indicated that most
riverine DOC was photochemically converted to dissolved inorganic carbon
(DIG) over a period of weeks in this region (Dagg et al., in press). More
terrestrially-derived components such as lignin had similar fates (Hernes and
Benner, 2003).
Some labile sedimentary organic matter, from in situ diatom production, was
rapidly (day to weeks) shunted to the Mississippi River Canyon (Bianchi et al.,
2006), essentially bypassing the hypoxic zone to the west. The supply rate of this
phytodetritus was sufficient to support macrobenthic polychaete populations that
do not exist in nearshore waters off the Louisiana coast. The removal of labile
OM by winter season and hurricane events may act as a cleansing mechanism,
reducing the potential for hypoxia (Bianchi et al., 2006).
There are plumes from rivers and local estuaries along the coast containing
colored dissolved organic matter (Chen and Gardner, 2004). DOC concentrations
are also generally high (Engelhaupt and Bianchi, 2001) but higher still in the
Atchafalaya River than the Mississippi River (Chen and Gardner, 2004; Pakulski
et al., 2000; Bianchi et al., 2004).
These brief comments hardly do justice to the vast amount of work completed
since the Integrated Assessment. However, they do provide evidence of improved
understanding and elaboration of the role of different forms of OM in the NGOM
ecosystem.
43
-------
Synthesis efforts regarding organic matter sources
In most environmental analyses, synthesis of diverse data sets is essential for
clarifying cause-effect couplings and sorting out primary from secondary effects.
Hypoxia and the role of various OM sources in NGOM hypoxia are no exception.
Fortunately, a variety of descriptive and more quantitative syntheses/reviews have been
developed since the Integrated Assessment.
Several studies, including those of Rabalais et al. (2002), Turner et al. (in press),
Justic et al. (in press) and Rabalais et al. (2007), largely reaffirm the primacy of river
nutrients in supporting high rates of in-situ primary production as the dominant source of
OM supporting intense ecosystem respiration and development of hypoxic conditions.
Walker and Rabalais (2006) analyzed SeaWiFS algal biomass data in relationship to river
flow, nitrate loads from rivers and hypoxia. Results confirmed strong relationships
between nutrient loading and algal biomass distributions; direct relationships to hypoxic
waters remained elusive for a variety of reasons. The importance of this work lies in the
fact that the whole hypoxic-prone zone was assessed in a synoptic fashion and data were
available for both low and high nutrient load periods. Dagg et al. (in press) also reviewed
data to determine Mississippi River plume contributions to hypoxia. Results were largely
consistent with those noted above, but Dagg et al. (in press) focused on the important role
of the plume in both producing and consuming organic matter and dissolved oxygen and
in building a case for the importance of coastal wetlands as an important organic matter
source. However, there are problems with the magnitude of wetland OM contributions
suggested by these calculations, including conversion of wetland sediment losses to OM
mass, no consideration for on-marsh respiration of this material, and no consideration of
the refractory nature of the parti culate material, a major portion of this OM. Based on
present understanding of the issue, it seems unlikely that wetland loss could be a prime
source of OM to the hypoxic zone.
Finally, there have been several quantitative assessments of OM for portions of
the hypoxic zone, and these are emphasized here because it seems that these types of
syntheses are especially useful in understanding hypoxia and could serve as templates for
designing future data acquisition programs. Several other studies, including those of
Rowe and Chapman (2002) and Dagg and Breed (2003) have proposed broader
conceptual models for the plume and the full hypoxic zone, respectively, and these might
also be useful in study design and improving our vocabulary when discussing the hypoxic
zone and the role of various OM sources. Gordon et al. (2001) used a variety of
measurements to evaluate the distribution and accumulation of organic matter on the
shelf west of the Atchafalaya River. They reported inputs from rivers and in-situ
production (in-situ production dominated), estimated OM losses due to water column and
sediment respiration (OM substrates being marine and riverine, respectively) and long-
term burial (< 5% of total inputs). Green et al. (2006b) used careful delineation of the
Mississippi River turbidity plume coupled to a biological model to investigate OM
budgets for this zone. They reported that labile OM was mainly from autochthonous
phytoplankton production and that riverine OM inputs to the plume were three times as
large but quite refractory. Losses of OM were mainly from microbial respiration, and,
44
-------
importantly, the plume as a whole was net autotrophic, again suggesting the primacy of
in-situ production. Finally, while the plume is a small fraction of the full hypoxic zone,
Green et al. (2006b) estimated that plume derived OM was equivalent to about 23% of
the OM needed to create observed hypoxia on the full shelf.
Table 1: A partial summary of papers published following the Integrated Assessment related to sources of organic
matter to the Gulf of Mexico.
General Topics and issues
Comments regarding OM/hypoxia
Reference
Landside Sources
POC in river sands
Sedimentation of river POC
Relict peats
Seasonal transport of POC
Sediment storage and transport
River OM loads
River inputs
Terrestrial OM
Riverine DON
Riverine OM and nutrients
Riverine DOM
Marsh/estuary DOC
OM distribution
Similar in magnitude to suspended POC load in river
high deposition of terrestrial POC in plume region
source of old organic matter to plume area
fluid muds are transported seasonally to GOM
seasonal transport of mobile muds from delta to shelf
DOC and DON loads to GOM
transport of river diatoms to plume area
fate of lignin
photoammoniiication of DON to DIN
Effects of flow through coastal wetlands
CDOM analysis
high DOC concentrations in these systems
sources and fate of OM from rivers to shelf
Bianchi et al., 2002
Corbett et al., 2004
Galleretal.,2003
McKee et al., 2004
Corbett etal., 2006
Bianchi et al., 2004; Duan et al., 2007
Wysocki et al., 2006; Duan and
Bianchi, 2006
Hemes and Berner, 2003
Pakulski et al., 2000
Xu, 2006
Chen and Gardner, 2004
Engelhaupt and Bianchi, 2001
Gordon etal., 2001
Water Column/ Sediment Processes
Flocculation and sedimentation
Light field
Plankton characteristics
Plume budget
OM source
Deposition
DOM characteristics
Sediment DOC
Fate of benthic diatoms
Hurricane effects
Sediment processes
Plankton composition
Plankton composition
enhanced process in plume area; high rates
light absorption/scattering limiting production
satellite-based relations between N- loads and
chlorophyll
CO2 budget in plume
high rates of plankton production west of plume
Influence of larvaceans on deposition
Lability of DOM in Region II
Release of DOC from shelf sediments
Benthic diatom shunted to MR canyon; cleansing effect
storm transport of deposited materials-decadal scale
ammonium flux from sediments important for plankton
Diatom occurrence in western regions of hypoxic zone
microbial processes in shelf waters
Dagg et al., 2004
D'Sa and Miller, 2003
Walker and Rabalais, 2006
Cai, 2003
Dagg etal., in press
Dagg and Brown, 2005
Benner and Opsahl, 2001
Sutula etal., 2004
Bianchi et al., 2006
Corbett et al., 2006
Eldridge and Morse, 2007
Warwick and Paul, 2004
Liu etal., 2004
Synthesis/Overviews
OM budget
Conceptual model/synthesis
Model analysis
Statistical model
Carbon budget for plume area
planktonic dynamics of region outside plume
differences between water and sediment respiration
relates N-load to hypoxia; phytoplankton OM implied
Green etal.,2006b
Dagg and Breed, 2003
Hetland and DiMarco, 2007
Scavia etal., 2003
45
-------
General Topics and issues
Water column synthesis
Review/synthesis
Nutrient/Organic loads
Forecasting hypoxia
Primary production-nitrate model
Concepts of hypoxic zones
Comments regarding OM/hypoxia
plume contributions to hypoxia; gaps in understanding
new monitoring data strengthens nutrient/hypoxia model
confirms Integrated Assessment, wetland loss small OM
examines models and suggests nutrients major driver
model indicates buffered response to N-load reductions
suggests spatial dimensions/processes in hypoxic zones
Reference
Dagg etal, in press
Rabalaisetal.,2007
Turner et al, in press
Justic etal., in press
Green et al., in press
Rowe and Chapman, 2002
* Entries are shown for a variety of topics and comments are focused on issues related to organic matter in
the GOM. This table is not a complete summary of all papers published on this subject; rather it provides
an indication of the great diversity of studies conducted since the Integrated Assessment.
Key Findings and Recommendations
The SAB Panel concludes this section with several findings. First, there is general
and strong support for the conclusion that riverine nutrients support levels of plankton
production capable of creating observed hypoxic conditions. However, some aspects of
the relationship between in-situ phytoplankton production and hypoxia remain uncertain.
There is need for additional study of the hypoxia issue that emphasizes process studies
and better coupling of physics to the chemical and biological features of the hypoxic
zone. The SAB Panel therefore provides the following recommendations.
Continued research should be conducted to further elucidate the role of N and P
from the MARB in stimulating phytoplankton production, the primary drivers
creating excess OM and thus hypoxia in the Gulf.
A series of consistent, well-placed, and well-timed process studies should be
conducted in the NGOM. Virtually all the OM review/synthesis papers referenced
above state that their analyses suffer from a lack of pertinent process data.
DOM and POM delivered to the NGOM by rivers and from coastal wetland losses
represent potential OM sources. The weight of evidence currently available
suggests that it is unlikely these were triggers for hypoxia development or primary
OM sources for hypoxia maintenance. However, the magnitude of river OM
sources is large, and hence further characterization of this material is warranted.
2.1.6. Denitrification, P Burial, and Nutrient Recycling
The availability of N and P in an ecosystem is controlled both by external
loadings and internal biogeochemical processes. Ideally information is needed on the
load of biologically available nutrients, which is not necessarily well reflected by either
the load of dissolved inorganic nutrients or the load of total nutrients. Internal
biogeochemical processes are poorly known for the NGOM. Some, but not all, of the
dissolved organic nutrients and particle-bound nutrients delivered to coastal waters
46
-------
become biologically available on ecologically meaningful time scales (days to months).
In the Mississippi River, the fate of the particle-bound P is of particular interest since it is
the most common form of P in the river (Sutula et al., 2004). The bioavailability of this
form of P is low within the freshwater portions of the Mississippi River, but, as the
particles encounter the increasingly more saline waters of the Gulf of Mexico, the high
ion abundances of seawater cause much of the adsorbed inorganic P to desorb, converting
it into highly bioavailable dissolved inorganic P (Fox et al., 1985; Froelich, 1988;
Howarth et al., 1995; Sutula et al., 2004). In addition, sediment diagenetic processes
further increase the biological availability of particle-bound P delivered to the Gulf
(Sutula et al., 2004).
For many coastal marine systems, the tendency is for benthic processes to make N
limitation more prevalent since the N sink through denitrification is relatively larger than
is the loss of P through permanent sediment burial (National Research Council, 2000;
Blomquist et al., 2004; Howarth and Marino, 2006). Phosphorus release from sediments
is frequently less than the rate of P remineralization, due to P adsorption and storage in
surface sediments (National Research Council, 2000; Howarth and Marino, 2006).
Variations in P release are probably due to differences in the amount and forms of iron in
the sediments, the extent of sulfate reduction, and mixing by the benthic fauna,
particularly as this affects micro-scale variation in pH (Howarth et al., 1995). The
dynamics of P-sediment exchanges in the Louisiana shelf region are sufficiently complex
that in a recently published model of sediment diagenesis (Morse and Eldridge, in press),
P processes were deliberately not considered (John Morse, personal communication,
10/27/06). Given the recent evidence of the role of P in controlling phytoplankton
production in the plume and near-plume regions, this process needs further examination.
Sulfate reduction is particularly important in affecting the P cycle of coastal
marine sediments, since it can transform highly adsorptive forms of iron (III) oxides and
hydroxides into non-sorptive iron (II) sulfides (Krom and Berner, 1980; Caraco et al.,
1989, 1990; Blomqvist et al., 2004). Sulfate reduction may also release P from
covalently bound minerals as diagenesis proceeds (Sutula et al., 2004). Sulfate reduction
dominates the metabolism of the sediments to the west of the Mississippi River on the
Louisiana shelf away from the immediate plume of the river (Rowe et al., 2002), as is
true for many coastal marine sediments (Howarth, 1984). Sutula et al. (2004) have
demonstrated that the P content of these sediments is only half that of the riverine
sediments in the Mississippi from which they are derived due to losses during diagenesis.
Sulfate reduction and the concomitant changes in sediment iron chemistry may not be the
only factor involved. Sutula et al. (2004) noted that significant sediment P is lost in the
immediate plume area of the Mississippi River, a high-energy environment subject to
physical mixing and sediment reworking, which may make sulfate reduction unlikely [the
"sub-oxic fluidized bed reactor" processes that Aller (1998) described for other riverine
plumes].
Studies in the Gulf of Mexico have shown that aerobic respiration in the
sediments is low during hypoxic events (Rowe et al., 2002). This result suggests that
anaerobic respiration, the accumulation of reduced compounds, and subsequent oxidation
47
-------
of these reduced species in the benthic boundary layer (BBL) and sediments may account
for a large percentage of the oxygen draw down in this area (Morse and Rowe, 1999).
Other work has found that the balance between the frequency of seabed disturbance, rate
of geochemical reactions, and reactant concentrations work together to promote efficient
remineralization through redox cycling in highly mobile muds near large river (McKee et
al., 2004; Aller et al., 2004; Chen and Gardner, 2004; Chen et al., 2005). This frequent
cycling of reduced and oxidized compounds is likely to have a profound effect on short-
term oxygen consumption in the BBL, which could influence development of bottom
hypoxia.
Hypoxia and bottom water oxygen deficiency influence not only the habitat of
living resources but also the biogeochemical processes that control nutrient
concentrations in the water column. Internal feedbacks on biogeochemical processes
occur with oxygen depletion. Increased P flux from sediments into overlying waters with
hypoxia is a classic response in freshwater systems (Mortimer, 1941) and has been well-
documented in coastal marine ecosystems (Nixon et al., 1980; Conley et al., 2002a,
2002b). However, relatively little work has been done on the Mississippi River shelf on
estimating the magnitude of enhanced P release with hypoxia and the impact on the
overall P biogeochemical cycle. Higher P levels do accumulate in the bottom waters of
the NGOM during hypoxia, but there is no evidence that this mixes into the overlying
photic zone where it could be available to phytoplankton. This is critical information as
P can be an important limiting nutrient in the plume (Sylvan et al., 2006).
Hypoxia also may influence rates of denitrification. Denitrification is one of the
major losses of fixed nitrogen in the oceans (Seitzinger and Giblin, 1996), however, its
measurement is difficult (Groffman et al., 2006). Denitrification is the reductive
respiration of nitrate or nitrite to N2 or N2O and includes the recently discovered
anaerobic ammonia oxidation (ANNAMOX) process (Dalsgaard et al., 2003). The rates
of denitrification are dependent on a variety of factors, but a major control is the
availability of starting products [e.g. nitrate (Kemp et al., 1990) and carbon (Smith and
Hollibaugh, 1989; Sloth et al., 1995)]. Note that denitrification is favored by the absence
of oxygen, but most coastal marine sediments are anoxic below the top few mm. Given
that large-scale increases in nitrate concentrations and in productivity that have occurred
on the Mississippi River shelf, it is likely that the rates of denitrification have also
increased through time. Very few measurements on this important process are available,
however.
An open question is how much hypoxia affects the annual rates of denitrification.
Few direct measurements of denitrification exist for the Mississippi River shelf, with
most previous estimates using potential denitrification rates. Lower rates of potential
denitrification were observed in the Gulf of Mexico zone of hypoxia when low oxygen
concentrations were encountered (Childs et al., 2002, 2003), although the observed rates
were at the low end of rates reported for other systems (Herbert, 1999). Denitrification
can be limited by the availability of nitrate, and hypoxia may reduce the supply rate of
nitrate by slowing rates of nitrification (the oxidation of ammonium to nitrate); however,
nitrate concentrations in the hypoxic area were high enough in the Childs et al. (2002)
48
-------
study not to be limiting. In addition, sulfide, which is commonly found in anoxic
environments, acts to inhibit nitrification (the oxidation of ammonium to nitrate) (Joye
and Hollibaugh, 1995), thus reducing the availability of nitrate. In Danish coastal waters,
rates of denitrification are highest during winter when nitrate concentrations are at their
annual maximum (Nielsen et al., 1995), and low rates are observed during the summer.
There are no seasonal measurements of denitrification available for the NGOM to
estimate the overall effect of hypoxia. In general, the overall rates of denitrification are
believed to be lower with hypoxia (S0rensen et al., 1987; Graco et al., 2001) and
eutrophication (Smith and Hollibaugh, 1989), although Vahtera et al. (2007) suggest that
denitrification has potentially increased with hypoxia. Water column rates of
denitrification in the oceans are high in mid-water hypoxia areas (Deutsch et al., 2007).
Further investigations of the effects of hypoxia on the rates of denitrification are sorely
needed on the Mississippi River shelf, as this is the major pathway of nitrogen loss.
Measurement of the fluxes of N and P from sediments provides a direct means to
assess the role of sediment processes on the relative balance of N and P in the overlying
water column. There are relatively few NGOM studies where both N and P fluxes from
sediments have been determined simultaneously. A compilation of these studies shows a
dissolved inorganic nitrogen/dissolved inorganic phosphorus (DIN:DIP) flux ratio that
varies from approximately 1:1 to 25:1, with a mean of-10:1 (Twilley et al., 1999).
Key Findings and Recommendations
The SAB Panel finds that additional information is needed on internal
biogeochemical processes controlling the availability of nutrients to support primary
production in the NGOM. The SAB Panel recommends that research be conducted in the
following areas.
The dynamics of sediment/water exchanges of P on the Louisiana shelf and their
relative role in P cycling. Information on both aerobic and anaerobic processes is
needed.
The effects of hypoxia on the rates of denitrification and on long-term burial and
regeneration of C, N, and P on the Louisiana shelf.
N and P biogeochemical processes in sediments that include analysis of oxygen
dynamics and the rates of supply of oxygen to the sediment surface.
2.1.7. Possible Regime Shift in the Gulf of Mexico
Hypoxia can act as a positive feedback to enhance the effects of eutrophi cation
(Vahtera et al., 2007). It has long been known in lakes (Mortimer, 1941) that the internal
P loading from sediments during anoxia can sustain eutrophi cation. In the Baltic Sea,
49
-------
which is one of the largest coastal areas in the world to suffer from eutrophication-
induced hypoxia, large internal P loading occurs with hypoxia. The amount of DIP
released from sediments in the Baltic is an order of magnitude larger than external inputs
from rivers (Conley et al., 2002a). Large sediment-water fluxes of DIP with hypoxia
must also occur in the Gulf of Mexico, returning DIP to a partially P limited water
column (Sylvan et al., 2006), stimulating phytoplankton growth and acting as a positive
feedback to increase hypoxia severity. As discussed earlier (Section 2.1.6), hypoxia has
the potential to reduce rates of denitrification, which would cause less N to be lost from
the system, and also act as a positive feedback to increase hypoxia severity.
Recent studies in other coastal marine ecosystems, including Chesapeake Bay
(Hagy et al., 2004) and Danish coastal waters (Conley et al., 2007), suggest that repeated
hypoxic events can help to sustain hypoxic conditions. Large-scale changes in benthic
communities occur with hypoxia, reducing the abundance of large, slow growing, deeper
dwelling animals and facilitating smaller, fast growing species that can colonize surface
sediments rapidly following hypoxia (Diaz and Rosenberg, 1995). Reductions in the
abundance and size structure of benthic organisms have been observed in the NGOM
with hypoxia (Rabalais et al., 2001a). These smaller, surface-dwelling species have less
capability to irrigate and bring oxygen downward into the sediments, helping to keep the
sediments anoxic. The loss of benthic communities and the inability of the communities
to recover with repeated hypoxic events (Karlson et al., 2002) may make ecosystems
more vulnerable to the development and persistence of hypoxia. In addition, with the
loss of sediment buffering capacity through the loss of electron acceptors (NOs, C>2, Fe2+,
Mn2+), there is a change in sediment metabolism from aerobic to anaerobic pathways,
changing the rates and processing of organic matter.
Wiseman et al. (1997) showed that the area of hypoxia along the Louisiana-Texas
shelf was correlated to Mississippi River flow. These relationships were similar to those
found for Chesapeake Bay (Boicourt, 1992) demonstrating the important role of river
inputs in providing both freshwater induced stratification and adding nutrients stimulating
phytoplankton production. However, this apparent relationship has broken down since
1993 (data provided by DiMarco, personal communication). It appears that the Gulf of
Mexico hypoxia has worsened following the record breaking 1993 spring floods, e.g.,
smaller river flows now induce a larger response in hypoxia (see Section 2.1.2). The first
large (>15,000 km2) hypoxic event occurred after the 1993 flood, with large hypoxic
areas over 15,000 km2 observed in most following years. This pattern of a more sensitive
system is also evident with May-June nitrate loading causing a larger hypoxic area in the
NGOM than prior to 1993 (data not shown). A similar pattern of an increasingly
sensitive system following the initial occurrence of hypoxia has been observed in Danish
coastal waters with worsened hypoxia following the first appearance of large-scale
hypoxic events (Conley et al., 2007).
Changes such as those described above suggest that a regime shift has occurred in
coastal marine ecosystems that have been affected by large-scale hypoxia (Conley et al.,
2007). Regime shifts are rapid transitions that change the structure and functioning of the
ecosystem from one state to another as a consequence of a change in an independent
50
-------
variable. Once a threshold is passed, the ecosystem changes to a new alternative state,
with changes in biological variables that can propagate through several trophic levels
(Scheffer et al., 2001; Collie et al., 2004). For example, an increase in certain pelagic
species (e.g., gelatinous carnivores) can disrupt top-down control of the food web
structure causing a regime shift to an alternative stable state. The new stable system may
not respond to changes in nutrient levels, a bottom up control, until nutrient input is
reduced to a point below which the regime shift occurred. A regime shift due to hypoxia
implies that, due to hysteresis in the system, nutrients will need to be reduced below the
level at which the threshold occurred in order to reduce hypoxia. The management
implications are that nutrients should be reduced as soon as possible before the even
larger nutrient reductions are required to reduce the area of hypoxia.
Regime shifts can have large consequences for fisheries (Collie et al., 2004; Oguz
and Gilbert, 2007). The Gulf of Mexico ecosystem is a tremendously valuable resource
from economic, ecological and social perspectives. In 2004, the value of commercial fish
harvest in the Gulf of Mexico was $670 million (NOAA, 2007). The Gulf of Mexico
shrimp fishery is among the most valuable fisheries in the nation, with a total value in
2004 of about $370 million, and about $140 million in Louisiana alone. Additionally, an
estimated 24.6 million recreational fishing days occurred in the Gulf of Mexico in 2004,
with about 4.8 million of those occurring in Louisiana waters (NOAA, 2007). The Gulf
of Mexico also serves as habitat for a host of other species, including endangered sea
turtles and marine mammals. Thus, the Gulf of Mexico is a tremendously valuable
resource that is potentially being threatened by hypoxia.
Earlier studies found it difficult to identify impacts of hypoxia in fisheries
landings statistics (Diaz and Solow, 1999; Rabalais and Turner, 2001), although there has
been a shift in relative population abundance from benthic to pelagic species (Chesney
and Baltz, 2001). A summary of published studies, as well as works in progress, on the
effects of hypoxia on living resources in the NGOM are mentioned in Appendix A.
There is strong scientific evidence that ecosystems in the northern Gulf of Mexico are
stressed by hypoxia (Diaz et al., 2003). Studies have found impacts ranging from the
molecular/genetic level (Brouwer, 2006; Hendon et al., 2006; Perez et al., 2006; Wells et
al., 2006), the organismal level (Brouwer, 2006; Zou, 2006) and the ecosystem level
(Craig et al., 2001; Rabalais and Turner, 2001a; Rabalais, 2006). Potential impacts have
been identified due to displacement from preferred habitat (Craig and Crowder, 2005;
Craig et al., 2005; Switzer, et al., 2006). There is also recent evidence that hypoxia has
affected the valuable brown shrimp fishery (Zimmerman and Nance, 2001).
There are some indications that the Gulf of Mexico has undergone a regime
shift. In the hypoxic/anoxic zone of the Louisiana inner shelf many taxa are lost during
the peak of hypoxia. Certain typical marine invertebrates are absent from the fauna, for
example, pericaridean crustaceans, bivalves, gastropods, and ophiuroids (Rabalais and
Turner, 200la). As noted above a shift has been observed in the relative abundance of
fish species. Changes in benthic and fish communities with the change in frequency of
hypoxia are cause for concern. If actions to control hypoxia are not taken, further
ecosystem impacts could occur within the NGOM, as has been observed in other
51
-------
ecosystems. The recovery of hypoxic ecosystems may occur only after long time periods
(Diaz, 2001) or with further reductions in nutrient inputs. Experience has shown
recovery to be greatly delayed, taking years to decades for ecosystems to recover after
nutrient inputs are reduced, and with probably less than complete recovery possible (e.g.,
Diaz, 2001; Diaz et al., 2003; Mee, 2006; Raloff, 2004). Some smaller organisms may
respondmore rapidly and on annual cycles. For example, in low load years there is less
hypoxia, lower phytoplankton biomass and presumably less organic deposition and lower
rates of sediment processes. On the other hand, larger benthic organisms respond more
slowly, and resident fish and shellfish populations will require more time to return to
previous conditions. One potential concern with regime shifts is that the condition is not
always reversible. The system can follow a different path to pre-impact conditions and
not return to its former state. This is called a hysteresis effect. However, given that the
Gulf of Mexico is an open shelf system, recovery should be more rapid than in enclosed
ecosystems. Thus, there are potentially large benefits that justify taking action to control
hypoxia, and thereby avoiding large-scale changes in the Gulf of Mexico ecosystem.
Key Findings and Recommendations
Hypoxia probably increases sediment-water fluxes of P and may reduce the
potential for denitrification, and change the degradation of organic matter in sediment
from aerobic to anaerobic metabolism. Biological changes have occurred in the benthic
communities of the NGOM, and there is evidence that the living resources are impacted
by hypoxia. The Gulf of Mexico ecosystem appears to have gone through a regime shift
with hypoxia such that today the system is more sensitive to inputs of nutrients than in
the past, with nutrient inputs inducing a larger response in hypoxia as shown for other
coastal marine ecosystems (Chesapeake Bay, Danish coastal waters). The SAB Panel
therefore provides the following recommendation.
Nutrients should be reduced as soon as possible before the system reaches a point
where even larger reductions are required to reduce the area of hypoxia.
2.1.8. Single Versus Dual Nutrient Removal Strategies
The Action Plan seeks to significantly reduce the size of the Gulf of Mexico
hypoxic zone by the year 2015, primarily through reductions in nitrogen (N) loadings
from the MARB to the NGOM. Increases in N loads have clearly been occurring
throughout the past decades, and there is ample evidence to conclude that N from the
MARB is a driving force in determining, at least in part, the timing, severity and extent of
the hypoxic zone. Since the mid-90s, N loadings from the MARB have decreased,
although they are still much elevated over historic levels. Total phosphorus loadings,
however, have not changed greatly during this period (Battaglin, 2006; Turner et al., in
press; Section 2.1.9 of this report). This trend in nutrient loadings has led to reduced
(albeit still very high by "Redfield" standards) N:P ratios. This evidence suggests that P
52
-------
is an additional nutrient of concern, in terms of input reductions. As conveyed in
previous sections of this report, a number of investigators (Sylvan et al., 2006; Dagg et
al., in press) have concluded that P is limiting primary production during key periods of
high productivity and in zones of high biomass accumulation in the NGOM adjacent to
hypoxic waters. Therefore, the role of P in the onset, extent, and duration of the hypoxic
zone is worthy of additional consideration.
Many factors influence the cycling and ultimate fate of both N and P. As both
play a significant role in driving primary production within the NGOM (and perhaps, in
conjunction with Si, in the composition of the primary producers and the likely fate of
produced organic carbon), it is logical to consider the potential for removal of either or
both elements as a means to reducing hypoxia. The 2001 Action Plan focuses on N
reductions but does not preclude either P reduction or dual removal strategies. For
example, the most recent report of the Mississippi River/Gulf of Mexico Watershed
Nutrient Task Force's Management Action Review Team (MART, 2006a) concludes that
most load reduction projects developed under the Clean Water Act Section 319 program
have targeted both N and P for reduction. Indeed, Howarth et al. (2005) noted that some
N control practices utilized in the U.S. effectively remove P as well, although the reverse
is not always the case. However, not all control practices will be effective as a dual
nutrient removal strategy; see specific discussion on this topic in Section 4.5.10.
Restoration plans that focus on N alone may not rapidly improve the situation in
the MARB where many streams and river segments are degraded by excess P
concentrations (Action Plan., MR/GMWNTF, 2001). Given recent discoveries
concerning the importance of P in production of organic carbon within significant
portions of the NGOM, focusing on N reduction alone may be insufficient to provide the
desired reduction in the hypoxic zone. However, some plans being undertaken to reduce
non-point sources of N [forested buffers, 319 programs, and others (see Section 4.4.2, for
example)] will also lead to P reductions, as well. Reductions in P alone will alleviate
some of the water quality issues facing freshwater regions of the MARB but are not
likely, given our current state of understanding, to significantly address the over-
enrichment of the NGOM. Therefore, greater emphasis on a dual nutrient removal
strategy is warranted, a conclusion that has been reached in other instances (e.g., National
Research Council, 2000; Boesch, 2002; Howarth and Marino, 2006).
Further work is necessary to examine how effectively current reduction strategies
target both elements. There may be areas where shifts in removal techniques could
improve P reduction. In addition, there is still much to be learned about the response of
autotrophic and microbial communities to shifts in nutrient loading and ratios. A better
understanding of how these communities have responded to the current loadings and
predictions of how they will continue to adapt to nutrient reductions will greatly improve
predictions of the likely response in the extent and duration of hypoxia to nutrient
reductions in the future.
53
-------
Key Findings and Recommendations
Recent information clearly indicates that P controls productivity in some portions
of the NGOM. The SAB Panel finds that restoration plans focusing on N alone may not
rapidly improve the situation in the MARB and may be insufficient to provide the desired
reduction in the hypoxic zone. Reductions in P alone will alleviate some of the water
quality issues facing freshwater regions of the basin but are not likely to significantly
address the over-enrichment of the NGOM. Therefore the SAB Panel offers this
recommendation.
In addition to the N reduction strategy currently in place, reduction strategies for
P should be implemented. Section 4.2 provides greater detail on the SAB Panel's
recommended targets for reducing both N and P.
2.1.9. Current State of Forecasting
There are several types of modeling efforts working toward a better understanding
of factors influencing the extent and duration of the Gulf of Mexico hypoxic zone. These
vary from the simple to the complex and are based on empirically observed relationships,
on mechanistic understanding, or some combination of both.
Empirical models are widely used in the aquatic sciences to establish relationships
between variables, with the most well known being the correlation between spring P
loading in lakes and summer chlorophyll concentrations (Vollenweider, 1976). This
work has been widely used in a management context to justify reductions in
anthropogenic phosphorus loading to lakes and to set goals for reductions for particular
lakes. Nixon et al. (1996) developed a similar correlation between annual loading of DIN
and rates of primary productivity for marine ecosystems. While establishment of
empirical models has greatly enhanced understanding of the structure and functioning of
aquatic ecosystems (Peters, 1986), the standard criticism of this approach is that
correlation does not imply causation. Although correlations between variables exist, they
do not explain why variables are correlated or the mechanisms of the relationship. They
do, however, provide some very useful predictive capability. In addition, when
ecosystem production is greatly different from that predicted, controls on productivity
other than nutrients may be dominating, such as light limitation or limitation from rapid
flushing (Howarth et al., 2006a).
Some new forecast modeling work has been completed since the Integrated
Assessment. Turner et al. (2006) developed simple linear and multiple regression models
to examine hypoxia in the NGOM. Empirical models require important decisions
regarding the choice of variables and of the time scales of model operation. Turner et al.
(2006) tested many different nutrient loading lag times and concluded that the best
54
-------
relationship was obtained two months (May) prior to the maximum observed extent of
hypoxia (July), with significant correlations for nitrate+nitrite, total nitrogen (TN), ortho-
P and total phosphorus (TP) (r2 values of 0.50, 0.27, 0.54, and 0.60, respectively). A
multiple regression analysis was also developed incorporating nutrient load and a new
variable "Year" to account for the increase in carbon in surface sediments after the 1970s
causing significantly more sediment oxygen demand. A lag of two months of nutrient
loading was, again, the most significant variable to describe hypoxic area with r2 values
of 0.82, 0.80, 0.69, and 0.64 obtained with nitrate+nitrite, TN, ortho-P and TP,
respectively. Turner et al. (2006) then used the nitrate+nitrite model to extrapolate
beyond the data range used to construct their models to predict hypoxic area prior to
available measurements. When the hindcasted values became negative, they were plotted
as zero values. In general, it is considered incorrect to extrapolate model results in this
manner beyond the range of the data supporting the model, as other mechanisms and
relationships may exist that may not be included in the regression analysis. Further, the
SAB Panel believes that the addition of the variable "Year" in the multiple regression
analysis is inappropriate as the addition of one more year will cause prediction of a
positive increase in hypoxia with time.
Among models that address Gulf of Mexico hypoxia and include some
consideration of processes and mechanisms, that of Scavia et al. (2003) is one of the
simplest. Their model uses a relationship between the nitrogen loading from the MARB
and the decay of oxygen "downstream" (i.e., in the NGOM - within the plume and the
nearshore reaches to the west of the Mississippi and Atchafalaya River outflows). When
used in a forecast mode, this model is able to only explain approximately 45-55% of the
variability in hypoxic length and area. This model explicitly addressed uncertainty in
prediction. The SAB Panel found this approach to be very useful. Recently, in
combination with a watershed model, the model of Scavia et al. (2003) has been used to
address how climatic variability and change may affect Gulf hypoxia (Donner and
Scavia, 2007). A similar model has also been applied very successfully to understand
hypoxia and anoxia in Chesapeake Bay (Scavia et al., 2006). The Scavia et al. (2003)
model focused on N loading and did not consider P. Consideration of P would seem to
be a timely addition to the model, and a manuscript including P recently was accepted for
publication by Scavia and Donnelly (Scavia and Donnelly, in press). This model
approach, and the modeling efforts of Bierman and colleagues and Justic and colleagues
(see below) all provide reasonably consistent guidance and suggest similar levels of N
reduction that might be required to reduce the extent of the hypoxic zone.
Other process-based models are more complex and attempt to model both
physical and biological controls occurring in the hypoxic region. Examples include those
of Bierman et al. (1994), Justic et al. (1996, 2002), and Green et al. (2006b). The
Bierman et al. (1994) model is the most complex of these approaches and simulates the
steady-state summertime conditions for the hypoxic area using three-dimensional
modeling of the physics as well as interactions between food web processes, nutrients,
and oxygen. The model of Justic et al. (1996, 2002) simulates oxygen dynamics at one
location within the hypoxic zone using a simple model that has two vertical layers and
meteorological conditions and nitrogen loads as drivers. The Green et al. (2006b) surface
55
-------
mixed layer model is based on food web dynamics and relatively simple two-dimensional
physics (no vertical dimensionality) of the Mississippi River plume. This model predicts,
among other things, the relationship between carbon sources and bottom-water oxygen
depletion; the model does not include changes to either N or P inputs or dynamics. None
of these more complex models explicitly presented analysis of uncertainty or sensitivity
analysis of potential biasing terms. As with the Scavia et al. (2003) model, Bierman et al.
(1994) and Justic et al. (1996, 2002) do not consider P loads or dynamics.
It should be pointed out that complex water quality models that could be very
useful in the NGOM have been developed and used in other environmentally stressed
regions like the Chesapeake Bay system (Cerco and Cole, 1993), Long Island Sound (St.
John et al., 2007), the New York/New Jersey Harbor/New York Bight complex
(Landeck-Miller and St. John, 2006), and the Massachusetts/Cape Cod Bays system
(Besitkepe et al. 2003). These models include a coupling to three-dimensional and time-
dependent hydrodynamics, a water column eutrophication submodel and a sediment
diagenesis/nutrient flux submodel. The water column eutrophication submodel includes
state-variables for three functional phytoplankton groups; dissolved inorganic nutrients
(ammonium, nitrate+nitrite, ortho-phosphate, and silica);, and labile and refractory forms
of dissolved and particulate organic nitrogen and phosphorus, biogenic silica; labile and
refractory forms of particulate and dissolved organic carbon; and dissolved oxygen. The
sediment nutrient flux submodel includes state-variables for labile, refractory, and inert
organic carbon, nitrogen, and phosphorus, as well as biogenic silica. Inorganic
substances tracked include ammonium, nitrate+nitrite, ortho-phosphate, silica, sulfide,
and methane. Processes tracked in the sediment flux model include: organic matter
deposition; sediment diagenesis; burial; the flux of inorganic nutrients between the water
column and the sediment bed; and the generation of sediment oxygen demand (SOD).
There is an inherent trade off between model simplicity (where many potentially
important factors are not considered) and complexity (where many coefficients and a
great amount of data are required). More complex models may have value to help devise
effective management strategies, especially if N reductions alone will not be sufficient to
control hypoxia and if the more complex models can reasonably capture the importance
of P. However, with complexity comes greater numbers of estimated parameters and the
uncertainty associated with them. Hence this type of model may not improve forecasting
capabilities dramatically. The development of more complex models is likely to prove
extremely valuable for understanding the physical factors controlling water and carbon
(C) transport, the dynamics of nutrient interactions with primary producers, and the
recycling and loss of C and nutrients from the system. There is also great value in
refining and further developing simple models, which may, in the end, prove most
valuable for making management decisions. Scavia et al. (2004) explicitly compared the
models of Scavia et al. (2003), Biermann et al. (1994), and Justic et al. (1996, 2002) for
use in managing Gulf of Mexico hypoxia and showed that all three models gave broadly
consistent guidance.
The physics of the NGOM region is complex, and there is clear value in
developing more complex models of physical processes for this region. Improved three-
56
-------
dimensional models with finer grid structure than present models would have many uses.
These uses include assisting the interpretation of monitoring data and serving as
platforms upon which improved models of biogeochemistry and ecological response
could be built. However, the level of complexity in the biogeochemistry and ecology
need not match the complexity of the physical models (Hetland and DiMarco, 2007).
Complex physical models could be very valuable in constructing simple box mass-
balance accounting models for C, N, P, Si, and O, for example. The importance of
developing such budget-based models is discussed further below.
In addition to statistical and simulation models, another modeling format that
should be considered involves construction and evaluation of material budgets or mass
balance models. These are basically quantitative input-output budgets with additional
complexity added by consideration of internal processes of production, recycling and
loss. These relatively simple budgets provide a quantitative mass balance framework to
test the understanding of how the systems work. These budgets should be developed on a
seasonal basis (e.g., summer hypoxic season) and evaluated for distinctive areas (e.g.,
Mississippi River Plume). These budgets are largely based on empirical observations and
are not simulated through time, although data used in a budget analysis are needed in
simulation models for both calibration and verification. As an example, an oxygen
budget (Equation 1) would involve DO inputs/outputs from air-sea diffusion, horizontal
advective/dispersive transport, and vertical transport between euphotic and sub-
pycnocline zones. In addition, DO is added through daytime photosynthesis and lost
through water column and sediment respiration. Evaluation of these pathways indicates
especially important processes, and imbalances in the budget point to areas where
understanding or measurements are inadequate. We suggest that conceptual mass
balance models also be used to provide a checklist of needed measurements for future
NGOM hypoxia research/monitoring.
Other general points regarding modeling efforts are summarized in Section 3.4 of
this report. An important conclusion for both models of the response of the NGOM to
nutrient inputs and watershed models generating estimates of nutrient loads is that a
diverse ensemble of models is needed, including both relatively simple and more
complex ones. No one best approach to modeling can be identified, and management of
Gulf hypoxia is best served by having multiple models with multiple outputs. The SAB
Panel suggests that modeling efforts, ranging from the simple to complex, be conducted
in parallel wherein there is the opportunity for cross-testing of results among model
formats. When predictions tend to agree, managers can have more confidence in
deciding upon courses of action. When models do not agree, dissecting the reasons for
divergence can lead to better understanding and, ultimately, better management.
57
-------
Key Findings and Recommendations
Since the Integrated Assessment, a number of modeling approaches have been
employed to characterize the onset, volume, extent, and duration of the hypoxic zone.
Models have been able to explain approximately 45-55% of the variability in hypoxic
length and area. However, the SAB Panel finds that model development, calibration, and
verification are hampered by the relative paucity of data on the duration and extent of
hypoxia and on rates of important biogeochemical and physical processes that regulate
hypoxia. In addition, the SAB Panel finds that a diverse ensemble of models is needed,
including both relatively simple and more complex ones. No one best approach to
modeling can be identified, and management of Gulf hypoxia is best served by having
multiple models with multiple outputs. The SAB Panel provides the following
recommendations to advance the science for characterizing the onset, volume, extent, and
duration of the hypoxic zone.
To the extent reasonable, future models (particularly more complex models that
rely on accurate representation of ecological and biogeochemical processes) of
hypoxia in the Gulf should consider nitrogen, phosphorus, and their interactions.
However, this is a significant challenge since these interactions are so poorly
studied in the NGOM at present.
The development of more comprehensive monitoring should be coordinated with
model development. For example, the more complex physical models of the
NGOM should be used to aid in interpretation of monitoring data on extent and
duration of hypoxia. These models can also feed into both simple and complex
biogeochemical and ecological models.
Because there is great value in developing simple mass balance models in the
NGOM for organic C, dissolved oxygen, and nutrients, mass balance models
should be used to provide a checklist of needed measurements for future NGOM
hypoxia research/monitoring.
Gulf hypoxia models should be designed so that they can be compatible with
watershed models. That is, there must be compatibility in 1) the time step
between a Gulf hypoxia model and a watershed model, and 2) the form of key
variables that serve as outputs from a watershed model and inputs for a Gulf
hypoxia model (e.g., a watershed model that predicts total nitrogen is not
compatible with a Gulf hypoxia model that requires specific forms of nitrogen).
58
-------
3. Nutrient Fate, Transport, and Sources
The SAB Panel was asked to review the available literature and information,
especially that developed since 2000, that would allow them to assess any changes and
improvements in the understanding of nutrient sources and flux estimates within the
Mississippi and Atchafalaya River basins (MARB) (see Figure 2) and the current ability
to use watershed models to route and predict nutrient delivery to the Gulf of Mexico.
The following sections discuss the current levels of understanding and provide brief
summaries of the SAB Panel's key findings and recommendations.
3.1. Temporal Characteristics of Streamflow and Nutrient Flux
The research needs identified in the Integrated Assessment to understand and
document the temporal characteristics of MARB riverine nutrient loads included 1)
studies on small watersheds to better document nutrient export on the short time scales
needed; 2) detailed information on tile drainage intensity; 3) increased monitoring of
stream sites; and 4) measurements of point source discharges rather than estimates from
permits. Only a limited number of these needs have been met.
However, more recent estimates of agricultural drainage appear to be more
representative than those used in the original assessment, and new procedures for load
calculations have resulted in changes in estimates of nutrient fluxes. A brief discussion
of each of the improvements follows.
Current Extent and Patterns of Agricultural Drainage
The Integrated Assessment relied largely on the 1987 USDA-ERS report (Pavelis,
1987), which based estimates of agricultural drainage on land capability class and crop
information from the 1982 Natural Resources Inventory (NRI). NRI estimates were
dropped after 1992, and NRI is statistically valid only at a watershed or county level.
Based on the USDA surveys, some degree of subsurface drainage is present on 13 million
hectares (over 32 million acres) in the Midwest states. However, there is considerable
uncertainty with respect to the actual extent and distribution of drainage of cultivated
cropland. In the absence of additional survey data, more recent estimates of the extent of
drained agricultural land have been developed based on land use and soil
class/characteristics (Jaynes and James, 2007; Sugg, 2007). This general approach needs
further development and validation but seems to provide the best current estimate of the
extent of agricultural drainage. The approach takes advantage of the now extensive and
detailed GIS coverages and provides a considerably finer level of spatial resolution than
previously available.
In the following example, USDA STATSGO soil data were used to estimate the
extent of agricultural drainage based on the distribution of row crops (primarily corn and
soybean) on soils with a drainage class of poorly drained soils and slopes 2% or less
59
-------
(Figure 10, per D. Jaynes, National Soil Tilth Lab, Ames, IA). The patterns of
agricultural drainage predicted using this approach are generally similar to patterns in
land use (Figure 11) and in-stream nitrate concentration estimated from STORET data
selected to exclude point source influences (Figure 12). Drainage estimates could be
further refined by using improved land use data and by using SSURGO rather than
STATSGOdata.
Figure 10: Estimated extent of agricultural drainage based on the distribution of row crops, largely corn
and soybean, and poorly drained soils (per D. Jaynes, National Soil Tilth Lab, Ames, IA).
60
-------
Upper Mississippi & Ohio River Basins
T-W2 Jjuid Cnvtr
Figure 11: Land cover based on Landsat data (adapted from Crumpton et al., 2006).
Upper Mississippi & Ohio River Basins
Stream nitrate concentration
Figure 12: Flow weighted average nitrate concentrations estimated from STORET data selected to exclude
point source influences (adapted from Crumpton et al., 2006).
The relationship between nitrate concentration and land use is further illustrated
in Figure 13 for 52 NASQAN stations (Alexander et al., 1998) in the upper Mississippi
and Ohio River basins selected to exclude sites with large upstream reservoirs or
61
-------
extensive upstream urban areas (Crumpton et al., 2006). See Section 4.5.7 for further
discussion on urban non-point sources. Percent cropland (corn or soybean) accounts for
90% of the observed variation in the average of 1980 to 1993 annual flow-weighted
average nitrate concentrations for the 52 stations examined. Reduced nitrogen
(calculated as total nitrogen minus nitrate) shows a slight, but statistically significant,
increase with percent crop land.
14
12
10
g
I
+j
-------
Change in the Flux Estimation Method
Riverine loads can be calculated with many different methods; the method chosen
is dependent on sampling frequency as well as river size, which determines how quickly
the concentration changes. A comparison of the estimates of annual N flux for the
combined Mississippi and Atchafalaya Rivers using five different methods is shown in
Figure 14. Goolsby et al. (1999) presented nitrate-N loads to the Gulf for 1955 through
1996. For the years prior to 1968, loads were calculated from either daily samples
composited at 10- to 30-day intervals for analysis. For the period 1968-1996, they used a
multiple regression approach to calculate daily concentrations based on about 10 to 15
samples per year (or less) and daily flow [shown as Goolsby et al. (1999)]. Goolsby et al.
(1999) calibrated one model (using a minimum variance unbiased estimator, MVUE) for
1968-1975, and one model for 1976-1997. This type of regression equation provides a
good measure of the overall flux of a nutrient for the entire period of fitting but is less
accurate for a given year. Since the Integrated Assessment, USGS has modified load
estimation procedures to reduce the bias in the regression models. These modified
procedures are all based on the rating curve method but differ in the form of the equation
and/or calibration periods. In July 2002, USGS posted load estimates for the entire
period of record using ESTIMATOR (Cohn et al., 1992; Gilroy et al., 1990), a
regression-model method using the same MVUE technique used by Goolsby et al. (1999)
with a 10-year moving window calibration period, and provided updated annual estimates
through June 2002, followed by annual updates through June 2005 (shown as LOADEST
10 yr). In this case the MVUE procedure used was equivalent to the adjusted maximum
likelihood estimate (AMLE, discussed below) used in later estimates because there were
no censored nitrate values in the calibration datasets. In 2006, the USGS posted new
estimates for the entire period of record using Load Estimator (LOADEST) (Runkel and
others, 2004) with the AMLE procedure and a 5-year moving window (shown as
LOADEST 5 yr). In addition to a shorter calibration period, the AMLE procedure
modifies the rating curve equation in an attempt to correct for transformation bias.
However, the AMLE procedure can still suffer from serial correlation in the residuals; so
when sufficient data are available, the USGS applies a period-weighted interpolation to
correct the AMLE estimate for the serial structure in the residuals (Aulenbach and
Hooper 2006). Results from this composite method for the mainstem Mississippi and
Atchafalaya Rivers are nearly the same as just using a period-weighted (or linear
interpolation) approach for nitrate-N (shown as Composite). This suggests that the
regression model in the composite method adds little when at least 10 samples are
available for a given year, as well as demonstrating that concentrations of nitrate-N
change slowly in these large rivers. (For additional information on methods used to
estimate nutrient fluxes see: http://toxics.usgs.gov/pubs/of-2007-1080/methods.html.)
63
-------
X
2.0
1.5 -
(D *-
~m V*
~ o 1-°H
(U O
(U E^ 0.5 -
0.0
Composite sampling prior to 1968
LOADEST(5yr, 1979 on)
Goolsby et al.
LOADEST 10yr
Composite
1950 1960 1970 1980 1990 2000 2010
Figure 14: MARB nitrate-N fluxes for 1955 through 2005 water years comparing estimates from various
methods for 1979 to 2005. Based on USGS data from Battaglin (2006) and Aulenbach et al. (2007).
Although the overall year-to-year pattern of N flux is consistent across the various
methods, there is considerable variability amongst the estimates of each annual N flux.
Figure 15 shows the percent difference between three of the methods and the current
LOADEST 5 yr method in both percent and metric tons for the entire period of record.
The LOADEST 10 yr method estimated N fluxes that ranged from as much as about 18%
less (1990) to 28% more (1994) than the N fluxes estimated by the LOADEST 5 yr
method. That translates into an underestimate of about 180,000 metric tonne or 198,000
ton of N that was delivered to the Gulf in 1990 and an overestimate of about 260,000
metric tonne of N (287,000 ton of N) in 1994. Research published since 2003 would
have used the LOADEST 10 yr fluxes in models predicting the Gulf hypoxic zone in
which case they likely used the more recent estimates (2003 and 2004 in Figure 14),
which ranged from only 3-10% or 25-50,000 metric tonne of N (28-55,000 ton of N)
more than the estimated flux using the current LOADEST 5 yr method. The flux
estimates presented in the following sections of this report are based on the new
LOADEST 5 yr method.
64
-------
Goolsby et al.
LOADEST(IOyear)
Composite
0.2 -
0.1 -
-0.1 -
_n o -
I
1
1
.1
1'
1
,
'
I1
1,
1975 1980 1985 1990 1995 2000 2005
Figure 15: Comparison (percent and absolute basis) of MARB nitrate-N fluxes to LOADEST 5 yr method
for 1979 through 2005 water years. Based on USGS data from Battaglin (2006) and Aulenbach et al.
(2007).
3.1.1. MARB Annual and Seasonal Fluxes
The following analysis is based on U.S. Geological Survey streamflow and water-
quality monitoring data described in Aulenbach et al. (2007) and available on the internet
at: http://toxics.usgs.gov/pubs/of-2007-1080/. The nutrient flux estimates were
calculated as the combined fluxes at the Mississippi River near St. Francisville, LA and
the Atchafalaya River at Melville, LA (Figure 16) using the LOADEST 5 yr method
discussed in the previous section.
65
-------
Omaha, Nfi
Hermann, MO
\
* ? M i'.i S C/ it i i I i s ? t i
Thebes, IL
Below Little Rock. AR
Clinton, IA
Graf ton, IL
Alton, IL
Grand Chain, IL
1 / Metropolis. IL Cannelton. IN
: * *^===
Arkansas Rive,
Knox Landing, LA
Alexandria. LA
Red '"
Melville, LA
Atiiiafa!a\a River '
near St. Francisvillc, LA
Figure 16: Schematic showing locations of MARB monitoring sites (Aulenbach et at., 2007).
Annual Patterns
Nitrogen During the last five years (2001 to 2005 water years), an average of
813,000 metric tonne (896,000 ton) of nitrate-N and 429,000 metric tonne (473,000 ton)
of total Kjeldahl N (TKN) were transported annually to the Gulf. There is considerable
inter-annual variability in these flux values, driven primarily by precipitation patterns and
resulting streamflow (Figure 17), which appears to have increased slightly since the
1950s. Since the mid-1990s, annual nitrate-N flux has steadily decreased, which is more
clearly shown by the 5-year running average. In addition, TKN has also shown a steady
decline since the mid 1980s, so the total N flux, although highly variable from year to
year, shows a very striking decline. The annual NH4-N flux also decreased during the
monitoring period (from 77,000 metric tons N/yr [85,000 tons N/yr] in 1980 to 1984 to
12,000 metric tons N/yr [13,000 tons N/yr] for 2001 to 2005) but was not the primary
reason for the decline in TKN, as particulate and organic N declined. The decline in
NH4-N is likely due to improvements in sewage treatment as is at least part of the decline
in particulate and organic N (Larson, 2001; Metropolitan Council, 2004). In addition,
reduced sediment loads, because of a reduction in soil erosion, may also be a driving
factor in reducing particulate N losses (Richards and Baker, 2002).
66
-------
^-v 1200000
CO
^ 1000000 -
g
= 800000 H
E
s-" 600000 -
IT 400000 -
s_
-^ 200000 -
> 0
2.5
2.0 -
1.5 -
1.0 -
Annual nitrate
Nitrate five year running average
E, 1.0-
0.5 -
o.o 4-
Ammonium-N
o Particulate/Organic N
T- Total N
1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 17: Flow and available nitrogen monitoring data for the MARB for 1955 through 2005 water years.
(LOWESS, Locally Weighted Scatterplot Smooth, curves shown in red). LOWESS describes the
relationship between Y and X without assuming linearity or normality of residuals, and is a robust
description of the data pattern (Helsel and Hirsch, 2002).
Phosphorus and Silicate Temporal trends in total P, soluble reactive P (SRP),
and dissolved silicate fluxes for the combined rivers are less striking than the trends in N
flux. The average annual total P flux (Figure 18) was 154,000 metric tons P/yr (170,000
tons P/yr) for the water years 2001 - 2005, with SRP flux 24% of total P flux. Battaglin
(2006) reported that total P flux increased during that period, but this was in comparison
to the average flux during the period 1980 - 1996. When total P flux is viewed during the
67
-------
entire period of 1980 - 2005 and a LOWESSS curve fit to the dataset, there appears to be
a slight increasing trend since the mid 1990s. The annual flux of dissolved silicate
appears to have declined slightly since the early 1990s.
1200000
1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 18: Flow, available phosphorus, and available silicate monitoring data for the MARB for 1955
through 2005 water years. (LOWESS curves shown in red). Based on USGS data from Battaglin (2006)
and Aulenbach et al. (2007).
Nutrient Ratios - Ratios of N to P and Si to N can be important in determining the
growth of various phytoplankton species in the Gulf. The Si:DIN (dissolved inorganic N)
ratio ranged from about 2 to 4.5 during the 1950s and 1960s but then greatly decreased as
silicate concentrations declined by about 50% between the 1950s and 1980s (Turner and
Rabalais, 1991; Rabalais et al., 1999). Ratios since 1980 of Si:DIN have been just above
1 annually (Figure 19), averaging 1.08 for 2001 to 2005 water years. Nitrogen to P ratios
averaged 18 for 2001 to 2005 have shown little variability since the early 1990s, with
68
-------
perhaps a declining trend. These ratios are useful to compare to the Redfield ratio (Si:N:P
= 16:16:1) and suggest, as Rabalais et al.(1999) concluded, that annual nutrient fluxes to
the Gulf are quite close to this ratio. However, spring ratios, discussed later, are
somewhat different and may have a more important effect on Gulf phytoplankton growth.
O 1.25 -
O
£ 1.00-
Q 0.75 -
CO
0.50
1980 1985 1990 1995 2000 2005 2010
Figure 19: Ratio of total N to total P and dissolved silicate to dissolved inorganic N for MARB for the
1980 through 2005 water years. Based on USGS data from Battaglin (2006) and Aulenbach et al. (2007).
Seasonal Patterns
Nitrogen -- Since the Integrated Assessment., greater emphasis has been placed on
the spring flux of nutrients (sum of April, May, and June fluxes) as a possible important
regulator of hypoxia, and, therefore, fluxes for this period were examined using the
available data for the period 1979 - 2006. Whereas the annual water flux showed a
slightly increasing trend since 1990 (Figure 17), the spring water flux, although highly
variable, appears to show a decreasing trend (Figure 20). Spring nitrate-N flux also has
declined, with even larger decreases in TKN flux and, therefore, total N flux.
69
-------
400000
E
c
o
I
O 300000 -
X
"co
200000 -
100000 -
to
c
o
CD
E
c
o
x
CD
C
CD
>
o:
0
1.0
0.8 -
0.6 -
0.4 -
0.2 -
0.0
1.0
0.8 -
0.6 -
0.4 -
0.2 -
0.0
Nitrate-N
1975
1980
1985
1990
1995
2000
2005
2010
Figure 20: Flow and nitrogen flux for the MARB during spring (April, May, and June) for the period 1979-
2005. (LOWES S curve shown in red). Based on USGS data from Battaglin (2006) and Aulenbach et al.
(2007).
Phosphorus and Silicate -- Spring P flux (both total and SRP) has changed
relatively little, with perhaps a small decrease in total P flux (Figure 21). The spring
dissolved silicate flux has shown a pronounced decline since 1990s, greater than the
decline in water flux. The reason for this decline is not known.
70
-------
400000
g 300000 -
I
^ 200000 -I
100000 -
jo
-------
further substantiates the conclusion drawn earlier that tile-drained fields are a primary
source of N, which is released beginning in winter (Ohio into central Illinois) to spring
(northern Illinois, Iowa and Minnesota). This influence was very evident in 2002, when
50% of the nitrate-N flux occurred during the 3 spring months. Royer et al. (2006)
pointed out how most of the N and P flux from tile drained watersheds occurred during a
few months during winter and spring each year, further supporting the trends at this
larger scale.
uu
X
LL 50 -
(U
-^ ^ 40 -
CD §
^ < 30 -
S= 0
<£2°-
o
E 10-
co
n -
T
f -f-
-^ ^^ " r^H
"T" I
i
4- ^^
i
-1- 4-
1 * -**-
Water Nitrate-N TKN Total P Silicate
Figure 22: Sum of April, May and June fluxes as a percent of annual (water year basis) for combined
Mississippi mainstem and Atchafalaya River. Box plots show median (line in center of box), 25th and 75th
percentiles (bottom and top of box, respectively), 10th and 90th percentiles (bottom and top error bars,
respectively) and values < 10th percentile and > 90th percentile (solid circles below and above error bars,
respectively). Based on USGS data from Battaglin (2006) and Aulenbach et al. (2007).
Nutrient Ratios - N to P ratios during spring flow to the Gulf averaged 22 for
2001 to 2005 (Figure 23), greater than the annual value of 18 for the same time period.
As discussed previously, nitrate transport is greater during this period than is P transport.
The Si:DIN ratio was also lower during the spring compared to the annual mean for 2001
to 2005 (spring ratio 0.84, annual ratio 1.08), reflecting greater transport of nitrate
compared to silicate. Turner et al. (1999) concluded that decreasing Si:DIN ratios to less
than 1.1 could greatly alter Gulf food web dynamics because the proportion of diatoms in
the phytoplankton community would be reduced, which would impact zooplankton and
higher trophic levels.
72
-------
0.50
1980 1985 1990 1995 2000 2005 2010
Figure 23: Ratio of total N to total P and silicate to dissolved inorganic N for the MARB during spring
(April, May, and June) for the period 1980-2006. Based on USGS data from Battaglin (2006) and
Aulenbach et al. (2007).
3.1.2. Subbasin Annual and Seasonal Flux
Annual Patterns
USGS estimates (Aulenbach et al, 2007) were used to examine nutrient fluxes
within subbasins of the MARB. Annual nutrient fluxes were calculated with an adjusted
maximum likelihood estimate (AMLE), a type of regression-model method, with a 5-year
moving average calibration period (composite method estimates were not made for
subbasin data). Figure 24 shows the location of nine subbasins comprising the MARB
and Table 2 lists site name and map number for the associated monitoring sites. Figure
16 shows a schematic of the MARB sampling stations to assist with the following
analyses. The initial analysis discusses the cumulative fluxes of five major subbasins: 1)
Upper Mississippi (upstream of Thebes, IL minus the inflow from the Missouri River), 2)
Ohio-Tennessee (upstream of Grand Chain, IL), 3) Missouri (upstream of Hermann,
73
-------
MO), 4) Arkansas-Red (combined flux from the Arkansas and Red Rivers), and 5) Lower
Mississippi.
A USGS Gaging Station
A USACE Gaging Station
0 500 Kilomdtrc
I I I I I
Figure 24: Location of nine large subbasins comprising the MARB that are used for estimating nutrient
fluxes (from Aulenbach et al, 2007).
74
-------
Table 2: Site name and corresponding map number for sites discussed in the following section.
Site name
Mississippi River at Clinton, Iowa
Mississippi River below Grafton, Illinois
Missouri River at Omaha, Nebraska
Missouri River at Hermann, Missouri
Mississippi River at Thebes, Illinois
Ohio River at Cannelton Dan at Cannelton, Indiana
Ohio River at Dam 53 near Grand Chain, Illinois
Arkansas River below Little Rock, Arkansas
Red River at Alexandria, Louisiana
Map Number
1
2
3
4
5
6
7
8
9
Annual Flux Estimates: The flux estimates from the five subbasins are listed in
Table 3. During the last five water years, most of the nitrate-N flux (84%) and TKN flux
(73%) was from the Upper Mississippi and Ohio-Tennessee subbasins. The Missouri
subbasin contributed 9.8% of the nitrate-N flux to the Gulf, with much smaller fluxes
coming from the Arkansas-Red and lower Mississippi River subbasins. These data
clearly illustrate that the source of both nitrate-N and TKN is from the upper Mississippi
River basin before the Missouri River enters. For total P flux, the Missouri subbasin was
more important and contributed 20% of the flux, compared to 26% and 38% for the upper
Mississippi and the Ohio-Tennessee subbasins, respectively.
Table 3: Average annual nutrient fluxes for the five large subbasins in the MARB for the 2001-2005 water
years. (Percent of total basin flux shown in parentheses.)
Subbasin
Upper Mississippi1
Ohio-Tennessee
Missouri
Arkansas-Red
Lower Mississippi1
Area
(km2)
493,900
525,800
1,353,300
584,100
183,200
Flow
(M m3/yr)
116,200
279,800
60,080
67,200
129,550
Nitrate-N
TKN
Total P
(in 1,000 metric tons)
349 (43%)
335 (41%)
78.6 (9.8%)
28.7(3.5%)
22.1(2.7%)
136 (32%)
175 (41%)
83.8 (20%)
43.9(10%)
-8.4 (-2%)
40.4 (26%)
58.7 (38%)
30.4 (20%)
8.7 (6%)
16.1 (10%)
Nutrient fluxes calculated by difference. Negative values occur where downstream site had a lower flux
than upstream site, the result of either error in the flux estimates or a real net loss of nutrients within the
subbasin (Aulenbach et al., 2007).
To further examine source areas of N, P and silicate, the nutrient fluxes in the
MARB were divided into ten smaller subbasins (see Figure 24 and Table 4), with some of
the values calculated as the difference between an upstream and downstream monitoring
station. The lower Mississippi River subbasin is again calculated by difference and is the
same in both the five and ten subbasin analysis (this subbasin is not shown in Figure 1,
but was included in the Table 3 analysis). These results are listed in Table 4. For nitrate-
N, this further breakdown of the basin indicates that the largest sources are the upper
75
-------
Mississippi and Ohio-Tennessee River subbasins. These subbasins represent about 31%
of the total land area within the MARB, yet they contribute about 82% of the nitrate-N
flux, 69% of the total Kjeldahl N, and 58% of the total P flux. Furthermore, when the
subbasins are further divided, the subbasin contributing to the upper Mississippi River
between Clinton, IA and Grafton, IL contributes about 29% of the nitrate-N flux, while
representing only 7% of the drainage area. The Missouri River at Hermann also was a
relatively large contributor of total P (14% of total flux). For dissolved silicate,
percentages did not include the Red River because estimates were not available. Again,
most of the silicate flux was from the upper Mississippi River and the Ohio-Tennessee
River, similar in proportion to water flux.
Table 4: Average annual nutrient fluxes for ten subbasins in the MARB for the 2001 -2005 water years..
Some subbasin fluxes are calculated as the difference between the upstream and downstream monitoring
station. (Percent of total basin flux shown in parentheses.)
Subbasin
Mississippi-Clinton
Mississippi-Grafton1
Missouri-Omaha
Missouri-Hermann1
Mississippi-Thebes1
Ohio-Cannelton
Ohio-Grand Chain1
Arkansas-Little
Rock
Red River-
Alexandra
Lower Mississippi1
Area
(km2)
222,000
221,700
836,000
517,000
50,300
251,000
275,000
409,300
175,000
183,200
Flow
(M m3/yr)
48,300
52,100
23,900
36,100
15,800
133,400
146,400
33,900
33,200
129,550
Nitrate-N
TKN
Total P
Si
1,000 metric tons
88.3(11%)
237 (29%)
24.1(3%)
54.6 (7%)
23.8 (3%)
160 (20%)
175 (22%)
21.9(3%)
6.8 (1%)
22.1(2.7)
50.1 (12%)
71.7(17%)
25.4 (5.9%)
58.4 (14%)
13.9 (3%)
92.1 (21%)
82.7 (19%)
19.5 (5%)
24.3 (6%)
-8.4 (-2)
8.5 (6%)
21.2(14%)
8.1 (5%)
22.3 (14%)
10.8 (7%)
35.2 (23%)
23.5 (15%)
4.4 (3%)
4.3 (3%)
16.1 (10%)
219(12%)
162 (9%)
102 (6%)
161 (9%)
8.5 (0.5%)
355 (20%)
320 (18%)
102 (6%)
757 (20%)2
:For these basins, fluxes were calculated as the difference between upstream and
2For these two subbasins, calculated by difference from overall basin flux minus
flux was estimated.
downstream stations.
eight subbasins where Si
Annual Yield Estimates: Similarly, the nitrate-N and TKN yields were dominated
by the Upper Mississippi and Ohio-Tennessee River subbasins, with nitrate-N values of
7.1 and 6.4 kg N/ha/yr (6.3 and 5.7 Ib N/ac/yr) and TKN values of 2.7 and 3.3 kg N/ha/yr
(2.4 and 2.9 Ib N/ac/yr) for the upper Mississippi and Ohio-Tennessee River subbasins,
respectively (Table 5). The Missouri and Arkansas-Red River subbasins had much lower
nitrate-N yields of 0.6 and 0.5 kg N/ha/yr (0.53 and 0.44 Ib N/ac/yr) for this five-year
period. Similar to N, yield of total P was much greater in the upper Mississippi and
Ohio-Tennessee River subbasins when compared to the Missouri River. The greater
yields from the upper Mississippi and Ohio-Tennessee River basins no doubt reflect the
relative sizes of the basins when compared to the Missouri River but also the importance
of point sources in the basins, as well as more intensive agricultural inputs.
76
-------
Table 5: Average annual nutrient yields for the five large subbasins in the MARB for water years 2001-
2005.
Subbasin
Upper Mississippi
Ohio-Tennessee
Missouri
Arkansas-Red
Lower Mississippi
Nitrate-N
TKN
Total P
(kg/ha/yr)
7.1
6.4
0.6
0.5
1.2
2.7
3.3
0.6
0.8
-0.5
0.8
1.1
0.2
0.1
0.9
When nutrient yields from the nine smaller subbasins are examined, the yields
from the upper Mississippi River between Clinton and Grafton and the entire Ohio River
basin were 10.7 and 6.4 kg N/ha/yr (9.6 and 5.7 Ib N/ac/yr), respectively (Table 6). The
largest total P yield (2.1 kg P/ha/yr or 1.9 Ib P/ac/yr) was from the subbasin measured on
the Mississippi River at Thebes, which would include row crop lands of Missouri River
and southern Illinois River along with sewage effluent from St. Louis. Greatest dissolved
silicate yields were from the Ohio River, followed by the upper and lower Mississippi
River, again reflecting water flux.
Table 6: Average annual nutrient yields for nine subbasins in the MARB for the 2001 - 2005 water years.
Some subbasin yields are calculated as the difference between the upstream and downstream monitoring
stations.
Subbasin
Mississippi-Clinton
Mississippi-Grafton
Missouri-Omaha
Missouri-Hermann
Mississippi-Thebes
Ohio-Cannelton
Ohio-Grand Chain
Arkansas-Little Rock
Red River- Alexandra
Lower Mississippi
Nitrate-N
TKN
Total P
Silicate
(kg/ha/yr)
4.0
10.7
0.3
1.1
4.7
6.4
6.4
0.5
0.4
1.2
2.3
3.2
0.3
1.1
2.8
3.7
3.0
0.5
1.4
-0.5
0.4
1.0
0.1
0.4
2.1
1.4
0.9
0.1
0.2
0.9
9.9
7.3
1.2
3.1
1.7
14.1
11.6
2.5
9.91
:Flux calculation available only for sum of two subbasins.
Subbasin Nitrate-N Yield Compared to Net NInputs: The complete time series
records were examined to better understand longer term patterns in subbasins
contributing the largest N and P fluxes. At the five subbasin level, the trend lines for
flow and N fluxes for the Ohio River basin have been relatively flat since the early 1980s
(Figure 25).
77
-------
X
iZ
CD
"03
CO
0
=
£
H>
^
^^
0
iZ
uuuuuu -
400000 -
300000 -
200000 -
100000 -
0 -
40 -,
\^ 30 -
^,
^_
i
J2 20-
~z_
S 10-
n -
Ohio River subbasin
9
^k ^
9 ** %
* * *
**
^&
^
Net N Inputs
O River Nitrate-N yield
m +
"*r~
0
*
4W 90 A
jsA* * ******» *
! *
O
0 QJ Q
''cP^LRj o .-T"^ oi4)
O O ° u HP
- 2.0
-15 "Q)
^ CO
>- c
.^ ^^
-1.0 °c
9 .0
-0.5 ^ E
. n n
1940 1950 1960 1970 1980 1990 2000 2010
Figure 25: Net N inputs and annual nitrate-N fluxes and yields for the Ohio River subbasin. (LOWESS
curves for riverine nitrate-N shown in red.) Based on USGS data from Battaglin (2006) and Aulenbach et
al. (2007).
However, the upper Mississippi River subbasin has experienced a decreasing trend in
annual flow since the mid 1990s (Figure 26). What appears to be only a slight decrease
in nitrate-N yield in the upper Mississippi subbasin in response to what the panel thinks
are greatly decreasing net N inputs, demonstrates the difficulty in predicting riverine
nutrient yields in tile-drained agricultural lands. Many interacting factors are at work,
which are difficult to estimate and/or measure. For example, there are uncertainties in
some of the estimates, such as biological N2 fixation (primarily soybean), as well as our
assumption that large soil N pools are in a steady state. The predominant soil types in the
upper Mississippi subbasin are Mollisols, which are high in organic matter with large soil
organic N pools (much larger than the Ohio River subbasin). As fertilizer rates have
stayed constant and yields have increased, several possibilities may account for the lack
of riverine response. These include increasing soybean N2 fixation percentages, net N
78
-------
mineralization of soil organic N (David et al., 2001), long lag times due to a buildup of
relatively easily degradable organic N (amino sugar N, Mulvaney et al., 2001) that is now
being released, or perhaps increasing tile drainage and loss of fall applied N. Figure 26
includes a recalculation of net N inputs for 1998 to 2005, increasing soybean fixation
rates from 50 to 70%, and assuming a corn acre net soil mineralization rate of 10 kg
N/ha/yr (8.9 Ib N/ac/yr). These two changes greatly alter the net inputs, pushing the
value back up to where it was during the 1980s.
250000
v ^ 200000 -|
1> CO
"-1- c 150000 -
0 .9
^ ^ 100000 -
50000 -I
0
40
Upper Mississippi River subbasin
TT; v. 30 -
o J2 20 -
X
LI- O)
10 -
o
Net N Inputs
Riverine Nitrate-N yield
Net N Inputs revised
00
0 O n ,o
_o O O i
2.0
h 1.5 7R
- 1.0
- 0.5
0.0
1940 1950 1960 1970 1980 1990 2000 2010
Figure 26: Net N inputs and annual nitrate-N fluxes and yields for the upper Mississippi River subbasin.
(LOWESS curves for riverine nitrate-N shown in red.) Shown in green is a recalculated net N input for the
upper Mississippi River basin, increasing soybean N2 fixation from 50 to 70% of above ground N, and a
soil net N mineralization rate from 0 to 10 kg N/ha/yr. Based on USGS data from Battaglin (2006) and
Aulenbach et al. (2007).
Soybean production is a net depletion to soil N pools and the fixation rate is a
function of available inorganic N (nitrate) in the soil (Gentry et al., 2001). When there
was more inorganic N left from corn production prior to the late 1990s, soybeans would
have fixed less N compared to recent growing seasons when corn yields have set records,
and little residual soil nitrate would be expected. This could be leading to increasing
soybean N2 fixation rates, which are not accounted for in typical net N input calculations.
79
-------
A second factor is soil mineralization. Net N input calculations assume that the
soil organic N pool is at a steady state (Mclsaac et al., 2002), with mineralization rates in
a year balanced by immobilization (both microbial and crop residue inputs). It is possible
that with greater corn production and steady fertilizer rates, increased mineralization rates
occur, so that there is a net depletion of soil organic N (one component of soil organic
matter, which is discussed further in Section 4.5.6). This depletion, as discussed earlier,
may be small (about 10 kg N/ha/yr or 8.9 Ib N/ac/yr) but over many acres would be an
important additional input.
Finally, another factor may be an increase in tile drainage intensity in the region,
combined with increasing fall fertilization and warmer winter temperatures. New and
replacement tile drainage is added every year to this region, although no data are
available to quantify the increase. Fall application of anhydrous ammonia in much of the
region has increased greatly since the 1980s (see later discussion in Section 4.5.6 for
supporting sales and USDA ARMS data). The four states of the upper Mississippi River
basin (Minnesota, Wisconsin, Iowa and Illinois) all show an increasing winter
(November through March) temperature (for the months following fall application of
anhydrous ammonia all show strong increasing trends in winter temperatures during the
last 30 years, data not shown). Warmer soils would increase nitrification rates and lead
to higher concentrations of soil nitrate that could be lost with late winter and spring
precipitation. Therefore, fall applied anhydrous ammonia could be a more important
source of spring nitrate-N flux in this subbasin during recent years and, when combined
with changing N input and output patterns, may be keeping the flux steady despite the
reduction in annual net N inputs.
Changes in subbasin P: As discussed previously, total P flux for the MARB has
increased during the monitoring period. Most of this increase was found to have
occurred in the Ohio River subbasin, particularly during the 2001 to 2005 time period
(Figure 27). In comparing the 2001 to 2005 period with 1980 to 1996, Ohio River total P
increased 51%, while water flux increased only 6%, and reactive P decreased by 20%.
This led to a large increase in particulate/organic P of 89% between these two time
periods. Because TKN decreased by 3% during this period, it does not seem that
increased erosion can explain this pattern (all indications are that erosion has decreased).
The 89% increase in particulate/organic P represents most of the increase in total P flux
to the NGOM between 1980 to 1996 and 2001 to 2005. Unfortunately, data are not
available because of monitoring limitations for smaller basins within the Ohio River
subbasin to further determine the source of this P flux. However, this trend seems to be
more widespread than just the Ohio subbasin.
80
-------
X 's_
-^ -^
U- Q_
100000
80000 -
60000 -
Q_ -b
si
"-" 20000 -
Total P
Particulate/organic P
0
1975 1980 1985 1990 1995 2000 2005 2010
Figure 27: Total P and particulate/organic P fluxes for the Ohio River near Grand Chain, Illinois.
(LOWESS curves shown in black and red). Based on USGS data from Battaglin (2006) and Aulenbach et
al. (2007).
The Missouri and Upper Mississippi River subbasins are following a similar trend
as the Ohio River, although their absolute increase in total P is much less than the Ohio
River. In both of these subbasins flow has decreased (by 10 and 31% for the Upper
Mississippi and Missouri River subbasins, respectively for 1980 to 1996 compared to
2001 to 2005), while total P flux has increased (about 10% in each subbasin). Again,
TKN flux has decreased. Therefore, in the Missouri, Upper Mississippi, and Ohio River
subbasins flow weighted total P concentrations have increased greatly during the last 15
years.
These observations are not consistent with overall TKN riverine fluxes in the
MARB, and at this time the SAB Panel has no explanation for this large, yet potentially
very important, change in total P concentrations and flux for these subbasins which could
influence management decisions.
Seasonal Patterns
Spring fluxes (sum of April, May, and June) were examined for the Mississippi
River at Grafton and the Ohio River at Grand Chain, and little change in water flux was
81
-------
detected (Figure 28). However, for nitrate-N, there seems to be a slight increasing
pattern of spring flux based on LOWESS curves.
"E
c
o
E
**~^s
X
_D
1 1
u_
l_
(D
CO
^
X « -N
^ z
LJ_ (f>
-r C
^ 2
2 o
2 5
~ "CD
Z E
CD C
- .9
CD :=
.> E
(JUUUU
60000 -
l
40000 -
20000 -
0 -
.*.*.
> *
.« * ' . * . -
^
Mississippi River at Grafton
i i i i i
1 pnnnn
I DUUUU
120000 -
i
annnn -
O\J\J\J\J
40000 -
n
OQn
0.25 -
0.20 -
0.15 -
0.10 -
0.05 -
Onn
0.30 -
0.25 -
0.20 -
0.15 -
0.10 '
0.05 -
n nn -
m
*
* * '
Ohio River at Grand Chain, IL
11111
.
"
t _^ ' '
" * *
9
0
Mississippi River at Grafton
11111
* A
* *
|_ * *-" ~ ^
***» « * '
Ohio River at Grand Chain, IL
1975
1980
1985
1990
1995
2000
2005
Figure 28: Spring water flux and nitrate-N flux for the Mississippi River at Grafton and the Ohio River at
Grand Chain, IL for water years 1975-2005. (LOWESS curves shown in red.) Based on USGS data from
Battaglin (2006) and Aulenbach et al. (2007).
When the sum of the upper Mississippi River at Grafton and Ohio River at Grand
Chain spring nitrate-N flux is plotted against the flux for the entire basin, an interesting
pattern emerges (Figure 29). During the 1980s into the early 1990s, some of the spring
flux was from other subbasins, mostly the Missouri River. However, the Missouri River
flux has greatly decreased so that now the upper Mississippi River above Grafton and the
Ohio River contribute nearly all of the spring flux. Sprague et al. (2006) discuss the
82
-------
riverine fluxes in the Missouri River basin (due to decreasing flow and management
practice changes) in a recent report that supports this observation.
1.0
08H
1
CO
i -i
±± 0
Z E
(Dc
- 9.
11
." 0.6 -
0.4 -
,
0.2 -
0.0
- MARB Basin
o Grafton + Ohio
oo
o
o
1975 1980
1985 1990 1995
2000 2005
Figure 29: Spring nitrate-N flux (sum of April, May, and June) for the Mississippi River at Grafton plus
Ohio River at Grand Chain subbasins compared to the combined Mississippi and Atchafalaya River for
1979 through 2005. Based on USGS data from Battaglin (2006) and Aulenbach et al. (2007).
3.1.3. Key Findings and Recommendations on Temporal Characteristics
Most of the research needs identified in the Integrated Assessment have not been
met, and fewer rivers and streams are monitored today than in 2000. Data continue to be
available for the large river sites, but many intermediate and smaller river monitoring
sites have been dropped from monitoring programs. Recently USGS has initiated real
time (every two hour) monitoring of three large river sites with field nitrate-N
measurement. These types of new efforts to provide expanded monitoring data are
critically needed. To more fully assess the response of the entire suite of management
programs and changes at the subbasin and large river scale in the MARB, we need more
robust monitoring programs that have adequate sampling intensities to allow the
composite method (the preferred one) of estimating stream loads to be utilized. At the
small watershed (1,000 to 50,000 ha or about 2,500 to 125,000 ac) scale, there have been
many studies, but they provide data for only the period of funding, which is often short.
A monitoring network is needed throughout the MARB focused on small watersheds with
larger N and P loads and that provides intensive, long-term data. This network will allow
determination of how effective particular individual or suites of management programs
are in reducing nutrient loads. However, because of year-to-year weather patterns and
the often slow response of changes in outputs, these programs will need to be in place for
decades. Finally, there is a critical need for the ability to document tile drainage
intensity, which requires that new techniques be developed and applied.
83
-------
Changes in USGS flux calculation methods have altered estimates of nutrient flux
as reported in the Integrated Assessment. LOADEST 5 yr and a new COMPOSITE
method seem to be the best estimation methods. Although water flux for the MARB has
increased slightly during the past 25 years, total N, primarily nitrate-N and
particulate/organic N, has decreased. The total N flux averaged 1.24 million metric
tons/yr (1.37 million tons/yr) from 2001 - 2005 (65% of the flux is nitrate), and the total
P flux averaged 154,000 metric tons/yr (170,000 tons/yr). During the spring (April-
June), water flux for the MARB appears to have decreased slightly, causing similar
decreases in total N (nitrate-N and TKN). Spring dissolved silicate flux has declined
more than water flux. Neither total P nor SRP fluxes show major annual or seasonal
trends during the full period of record.
The subbasin analysis provides clear evidence that while the upper Mississippi
and Ohio-Tennessee River subbasins represent about 31% of the total drainage area of
the MARB, they contribute about 82% of the nitrate-N flux, 69% of the TKN flux, and
58% of the total P flux to the Gulf. Furthermore, when the subbasins are further divided,
the subbasin contributing to the upper Mississippi River between Clinton, IA and
Grafton, IL contributes about 29% of the nitrate-N flux while representing only 7% of the
drainage area. Perhaps more importantly, the upper Mississippi and Ohio-Tennessee
River subbasins currently represent nearly all of the spring N flux to the Gulf. These
subbasins represent the tile-drained, corn-soybean landscape of Iowa, Illinois, Indiana,
and Ohio and illustrate that corn-soybean agriculture with tile drainage leaks considerable
N under the current management system. The source of riverine P is more diffuse,
although these subbasins are also the largest sources of P. A large increase in the Ohio
River subbasin parti cul ate/organic P flux occurred during the 2001 to 2005 time period,
which was the source of nearly all of the increase in total P to the NGOM. At the same
time flow weighted total P concentrations increased in the Upper Mississippi and
Missouri River subbasins as well, although increases in flux were smaller than the Ohio
River due to decreased water flux. The SAB Panel has no explanation for this striking
change in P concentrations in these subbasins.
Based on these findings, the Panel recommends the following:
Establishment of a monitoring network (20 to 100) of small watersheds will
provide long-term (tens of years), intensive flux data to determine the response of
management programs and decisions in the MARB.
More intensive monitoring of larger rivers at the subbasin and entire MARB scale
is needed to allow for monthly calculation of fluxes using the composite
estimation method, the most accurate method estimating fluxes.
Further research is needed to determine why riverine spring nitrate-N fluxes are
not declining in response to annual net N input decreases, which will inform
management decisions for corn/soybean agriculture.
84
-------
The increase in riverine total P concentrations needs to be fully explored to verify
the increase and to further document the source, potentially having great
management implications for control of P in the MARB.
The tile-drained Corn Belt region of the MARB is an important target for
reductions in both N and P, focusing on both surface (P) and sub-surface losses
(N).
Additional research is needed to better define the extent, pattern, and intensity of
agricultural drainage, including cropland drained by field tile as well as cropland
not directly drained by field tile but contributing to drainage networks.
3.2. Mass Balance of Nutrients
Mass balance can be used to better understand sources, sinks, and transformations
of nutrients in ecosystems, although losses to stream water are not specifically
determined. Goolsby et al. (1999) constructed a detailed annual N mass balance for 1960
- 1996 and a P mass balance for 1992. Improving flux estimates was identified as a
research need. In particular, better estimates are needed for soil N mineralization, soil
immobilization, plant N volatilization, denitrification, and biological N2 fixation.
Cropping Patterns
Mass balances reflect the types and areas of crops grown across the MARB.
There were large changes in these crops over the past half century (Figure 30). Earlier
cropping systems had more diverse rotations, including corn, wheat, hay, and oats. With
the onset of modern agriculture and large fertilizer inputs, much of the MARB is now in a
corn and soybean rotation. By the late 1990s, corn and soybean areas were equal but
more recently corn acreage has increased and soybean has decreased, with this trend very
apparent in 2007. This trend is expected to continue as demand for corn increases due to
expanding ethanol production, the implications of which are discussed in detail in Section
4.5.9.
85
-------
CO
.c
o
o
o
CD
35000
30000 -
25000 -
20000 -
15000 -
§- 10000 -
s_
o
5000 -
1940 1950 1960 1970 1980 1990 2000 2010
Figure 30: Area of major crops planted in the MARB from 1941 through 2007. Adapted from Mclsaac,
2006.
Non-Point Sources
Nitrogen: The N mass balance described in the Integrated Assessment indicated
that there was a greater surplus of N during the 1950s than during the 1980s and 1990s
(Goolsby et al., 1999). Mclsaac et al. (2001, 2002) used the same data set to determine
the N mass balance using a method described by Howarth et al. (1996) that also has been
used by many others (e.g., David and Gentry, 2000; David et al., 2001; Mclsaac and Hu,
2004 for Illinois). Net anthropogenic N inputs (NANI) were calculated (sum of fertilizer,
NOy deposition, N2 fixation, minus net food and feed imports) from existing MARB data
bases, assuming that the large soil organic N pool is in a steady state. Manure is included
in this calculation as part of the feed imports, where grain consumed and excreted as a
part of animal agriculture is estimated. NANI is N that should be available for
denitrification, loss to groundwater, or leaching and transport in streams.
The recalculated NANI for the MARB showed a clear increase from about 9 kg N
ha/yr (8 Ib N/ac/yr) in the 1940s to about 16 kg N/ha/yr (14 Ib N/ac/yr) from the early
1980s to present, with a maximum value of 20.9 kg N/ha/yr (18.7 Ib N/ac/yr) in 1988
(Figure 31). This increase was due to increasing fertilizer N inputs (from 0 to ~ 20 kg
N/ha/yr or 17.9 Ib N/ac/yr)) and higher N2 fixation from the increased soybean
production (from about 8-14 kg N/ha/yr or about 7-12.5 Ib N/ac/yr). Atmospheric
deposition appears to be the greatest in the Ohio River basin (about 16% of NANI) and
shows a slight increase basin-wide but generally is a small component of the NANI (for a
86
-------
more detailed discussion see Appendix B: Flow Diagrams and Mass Balance of
Nutrients. Manure shows a slight decrease across the MARB, as extensive animal
production has moved to feedlots further west, but represents only about 16% of the total
inputs. However, animal production has become concentrated in specific regions of the
MARB, creating localized nutrient surpluses compared with crop needs and offtake
(USDA, 2003). Up to now, this has led to water quality impairment at a local rather than
MARB scale, due to where the animal operations have become concentrated (for more
information on distribution see Section 4.5.5 and Appendix E: Animal Production
Systems). Therefore, the major changes in inputs were due to fertilizer and N2 fixation.
However, when compared to the amount of N removed during crop harvest, which has
dramatically increased since 1940, the increase in N inputs from fertilizer and N2 fixation
don't appear to have increased proportionately. In fact, this rapid increase in crop
production has led to a small decrease in NANI from about 17 kg N/ha/yr (15 Ib N/ac/yr)
87
-------
in 2000, to net N inputs of 14 kg N ha/yr (12.5 Ib N/ac/yr) in 2004 and 2005 (Mclsaac,
30 -
o
O
O
CD 10
o
JO
CO 0
^ 40
CD
D)
O
30 -
20 -
10 -
Manure
Human consumption
Harvest
Net Nitrogen Inputs
- Annual value
<> Previous 2 to 5 year average
0
1940 1950
1960
1970
1980
1990 2000
2010
Figure 31: Nitrogen mass balance components and net N inputs for the MARB, as calculated by Mclsaac
et al. (2002) and updated through 2005 by Mclsaac (2006).
2006).
The subbasins that contribute the greatest N flux to the Gulf are the upper
Mississippi and Ohio River basins, due largely to the intensity of agriculture with
concomitant large inputs of N from fertilizer and fixation combined with the system of
tile drains. Therefore, when the nitrogen balance is presented by subbasin (Figure 32) the
highest net nitrogen inputs are to those subbasins.
88
-------
0)
D)
(D
40 -
30 -
20 -
10 -
_>» 0
"m 40
40
D) 30 H
I2OH
-in -
1
30 -
20 -
10 -
Entire MRB
.JiAMJW^
*f+*H*/*itr^
0
1940 1950 1960 1970 1980 1990 2000 2010
Figure 32: Net N inputs for the four major regions of the MARB through 2005. Adapted from Mclsaac,
2006.
However, a closer look at the inputs to the upper Mississippi River basin shows
that, even though N inputs from fertilizer and N2 fixation appear to be fairly level during
recent years, the amount of N removed during harvest continues to increase, resulting in a
substantial decline in NANI (Figure 33). These changes are not reflected in the other
subbasins, which lead to a small decline in NANI to the overall basin. However, given
the importance of the upper basin as a source of nitrate-N, it might be expected that the
riverine flux of N would start to decrease.
89
-------
CO
O)
.*;
0
00
60 -
O
£~ 40
O
O
0 20
O
JO
CO 0
50
Manure
Human consumption
Harvest
Trrrrrrrn
m^
0
O) An
o 40
30 -
20 -
10 -
- Annual value
o Previous 2 to 5 yr
average
Net Nitrogen Inputs
1940 1950 1960 1970 1980 1990 2000 2010
Figure 33: Nitrogen mass balance components and net N inputs for the upper Mississippi River basin, as
calculated by Mclsaac et al. (2002) and updated through 2005 by Mclsaac (2006).
Mclsaac et al. (2001, 2002) showed that net N inputs could be used, in
combination with riverine water flux, to predict export of nitrate-N to the Gulf. They
found that a 2-5 year lagged net N input explained the most variation in nitrate-N export,
with 6-9 year lagged net N inputs explaining less, but a significant amount of the
variation. Therefore, given the large decrease in net N inputs in the upper Mississippi
90
-------
River subbasin, it is reasonable to expect riverine export of nitrate should decrease.
However, there is a factor that is not assessed in the net N input mass balance that may be
important.
Mclsaac and Hu (2004) showed that, for tile and non-tile drained regions of
Illinois, net N inputs were similar but that riverine export of N was much greater in the
tile drained watersheds. They found that during the 1990s net N inputs were equal to
riverine N flux, about 27 kg N ha/yr (24 Ib N/ha/yr). This would leave no N available for
other fluxes that are thought to be important, such as terrestrial and aquatic
denitrification. More recent net N inputs in these same tile drained watersheds are about
zero, yet riverine N export has continued. Given that there are denitrification losses (that
are unmeasured), this result indicates that N must be coming from a depletion of soil N
pools, as suggested by Jaynes et al. (2001). With steady fertilizer N rates, high corn and
soybean yields, and high stream N export, the only source available to supply N would be
the large soil N pool (often 10,000 to 15,000 kg N/ha or 8,930 to 13,400 Ib N/ac) in the
Mollisols of the upper Midwest. Techniques are not yet available to document the small
change that would be occurring in this N pool from a small annual depletion of 25 to 50
kg N/ha/yr (22 to 45 Ib N/ac/yr); however, this possibility has critical implications for the
sustainability of production.
Another possibility raised by Mclsaac et al. (2002) is that estimates of crop
harvest N, N2 fixation, or animal consumption of N and manure production could be
inaccurate. Although Goolsby et al. (1999) recommended that we improve estimates of
the N mass balance, we have not made progress in our methods or data available to
calculate individual fluxes of N. Manure is an important component of the mass balance
and can be thought of as N that is not exported in grain (or forage that is consumed) or,
therefore, the N that is returned to the landscape in the MARB. There are many
assumptions in calculating the manure flux that could also alter our interpretation of the
overall mass balance.
Phosphorus: A P mass balance for 1992 was included in the Integrated
Assessment that incorporated fertilizer, manure, grain harvest, hay harvest, and pasture
grazing (Goolsby et al., 1999). Small but potentially important changes in the large soil
pool were not included because methods are not available for making this estimate for
short-time spans.
A P mass balance was calculated using the extended N mass balance (Mclsaac,
2006) for 1951 - 2005 for each state, and these values were then summed for the MARB
(Figure 34). P fertilizer inputs have decreased since the 1970s such that the increased
harvest now exceeds fertilizer inputs (and manure retention) most years, so large soil P
pools are being utilized by crops. The large buildup of soil P in the 1970s and 1980s led
to a large positive net P balance, but decreased fertilizer inputs and high crop yields result
in the current negative balance.
91
-------
O)
0
c
o
Q.
O
O
8
_co
CD
CO
CO
Q.
CO
O
10
8 -
6 -
4 -
2 -
Fertilizer
10
Manure
Human Consumption
Harvest
8 -
6 -
4 -
2 -
0 4oooonoocogoc
-------
-2
1950
1960
1970
1980
1990
2000
2010
Figure 35: Net P inputs for the four major subbasins of the MARB through 2005. Adaptive from Mclsaac,
2006.
93
-------
A closer look at the upper Mississippi River basin (Figure 36) shows an even
larger decline in P from fertilizer and a steady decline in P from manure.
15
10 -
5 -
(0
.c
Q_
O)
0
15
(U
o 10
o
O
(D 5
CO
00
Q.
V)
O
Fertilizer
1950
1960
1970
1980
1990
2000
2010
Figure 36: Phosphorus mass balance components and net N inputs for the upper Mississippi River basin.
Adapted from Mclsaac, 2006.
94
-------
Point Sources
In the Integrated Assessment, point sources were estimated to contribute about
11% of the total nitrogen and an undefined, though likely somewhat lower, total
phosphorus flux to the MARB. This assessment (Tetra Tech, Inc., 1998) was based on
1996 information, and it estimated fluxes at 321,000 metric tons N/yr (354,000 tons N/yr)
and 91,500 metric tons P/yr (101,000 tons P/yr).
A reassessment (MART, 2006b) was based on 2004 permit information, adjusted
assumptions, evaluated more facilities, and revised estimated fluxes downward to
233,000 metric tons N/yr (257,000 tons N/yr) and 39,500 metric tons P/yr (43,500 tons
P/yr). Municipal treatment plants (STP) were thought to account for about 65% of the
total point-source fluxes for both N and P. However, few permits have suitable data for
direct flux calculations, and only 11.1% of the mass flux was directly calculated from the
permit information. The rest of the mass flux was estimated using "typical pollutant
concentrations" (TPC) and estimated daily water flows from point sources. The TPCs
used in the MART (2006) estimates are lower than those used by other water quality
programs, therefore, the SAB Panel has re-calculated the contribution of N and P from
municipal sewage treatment plants based on effluent concentrations that better reflect
measured nutrient concentrations from point sources during 2004. These calculations
also assume that the point source load is delivered to the NGOM without any in-stream
losses. Therefore, they are the upper estimate for the contribution of point sources to the
total N and total P riverine load. The SAB Panel's calculation indicates that load
estimates would need to be revised upward to 267,000 metric tons N/yr (294,000 tons
N/yr) (72% from STPs and 28% from industrial sources) and 53,000 metric tons P/yr
(58,500 tons P/yr) (77% from STPs and 23% from industrial sources). (See Appendix D
for a more detailed discussion of the SAB Panel's estimates.) When the contributions
from all point sources are compared to the average annual N and P fluxes for the period
2001 - 2005, these new estimates indicate that point sources contribute to the Gulf about
22% and 34% of the average annual N and P flux, respectively. When compared to 2004
N and P fluxes (slightly higher than average fluxes), the percentage of the N flux
contributed by point sources drops to about 20%, and the P flux remains constant at about
34%. Fluxes from point sources are equally distributed throughout the year, but spring
flux is critical to the Gulf. Assuming equal monthly loads from point sources, the SAB
Panel's estimates indicate that point sources are responsible for approximately 14% of
spring N flux and 27% of spring P flux for 2001 - 2005. Again, the Panel emphasizes
that these are rough estimates, as measured data are not available at this time to make
more accurate determination of point source contributions.
A summary of the percent of P fluxes by major hydrologic region, based on the
new estimates, is shown in Figure 37. Collectively, the upper Mississippi and Ohio River
basins account for about half the P flux from point sources in the MARB.
95
-------
11 Arkansas-
Red-White
13%
Unknown
2%
10 Missouri
16%
08 Lower
Mississippi
13%
05 Ohio
24%
06 Tennessee
5%
07 Upper
Mississippi
27%
Figure 37: Total phosphorus point source fluxes as a percent of total flux for the MARB for 2004 by
hydrologic region.
This analysis suggests that point source P fluxes are a significant source of both
annual and spring fluxes to the MARB and the Gulf and that substantial reductions in P
fluxes in the MARB are likely if P fluxes from point sources are reduced. Point sources
are a less important source of spring and annual N flux; however, reduction in N fluxes
from point sources may offer a certain and cost effective means of achieving some of the
N reductions needed in the MARB. It is important to emphasize that the differences in
assumptions used to estimate fluxes based on TPC have a major impact on annual and
seasonal flux estimates for the MARB and would likely affect the estimated cost
effectiveness of requiring N or P removal from point sources in the MARB (discussed
further in Section 4.5.8).
96
-------
Key Findings and Recommendations
Although N mass balances have been recalculated since the Integrated
Assessment, the research needs described in that report remain. Components of the N
mass balance such as denitrification, N2 fixation, manure N, and soil N pool processes
such as mineralization and immobilization are not measured each year. Only N2 fixation
and manure N can even be estimated, with the other fluxes having little data available to
make calculations. Point sources export N and P directly to rivers, yet their contributions
continue to be estimated from permits.
New methods have been used to calculate N mass balances in this report (net
anthropogenic N inputs, NANI). NANI and net P inputs for MARB have increased
greatly since the 1950s; but have decreased in the past decade because of steady fertilizer
applications and increased crop yields for N, and reduced fertilizer applications and
increased crop yields for P. Mass balances in the upper Mississippi River subbasin
suggest that under the current tile drained corn and soybean management system
depletion of soil organic N pools may be occurring. From a sustainability viewpoint, this
needs to be fully documented and decreased as new systems are put in place to reduce N
export in rivers. Point sources represented 22% of riverine N flux and 34% of P flux
delivered to the Gulf. Manure is a more significant source of P than N; and where
riverine N flux is greatest, excess manure N tends to be a less important input. Manure is
likely more important basin-wide to local water quality problems, rather than a large
component of MARB export of N or P, because of where concentrated animal production
has relocated. The greatest decrease in net N and P inputs was seen in the upper
Mississippi River basin. From 1999 - 2005, 54% of N inputs were from fertilizer, 37%
from fixation, and 9% from deposition for the entire basin. Deposition was most
important in Ohio basin (16% of inputs). Based on these findings, the Panel offers these
recommendations.
Continue and expand research to more accurately and fully measure the N
mass balance in the MARB by developing methods and gathering data for
improving the estimates of critical fluxes such as N2 fixation, manure,
denitrification, and soil N pool changes.
Sustainability of soils in the MARB must be fully addressed by research to
improve measurement of changes in soil N pools as a result of new
management systems, with changes in soil N pools incorporated into more
complete N mass balances. Section 4 discusses the need for research on
changes in N pools associated with different management practices, e.g.
tillage systems and other practices.
N and P from point sources should be estimated from direct measurements,
rather than relying on estimated values based on permits, so that more
accurate calculations can be made of their contributions to the nutrient fluxes.
97
-------
3.3. Nutrient Transport Processes
Aquatic processes
Studies conducted since the Integrated Assessment have addressed many of the
research needs that were identified for nutrient transport processes: quantification of in-
stream processes such as denitrification (particularly in small streams), research in small
watersheds to identify dynamics and timing of N transport and to better understand the
impact of drainage practices on nutrient flux, and development of a better understanding
of N behavior during floods. We review these advances for nitrogen, phosphorus, and
silicate transport and transformation.
Nitrogen. In-stream nitrogen removal in river networks is variable, but it can be
substantial, particularly in river networks with relatively low nitrogen concentrations. In
sixteen river networks in the northeastern United States, the Riv-N model predicted that
37 to 76% of nitrogen inputs were removed within streams (Seitzinger et al., 2002), and
the SPARROW model predicted that 7 to 54% of nitrogen inputs were removed
(Alexander et al., 2002b). Estimates of the percentage of annual N inputs removed by in-
stream processes in regional drainages in the Mississippi River basin range from 20 to
55% (SPARROW model, Figure 38). The Ohio and White River basins removed the
lowest percentage and the Arkansas and Missouri River basins the highest. Although
these are estimates of the role of in-stream processes on an annual basis, the SPARROW
model results strongly reflect the effects of seasonal pulses, especially the high spring
values, because the mean annual flux is a flow-weighted estimate (R. Alexander, personal
communication).
Total Nitrogen
Total Phosphorus
Regional Watersheds
Figure 38: Percentage of nutrient inputs to streams that are removed by instream and reservoir processes as
predicted by the SPARROW model (Alexander et al., in press).
98
-------
In-stream N removal accounts for a much smaller fraction of annual N export in
tile-drained agricultural regions and other areas where stream water nitrogen
concentrations are extremely high and water residence time is short. The proportion of
the nitrate flux that was denitrified was highest in forested systems, lowest in urban, and
intermediate in agricultural streams in Michigan (Inwood et al., 2005). Denitrification
removed a greater fraction of N in meandering than in channelized reaches, but removal
never exceeded 15%/day except during periods of low flow and warm temperature
(Opdyke et al., 2006). Denitrification is a significant pathway for N removal in mid-
western tile-drained streams during low flow, warm periods (summer and autumn), which
improved local water quality at those times (Royer et al., 2004; Schaller et al., 2004).
However, most of the nitrate is exported to the Gulf during high flows from January to
June (Royer et al., 2006), and denitrification removes an insignificant fraction of this flux
(Royer et al., 2004; 2006). Because in-stream removal is a small fraction of total flux at
high flows, enhancing N removal by 50% during low flows (Q < median) would reduce
annual N export only by less than 2% in Illinois agricultural streams; whereas enhancing
removal by 25% during high flows (greater than 75th percentile flows) would reduce
annual N export by 21% (Royer et al., 2006).
Recent research on streams in predominantly forested watersheds has shown that,
in comparison to larger rivers, small streams remove a higher proportion of their
incoming nitrogen per unit of water travel time (Alexander et al., 2000), per stream reach
(Seitzinger et al., 2002), and per unit length (Wollheim et al., 2006; Helton, 2006).
However, larger streams remove larger masses of nitrogen because more nitrogen passes
through them (Seitzinger et al., 2002, Wollheim et al., 2006, Helton, 2006). Small
streams receive and transport a significant amount of N to larger rivers, e.g. N loads to
headwaters account for 45% of the load delivered to the entire river network in the
northeastern U.S. (Alexander et al., 2007a). Similar calculations have not yet been done
for the Mississippi River basin (R. Alexander, personal communication). Enhancing
nutrient removal in small streams by restoring stream length that has been lost to
straightening or burial could improve local water quality and decrease both N and P load
to larger rivers (Bernot and Dodds 2005); however these reductions would be greatest at
low stream flows, and less effective at high discharges when the bulk of nutrient load is
being transported to the Gulf.
Denitrification is not the only pathway for N removal in streams, although it is the
most permanent. Removal of nitrate from stream water and its assimilation into
biological tissues transforms N from dissolved to particulate form, which reduces the rate
at which it is transported downstream. Particulate N can be deposited and stored in
sediments, where it can be mineralized and potentially denitrified.
Effectiveness of N removal in aquatic systems increases with water residence
time, so reservoirs can make a significant contribution to N removal in river networks.
Denitrification in an Illinois reservoir reduced average annual N export by 58%, but the
percent reduction in annual export over a 23-year period varied from 31 to 91 % as
retention time increased (David et al., 2006). N retention in Illinois reservoirs is higher
99
-------
than observed from rivers and reservoirs with lower nitrate concentrations (Figure 39).
The difference can be attributed to lower removal efficiencies in natural lakes than in
reservoirs where elevated inputs of N support high rates of denitrification in the
sediments (David et al., 2006). Denitrification in aquatic sediments (80% in reservoirs in
the tile-drained part of Illinois and 20% in streams) was estimated to reduce N export
from Illinois by 25 % (David et al., 2006). Existing floodplain backwaters on the upper
Mississippi River basin are limited in their effectiveness in N removal by denitrification
because of short water-retention times and a lack of hydrologic connectivity with the
main stem (Richardson et al., 2004; David et al., 2006). Enhancing connectivity and
water-residence time on floodplains during periods of high discharge and high nitrate
concentrations in the spring has been suggested as an effective way to reduce N loading
to the Gulf (David et al., 2006).
100
CD
>
O
E
CD
80 -
60 -
40 -
20 -
0
A
D
O
y=88.453x
r^O.73
Lake Shelbyville
Seitzingeret al. rivers
Gamier et al.
Opdyke et al.
Royeret al.
Seitzinger Eq. 2
Our equation
-0.3677
y=230.2.0x
r^O.91
-0.5440
0.1
1 10 100 1000 10000
Depth/Time of Travel (m/yr)
100000
Figure 39: N removed in aquatic ecosystems (as a % of inputs) as a function of ecosystem depth/water
travel time (modified from David et al., 2006). Values shown are for 23 years in an Illinois reservoir (David
et al., 2006), French reservoirs (Gamier et al., 1999), Illinois streams (an average from Royer et al., 2004),
agricultural streams (Opdyke et al., 2006), and rivers (Seitzinger et al., 2002). The curve from Seitzinger et
al. (2002) is not as steep as the curve that includes information from reservoirs in an agricultural region.
Because N2O is a potent greenhouse gas, whether the end product of
denitrification is N2O or N2 is of importance. The IPCC estimates that 1.25% and 0.75%
of N that enters agricultural soils and rivers, respectively, is converted to N2O (Mosier et
100
-------
al., 1998). However, that fraction includes N2O production via both nitrification and
denitrification. IPCC assumes that only 0.5% of N that is denitrified in rivers is
converted to N2O (Mosier et al., 1998), but they do not estimate this fraction for soils. A
review of 32 studies of terrestrial denitrification reported the fraction of denitrified N
converted to N2O to be highly variable (0 to 100%) with a mean of 27% (Stevens and
Laughlin, 1998). Thus available data suggest that denitrification in aquatic systems
produces less N2O as a fraction of denitrified N than terrestrial systems. Therefore,
where denitrification occurs on the landscape will influence its contribution to
greenhouse gases. However, enhancing denitrification to reduce water quality impacts of
leached nitrogen will increase greenhouse gas emissions if nitrogen leaching rates remain
high.
Phosphorus. An understanding of P transport and transformation in streams and
rivers has developed in parallel with the studies on N just described. Stream networks
alter the timing, magnitude, and bioavailability of edge-of-field P loss during transport to
the Gulf via geochemical and biological processes: sediment sorption and desorption,
precipitation and dissolution, microbial and algal uptake, and riparian floodplain and
wetland retention. Many of the geochemical processes are mediated by biota; e.g., co-
precipitation of dissolved P with calcite may be biologically mediated during active
photosynthesis (Neal, 2001), and aquatic biota accounted for 30 - 40% of sediment P
uptake and release in wetland (Khoshmanesh et al., 1999) and stream sediments
(McDowell and Sharpley, 2003).
Fluvial sediments come from overland flow and erosion of stream channels and
banks. High discharge events that generate overland flow in agricultural regions
commonly account for most of the annual phosphorus load (e.g., Gentry et al., 2007).
Soils eroding from stream banks may be subsoils poor in P, which is less available for
release to water; hence the subsoils will likely represent a net sink for P (McDowell and
Sharpley, 2001). Land-disturbing activities (e.g., urban development and mining) can be
a significant source of sediment P, particularly when eroded sediments are rich in
nutrients because of past agricultural practices. For example, construction of one side
channel on the Missouri River floodplain has been calculated to contribute ~ 4,000 metric
ton P (4,400 ton P) to the river (Kristin Perry, Missouri Clean Water Commission,
presentation to SAB Panel, June 2007).
Regardless of sediment source, particulate P is the predominant form in transport.
Both fluvial hydraulics and adjacent land use influence the properties of sediment within
river systems (McDowell et al., 2002). To link P loss from the landscape to channel
processes, variability in flow, local sources of P, sediment properties, and changes in P
forms and loads should be simulated in models that estimate P loss from catchments,
although this is rarely done.
In tile-drained agricultural regions, P is transported to streams by both overland
flow and by the artificial drainage systems, which have been associated with elevated
dissolved reactive P (DRP) concentrations (Xue et al., 1998). DRP concentrations
remained high in successive tile flow events, suggesting a pool of soil P that is readily
101
-------
desorbed (Gentry et al., 2007). In a tile-drained Illinois watershed, P loss via tiles
represented 45% of total P loss in one year and 91% during a wetter year (Gentry et al.,
2007). One rain-on-snow event transported about 40% of the annual P load in one week,
80% of which was DRP (Gentry et al., 2007). Clearly artificial drainage alters both the
amount and form of P exports, and the amount exported is dependent on both the
magnitude and timing of storms.
In fluvial systems with good hydraulic mixing (e.g., shallow streams), P
availability in sediments can be estimated by the equilibrium P concentration (EPCo). At
low flow, EPCo will have a major influence on soluble P concentration, for P will desorb
from sediments if P concentration in water is less than the sediment's EPCo, or P will
adsorb to sediments if P concentration is greater than EPC0. P desorbed from sediments
will be available for biological uptake. Bioavailable P from desorption is likely to be
most significant as salinity increases in the estuary (Sutula et al., 2004).
Although cellular uptake and growth rates are generally saturated at low P
concentrations, maximum biomass accrual in streams often occurs at somewhat greater
concentrations (0.015-0.050 mg PCvP/L, Popova et al., 2006). This range of dissolved
P concentrations might be more typical of streams draining agricultural catchments, and
therefore, algal and microbial uptake likely plays a significant role in dissolved P
retention, especially at low flow. Dissolved P uptake rates of algae vary with light, water
velocity, temperature, grazing, and time following in-stream disturbances (Mulholland et
al., 1994).
Estimates of the percentage of total P inputs removed by these in-stream
processes in regional drainages in the Mississippi River basin range from 20-75%
(SPARROW model, Figure 38). The Ohio River basin removed the lowest percentage
and the Arkansas River basin the highest. These percentages are considerably higher
than what was used in the Integrated Assessment (28 to 37% in small streams and
negligible in the mainstem).
P concentrations and loads generally increase with increasing discharge and are
greatest on the rising limb of the hydrograph (e.g., Green and Haggard, 2001; Novak et
al., 2003; Richards et al., 2001). Although P concentrations are greater during high
flows, the importance of in-stream P retention is minimized at those times because of
sediment resuspension and scouring within the channel. However, P deposition on
floodplains may be a significant P sink during storms. Many streams export most of their
P loads during episodic storm events; e.g., in Illinois agricultural watersheds, extreme
discharges (>90th percentile) are responsible for 84% of P export and 98% of P export
occurred at discharges > median (Royer et al., 2006). This export is primarily particulate
P; in contrast, over half of dissolved P export can occur during base flow conditions
(Novak et al., 2003). Dissolved P constitutes a larger proportion of P export in
watersheds with extensive tile drainage (Royer et al., 2006). Because most P transport
occurs at high flows, models from Illinois agricultural watersheds suggest that enhancing
in-stream P removal by 50% during low flows (e.g., less than the median) would reduce P
102
-------
export by less than 1%, whereas enhancing P removal by 25% during high flows (more
than the 75th percentile) would increase P removal by 24% (Royer et al., 2006).
Silicate. Understanding of Si transport and transformations in rivers and streams
lags far behind that of N and P. Although first generation models for Si transport and
transformations are available (Gamier et al., 2006; Sferratore et al., 2006), there are
currently no models in the Mississippi River basin to predict the transport of dissolved
silicate or biogenic Si (amorphous Si contained in diatoms and phytoliths). Once
dissolved silicate is weathered, there are a number of transformations that occur including
inorganic transformations (such as new clay formation and precipitation as amorphous Si
in soils) and biological transformations (such as the uptake and deposition in terrestrial
plants, uptake and deposition in diatoms in aquatic systems) (Conley, 2002). Unlike
models developed for N and P, there are no models that describe the complexity of
biological transformations that occur with Si. In addition, significant reductions in the
transport of Si have occurred with the building of dams along the Mississipi River
leading to potentially significant changes in food webs on the Mississippi River shelf
(Turner and Rabalais, 1994).
Freshwater wetlands
The Integrated Assessment recognized the historical loss of many freshwater
wetlands as one of the primary land-use changes contributing to excess nutrient loads in
the Mississippi River basin. Mitsch et al. (1999) suggested the creation and restoration of
wetlands for the specific purpose of controlling non-point source nutrient loads and
emphasized the importance of targeting wetland creation and restoration in areas where
nitrogen concentrations and loads were highest. They estimated that restoring about 2
million ha (5 million ac) of wetlands would reduce N loads to the Gulf of Mexico by
20%, assuming a denitrification rate of 150 kg N/ha (134 Ib N/ac) of wetland /yr.
Subsequent research (Section 4.5.2 of this report) suggests that wetlands can achieve
substantially higher N removal rates in areas with elevated nitrate concentrations (Figure
12), underscoring the importance of targeting restorations.
Wetland restoration is a particularly promising approach for heavily tile drained
areas like the Corn Belt (Figure 10). This region was historically rich in wetlands, and in
many areas, farming was made possible only as a result of extensive wetland drainage
(Dahl, 1990; Pavelis, 1987). There are widespread opportunities for wetland restoration
in the Mississippi River basin, and since the CENR reports, approximately 570,000 ha
(1.4 million ac) of wetlands have been restored, created, or enhanced within the basin
under the Wetland Reserve Program (WRP), Conservation Reserve Program (CRP),
Conservation Reserve Enhancement Program (CREP), Environmental Quality Incentive
Program (EQIP), and Conservation Technical Assistance (CTA) (Table 7). However, the
vast majority of wetland restorations have been motivated primarily by concern over
habitat loss, and site selection criteria for wetland restorations have not primarily
considered water quality functions. This past emphasis does not lessen the promise of
wetlands for water quality improvement but rather underscores the need for programs
103
-------
focused on restoring wetlands explicitly for the purpose of reducing non-point source
nutrient loads.
Table 7: Acres of wetlands created, restored or enhanced in major subbasins of the Mississippi River from
2000-2006 under the Wetland Reserve Program (WRP), Conservation Reserve Program (CRP),
Conservation Reserve Enhancement Program (CREP), Environmental Quality Incentive Program (EQIP),
and Conservation Technical Assistance (CTA). (Personal communication, Mike Sullivan, USD A).
2-digit Watershed
Ohio River basin
Tennessee River basin
upper Mississippi River basin
lower Mississippi River basin
Missouri River basin
Arkansas, White, and Red River basins
Total
Hectares
33,300
2,130
133,227
241,868
93,108
68,161
571,794
Nutrient Sources and Sinks in Coastal Wetlands
The general conclusion in the Integrated Assessment was that coastal wetlands are
of secondary importance as nutrient sinks in comparison to other sources and sinks.
Their role as a source of organic matter was discussed in an earlier section, and a more
detailed review of that subject is in Section 2.1.5. Mitsch et al. (1999) assessed the utility
of wetlands as nutrient and sediment sinks and concluded that 1) potential NOs reduction
by coastal wetlands was likely less than 10-15% of the total river load; 2) water passage
through coastal wetlands would likely decrease water column N:P and N:Si ratios; 3) the
concept of coastal wetlands as net nutrient sinks remains controversial (e.g., Turner,
1999) so more large-scale measurements are needed; 4) deltaic systems might become N-
saturated or begin to release N in forms other than NOs; and 5) research and modeling
was needed to better understand relationships between land subsidence, river diversions
into wetlands, and N uptake in the coastal wetland/delta area. The Integrated Assessment
concluded that although coastal denitrification rates were substantial (10-25 g N/m2/yr)
relative to many shallow estuarine areas, diversion of river water into coastal wetlands
might lead to N removal rates of 50-100 metric ton N/yr (55-110 ton N/yr), which is a
relatively small fraction of N reduction goals.
A number of papers have been produced concerning nutrient sources and sinks in
coastal wetlands since the Integrated Assessment. Lane et al. (2002) reported large
decreases in nitrate as river water passed through an estuarine/wetland complex
(Fourleague Bay); this estuarine-marsh complex appears to buffer the impact of the
Atchafalaya River on coastal waters by causing an estimated 41to 47% reduction in river
nitrate concentrations. Denitrification rates in coastal wetlands ranged from 30 to 40 g
N/m2/yr (larger than rates typically measured in adjacent estuaries), accretion rates of 8-
11 mm/yr or about 2,300 g dry sediments m2/yr (approximating sea level rise), and N
burial rates of about 7 g N/m2/yr. Day et al. (2003) and others argued for river diversions
104
-------
to wetlands to prevent land losses and remove nutrients via denitrification, burial, and
plant uptake. Nitrogen reductions of about 4 g N/m2/yr and 10-20 g N/m2/yr have been
recorded for forests and wetlands, respectively. Particulate N burial rates of 13 to 23 g
N/m2/yr have been measured in some wetlands. These are substantial rates by estuarine
standards but modest relative to wetlands/reservoirs in the upper MARB connected to or
adjacent to agricultural drainage. However, Turner (1999) reported very small N
concentration reductions and modest TSS, POC, and parti culate P concentration
reductions in waters flowing through the Atchafalaya system and hence concluded river
diversions would remove small amounts of nutrients relative to nutrient input loads.
The recent literature supports the importance of forested and other types of coastal
wetlands for nutrient uptake and sediment accretion, both of which would lead to
reductions in loads to the GOM. Rates appear to be substantial compared to most sub-
tidal estuarine locations (excluding areas like the Mississippi River plume) and moderate
to small relative to many freshwater natural and created wetlands. Rates lower than those
observed in more northern wetlands of the MARB may be due to the generally lower
nutrient loading rates to these coastal wetland systems (< 10 g N/m2/yr). However, given
the data currently available, it is doubtful that a predictive model of nutrient losses in
these coastal wetlands can be developed following the general form of statistical models
used for predicting nutrient losses in freshwater wetlands (Saunders and Kalff, 2001;
Spieles and Mitsch, 2000).
Missing from the GOM hypoxia analysis is a regional scale (i.e., larger spatial
scale) analysis of both nutrient (N and P) and OM losses associated with coastal
wetlands. It would appear that sufficient information is currently available to delineate
the spatial extent of various coastal wetland habitats. It seems less certain that essential
nutrient and OM loss rate estimates (e.g., long-term burial of C, N and P or
denitrification) are available to achieve this goal.
There appear to be few nutrient, sediment, or organic matter budgets available for
these coastal wetlands that can be used to judge the effectiveness of wetlands as either
sinks or sources. For example, nutrient sink behavior of wetlands has been inferred from
nutrient concentration reductions with distance from a nutrient source. While this
approach has appeal, it would be more convincing if nutrient loads (i.e., concentrations
coupled to water flows) entering and leaving wetland systems were compared in a mass
balance format. Additionally, more emphasis on process measurements (e.g., burial,
denitrification, and plant uptake rate) would allow for better understanding of observed
differences between wetland inputs and outputs. It appears that process measurements in
these coastal wetlands lag behind those made in natural and created wetlands in other
parts of the MARB.
105
-------
Key Findings and Recommendations
The percentage of annual N and P inputs removed by in-stream processes varies
by MARB sub-basin and ranges from 20 to 55% for N and 20 to 75% for P based on
model estimates. There currently are no models to predict the transport of dissolved
silicate. Denitrification can be a significant pathway for N removal in small streams
during low flow, warm periods, thereby enhancing local water quality. However, most
nitrate is exported to the Gulf during high flows in the period from January to June, when
denitrification is not effective in removal. Since the effectiveness of N removal in
aquatic systems increases with water residence time, enhancing the connectivity and
water residence time in floodplains and backwater areas on the upper Mississippi River
during periods of high flow and high nitrate concentrations could increase the
effectiveness of N removal by denitrification and, therefore, reduce the N flux to the
Gulf. Likewise, since high flow events that generate overland flow in agricultural
regions generally account for most of the annual P flux (as much as 84%, primarily as
particulate P), deposition on floodplains and in backwater areas could represent a
significant P sink. However, in tile drained areas, dissolved reactive P represents a much
larger percentage of P flux (45-91%), and deposition is a less significant sink.
There has been substantial wetland restoration within the MARB since the CENR
reports, but restorations have not been targeted for water quality benefits. The greatest
water quality benefits will be realized in areas of the Corn Belt with highest nitrate
concentrations and loads.
Although current estimates of denitrification rates in coastal wetlands are higher
than the estimates used in the Integrated Assessment, current studies still conclude that
river diversions to coastal wetlands would remove only small amounts of nutrients
relative to the total fluxes. However, better estimates of nutrient and organic matter loss
rates (denitrification; long-term burial of C, N, and P; and plant uptake) are needed to
better understand observed differences between wetland inputs and outputs in coastal
areas. Based on these findings, the SAB Panel offers these recommendations.
Removal of both N and P can be increased by implementing management
strategies that include enhancing hydrologic exchange and retention on
floodplains and in backwater habitats when discharge, total P and nitrate
concentrations are high (e.g., during spring), particularly in rivers of
intermediate size.
More reliable and process-driven models that simulate fluvial processes
and estimate N and P transfer to stream channels need to be developed to
more accurately predict land management or BMP impacts on nutrient
inputs to receiving waters.
First-generation models need to be developed to describe the transport and
transformations of Si in the MARB.
Programs focused on restoring wetlands explicitly for the purpose of
reducing non-point source nutrient loads need to be implemented and
106
-------
targeted in areas of the Corn Belt with highest nitrate concentrations and
loads.
Better measurements of key processes (e.g., burial, denitrification, plant
uptake rate) are needed in coastal wetlands to provide a better
understanding of observed differences between inputs and outputs.
Regional scale studies of coastal wetlands are needed to develop nutrient,
sediment, and organic matter budgets that can be used to better evaluate
the effectiveness of coastal wetlands as sinks or sources.
3.4. Ability to Route and Predict Nutrient Delivery to the Gulf
The SAB Panel concurs with the Integrated Assessment's identification of
modeling as a critical component of an adaptive management approach to improving Gulf
hypoxia. Along with monitoring, interpretation, and research, modeling can improve the
scientific understanding of the impacts of land and nutrient management actions on
watershed and Gulf of Mexico environmental quality. The Integrated Assessment used
only limited modeling results at the field-scale and none at the watershed scale in its
assessment.
Research was proposed in the Integrated Assessment to develop an effective
modeling framework, including improved watershed and basin-scale simulation of
nutrient transport and transformations from natural, urban, and agricultural landscapes,
improved estimates of nutrient mass balances throughout the landscape and improved
understanding of biogeochemical cycling within the basin. Within this modeling
framework, further research was called upon to assist in four areas: 1) to characterize the
dynamics and timing of nutrient movement from the edge of the field in agricultural
landscapes to small streams and tributaries, particularly from agricultural tile drainage
systems; 2) to scale up from experimental plots to watershed/farm-scale studies of on-
farm practices and edge-of-field strategies to reduce and intercept nutrients; 3) to assess
the effects on nutrient loads and hypoxia of long-term change in climate, hydrology, and
population; 4) to evaluate the role of flood events and the potential role of flood
prevention strategies on nutrient transport to the NGOM; and 5) to improve
understanding of the social and economic trade-offs and impacts of various management
and policy alternative strategies.
Numerous models have been used to describe sources, transport, and delivery of
nutrients at various spatial and temporal scales within the MARB (Table 8). Several of
these studies address needs identified in the Integrated Assessment., including improved
understanding of basin-scale nutrient transport, nutrient cycling processes, tile-drainage
nutrient transport, watershed-scale simulation of in-field and edge-of-field practices,
climate effects, and loading from high-flow events. Issues associated with social and
economic tradeoffs of alternative strategies are discussed in Sections 4.3 and 4.4.
107
-------
While each of the models listed in Table 8 may prove useful for developing
adaptive management to mitigate hypoxia, we single out three models for further
discussion based on the fact that they have been applied at the basin wide scale within the
MARB. In so doing, we do not intend to suggest that only these three models should be
relied upon for insight into the processes of nutrient fate and delivery.
Table 8. Attributes of models used to estimate sources, transport and/or delivery of nutrients to the Gulf of
Mexico.
Model
ADAPT
AnnAGNPS
DAFLOW/BLTM
DRAINMOD
EPIC
GLEAMS
HSPF, LSPC
IBIS/THMB
L-THIA
NANI
PLOAD
REMM
RZWQM
SPARROW
SWAT
WARMF
WEPP
T 2
Type
E,M
E,M
M
E,M
E,M
E,M
E
E,M
C,E
B
C
M
E,M
S,M
E,M
EM
M
T- 3
Time
D
D
D
D
D
D
D
D
A
A
M
D
S
A
S,D
D
S
Space
F,W
H,W
R
F
F
F
W
W,B
F,W
W,B
W
F
F
W,B
H,W,B
W
F,W
Components
R,D,S,E,N,P,U
R,E,N,P,U
R,N,Q
RAN
R,S,E,N
R,E,N
R,D,S,E,N,Q,U
R,D,S,E,N,Q,P
R,E,N,U
N,P
R,E,N,U
R,D,E,N
R,D,S,N
R,D,N,Q,P,U
R,D,S,E,N,Q,P,U
R,E,N5P,U
R,D,S,E,N
Inputs
C,F,P,A,S
C,F,P,A,S
C,G
C,F,L
C,F,A,L
C,F;A,L
C,F,P,A,S
C,F,P,A,S
C,P,N,F
F,P,A,S
F,C,P,S
C,F,L
C,F,A,L
C,F,P,A,S,G
C,F,P,A,S
C,F,PAN
C,F,P^V,S
Outputs
F,S,N,P
F,S,N,P
F,N
F,N
F,S,N,P
F,S,N,P
F,S,N,P
F,S,N,C
F,S,N,P
N
F,S,N,P
F,S,N,P,C
F,S,N,P
F,N,P
F,S,N,P,C
F,S,N,P
F,S,N,P
0
Predicts
C,L,M
C,L,M
C,L
C,L,M
C,L,M
C,L,M
C,L,M,R
C,L,M
L
C,L,M
C,L,M
C,L,R
C,L,M
C,L
C,L,M,W,R
C,L,M
C,L,M,R
Strength9
Overland/drainage
In-field sediment
River routing
Drainage
In-field practices
Overland
Overland/stream
Ecosystem
W-s land-use
change
Process accounting
Distribute w-s loads
Riparian ecosystem
Subsurface/plants
Data-driven
Overland/w-s
TMDL study
Hillslope
MARB
_ 10
Refs
1
2
3
4
5
6
-
8
-
10
-
12
-
14
15
-
17
NOTE: Although all models cited above (as well as numerous other models not included) are relevant to
MARB and Gulf of Mexico hypoxia issues, not all of the above models have been discussed within
this report. In this section, the discussion was focused on models considered most applicable to
MARB basin-scale processes. More details on applications of these models within the MARB can be
found in cited references.
Typemodel classification: S=statistical/stochastic, C=export coefficient, B=mass balance,
E=empirical/process-based, M=mechanistic/process-based
Timesmallest time-scale for output. S=subdaily, D=daily, M=monthly, A=annually
4
Spaceorganizing spatial scale: F=field, H=hydrologic resource unit, W=watershed, B=basin, R=river
network
Componentsmodel features: R=runoff, D=drainage, S=snowmelt, E=erosion, N=nutrients, Q=stream
processes, P=ponds/reservoirs, U=urban
Inputsinput types: C=climate (temperature, precipitation), F=fertilizers, P=point sources,
A=Atmospheric deposition, S= spatial land-use, L=single-field land use, G=stream gage data
Outputsconstituents modeled: F=flow, S=sediment, N=nitrogen, P=phosphorous, C=carbon
108
-------
Predictspredictive capability: C=climate change, L=land-use change, M=land-management change,
W=wetland change, R=riparian change
9
Strengthapplication for which model tends to be well suited (w-s=watershed)
MARB RefsRecent and key references from Mississippi/Atchafalaya River basin model applications:
1ADAPT: Dalzell, B.J., P.H. Gowda, D.J. Mulla. 2004
Gowda, P.H., D.J. Mulla. 2006.
Gowda, P.H., B.J. Dalzell, D.J. Mulla. 2007
Sogbedji, J.M.,G.F. Mclsaac. 2006.
2AGNPS:
Yuan, Y., R.L. Bingner, R.A. Rebich. 2001
Yuan, Y., R.L. Bingner, F.D. Theurer. 2006
Yuan, Y. R.L. Bingner, F.D. Theurer, R.A. Rebich, P.A. Moore. 2005
3DAFLOW/BLTM:
Broshears, R.E., G.M. Clark, H.E. Jobson. 2001
4DRAINMOD:
Northcott, W.J., R.A. Cooke, S.E. Walker, J.K. Mitchell, M.C. Hirschi. 2001
5EPIC:
Atwood, J.D., V.W. Benson, R. Srinivasan, C. Walker, E. Schmid. 2001
Chung, S.W., P.W. Gassman, R. Gu, R.S. Kanwar. 2002
6GLEAMS:
Wedwick, S., B. Lakhani, J. Stone, P. Waller, J. Artiola. 2001
8IBIS/THMB:
Donner, S.D., M.T. Coe, J.D. Lenters, I.E. Twine, J.A. Foley. 2002
Donner, S.D., C.J. Kucharik. 2003a
Donner, S.D., C.J. Kucharik, J.A. Foley. 2004.
Donner, S.D. 2006
10NANI:
Mclsaac, G.F., M.B. David, G.Z. Gertner, D.A. Goolsby. 2002
Howarth, R. W., G. Billen, D. Swaney, A. Townsend, N. Jarworski, K. Lajtha, J. A. Downing, R. Elmgren, N.
Caraco, T. Jordan, F. Berendse, J. Freney, V. Kueyarov, P. Murdoch, and Zhu Zhao-liang. 1996.
12REMM:
Graff, C.D., A.M. Sadeghi, R.R. Lowrance, R.G. Williams. 2005
14SPARROW:
Alexander et al, in press.
Alexander, R.B., R.A. Smith, G.E. Schwarz., 2004.
Smith, R.A., G.E. Schwarz, R.B. Alexander, 1997
Alexander, R.B., R.A. Smith, G.E. Schwarz., 2000
15SWAT:
Anand, S., K.R. Mankin, K.A. McVay, K.A. Janssen, P.L. Barnes, G.M. Pierzynski. 2007
Du, B., A. Saleh, D.B. Jaynes, J.G. Arnold. 2006
Gassman, P.W., M.R. Reyes, C.H. Green, J.G. Arnold. 2007
Green, C.H., M.D. Tomer, M. DiLuzio, J.G. Arnold. 2006a
Hu, X., G.F. Mclsaac, M.B. David, C.A. Louwers. 2007
Jha, M., J.G. Arnold, P.W. Gassman, F. Giorgi, R.R. Gu. 2006
Kirsch, K., A. Kirsch, J.G. Arnold. 2002.
Santhi, C., J.G. Arnold, J.R. Williams, W.A. Dugas, R. Srinivasan, L.M. Hauck. 2001
Shirmohammadi A., I. Chaubey, R. D. Harmel, D.D. Bosch, R. Munoz-Carpena, C. Dharmasri, A. Sexton, M. Arabi,
M.L. Wolfe, J. Frankenberger, C. Graff, T.M. Sohrabi. 2006..
Stone, M.C., R.C. Hotchkiss, C.M. Hubbard, T.A. Fontaine, L.O. Mearnes, J.G. Arnold. 2001
Vache, K.B., J.M. Eilers, M. V. Santelman. 2002
VanLiew, M.W., T.L. Veith, D.D. Bosch, J.G. Arnold. 2006
Wang, X., A.M. Melesse. 2005
17WEPP:
Tiwari, A.K., L.M. Risse, M.A. Nearing. 2000
109
-------
SPARROW Model
The SPARROW (SPAtially Referenced Regressions On Watershed attributes)
model is a hybrid mechanistic/empirical, basin-scale simulation model developed by U.S.
Geological Survey (Smith et al., 1997; Alexander et al., 2007a). The model uses spatially
distributed data on nutrient sources, climate, soils, topography, and natural and artificial
drainage densities to estimate N and P delivery to streams and removal processes in
streams and reservoirs under long-term steady-state conditions. Nutrient sources include
atmospheric deposition (N only), urban/human sources, agricultural runoff and
subsurface drainage, and natural sources from forest, barren, and shrub lands.
Wet-deposition data are the basis for atmospheric deposition N source, which
assumes that dry deposition and ammonium deposition are spatially correlated to wet
deposition. Urban nutrient sources include all human-population-dependent nutrient
sources: municipal and septic-system wastewater, stormwater runoff, and other sources
that are spatially correlated to human population data (such as wet and dry deposition
from vehicles, power plants, etc.). Agricultural nutrient sources include commercial
fertilizers, livestock manure, and biological N2 fixation. Soil transformations of N and P
are not considered and assumed to be in equilibrium between immobilization and
mineralization.
SPARROW simulates N and P fluxes (mass) and yields (mass per unit area)
within sub-catchments using three first-order attenuation terms, with mass-balance
constraints, to represent nutrient losses in overland transport, riverine processes, and
reservoir trapping. Model parameters are calibrated based on the source conditions for a
base year and the flow-adjusted, long-term mean annual loads of total N and total P
estimated using rating curves fit to stream monitoring data. The calibrated SPARROW
model can be used to assign these loads to specified nutrient sources and sub-catchments
with quantifiable uncertainty.
SPARROW has been applied to diverse watersheds including the MARB (Smith
et al., 1997; Alexander et al., 2000; Alexander et al., 2007a), the Chesapeake (Preston
and Brakebill, 1999), the Neuse (McMahon et al., 2003) in North Carolina, and the
Waikato (Alexander et al., 2002a) in New Zealand. The most recent application of
SPARROW to the MARB includes more and better data for model parameter estimation
and greater detail in the model specification (Alexander et al., in press). The result is an
increased number of nutrient-source terms and 20% less model error compared to
previous applications of SPARROW (Smith et al., 1997; Alexander et al., 2000). Several
important assumptions are embedded in the modeling approach, however, and these must
be considered in interpretation of model results for the MARB.
SPARROW does not assess flow-related changes in nutrient loads, which are
important to Gulf hypoxia extent and severity. In comparing SPARROW predictions
with observed loads for a particular period or location, SPARROW does not estimate
nutrient load for any particular year, but rather a flow-adjusted or flow-independent load.
110
-------
This is the load predicted under long term average flow conditions for the source input
conditions of a particular year (the base year). For example, the SPARROW estimates
for the 1992 base year in Alexander et al. (in press) are the mean annual loads that would
be predicted under the source conditions of 1992 and the mean annual flow of the period
from 1975-2000. These represent the loads that would have been expected in 1992 if
1992 had had the mean annual flow of the period 1975-2000. They are not an estimate of
the nutrient loads in!992. As a result, the comparisons between 1992 and 2002 in
Alexander et al. (in press) are not based on the loads predicted for 1992 and 2002 but
rather the loads that would have been expected in 1992 and 2002 under the different
source inputs for those two years, but assuming both years had had exactly the same flow
patterns, i.e. the mean annual flow of the period 1975-2000.
Model input coefficients for each nutrient source are statistically estimated by the
model, and as such, are influenced by all sources that are spatially correlated to these
sources (whether these correlated sources are in the model or not). For example, wet-
deposition N was spatially characterized from monitoring data, but no data for dry
deposition were used; and urban sources were modeled assuming that all sources were
correlated and spatially distributed similar to the model input of population. Particularly
in these two cases, coefficients may be artificially high as they include the effects of other
spatially correlated sources that are not in the fitted model. The model does not account
for soil storage of nutrients, but assumes that stream inputs are correlated to the
agricultural nutrient source inputs. The lack of a soil storage term may ignore nutrient
carry-over effects that are often important in determining stream export (David et al.,
1997; David and Gentry, 2000; Mclsaac et al., 2001; Mulvaney et al., 2001). These
limitations and others discussed in Alexander et al. (in press) should be considered in
interpretation of the SPARROW model results.
The most recent version of SPARROW (Alexander et al., in press) is thought to
have improved many aspects of this statistical model. For example, the percentage of N
and P that enters streams and is actually delivered to the NGOM has increased, as in-
stream removal terms have been reduced. SPARROW can be used to examine source
inputs for the nutrients being transported by streams, and with each new version these can
change. Source areas are quite dependent on land-to-water transfer coefficients, and the
way the model represents inputs and their availability to be transferred to a stream. As
expected, agriculture was found to be the major source of nutrients to the NGOM in this
recent application (Alexander et al., in press). Important non-agricultural nutrient
contributions were from atmospheric deposition and urban sources. The largest source of
N is attributed to fertilizer inputs to corn and soybean fields (52%) followed by
atmospheric deposition (16%). In contrast, the largest source of P is attributed to animal
manure on pasture and rangelands (37%) followed by corn and soybeans (25%), other
crops (18%), and urban sources (12%) (Alexander et al., in press). It is important to note
that in the model structure, manure is the only source of P that is available for transport
from pasture and rangelands. These lands are otherwise assumed to be in steady state.
Similarly, fertilizer, N fixation, and manure N are the only source of N that is available
for transport from corn and soybean. These lands are assumed to be in long term steady
state, and there is assumed to be no net soil mineralization. Statistical coefficients for
111
-------
agricultural sources suggested N delivery to streams ranging from 6% of applied nutrients
for pasture/rangeland to 16% for corn and soybeans, and the opposite trend for P
delivery, ranging from 2% for corn and soybeans to 14% for pasture/rangeland. These
results give a very different picture of important inputs to the basin and their effects on
riverine N and P fluxes. For example, atmospheric deposition (for N) and manure (for P)
are thought to much more important, and point sources and corn and soybean production
much less important than mass balance type calculations would suggest (see Section 3.2).
SPARROW is continually being developed and improved. However, the Panel cautions
against the sole use of SPARROW (or any model) for making decisions about where to
target management efforts given the current stage of development of this approach.
SWAT Mode I
The Soil and Water Assessment Tool (SWAT) model is a physically based,
deterministic, continuous, watershed-scale simulation model developed by the USDA
Agricultural Research Service (Neitsch et al., 2004; Arnold et al., 1998). It uses spatially
distributed data on topography, soils, land cover, land management, and weather to
predict water, sediment, nutrient, and pesticide yields. A modeled watershed is divided
spatially into subwatersheds using digital elevation data according to the density
specified by the user. Subwatersheds are modeled as having uniform slope and climatic
conditions, and they are further subdivided into lumped, nonspatial hydrologic response
units (HRUs) consisting of all areas within the subwatershed having similar soil, land
use, and land management characteristics. The use of HRUs allows soil and land-use
heterogeneity to be simulated within each subwatershed but ignores pollutant attenuation
between the source area and stream and limits spatial representation of wetlands, buffers,
and other BMPs within a subwatershed.
The model includes subbasin, reservoir, and channel routing components. The
subbasin component simulates runoff and erosion processes, soil water movement,
evapotranspiration, crop growth and yield, soil nutrient and carbon cycling, and pesticide
and bacteria degradation and transport. It allows simulation of a wide array of
agricultural structures and practices, including tillage, fertilizer and manure application,
subsurface drainage, irrigation, ponds and wetlands, and edge-of-field buffers. The
reservoir component detains water, sediments, and pollutants, and degrades nutrients,
pesticides and bacteria during detention. The channel component routes flows, settles
and entrains sediment, and degrades nutrients, pesticides and bacteria during transport.
SWAT typically produces daily results for every subwatershed outlet, each of which can
be summed to provide monthly and annual load estimates.
The SWAT model has been tested for a wide range of regions, conditions,
practices, and time scales (Gassman et al., 2007). Evaluation of monthly and annual
streamflow and pollutant outputs indicate SWAT functioned well in a wide range of
watersheds. Relatively poor results in some cases, particularly for daily flow and
pollutant outputs, were attributed partly to input and calibration data uncertainty and
partly to model limitations. In general, the model had more difficulty simulating wet
112
-------
years than dry years and tended to overestimate soil water in dry soil conditions and
underestimate in wet soil conditions.
Numerous studies have applied the SWAT model in the Mississippi River basin.
Several recent studies have addressed issues identified in the Integrated Assessment., such
as application of field-scale hydrologic processes to large watershed scale (Arnold et al.,
1999; Anand et al., 2007), effectiveness of various nutrient-reduction strategies in
agricultural watersheds (Santhi et al., 2001; Vache et al., 2002; Hu et al., 2007), model
enhancements to address tile-drained cropland (Du et al., 2006; Green et al., 2006a), and
assessment of the impacts of climate change on large-basin hydrology and nutrient export
(Jha et al., 2006). These studies are discussed below.
Studies from field scale (Anand et al., 2007) to basin-scale (Arnold et al., 1999) in
the MARB have demonstrated the ability of SWAT to scale-up processes to the large-
watershed scale. Arnold et al. (1999) validated the water-balance component of SWAT
in a large-scale modeling study of the conterminous U.S. and concluded that it would be
useful in studying the effects of climate and BMPs on annual and seasonal runoff. The
long-term effects of various BMPs was assessed in a 4,277 km2 (1,651 mi2) pasture-
rangeland-dominated watershed experiencing urban growth in Texas (Santhi et al., 2001).
They found future (2020) loads could be reduced by about 50% by implementing a
combination of practices, including a 1 mg/L limit for wastewater treatment plant P
effluent, limiting dairy manure land applications to the P rate, exporting 38% of manure
from the watershed, and reducing P in livestock diet. Vache et al. (2002) explored the
impact of traditional and alternative agricultural practices on water quality in two
agricultural watersheds in Iowa. Continuing current trends in Midwestern agricultural
production to 2025 (including increased conservation tillage, increased farm size and
total acres, and current BMPs) resulted in simulated increase of nitrate export and
decrease of sediment export relative to present. Two other scenarios representing
different combinations of practices, such as complete conversion of cropland to no-till,
implementation of riparian buffers on all streams, and increased use of perennial cover
(CRP, pasture, and alfalfa), resulted in reductions of nitrate loads by 54 to 75% and
sediment load by 37 to 67% simulated by SWAT. In a tile-drained watershed in east-
central Illinois, reductions in N fertilizer resulted in 10 to 43% decrease in riverine nitrate
export (Hu et al., 2007). However, SWAT overestimated nitrate export during major wet
periods and had several other unrealistic aspects of N cycle components. Recent
enhancements have been made to allow better simulation of tile-drainage in agricultural
fields by SWAT (Du et al., 2006; Green et al., 2006a). This change indicates that
previous modeling results by SWAT in heavily tile-drained watersheds should be
reassessed using the revised model.
Shifts in future precipitation and climate may impact flow and nutrient loads from
the MARB. Jha et al. (2006) used SWAT to assess the effects of future climate change on
UMRB flows. They found a doubling in CC>2 (to 660 ppmv) to result in a 36% increase
in average annual streamflow and a 20% increase in precipitation to increase streamflow
by 58%. Similar increases were found in average monthly streamflow in the April to
May period, which is in the critical period for hypoxia development. Mean annual
113
-------
streamflow changes in response to six general circulation model scenarios ranged from
-6% to +51%. Results indicated increases in rainfall and snowmelt in January and
February and large increases in spring stream flow.
IBIS/THMB Model
The Integrated Biosphere Simulator (IBIS) land-surface and terrestrial ecosystem
model and the Terrestrial Hydrology Model with Biogeochemistry (THMB, an enhanced
version of Hydrological Routing Algorithm, HYDRA) are two physically based models
that have been linked to model large basin-scale hydrological, carbon, and nutrient
processes (only nitrogen at this time) (Donner et al., 2002; Donner, 2006). The IBIS
model represents phenomena such as land-surface biophysical processes, canopy
physiology, vegetation phenology, and long-term ecosystem dynamics at different time
steps, ranging from 60 minutes to 1 year, to simulate time-transient surface and
subsurface hydrological fate and transport processes. IBIS requires spatially distributed
inputs of climate, soil texture, vegetation type and associated management information.
It uses these inputs to simulate terrestrial processes at a user-defined grid-cell scale. The
terrestrial model is coupled with THMB, which represents phenomena such as solute
transport, surface and subsurface leaching, point-source inputs, and in-stream chemical
and biological transformations to simulate river, wetland, lake, and reservoir flow and
storage of water and nutrients.
The IBIS/HYDRA model, with a simplified nitrogen leaching algorithm, was
found to represent much of the spatial and temporal variability in stream discharge and
nitrate export within the MARB (Donner et al., 2002). A study of 29 stations in the
MARB from 1965-1994 found interannual errors in simulated river discharge were less
than 20% for the majority of the data (76%), although the seasonal errors were greater
than 20% for 65% of the station months and particularly underestimated the magnitude of
spring discharge. A similar analysis found simulated annual mean nitrate export of the
Mississippi River at St. Francisville was within 1% of the USGS estimate, but annual
errors at various stations varied widely.
Results of the IBIS/HYDRA modeling study indicated that nitrate export from the
MARB was significantly greater during the latter half of the 1955-to-1994 period, largely
due to the increase in N fertilizer application, with greatest contribution from the central
and eastern subbasins (Donner et al., 2002). This analysis made many simplifying
assumptions about nitrogen inputs, fate and transport in order to isolate the impact of
hydrology on nitrate export variability. Donner et al. (2002) concluded that the observed
increase in river discharge was responsible for about 25% of the increase in nitrate export
between 1966 and 1994, with an error of 7%. The remainder of the increase was inferred
to arise predominately from an increase in fertilizer N inputs. In the Upper MARB
(1974-1994), Donner and Kucharik (2003) found that a +/-30% change in N fertilizer
application resulted in little change in corn yields (+4%/-10%) but greater sensitivity in
dissolved inorganic N subsurface drainage (+53%/-37%). They note that soil N storage
resulting from the +30% fertilizer N case appeared to lead to almost 60% increase in
nitrate export after 20 years and that this effect was greatest during wet years.
114
-------
Further work for the entire MARB based on IBIS/HYDRA results led Donner et
al. (2004) to conclude that the doubling of nitrate export to the Gulf of Mexico over the
1960-to-1994 period resulted largely from an increase in fertilizer application rates,
particularly to corn, an increase in runoff across the basin, and the expansion of soybean
cultivation. Their results indicated that by the 1990s, fertilized cropland (particularly in
Corn Belt hot spots across Iowa, Illinois, and Indiana) became the overwhelming nitrate
source in the river system, contributing almost 90% of the nitrate from just 20% of the
watershed area. Changes in MARB crop production systems associated with a shift away
from meat production were simulated by IBIS/THMB (Donner, 2006). Results indicated
a reduction in total land and fertilizer demands by over 50% and N export by 49-54%
without any change in total production of human food protein.
Discussion and Comparison of Models
The SAB Panel found only one study that compared any of the three focus
models. A study comparing SWAT and a statistical approach based on SPARROW
within the Great Ouse watershed in the United Kingdom found similar total oxidized
nitrogen load estimations and similar statistical reliability of the two models (Grizzetti et
al., 2005). They suggested using SPARROW as a screening tool for identifying sources
and using SWAT for testing management practice scenarios but found that both models
demonstrated utility for nitrogen load estimation.
Different modeling approaches resulted in different assessments of nutrient
sources and distribution within the MARB among the models, where comparisons were
possible. Cropland was found to contribute 90% of nitrate within the MARB by
IBIS/HYDRA (Donner et al., 2004) compared to 66% of total N for all crops by
SPARROW (Alexander et al., in press). Both models suggested the major nutrient source
yields (mass per unit area) originated from the central Mississippi and Ohio River basins.
Future work with SWAT (J. Arnold, personal communication) applied to the entire
MARB will provide another estimate of nutrient sources and distribution to assist with
watershed planning and management decisions.
Targeting
The models cited in Table 8 vary considerably in type, scale, and approach. The
Gulf hypoxia issue requires a diversity of model types, scales, and approaches. Models
that can support adaptive management of Gulf hypoxia and within-region water quality
are those that can best inform targeting of the most effective actions at the lowest cost.
Three forms of targeting are especially important:
targeting sub regions or watersheds (of perhaps the 8 or 12 digit HUC size)
that have a disproportionate effect on hypoxia and local water quality;
targeting the type and placement of conservation practices within those
watersheds to achieve the greatest gains at the lowest cost; and
115
-------
targeting the timing of nutrient flows to best attenuate the hypoxic zone.
Both SWAT and IBIS/THMB models directly address targeting of practices by
simulating the effects of farm/plot scale best management practices (BMPs) directly,
whereas the absence of BMP simulation capability is a weakness in SPARROW
(acknowledged in Alexander et al., in press). Simulation of these BMPs is vital to the
evaluation of successful management of nitrogen and phosphorus runoff in MARB.
SWAT and IBIS/THMB include these practices but would benefit from additional
verification that their mechanistic characterizations represent the range of field and
watershed-scale processes present in the MARB. SPARROW shortcomings in
simulation of BMPs are being addressed by ongoing efforts to evaluate farm and plot
scale BMPs by USGS and others using data in the same sense as SPARROW is fitted
(e.g., identifiability). When coupled with SPARROW, the results should yield a useful
predictive model for the impact of BMP and other practices within MARB on Gulf
hypoxia and should include uncertainty analysis.
Practices should be evaluated both for their impacts on total annual or long-term-
average nutrient loads as well as loads on a seasonal or other short-term time frame.
Within-year timing of pollutant loads is simulated by SWAT and IBIS/THMB (both
models operate on a daily basis; see Table 8) but not the annual-based SPARROW.
Timing issues are critical for addressing seasonal water quality concerns both locally as
well as for the April-to-June loads that appear to govern hypoxia development.
All three models address spatial targeting of sources and implementation.
SPARROW spatial resolution is tied to the resolution of available monitoring data; recent
studies of the MARB have been conducted at the HUC8 level. SWAT has been applied
at a range of watershed scales from collections of fields to Mississippi River basin,
whereas IBIS/THMB tends to be most applicable at the larger watershed scales. Both
SWAT and EVIIB/THMB spatial resolutions currently are dictated by computing capacity
for larger-scale basins. Spatial-scale issues are critical in targeting implementation
actions, funding, and resources to areas with the greatest potential for improvement.
Model interpretation must consider the relationship of watershed spatial heterogeneity
and model averaging of input variables, process algorithms, and outputs. All three
models provide information to assist with spatial targeting of actions.
Finally, there is a need to integrate watershed models with economic models in
making targeting recommendations. Due to the ability to assess the effectiveness of
specific conservation practices in a sub watershed context, integrated economic-
watershed models have largely relied on mechanistic models such as EPIC and SWAT.
More discussion of these integrated models can be found in Section 4.3.
Model Uncertainty
Model predictions all have a degree of uncertainty, which should be addressed in
presenting model results. Model uncertainty may be due to variability, inaccuracy, or
116
-------
inappropriateness of multiple factors: 1) model algorithms or methods, 2) inputs (known
or measured values, such as climate data), 3) parameters (values estimated based on
functional relationship with known inputs, such as soil hydraulic conductivity or runoff
curve number), 4) calibration data (including measurement errors associated with
streamflow estimation and sample collection, storage, and analysis), 5) boundary
conditions (such as initial soil moisture), 6) temporal scale (such as rainfall intensity),
and 7) spatial scale (such as topography) (Shirmohammadi et al., 2006). Model
uncertainty can be assessed by describing the impact of uncertain inputs and parameters,
treated as random variables, on output variability.
Uncertainty varies among models and differs among watersheds, depending on
availability of data, appropriateness of model assumptions for the given watershed and
climate, and skill of the modeler in applying the model and interpreting the results. One
study evaluated the impact of input uncertainty on SWAT2000 model output variation
and found that input uncertainty was transferred nonlinearly through the model
(Shirmohammadi et al., 2006). Coefficients of variations (CVs) of 34 input parameters
ranging from 10 to 76% resulted in a single-year output CV of only 28% for streamflow,
lower CVs for ammonium and organic N (6-7%) and mineral P (12%), and higher CVs
for nitrate (101%), organic P (58%), and sediment (36%). Measured streamflow, nitrate,
and ammonium values were within one standard deviation (SD) of the mean modeled
output, whereas sediment was within 1.5 SD.
An advantage of SPARROW (and other models that are parameterized using
optimization criteria, such as least squares or maximum likelihood) is that the model
provides error terms for prediction with respect to parameter uncertainty. Process
(mechanistic) models, such as SWAT and IBIS/THMB, are "overparameterized," which
means that the observational data are insufficient to provide unique/optimal estimates of
model parameters. Thus, these process models typically have been parameterized
according to modeler's best judgment, with important ramifications on model
uncertainty. New approaches acknowledge that many different parameter sets may fit
equally well for these mechanistic and process-based models (e.g., Beven, 2001). This
approach avoids the difficult question of how the modeler chooses an optimal, single set
of parameters.
Key Findings and Recommendations
Interactions of climate, land, water body, and management factors on nutrient
yields and loads are incredibly complex. As such, management decisions should always
consider multiple models with different modeling approaches. The models discussed in
this report are capable of nitrogen and phosphorus load estimation on the scale of the
MARB, yet each has strengths and weaknesses. Other models and modeling approaches
also exist, and each has inherent strengths, limitations, and value to improving
understanding of and informing decision-making related to the MARB and Gulf hypoxia.
Thus, a diversity of models needs to be developed and applied for load estimation, BMP
evaluation, implementation targeting, and forecasting. Models should provide
117
-------
information about the direction, magnitude, and uncertainty of the impact of current and
planned actions on ecosystem services at the appropriate temporal and spatial scale and at
a resolution and precision that is appropriate to guide these decisions. With an enhanced
modeling toolbox at their disposal, decision makers will need to select the model or
models best suited to answering their questions and guiding their decisions.
The uncertainty of results for each model reflects the uncertainty of the model
structure and algorithms, as well as that propagated by the input data, user
parameterization, calibration process, and other user-defined conditions. Other than the
model itself, each of these factors is influenced by the skill of the model user, making it
difficult to make blanket generalizations about reliability or applicability of the models
discussed.
Adaptive management will be more informative, particularly in the initial years
post implementation, if monitoring data are used to improve models for the next iterative
prediction. This requires that the monitoring be designed, at least in part, for this task.
These monitoring data will also enhance the modeling effort. Rigor of model validation
can be assessed through statistical comparison of calibration data with validation data
provided through monitoring. Greater availability of monitoring data will allow a greater
difference between calibration and validation data sets and provide a more rigorous
model validation. For example, applying a model to a different watershed with different
climate will better test the robustness of the model than a validation using a different
period of climate data within the same watershed.
Adaptive management will require modeling flexibility as well as consideration of
the compatibility between watershed models, economic models, and Gulf of Mexico
hypoxia models. The various models need to have the capability to translate across
temporal and spatial scales and to communicate how factors affecting ecosystem services
are simulated in order to have a smooth interface. For example, watershed model output
of total N at annual scales should be able to interface with a Gulf model requiring daily
conditions of inorganic N. In addition, models need to be developed and used to assess
effects of policy decisions and management practices. Characterization of the degree of
uncertainty would assist interpretation of results and application of these results within an
adaptive management framework. Based on these findings, the SAB Panel offers the
following recommendations.
A diversity of watershed modeling approaches, ranging from simple forecasting
to complex statistical and mechanistic approaches, will be useful for describing
loading and timing of nutrients to the NGOM.
Model selection should depend on the question(s) being asked and the associated
strengths and weaknesses of the various models; for Gulf hypoxia, watershed
models should address issues of management option selection, spatial targeting of
actions, and temporal delivery of nutrient loads to the NGOM.
Water-quality monitoring and the documentation of critical ancillary information
118
-------
(i.e., inputs and management practices) should be designed, at least in part, to
support model use and assessment, and adaptive management.
Uncertainty of model results should be assessed and reported. As much as
possible, all potential sources of error should be explicitly recognized and
discussed in reporting results. Further, confidence bounds should be reported for
all applicable sources (e.g., parameter uncertainty). For those sources for which
formal confidence intervals cannot be computed, sensitivity analysis or another
form of uncertainty analysis should be undertaken and reported.
119
-------
4. Scientific Basis for Goals and Management Options
4.1. Adaptive Management
Adaptive management offers a way to address the pressing need to take steps to
manage for factors affecting hypoxia in the NGOM in the face of uncertainties. The
authors of a recent study undertaken by the National Research Council of the National
Academy of Sciences identified six elements of adaptive management that are directly
relevant to goal setting and research needs (National Research Council, 2004): 1)
resources of concern are clearly defined; 2) conceptual models are developed during
planning and assessment; 3) management questions are formulated as testable hypotheses
to guide inquiry; 4) management actions are treated like experiments that test hypotheses
to answer questions and provide future management guidance; 5) ongoing monitoring
and evaluation is necessary to improve accuracy and completeness of knowledge; and 6)
management actions are revised with new cycles of learning.
Perhaps the most important "take-home" lesson from their work is contained in
the following statement:
Adaptive management does not postpone actions until "enough" is known about a
managed ecosystem (Lee, 1999), but rather is designed to support action in the
face of the limitations of scientific knowledge and the complexities and stochastic
behavior of large ecosystems (Holling, 1978). Adaptive management aims to
enhance scientific knowledge and thereby reduce uncertainties. Such
uncertainties may stem from natural variability and stochastic behavior of
ecosystems and the interpretation of incomplete data (Parma et al., 1998; Regan et
al., 2002), as well as social and economic changes and events (e.g., demographic
shifts, changes in prices and consumer demands) that affect natural resources
systems.
Thus adaptive management provides an appropriate way for decision makers to deal with
the uncertainties inherent in the environmental repercussions of prescribed actions and
their influences on hypoxia.
Adaptive management can be conducted at the several management scales that
occur in the NGOM and MARB. On the basin scale, adaptive management requires
measurements of both nutrient loadings and hypoxia extent (area). Although it will not
be possible to relate these changes to specific changes in the basin, these data will
provide better understanding of the relationships between nutrients and hypoxia. On
smaller scales, specific management actions can be treated as experiments that test
hypotheses, answer questions, and thus provide future management guidance at that scale
(for example, small watersheds).
The adaptive management approach requires that conceptual models are
developed and used and relevant data is collected and analyzed to improve understanding
of the implications of alternative practices (e.g., Ogden et al., 2005). To help illustrate
120
-------
what is meant by a conceptual model, the SAB Panel has developed a diagram that shows
major factors that affect hypoxia in the NGOM (Figure 40). The corresponding
Urban/ suburban
(non-point sources
Societal demands and
national policy
Agricultural subsidies & incentives
Energy policies
Market conditions
Atmospheric sources
and sinks
Farm production choices
Farming systems and practices Agricultural
On-site conditions (slope, soil, etc.) Sources
Weather
Urban/ Industrial
(point sources)
(ream Flow
Hydrology
Climate change
Interannual variability Denitrification
Co-benefits
To Farm
- Lifestyle
To Society
- Recreation
- Rural amenities
- Flood control
- Ecosystem services
(water and air quality)
- Food, fiber, and energy
Hypoxia Impacts
Landscape structure
& function
(drainage)
Fisheries
Recreation
Ecosystem services
Lifestyle
Biogeochemistry
Gulf physics & biology
£ i^ Food web structure
Winds
Shelf circulation
(currents)
Shelf break eddies
Freshwater plume
dispersal
-------
conceptual model would estimate the relative contribution of each influence. Those
estimates could serve as hypotheses of relative effects, and the diagram could illustrate
hypothesized interactions and feedbacks. Such a conceptual model organizes how
adaptive management research is conducted in a framework where the testing of
hypotheses and the new knowledge gained is then used to drive management adaptations,
new hypotheses and the collection of new data on endpoints. Unlike the traditional
model of hypothesis driven research, adaptive management implies coordination with
stakeholders and consideration of the economic and technological limitations on
management. Unlike traditional demonstration projects, adaptive management implies an
understanding that complex problems will require iterative solutions that will only be
possible through generation of new knowledge as successive approximations to problem
solving are attempted.
Successful implementation of the adaptive management process is occurring in
the Grand Canyon (Meretsky et al., 2000) and the Everglades (Sklar et al., 2005). In
addition, steps toward adaptive management are being examined in the Upper Mississippi
River basin (O'Donnell and Galant, in press). That work documents the need for greater
collaboration between scientists and management agencies to plan, design, and monitor
river enhancement programs. Problems exist in setting quantifiable success criteria,
developing appropriate monitoring designs, and disseminating information. The SAB
Panel expects similar difficulties in implementing adaptive management to occur
elsewhere in the MARB.
There needs to be a better understanding of the spatial and temporal aspects of
basin-level responses to management practices and also a focus on other scales at which
response can occur in a more timely fashion. Yet observations of a basin-level response
to practices cannot be expected for some time, which calls for management and
evaluation to be focused on a sub-basin scale. Therefore it is important to obtain
information at a scale where practices can be broadly and appropriately applied and
where results are "meaningful and interpretable." The relevant scale would likely be at
smaller sub-watershed scales, where local water quality and quantity benefits may
become evident more quickly. Furthermore, the demonstration of adaptive management
within a small sub-watershed may enhance practice adoption at other locations. Thus
conceptual models need to be developed for this scale of resolution as well. Focus at the
small-watershed scale will also provide local water quality and quantity benefits. The
results from small watershed studies must be able to be extrapolated to other small
watersheds in the sub-basin and, preferably, the entire MARB, if they are to be useful in
reducing hypoxia in the NGOM.
Experiments that could be applied at small watersheds to help improve
understanding of the effects of different practices have the following characteristics:
Practices applied on the small watersheds should conform to accepted
practice standards or make specific modifications of practices that can be
implemented in new standards;
122
-------
Monitoring should be at appropriate intensities (time and space) to
determine effects of practices on water quality and quantity;
Monitoring should also measure co-benefits, including carbon
sequestration, wildlife habitat, flood control, etc.;
Practices should be applied in suites or systems, and components should
be monitored to determine effects of component practices;
Changes in hydrology and crop productivity must be measured in addition
to changes in water quality. Even at the small-scale, too many studies
have focused just on nutrient concentrations in outflow water and
neglected hydrologic or productivity changes;
All components of the cost of adopting and maintaining these practices
should be measured and monitored. Such costs include direct equipment
and structural costs, yield effects, changes in management time, changes
in risk, and other costs;
These studies should be designed to improve our understanding at local,
medium and broad basin scale. Thus the experiments should be designed
so that they can feed into conceptual models that operate at different
scales; and
Within practical limits, studies should be part of an adaptive management
research strategy for the MARB to optimize the efficiency of research
investments and to assure that results are coordinated, complimentary and
consistent.
Integrated modeling and monitoring play an important role in adaptive
management. The cornerstone of adaptive management is the concept of learning about
the impacts of actions and using that new understanding to guide future actions. Models
can assist that learning by being used to evaluate impacts and uncertainties of proposed
actions, such as targeted practices and locations or proposed policies, on both MARB and
NGOM responses. In addition, monitoring must also be part of an adaptive management
strategy in order to verify that the actions are addressing the stated goals or to test
hypotheses. Monitoring is needed to improve the next generation of models and model
assessments and to eventually verify that projected changes occur.
Adaptive management is also important to building infrastructure and to strategic
planning and policy development of mechanisms of conservation practice
implementation. For example, adaptive management can be used to evaluate if incentive-
based programs are effective at bringing about changes in conservation practice
acceptance and adoption at a local or small watershed level. At a basin level, other
programs might be needed to facilitate adaptation of strategies and policies, and there
must be constant feedback among all vested parties. As the scale of system increases
123
-------
(i.e., from a small watershed to the entire MARB), the complexity of adaptive
management increases dramatically.
Key Findings and Recommendations
Adaptive management can be used at several scales of resolution in the NGOM
and MARB to provide a framework under which management activities can occur while
monitoring and modeling the outcomes in order to provide information so subsequent
management can be improved. Therefore, the SAB Panel offers these recommendations.
An adaptive management approach should be adopted to evaluate the
success of reaching goals and for testing hypotheses (at the relevant scale).
Conceptual models should be developed at appropriate scales of resolution
to frame the adaptive management process in addressing factors affecting
hypoxia in the NGOM.
Both the use of quantitative models and the collection of data should be
conducted within an adaptive management framework and at appropriate
management scales so that the information gained from models and data
are related to the critical questions about managing and understanding the
system.
Management actions should be designed as experiments within the context
of evolving conceptual understanding of the system. The repercussions of
management actions need to be monitored so the outcomes can be used to
enhance learning and thus to improve future management actions.
124
-------
4.2. Setting Targets for Nitrogen and Phosphorus Reduction
To reduce hypoxia in the bottom waters of the NGOM, the Integrated Assessment
set a target that N loading should be reduced by 30% in order to shrink the five-year
running average size of the hypoxic zone to below 5,000 km2 (1,930 mi2) by 2015. This
reduction is significantly less than the three- to five-fold increase in N loading to the Gulf
of Mexico due to human activity during the 20th century, and particularly in the last 30-
to-50 years (Goolsby et al., 2001; Boyer and Howarth, in press). Since the Integrated
Assessment, a number of modeling efforts have provided a better depiction of how the
area of hypoxia may respond to reduced N loading. The three available models were
compared by Scavia et al. (2004), who concluded from these models that the 30%
reduction in N is probably not sufficient to reach the goal of a hypoxia area of 5,000 km2
or less (Scavia et al., 2004). The consensus from these models is that N loads probably
need to be reduced by 40 to 45% to reach the hypoxia reduction goal. In addition, a
number of studies suggest that the consequences of climate change need to be considered,
and this may require an N load reduction on the order of 50 to 60% to meet the original
Integrated Assessment goal for hypoxic area (Justic et al., 2003; Donner and Scavia,
2007). However, predicting the consequences of climate change on nutrient fluxes and
hypoxia remains a very uncertain business (Howarth et al., 2006). The SAB Panel finds
that the consensus of models reported by Scavia et al. (2004) and the new model of
Scavia and Donnelly (in press), which uses the latest available load estimates from the
USGS, supports a target of reducing the five-year running average of N loadings by at
least 45%. This target should be re-assessed as more monitoring data are obtained,
current models are refined, and new models are developed.
Only recently has new evidence emerged for the need to control P inputs as well
as N in the NGOM. Work by Sylvan et al. (2006) has shown P to be the limiting nutrient
during periods of maximum primary production in the near-shore NGOM high
productivity zone. Because previous attention has focused on N, there has been limited
effort to model the effects of P on hypoxic area. Scavia and Donnelly (in press) used the
previously developed and calibrated model (Scavia et al., 2004) to evaluate both the
effects of new USGS load estimates and to assess the potential for P to control hypoxia
dynamics under current and historical conditions. Confirming the results of Sylvan et al.
(2006), Scavia and Donnelly found that P could have become limiting in some areas and
times because of the relative increase in N loads during the 1970s and 1980s. While they
concluded that P did frequently control hypoxia in near field zone of NGOM, they noted
that a P only strategy would likely reduce production in the near field but possibly
increase production in down-field N controlled areas of NGOM. Their work, using the
new USGS load estimates, reinforced the need for a dual nutrient strategy combining a
45% reduction in N with a 40 to 50% reduction in the five-year running average of P
loading. While the far field effects could possibly be reduced through an N only strategy,
they suggested that a prudent approach would be to reduce both N and P, simultaneously.
They also noted that an N and P reduction strategy would not only reduce hypoxia in the
NGOM but would also help to remove P-induced Clean Water Act impairments in the
MARB. Based on this recent modeling work, the SAB Panel finds that a comparable P
125
-------
reduction is needed, again based on 5-year running average fluxes. As with the N target,
this P target should be re-assessed over time as more monitoring information is gained
and new models are developed.
Baseline for Reductions
The CENR report and Scavia et al. (2004) made recommendations on an N
reduction target with reference to average fluxes for 1980 to 1996. These fluxes were
calculated using different methods (see Section 3.1) than in this report, but the N
reduction target proposed recently by Scavia and Donnelly (in press) used a combination
of the newer USGS five-yr LOADEST and composite estimates since 1980. In this
report we only use the five-yr LOADEST results, since the composite estimates are
incomplete; however, they are very similar to each other (again, see Section 3.1).
During the last five years of record, annual water flux to the NGOM has declined
by 5.8%, whereas nitrate-N and TKN have declined even more, leading to a total annual
N reduction of about 21% (Table 9). Considering the original reduction target of a 30%
reduction in total N, it would seem that substantial progress was made beyond the
reduction that would occur from less flow alone. However, the largest reduction was in
TKN, with a large part of this decrease from the Missouri River (discussed in Section
3.1). For the important spring flux of N, there was little reduction in nitrate-N beyond the
reduced water flow (-11 and -12.4 % declines in water and nitrate-N flux, respectively).
Again, TKN was greatly reduced (-31.5%) during spring flows, leading to most of the
decline in total N (-19.2%), beyond the reduction in water flux. This suggests that during
the important high flow spring period (April, May, June), reductions in nitrate-N flux to
the NGOM have not occurred under management systems and programs now in place
since the last report. However, the annual nitrate-N reduction indicates that the tile-
drained corn and soybean systems in the Upper Mississippi and Ohio River subbasins
seem responsive on an annual basis to the recent reductions in net N inputs, as discussed
in Section 3.2. Whether spring nitrate-N loads will respond to these changes in NANI is
uncertain at this time.
126
-------
Table 9: Annual and spring (sum of April, May, June) average flow and N and P fluxes for the MARB for
the 1980 to 1996 reference period compared to the most recent five year period (2001 to 2005). Load
reductions in mass of N or P also shown.
Annual
Water
Nitrate-N
TKN
Total N
Total P
Spring
Water
Nitrate-N
TKN
Total N
Total P
1980 to
1996 flux
2001 to 2005
flux
million m (water) or million
metric tons
692,500
0.96
0.61
1.58
0.137
236,800
0.38
0.21
0.59
0.046
652,500
0.81
0.43
1.24
0.154
210,600
0.33
0.14
0.48
0.050
change
%
-5.8
-15.4
-30.0
-21.1
+12.2
-11.0
-12.4
-31.5
-19.2
+9.5
45%
reduction N
target flux
45% reduction
P target flux
million metric tons
0.53
0.34
0.87
0.21
0.12
0.32
0.075
0.025
For total P flux, both annually and during the spring, there were increases of 12.2
and 9.5%, respectively. It is not clear why total P fluxes are increasing (with
corresponding smaller water fluxes), and the result suggests that the reduction target of
45%, relative to the 1980 to 1996 period, is close to 50% for the 2001 to 2005 period.
Likewise, the 45% N load reduction target, relative to the 1980 to 1996 period, is
equivalent to a 30% reduction relative to the 2001 to 2005 period. Fertilizer P
consumption in the MARB has been relatively constant since about 1984 and is similar to
consumption during 1970-to-1975 period. Net P inputs to the MARB have declined since
the 1970s and have been predominantly negative since the mid-1990s (see Section 3.2
and Figure 34). Table 9 also indicates N and P reduction recommendations in units of
mass with reduction targets of 45% N and 45% P, assuming the reduction were spread
across all forms of N and P, that occur both annually and during the spring.
While the SAB Panel finds that both N and P reductions are warranted, additional
modeling and dose response research is needed to refine the reduction targets,
particularly for P loading. Scavia and Donnelly (in press) presented the only model
results that relate P loads to hypoxia in the NGOM. Further, there are no experimental
data relating phytoplankton responses there to different levels of P. Ideally, targets for
reducing P based on water quality should have greater model support, and should
consider dose response relationships for P responses by the in situ phytoplankton
communities. In the meantime, the response of the Gulf system to a specific amount of P
127
-------
reduction remains uncertain and must await the formulation of new models and dose
response relationships for the receiving waters. Water quality models aimed at
evaluating the effects of these reductions will also rely on this information. Dose
response relationships should be developed using in situ bioassays designed to "ask the
phytoplankton" what the response relationships and bloom thresholds are. These
bioassay experiments are a logical follow-up to the work of Sylvan et al. (2006), which
has shown P to be the limiting nutrient during periods of maximum primary production in
the near-shore NGOM high productivity zone. Bioassays are needed on a seasonal basis,
where the effects of hydrologic variability and changing N:P input (loading) ratios on
primary production, phytoplankton community composition, and biogeochemical and
trophic fate can be evaluated.
In Section 4.5.8 on Most Effective Actions for Industrial and Municipal Sources,
the SAB Panel provides some ballpark estimates of possible N and P reductions from
upgrading major municipal wastewater treatment plants. The SAB Panel's example
calculations demonstrate that sewage treatment plant upgrades to achieve total N
concentration limits of 3 mg/L and total P concentrations of 0.3 mg/L could create
reductions in total annual N flux to the Gulf by about 10% and the total spring N flux by
about 6%. Upgrading to achieve P concentrations of 0.3 mg/L would create reductions in
P fluxes from sewage treatment plants from 41,000 metric tons P/yr (45,000 ton P/yr) to
10,500 metric tons P/yr (11,600 ton P.yr) or about a 75% reduction in annual flux from
sewage treatment plants to the MARB. These reductions, in turn, would translate into
reductions of total annual P flux to the Gulf by about 20% and the total spring P flux by
about 15%. If further investigation and data collection confirms the SAB Panel's
calculations, upgrades to major wastewater treatment plants in the MARB could
accomplish nearly half of the Panel's recommended P reduction targets. This would
represent very significant progress for both improving water quality in the MARB and
reducing hypoxia in the NGOM.
Despite the need for additional model and bioassay work, the proposed target of a
45% reduction in annual P load should be used in an adaptive management framework to
allow development of strategies that optimize both N and P reductions while more
knowledge is acquired on P reduction impacts on near-field hypoxia. Unlike N, the P
reduction strategy will help address water quality impairments in the MARB. Given the
evidence that both N and P should be reduced in the NGOM, setting a goal for P
reduction should not await the development of new models and availability of new
experimental data. Enough information exists now to set a goal in an adaptive
management context beginning with the P reductions that are already feasible given
existing technologies and options.
In 2000, EPA recommended nutrient criteria to States and Tribes for use in
establishing their water quality standards consistent with Section 303(c) of the Clean
Water Act (CWA) (USEPA, 2000). EPA's recommended criteria represent an estimated
"reference condition," and it is assumed that the reference condition concentration would
protect all designated uses (including the most protected uses, such as high quality
fisheries, sensitive aquatic life, etc.). The SAB Panel asked EPA for a comparison of the
128
-------
SAB Panel's recommended 45% reductions for TN and TP flux to the reductions in
nutrient levels that would correspond to EPA's ecoregional nutrient criteria for reference
conditions (U.S. EPA, 2006b). This comparison is provided in Appendix C: EPA's
Guidance on Nutrient Criteria. Although a number of assumptions were required to
make this comparison (see the caveats in Appendix C), EPA's preliminary analysis
suggests that the SAB Panel's recommended targets for reducing TN and TP are, for most
regions, not likely to be as stringent as would be obtained if states adopted EPA's
recommended reference condition values into state water quality standards for all waters.
This comparison should not be interpreted as the SAB Panel's endorsement of EPA's
recommended nutrient criteria but rather an emphasis on the need to consider both within
basin nutrient criteria and NGOM load reduction goals. Numeric nutrient standards being
developed by the states of the MARB will almost certainly be concentration rather than
load based and may be most stringent during warmer, lower flow periods when absolute
loads can be relatively low but when local waters are most frequently impaired by excess
nutrient levels. It will be important for EPA and other agencies to evaluate and, if
necessary, reconcile within-basin water-quality standards with load-reduction goals for
the NGOM. Strategies are needed for integrating standards throughout the MARB to
better manage hypoxia as well as local water quality.
A mechanism in the Clean Water Act for addressing water quality impairments is
the development of Total Maximum Daily Loads (TMDLs), though it is important to note
that the focus of TMDL development is identification of the source and causes of water
quality impairment, rather than on implementation of change for improving water quality.
Under Section 303(d) of the Clean Water Act, states, territories, and authorized tribes are
required to develop lists of impaired waters (i.e., waters that have not met water quality
standards). The law requires that the appropriate jurisdictions develop TMDLs for these
impaired waters. The TMDLs specify the maximum amounts of pollutants that
waterbodies can receive and still meet water quality standards. In addition, TMDLs
allocate pollutant loadings among point and non-point sources.
The status of nutrient criteria and TMDL development along the Mississippi
River has been reviewed by the National Academy of Sciences (National Academy of
Sciences, 2007). The National Academy of Sciences notes that none of the 10
Mississippi River mainstem states currently have numeric criteria for nitrogen or
phosphorus applicable to the River, and that without such standards, there is little
prospect of significantly reducing or eliminating hypoxia in the Gulf of Mexico. The
National Academy of Sciences also describes how the process of developing numeric
nutrient criteria and TMDLs for the Mississippi River could lead to water quality
improvements in the Gulf of Mexico. NAS suggests that through such a process, EPA
could adopt the necessary numerical nutrient criteria for the terminus of the Mississippi
River and waters of the northern Gulf of Mexico. Maximum nutrient loads could be
assigned to each state and the loads could be translated into water quality criteria. Each
state would then be required to develop a TMDL for waters that failed to meet the
applicable criteria, and a coordinated effort could be undertaken to reduce point and non-
point source loads to meet allocations established by the TMDLs. Thus, the NAS report
identifies an approach through existing legislation (the Clean Water Act) that could be
129
-------
used to redress Gulf Hypoxia, but the SAB stresses that a great many steps exist between
calling for "a coordinated effort" and implementing the full set of actions that must be
undertaken for water quality to actually improve in the Gulf.
Key Findings and Recommendations
Based on findings since the Integrated Assessment, a N reduction target of
greater than 30% will be needed to reduce the hypoxic area to 5000 km2 (1,930 mi2).
Recent research indicates N reductions of at least 45% will be needed to achieve the
target in most years and reductions may have to exceed 50% due to effects of climate
change. Research by several investigators provides evidence that P may limit primary
production in the river outflow, near-field areas of the Gulf. Based on new research
with the same model used to establish the N target, reductions in P loads of 40 - 50 %
are needed to reduce P-controlled hypoxia in the near-field areas of NGOM. P
reductions in the MARB will not only benefit the NGOM but will also help to address P
impairments in the MARB. Based on these findings, the SAB Panel offers the following
recommendations.
To reduce the size of the hypoxic zone, the total N flux to the NGOM from the
combined Mississippi and Atchafalaya Rivers must be reduced by at least 45%
from 1980 to 1996 average fluxes, to no more than 790,000 metric tonne N/y
(870,000 ton/yr), and 290,000 metric tonne N (320,000 ton) during the spring
(April, May, June), both on a five-year running average.
To reduce the size of the hypoxic zone, commensurate reductions in P are
needed. The total P flux to the NGOM from the combined Mississippi and
Atchafalaya Rivers should be reduced by at least 45% from 1980 to 1996
average fluxes, to no more than 68,000 metric tonne P/yr (75,000 ton P/yr) on a
five-year running average.
130
-------
4.3. Protecting Water Quality and Social Welfare in the Basin
The SAB Panel has been asked whether social welfare can be protected while
reducing hypoxia and improving water quality in the Basin. To thoroughly answer this
question would require quantification of the full costs of all activities undertaken to
reduce the necessary nutrient loading into the Gulf (from agricultural sources, point
sources, air deposition, etc.) and the full benefits accruing from those activities. The
benefits would include the direct benefits of reducing the size of the hypoxic zone
(commercial fishery effects, recreational fishery gains, the value placed on preserving
intact ecosystems, biodiversity, etc.) and the "co-benefits" (such as improved local water
quality, increased wildlife habitat, flood control, aesthetic values, etc.).
Since the costs, benefits, and co-benefits will depend on the extent of coverage
and specific locations that control options are located, a complete answer to the question
would require knowing the details of how such nutrient reductions would occur. For
example, if these reductions are to be achieved entirely through restoration of wetlands
and tighter municipal source controls, it would be necessary to know where the wetlands
would be located and where the point source reductions would occur in order to estimate
their costs and their co-benefits. In contrast, an entirely different set of co-benefits and
costs would likely result from relying on a broader array of control options that also
included nutrient management, increased perennials, riparian buffers, drainage
management, and reductions in air deposition. Further, the exact policy approach (e.g.,
expanded EQIP funding, mandates, or taxes) would need to be specified if estimates of
the incidence of the costs are to be estimated (i.e., whether the costs would ultimately be
borne by taxpayers, consumers, or by farmers and landowners).
To date, no set of models and/or studies have been undertaken that address all of
the necessary components on a basin-wide scale to estimate the effects on social welfare.
However, a number of studies, beginning with the research in the Integrated Assessment.,
have been done that address substantial components of this question. More complete
efforts at quantifying the control costs than the benefits have been undertaken, though
there remains a need for much more work on both sides of the equation. Integrated
models at multiple levels and scales are needed to support this effort. The existing
research focuses largely on agricultural non-point source control. This section
summarizes findings from the limited set of large-scale economic-watershed models of
agricultural non-point sources that have been applied to date.
Assessment and review of the cost estimates from the CENR Integrated Assessment
Doering et al. (1999) in the Integrated Assessment undertook an ambitious cost-
effectiveness analysis of several policy approaches to reach the N loss reduction goal of
20% established as part of the Integrated Assessment. The central modeling system they
used was the U.S. Mathematical Programming (USMP) model, which represents the
agricultural sector in 45 production regions throughout the United States with 10 crops,
16 animal products, retail and processed products, and a range of domestic and
131
-------
international supply and demand relationships. Management practices include crop
rotations, five tillage options, and varying fertilizer rates.
The environmental effects of various management practices and land uses in
USMP are predicted by the EPIC model (the Environment Productivity Impact
Calculator). USMP uses EPIC to predict changes in N loss, P loss, and sediment loss at
the edge of the field from changes in land use and conservation practices. Donner et al.
(1999) chose a 20% N loss reduction goal as "the best combination of sizable nitrogen-
loss reductions and acceptable economic costs" (Doering et al., (1999) page 37). The
remainder of their analyses focused on the evaluation of several policies that might
achieve this environmental goal. Some key predictions from the modeling system
include:
A 20% reduction in fertilizer N application rates would result in the reduction
of edge-of-field N loss by about 11%. In contrast, a 45% reduction mandate
and fertilizer tax set to achieve a 45% reduction is predicted to result in the
target goal of N loss reduction of about 20%. The less than proportional
reduction in N loss coming from reduced fertilization in this modeling system
is a result of predicted changes in acreage resulting from the feedback effect
of price changes. Specifically, higher crop prices due to lower yields from the
reduced fertilization rates induce more acreage planted to the fertilized crop,
thereby partially offsetting the reduction in N. Whether the magnitude of the
yield effects embedded in these models is accurate is an important question.
For further discussion of this issue, see Section 4.5.6.
Some 7.29 million hectares (18 million acres) of wetland restoration would
achieve the 20% reduction in N loss goal at a cost of over $30 billion.
Restoration of 10.9 million hectares (27 million acres) of riparian buffers was
estimated to cost over $40 billion and generated relatively small reductions in
N losses, suggesting that this strategy is not cost-effective for hypoxic zone
control. In light of current evidence that phosphorous is also of concern, this
result should be reconsidered as there is significant evidence that buffers can
be quite effective in holding sediment and phosphorous in field.
A "mixed policy" with a 2.02 million hectares (5 million acre) wetland
restoration program in conjunction with a 20% fertilizer reduction is more
cost-effective than most of the previous approaches, but the 45% reduction in
fertilizer is more cost-effective yet.
The introduction of point-non-point source trading across the basin where the
cap applies only to point sources will not achieve the 20% N loss reduction
due to the relatively small magnitude of N contribution from point sources.
Even with a stringent standard on point sources, only about 5% of the needed
reductions occur.
132
-------
These policies are likely to produce large "co-benefits" (i.e., other
environmental benefits occurring within the basin and on-farm productivity
benefits not immediately captured in the current profitability resulting from
the policies). For example, the authors estimate that restoration of 405,000
hectares (1,000,000 acres) of wetlands would yield total benefits in the basin
that exceed the costs, even without considering any benefits of hypoxia
reduction.
Cost estimates used for the Integrated Assessment for a 20% reduction in N
discharge coming from agricultural non-point sources range from $15 billion to $30
billion; however these estimates suffer from a number of shortcomings including
consideration of only a few options for reducing nutrient discharge and limited targeting.
More inclusive assessments with better targeting of options to locations where they are
most appropriate may reduce these costs.
In follow up research, some of the same study coauthors (Ribaudo et al., 2001)
compare nitrogen reduction methods with wetland restoration and low and high levels of
N loss reduction. They find that nutrient management is more cost-effective at low levels
of N loss reduction while wetlands restoration is more cost-effective at high levels. Table
10 and Table 11 (listed at the end of this discussion) briefly summarize the key
components of these studies and the other large-scale studies that are reviewed in the
following discussion.
Due to limits on the understanding of the economics and natural science at the
time, the work in the Integrated Assessment and its follow up is based on assumptions
that, in light of more recent research and availability of data, could be improved upon in
future work. The USMP model represents a wide variety of agricultural raw inputs and
intermediate products at a relatively aggregate scale. However it does not contain
detailed description of land use, soil characteristics, yields, etc. at the individual field
and/or sub basin scale. This inability to target finer scales could result in overstating the
costs of meeting a particular reduction goal because significant cost savings can accrue
from targeting land-management strategies.
The Integrated Assessment assumed a one-to-one relationship between the
reduction in edge-of-field nitrogen loss and reduced loadings to waterways without
incorporating the geographic differences in movement of N from the field of origination
to the Gulf. Whether this shortcoming over- or under-states the costs is an empirical
question, but the results coming from a model that explicitly incorporates the fate and
transport of nutrients and sediment might suggest very different results concerning the
cost-effectiveness.
Other large scale integrated economic and biophysical models for agricultural non-point
sources
Since completion of the Integrated Assessment, several basin-wide studies have
evaluated policies that might reduce Gulf hypoxia and/or have effects on other
133
-------
environmental amenities that could be considered co-benefits (including carbon
sequestration and upstream, local water quality indicators). The models can be divided
into those that use the USMP modeling framework and those based on econometric
estimates of behavioral response to economic drivers.
Booth and Campbell (2007) used a regression model to estimate the cost of
reducing N losses when targeting conservation dollars to those areas with the highest
proportion of fertilizer use. They modeled a hypothetical case in which conservation
enrollment rises in direct proportion to the nonlinear rise in nitrate flux that occurs as
fertilization intensity increases. The result was an increase in the amount of land in the
high-fertilizer watersheds enrolled in the Conservation Reserve Program by 2.7 million
hectares (6.67 million acres) (a 29% increase over 2003 CRP levels) at a cost of $448
million. Booth and Campbell (2007) describe this as a 6.2% increase over the combined
cost of commodity support and conservation programs. They account for the drop in
commodity support spending that would accompany the enrollment of commodity-
farmed land in the CRP. Booth and Campbell (2007) do not specify the percentage
reduction in nitrate loading that would result from this scenario.
Wu et al. (2004) and Wu and Tanaka (2005) developed an econometric model of
crop choice and tillage choice using the National Resources Inventory for the upper
Mississippi River basin. They estimated the probability of adopting conservation tillage
and crop choice based on a variety of physical and economic variables including land
quality, slope, climate conditions, and profits. They used over 40,000 crop land points
observed for 16 years, although only a subset of the observations were used for model
fitting. These adoption models then simulate adoption profiles under alternative policies.
Finally, the environmental effects of the policies are predicted with a biophysical model.
Wu et al. (2004) used a set of environmental production functions estimated via a meta-
modeling approach (Wu and Babcock, 1999), based on data generated from the EPIC
model. They found that crop rotations are not a cost-effective strategy to N reduction.
Wu and Tanaka (2005) used the SWAT model to predict water quality changes
from the policies. They considered the same two policies as Wu et al. (2004), as well as
a policy that would increase the amount of land set-aside in a Conservation Reserve-type
program and a fertilizer tax at various rates. They found a fertilizer tax to be the most
cost-effective of policies they considered.
Kling et al. (2006) employed a similar econometric modeling approach. Like Wu
et al. (2004), they used the National Resource Inventory data to link the cost data with the
SWAT model. They estimated the costs and water quality benefits of implementing a set
of conservation practices associated with implementation rules based on distances to a
waterway, slope, and erodibility indices. The conservation practices assessed include
grassed waterways, nitrogen management, terraces, buffers, land retirement and
conservation tillage. They estimated that this placement of conservation practices on the
landscape would cost over $800 million annually (or roughly $16 billion if viewed as a
lump sum cost assuming a 5% rate of discount) and would achieve a 22% reduction in N
loadings into the upper Mississippi River basin at Grafton 111. Within the UMRB, they
134
-------
estimated a 40-66% reduction in sediment loads, a 6-47% reduction in P loads, and a 9-
29% reduction in N loads. These estimates (like those from all of the studies reviewed
here) are likely to be very sensitive to the set of conservation practices included and the
specific scenarios studied.
Greenhalgh and Sauer (2003) used the USMP, augmented in two important ways:
1) they configured the model by watersheds and added information on municipal waste
water treatment plants and 2) they included "attenuation" coefficients derived from the
SPARROW model to reflect the transport component of N flows between watersheds.
The focus of their work was on policy options for hypoxia that also contribute to
greenhouse gas reductions. The policies they considered include N trading between point
and non-point sources, GHG trading assuming external carbon prices of $5/ton and
$14/ton, N trading with additional payments for GHG emission reductions, an N fertilizer
tax, a subsidy to farmers willing to shift from conventional to conservation tillage, and an
expansion of the CRP program to 16 million ha (40 million ac) nationwide. Of the
policies evaluated, none achieved the 20% reduction goal of the Doering et al. (1999)
analysis. The largest reductions were achieved in their simulation of point/non-point
source trading with a stringent N standard. The most cost-effective policies were also the
trading programs.
Ribaudo et al. (2005) also considered the possibility of N trading between point
and non-point sources using the USMP model. They found that trading has significant
potential to reduce costs relative to a requirement that wastewater treatment plants be
required to install stringent nutrient removal technology.2
These studies shed light on the costs of addressing the hypoxia problem from
conservation practices in the agricultural sector, and the way these costs may vary
depending on the policy instrument chosen (trading program, conservation payment, tax,
etc.). These studies also directly bear on the question of how much it will cost to address
local water quality in the MARB. However, as noted above, shortcomings of the
integrated models have prevented assessment of many policies as well as conservation
practices and sinks. None of the models include point source and non-point source
control options. With the exception of Booth and Campbell (2007), most models have
not adequately addressed the cost savings associated with targeting. Nonetheless, results
to date suggest that there is large variability in the costs of alternative policies. The issue
of who pays these costs may also be important to consider since the incidence (who must
pay the costs) may differ dramatically across policies. A notable example is a fertilizer
tax, which has the same social costs as a restriction but which may have a much higher
incidence on farmers.
Improved estimates of the costs of installing and maintaining conservation
practices could be generated with the current suite of models by considering alternative
sets of conservation practices. This can be accomplished using the following steps: 1)
identifying conservation practices that are most likely to be effective in reducing nutrients
2 It is important to recognize that these studies assume a perfectly efficient water quality trading program
with no trading restrictions; current water quality trading programs do not match the modeled system.
135
-------
important for hypoxia, and 2) identifying scenarios that place these conservation practices
on the landscape. These scenarios could be based on rules of thumb (identifying for
example a particular conservation practice to be used on cropland with specific climate
and soil characteristics), algorithms for optimal placement to minimize costs, multiple
goals such as maximizing in basin co-benefits or income support, or policy relevant
methods such as the use of an environmental benefits index, etc.; and 3) computing cost
estimates from economic models and water quality changes from watershed models.
Research Assessing the Basinwide Co-Benefits
As noted above, many of the same practices that could contribute to reductions in
the hypoxic zone could also have significant effects on local water quality, carbon
sequestration, wildlife habitat, flood control, and other ecosystem services. The physical
co-benefits of many conservation practices and sinks are described in Section 4.5.10. On
the basin-wide scale, there are a few studies that provide physical measures of one or
more co-benefits that are associated with implementation of conservation practices that
would address hypoxia, particularly related to carbon sequestration and water quality (see
for example, Feng, et al. (2005), Lewandrowski et al. (2004), Greenhalgh and Sauer
(2003)). These studies consistently indicated that significant co-benefits are present, but
these estimates are not monetized and are reported in physical units. Further, the policies
analyzed are not focused on hypoxia reduction.
Thus, the work reported in the Integrated Assessment remains the most complete
coverage to date of the potential value to MARB residents of the water quality and other
co-benefits. The estimates provided there suggested that the monetized value of the
benefits to the basin were larger than the costs based primarily on benefit estimates of the
value of erosion control and wetlands restoration. A more complete accounting of these
benefits could be developed using benefits transfer techniques, although there are many
ecosystem services for which currently accepted methods are not likely to adequately
fully capture the value of the benefits. But, in any case, because the Integrated
Assessment was not able to quantify all co-benefits, total co-benefits within the basin
would almost certainly be larger than those estimated.
Due to the incredible complexity in this system, as well as limits in data,
modeling and research, definitive statements on social welfare are not possible. For
example, there is incomplete information on the costs of farm-level actions to reduce
edge-of-field nutrient losses. There is even greater uncertainty in quantifying the
effectiveness of farm-level nutrient control actions in reducing watershed-level nutrient
flux and about the relationship between watershed-level nutrient flux and the spatial and
temporal dimensions of the hypoxic zone. These uncertainties are further exacerbated by
the possibility of regime shift in the Gulf of Mexico, whereby the system could become
more susceptible to hypoxia following the initial occurrences. If regime shift is a factor,
then historic data on the relationship between nutrient flux and the size of the hypoxic
zone does not provide guidance on the decrease in nutrients required to achieve a given
reduction in the size of the hypoxic zone. Hence, a return to historic lower levels of
136
-------
nutrient fluxes might not be adequate to return to a corresponding size of the hypoxic
zone.
There are many sources of uncertainty in the economic, hydrologic, and Gulf
systems that make it difficult to render definitive conclusions about social welfare
Indeed, it is precisely because of these many uncertainties and need for additional
research that we recommend an approach based on an adaptive management strategy that
aims to move in a "directionally correct" fashion, rather focusing on achieving a precise
outcome.
While we cannot definitely say that we can achieve the 5,000 km2 (1,930 mi2)
goal while maintaining social welfare, there is evidence that suggests it is feasible to do
so. First, and perhaps most importantly, welfare losses in the Basin will be at least
partially or even totally offset by co-benefits of nutrient reduction actions. For example,
if wetlands restoration is used to control nutrient flux, it will result in improvements in
wildlife habitat and local water quality, both of which will improve welfare in the Basin.
Findings from the Doering et al. (1999) assessment point out that the benefits accruing
locally from wetlands restoration might well exceed the costs, even without any Gulf
hypoxia reductions. Similar estimates are reported in Hey et al. (2004) for substantial
restoration of wetlands in flood plains (see Section 4.4.2). Management actions that
reduce farm-level nutrient losses may lead to better local water quality, thereby
improving welfare for affected residents within the Basin. If management actions are
undertaken to control air emissions, thereby reducing atmospheric deposition of nitrogen,
it will result in improvements in air quality, reduction in acid precipitation, lower
emissions of greenhouse gasses, etc. Thus, co-benefits within the Basin will at least
partially and perhaps fully offset welfare losses associated with the costs of implementing
management actions. And in the longer term, a transition from corn to perennial crops
could benefit farmers and other Basin residents. Thus, there may be larger scale
transitions in the agronomic system that provides opportunities to reduce nutrient flux
while maintaining welfare in the Basin.
A second reason for optimism is that cost-effective approaches, such as targeting
low cost sources and using emissions trading, have not yet been applied. These
approaches have the potential to reduce the costs of nutrient control, possibly
considerably, thereby reducing the burden of complying with the goal. Thus, there may
be opportunities to control the cost of nutrient reduction.
137
-------
Table 10: Summary of Study features of Basin wide Integrated Economic-Biophysical Models
Authors
Doering et al.
(1999)
Ribaudo et al.
(2001)
Greenhalgh and
Sauer (2003)
Wu et al. (2004)
Ribaudo et al.
(2005)
Wu and Tanaka
(2005)
Kling et al. (2006)
Study Region
Entire U.S.
with policies
simulated in
Mississippi
River basin
Entire U.S.
with policies
simulated in
Mississippi
River basin
Entire U.S.
with policies
simulated in
Mississippi
River basin
upper
Mississippi
River basin
Entire U.S.
with policies
simulated in
Mississippi
River basin
upper
Mississippi
River basin
upper
Mississippi
River basin
Models used
USMP and EPIC
USMP and EPIC
USMP and EPIC
with Sparrow
derived transport
coefficients
Econometric model
and EPIC based
metamodels
USMP and EPIC
Econometric model
and SWAT
Econometric model
and SWAT
Environmental
measures
N, P, and sediment in
the MARB ( not
delivered to the
NGOM)
N, P, and sediment in
the MARB ( not
delivered to the
NGOM)
N delivered to the
Gulf, greenhouse gas
emissions, P and N,
soil erosion in the
MARB
N leaching, N runoff,
wind erosion, and
water erosion in
UMRB
N in MARB
N delivered to the
NGOM
N, P, and sediment in
UMRB and N
delivery to the
NGOM
Comments
Original CENR
study
Extension of
CENR study
Study focuses on
co-benefits of
policies
Finer spatial detail
than USMP but no
price feedbacks
Follow up to
original CENR
study
Finer spatial detail
than USMP but no
price feedbacks
Finer spatial, but
no price feedbacks
138
-------
Table 11: Summary of Policies and Findings from Integrated Economic-Biophysical Models
Study
Policies/Actions Evaluated
Key Findings"
Doering et al.
(1999)
1. Fertilizer reduction
mandates/fertilizer taxes
2. Wetland restoration
3. Riparian Buffer
4. Mixed Policy (wetlands and
fertilizer reduction)
5. Water Quality Trading
1. Cost effective approaches exist to reducing nitrogen
losses in the 20% range
2.Wetland-based strategies are more expensive than
fertilizer reduction
3. Buffers are not cost-effective for reducing N losses
4. A combination of 5 million acre wetland restoration
with 20% fertilizer reduction is most cost-effective
5. These cost-effectiveness measures do not take into
account the transport of nitrogen to the Gulf and the
rankings of preferred alternatives could change
Ribaudo et al.
(2001)
1. Reduce fertilizer rates
2. Wetland restoration
1. below 26% reduction in N losses, fertilizer
reduction/management is most cost-effective
2. Above this rate, wetland restoration is most cost-
effective
Greenhalgh
and Sauer
(2003)
1. N trading between point and non-
point sources,
2. greenhouse gas trading
3. N trading with additional
payments for GHG reduction
4. N fertilizer tax,
5. conservation tillage payment
6. expansion of CRP to 40 million
acres nationwide
1. Nutrient trading (point/non point) with tighter
discharge limits could reduce nitrogen reach the
NGOMby 11% annually
2. Nutrient and greenhouse gas trading were the lowest
cost policies, but nutrient trading was the most cost-
effective
3. The co-benefits of these policies in terms of
greenhouse gas reductions, phosphorous, and sediment
can be significant
j,
Crop rotations not a cost-effective strategy for N
reduction
Wu et al.
(2004)
1. Conservation payments for
conservation tillage and
2crop rotations
Wuand
Tanaka (2005)
1. Fertilizer tax
2. Payments for conservation tillage
3. Payments for land retirement
4. Payments for crop rotations
Fertilizer tax is the most cost-effective of policies
considered
Booth and
Campbell
(2007)
Targeting CRP to watersheds with
the greater proportion of fertilizer
used. Hence CRP rises in direct
proportion to fertilizer/cropping
intensity.
Targeting CRP and enrolling an additional 2.7 million
hectares in those areas with the greatest fertilizer
intensity would increase annual agricultural subsidies to
the MARB by 6.2% (over the combined commodity
support and conservation funding in 2003).
Ribaudo et al.
(2005)
N trading between point and non-
point sources
Trading between waste water treatment plants and non-
point/agricultural sources to meet the reductions
achievable by installing advance nutrient removal
technology at treatment plants would have large welfare
gains
Kling et al.
(2006)
Implementation of a set of targeted
conservation practices including
conservation tillage, land
retirement, terraces, contouring,
grassed waterways, and reduce
fertilization rate on corn
1. Annual costs of $800 million per is predicted to
achieve 22% reduction in N loading to the NGOM,
2. within the UMRB sediments loads were reduced by
40-66%, total P was reduced by 6-47% and N by 9-29%
a) Doering et al. (1999) also conclude that fertilizer restrictions are more cost-effective than a fertilizer tax, but they
apparently incorrectly count tax revenues as a cost rather than a transfer. The restrictions and tax have the same welfare
effects, though different distributional implications.
139
-------
Principles of Landscape Design
Another perspective for protecting social welfare can be drawn from the
principles of landscape design. A landscape perspective involves broad-scale
consideration of how decisions affect resources, particularly in the long run. Guidelines
have been proposed as a way to facilitate land managers considering the ecological
ramifications of land-use decisions (Dale et al. 2000). These guidelines are meant to be
flexible and to apply to diverse land-use situations, yet require that decisions be made
within an appropriate spatial and temporal context. These landscape design guidelines
can serve as a checklist of factors to be considered in making decisions that relate to
implications for hypoxia in the Gulf.
Examine the impacts of local decisions in a regional context. The spatial array of
habitats and ecosystems shapes local conditions and responses (e.g., Patterson, 1987;
Risser, 1985), and local changes can have broad-scale impacts over the landscape.
Hypoxia is a classic example of such impacts, for fertilizer applications in the
Midwestern states can affect oxygen conditions in the Gulf of Mexico. This
guideline notes that it is critical to examine both the constraints placed on a location
by the regional conditions and the implications of decisions for the larger area.
Therefore, it is critical to identify the surrounding region that is likely to affect and
be affected by the decision and examine how adjoining jurisdictions are using and
managing their lands. Forman (1995) suggests that land-use planning should first
determine nature's arrangement of landscape elements and land cover and then
consider optimal spatial arrangements and existing human uses. Following this
initial step, he suggests that the desired landscape mosaic be planned first for water
and biodiversity; then for cultivation, grazing, and wood products; then for sewage
and other wastes; and finally for homes and industry. Of course, planning under
pristine conditions is typically not possible. Rather, the extant state of development
of the region generally constrains opportunities for land management.
Plan for long-term change and unexpected events. Impacts of decisions can, and
often do, vary over time as a result of delayed and cumulative effects. Future options
are often constrained by the decisions made today as well as by those made in the
past. For example, areas that are urbanized are unlikely to be available for any other
land uses because urbanization locks in a pattern on the landscape that is hard to
reverse. Thus, management actions should be implemented with some consideration
as to the physical, biological, aesthetic or economic constraints that are placed on
future uses of resources. External effects can extend beyond the boundaries of
individual ownership and thus have the potential to affect surrounding owners.
Planning for the long term also requires consideration of the potential for unexpected
events, such as variations in temperature or precipitation patterns or disturbances.
Long-term planning must also recognize that one cannot simply extrapolate
historical land-use impacts forward to predict future consequences of land use. The
transitions of land from one use or cover type to another often are not stable over
time because of changes in demographics, public policy, market economies, and
technological and ecological factors.
140
-------
Preserve rare landscape elements, critical habitats, and associated species. This
guideline implies a hierarchy of flexibility, and it implicitly recognizes ecological
constraints as the primary determinants in this hierarchy. For example, a viable
housing site is much more flexible in placement than an agricultural area or a
wetland dedicated to improving water quality and sustaining wildlife. Optimizing
concurrently for several objectives requires that planners recognize lower site
flexibility of some uses than others. However, given that most situations involve
existing land uses and built structures, this guideline calls for examining local
decisions within the regional context of ecological concerns as well as in relation to
the social, economic, and political perspectives that are typically considered.
Avoid land uses that deplete natural resources over a broad area. Depletion of
natural resources disrupts natural processes in ways that often are irreversible over
long periods of time. The loss of soil via erosion that can occur during agriculture
and the loss of wetlands and their associated ecological processes and species are
two examples. This guideline requires the determination of resources at risk, which
is an ongoing process as the abundance and distribution of resources change. This
guideline also calls for the deliberation of ways to avoid actions that would
jeopardize natural resources and recognition that some land actions are inappropriate
in a particular setting or time, and they should be avoided.
Avoid or compensate for effects of land use on ecological processes. Negative
impacts of land use practices might be avoided or mitigated by some forethought.
To do so, potential impacts need to be examined at the appropriate scale. At a fine
scale, farm practices may interrupt ecoregional processes. At a broad scale, patterns
of watershed processes may be altered, for example, by changing drainage patterns
as part of the land use. Therefore, how proposed actions might affect other systems
(or lands) should be examined. For example, human uses of the land should avoid
uses that might have a negative impact on other systems; at the very least, ways to
compensate for those anticipated effects should be determined. It is useful to look
for opportunities to design land use to benefit or enhance the ecological attributes of
a region.
Implement land-use and -managementpractices that are compatible with the
natural potential of the area. Local physical and biotic conditions affect ecological
processes. Therefore, the natural potential for productivity and for nutrient and
water cycling partially determine the appropriate land-use and management practices
for a site. Land-use practices that fall within these limits are usually cost-effective in
terms of human resources and future costs caused by unwarranted changes on the
land. Nevertheless, supplementing the natural resources of an area by adding
nutrients through fertilization or water via irrigation is common. Even with such
supplements, however, cost-effective management recognizes natural limitations of a
site. Implementing land-use and -management practices that are compatible with the
natural potential of the area requires that land managers understand a site's potential.
For example, land-management practices such as no-till farming reduce soil erosion
141
-------
or mitigate other resource losses. Often, however, land uses ignore site limitations or
externalize site potential. For example, building shopping malls on prime agriculture
land does not make the best use of the site potential. Nevertheless, land products are
limited by the natural potential of the site.
Together these guidelines form the basis of a landscape design perspective that
should improve the ability to understand and manage the complex system that is affecting
hypoxia in the Gulf of Mexico.
Key Findings and Recommendations
The large-scale policy models that have been developed to date each have
strengths and weaknesses. None of the models adequately address the full range of
management options (wetlands, buffers, nutrient management, etc.) or the full range of
policy instruments in a geographically explicit manner. In fact, no single model is likely
to be adequate for the full range of decision making that adaptive management of this
complex system requires. Moreover, the focus of prior analyses was on cost-effective
strategies to reduce N loss, which was the concern at the time. Given that the best current
science suggests P is also a limiting nutrient in the Gulf, it is important to seek cost-
effective practices that affect both N and P while considering possible tradeoffs between
them.
The CENR study remains the only research effort to consider the overall costs and
benefits of controlling hypoxia in the Gulf of Mexico. The study suffers from a number
of shortcomings (many control options and sources of nutrients were not considered, the
hydrology of fate and transport was ignored, and no sensitivity analysis concerning key
assumptions was undertaken to name a few). The evidence from this work and other
studies suggests that it is probable that social welfare in the basin can be maintained
while achieving the goal of a 5-year running average of 5000 km2 for the hypoxic zone.
Most importantly, welfare losses from costs incurred to control hypoxia in the Basin will
be offset, at least in part, by co-benefits of nutrient reductions. For example, research on
wetlands in the MARB suggests that the benefits of large scale restoration efforts would
exceed the costs. Second, only limited targeting of control options that focus on hypoxia
reduction and its co-benefits have been undertaken. Given the significant gains in cost
savings that targeting can achieve, this suggests that it may be possible to achieve
hypoxia reduction at lower cost than predicted in models that do not consider complete
targeting. Based on these findings, the SAB Panel offers the following
recommendations.
The management of factors affecting hypoxia within the MARB should be viewed
as components of a designed landscape so that costs and benefits at various spatial
and temporal scales are explicitly considered.
Integrated economic and watershed models are needed to support an adaptive
management framework. Models are needed that represent land use and costs of
142
-------
conservation at both the fine scale, such as the 8 or 12-digit HUC size, as well as
a larger scale that encompasses the entire MARB.
Research that assesses the optimal suites of conservation practices to maximize
both local water quality and other co-benefits and Gulf hypoxia reduction is
needed. This will require improved understanding of the watershed scale benefits
of these control measures and their costs.
To reduce hypoxia and protect social welfare in the MARB, control measures that
both reduce hypoxia cost-effectively and provide co-benefits in the MARB should
be targeted whenever possible. Targeting control measures can reduce the costs
and increase co-benefits associated with measures to control hypoxia in the Gulf
of Mexico.
4.4. Cost-Effective Approaches for Non-point Source Control
While the Action Plan and this Advisory urge the reliance on adaptive
management principles, a variety of tools can be used as the vehicle for implementation
within adaptive management. The current Action Plan indicates a principle of
encouraging "actions that are voluntary, practical, and cost-effective" (page 9).
Additionally, the plan will "utilize existing programs, including existing State and
Federal regulatory mechanisms," as well as identify needs for additional funding. These
statements include a variety of tools ranging from purely voluntary programs (those with
no associated financial incentives) to current conservation programs funded by state and
federal agencies (such as the Conservation Reserve Program (CRP) and the
Environmental Quality Incentive Program (EQIP)) to water quality trading. Research
assessing the costs and effectiveness of these approaches is addressed in this section.
Complicating the design of cost-effective approaches is the geographic distance
between the sources of nutrients and the receiving waters downstream. Two identical
farm fields in different locations (with resulting differences in the hydrology of the local
watershed) will send differing amounts of nutrients to the Gulf. Hence, the effectiveness
of a practice or sink in a particular location depends on what sources and sinks are
present elsewhere in the watershed. Whether it is cost effective to install a buffer at a
particular location may depend upon whether there is a wetland at the base of the
watershed, whether conservation tillage is being practiced elsewhere, etc. Thus, rather
than focus on individual practices, policy options that can simultaneously encourage the
adoption of practices and sinks that are jointly cost effective will best protect social
welfare in the Basin.
It is important to clarify the concept of "costs." Here, "costs" refers to the least
amount of compensation needed to effect change, e.g., the compensation that would be
necessary for a landowner or farmer to adopt a conservation practice. This is the standard
concept of economic cost, relevant to any good or service. This cost includes "direct"
143
-------
costs such as the cost of new equipment, building of structures, and labor to manage a
practice, as well as a myriad of potential "indirect" costs such as lost profits from
adopting the practice, compensation for added risk from the practice, etc. Components of
these costs can be negative; i.e., it may actually increase profitability to adopt some
practices (conservation tillage in certain circumstances is a notable example).
Second, the focus of most economic studies is on total costs with little or no
consideration paid to what subset of society actually bears the costs (incidence) of the
policy. This focus on efficiency (seeking the lowest cost approach) is based on the
premise that compensation could always be paid to those bearing the cost in some form
so that society will be best off if the lowest cost option is pursued. However, since such
compensations are rarely paid, the issue of who pays is likely to enter the policy decision.
Complete information on the incidence of alternative tools in this context is not available,
but where appropriate, we note the likely incidence considerations.
4.4.1. Voluntary programs - without economic incentives
There is a small and growing literature concerning the effectiveness and optimal
design of voluntary agreements that do not have positive or negative financial incentives
associated with them (National Research Council, 2002; Morgenstern and Pizer, 2007).
Key insights were presented in a game-theoretic model by Segerson and Miceli (1998),
who identified the conditions under which voluntary agreements are likely to yield
efficient pollution levels without significant economic incentives. They studied
voluntary agreements that are based on threats of harsher outcomes if the goals are not
met, using the example of mandatory abatement requirements if the voluntary agreement
does not succeed in meeting the pollution goal. The premise is that firms will voluntarily
agree to reduce pollution if they can avoid the costs that future mandatory controls would
otherwise bring. In the absence of financial compensation, the presence of a positive
probability of a penalty (or cost in the form of mandatory control) is required to support
Segerson and Miceli's findings that there are situations in which efficient levels of
pollution control can be achieved with voluntary agreements (without economic
incentives). They found that pollution reduction is likely to be small when the
background threat is weak.
Empirical work also sheds light on the efficacy of voluntary agreements that do
not have financial incentives. Mazurek (2002) identified 42 voluntary environmental
initiatives sponsored by the federal government since 1988. Although the programs she
identifiee are largely outside the realm of agriculture, her conclusions are relevant.
Mazurek concluded that a variety of implementation problems have led to "lower-than-
expected" environmental results for voluntary (without financial incentive) agreements, a
result consistent with findings of a 1997 USGAO (1997) report concerning four voluntary
agreements related to climate change.
In the same National Research Council report (2002), Randall identified three
essential functions for government if voluntary agreements (without financial incentives)
144
-------
are to be effective. These key functions are meaningful monitoring to back up a threat of
government inspection, "credible threat of regulation" if the goals are not met, and a clear
liability system to punish "blatant polluters and repeat offenders." Randall concluded
that "voluntary (or negotiated) agreements, industry codes, and green marketing should
be viewed as promising additions to the environmental toolkit, but they should
supplement, not supplant, the regulatory framework. They make a nice frosting on the
regulatory cake. But the cake itself must be there (pages 317-318)."
Finally, Morgenstern and Pizer (2007) presented seven case studies on voluntary
agreements (without economic incentives) in the U.S. and elsewhere. Point estimates of
environmental improvements attributable to the voluntary programs ranged from negative
values (actual declines in environmental performance) to a maximum of 28%
improvement in environmental performance. Morgenstern and Pizer concluded "that
voluntary programs have a real but limited quantitative effect... (page 182)."
Given the historical aversion to imposing mandatory requirements in agriculture,
the collective weight of these studies suggest that voluntary agreements that do not have
incentives associated with them are not likely to be adequate on their own to achieve
significant reductions in nutrient runoff. In short, voluntary programs without incentives
can have small effects but cannot be relied upon to induce major environmental
improvements.
4.4.2. Existing Agricultural Conservation Programs
Currently, the largest incentive-based conservation programs related to agriculture
are the EQIP and CRP. A potentially significant program introduced in the 2002 Farm
Bill was the Conservation Security Program (CSP), which has been funded only partially
and implemented incrementally. The CRP pays farmers to retire land, and the other two
pay farmers to implement conservation practices on their farms (EQIP is a cost-share
program; CSP was intended to cover the full costs of adoption). Numerous studies
undertaken by USDA's Economic Research Service and others have estimated the
magnitude of environmental benefits from these programs in physical terms (e.g., tons of
erosion reduction, acres of habitat preserved, acres of wetlands restored, etc) and some
efforts have been made to monetize these benefits [see Claassen et al. (2004) for a
summary of CRP studies as well as Haufler (2005)]. The Conservation Effects
Assessment Program (CEAP) was initiated in an attempt to provide nationwide estimates
of the benefits provided by the full suite of conservation programs; a national assessment
of the water quality benefits is being developed currently (Bob Kellogg, presentation to
SAB Hypoxia Advisory Panel, December 6, 2006).
The CRP pays landowners to take their land out of crop production and place it in
perennial vegetation or trees, depending on the region of the country, with a goal of
creating wildlife habitat and reducing erosion (and originally to reduce crop production).
The CRP enrolls about 10% of total US cropland, nearly all in ten-year contracts
although there is significant concern that high corn prices due to ethanol expansion may
145
-------
rapidly reduce this amount. A number of studies have identified large environmental
benefits associated with the CRP [Smith and Alexander (2000), Feather et al. (1999)].
The program has used an Environmental Benefits Index (EBI) since 1990 to prioritize
parcels for inclusion in the program that gives points to land based on particular
environmental attributes and cost. The movement from targeting erodible lands (prior to
1990) to the use of the EBI for targeting has been estimated to have doubled the benefits
from the program (Feather et al., 1999). Ribaudo (1989) estimated that a CRP enrollment
that targets lands based on environmental damages (benefits) would have significantly
greater benefits still. By redesigning the weights in this index, the program could target
land that is predicted to contribute high nutrient loadings to the Gulf.
Many other studies have addressed the cost-effectiveness of land retirement to
achieve environmental benefits within the context of the CRP. In a series of papers
assessing the efficiency of the Conservation Reserve Enhancement Program (CREP) in
Illinois, Khanna et al. (2003) linked the AGNPS model with site specific characteristics
of parcels to examine the relative efficiency of alternative targeting mechanisms (Yang et
al., 2003, 2004, and 2005). Extremely large gains from targeting were reported; for
example, Yang et al. (2004) estimated that with targeting, 30% less cropland could have
been retired (at almost 40% less total cost) while achieving 20% reductions in erosion
instead of the actual 12% reduction.
The EQIP program is a cost share program for conservation practices in livestock
facilities and on land that remains in agricultural production. A prospective benefit cost
analysis (as required by Executive Order 12866) predicted over $5 billion in net benefits
from the EQIP program as implemented under the 2002 Farm Bill, even though not all of
the benefits could be monetized (US Department of Agriculture, 2003).
The Wetland Reserve Program (WRP), Grassland Reserve Program (GRP), and
Wildlife Habitat Incentive Program (WHIP) are all smaller land retirement programs that
also could potentially benefit efforts to reduce Gulf hypoxia. Additional information on
the large-scale potential for wetlands is provided by Hey et al. (2004), who addressed the
question of whether the social benefits from restoring up to 2.83 million hectares (7
million acres) of cropland in the 100 year floodplain of the upper Mississippi River basin
to wetlands exceed the costs. The benefits include reduced flood related crop damages,
reduced crop subsidies and non-flood related recreation benefits of wetland conversion
including fishing, hunting, and general recreation usage. These benefits were compared
to estimates of the costs of cropland conversion comprised of farm rental rates
(representing the present value of farmland income) and the costs of wetland construction
and maintenance. Hey et al. (2004) estimated that the benefits exceed the costs in all
locations considered except one county in Missouri. In the context of NGOM hypoxia,
this difference is especially striking because the benefits exceed the costs for this
conversion even without considering any benefits from reduction of the hypoxic zone.
As the authors carefully pointed out, the social efficiency of converting 2.83 million
hectares (7 million acres) does not mean that private benefits will exceed the private costs
for all parties. Individual landowners would stand to lose while recreationists accrue
benefits.
146
-------
These findings represent an important addition to the assessment of wetlands in
the Integrated Assessment. While Doering et al. (1999) concluded that wetland
restoration was less cost-effective than fertilizer reductions, their analysis did not include
cost savings from crop subsidy reductions nor flood related crop damages. In addition,
the Hey et al. (2004) work focuseD on wetlands targeted in flood plains. The study
suggests two points of key importance for NGOM hypoxia; 1) there is a large amount of
acreage that is situated in locations that potentially could serve as nutrient sinks in the
upper Mississippi River basin, and 2) the co-benefits of this action are large enough, in
and of themselves, to justify the social efficiency of converting this land to nutrient sinks
even without considering the benefits associated with reducing Gulf hypoxia.
The programs mentioned above can be categorized into one of two groups: land
retirement programs and "working" land programs. Both the CRP and WRP are
examples of land retirement programs, since landowners receive payments in exchange
for taking land out of active agricultural production and putting the land into perennial
grasses, trees, or wetlands restoration. In contrast, EQIP and the CSP are examples of
working land programs whereby landowners or producers receive payments to cover part
or all of the costs of making changes in conservation practices or management decisions
on their land that remains in agricultural production. Some research has addressed the
cost-effectiveness of working land programs vs. land retirement programs. For example,
Feng et al. (2006) found that a cost-effective allocation of resources to sequester carbon
in agricultural soils favors working land (via conservation tillage subsidies) over land
retirement (via payments to retire land and plant it in perennial grasses). It is important
to note however, that this study focused on stylized working land and land retirement
programs rather than attempting to address the cost-effectiveness of existing conservation
programs as actually implemented.
The existing working land and land retirement programs are implemented with
features that likely affect the cost-effectiveness of the programs for achieving
environmental gains in different ways. For example, the CRP uses an EBI that favors
admitting land into the program that achieves environmental benefits at relatively low
costs. All else equal, this component of the program will improve its cost-effectiveness.
In contrast, the CSP provides payments for ongoing stewardship of farmers so that
program expenditures are used to reward past behavior rather than to change existing
behavior. This, all else equal, will reduce the program cost-effectiveness for achieving
environmental gains. The lack of competitive bidding and clear targeting also reduces
the cost-effectiveness of this program. Finally, it is worth noting that targeting and
competitive bidding were explicitly disallowed in the EQIP program during its last
reauthorization. Again, this will reduce its cost-effectiveness.
4.4.3. Emissions and Water Quality Trading Programs
Emission trading is a regulatory approach that sets a maximum allowable level of
overall emissions and then allows sources to exchange pollution allowances. A properly
147
-------
structured trading program can reduce the costs of achieving emission standards by
allowing the flexibility necessary to focus pollution reductions on sources that are less
expensive to control. In theory, a broad based emissions trading program could help to
reduce the air and water contributions of nutrients to the NGOM. Water quality trading is
simply the name given to the extension of emissions trading to achieving water quality
objectives.
In a recent survey of the programs to support water quality trading in the U.S.,
Breetz et al. (2004) identified 40 water trading initiatives and an additional six state
policies with specific programs related to water quality trading. EPA has supported these
programs (US EPA, 2004a) and has produced explicit policies related to their
implementation. Many states and regions also have explicit policy guidance. However,
the effectiveness of these programs appears to have been quite limited as very few trades
are actually occurring. Further, little evidence of environmental improvement associated
with these programs exists (Breetz et al., 2004).
A key problem with these programs is the lack of a required water quality
improvement necessary to generate adequate demand for credits (King, 2005). To
achieve "cap and trade," an effective cap is necessary. A cap could come from a tight
enough cap on point sources such that they would find it cost-effective to purchase
credits from agricultural non-point sources. Alternatively, the cap could be extended to
agricultural sources. While some have conjectured that the Total Maximum Daily Load
(TMDL) program may eventually play this role, there is no current mandate for
agricultural sources to restrict nutrient runoff. Also problematic are a range of
restrictions on allowable trading such as requirements that a particular baseline set of
conservation practices be in place with credits accruing only for additional conservation
activity.
While trading could be a significant contributor to cost-effective nutrient control,
the necessary institutions for water and/or air emissions trading to be an effective policy
instrument are not broadly in place. In addition to clear and enforceable limits on
emissions or water quality contributions (from point and/or non-point sources),
enforceable rules concerning trading ratios, liability when standards are not met,
monitoring, etc. must be established before these markets can flourish. Ideally, a trading
program to address NGOM hypoxia would be broad based and include highly diverse
sources (such as air deposition and many agricultural non-point sources) to maximize the
potential for cost savings.
4.4.4. Agricultural Subsidies and Conservation Compliance Provisions
U.S. farmers have been the recipients of farm payments for decades. These
payments support prices and/or income, especially of farmers growing bulk commodities
such as corn and soybeans. Economic theory suggests that, all else equal, such payments
will increase the intensity and acreage of farming, possibly resulting in increased water
quality problems. Research by Reicheldorfer (1985) provided empirical evidence that
148
-------
these payments encourage crop production on highly erosive land. Likewise, a recent
study from USDA's Economic Research Service (Lubowski et al., 2006) quantified the
effect of one major program, subsidized crop insurance, on the location and acreage of
cropland and its environmental effects. Lubowski et al. (2006) estimated that about a
million hectares (2.5 million acres) were brought into production as a result of the
program and that these lands are more vulnerable to erosion, are more likely to include
wetlands, and have higher levels of nutrient losses than average.
To some extent, USDA's conservation programs (see Section 4.4.2) exist to
counteract the "perverse effects" or unintended consequences of its crop subsidies
inasmuch as government financial support has encouraged farmers to choose commodity
crops that require more fertilizer, maximize yield without regard to soil and water quality
consequences, and cultivate marginal land. Re-structuring or eliminating existing
subsidies could serve to mitigate some of these perverse effects (e.g., by shifting
subsidies to reward less fertilizer-intensive crops as well as by requiring, as a condition of
receiving subsidies, certain conservation practices).
Taheripour et al. (2005) provided additional evidence on this point. First, their
model suggests that removal of all crop subsidies would reduce nitrogen pollution by
8.5% and that the reduced need for distortionary income taxes to support these subsidies
could increase social welfare by $1.2 billion. Further, they found that tax neutral policies
to achieve nitrogen reduction can generate significant double dividends (a double
dividend refers to a situation where a policy not only internalizes an externality but also
reduces the deadweight losses associated with distortionary taxation such as an income
tax). They provide an estimate of the magnitude of the double dividend for a range of
nitrogen reduction goals and policy approaches including a nitrogen tax, a nitrogen
reduction subsidy, a tax on output, and a combined output tax and nitrogen reduction
subsidy and find that a double dividend from these instruments can be significant.
While environmental improvements associated with agriculture have largely been
pursued via cost-share or subsidy programs, one significant regulatory approach has been
the implementation of environmental compliance provisions that require farmers who
receive farm program payments (including price support and income support) to
undertake some environmental performance practices. Specifically, in the 1985 Food
Security Act, conservation compliance provisions required owners of highly erodible
land (a categorization of land based on its slope and soil type) to implement soil
conservation plans and a "swampbuster" provision disallowed payments to go to farmers
who converted wetlands to crop land. Claassen et al. (2004) estimated that up to 25% of
the reduction in soil erosion that occurred between 1982 and 1997 was attributable to
conservation compliance. Many believe these gains could have been higher if there had
been stronger enforcement of the mechanism. While no direct estimates are available of
the increased benefits that could come from more enforcement, there is evidence of very
limited reporting and penalizing of violations (Claassen, 2000).
Claassen et al. (2004) assessed the prospect for reducing nutrient losses from the
Mississippi River basin by extending compliance requirements to nutrient management.
149
-------
They used "nutrient management" to refer to the range of activities related to the timing
and level of fertilization decisions that best minimizes soil nutrients in excess of crop
needs at any point in time. They noted that the ideal set of nutrient management practices
will vary considerably across farms and regions and that the costs of these activities will
also vary notably across this space. Using data from the EQIP program, they summarized
the distribution of incentive payments needed to induce willing adoption of nutrient
management practices as defined under EQIP. For the Heartland region (ERS Farm
Resource Region), the average annual incentive payment is about $7 per acre, and 95% of
the payments are $12 per acre or less.
While these data provide an excellent starting point for assessing the cost
effectiveness of nutrient management methods addressing local water quality and NGOM
hypoxia, several additional pieces of information would be needed for a full assessment.
First, these costs represent the compensation needed for those farmers who have already
adopted practices under the EQIP program; those who have not adopted are likely to have
at least as high costs, possibly substantially higher. In this regard, these costs could be
viewed as a lower bound. Second, these costs are specific to the EQIP requirements for
nutrient management. Whether these requirements are effective enough to yield
substantial off-site benefits is not addressed. Nonetheless, based on this cost assessment
and a comparison with the annual commodity program payments farmers typically
receive, Claassen et al. (2004) concluded that substantial nutrient management could
occur with extension of conservation compliance provisions to nutrients.
Claassen et al. (2004) also considered whether buffer practices could be induced
under conservation compliance provisions. They included riparian buffers, filter strips,
grassed waterways, and contour grass strips in their discussion of buffer practices. To
assess the costs of these practices and how they vary across locations, they looked at
information on producers' willingness to accept compensation for adoption of the
practices from the priority areas sign up of the continuous CRP. Owners of these lands
received an average payment of about $90 per year in addition to 50% cost share for
installation of the buffer practice. Based on this analysis, as an example, Claassen et al.
(2004) computed the annual costs per area for a filter strip and concluded that, in many
cases, this payment would be below the average subsidy received by producers, thereby
suggesting that buffer practices might also be successfully adopted under nutrient
compliance provisions.
Finally, Claassen et al. (2004) noted that conservation compliance provisions are
likely to have few transaction costs relative to other policies (although enforcement costs
would need to be considered) and require very low budgetary outlays beyond the
payments that are already provided for commodity or insurance programs. Claassen et al.
(2004) also argued that conservation compliance requirements have been relatively cost-
effective due to the flexibility with which they can be implemented. Producers in
different regions of the country, with differing soil and weather conditions, can meet their
compliance obligations with different practices. This flexibility means that the most
appropriate technologies can be used for the location of the practice.
150
-------
4.4.5. Taxes
The use of a per unit tax to internalize the costs of externalities of production is
well known to be highly cost effective when the tax is placed directly on the externality
generating activity; these "Pigouvian" taxes are the equivalent of placing the appropriate
price on the pollutant (Baumol and Gates, 1988). Taxes can be a powerful market signal,
communicating the need to change behavior, Baumol and Gates (1988) demonstrated that
subsidies (essentially just negative taxes) can also be designed that provide the equivalent
market signals for changes in behavior. This argument is often used to support the design
of environmental programs that pay participants for the provision of environmentally
friendly practices rather than using taxes to change behavior. A potentially important
exception to this equivalence can occur when the provision of a positive payment induces
entry into the farming sector generating production on otherwise unprofitable lands. This
possibility was addressed in Section 4.4.4 in the context of general agricultural subsidies
and conservation compliance.
A tax directly on an input into production that is highly correlated with the
pollutant can be an efficient second-best policy. The possible use of a nitrogen fertilizer
tax was considered in Doering et al. (1999) and found to be as cost-effective as any of the
policies they considered (they note that the initial incidence falls on farmers). Fertilizer
taxes already exist in some states, but are set at much smaller levels than those studied by
Doering et al. (1999). The inelastic demand for fertilizer (Denbaly and Vrooman, 1993)
means that the magnitude of taxes needed to induce behavioral change would likely be
large.
The incidence of a tax (and thus determination of who pays the costs) is likely to
fall on farmers and consumers of food products made from crops that use fertilizer. In
contrast, the incidence of conservation program payments is largely on taxpayers.
Finally, it is important to note that tax instruments will be more efficient the more
broadly they are applied to the various nutrient sources identified as pollutant
contributors; so ideally a tax would be applied to all nutrient sources rather than singly to
fertilizer.
4.4.6. Eco-labeling and Consumer Driven Demand
The idea that environmentally friendly producer behavior can be induced by
consumer demand is one basis for eco-labeling and certification programs. Dolphin safe
tuna (Teisl et al., 2002) and organic fruits and vegetables (Loureiro et al., 2001) are two
successful examples. Research analyzing the effectiveness of eco-labeling suggests some
promise.
Thogersen (2002) summarized three schemes, all implemented in Europe, that
have been credited with significant reductions in emissions from heating appliances and
paint solvents (the German "Blue Angel" brand) and reductions in pollutants from paper
151
-------
production and household chemical and laundry emissions (the Swedish "Good
Environmental Choice" label and the Nordic "Swan" label). Although not specific to a
particular product, Clark and Russell (2005) noted that several studies of the Toxic
Release Inventory have shown that information can affect firms' choices.
Could consumer driven demand affect the changes in land use and agricultural
management necessary to contribute notably to nutrient flows into the Gulf? This
approach would require the labeling of food and fiber products made from agricultural
outputs in the MARB to indicate that they were produced in such a way as to reduce or
eliminate nutrient contributions to hypoxia. Consumers would then need to respond to
this labeling by purchasing products, presumably at a higher cost, in adequate quantity to
change the market behavior. Given that much of the grain produced in the Corn Belt is
used for livestock feed and not directly traceable to its field of origin, it will be difficult
to distinguish products that were produced with "hypoxia-friendly" production practices
from those that were not. It is not clear that labeling can credibly be produced without
significant government involvement and expense (Crespi and Marette, 2005). Nor is it
clear that consumer response would be adequate to drive changes in production practices,
even if the labeling challenges could be overcome. One area in which labeling may
prove effective is in animal agriculture where the tracking of an individual unit from
producer to final consumer is more straightforward.
4.4.7. Key Findings and Recommendations on Cost Effective Approaches
Voluntary agreements with no accompanying economic incentives are not likely
to be adequate to obtain significant reductions in N and P. While there may still be some
low-cost conservation practices that can be implemented in some locations (better
"crediting" for manure spreading for example), nutrient reductions that face agricultural
producers with costly tradeoffs cannot be expected without strong economic signals.
These economic incentives can take many forms: conservation payments such as those in
many current agricultural conservation programs, taxes, restructuring or removal of
subsidies (such as conservation compliance provisions), etc.
Water quality trading programs have not yet demonstrated the ability to improve
environmental performance and/or reduce costs of meeting environmental targets
primarily due to an absence of effective emissions restrictions. However, with clearer
water quality improvement mandates and more flexible rules for trading, these programs
could develop into cost-effective instruments.
Numerous studies have demonstrated that existing incentive-based conservation
programs, specifically the CRP, WRP and EQIP, have provided significant
environmental benefits. However, these programs can be much more cost-effective with
additional targeting and competitive bidding mechanisms. Given the menu of existing
programs, it is possible to reduce hypoxia and protect water quality in the MARB without
significant new government funding, although the distributional consequences of the
various approaches will differ. Based on these findings, the SAB Panel offers the
following recommendations.
152
-------
To achieve N and P reductions from agricultural sources of the magnitude needed
to affect hypoxia, economic incentives are needed to induce adequate adoption of
conservation practices. These incentives can take many forms: conservation
payments, taxes, and/or restructuring of existing farm subsidy and compliance
requirements.
To maximize the N and P reductions achieved with federal and state conservation
dollars (e.g., CRP, WRP and EQIP), targeting and competitive bidding
mechanisms are needed so that lands enrolled in these programs achieve
maximum environmental benefits at lowest cost. Strategically placed wetlands in
the upper Mississippi River basin could serve as effective nutrient sinks.
Research has demonstrated that the local co-benefits are large enough, in and of
themselves, to justify restoring these wetlands. The additional benefits associated
with reduction in Gulf hypoxia reinforce the conclusion of the desirability of
wetlands restoration.
Water quality trading programs hold promise, but, without enforceable caps
(water quality standards), these programs cannot be expected to achieve much
nutrient reduction.
To minimize the adverse effects of existing agricultural subsidy programs,
conservation compliance requirements that target reductions in nutrients could be
very cost-effective, but only with adequate enforcement.
To select policies and programs with maximum economic efficiency, all co-
benefits should be considered regardless of which policy tools are used. For
example, since wetlands provide valuable habitat and flood control in addition to
water quality benefits, there may be instances in which it is desirable to control
nutrients by restoring wetlands, even if it is less costly to reduce nutrients by
managing croplands.
153
-------
4.5. Options for Managing Nutrients, Co-benefits, and Consequences
4.5.1. Agricultural drainage
The Integrated Assessment reports identified several research needs related to
agricultural drainage. Brezonik et al. (1999) emphasized the importance of agricultural
drainage in nutrient transport from cropland and identified increased spacing of
subsurface drainage tile and controlling water table levels (controlled drainage) among
those practices that could potentially reduce nitrate losses from cropland. Mitsch et al.
(1999) noted that controlled drainage was not widely practiced in US Corn Belt and that
most of the research on controlled drainage had been conducted in more southern
climates.
Alternative drainage system design and management
Relatively few field studies have addressed the effects of subsurface drain depth
and spacing on N losses from cropland. Overall, results suggest a trend of decreased
subsurface flow and decreased N loss at wider tile spacing or decreased tile depth.
Reported reductions in nitrate export are primarily due to reductions in the volume of
flow rather than reductions in nitrate concentration. Drain flows and N loss can be
affected by both drain spacing and depth (Hoffman et al., 2004; Kladivko et al., 2004;
Skaggs and Chescheir, 2003; Skaggs et al., 2005), and use of drainage intensity (Skaggs
et al. 2005) normalizes some of the variability in results of drainage spacing studies.
Drainage intensity increases with deeper tile depths and closer tile spacing. Research
suggests that reducing drainage intensity by either shallower tile depth or wider tile
spacing will reduce subsurface flow and nitrate loss. However, adjustments in tile
spacing and depth are only possible when drainage systems are being installed, and the
Corn Belt is already extensively drained. As these systems are replaced, repaired, and
upgraded over the next few decades, there will be opportunities to consider alternative
drainage designs to minimize nutrient losses. In the meantime, there may be
opportunities to achieve similar benefits by retrofitting existing drainage systems with
control structures that allow some management of subsurface drainage.
Drainage management (controlled drainage) is currently an area of active research
and development (http://extension.osu.edu/~usdasdru/ADMS/ADMSindex.htm).
Research suggests that drainage management could reduce nitrate transport from drained
fields by 30% for regions where appreciable drainage occurs in the fall and winter
(Cooke et. al., in press). Although water table management could potentially alter
nitrification and denitrification reactions, reported reductions in nitrate export with
controlled drainage are primarily due to reductions in the volume of flow rather than
reductions in nitrate concentration. Some uncertainty arises from difficulties in closing
water balances (and therefore N balances) in field studies, and an unknown amount of
subsurface flow reduction could be due to lateral seepage and/or increased surface runoff
(Cooke et al., in press). Simulation studies predict increased surface runoff when higher
water tables are maintained using controlled drainage (Skaggs et al., 1995; Singh and
154
-------
Helmers, 2006) suggesting a potential tradeoff between reduced subsurface drainage and
increased surface runoff. Although raising the water table can decrease the volume of
infiltrating water entering drainage tile, higher water tables can also increase surface
runoff resulting in increased erosion and loss of particulate contaminants such as soil
bound phosphorous.
Controlled drainage requires relatively flat and uniform topography, and slopes of
less than 0.5% or 1 % are recommended (Cooke et al., in press; Frankenberger et al.,
2006). Concerns for erosion and surface runoff increase with increasing slope, and
slopes greater than 0.5-1% can require an impractical number of control structures.
There has been speculation that new technologies could make the practice economically
feasible at slopes of 2% or more, but this would raise even greater concerns over surface
runoff. Although tile drainage is widespread throughout the Corn Belt, it is not clear
what portion of this tile drainage can be retrofitted with structures for controlled drainage.
A first approximation might be an estimate of the fraction of tile drained lands with
slopes less than 0.5-1%, but this approach requires higher resolution topography than is
generally available in the Corn Belt. These estimates are available for a few large
drainage districts in north central Iowa for which very high resolution topography were
developed. Although 50 to 75% of the cropland in these drainage districts is tile drained,
only about 10% has a slope less than 1% and only about 3% has a slope less than 0.5%
(Matt Helmers, Iowa State University, Ag Drainage Website,
http://www3.abe.iastate.edu/agdrainage). These results suggest that controlled drainage
may be applicable to a relatively small fraction of tile drained land in Iowa, but this may
not be representative of other regions of the Corn Belt. Based on STATSGO soils data,
Illinois, Indiana, and Ohio may have twice as much cropland suitable for controlled
drainage as Iowa (Dan Jaynes, National Soil Tilth Lab, Ames, IA). High resolution
topography could provide a much better basis for this assessment.
Bioreactors
Denitrification bioreactors have been installed in the field as treatment systems
for tile drain effluent (Van Driel et al., 2006) and as denitrification walls (a trench filled
with carbonaceous material to intercept subsurface flow) (Schipper and Vojvodic-
Vukovic, 1998; Robertson et al., 2000; Schipper and Vojvodic-Vukovic, 2001; Schipper
et al., 2004; Schipper et al., 2005). Bioreactors on tile drains are typically bypassed
during high flows and "are most usefully applied in the treatment of baseflows rather than
peak flows." Current knowledge indicates that denitrification walls are effective for at
least 5 to 7 years with little or no loss of nitrate removal capacity (Robertson et al., 2000;
Schipper and Vojvodic-Vukovic, 2001). A variety of materials such as corn stalks, wood
chips, and sawdust are potential organic amendments to enhance denitrification in
bioreactors. Continued research is needed to determine whether denitrification
bioreactors could be installed around lateral tile drain lines and whether this would be
technically and economically feasible. Future re-design of tile drain systems may include
integrated denitrification enhancements around tile lines and at the outlets of smaller tile
lines.
155
-------
Key Findings and Recommendations:
Alternative drainage designs with reduced drainage intensity due to shallower tile
depths and/or wider tile spacing could significantly reduce nitrate losses but can be
expected to increase surface runoff and losses of particulate contaminants. Controlled
drainage could significantly reduce nitrate losses where appreciable drainage occurs in
the fall and winter but can be expected to increase surface runoff and losses of parti culate
contaminants. Controlled drainage is most appropriate for areas having slopes of less
than 0.5-1%, and it is not clear what fraction of tile drained lands are suitable for
application of controlled drainage. In some areas, slope could seriously constrain
applicability of the practice. Bioreactors can significantly reduce nitrate concentrations
but typically must bypass peak flows during which much of the nitrate load is
transported. Based on these findings, the SAB Panel offers these recommendations.
Additional research is needed to evaluate topographic constraints on the
applicability of controlled drainage including developing high resolution
topography for the Corn Belt.
Additional research is needed to fully characterize water and nutrient balances for
alternative drainage design and management most critically using small watershed
scale studies (less than 2,500 hectares or about 10,000 acres) to document effects
when scaled up.
A strategy for implementation of alternative drainage design or management
should be developed that includes consideration of potential trade-offs between
reduced nitrate loss through tile drains and increased P loss through surface
runoff.
4.5.2. Freshwater Wetlands
If wetlands are to serve as long-term "sinks" for nutrients, reductions in nutrient
loads must reflect net storage in the system through accumulation and burial in sediments
or net loss from the system, for example through denitrification or vegetation removal.
The effectiveness of wetlands in reducing N export from agricultural fields will depend
on the magnitude and timing of NOs loads and the capacity of the wetlands to remove
NOs by denitrification. In contrast to NOs, gaseous losses of P are insignificant, and
sediment accretion of bound inorganic P and unmineralized organic P is the primary
mechanism by which wetlands serve as long-term P sinks. With the exception of P
associated with suspended solids, wetlands are generally less effective at retaining P than
at removing NO3 (Reddy et al., 1999).
156
-------
Nitrogen
The effectiveness of wetlands in NOs reduction is a function of hydraulic loading
rate, hydraulic efficiency, NO3 concentration, temperature, and wetland condition. Of
these, hydraulic loading rate and NOs concentration are especially important for wetlands
intercepting non-point source loads. Hydrologic and NOs loading patterns vary
considerably for different landscape positions and different geographic regions. The
combined effect of variation in land use, precipitation, and runoff means that loading
rates to wetlands receiving non-point source loads can be expected to vary by more than
an order of magnitude and will, to a large extent, determine NOs loss rates for individual
wetlands.
Mitsch et al. (2005a) examined NOs retention in Mississippi River basin wetlands
receiving non-point source NOs loads either directly or through diversion of river water.
Their study extended the earlier analysis of Mitsch et al. (1999) to include additional
wetlands and to include wetlands outside the agricultural regions of the Corn Belt. They
found that 51% of the NOs mass reduction by the wetlands examined could be explained
by a nonlinear regression based on annual mass load of NOs per area of wetland.
However, when the analysis is restricted to Corn Belt wetlands that receive seasonally
variable water and nutrient loads (i.e., subjected to non-point source loading regimes), the
relationship is much weaker (Crumpton et al. 2006, in press). Based on 34 "wetland
years" of available data (12 wetlands with 1-9 years of data each) for sites in Ohio
(Mitsch et al., 2005a; Zhang and Mitsch, 2000, 2001, 2002, and 2004), Illinois (Hey et
al., 1994; Kovacic et al., 2000; Phipps, 1997; Phipps and Crumpton, 1994), and Iowa
(Crumpton et al., 2006; Davis et al., 1981), percent mass NOs removal is much more
closely related to hydraulic loading rate (HLR) (Figure 41, R2 = 0.69) than to mass
loading rate (R2 = 0.22).
O
e
+- »
fin
100
90
80
70
60
50
40
30
20
10
0
10 20 30 40
Hydraulic loading rate (m yr1)
157
-------
Figure 41: Percent mass nitrate removal in wetlands as a function of hydraulic loading rate. Best fit for
percent mass loss = 103*(hydraulic loading rate)"033. R2 = 0.69. Adapted from Crumpton et al. (2006, in
press).
Hydraulic loading rate explains relatively little of the variability in NOs mass
removal, which can vary considerably more than percent NOs removal among wetlands
receiving similar hydraulic loading rates. However, much of the variability in mass NOs
removal can be accounted for by explicitly considering the effect of HLR and flow
weighted average (FWA) NOs concentration (Crumpton et al., 2006, in press). For the
wetlands in Figure 41, mass NOs removal rate can be predicted as the product of percent
removal (estimated as 103 * HLR"033) and mass load (estimated as HLR*FWA). This
simplifies to the function [mass removal in kg N/ha/yr = 10.3 * (HLR in m/yr)°'67 * FWA
NOs concentration in g N/m3] and explains 94% of the variability in mass NOs removal
for the wetlands considered here (Figure 42). The isopleths on the function surface in
Figure 42 represent the combinations of HLR and FWA that can be expected to achieve a
particular mass loss rate and illustrate the benefit of targeting wetland restorations in
areas with higher NOs concentrations. The wetlands examined by Mitsch et al. (2005a)
had a median loading rate of 600 kg NCVN/ha/yr, at which they predicted losses of 290
kg NOs-N/ha/yr. This mass loss rate is near the lower mass loss isopleth of Figure 42 as
would be expected for either low FWA concentrations at moderate to high HLRs or
higher FWA concentrations at lower HLRs. Half of the wetlands considered by Mitsch et
al. (2005a) had NOs concentrations below 3 mg N/l. NOs concentrations in tile drainage
water commonly exceed 10 to 20 mg N/l (Baker et al., 1997, 2004, in press; David et al.,
1997; Sawyer and Randall, in press). The greatest benefit of wetlands for mass NOs
reduction will be found in those extensively row-cropped and tile-drained areas of the
Corn Belt where NOs concentrations and loading rates are highest. For these areas, NOs
mass removal rates could be several times higher than predicted by Mitsch et al. (1999,
2005a).
158
-------
>, 3000
is
F 2500 -
£, 2000
-------
than 0.13% of total nitrate loss in a wetland recharged by GW with elevated nitrate levels
(based on maximum flux rates reported by Paludan and Blicher-Mathiesen 1996). N2O
emission rates in wetlands receiving non point source nitrate loads average around 1
umole N2O m"2 hour"1 (Hernandez and Mitsch 2006; Paludan and Blicher-Mathiesen
1996) which is very similar to rates reported for cultivated crops in the Midwest (1-2
umole N2O m"2 hour"1 (Parkin and Kaspar 2006; Grandy et al. 2006). The available
research suggests that wetlands restored on formerly cultivated cropland for the purpose
of nitrate removal would have little or no net effect on N2O emissions.
Phosphorus
P removal in wetlands is controlled by three sets of processes: 1) sorption or
release of P by existing sediments, 2) accumulation of P in new biomass, and 3)
accumulation of P associated with the formation and accretion of new sediments/soils
(Reddy et al., 2005). Existing sediments will have a finite capacity for sorption of P,
determined in part by Al and Fe content in acid soils and by Ca and Mg content in
alkaline soils. There will also be a finite capacity for the accumulation of P in new
biomass. Of the three sets of processes, only the last contributes to long-term, sustainable
P retention by wetlands: the accumulation of bound inorganic P and unmineralized
organic P associated with the formation and accretion of new sediments and soil.
P sorption on both antecedent and newly accreting wetland soil is largely
controlled by Fe, Al, and Ca. Reducing conditions found in wetlands may decrease
sorption of P as insoluble complexes formed with Fe+3 are released upon reduction to
Fe+2, solubilizing the P (Patrick et al., 1973). High S levels may enhance P flux from
soils due to the binding of iron by sulfides (Bridgham et al., 2001; Caraco et al., 1989).
Alkaline wetland soils are more conducive to P sorption than acidic wetland soils due to
the presence of Ca in the alkaline wetland soils and the formation of insoluble Ca-bound
P (Bruland and Richardson, 2006; Richardson, 1999). These two studies indicate that
wetlands developed on soils rich in calcite and exchangeable Ca are likely to be more
effective sinks for P under the reducing conditions necessary for denitrification. More
research is needed to understand 1) the effects of wetland creation such as is being done
in the upper Mississippi River basin and 2) whether wetlands created/restored on
Mollisols will be effective P sinks due to formation on Ca-P complexes in addition to
sedimentation and SOM formation. Bruland and Richardson (2006) determined that
marshes with a higher soil P sorption index (amount of P sorbed by soil from a phosphate
solution in 24 hour incubation) would be the best P sinks and that specific marshes could
be targeted based on this index. It is important to remember, however, that antecedent
soils of restored wetlands have a finite P retention capacity. The long-term sustainable
capacity of these systems to retain P is determined primarily by the accumulation of P
associated with the formation and accretion of new sediments and soils. Studies of
wetlands constructed to intercept non point source nutrient loads in the MARB confirm
the importance of sediment accretion for P retention (Anderson et al., 2005; Mitsch et al.
2005b) but also demonstrate that wetlands can become a P source if sediments are
remobilized (Mitsch et al., 2005b). Most of the MARB studies represent recently
160
-------
constructed wetlands, and the long-term sustainable capacity of these systems to reduce P
loadings is unclear.
Wetlands created, enhanced, and restored for N removal could also function for P
removal, but limits to sustainable P removal must be recognized. Both NOs and P
removal in wetlands will be enhanced by longer retention times and accretion of organic
rich sediments. Long-term solutions for P load reduction in the MARB will likely
depend more on reduction in sources than will long-term N load reduction. It will be
important to manage restored wetlands so they do not become long-term sources of P
after non-point sources of P have been reduced.
Key Findings and Recommendations
As concluded in the Integrated Assessment, wetlands can be very effective in
removal. Recent data, though limited, support the IntegratedAssessment's conclusion
that N2O evolution from wetlands restored as NOs sinks would be a low percentage of
total denitrification. Wetlands receiving significant non-point source NOs loads at
moderate to high NOs concentrations export comparatively small amounts of organic N
and are nearly as effective in reduction of total N as in reduction of NO3. This situation
is less true for wetlands receiving loads at low NOs concentrations. Hydraulic loading
rate and NOs concentration are especially important determinants of NOs removal rates in
Corn Belt wetlands. Additional information is needed on created, restored, and enhanced
wetlands including long-term monitoring for total N and P retention. Based on these
findings, the SAB Panel offers the following recommendations.
Wetland restoration should be evaluated for its full range of benefits.
For greatest basin wide reduction in nitrate load, wetland restorations should be
targeted in those extensively row-cropped and tile-drained areas of the Corn Belt
where nitrate concentrations and loading rates are highest and sized based on
expected hydraulic loading rates and load reduction goals. For these areas, nitrate
mass removal rates could be several times higher than previously predicted.
Although limits to sustainable P removal by wetlands must be recognized,
wetlands restored for N removal should be managed for P retention as well.
4.5.3. Conservation Buffers
Conservation buffer practices include riparian buffers (forests and herbaceous
cover), field borders, filter strips, contour buffer strips, grass waterways, windbreaks,
hedgerows, and other practices. They are part of the suite of conservation practices that
are applied by farmers to achieve productivity, stewardship, and environmental quality
goals. Conservation buffers differ from other conservation practices in that they will
require long-term set aside of critical lands from continued agricultural production.
161
-------
Although often installed under the Conservation Reserve Program (CRP), conservation
buffers differ from other uses of CRP because conservation buffers allow most land to
remain in production while using critical areas as buffers for the agricultural land.
Prior analysis of nutrient control in the MARB focused on riparian forest buffers,
one prominent type of conservation buffer (Mitsch et al., 1999). Studies conducted over
the past decade in the Corn Belt have shown conservation buffers, especially riparian
forest buffers and riparian herbaceous buffers, to be effective sinks for nutrients and
sediment in landscapes with a significant portion of water moving as either surface runoff
or shallow subsurface flow. If nitrate is transported from crop land primarily in tile drain
flow as in much of the Corn Belt, riparian buffers and vegetated filter strips will have
little opportunity to intercept nitrate loads. It is likely that if drainage management is
changed to limit subsurface discharge through tile drains with concomitant increases in
surface runoff and shallow water table flow, riparian buffers will be critical to achieve
water quality goals.
Reduction of nitrogen by riparian buffers is generally determined by soil type,
watershed hydrology (artificial drainage, groundwater flow paths, saturation); and
subsurface biogeochemistry (organic matter supply, redox conditions) (Mayer et al.,
2006). Control of P depends more on infiltration, surface roughness and runoff retention.
Many riparian buffers have been restored or established, but few have been studied to
quantify water quality benefits. Richard Schultz, Tom Isenhart and others developed the
Riparian Management System for application in areas of the Corn Belt dominated by tile-
drain systems. Modifications to the original USDA Riparian Buffer specification
included integration of wetlands to intercept and remove tile drainage nitrate. Lee et al.
(2000, 2003) reported rates of nutrient and sediment removal in multi-species buffer
strips intercepting surface runoff in these systems. They found that switch grass and
switch grass/woody buffers retained 50-80 % of total N, 41 to 92% of NO3-N, 46-93% of
Total P, and 28-85% of dissolved reactive P from surface runoff produced in simulated
rainfall events.
Riparian herbaceous cover helps reduce sediment and other pollutants in surface
runoff through the combined processes of deposition, infiltration, and dilution. Those
functions are due to the cascading influence of perennial vegetation on soil quality when
compared to soils under annual row-crops. A series of studies on Bear Creek compared
soil quality and related processes within riparian soils in a corn-soybean rotation with
those soils in which perennial herbaceous vegetation had been reestablished (Schultz et
al., 2004). Six years after establishment of riparian switch grass, those soils contained
more than eight times the belowground biomass as adjacent crop fields (Tufekcioglu et
al., 2003). As a result, soils in riparian herbaceous cover amassed up to 66 % more total
organic carbon in the top 50 cm (20 in) than crop-field soils (Marquez et al., 1999). This
resulted in a two-and-a-half-fold increase in microbial biomass and a four-fold increase in
denitrification in the surface 50 cm (20 in) of soil when compared to crop-field soils of
the same mapping unit. As a result of increased soil quality, infiltration was nearly five
times faster in soils under perennial vegetation than in row-cropped fields (Bharati et al.,
2002). Riparian Management Systems such as those on Bear Creek are well-suited to
162
-------
intercept increased overland flow that might be associated with changes in drainage
management.
Several researchers have investigated the combined effects of these processes
within riparian herbaceous vegetation and reported that sediment and nutrients in surface
runoff can be reduced in the range of 12 to 90 % compared to unbuffered crop fields
(Dosskey, 2001; Lee et al., 2003). Major differences in impacts on the soil ecosystem
depend upon the photosynthetic pathway of the dominant vegetation [e.g., C3 (cool-
season grasses) or C4 (warm-season grasses)] in a buffer. Riparian herbaceous cover can
help improve the quality of shallow groundwater, much like filter strips or riparian forest
buffers. Hydrogeologic setting, specifically the direction of groundwater flow and the
position of the water table in thin sand aquifers underlying the buffers, generally is the
most important factor determining buffer efficiency (Dosskey, 2001).
When applied as part of a conservation management system, the effectiveness of
conservation buffers can be enhanced. There are few data on the field or landscape level
effectiveness of conservation buffers applied with or without other conservation
measures. Most data are from plot studies. Plot studies are inadequate, especially for
studies of grass waterways (GWW), which are designed to convey overland flow from
fields and stream bank restoration designed to reduce loss of sediment and sediment
bound chemical from unstable banks. Because GWW are installed in areas of known
water flow, they avoid problems of runoff bypassing filter strips and field borders. The
few studies of GWW conducted at the field scale show that they are very effective at both
runoff reduction and sediment trapping. In Germany, unmanaged grass waterways
reduced runoff and sediment delivery by 90 and 97% respectively compared to adjacent
fields with no GWW (Feiner and Auerswald, 2003). A GWW that was mowed closely
was less effective, with reductions of 10 and 27% for runoff and sediment delivery,
respectively. In New Brunswick, Canada, Chow et al. (1999) compared up- and down-
slope cultivation of potatoes and grain to the same crops with a terrace and grass
waterway system. The conservation system reduced runoff by 31% and sediment
delivery by 78%. On three small watersheds in the claypan soils region of Missouri,
sediment and TP loss increased as the extent of GWW decreased (Udawatta et al., 2004).
There are ongoing efforts by USDA to estimate the impacts of conservation
buffers on water quality in all watersheds with significant amounts of agriculture. The
Conservation Effects Assessment Project (CEAP) will eventually provide model-based
estimates of the water quality impacts of conservation practices in the MARB (Kellogg
and Bridgham, 2003). Conservation buffers are an important component of USDA
conservation programs. Table 12 summarizes the extent of seven major conservation
buffer practices installed in the six sub-basins of the MARB in federal fiscal years 2000
through 2006 (October 1999 - October 2006) (M. Sullivan, personal communication,
based on USDA-NRCS-Performance Results System,
http://ias.sc.egov.usda.gov/prshome). An estimated 0.94 million ha (2.31 million ac) of
conservation buffers were installed in the MARB in 1999-2006. As shown, each ha of
conservation buffer treats one or three ha of adjacent agricultural land, giving an
163
-------
estimated 3.46 million ha (8.55 million ac) of agricultural land has been treated by these
six conservation buffer practices (Table 12).
Table 12: Areas (ha) of conservation buffers installed in the six sub-basins of the MARB for FY 2000 -
FY2006.
Subbasin
Ohio
Tennessee
Upper
Mississippi
Lower
Mississippi
Missouri
Arkansas
White-Red
Sum
Area
treated
(ratio)
Area
treated
Contour
Buffer
Strips
(ha)
3,362
196
22,217
165
7,374
1,883
35,196
1:1
70,393
Field
Border
(ha)
5,441
1,914
7,357
7,541
16,413
15,631
54,298
1:1
108,595
Filter Strip
(ha)
50,617
10,724
159,604
10,274
116,755
79,658
427,631
3:1
1,710,52
5
Grassed
Waterway
(ha)
21,346
817
43,421
661
31,067
8,197
105,507
3:1
422,030
Riparian
Forest
Buffer (ha)
32,497
10,752
75,139
56,106
31,492
29,745
235,731
3:1
942,926
Stream
bank
Protection
(km)
755
418
722
503
470
287
3,155
NA
NA
Windbreaks
and
Shelter-belts
(ha)
794
2
8,448
391
39,377
2,173
51,185
3:1
204,739
Conservat
ion
Buffers
Applied
(ha)
114,832
26,025
317,422
75,486
256,693
145,290
935,748
3,459,207
* Kilometers are shown for stream bank protection. Conservation buffers applied includes areas in other practices not
shown here that are cumulatively small areas compared to the practices shown. The areas treated are based on the
ratios shown and assumes that each ha of buffer treats either one ha or three ha of adjacent agricultural land. Areas of
practices are from Mike Sullivan, USDA-NRCS, Personal Communication, and are derived from NRCS-PRS,
http://ias.sc.egov.usda.gov/prshome.
Information on the extent of other conservation practices established from FY
2000 through FY 2006 is also available from the NRCS Performance Results System.
Practices that are applied each year such as conservation tillage, residue management,
and nutrient management may be reported more than once during the record period if
there is a change in owner/operator, a new conservation plan is developed, and associated
practices are reported. There may have also been some systematic annual reporting in the
early years of the record period (2000-2003) (Personal communication, Mike Sullivan,
USDA-NRCS). All conservation tillage and residue management practices combined
were applied on as much as 8.42 million ha (20.8 million ac) and nutrient management
was applied on as much as 7.4 million ha (18.3 million ac) in the MARB in FY 2000 to
FY 2006 (Mike Sullivan, USDA-NRCS, Personal Communication, based on NRCS-
PRS). Wetland creation, enhancement and restoration was applied on 0.57 million ha
(1.42 million ac), drainage water management was applied on 756 ha (1,867 ac), and
stream bank restoration was installed on 3,155 km (1,972 mi). The values for 2002-2005
were reported in the USEPA Management Action Review Team report (MART 2006a)
and are similar to the above numbers when put on the same year basis.
164
-------
Currently no national databases allow a more detailed estimation of the
environmental benefits of these conservation practices, including conservation buffers.
This is the goal of the CEAP project. Estimates can be made based on acreage values,
but these cannot take into account either placement or efficacy of practices.
Cumulatively, conservation buffers, residue management, nutrient management, and
wetlands have impacted up to 21 million ha (51.9 million acres) of agricultural land in the
MARB based on the FY 2000 - FY 2006 areas of conservation practices. This area is the
sum of residue management, nutrient management, conservation buffer acreage, wetland
acreage and the potential land treated by conservation buffers (Table 12) and wetlands
(assuming 3 hectares treated for 1 hectare of wetlands). In reality, conservation practices
are applied as a system of practices, and it is likely that the total area treated through
these practices is less than 21 million ha (51.9 million ac). Additionally the data bases
used are likely to include some duplicate reporting for the annual practices. The nutrient
load reductions for these practices could be estimated based on amounts of N and P load
retained. Although these would be crude estimates, they would provide numbers for
comparison to the nutrient load reduction goals and provide a rough idea of where
conservation programs stand relative to those goals.
Key Findings and Recommendations
Conservation buffers and other conservation practices have affected a significant
acreage of MARB cropland through existing federal, state, and private programs. The
SAB Panel offers the following recommendations.
Continued, new, and enhanced small watershed based studies of suites of
conservation practices as applied on farms and in agricultural watersheds are
necessary. Analysis of effects of conservation buffers and other conservation
practices in the MARB should be coordinated with the ongoing USDA
Conservation Effects Assessment Project.
Conservation buffers and other conservation practices in the MARB should be re-
focused on N and P retention with special attention given to the interactions of
buffers with other practices. Environmental benefits indices should be calculated
in a way as to provide extra weight for N and P retention.
4.5.4. Cropping systems
Current cropping systems within the MARB are well established, but advances in
N fertilizer production technology, innovative crop rotations, inter-seeding with cover
crops, and alternative mulches or crop residues provide opportunities to improve water
and nutrient use efficiency as well as to decrease leaching and runoff of nutrients and
sediments. For example, inter-seeding of a leguminous cover crop within existing crop
rotations could enhance N and P use efficiencies, as long as the cover crop is carefully
165
-------
managed. Also, greater adoption of perennial systems, which could include cellulosic
production, have the potential to influence nutrient export via reduced N and P
applications as well as altered water budgets. Evapo-transpiration and infiltration will
likely be greater with perennial than annual cropping systems, contributing to a decrease
in potential runoff. Hydrologic and water quality issues related to perennials and
cellulosic production are discussed in more detail in Section 4.5.9. - Ethanol and Water
Quality in the MARB.
A continuous corn rotation typically results in annual N fertilizer applications
between 150 and 250 kg N/ha (134 and 223 Ib N/ac). This is a large amount of N
fertilizer relative to amounts applied to other crops. Including other crops (particularly
legumes) in a crop rotation usually reduces annual N fertilizer applications needed. In
addition to applying less N, perennial crops, such as alfalfa or other grass mixtures, have
longer effective growing seasons and are more efficient N users than annual crops, which
translate to greater water use and less nitrate leaching.
Randall et al. (1997) compared tile drainage and nitrate loss for corn-soybean and
corn-corn rotations to alfalfa and Conservation Reserve Program (CRP) grassland. From
770 to 905 mm (30 to 36 in) of tile water was recorded for the corn-corn and corn-
soybean rotations froml988-1993, whereas 416 to 640 mm (16 to 25 in) of tile water was
recorded for alfalfa and CRP. Flow-weighted nitrate-N concentrations were less than 5
mg/L for alfalfa and CRP but ranged between 13 and 40 mg/L for the rotations including
corn and soybean. The four-year nitrate-N loss from continuous corn or corn-soybean
rotations was 202 to 217 kg N/ha (180 to 194 Ib N/ac), while for alfalfa and CRP the loss
was less than 7 kg N/ha (6 Ib N/ac). Similarly, Jaynes et al. (2001), showed for a corn-
soybean rotation in central Iowa that even at economically optimum N fertilizer rates for
corn (67 to 172 kg N/ha or 60 to 154 Ib N/ac), NOs loss in tile drainage water increased
from 29 to 43 kg N/ha (26 to 38 Ib N/ac) with application rate. Also, a net N mass
balance indicated that N was being mined from the soil at economically optimum N
fertilizer rates and the system would not be sustainable (Jaynes et al., 2001).
Besides crop selection to enhance N and P removal, crop rotation also can be
managed to maximize nutrient removal and minimize leaching. Together, crop selection
and rotation can influence the amount of N and P in a soil profile as well as water
available for nutrient leaching. As mentioned, legumes, such as alfalfa and soybean, that
do not require supplemental N, can effectively use or "scavenge" residual inorganic N
remaining in the soil from previous crops. Some crops take up more P, and deep-rooted
crops can remove N and P from subsoil horizons. For example, root development of a
typical 3-year continuous corn system (maximum depths in May through September)
does not always coincide with time of high NOs leaching potential (generally February to
April). An alternative cropping system comprised of corn-winter wheat-alfalfa provided
a much different root development pattern, one that should more efficiently retain N
because it has deeper roots that are present most of the year (Sharpley et al., 2006b).
Olson et al. (1970) found thatNO3 concentrations at a depth of 1.2 to 1.5 m (3.9 to 4.9 ft)
in a silt loam soil were lower for an oat-meadow-alfalfa-corn rotation than for continuous
corn when ammonium nitrate was applied to both systems. The reduction in
166
-------
leaching was directly proportional to the number of years that oats, meadow, or alfalfa
was grown in rotation with corn. The reduction was attributed to the combined recovery
of NOs by shallow-rooted oats, followed by deep-rooted alfalfa (Olson et al., 1970). The
potential for NO3 leaching in such rotations is, therefore, less when compared with
continuous annual monocropping systems.
Clearly, including perennial crops in a rotation, as well as conversion to perennial
systems, can reduce NO3 leaching, partly due to the fact that perennials are generally
more efficient users of N than annuals. As a result, Randall and Vetsch (2005) raises a
key question of whether significant reductions in nutrient (especially NO3) loadings to
surface waters are possible without changing from the predominant annual cropping
system of corn-soybean rotation to a mixed system that includes perennials. While
annual grain crop production is an essential component of agricultural systems in several
areas of the MARB, the development of economically viable continuous cropping
systems will help improve in-field nutrient use efficiency and decrease off-site loads.
Additional co-benefits of perennials such as switchgrass, are that they have the potential
to accumulate large amounts of below-ground biomass and are effective in sequestering
C (McLaughlin and Walsh, 1998; McLaughlin and Lszos, 2005).
Retirement of land through the Conservation Reserve Program has demonstrated
different results for various cropping systems. For lands previously in corn, the reduction
in N delivered to the Mississippi River may have been as much as 25 to 30 kg N/ha/yr
(22 to 27 Ib N/ac/yr). For soybean it would have been somewhat less, and for small
grains, particularly wheat in the High Plains, smaller reductions, in the range of 10 kg
N/ha/yr (8.9 Ib N/ac/yr) may have been realized (see Section 3.1.2. - Subbasin Annual
and Seasonal Flux). Where CRP has been used to establish buffers, not only are
reductions from the retired lands realized, but the buffers can also be effective in
reducing inputs of N and P from upslope cropland entering water courses via surface
runoff and shallow subsurface flow. It should be noted, however, that most land enrolled
in CRP is primarily sloping, erosive land that is not tile drained. For instance, Mclsaac
and Hu (2004) studying N flux in several Illinois rivers between 1977 and 1997 found
that riverine N flux was about 100% of net N input for the tiled drained region (27 kg
N/ha/yr or 24 Ib N/ac/yr). In the non- tile drained region, riverine N flux was between 25
and 37% of net N input (23 kg N/ha/yr or 20 Ib N/ac/yr).
Key Findings and Recommendations
Cover crops and other living mulches can improve water and nutrient use
efficiencies and reduce nitrate leaching. Further research and demonstration is needed in
the MARB in several areas: examining the benefits of intercropping cover crops with
annuals such as corn; determining if leguminous cover crops reduce fertilizer N
requirements; and assessing how changes in cropping patterns can impact nutrient loss at
both local and basin-wide scales. If farmers could be encouraged to switch to a rotation
of perennial crops as compared to the predominant corn-soybean rotation system,
167
-------
significant N and P reductions would result. Based on these findings, the SAB Panel
offers these recommendations.
Cover, relay, and perennial crops should be considered in alternative cropping
systems that will reduce nutrient loss. Cropping systems that efficiently include
cover crops in grain and row cropping should also be encouraged in the Corn Belt
region of MARB. This should focus on the use of fall planted small grain cover
crops more suited to the short growing season after harvest and cold winters of the
upper Midwest.
Where corn-soybean production systems exist and/or where it is not feasible to
plant cover crops, it is even more important to encourage off-field conservation
practices.
4.5.5. Animal Production Systems
System development and nutrient flows
While overall production livestock numbers in the MARB have declined (see
Section 3.2. - Mass Balance of Nutrients), there has been an intensification of operations
in certain areas (see Figure 43, Figure 44, and Appendix E: Animal Production Systems).
Farmers adopted the AFO paradigm because of competitive pressures, changing
marketing practices, a need to be responsive to consumer demand for quality meat
products at a low cost, and declines in income from traditional grain crops in certain areas
of the MARB with inherently infertile soils (Lanyon, 2005). This critical socioeconomic
shift must be considered when proposing changes within the MARB that decrease the
impact of AFO and manure management on nutrient export.
168
-------
tonnes N per county or
combined counties'
D «25
D 2510100
D 100 to 250
250 to 1000
> 1000
'Some counties are combined to meet disclosure criteria.
Figure 43: Recoverable manure N, assuming no export of manure from the farm, using 1997 census data.
Adapted from USDA (2003) with the author's permission.
As a consequence of the spatial separation of crop and animal production systems,
fertilizer N and P is imported to areas of grain production. The grain (harvested N and P)
is then transported to areas of animal production, where inefficient animal utilization of
nutrients in feed (less than 30% is utilized) are excreted as manure. This system has led
to a large-scale, one-way transfer of nutrients from grain- to animal-producing areas
within the MARB and dramatically broadened the emphasis of nutrient and manure
management strategies from field to watershed to basin scales. For the MARB, farm-
level nutrient excesses are estimated at 337 million kg N (743 million Ib N) and 242
million kg P (534 million Ib P) (Gollehon et al., 2001).
169
-------
lonnes P per counly or
combined counties"
25 lo 100
10010250
250101000
>1000 'Some counties are combined to meet disclosure criteria.
Figure 44: Recoverable manure P, assuming no export of manure from the farm, using 1997 census data.
Adapted from USDA (2003) with the author's permission.
The land application and discharge of nutrients in manure from AFOs are
regulated under the National Pollutant Discharge Elimination System (NPDES), which
generally define an AFO as an operation where livestock are confined for an extended
period of time (at least 45 days in a 12-month period) and there's no grass or other
vegetation in the confinement area during the normal growing season (U.S. EPA, 2000a).
This definition is intended to differentiate confinement-based operations from pasture-
based operations, which are excluded from the Confined Animal Feeding Operations
(CAFO) regulations. The NPDES permit is required to control pollutants at an AFO and
keep them from entering surface waters. More explicitly, the U.S. EPA (2000a) defines
CAFOs as livestock operations that meet one of the following characteristics:
Confine more than 1,000 animal units (AU), where 1,000 AUs are defined as
1,000 slaughter and feeder cattle, 700 mature dairy cows, 2,500 swine (other than
feeder pigs), 30,000 laying hens or broilers if the facility uses a liquid system, and
100,000 laying hens or broilers if the facility uses continuous overflow watering.
Confine between 300 and 1,000 AU (as defined above), and either a man-made
ditch or pipe carries manure or wastewater from the operation to surface water or
animals come into contact with or surface water running through the area where
they are confined.
These regulations are enacted at a national level, and thus, there are recommendations
and controls on the land application or utilization of manures and their component
nutrients are in place at a state level in the MARB. Based on an US EPA summary of
170
-------
CAFO permit implementation completed in the first quarter of 2007, less than half of the
CAFOs in the MARB were permitted (46%; Table 13). States included are in Table 13,
if part of the state drains into the MARB. The approximate number of permitted CAFOs
in the MARB is similar to the national average (44%; U.S. EPA, 2007) but clearly, rule
implementation varies among states.
Table 13: Status of implementation of permits under the 2003 CAFO rule for states within the MARB.
Data provided by EPA Office of Wastewater Management, 2007.
State
Alabama
Arkansas
Colorado
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Michigan
Minnesota
Mississippi
Missouri
Montana
Nebraska
New Mexico
North Carolina
North Dakota
Ohio
Oklahoma
Pennsylvania
South Dakota
Tennessee
Texas
Virginia
West Virginia
Wisconsin
Wyoming
Total
Number of
CAFOs
558
2,110
225
500
584
1,859
476
150
150
198
1,007
433
492
TBD
1,000
151
1,222
47
162
625
462
369
129
1,204
150
30
161
51
14,505
Number of CAFOs
with permits to date
440
70
33
8
413
113
462
67
2
56
1,000
190
492
75
303
47
1,200
0
64
163
165
303
130
639
0
0
161
47
6,643
Permit coverage for
CAFOs under 2003 rule
79
3
15
2
71
6
97
45
1
28
99
44
100
TBD
30
31
98
0
40
26
36
82
101
53
0
0
100
92
46
171
-------
Manure as a Component of N and P Mass Balances
Within the MARB, counties with the greatest excess of recoverable manure N and
P (if applied on the farm where it is generated) tend to be in the western and drier areas of
the basin, Arkansas, and central Minnesota (Figure 43 and Figure 44). Recoverable
manure is defined as the portion of manure as excreted that could be collected from
buildings and lots where livestock are held and, thus, would be available for land
application. Recoverable manure nutrients are the amounts of manure N and P that
would be expected to be available for land application (USDA, 2003). They are
estimated by adjusting the quantity of recoverable manure for nutrient loss during
collection, transfer, storage, and treatment. Recoverable manure nutrients are not
adjusted for losses of nutrients at the time of land application. Where riverine N export is
the greatest (upper Mississippi and Ohio River basins with tile drainage), manure N
excess tends to be less, lower Mississippi River basin states, particularly Arkansas and
northern Missouri, clearly have more manure P on some farms than land area to apply it
(Figure 43). Although N from manure can be important in specific areas, basin-wide N
loss is a result of the dominant inputs of fertilizer and N2 fixation on tile-drained corn
and soybean fields. For P, manure is a more important source, particularly on the western
side of the basin (Figure 44).
Large-scale consolidation has created much larger AFOs, which makes
economical utilization and re-distribution of manure to croplands difficult and has
profound consequences for regional nutrient transfer and management within the MARB.
Furthermore, the potential for co-locating AFOs with areas of the corn production for
ethanol generation may exacerbate the accumulation of manure-based nutrients in these
areas. This co-location stems from the use of by-products from ethanol production
(distiller's grain) as animal feed (for more information see Section 4.5.9).
Remedial Strategies
Manure is a valuable resource for improving soil structure and increasing
vegetative cover, thereby improving water quality via reduced runoff and erosion
potential. Manures have been historically applied at rates designed to meet crop N
requirements. This has resulted in the accumulation of soil P above levels required for
crop production, and a concomitant increase in the potential for N and P loss via runoff,
leaching and N2O emission within the MARB (Table 14; Aillery et al., 2005; Sharpley et
al., 1998). In the past, separate strategies for either N or P have been developed and
implemented at farm or watershed scales. The SAB Panel recognizes that this approach
needs to change; N and P need to be managed jointly in order to improve water quality.
Because of different critical sources, pathways, and sinks controlling N and P export,
remedial strategies directed at only N or only P control can negatively impact the other
nutrient. For example, basing manure application on crop N requirements to minimize
nitrate leaching can increase soil P and enhance P losses (Sharpley et al., 1998; Sims,
1997). In contrast, reducing surface runoff losses of P via conservation tillage can
enhance nitrate leaching in some cases (Sharpley and Smith, 1994).
172
-------
Table 14: Estimates of manure production and N and P loss to water and air from Animal Feeding
Operations within the Mississippi River basin, on information from the 2002 U.S. Census of Agriculture
(adapted from Aillery et al., 2005).
K ' fMARR # Total N N N Total P
Kegion or IVlAKtS Operations Manure Runoff Leached Emissions N loss Runoff
MN^rf8 (MI' 52'498 62-52 32-89 °-36 164-45 198 5-58
iN^M^oro ^ 71'252 85-09 39-73 °-47 234-89 275 1L78
^"^"SD) ^ 7227 3«' "" 16844 2°5 6"
Appalachia (KY, 25916 315 1579
NC, TN, VA, WV) zz'//0 /y--:)/ :)H-0-:) U-V1 ^y.io ju u./y
LA^sT68 (AR' 12'252 19'9? 8'92 °'15 62'5? ?2 4'4?
?°"thf^PlainS 10,500 49.19 21.96 0.20 119.74 142 7.72
(OK, IX)
Total 195,365 368.63 194.46 2.46 1009.26 1206 52.34
Long-term sustainable management of nutrients in manure begins with sound feed
decisions, which generally lie with the integrator in the CAFO industry rather than the
individual farmer. Nutrient inputs to a farm should be matched as closely as possible
with export as animal or crop products. If a farm's N and P budget is rich in imports,
regardless of any other management decisions, there will be an ongoing accumulation of
N and P on the farm, which in the long-term will ultimately increase the potential for
nutrient loss to water or air when manure is land-applied. Nevertheless, the short-term
impacts of land-applying manure or litter on nutrient loss can be reduced by the adoption
of conservation practices detailed by USDA-NRCS (ftp://ftp-
fc.sc.egov.usda.gov/NHO/practice-standards/standards/590.pdf). However, conservation
measures at both farm and watershed scales involves a complex suite of options, which
must be customized to meet site-specific needs (for more information see Section 4.5.10:
Integrating Conservation Options and Appendix E: Animal Production Systems).
Alternative manure management technologies
Reducing farm-gate inputs of N and P in animal feed presents one of the best
nutrient management opportunities to effect a lasting reduction in N and P loss
(Appendix E: Animal Production Systems). Other measures, generally aimed at
173
-------
reducing the potential for N and P losses, are seen as short- rather than long-term
solutions to environmental concerns. For instance, long-term monitoring of P budgets in
Ohio showed that after nearly 20 years of BMP adoption and despite continually
increasing soil test P levels, manure applications and timing have been managed better,
resulting in more efficient use of P and reduced P loss to surface waters (Baker and
Richards, 2002). Manure-related conservation practices include:
Manure amendments, such as alum, to reduce ammonia volatilization and
sequester P in less soluble forms;
coagulant and flocculent techniques to separate and concentrate nutrients in liquid
manure systems; and
combining manure with biosolids and woodchips to reclaim soils that have been
disturbed (e.g., by mining or urban development).
As the cost of N fertilizer increases, it is clear that new markets for alternative
uses or products for manure will open up. For example, on-farm and regional energy
production via burning of manure is of increasing cost-effectiveness. Ash production via
burning, while rich in P, will be appreciably less bulky and, thus, enable cost-effective
transportation further from the source of generation. The bulky nature of manures and
resulting high cost of transportation has always been a major limitation to more effective
redistribution of N and P to nutrient deficient areas of the MARB.
Recent efforts to exclude cattle from streams as part of the Conservation Reserve
Enhancement Program (CREP) were estimated to have resulted in a 32% decrease in P
loadings to streams within the Cannonsville watersheds in south central, New York
(James et al., 2007). Thus, exclusionary programs like CREP and stream bank fencing
are working to reduce nutrient loading by fencing cattle out of the stream and adjacent
riparian zones. Clearly, grazing management and placement of stream bank fencing is
important to minimizing watershed export of P. For instance, herd size, pasturing time,
and cattle type could all be used to prioritize sites for stream bank fencing installation. In
addition, field observations [such as those by James et al. (2007)] show installation of
alternative watering sources do not necessarily preclude continued use of streams as a
preferred water source.
The wider adoption of manure hauling that links producers with buyers will
greatly enhance the sustainability of AFOs. At a state level, the Discovery Farms
program is conducting research on privately-owned Wisconsin farms in different
geographic areas, facing different environmental challenges (see
http://www.uwdiscoveryfarms.org/new/index.htm). The Discovery Farms program has
been very successful at gaining farmer support in at-risk catchments in efforts to find the
most economical solutions to overcoming the challenges environmental regulations place
on farmers. At a watershed level, the Illinois River Watershed Partnership (see
http://www.irwp.org/index.html) was established in 2005 to improve and protect water
174
-------
quality in the Illinois River in Arkansas and Oklahoma by working at a grassroots level
with watershed citizens and other organizations.
Key Findings and Recommendations
The impacts of animal production systems are mainly expressed at a local rather
than MARB scale. Overall, numbers of animals in the MARB have decreased, but
localized increases have occurred in several regions, which have had an impact on local
water resources. The economic and environmental sustainability of AFOs hinges on
reducing the nutrient imbalance at farm and watershed scales through carefully managed
feeding strategies. The wider adoption of manure transportation that links producers with
buyers will greatly enhance the sustainability of AFOs. The large-scale consolidation of
AFOs, co-siting with biofuel production facilities (byproduct grains used as animal feed),
and increases in N fertilizer prices will likely create the economies of scale and
alternative technologies for on-farm or localized manure use and management more
feasible.
The success of non-profit programs supported by watershed agricultural councils,
industry, and state agencies, should provide valuable demonstration models. If energy
prices remain at current levels, bioenergy production from manures could provide an off-
farm market for manures and reduce localized nutrient surpluses. Continuing educational
efforts with farmers and the public regarding the importance and impact of conservation
practices will be essential to reach environmental goals. Based on these findings, the
SAB Panel offers the recommendations below.
Strategies need to be implemented to encourage further development of
alternative uses for manures such as in composting, pelletizing, and granulation,
and as a soil amendment in nutrient deficient areas of the MARB.
Land-management planning and implementation of conservation practices should
be designed to identify and avoid applications in critical loss areas, to use buffers
or riparian zones, to manage grazing, to exclude stream banks, and to use
subsurface injection with innovative applicators.
Incentives to encourage on-farm and local bioenergy production from manure
sources should be provided.
4.5.6. In-field Nutrient Management
Fertilizer sources
The principal fertilizer N sources (>90% of fertilizer N) used in the MARB are
anhydrous ammonia, urea-ammonium nitrate solutions, and urea. Anhydrous ammonia
use in several leading corn-producing states (IL, IN,IA, MN, NE, OH) has tended to
decline in recent years, perhaps with the exception of consumption in Illinois and Indiana
(Figure 45) (Sources: Association of American Plant Food Control Officials; H. Vroomen
175
-------
with TFI-personal communication, 2007). The largest decline has been in Nebraska,
where use of anhydrous ammonia N has declined about 40% since the mid-1980s.
700000
600000 -
CD
'c
O g- 500000 H
E £
5 £ 400000 i
'
E
300000
200000 H
100000 -
0
1970 1975 1980 1985 1990 1995 2000 2005 2010
Illinois
Indiana
Minnesota
Iowa
Ohio
Nebraska
Figure 45: Fertilizer N consumption as anhydrous ammonia in leading corn-producing states for years
ending June 30.
The combined N consumption of urea and urea-ammonium nitrate solution has increased
and recently surpassed anhydrous ammonia tonnage in these six leading corn-producing
states (Figure 46). Although these data illustrate shifts in fertilizer N sources used, they
do not allow conclusions about the portion of the annual anhydrous ammonia
consumption that may be applied in the fall.
176
-------
(A
O
4,000,000
3,500,000
3,000,000
2,500,000
2,000,000
1,500,000
1,000,000
500,000
0
Anhydrous ammonia
Urea+ UAN Sol.
A Anhdrous ammonia + urea + UAN
1965 1970 1975 1980
1985 1990
Year
1995 2000 2005 2010
Figure 46: Changes in the consumption of principal fertilizer N sources used in the six leading corn-
producing states (IA, IL, IN, MN, NE, and OH) for years ending June 30.
Fertilizer use and application technology
The Integrated Assessment (CENR, 2000) concluded that "discharges of nitrogen
from farms to streams and rivers could be reduced by implementing a wide variety of
changes in management practices". These practices include switching from fall fertilizer
N to spring N applications, and applying nitrogen fertilizer and manure at not more than
agronomically recommended rates. Application rate and timing are linked for N because
the closer application is to the time of crop need, less N is lost to the atmosphere and
water, and less N is needed. Research at five Management System Evaluation Area
(MSEA) sites in the MARB (OH, IA, MN, MS, NE) reaffirmed BMPs for water quality,
including soil nitrate tests, improved water management, and improved N timing and
placement relative to crop needs (Power et al., 2000). Determining N sufficiency by
monitoring for plant greenness and use of field or remote-sensing technologies followed
by site-specific N applications hold promise to manage N more precisely.
Application timing. The risk of N loss with corn is greatest when fertilizer is
applied some time before the period of rapid plant growth. Data on fall application are
not directly available for the MARB and even seasonal data on fertilizer sales are not
kept by all states in the MARB (Terry, 2006). Fertilizer sales records for Iowa (from July
2002 to June 2006) showed that 48% of N fertilizer was sold in the period from July to
December and 52% from January to June. For anhydrous ammonia, the most common N
form used and the primary form applied in the fall, 54% was sold in the period from July
to December and 46% was sold from January to June. July to December sales of
177
-------
anhydrous ammonia accounted for 273,000 tons of actual N (data from
http://www.agriculture.state.ia.us/fertilizerDistributionReport.htm). For Illinois, there
has been an increase in fall N sales from the 1970s and 1980s to present, from about 25%
to 40 to 50% (Figure 47).
Although it is not possible to correlate fall N application directly with fall
fertilizer N sales, it is likely that a large fraction of the fall N sales represents N applied in
the fall (Czapar et al., 2007). A portion of this fall N tonnage sold may also be stored at
dealerships or in on-farm storage vessels for application the following spring. The
USDA Agricultural Management Resource Survey (ARMS-
http://www.ers.usda.gov/Data/ARMS/app/Crop.aspx ) data provide some insight into fall
N applications, yet they are not sufficiently complete (i.e. key years are missing) to
determine if the percentage of the acreage that receives some amount of fall N is
increasing, decreasing, or remaining static (Figure 47). The data do indicate that
Minnesota, Iowa, and Illinois tend to fall apply some N on a larger fraction of their corn
acres, compared to the other three states shown in (Figure 47). For three states, USDA
ARMS data were used to calculate the total fraction of N applied to corn in the fall, and
IL sales were also compared (Figure 49). As fewer producers in the Corn Belt farm the
existing acreage, there has been greater pressure to complete fertilization in the fall,
because of the numerous logistical challenges (labor demands, transportation and
application equipment availability, weather uncertainty, and fertilizer supply and cost
uncertainty) in the spring.
. , ,, on -,
Z £ yu
c 3 an
.s o ou
5 E 70
-o re ^ /U
0) 0) ^ fin
N g OU
'P S re ^n
t (A >i- OU
* ~Z - 40
°> > -2.
% 1 ^
S o o JU
o S! ?n
re £
E/s 1 n
ft i n
»
A
» . '
* : ' .
m :
₯ * ^ X *
I . ^
IL
IN
IA
MN
XNE
OH
Average
1995 1997 1999 2001 2003 2005 2007
Year
Figure 47: Percentage of N fertilized corn acreage which received some amount of N in the fall.
178
-------
o
o
s_
,0
M
o
0
Q.
Q.
^
Fall Fertilizer
c
o
CO
uu
50 -
40 -
30 -
20 -
10 -
0 -
50 n
40 -
30 -
20 -
10 -
0 -
50 -i
40 -
30 -
20 -
10 -
0 -
19
0 o o 8 o °
0 ° 0
0 0
ARMS data
0 ILSales Illinois
.
Iowa
* *
Minnesota
94 1996 1998 2000 2002 2004 2006 2008
Year
Figure 48: USD A ARMS data for the three states with highest fall N application, showing total amount of
fall applied N for that crop. Also shown are Illinois sales data for the same period.
Randall and Sawyer (2005) contacted State Extension soil fertility specialists and
State Fertilizer Associations to determine the fertilizer N amount that is applied in the
fall. Based on these data, they estimated 25% (5.1 million ha or 12.9 million ac) of the
20.5 million ha (50.6 million ac) of corn in an 8-state area (IA, IL, IN, MI, MO, MN, OH,
WI) received N in the fall. States with the largest amount of fall-applied N were
Minnesota (1.85 million ha or 4.56 million ac), Iowa (1.42 million ha or 3.52 million ac),
and Illinois (1.33 million ha or 3.28 million ac). It is likely that tile-drained portions of
these eight states have higher proportions of N applied in the fall either because of a
179
-------
greater dominance of corn/soybean agriculture or because regional soil temperatures are
also cold enough to help minimize the conversion of ammonium-N to nitrate-N
(nitrification) in the fall. Fall N application for corn as anhydrous ammonia, is currently
a recommended practice by virtually all Land Grant universities in the cornbelt, where
soil temperatures are consistently below 50° F at the 1.2 to 1.8 cm depth (0.47 to 0.72 in
or about /^ to % in), and the risk of environmental loss is not considered high or a
pragmatic concern (Snyder et al., 2001). Additional guidance is usually provided in
publications by Land Grant universities to maximize the benefits of fall N application and
to help minimize the risk of economical and environmental N losses (e.g. Shapiro et al.,
2003; Bundy, 1998).
In a 2003 phone survey of Champaign Co., IL (a dominantly tile-drained area),
61% of the 352 respondents reported applying some N in the fall, and 49% of
respondents applied all of their N in the fall (von Holle, 2005). Overall, the farmers who
fall fertilized applied an average of 79% of their annual N needs before January 1, 2003.
Data from 11 tile drained central-Illinois counties showed generally greater fall N
fertilizer sales than the state as a whole (Illinois Department of Agriculture fertilizer
tonnage reports). This difference was primarily due to the southern and non-tile drained
portion of the state having winter soil temperatures that are too warm for fall application,
where it is not recommended.
70
75
60 -
50 -
40 -
30 -
20 H
10
Illinois
11 tile drained
central-Illinois counties
b°
1975
1980
1985
1990
1995
2000
2005
2010
Crop Year
Figure 49: Fraction of annual fertilizer N tonnage in Illinois sold in the fall.
The effects of fall N application versus spring N application on nitrate transport in
tile drainage depend on many factors, including soil temperatures, soil texture,
precipitation, and drainage intensity. Randall and Sawyer (2005) reviewed the timing of
180
-------
N application and determined that spring application in Minnesota will typically result in
15% less nitrate-N loss than with fall application. In areas with warmer non-growing
season temperatures (such as central Illinois) that are tile-drained, losses of fall applied N
may be greater. Watershed-scale studies of changing from fall to spring application
(side-dressed) and changing the rate to account for more efficient use of spring applied N
showed at least a 30% reduction of nitrate concentrations in tile drain water (Jaynes et al.,
2004). These studies indicate there is a great potential in some years for substantial
reductions in N loss by applying N closer to when the crop can utilize it efficiently.
If these various estimates of N use and nitrate-N loss are combined, changing
from fall to spring application may affect at least 25% of the corn acreage and reduce
nitrate-N losses to streams from those acres by perhaps 10 to 30%. Split applications of
N do not always result in increased N efficiency and reduced nitrate-N losses, just
because of improved N synchrony with crop uptake demands. The literature to support
this practice indicates mixed results (Randall and Sawyer, 2005).
Nitrification inhibitors delay the conversion of ammonium to nitrate in soil. In
Illinois, it is estimated that a nitrification inhibitor is added to about 50% of the fall
applied anhydrous ammonia (Czapar et al., 2007). Application of a nitrification inhibitor
with anhydrous ammonia in the fall increased apparent recovery of N fertilizer in the corn
grain from 38% without a nitrification inhibitor to 46% with an inhibitor, compared to
47% with spring application with no nitrification inhibitor in long-term research results in
Minnesota (Randall and Sawyer, 2005, Randall et al., 2003). Ferguson et al. (2003)
found that in Nebraska the benefits of nitrification inhibitors (either increased yield or
reduced NOs-N leaching) are strongly dependent on specific conditions and are most
likely to be observed at suboptimal N rates (i.e. Economically optimum N rate (EONR;
the point where the last increment of N returns a yield increase large enough to pay for
the additional N)). They also reported that nitrification inhibitors can reduce crop yields
with late sidedress N applications. It is well known that time of N application will
largely govern any benefits from the use of nitrification inhibitors. Assuming increased
N recovery by the crop translates to less nitrate leaching, nitrification inhibitors can
potentially provide an economic benefit to farmers while reducing leaching.
Although the fertilizer N use trends indicate increased urea and urea-ammonium
nitrate (UAN) solution use in the Cornbelt and lower anhydrous ammonia use (Figure
46), there is a need for more research to document the benefits of split N applications of
these two sources vs. the more traditional fall anhydrous N applications. Use of urea and
UAN solutions may provide greater flexibility in N management than has been
experienced with anhydrous ammonia. Studies are underway to evaluate the crop and
water quality effects associated with different N sources and time of application (e.g. see
reports of work by Gyles Randall and others at the U. of Minnesota:
http://sroc.cfans.umn.edu/research/soils/index.html). In years when corn growth
proceeds rapidly, timely side-dressing can be difficult and delayed application can
severely reduce yields (personal communication, G. Randall, 2007).
181
-------
Application rate. Current N recommendations are usually applied across large
geographic regions and may provide erroneous results for field-specific soil-crop-climate
conditions (Gehl et al., 2005; Sawyer and Nafziger, 2005). Grouping soil types with
similar drainage characteristics, rooting depth, and organic matter content is a feasible
approach for determining more localized N recommendations and may result in more
environmentally friendly N management (Oberle and Keeney, 1990). Remote sensing,
geographic information systems, and variable application technologies offer an
opportunity to develop and implement site-specific N recommendations, but the
agronomic understanding of yield response to N on a site- and season-specific basis lags
behind the technological innovations. There are instances, however, where considerable
progress has been made in developing site-specific N recommendations (Raun et al.,
2005).
Application of N near rates that provide the EONR usually results in drainage tile
flow having nitrate-N concentrations in the range of 10-20 mg/L NOs-N for soybean-corn
rotations and 15-30 m/L NO3-N for continuous corn (Sawyer and Randall, in press).
Application of N above the EONR further increases MVN losses and reduces net
economic return. To the extent that N is being applied above the the EONR, reductions
in N loss through tile drains can be achieved with concurrent positive effects on net
return (Sawyer and Randall, in press).
A review of the effects on N rates on corn-soybean systems in the upper MARB
was conducted by Sawyer and Randall (in press) who found that in order to achieve a
30% reduction in tile drainage nitrate-N load, based on a study in Illinois, the N rate had
to be reduced by 78 kg N/ha (70 Ib N/ac) below the EONR, resulting in a large net
economic loss ($67/ha or >$27/ac). These results illustrate an example of the risk of
potentially large economic losses to farmers (and their communities) if they are asked to
reduce N rates below their maximum net return or EONR (Sawyer and Randall, in press).
The potential environmental benefits of any N rate reductions are highly site-specific, and
will also depend on how farmer's past N rates match their site-specific EONRs.
Economically optimum N rates are not the same across the cornbelt states, and the
same is true for other crops because of differences among soils, adapted crop varieties,
climate, management and many other factors that influence production and crop N
requirements (Hong et al., 2006; Sawyer and Nafziger, 2005). Corn N needs vary widely
both among and within fields (Scharf et al., 2005; Lory and Scharf, 2003). In some
fields, in some areas of the MARB, where farmer's N rates have exceeded the EONR
(especially where elevated N concentrations have been observed in water resources) there
may be opportunities to reduce N rates for corn (Mamo et al., 2003) and other crops.
Nitrogen application rate reductions must be economical for the farmer while also
protecting water resources. Prior history of many management inputs including fertilizer
N, manure, and tillage can affect crop N response and EONR interpretations. Farmers
should carefully consider N rates and evaluate results over several years, in the same
fields or plot areas. Rate reduction results obtained in one year can be highly affected by
environmental conditions. For example, it is not uncommon to observe year to year
variations in rain-fed corn yields ranging above 3.1 to 4.5 Mg/ha (50 to 90 bu/ac), and
182
-------
economic N rates associated with those yields to vary by more than 60 to 84 kg N/ha/yr
(54 to 75 Ib N/ac/yr) (Sawyer and Randall, in press; Mamo et al., 2003; Jaynes et al.,
2001).
As discussed in Section 3.2, higher crop yields (Figure 50) have resulted in
increased N removal in harvested grain, without increased N fertilization. Greater crop
harvest N removal may have helped contribute to slight reductions in net N inputs in the
entire MARB since about 2000; particularly in the Ohio and Upper Mississippi River
subbasins (see Section 3.2), the two sub-basins which also contribute the greatest annual
and spring N flux to the NGOM. Increased crop yield trends, improved plant genetic
selection, and pest control may also be contributing to the reduced nitrate-N transported
to the NGOM since the mid-1990s, and the steady decline in total N delivered to the
NGOM since the 1980s (see Section 3.1.1 and Figure 17). Any reductions in N
application rates could threaten attainment of high crop yields, which are vital to
profitable production, and which have contributed in some measure to the reductions in
net N inputs and riverine N discharge mentioned above.
19 -,
IZ
in
IU
re
r- p
^ o
f C
D
(1) /I
*t
i
^
* A_^_A ^« X*^ y^
A /\ 4^^^""^ ^^ X i
^v / \ / \s^ 160bu/A
^ Nf Y ^
126bu/A
i i r i i i i i i i r i i i i i
O^ O} O} O} O^ O^ O} O} O} O} ^D ^D ^D ^D ^D ^D ^D
O^ O^ O^ O^ O} O} O} O} O} O} ^D ^D ^D ^D ^D ^D ^D
Year
Figure 50: Average corn yields in six leading corn-producing states (IA, IL, IN, MN, NE, and OH), 1990-
2006 (Source:USDA National Agricultural Statistics Service).
Challenges and complexities of determining the EONR in individual fields and
farms, prevent the ability to make any general conclusions regarding N rate reductions
across the MARB that will achieve specific N load reductions to the NGOM. Because of
the complexity and dynamic nature of the N cycle, soil tests for N (nitrate, mineralizable
N) have not met with much success in practical field applications (e.g. Scharf et al.,
183
-------
2006a). Some, like the Pre-Sidedress Nitrate Test (PSNT), have resulted in modest
successes in N rate adjustments, particularly where there is a long history of manure
applications and there has been a build-up of residual soil N (organic and inorganic). A
new soil N test (ISNT) developed in Illinois offered promise of more reliably predicting
mineralizeable soil N pools (Khan et al., 2001; Mulvaney et al., 2001); however, a recent
report indicates the ISNT does not work well elsewhere (Barker et al., 2006a and 2006 b;
Laboski et al., 2006).
One of the key challenges in managing N in farm fields is to minimize
unnecessary N applications in low-yielding years and to provide adequate N in high-
yielding years to meet crop demands. Historically, it has been very difficult for even
experts to predict residual soil N, recently applied fertilizer N, and mineralized N
accessible by plants during a given growing season (e.g., Schlegel et al., 2005;
Shehandeh et al., 2005). Furthermore, the inability to accurately predict the amount,
intensity, or duration of rainfall in a given year, makes it difficult to adjust N rates each
year for a specific soil, crop variety/hybrid, tillage system, or cropping system.
Watershed-scale fertilizer management
The first watershed-scale study of changing from fall to spring N application
involves changes in both rate and timing (Jaynes et al., 2004). The Late Spring Nitrate
Test (LSNT) is designed to help farmers add appropriate amounts of N in the spring
instead of fall. Use of the LSNT for corn grown within a 400 ha tile-drained watershed in
Iowa resulted in at least a 30% reduction of nitrate-N concentrations in tile-drain water.
The LSNT involved changing timing, rate, and source of N fertilizer. Another Iowa
study concluded that although watershed-scale implementation of LSNT had the potential
to reduce nitrate loss through drainage water, it could also increase grower risk,
especially when above-normal rainfall occurs shortly after the side-dress N is applied and
N is lost to tile drainage or denitrification (Karlen et al. 2005). Development of
affordable risk insurance or some other financial incentive by federal, state, or private
agencies may be needed to stimulate adoption of the LSNT.
Contr oiled-release fertilizers
Controlled- and slow-release N fertilizers (CRN) are fairly commonly used in
high-value applications, such as horticultural crop and turf production. Products include
urea formaldehyde and isobutylidene diurea, and sulfur- and polymer-coated products.
Use of CRN fertilizer is limited because of the high cost, with world-wide consumption
less than 1% of all fertilizer N products. However, recent advances have brought some
CRN products to an economical level for many agricultural crops. Controlled-release N
fertilizers have the potential to significantly improve N use efficiency, maintain crop
productivity, and minimize the potential for nitrate loss from fields (Blaylock, 2006).
184
-------
Effects of N management on soil resource sustainability
It is well known that soil organic carbon (SOC) storage in Corn Belt Mollisols has
been decreased by long-term cropping. For instance, in an Iowa study to determine the
effects of cropping systems on SOC, there was 22 to 49% lower SOC than native prairie
sampled in fence-rows for all cropping systems that had been in place for 12 to 36 years
[including continuous corn (CC); corn soybean rotation (CS); corn, corn oats, alfalfa; and
corn oats alfalfa, alfalfa] (Russell et al., 2005). Current efforts to sequester carbon by
restoring SOC and to obtain benefits of fertility and tilth associated with higher SOC in
Mollisols should be considered in achieving nutrient load reductions from these crop
production systems.
Nutrient management practices need to be assessed for their ability to enhance or
maintain SOC content in addition to their impact on profit, yield, and water quality
(Jaynes and Karl en, 2005). A careful review of the literature on this subject is warranted
because of the potential that fertilizer management to achieve water quality
improvements may lead to further soil quality degradation. Jaynes and Karlen (2005),
based on Jaynes et al. (2001), find a partial N mass balance for three fertilizer N levels in
a corn/soybean rotation on Mollisols in the Des Moines lobe region of Iowa. Tillage
consisted of either moldboard or chisel plowing in the fall and use of a field cultivator for
seed-bed preparation and for weed control several times during the early growing season.
The partial N mass balance shows that the IX and 2X fertilizer N rates have a negative N
mass balance and the 3X rate had a positive mass balance. Although the 2X rate (134 kg
N/ha or 120 Ib N/ac on corn, no N applied to soybeans) was the economic optimum, the
negative N mass balances may indicate a long-term decline in soil fertility. According to
the authors, "The lower two N rates were thus effectively mining N from the SOM,
which would result in a measurable decrease in SOM and a degradation of the soil
resource over the long term." Although all treatments had average nitrate-N
concentrations above 10 mg/L nitrate-N, there were large and consistent differences
among N loads in drain tile (Table 15). The IX and 2X treatments achieved drain tile
nitrate-N load reductions of 39% and 27%, respectively, compared to the 3X fertilizer N
rate (201 kg N/ha or 179 Ib N/ac).
185
-------
Table 15: Partial N balance for 4-year rate study by Jaynes et al. (2001). The last two columns added here
and were not part of original table.
Fertilizer
Rate
IX
2X
3X
Total
Fertilizer
Applied
Total Wet
and Dry
Deposition
Total
Fixed
Total
Grain
Removed
Total
Drainage
Loss
Total
Runoff
Change
of
Residual
Mineral
N
N
Balance
Residual
ktr N/ha
144
289
414
43
43
43
395
397
394
522
590
606
119
142
195
0
0
0
6
13
-7
-55
-26
47
(Residual/
Fixed)* 100
(Residual
/total
flux)* 100
%
-14
-6.5
12
-4.4
-1.8
2.8
The N mass balance approach to determining long-term changes in SOC or SOM
presents numerous problems. First, there is no mechanism for lower fertilizer N
applications to directly stimulate increased SOM mineralization. Any effect on SOC
would be due to lower residue, particularly during the corn phase of the rotation and
during soil tillage. Secondly, although a very high quality study, the partial N mass
balances shown are subject to different interpretations if only small errors exist. For
instance, the total mass balance residual is less than 5% of the total fluxes measured and
is 6 to 14% of the estimated N fixation. Therefore, small imprecision in estimated or
measured values could lead to different interpretations.
A number of studies have made direct measurements of SOC over long-term
studies of fertilizer rates. At least six relevant studies (three in IA and one each in KS,
MN, NE) have been conducted on Mollisols in the Corn Belt. The general conclusion
from these studies is that high fertilizer N rates on continuous corn will lead to SOC
increases and that sub-optimal N rates lead to SOC depletion. There is no direct evidence
for an effect of lower non-zero fertilizer rates, near the economic optimum, leading to
decreases in SOC from these studies.
Russell et al. (2005) analyzed studies of two Iowa sites (Kanawha and Nashua)
for the impact on SOC of four N fertilization rates (0, 90, 180, and 270 kg N/ha/yr or 0,
80, 161, and 241 Ib N/ac/yr) and four cropping systems [continuous corn (CC), corn
soybean (CS); corn-corn-oat-alfalfa (CCOA), and corn-oat-alfalfa-alfalfa (COAA)]. One
study had been ongoing for 23 years and the other for 48 years at the time of sampling of
SOC in 2002. The only difference related to fertilizer rate was for the 23 year experiment
(the Nashua site). In this experiment, the 270 kg N/ha/yr (241 Ib N/ac/yr) for CC had
higher SOC for only the 0-15 cm (0-5.9 in) depth. There were no differences among the
0, 90, and 180 kg N/ha/yr rates for CC at the Nashua site for any depths. There were also
no differences for the 0-100 cm (0 - 39 in) soil for any N rates used for CC, including the
highest rate of 270 kg N/ha/yr (241 Ib N/ac/yr). There were no other significant fertilizer
N rate effects found in the study (Russell et al., 2005).
186
-------
An earlier Iowa study that included the Nashua and Kanawha sites and a third site
(Sutherland) reached similar conclusions as those of Russell et al. (2005). In that study,
Robinson et al. (1996) found that N fertilizer rate on corn (0-180 kg N/ha/yr or 0 -161 Ib
N/ac/yr) was not significant in determining SOC but only whether fertilization occurred.
In both studies (Russell et al., 2005; Robinson et al., 1996), the cropping systems with
alfalfa [termed meadow in Robinson et al. (1996)] had the highest SOC. Corn silage
treatments and no fertilizer treatments had the lowest SOC (Robinson et al., 1996). A
third Iowa study did not compare SOC under different fertilizer rates but did show that
high fertilizer N (206 kg N/ha/yr or 184 Ib N/ac/yr) resulted in increases in SOC over 15
years with continuous corn (Karlen et al. 1998a). The general conclusion from the Iowa
studies is that for either CC or CS systems, fertilizer rate has little or no effect in the 90-
180 kg N/ha/yr (80-161 Ib N/ac/yr) range. Given that the average N fertilizer application
to corn in Iowa was 158 kg N /ha (141 Ib N/ac) in 2005 (USDA ERS:
http://www.ers.usda.gov/Data/ARMS/app/CropResponse.aspx) and the economic
optimum rate ranged between 67 and 172 kg N/ha or about 60 and 154 Ib N/ac
(approximate mean of 137 kg N/ha or 122 Ib N/ac) during 1996 and 1998 in the Iowa
study by Jaynes et al. (2001), it seems unlikely that these rates would lead to a depletion
of SOC due to a N rate effect. Corn yields with the moderate N rates in the Jaynes et al.
(2001) study ranged around 10 Mg/ha (159 bu/ac) and the Iowa state average corn yield
in 2005 was about 10.9 Mg/ha (173 bu/ac).
Results from other studies in the Corn Belt are mixed and have found no
consistent effect of N rate on SOC. In Kansas, Omay et al. (1997) found no effect of
either 224 or 252 kg N/ha (200 or 225 Ib N/ac) versus no N for over 10 years of CC or
CS. A small significant difference in SON (less than 5% decrease) was found on one soil
for the 0 N treatment. Increased residue inputs were attributed to N fertilization, and
inclusion of soybean in the rotation reduced SOC and soil organic N. In contrast, CC
receiving 200 kg N/ha/yr (179 Ib N/ac/yr) for 13 years had higher SOC than in the 0 N
treatment on a Minnesota Mollisol (Clapp et al., 2000). In an 18-year experiment in
Nebraska, N rate (0, 90, 180 kg N/ha/yr or 0, 80, 161 Ib N/ac/yr) had an effect on SOC in
the 0 to 7.5 cm (0 to 2.9 in) soil after eight years but had no effect after 18 years,
presumably due to tillage differences (Varvel 2006).
Recent work in Nebraska on an irrigated Mollisol compared long-term (initiated
in 1999) continuous corn and corn-soybean rotations under recommended and intensive
management and found that SOC was increased under recommended and intensive
management of CC but not in the CS systems (Adviento-Borbe et al., 2007; Dobermann
et al., 2007). These scientists also reported that greenhouse gas (GHG) emissions from
agricultural systems can be kept low when management is optimized towards better
exploitation of the yield potential. To accomplish SOC increases while keeping GHG
emissions low, Dobermann et al. (2007) reported the following required factors: 1)
choosing the right combination of adapted varieties or hybrids, planting date, and plant
population to maximize crop biomass production; 2) tactical water and N management,
including frequent N applications to achieve high N use efficiency and minimized N20
emissions; and 3) a deep tillage (non-inverting) and residue management approach that
favors a build-up of SOC as a result of large amounts of crop residues returned to the soil.
187
-------
If a fertilizer effect on SOC exists, it is more likely to occur under CC than CS
because increased fertilizer generally leads to increased corn production. It is logical to
assume that increased corn production (including grain, stover, and roots) should lead to
increased SOC. In general in the published studies, this relationship does not hold,
although applying zero N fertilizer generally leads to less SOC over time than high
fertilizer N rates. In summary, although it is beyond the scope of the SAB Panel to
review all the research relevant to changing SOC in Corn Belt soils, it is clear that
inclusion of alfalfa in a rotation is very effective at building SOC. The effects of tillage
are not clear. Based on the existing literature, there is evidence that changes in fertilizer
rates within the range of those optimum for corn production are unlikely to lead to long-
term SOC and SON declines. Although it is possible to build SOC under CC with
relatively high fertilizer additions [e.g., 201 to 299 kg N/ha/yr or 179 to 267 Ib N/ac/yr
(Adviento-Borbe et al., 2007; Dobermann et al., 2007) and 206 kg N/ha/yr ( 184 Ib
N/ac/yr) Karl en et al. 1998b) care must be taken to ensure that these fertilizer additions
are sustainable economically and that they do not harm water quality. From a global C
balance perspective, it is also worth noting that there is a C emissions cost of producing
N fertilizer that would need to be taken into account when doing C mass balances for
higher fertilizer N rates on corn. However, if high-yield production is achieved, with
good N use efficiency, these fertilizer C emissions may be offset (Adviento-Borbe et al.,
2007). More research on the net effects of N fertilizer rates on SOC and GHG emissions
is needed.
Precision agriculture management tools for Nitrogen
Global positioning system (GPS) and geographic information system (GIS)
technologies are becoming more widely adopted by farmers and show promise for
developing management zones in fields that could target application rates for low- versus
high-yielding areas (Schlegel et al., 2005) and reduce N applications in areas of the field
most prone to N losses (Chua et al., 2003). Field-transect apparent electrical conductivity
(ECa) or electromagnetic induction measurements can help define management zones,
based on surrogate detection of soil texture differences (Davis et al., 1997; Kitchen et al.,
1999). Reductions in N application rates for corn range from 6 - 46% when using site
specific management zone approaches as opposed to a uniform rate of N application
(Koch et al., 2004). Dividing fields into a few management zones might reduce N loss,
but because of within-field variability, more spatially intensive N management might
provide greater economic and environmental benefits (Hong et al., 2006; Scharf et al.,
2005).
Basing N applications on past yields has not proven to be an effective approach to
variable rate fertilization of N (Murdock et al., 2002; Scharf et al., 2006b). In-season
crop N sensing research (chlorophyll meter, remotely-sensed multispectral color images,
on-the-go and hand-held optical reflectance sensors) (Scharf et al., 2006a), using
reference "N- rich" or calibration strips or plots in targeted areas within fields (Raun et
al., 2005) has shown the potential benefits of these newer technologies in providing in-
season guidance to farmers and crop advisers for improved N nutrition management.
188
-------
This "N-rich" calibration approach appears to have been more successful with winter
wheat than for corn, to date. Chlorophyll meters and remotely sensed crop reflectance
have been used as an index for plant N status, and N-fertilizer use efficiency improved
when these techniques were used (Osborn et al., 2002; Varvel et al., 1997). Crop N-
sensing technologies present opportunities to reduce and better time fertilizer N
applications; however, there have been few direct assessments of impacts of these
approaches on residual soil N and nitrate losses. Further verification of the performance
of these techniques is needed in order for implementation by farmers to be more
widespread.
When technology costs are considered, economic returns to farmers are often
inadequate to justify adoption of variable rate N management. Frequently, the costs of
spatial N management technologies exceed the cost of the fertilizer N saved, which are
dependent on fertilizer prices. As a consequence, adoption of these technologies has
proceeded at a slower rate than anticipated, partly because of high technology and
equipment costs and spatially variable economic returns. Economics research suggests a
number of reasons for this low slow adoption including high fixed costs of adoption and
uncertainty in returns. These factors suggest that incentives to encourage adoption may
need to cover option values and that revenue insurance programs to address the risk may
be appropriate instruments (Khanna et al., 2000; Khanna, 2001; Isik and Khanna, 2002,
and Isik and Khanna, 2003).
Incentives have been used in Missouri in cost-sharing some of the expenses of
precision technologies within the USDA EQIP program (Agronomy Technical Note MO-
35, September 2006). Cost-share in this Missouri USDA NRCS Code 590 nutrient
management program provides a farmer $49/ha ($20/ac) per year for a three-year
contract, with the full $148/ha ($60/ac) provided at the end of the first year. Farmers in
this Missouri EQIP precision N-sensing program are advised to follow guidance for N-
sensing interpretation based on work by Scharf et al. (2006a and 2006b).
Precision Agriculture Management Tools for Phosphorus
Spatial variability in soil test phosphorus (P) levels can be large, with levels often
ranging from very low to very high (agronomic interpretation) in the same field
(Bermudez and Mallarino, 2007; Wittry and Mallarino, 2004; Reetz et al., 2001; McGraw
and Hemb, 1995). This variability can also be large in fertilized, manured, and grazed
pastures (Snyder and Leep, 2007; Mallarino and Schepers, 2005). With the advent of
commercially available GIS and GPS technologies in the early 1990s, crop advisers and
farmers began to more precisely define the spatial variability of soil fertility levels,
including soil test P (Figure 51). In recent years, zone or grid (e.g., 0.25 to 1 ha or .6 to
2.5 ac) sampling has been used to better define management units to receive different P
application rates (Reetz et al., 2001), as opposed to the formerly recommended practice
of whole-field composite sampling (e.g., Thorn and Sabbe, 1994). In spite of
considerable research effort, no widely accepted standard for soil sampling fields for
precision or site-specific management has been established (Mallarino and Schepers,
2005), because soils are naturally heterogeneous and their spatial variability occurs at
189
-------
many scales. Recent soil sampling summary results for more than 3.3 million soil
samples in North America from both public and private soil testing laboratories also
showed wide variability in soil test P levels within and among states in the U.S.
(PPI/PPIC/FAR, 2005). Snyder (2006) summarized the soil test results for the 20 major
MARB states (over 2.1 million samples) and reported 1) 40% of the states have
experienced a decline in soil test P since 2001, and 2) 78% of the samples tested below
50 mg/kg (ppm) Bray 1 equivalent-extractable P and 94% tested 100 ppm or below. In
fact, crop harvest removal of P exceeds fertilizer plus recoverable manure P in 11 of the
20 states (PPI/PPIC/FAR, 2002). These data are in agreement with the trends in net
anthropic P input in the MARB, discussed in Section 3.2 of this report.
Bray 1 Soil Test P, 0 - 6 inches
Fall Soil Samples 2001
W. A. Field
R.W. Field
0 102030405060708090
Range 1-93 ppm
Avg. 28
0 10 20 30 40 50 60
Range 1-59 ppm
Avg. 15
Figure 51: Variability in soil test P levels in typical farmer fields in Minnesota (2007 personal
communication with Dr. Gary Malzer, University of Minnesota)
Early season detection of corn P deficiency may be possible with remote sensing,
but detection of deficiencies later in the season, which correlate better with crop yield,
has not been successful (Osborne et al., 2002). At this time, remote sensing or on-the-go
sensing of plant P status does not appear to be as commercially viable as plant N sensing.
Variable-rate fertilization can result in better P fertilizer management. For
example, Burmedez and Mallarino (2007) found that variable-rate technology applied 12
to 41% less fertilizer and reduced soil test variability on farmer's fields in Iowa,
190
-------
compared with the traditional uniform rate fertilization method. Perhaps one of the most
important aspects of intensive soil sampling and variable-rate P application technologies
is the capability to apply P fertilizer where it is needed while minimizing or reducing P
applications in field areas which have elevated soil test P. In Iowa, variable-rate P
application helped decrease soil test P in field areas with high soil test P, when applying
manure (Figure 52) or fertilizer (Figure 53). As of yet, however, variable rate or
precision P fertilization has been shown to have little economic benefit in the major corn
and soybean producing states compared to uniform applications (Lambert et al., 2006;
Mallarino and Schepers, 2005). Further, there are ongoing efforts to update soil test P
crop response calibrations and fertilizer recommendations to optimize P fertilization
(Beegle, 2005).
Manuring Method
=i No manure
=1 Uniform rate
^^B Variable rate
Figure 52: Effect of variable-rate versus uniform rate application of liquid swine manure on changes in soil
test phosphorus in Iowa fields [2007 personal communication with Dr. Antonio Mallarino, Iowa State
University and Wittry and Mallarino (2002)].
191
-------
0.
Qi
a>
c
03
f
o
a.
*-)
en
03
O
V)
Fertilization Method
Variable rate
i i Uniform rate
VERY LOW
SOILP
OPTIMUM
SOILP
LOW
SOILP
HIGH
SOILP
Figure 53: Effect of variable rate versus uniform rate application of fertilizer P on soil test P in multiple
Iowa fields across multiple years (2007 personal communication with A. Mallarino, Iowa State University).
Numerous studies have shown a strong relationship between soil test P levels and
the concentration of dissolved P in runoff (Sharpley et al., 2006a, 2006b; Andraski and
Bundy, 2003; Pote et al., 1999) and tile drainage (Heckrath et al., 1995). Recent work by
Gentry et al. (2007) showed that tile drainage P losses in Illinois can exceed one kg
P/ha/yr (0.9 Ib P/ac/yr), with much of the loss occurring during a few peak storm events
in the spring. However, annual manure or fertilizer P applications can control the
concentration of total and dissolved P in surface runoff (Pierson et al., 2001; Sharpley et
al., 2001).
Soil test P thresholds alone cannot define the potential or risk of P losses from
agricultural fields. Slope, hydrologic characteristics, tillage, P rate, and time after P
application before a runoff producing rainfall, and other factors also affect the risk of P
loss (Sharpley et al., 2006a). Soil test P thresholds alone cannot define the potential or
risk of P losses from agricultural fields. Slope, hydrologic characteristics, tillage, P rate,
and time after P application before a runoff producing rainfall, and other factors also
affect the risk of P loss (Sharpley et al., 2006a). To address all factors influencing P loss
from agricultural fields, an environmental risk assessment tool (the P Index) was
proposed by Lemunyon and Gilbert (1993), which has been regionally modified and
adopted by 49 of 50 states in the U.S. to identify and delineate the risk for agricultural P
192
-------
loss for use in the development of Comprehensive Nutrient Management Plans (Sharpley
et al., 2003). Use of P Indices has also been encouraged by industry, in recognition of the
spatial variability in soil test P levels within fields, and the spatial variation in source and
transport factors (Snyder et al., 1999). "Variable rate P application can be practically
implemented on the basis of P index ratings for field zones, not just based on soil test P"
(Wortmann et al., 2005). Variable rate fertilizer P application is becoming more common
in Nebraska, Iowa, Missouri, Kansas, and other states, and some custom applicators are
beginning to apply manure at variable rates.
Nutrient management planning strategies
A survey of 127 farms (90% of all farms) in two northeastern Wisconsin
watersheds offers some insight into how successful nutrient management has been in
reducing nutrient applications (Shepard, 2005). Farmers with a nutrient management
plan (53% of farms) applied less N and P (139 kg N/ha and 31 kg P/ha or 124 Ib N/ac and
28 Ib P/ac) than farms without a plan (188 kg N/ha and 44 kg P/ha or 168 Ib N/ac and 39
Ib P/ac), but only half the farmers credited on-farm manure N, and only 75% fully
implemented their plans on most of their acres.
For nutrient management planning to decrease nutrient loss, technical and
financial assistance programs need to focus on plan implementation and maintenance in
the MARB rather than on targeting the number of plans written in a given period.
Despite programs subsidizing plan writing, a critical limitation is the lack of certified
plan writers to meet the demand and deadlines. Further, there needs to be an effective
mechanism to ensure plan adoption and regular updating of plans. Efforts are underway
in the Heartland states of the MARB (IA, KS, MO, NE) to develop nutrient management
plan assessment protocols. This aims to identify key factors that limit plan
implementation so that practical solutions can be developed. One option is preparation of
a simplified plan that farmers can quickly refer to. Also, documenting nutrient
management plan implementation is being rewarded with financial credits in New York
drinking water supply watersheds (Watershed Agricultural Council, 2004). These credits
can be used to purchase or upgrade equipment that would need to be used to implement
the plan, such as manure spreaders, injectors, etc.
An assessment is needed of the socioeconomic barriers to successful adoption of
nutrient management planning strategies in the MARB as well as the N and P loss
reductions achievable. Such an assessment has been done in a drinking water supply
watershed for New York City that claims a 93% participation in volunteer conservation
programs (Watershed Agricultural Council, 2004). A survey of CREP participants
showed they were generally older and more likely to obtain information from extension
agents, consultants, and watershed council personnel than non-participants, but there was
no difference in educational level or farming status (full or part time) (James, 2005).
Overall, negative attitudes toward voluntary adoption of BMPs were a result of the loss
of productive land and loss of being able to decide independently what to do on their own
land. These survey results illustrate the difficulties in gaining adoption of nutrient
193
-------
management BMPs by farmers in any watershed, transferring new BMP technology, and
the socioeconomic pressures faced.
Key Findings and Recommendations
Reductions in N losses and residual soil NOs-N are possible with attention to
improved in-field N management. It may be possible to reduce N rates and alter N
timing in some portions of the MARB. Such rate reductions may be accomplished
through implementation of refined management, but they must be economical for farmers
and care must be taken to protect soil resource sustainability. Crop N sensing and
variable rate N management implementation, using management zone approaches may
prove useful in attainment of economic optimum N rates in individual fields, which may
also help reduce N losses. Higher fertilizer, fuel, and machinery costs have stimulated
increased interests in some newer N management technologies, as well as other means to
improve fertilizer N effectiveness and efficiency; however use of site-specific or
precision technologies has not yet proven financially rewarding to many farmers, due to
the high cost of sampling, ground- truthing, and application technology. Based on these
findings, the SAB Panel offers the following recommendations.
Because of the importance of both N and P to Gulf hypoxia and as various
cropping systems can have different positive and negative effects on N and P
export reduction, remedial strategies must be directed at system-wide nutrient
management rather than either N or P applications alone. Future research to
evaluate the effects of different nutrient management impacts on crop production
should include measures of water and air quality effects.
There is a lack of consistent year-to-year USDA nutrient management survey
data, which hinders any broad nutrient use and management evaluation and
interpretations. These data will become more important in monitoring and
understanding changes in nutrient management practices as biofuel markets
expand. Consistent year-to-year data collection on nutrient management of major
crops and emerging energy crops is recommended.
Cost-share incentives like the USDA payment support for crop N -sensing and
precision N management in Missouri, intensive educational programs (e.g., on-
farm demonstrations), and/or other means should be explored to encourage the
agricultural community to improve nutrient use efficiency and effectiveness with
all nutrient sources (i.e., fertilizer, manure, biosolids, composts, by-products,
etc.). Such programs may be especially helpful in corn systems in the upper
Mississippi and Ohio River subbasins, which have been identified as major
contributors of spring nitrate-N flux to the NGOM.
Although the economic and water quality impacts of controlled release fertilizers
in commercial field crop systems have not been fully proven, their beneficial use
should be explored through additional research and demonstrations at field and
watershed scales. Programs to stimulate greater adoption of locally-proven
194
-------
technologies like urease and nitrification inhibitors (and controlled release
fertilizers, once proven economically and environmentally effective) to enhance
crop nitrogen recovery and use efficiency, should be considered as the shift
towards greater urea and urea-ammonium nitrate N use continues.
Watershed-scale evaluations of split applications of N in the spring for corn
should be conducted to determine watershed-scale benefits of this N management
approach compared to the more traditional application of anhydrous ammonia in
the fall, especially in the upper Mississippi and Ohio River subbasins.
More research on the net effects of N fertilizer rates on soil organic carbon (SOC)
and greenhouse gas (GHG) emissions is needed.
Crop and animal production systems are essential to the economic viability of
agriculture in the MARB. Thus, an infrastructural assessment of how animal
production can co-exist with grain and forage production is needed. Long-term
strategies should be explored whereby more effective crop and animal production
systems remedy or avoid excessive N and P loading to water and air resources.
Cost-benefit ratios vary among farmers; with for example, labor availability, farm
organization, and financial situation. However, past experience shows that
adoption of conservation practices is not solely dependent on cost-effectiveness.
Thus, there needs to be consideration of the socioeconomic barriers to, and
impacts of, adoptions of nutrient management planning strategies in the MARB.
New approaches should be investigated to overcome socioeconomic barriers,
including incentive programs.
4.5.7. Effective Actions for Other Non-Point Sources
Atmospheric Deposition
This section reviews actions for reducing NOx emissions that contribute to
atmospheric deposition of nitrogen. For the United States as a whole, atmospheric
deposition of oxidized nitrogen compounds released during fossil fuel combustion
contributes an estimated 30% of the entire inputs of new nitrogen (Howarth et al., 2002).
As discussed earlier in Section 3.2, atmosphere deposition of oxidized nitrogen is less
important in the MARB but still accounts for an estimated 8% of nitrogen contributions
to the upper MARB and 16% of the nitrogen inputs to the Ohio River basin. NOx
emissions to the atmosphere in the United States could be virtually eliminated at
reasonable cost using currently available technologies (Moomaw, 2002; Howarth et al.,
2005). In addition to potential benefits concerning Gulf hypoxia, reducing NOx
emissions in the MARB can contribute to improved local air and water quality and can
reduce atmospheric transport of nitrogen to the northeastern States, where atmospheric
deposition is an even more significant problem.
195
-------
In addition to deposition of oxidized nitrogen, there is significant deposition of
ammonia and ammonium (NHx) in some regions of the MARB. These are not
considered in the mass balance approach for nitrogen in Section 3.2 because the NHx
originates largely from volatilization from animal wastes and other agricultural sources
and so does not represent new nitrogen inputs to the basin, but rather a recycling of
nitrogen within the basin (Howarth et al, 1996). Nonetheless, high rates of volatilization
followed by conversion to ammonium nitrate or sulfate can lead to significant long-
distance transport and contribute to reactive N distribution in other sensitive areas.
Furthermore, high rates of NHx deposition in the basin can result in increased leakage of
nitrogen to downstream aquatic ecosystems. In Iowa, Minnesota, and Wisconsin, NHx
deposition exceeds NOy deposition, and averages over 7.5 kg N/ha/yr (6.7 Ib N/ac/yr) in
Iowa (results from CMAQ model, Robin Dennis, NOAA, unpubl.)
Mobile sources account for approximately 55% of NOx emissions to the
atmosphere on a national level (Melillo and Cowling, 2002). While automobiles have
been subject to fairly strict NOx standards in recent years, emissions from light trucks
have not historically been as strict. Tightening regulations on light trucks represents an
opportunity for significant reduction in NOx emissions, as approximately half of new
vehicle sales in recent years have been light duty trucks (Moomaw, 2002). Heavy diesel
trucks, buses, and trains have accounted for a growing fraction of NOx emissions because
of strict NOx standards on automobiles and the absence of similarly strict controls on
heavy diesel vehicles.
Stationary sources account for approximately 45% of NOx emissions, with
electric generating facilities accounting for roughly half of all stationary source
emissions, and industrial fuel combustion account for slightly less than one-third. The
remainder of stationary-source NOx emissions are from non-fuel industrial processes
(12%) and from commercial, institutional, or residential fuel combustion (8%) (U.S.
EPA, 2006a).
Stringent new source performance standards have greatly reduced emissions from
new electric generating facilities. Low emission, combined-cycle gas turbines account
for most new electric generating capacity in recent years (Bradley and Jones, 2002).
Unfortunately, some existing policies provide incentives that discourage more
widespread adoption of new, cleaner technologies. For example, under the Clean Air
Act, high NOx emissions by older, coal fired power plants are "grandfathered," and
therefore not subject to the stringent emission standards of new generating capacity. As a
consequence, electric utilities have the incentive to keep older coal plants running far
beyond what would otherwise be their economic lifespan (e.g., Ackerman et al., 1999;
Nelson et al., 1993; Maloney and Brady, 1988). As a result, while 90% of new electric
generating capacity is produced with gas turbines, coal still produces 55% of the
electricity in the US (Moomaw, 2002). And it was estimated that in 1998, coal-fired
power plants were responsible for nearly 90% NOx emissions from electric power
generation (U.S. EPA, 2000b; U.S. EPA, 2006a). About a quarter of the coal-fired
196
-------
electric generating capacity in 1996 was constructed prior to 1965, and almost one-half
was constructed prior to 1975 (Ackerman et al, 1999).
Considerable reductions in NOx emissions can be achieved with existing
commercial technologies by replacing outdated coal-fired capacity with modern gas-fired
combined-cycle power plants (Howarth et al., 2005). Existing coal plants can also be
retrofitted with new control technologies, such as Low-NOx burners (Bradley and Jones,
2002; Ackerman et al., 1999). Other promising technologies for reduction emissions
from coal-fired power plants include fluidized bed boilers (Co-Generation Technologies,
2006), and gasified coal combined-cycle power plants, and sequestration of emissions
(U.S. DOE, 2006).
For the most part, NOx emissions in the United States are regulated because of
concerns over formation of smog and ozone and seldom because of water-quality
concerns (Moomaw, 2002; Melillo and Cowling, 2002). Since smog and ozone pollution
occur mostly in summer months, regulation of NOx emissions from stationary sources
has often focused on summer-time only regulation (Howarth et al. 2005). Since the
largest cost of controlling NOx from power plants is the capital cost of building scubber
systems, the additional cost of requiring year-round NOx control from power plants is
small compared to that for summer-time only controls. Thus, year-round operation of
existing control technologies represents a cost effective approach for reducing NOx
emissions. Some local and state governments, such as New York State, have recently
moved towards year-round regulation of NOx because of concern over coastal nitrogen
pollution (Ron Entringer, NY State DEC, personnel communication).
Residential and Urban Sources
Urban and suburban runoff comes from a variety of sources, including impervious
surfaces like roads, rooftops and parking lots, as well as pervious surfaces like lawns.
Urban and suburban runoff can be important sources of pollutants, especially for local
water quality effects. For example, the National Water Quality Inventory: 2000 Report
to Congress concluded that urban runoff is a major source of water quality impairment in
surface waters (U.S. EPA, 2002). There are a variety of actions to control non-point
urban sources, including both structural and non-structural practices (e.g., U.S. EPA,
2005).
Although controlling urban non-point sources can provide significant benefits
from improvements to local water quality, these non-point sources are not significant
determinants of hypoxia in the Gulf of Mexico, both because concentrations tend to be
lower than those from agricultural sources and because the urban land comprises less
than 1% of the Mississippi River basin (e.g., Mitsch et al., 1999). Thus, although actions
to reduce urban non-point sources may be justified, these control actions will not likely
contribute significantly to reductions in the size of the Gulf of Mexico hypoxic zone.
Since control of urban non-point sources will not have an important role in reducing
hypoxia, we do not focus on actions to reduce urban non-point sources of nutrients in this
report.
197
-------
Key Findings and Recommendations
Atmospheric deposition is a small but significant (8% in Upper Mississippi and
16% in Ohio River subbasins) contribution to N inputs in the Mississippi River basin.
Opportunities exist to lower NOx emissions in a number of ways, but it is not likely that
hypoxia will drive most of these regulatory decisions. Rather, hypoxia reduction and
other water quality benefits should be incorporated in a number of regulatory decisions
regarding air pollution. Based on these findings, the SAB Panel offers the following
recommendation.
Water quality benefits and effects on hypoxia should be incorporated into
decisions involving retirement or retrofitting of old coal-fired power
plants, NOx controls such as the extension of the current summertime
NOx standards to a year-round requirement, and emissions standards and
mileage requirements for sport utility vehicles, heavy trucks and buses.
4.5.8. Most Effective Actions for Industrial and Municipal Sources
Sewage treatment plants and industrial dischargers represent a more significant
source of N and P in the MARB than was originally identified in the Integrated
Assessment. Although most point sources in the MARB do not have permits that require
removal of N or P from discharged effluent, as local water quality standards for these
nutrients have not yet been developed, states are charged with developing water quality
criteria for achieving and maintaining designated beneficial uses of surface waters,
including those waters that receive sewage treatment plant effluent. However, the
process by which these criteria are translated into quantitative and enforceable nutrient
limits from regulated point sources remains unclear.
Based on data from the recent MART (2006b) report, the SAB Panel has
estimated that permitted point-source discharges represented approximately 22 and 34%
of the average annual total N and total P flux to the Gulf, respectively, for the 2001 to
2005 water years (for a detailed discussion see Appendix D: Calculation of Point Source
Inputs of N and P). These point sources represent a significant opportunity to reduce N
and P loadings that should be fully evaluated in the context of other potential
management changes in the MARB.
Encouraging behavioral changes of non-domestic sewer users as well as
increasing capital investments in sewage treatment and industrial treatment plant
upgrades have proven to be effective approaches to managing nutrient discharges in other
areas of the U.S. (U.S. EPA, 2004b; U.S. EPA, 2003a; Chesapeake Bay Commission,
2004). The use of Biological Nutrient Removal and Enhanced Nutrient Removal
technologies for N and P removal are being implemented to reduce N and P
198
-------
concentrations in sewage treatment plant effluent discharge by 50 to 80% (Maryland
Department of Environment, 2005; U.S. EPA, 2004b). Sewage treatment plant upgrades
designed to remove phosphorus typically include enhanced chemical precipitation
applied alone or in combination with biological phosphorus treatment and membrane
filtration. These types of sewage treatment plant unit operations, which can achieve
effluent discharge phosphorus concentrations as low as 0.1 mg/L total phosphorus or less,
now constitute the BMP for phosphorus removal at sewage treatment plants. Removing P
to a 0.1 mg P/L limit is most commonly implemented where there is a market for water
recycling, such as in communities located in the desert Southwest, and the increased cost
can be justified. In locations where there is no market for recycled water, higher limits for
P (for example, 0.3 or < 1.0 mg P/L) will be more cost effective.
The SAB Panel presents an example calculation to demonstrate the magnitude of
reduction possible in riverine total N and P fluxes to the NGOM if technology for N and
P removal from sewage effluent were implemented for large sewage treatment plants (0.5
million gallons per day and above) across the MARB. Based on the SAB Panel's
adjustment to the MART report's estimates of N and P effluent from sewage treatment
plants (MART, 2006b), the SAB Panel has calculated that upgrades for large sewage
treatment plants in the MARB to achieve total N concentration limits of 3 mg/L could
create reductions in N flux from sewage treatment plants from 192,000 metric tonne N/yr
(212,000 ton N/yr) to 70,000 metric tonne N/year (77,000 ton N/yr), about a 64%
reduction in annual N flux from sewage treatment plants. This translates into a reduction
of total annual N flux to the Gulf by about 10% and the total spring N flux by about 6%.
Upgrading to achieve P concentrations of 0.3 mg/L would create reductions in P fluxes
from sewage treatment plants from 41,000 metric tonne P/yr (45,000 ton P/yr) to 10,500
metric tonne P/yr (11,600 ton P/yr) or about a 75% reduction in annual flux from sewage
treatment plants to the MARB. These reductions, in turn, would translate into a decrease
in the total annual P flux to the Gulf by about 20% and the total spring P flux by about
15%. It is important to recognize that these estimates assume that the changes in
biosolids quality and production rates resulting from the capital improvements to the
sewage treatment plant do not adversely impact nutrient management procedures
implemented at biosolids land application sites.
In the Chesapeake Bay watershed, nutrient reductions from sewage treatment
plant upgrades were determined to be as cost effective as, and more predictable than, the
estimated reductions achieved through implementation of agricultural non-point source
BMPs. The Chesapeake Bay Commission (2004) found average point source costs to
remove N and P of $8.56/lb and $75/lb, respectively, which was within the range of most
widely implemented agricultural BMPs (U.S. EPA, 2003b). The Commission stated that
"this technology-based approach provides the highest degree of confidence for consistent,
long-term reductions. Furthermore, the cost of this technology has continued to decline
in recent years."
However, there are many differences in point source distribution, population, and
income in various sub basins of the MARB compared to other areas of the country where
point sources have had total N and P reductions (such as the Chesapeake Bay or Long
199
-------
Island Sound). Therefore, a cost effectiveness analysis of point-source controls of N and
P in the MARB is needed to fully evaluate this particular method of reducing nutrient
inputs to rivers in the context of non-point source control costs. A part of that analysis
should consider the cost of N and P removal that could be optimized by establishing
loading caps for individual treatment plants and/or groups of plants within river basins
and by allowing nutrient credit trades between the plants. This "point-to-point" trading
allows those plants that can most efficiently achieve reductions to sell nutrient reduction
credits to plants that would incur much higher costs to achieve their loading cap. This
approach is being used in Long Island Sound and in the Chesapeake Bay watershed
within Virginia. These point-to-point trading programs are consistent with an overall cap
and trade program as discussed in Section 4.4.3.
Another potential approach for reducing the nutrient discharge from sewage
treatment plants, that could be applied alone or in combination with plant upgrades, is to
encourage local sewer districts to establish more stringent nutrient pretreatment standards
for private industries and other non-domestic sewer users. Meat packing, chemical
manufacturing and food processing are examples of the types of industries that generate
wastewater containing large amounts of N and P. Through the regulatory authority
granted to them under the National Pollutant Discharge Elimination System (NPDES)
program, sewer districts can encourage industries to reduce their nutrient discharge to
sewage treatment plants through the establishment of local sewer discharge nutrient
limits as well as by the judicious development of technology-based wastewater surcharge
rates.
The overall decrease in the mass of nutrients discharged into the local sewer
system due to pretreatment will improve the quality of both the sewage treatment plant
effluent and biosolids and will result in a net reduction of nutrients entering the MARB.
A feasibility study is needed to evaluate the regulatory and economic options that could
be applied to provide incentives for major industries to identify and implement pollution
prevention measures to reduce and/or recycle nutrients that would otherwise be
discharged into the local sewer system.
In addition, industrial treatment plant upgrades designed to remove nutrients can
also reduce nutrients that are directly discharged to the MARB and the Gulf. Industrial
discharges account for about 28% of the point source N flux and 23% of the point source
P fluxes, or 75,000 metric tonne N/yr (83,000 ton N/yr) and 17,000 metric tonne P/yr
(18,700 ton P/yr). Experience in other regions has shown that industrial sources could be
targeted on a permit-by-permit basis since frequently a limited number of permitted
facilities are responsible for a large part of the load. This approach could be
recommended for the MARB. It would be useful to design initial efforts to focus on
discharge categories likely to have high nutrient discharges. Examination of discharge
information (Table 3, MART, 2006b) reveals that two categories (industrial organic
chemicals and plastic materials/synthetic resins) account for about half of industrial N
discharges, about 45,000 metric tonne N/yr (50,000 ton N/yr). For P, four categories
(crude petroleum and natural gas, electrical services, refuse systems, and wet corn
milling) account for about 40% of the industrial load or about 5,500 metric tonne P/yr
200
-------
(6,000 ton P/yr). Industries in these categories should be evaluated for opportunities to
reduce N and P discharges through pollution prevention, process modification or
treatment.
While P removal is technologically feasible and widely implemented elsewhere,
advanced treatment increases the amount of biosolids generated and, therefore, the land
area needed to manage a given amount of biosolids based on P and N needs of the crop,
rather than just the N requirements. This will create additional costs for biosolids-
management programs in the MARB and needs to be considered when evaluating the
total cost of implementing P removal at sewage and industrial treatment plants in the
basin.
Unlike nitrogen, which can be biochemically transformed and removed from the
sewage treatment plant as a volatile gas (N2 and/or N2O) through the
nitrification/denitrification process, phosphorus is simply moved from the liquid to solid
phases and accumulates in the biosolids. Physical upgrades in sewage treatment plants
specifically aimed at reducing the phosphorus concentration in the effluent discharge
typically include substantial additions of precipitating chemicals (e.g., alum) alone, or in
combination with, higher efficiency membrane filtration. The net effect of these capital
improvements is a significant increase in the mass of biosolids requiring handling and
management. Most biosolids are beneficially used in crop production on land located as
near to the treatment facility as feasible to minimize transportation costs. Transportation
distances range from essentially zero to several hundred kilometers depending on plant
location, size and the amount of biosolids or biosolid nutrient content. Phosphorus
removal will increase both the mass of biosolids and the P content of the biosolids.
Biosolids application to agricultural land is regulated through the NPDES permit
of the treatment facility. In many places in the MARB, land application of biosolids is
based on the N needs of the crop. As with animal manures, biosolids application to meet
crop N needs results in over application of P and build-up of bio-available P in the soil
surface. Research during the last two decades has indicated that soil P levels substantially
in excess of crop needs can cause elevated P concentrations in runoff; particularly from
critical source areas within fields. As a result, recommendations for application of
organic nutrient sources, such as manure or biosolids, suggest that applications be limited
based on P where the risk of loss is moderate to high. This will minimize the opportunity
for P removed from discharged effluent to be lost in runoff when biosolids are land
applied. All states now have a tool to estimate the potential for P loss from application of
manure or biosolids. Nearly all states use a locally adapted version of the Phosphorus Site
Index (PSI) to estimate P loss risk. Since biosolids currently contain more P relative to N
than crops require, land application of biosolids should routinely involve an evaluation of
the risk of P loss using the PSI or another risk assessment tool.
201
-------
Key Findings and Recommendations
Sewage treatment plants and industrial dischargers represent a more significant
source of N and P in the MARB than was originally identified in the Integrated
Assessment. Tightening effluent limits on large sewage treatment plants together with
establishing more stringent pretreatment nutrient standards on non-domestic sewer users
may offer some of the most certain short-term and cost-effective opportunities for
substantial nutrient reductions, particularly for P, but a full analysis of costs needs to be
conducted in the context of non-point source reduction costs. Based on these findings,
the SAB Panel offers the following recommendations.
Tighter limits on N and P effluent discharge concentrations for major sewage
treatment plants, together with concomitant reductions in nutrient discharges from
non-domestic sewer users, should be considered, following an analysis of the cost
and technical feasibility for a particular basin.
A review of discharge data, including N and P loads, for industrial dischargers
could identify possible industrial facilities to target for cost-effective reductions.
Regulatory authorities should encourage or require sewage treatment plants to
utilize phosphorus-based biosolids land application rates rather than the nitrogen-
based rates in beneficial-use programs.
4.5.9. Ethanol and Water Quality in the MARB
The production of renewable fuels has been of interest since the 1973 oil price
shocks, and technologies for the conversion of crops into ethanol and bio-diesel have
existed since the 1940s. Currently about 99% of renewable transportation fuel produced
domestically is ethanol from grains and oil crops, primarily corn (Institute for
Agricultural and Trade Policy [IATP] 2006). This section focuses on the potential water
quality implications of both ethanol production from corn and its potential production
from lignocellulosic feedstocks.
The rapid growth in corn prices is a result of increased ethanol production,
projected to rise from less to 2 billion gallons in 2001 to more than 19 billions gallons in
2009, a 950% increase (IATP, 2006). Current estimates are that about 75% of that
production will be in the nine Upper Mississippi River Corn Belt states (IATP 2006).
The Food and Agricultural Policy Institute (FAPRI) projects that ethanol production from
corn will increase from about 6.8 billion gallons in 2007 to over 14 billion gallons by
2012. Associated with this increase in ethanol production, FAPRI projects an increase in
corn acreage from about 80 million acres to about 94 million acres in the same time
period (www.fapri.missouri.edu ). This growth of grain-based ethanol production may
have major water quality implications for the MARB and the country.
202
-------
Cellulosic ethanol is an alternative fuel made from a variety of non-food
feedstocks (such as agricultural residuals like corn stover and cereal straws, industrial
plant byproducts like saw dust and paper pulp, and crops grown specifically for fuel
production like switchgrass, Panicum virgatum). By using a variety of regional
feedstocks for refining cellulosic ethanol, the fuel can be produced in nearly every region
of the country. Though it requires a more complex refining process, cellulosic ethanol
produces less impacts on water quality, contains more net energy, and results in lower
greenhouse emissions than traditional corn-based ethanol (Mclaughlin and Walsh, 1998).
One of the challenges for wider use of cellulosic ethanol is that the cost of production is
higher than current prices for corn ethanol and gasoline. Another challenge is that
technology has not yet developed the fermentation efficiency for conversion of cellulosic
feedstocks to the level at which it is commercially viable. Contributing to the high cost is
the need to consolidate enough feedstock close to the plant to produce an adequate supply
as well as the cost of transporting the heavy and bulky feedstock (Perlack and Turhollow,
2003).
Many hope that the heightened interest in biofuels will lead to a more sustainable
mode of energy production by reducing impacts on water quality, recycling biomass
residuals and emitting little, if any, greenhouse gases. The vision is that future
biorefineries will use tailored perennial plants in increasing amounts (Perlack et al.,
2005). Integration of agroenergy plant resources and biorefinery technologies can lead to
a new manufacturing paradigm (Ragauskas et al., 2006). While these possibilities exist,
much is unknown concerning how this future might develop and whether it is
economically and technically viable.
Water Quality Implications of Projected Grain-based Ethanol Production Levels
The SAB Panel could find no published estimates of the likely impact of the
consequences of expanded corn based ethanol production on nutrient flows from the
MARB. To characterize the short-term potential impact, a set of simple calculations is
reported in Table 16 that combine acreage projections from the FAPRI baseline for CRP
and three major field crops in the U.S. with estimates of the per acre nutrient losses from
these crops (CEAP 2007). The second and third columns in the table report the projected
nationwide acreage for the years 2007 and 2013 for corn, soybeans, wheat, and CRP and
the fourth column reports the projected change in acreage for each. As can be seen, the
FAPRI baseline projects a sizable increase in corn acreage, with that increase coming
largely from soybeans and the CRP (totals do not add up since other cropland is omitted).
The fifth column estimates per acre N loss for corn, soybeans, and winter wheat
based on the sum of waterborne losses reported in the CEAP assessment
(http://www.nrcs.usda.gov/technical/nri/ceap/croplandreport/ table 36, page 117) for the
Upper Midwest region. The CEAP report did not estimate N loss from CRP, but for the
current analysis, losses from CRP are assumed to be 10 % of the average loss from
cropland. The sixth column reports the estimated change in total N losses due to the
change in acreage of CRP and each respective crop, with the sum in the bottom row
203
-------
representing the total projected increase in N loss. By this calculation, N losses
nationwide could increase by 297 million pounds N /year between 2007 and 2013.
Implications for nutrient loads to the Gulf of course depend on how much of the
predicted acreage change will occur in the MARB. Assuming the MARB accounts for
80% of the change in cropping systems, additional losses of 238 million pounds N / year
could be expected for the MARB.
While these estimates are rough and omit numerous factors that could affect the
nutrient loss from these lands (policy changes, e.g. higher mandates for the ethanol
content of gasoline, farming practices, energy prices, and climate change) they provide an
idea of the magnitude of the possible short-term nutrient consequences from increased
corn-based ethanol production.
204
-------
Table 16: Estimated changes in N losses from cropping changes
Corn
Soybeans
Wheat
CRP
Total
2006 - 2007
FAPRI
Baseline
(million
acres)
78.3
75.5
57.3
36.0
247.2
2012-2013
Acreage
Projections,
FAPRI1
(million
acres)
93.7
67.9
58.3
30.0
249.9
Projected
Change in
Acreage2
(million
acres)
15.4
-7.6
0.9
-6.0
predicted by FAPRI from 2006-2013.
NLoss
Estimate per
acre3 (Ibs./acre)
28.1
17.7
12.9
2
Diff. in Total
N Losses -
million Ibs.4
431.6
-134.2
11.7
-12
297
1. These projections for U.S. agricultural markets are from the August, 2007 baseline
http://www.fapri.missouri.edu/outreach/publications/2007/FAPRI MU Report 28 07.pdf
2. This column is the difference between columns 1 and 2.
3. Per acre estimates of N loss for corn, soybeans, and winter wheat are the sum of waterborne losses
reported in the CEAP assessment (http://www.nrcs.usda.gov/technical/nri/ceap/croplandreport/ table 36,
page 117) for the Upper Midwest region. The CEAP report did not estimate N loss from CRP, but for the
current analysis, losses from CRP are assumed to be 10 % of the average loss from cropland. The CEAP N
loss rates are based on simulations using the Erosion Productivity Impact Calculator (EPIC) model. The
CEAP estimates tend to overestimate surface losses and underestimate subsurface losses because EPIC
does not estimate tile drainage losses that increase the dissolved subsurface loss of nitrate.
4. The difference in total N losses is computed by multiplying the projected changes in acreage (column 3)
by the N loss estimate per acre (column 4).
Impacts on Nutrient Application to Corn
In the simple calculations made in Table 16, it was implicitly assumed that N
application rates will remain unchanged. However, reductions in N application rates have
been identified as one tool to reduce N loss from corn (CERN, 2000). The level of
nitrogen application that maximizes farm profits for a given soil and climate is a function
of price and input costs. Corn price has increased, but fertilizer N costs have also
skyrocketed in recent years so it is not possible, without further analysis, to determine the
net effects of these two price trajectories on fertilizer application rates. Further, as
Sawyer and Randall (2006) point out, simply applying N at economically optimal rates
will not resolve the issue of nitrate movement from fields in subsurface drainage, for
nitrate losses occur in corn production systems even when no N is applied.
High corn prices associated with market impacts of increased ethanol production
will make it less profitable for farmers to manage N conservatively. Higher corn prices
are likely to reinforce the perception that assurance of adequate N is worth the cost, since
farmers are more likely to be adverse to risks of yield loss when corn prices are high.
Based on economic optimum yield and historic response to high corn prices by farmers,
205
-------
$4/bushel corn may tend to increase N application rates to levels where N use efficiency
is lower. High corn prices also provide a disincentive for cropland retirement or
conversion to perennials.
Finally, it is worth noting that a large literature exists on the likely magnitude of
yield drag associated with continuous corn and other crop rotations. These effects may
also mean higher fertilization over the levels assumed in the CEAP study used in Table
16. See Katsvairo and Cox. (2000a and 2000b) and Pikul, Hammack, and Riedell (2005).
Grain versus Cellulosic Ethanol and Water Quality
Cellulosic ethanol produced from perennial grasses, fast-growing woody species,
manures and other biomass residuals such as corn stover could allow the US to meet
renewable transportation fuel goals while improving water quality (Mann and Tolbert,
2000; Perlack et al., 2005). Yet the rapid expansion of grain-based ethanol products may
be a disincentive to development of perennial crops or crop residual-based ethanol. The
technology to produce ethanol from cellulosic materials is rapidly improving but is not
yet operational. The production, storage, and handling infrastructure are in place for
grain but not for perennial crops or residuals. Cellulosic material is harder to handle and
only biomass sources such as forestry residuals and corn stover are in sufficient
abundance to provide reliable supplies.
Grain-based ethanol producers are interested in the development of technology
using corn stover and other crop residue as feedstock. Crop residues represent the largest
potential source of feedstock, projected to be 354 million metric tonne/yr (390 million
ton/yr). Graham et al. (2007) estimated about 58 million dry metric tonne/yr (64 million
dry ton/yr) could be removed with soil loss at "tolerable levels" (T) levels, but at Va T soil
loss removals could only be about 18 million metric tonne/yr (19.8 million ton/yr) (at
1995-2000 corn production levels). However, soil losses could increase 2 to 20 fold and
still be below T. Therefore harvesting corn stover to keep soil losses just below T would
result in substantial increases in erosion and associated N and P losses compared to
current conservation or no-till production.
English et al. (2006) proposed that corn stover may be the largest potential source
of cellulosic materials for ethanol production once cellulosic technologies are cost
competitive. However, the contribution of returning stover to soil quality and quantity
has long been recognized. Wilhelm et al. (2004) conclude that corn stover can be
harvested for ethanol production, but recommendations for removal vary depending on
regional yield, climatic conditions, and cultural practices.
Perennial grasses, including switchgrass and high biomass-producing trees, are
currently considered the most promising energy crops (Tolbert, 1998; Kurt et al., 1998;
McLaughlin and Kszos, 2005). Miscanthus and sweet sorghum have also been suggested
as possible perennial feedstocks. This discussion focuses on switchgrass, which is a
warm season perennial native prairie grass that produces high biomass in its above
ground growth and in deep roots. Switchgrass requires some N and P for optimal
206
-------
production, but less than corn. Switchgrass normally requires two growing seasons to
become fully productive, but then it can grow for 20 years or more without replanting.
Thus, either expected profitability from switchgrass production must be large enough to
overcome early lower yields or an incentive program will be needed to compensate the
farmer during the two-year transition. As mentioned previously, the transport and storage
infrastructure needed to handle the large quantities of materials for an ethanol facility will
need to be developed.
The evidence thus far suggests that switchgrass is a more favorable energy crop
for reducing impacts on the land and climate, however the technology for converting
switchgrass to ethanol is not yet commercially viable. The fermentation co-product is a
lignocellulosic material that can be dried and burned to provide part of the energy for the
facility with net positive energy returns (Farrell et al., 2006). It is very low in nutrients,
is not suited as a feed amendment, and poses little threat to water quality. If it is grown
instead of corn on productive soils, N and P losses are expected to be reduced by over
50% (Chesapeake Bay Program, 2003). Switchgrass will also sequester carbon, increase
soil organic matter, and improve soil quality through its extensive, deep root system.
These positive environmental attributes have substantial potential to provide multiple
revenue streams. Lower production cost, greater net energy production, multiple revenue
streams and environmental benefits of switchgrass all favor its long-term use as a
dedicated energy crop. However, the lag in development of fermentation technology and
the lack of existing infrastructure prevent it from replacing corn as the major ethanol
feedstock for the near future.
Increasing grain prices have increased the relative economic advantage that row
crops, particularly corn, have over switchgrass. Substantial incentives will be needed
before farmers would convert row crop land to switchgrass or other perennials at current
market conditions. Babcock et al. (2007) estimated that the magnitude of subsidies
would be significant and that conversion of all cropland to switchgrass in a watershed in
northeastern Iowa would result in an 84%, 83%, 44% and 53% reduction respectively in
sediment, total phosphorus (TP), nitrate (NO3) and total nitrogen (TN) at the watershed
outlet compared to existing conditions. Model results also indicated that conversion of
all cropland in the watershed to continuous corn would increase sediment, TP, NO3, and
TN from current levels by 23%, 128%, 147% and 150% respectively. They also
evaluated the impact of growing switchgrass on all Highly Erodible Land (HEL) and
continuous corn on other cropland. Careful placement of the switchgrass on other
sensitive landscapes and as a buffer on non-HEL land could provide additional water
quality benefits.
207
-------
Key Findings and Recommendations
Expansion and intensification of corn production to support grain-based ethanol
production and impacts of ethanol co-products from the animal production sector are
likely to cause major increases in N and P losses in the MARB. The opportunity still
exists to make choices that result in a renewable energy strategy that achieves energy
goals with a reduced impact on the environment. Grain-based ethanol production is
rapidly expanding, and the SAB Panel's preliminary calculations demonstrate a
significant short-run increase in N and P losses to water resulting from current market
incentives favoring corn.
Cellulosic ethanol production can be less environmentally detrimental, but
current technology and infrastructures do not make it competitive with grain-based
ethanol. Harvesting corn stover as a feedstock for cellulosic ethanol has water and soil
quality implications. Switchgrass or other perennial grasses or woody biomass provide
greater net energy and lower production costs and potentially higher total revenue with
substantial environmental benefits when compared to corn and could become the
dominant feedstock if investment, policy, and market conditions do not keep renewable
energy policy focused on grain feedstocks. Based on these findings, the SAB Panel
offers the following recommendations regarding biofuel production.
Life cycle analysis, examining all impacts to air, water and climate, is needed to
compare the various feedstocks for ethanol production.
Research and development should focus on biofuel production systems that are
both economically viable and ecologically desirable.
If research continues to support the potential of cellulosic materials to meet
energy and environmental goals, incentives (or the removal of disincentives)
should be provided to promote ethanol production with more environmentally
benign feedstocks.
4.5.10. Integrating Conservation Options
The previous sections have described land management and conservation
practices that can enhance nutrient loss reduction and water quality locally and in the
Gulf. As discussed, these practices vary, sometimes substantially, in their effectiveness
among watersheds and subbasins in the MARB. Furthermore, there can be synergistic
effects on nutrient loss reductions, where combinations of these practices can produce
more (or less) than the sum of their individual reductions. In evaluating suites of
management options, it is crucial to determine whether the nutrients that are not released
to waters are being lost instead to other systems, so that reactive N and P are not actually
208
-------
removed from the environment, but just redistributed. These facts are an important part of
the basis for our recommendation that watershed based modeling approaches continue to
be developed and that they be explicitly used to design optimal land management systems
within an adaptive management context. As noted in Sections 2.1.9 and 3.4, watershed-
based models can be a key source of information for considering alternative sets of
conservation practices and implementation approaches. Ideally, integrated modeling
systems would be used to evaluate whether it is more cost-effective to reduce nutrient
loadings with targeted nutrient management practices on the farm, to subsidize edge-of-
field buffers in targeted watersheds, to change cropping patterns or to focus financing on
well-placed off-site freshwater wetlands, or to implement some carefully chosen
combination of these practices. However, while such models exist and are continuously
being further improved, there remain limitations of these models in their current state (see
Sections 2.1.9 and 3.4).
In Table 17, we provide a summary of the potential total nitrogen (TN) and
phosphorus (TP) reduction efficiencies (percent, %) in surface runoff, subsurface flow,
and tile drainage that can be realized where the various conservation practices could be
implemented within the MARB. The cost-effectiveness of these measures will vary from
site to site and with current and future land- and water-use designations. To a large
extent, these estimates are based on relevant sections of this report and on reports by
Devlin et al. (2003), Dinnes (2004), and Gitau et al. (2005). Where numeric values for
reduction efficiency were not included in these reports, relative effects of practices were
estimated based on expert opinion as negative (-, indicating increased export expected),
positive (+, indicating reduced export expected), or neutral (±, indicating no significant
effect expected).
209
-------
Table 17: Potential total nitrogen (TN) and phosphorus (TP) reduction efficiencies (percent change) in
surface runoff, subsurface flow, and tile drainage. Estimates are average values for a multiple year basis,
and some of the numbers in this table are based on a very small amount of field information.
Conservation Practice
Nutrient-use efficiency
Nitrification and urease inhibitors
Nitrogen timing, rate, and method of application
Spring versus Fall application
Recommended rate versus. Above
recommended rate
Subsurface versus. Surface broadcast
Phosphorus timing, rate and method of
application
Avoid runoff producing rainfall
Rate balanced to crop use versus Above
recommended rate
Subsurface versus Surface broadcast
Manure management
Bioenergy, treatment, alternative use, transport
to nutrient-deficit areas
Adoption of comprehensive farm nutrient
management plan
In-field management
Conservation tillage
No-Till versus Conventional tillage
Cover crops
Diverse cropping systems and rotations within
row cropping (78)
Contour plowing and terracing
Drainage management
Standard tile drainage versus undrained
Water table management versus uncontrolled
drainage
Shallow and/or wide versus standard tile
placement
Conversion to CRP
Conversion to perennials crops
Pasture/grassland management
Livestock Exclusion from Streams versus
Constant Intensive Grazing
Surface runoff
TN
+ 1
+
28 to 44
2
50 5
±
0 to 25 5
±
+ 6
0-65 5 7
0 to 25
25
50 2
25 to 70
257
20 to 55
36
25 7
-
40 2
+60 to
90 9
10 to 80
27
TP
±
±4
. 4
20 6
28-57 2
15 to
47 2
8 to 92
12
+ 6
0-45 57
35 to
70 25
7 to 63
2
25 to
88 2
30 to
75 57
70 7
-
+
+75 to
95 9
32 to
767911
Subsurface
flow
TN
+
Oto25
3
+
-
±
±
±
+ 6
+ 6
-
+
±
-
+
+
40 2
+90 10
+
TP
±
±
±
±
+
36 2
-
+ 6
+ 6
±
48 2
±
±
±
+
+
+
75 2
Tile drainage
TN
Ito
21 2
10 to
30%2
27 to
50 2
16 2
±
±
±
+ 6
+ 6
-
13 to
50 2
52 to
93 2
±
-
25 to
54 2
39 2
40 to
97 2
+
±
TP
±
±
±
±
+
25 2
-
+ 6
+ 6
±
+
±
±
-
25 to
42 2
+
+
±
210
-------
Managed Grazing versus Constant Intensive
Grazing
In-field vegetative buffers
Off-site measures
Sedimentation basins
Riparian buffers Total N Total P
Nitrate-N Dissolved P
Wetlands Total P
Dissolved P
-100 to
80 27
12 to 51
257
55 7
50 to 82
79
41 to 92
911
61 to 92
27
Oto78
27
4 to 67
257
65 7
40 to
937911
28 to
85 9n
Oto79
7911
22 to
867911
+
±
±
+
9 to 74
2
+
±
±
+
-
+
-
±
±
20 to
90 9n
±
-
±
±
+
Tile drainage loss of N and P
andP
Subsurface loss of N
Surface runoff of N and P
Relative effects of practices estimated based on expert opinion as negative (-, indicating increased export expected),
positive (+, indicating reduced export expected), or neutral (±, indicating no significant effect expected).
2 From Dimes (2004) report or from SAB Panel report. Values from IA, IL, MO, MN, NE, OH, and OK are included.
3 From Randall and Sawyer (2005), Nitrogen application Timing, Forms and Methods, p. 73-84. Session 6,
UMRSHNC (2006) report.
4 Increased crop yields afforded with N fertilizer, likely to increase P uptake by crop and removal if harvested.
5 From Devlin etal, 2003.
6 Improved manure management leads to lower land application and thereby less potential for loss in any pathway.
7 Values based on data included in Gitau et al. (2005).
8 Studies with only corn-soybean systems are not included, although they were included in Dinnes (2004).
Values from Smith et al. 1992.
10 Y?!u?? ft?!? ???4all et al., 199_7.
'' Values are modifications of values in Dinnes (2004) based on values in SAB Panel report.
Other comments on Table 17:
Values for percent nutrient loss reductions are basin-scale averages, derived from edge-of-field and small
watershed studies and not from widespread implementation. It must be emphasized that there is a great deal of
site-specificity (spatial and temporal), which results in a wide range in observed conservation practice efficiency.
Conservation practices shaded red are likely to have the greatest reduction efficiency on N loss from tile drainage.
Conservation practices shaded green are likely to have the greatest reduction efficiencies for N and P loss in
surface runoff.
Conservation practices shaded blue are likely to have the greatest reduction efficiencies for N and P loss in
subsurface flow.
While some of the conservation practices detailed have large local water quality benefits, they may not have a
major impact on nutrient loss to the Gulf. To help facilitate implementation of practices that reduce nutrient loads
to the Gulf, local water quality benefits are an essential to MARB-wide adoption of these strategies.
Estimates of N and P reductions are only appropriate to areas where a specific conservation practice can be
implemented. For instance, it would not be effective to implement surface runoff control practices such as
sedimentation basins on flat lands with no concentrated surface flow of water. To a certain extent, N and P risk
assessment tools that identify and quantify site vulnerability to N or P loss should be used at a local or field level
to effectively target practices and to maximize reduction.
Implementation of any one of the tabulated conservation practices can positively or negatively influence the
effectiveness of another.
Awareness of the weather forecast in planning any nutrient application or tillage operation is important to
avoiding rainfall-induced runoff of applied nutrients and erosion.
211
-------
The conversion of cropped acres to perennial crops is distinguished from conversion to CRP lands, in that
perennial crops will include grasses harvested for cellulosic biofuel production, which may receive maintenance or
low fertilizer N and P inputs.
The conversion of lands to CRP and from annual cropping to perennials is expected to decrease N and P loss in
surface runoff (shaded green), subsurface flow (shaded blue), and tile drainage (shaded red) due to reduced
fertilizer and manure nutrient inputs and to reduced erosion afforded by increased vegetative cover.
Improved N-use efficiency via appropriate timing, rate, and method of application is expected to benefit P loss
reductions by increasing crop P uptake and removal if harvested.
The estimated reduction efficiencies in Table 17 are based on edge-of-field losses
for studies conducted within the MARB and do not represent expected whole basin
reductions. These values represent potential reductions only for those areas where the
particular practices could be implemented and do not address how broadly a practice
could be applied. The shaded areas indicate those practices expected to have the greatest
impact on reducing nutrient export from the MARB as a whole: red shading indicates
conservation practices that translate into N loss reduction in tile drainage, green shading
is for surface runoff of N and P, and blue shading for nutrient loss in subsurface flow. It
is clear that where edge-of-field loss estimates are available, there is a large variability in
reduction efficiencies, which is both temporally and spatially dependent. This inherent
variability must be recognized when developing conservation or remedial strategies for
the MARB, in the context of probability of expected outcomes. It is also a key
component of the conservation premise that there is no "one size fits all" rationale for
adaptive management.
As a complement to the information summarized in Table 17, a second summary
of the likely environmental benefits is provided in association with the conservation and
land management. In Table 18 and Table 19, the focus is on the broader contribution
these practices can have with respect to a wide variety of environmental services
including local water quality, carbon sequestration in agricultural soils, wildlife habitat,
biodiversity, general recreational activities, and air pollution. These effects are based on
the scientific literature and professional judgment, and potential repercussions are
indicated only as being positive (+) or negative (-) or having no effect (0).
212
-------
Table 18: Anticipated benefits associated with different agricultural management options.
Agricultural
management
option
Decrease
drainage
intensity
Increase
freshwater
wetlands
Forested
riparian
buffers
Herbaceous
riparian
buffers
Improve
manure
mgmt.
Increase
acreage of
perennials
Increase
acres of
farmland
retired
Reduce
fertilizer N
and/ or P
application
Spring
fertilizer N
and/or P
application
Expand
corn-based
ethanol
production
Expand
cellulosic
ethanol
production
Reduce
Nload
to Gulf
+
+
+
+
+
+
+
+
+
-
+
Reduce
Pload
to Gulf
-
«*
+
+
+
+
+
+
0
-
+
Local
surface
WQ
N
+
+
+
+
+
+
+
+
+
-
+
p&
seds
-
«*
-
-
-
-
+
+
0
-
-
GW
quality
0
0
-
-
-
-
-
-
-
-
+
Carbon
seques-
tration
0
-
-
-
0
-
-
0
0
-
-
Local
wildlife
habitat1
+
-
-
-
0
-
-
-
0
-
-
Bio-
diversity1
+
-
-
-
0
-
-
-
0
-
-
Recreat-
ional
activities
+
-
-
-
0
-
-
-
0
-
-
Air
pollution
reduction
0
-
-
-
-
-
-
-
0
-
-
Soil
quality
0
0
-
-
-
-
-
0
0
-
-
Note: + = will lead to improvements in conditions;
effect; ? = effect unknown.
- = likely to be further degraded; 0 = will have little
213
-------
Table 19: Anticipated benefits associated with other management options.
Management
option
Decrease
NOX
emissions
Reduce
point source
loads
Reduce
urban non-
point source
loads
Enhance
floodplain
connectivity
Atchafalaya
diversion
Increase
coastal
wetlands
Reduce
Nload
to Gulf
+
+
+
+
7
?
Reduce
Pload
to Gulf
0
+
+
+
7
+
Local
surface
WQ:
N
+
+
+
+
7
?
Local
surface
WQ:P
& seds
0
+
+
+
7
?
GW
quality
0
0
+
0
0
0
Carbon
seques-
tration
0
0
0
+
0
+
Local
wildlife
habitat1
0
+
+
+
0
+
Biodiversity1
0
+
+
+
0
+
Recreat-
ional
activities
0
+
+
+
7
+
Air
pollution
reduction
+
0
0
0
0
0
Soil
quality
0
0
+
0
0
+
Note: + = will lead to improvements in conditions;
effect; ? = effect unknown.
- = likely to be further degraded; 0 = will have little
In each of these tables, the effects predicted assume that conservation practices
are implemented and managed (maintained) as designed to maximize effectiveness and
life expectancies. Inadequate implementation and maintenance can lead to poor
performance of such systems. Further, these strategies need to be carefully targeted at an
appropriate level of intensity and over sufficient time in order to effectively reduce
nutrient export.
Finally, when considering these tables it is important to note there are synergistic
effects of combinations of conservation practices that result in greater nutrient loss
reductions than do individual practices (Table 18). For example, N application
management that minimizes the potential for excess N available to be leached (nutrient
management, Table 17) should be combined with efforts to reduce the potential off-site
movement of water (in-field management, Table 17). Conversely, there are potential
tradeoffs. For example, reduced-till, no-till, and tile drainage can decrease runoff,
erosion, and P loss but can enhance NOs nitrate leaching potential. As another example,
while N-based manure application can be a cost-effective N source to meet crop N needs,
P may be over applied, increasing the potential for increased runoff and loss of P.
214
-------
Key Findings and Recommendations
A number of conclusions concerning the appropriate use of conservation practices
can be drawn from these tables. First, there is no "one size fits all" land use or
conservation practice strategy that will be cost-effective in all locations. Rather, site
specific and regional optimization of conservation practices and appropriate targeting of
conservation practices and measures will be needed and will include a broad range of
alternative practices and land uses such as crop, animal, fertilizer, and drainage
management measures targeted to appropriate areas. The reduction efficiencies of these
practices are spatially and temporally variable, making it impossible to assign a specific
reduction efficiency for any given conservation practice. As information from ongoing
monitoring of nutrient loss reduction efficiencies becomes available, we will be better be
able to determine what major factors influence reduction efficiencies. This learning and
integration of new knowledge is important and will enhance the process of adaptive
management.
Second, practices that are likely to address NGOM hypoxia effectively in tile-
drained landscapes can differ markedly from those appropriate in non-tiled lands.
Further, while there are no-one-size-fits-all strategies, there are some approaches that
appear particularly promising. For example, inter-seeding of leguminous cover or relay
crops within corn and other grain rotations can decrease fertilizer N requirements, reduce
soil profile N at critical loss times of the year, and mine excess soil P. Reconnecting the
floodplain with managed agricultural lands, by managing hydrology to increase the
amount of time water is retained on the land (wetland) prior to entering the major fluvial
systems, should be considered an important part of an adaptive management plan to
reduce NGOM hypoxia.
Third, practices that are likely to be cost-effective in addressing NGOM hypoxia
may not be the same that yield the highest benefits in other environmental dimensions.
This has important planning and implementation implications, for it suggests that, when
considering implementation strategies, the optimal set of conservation practices and sinks
needs to be considered with respect both to NGOM hypoxia and to the suite of other
environmental concerns that are likely to vary regionally.
Finally, in considering information from the tables and "optimal" sets of
practices, the principles of adaptive management imply that approaches need to be
changed and updated with time to maximize overall efficiency. In the process, more
information can and will be learned about the effectiveness of these practices. This
information can be used both to improve the performance of water quality models to aid
in better implementation strategies and directly to improve targeting of conservation
practices and actions. Based on these findings, the SAB Panel offers the following
recommendations.
There is great temporal and spatial variability in nutrient loss reduction
efficiencies of the various conservation practices available. Thus, continued, new,
and enhanced small watershed based studies of suites of conservation practices as
applied in the real-world are necessary and should be set in a context of research,
215
-------
monitoring, and demonstration to stakeholders so that progress (or lack thereof) in
response to management change can be assessed. A variety of response measures
relevant to different watershed scales and environmental concerns should be
monitored. These measures should include both performance measures (e.g.,
nutrient loading at sub watershed levels, estimates of carbon sequestered on the
landscape) and practice-based measures (e.g., number of acres of wetlands
installed, miles of conservation buffers installed, etc.).
To reduce spring nitrate loss from tile drained regions, alternative and more
complex cropping systems (including perennials) are thought to be the most
effective method of reducing losses. However, given current constraints in
cropping systems, the SAB Panel recommends reducing or discontinuing fall N
application for corn, improved N fertilizer management techniques, use of cover
crops, wetland establishment, and drainage management where appropriate.
For P loss reduction, the Panel again finds that alternative and complex cropping
systems are most effective. For current cropping systems, the Panel recommends
that riparian buffer strips, improved P fertilizer and manure management, and
where appropriate, cover crops be implemented.
Where appreciable drainage occurs in the fall and winter, controlled drainage
could significantly reduce nitrate losses but can be expected to increase surface
runoff and losses of particulate contaminants.
If precision agriculture and controlled release fertilizer technologies are proven to
provide reductions in losses of N and P to water resources, then incentives should
be considered to stimulate their adoption.
Incentives for conversion to perennials, which have potential future use as
cellulosic biofuels production, should be established to promote the co-benefit of
greatly reduced nitrate and P loss from agricultural systems.
There should be a focus on conservation practices and implementation strategies
that appropriately match the nutrient reduction strategies with the goals of
reducing NGOM hypoxia as well as local/regional environmental goals (carbon
sequestration, wildlife, air quality, local water quality, etc.). Given the breadth
and magnitude of these additional environmental goals, these "co-benefits" should
be incorporated in the planning process.
Information on effectiveness and geographic appropriateness of various
conservation practices and nutrient reduction strategies should be used in
conjunction with formal models to plan implementation strategies for
conservation measures that effect a reduction in nutrient loading to the NGOM.
216
-------
5. Summary of Findings and Recommendations
This SAB report provides responses to charge questions in three general areas:
characterization of hypoxia; characterization of nutrient fate, transport and sources; and
the scientific basis for goals and management options. In the sections below, charge
questions are addressed very briefly with references to those sections of this report where
more detailed science on that particular charge question may be found.
5.1. Charge Questions on Characterization of Hypoxia
I. Characterization of Hypoxia - The development, persistence and areal extent of
hypoxia is thought to result from interactions in physical, chemical and biological
oceanographic processes along the northern Gulf continental shelf; and changes
in the Mississippi River basin that affect nutrient loads and fresh water flow.
A. Address the state-of-the-science and the importance of various processes in the
formation of hypoxia in the Gulf of Mexico. These issues include:
i. increased volume and/ orfunneling of fresh water discharges from the
Mississippi River;
ii. changes in hydrologic or geomorphic processes in the Gulf of Mexico and the
Mississippi River basin;
As discussed in Section 2.1, the hydrologic regime of the Mississippi River and
spatial distribution and timing of freshwater inputs to the Gulf of Mexico relative to the
occurrence of energetic currents and waves are critical to vertical mixing intensity,
stratification, and hypoxia in the Gulf. Alteration of the hydrologic regime of the
Mississippi and Atchafalaya Rivers from the 1920's to 1960's has likely increased the
residence time of freshwater on the Louisiana-Texas shelf as well as the area of the
NGOM shelf that is conducive to hypoxia.
Hi. increased nutrient loads due to coastal wetlands losses, upwellingor
increased loadings from the Mississippi River basin;
As discussed in Section 2.1, increased nutrient loadings from the Mississippi
River basin have triggered hypoxia by stimulating in-situ phytoplankton production of
labile organic matter in shallow near-shore receiving waters of the Gulf. Nutrients also
enter this region of the Gulf by advective transport from deeper offshore sources and
from atmospheric deposition. However, advective imports and atmospheric deposition
are relatively minor sources of nutrients in comparison with those from the Mississippi
River basin. The extent to which coastal wetland losses have changed nutrient processing
and loading to the Gulf of Mexico is a subject of continued study but is largely believed
to be of secondary importance.
iv. increased stratification, and seasonal changes in magnitude and spatial
distribution of stratification and nutrient concentrations in the Gulf;
217
-------
As discussed in Section 2.1, increased phytoplankton production, coupled with
stratification and suppressed vertical mixing associated with fresh water discharge has
caused hypoxia in bottom waters of the northern Gulf of Mexico. However, historic
analyses indicate a great deal of variability in seasonal, inter-annual, and decadal scale
patterns of primary productivity, phytoplankton biomass, and the amounts of freshwater
and nutrients discharged to the Gulf. Therefore, trends for nutrient-driven eutrophication
and hypoxia on these time scales have been difficult to interpret.
v. temporal and spatial changes in nutrient limitation or co-limitation, for
nitrogen or phosphorus, as significant factors in the development of the
hypoxic zone;
As discussed in Section 2.1.3, studies of waters overlying the hypoxic region of
the northern Gulf of Mexico indicate that N limitation characterizes offshore waters, but
inshore productivity appears to be P limited and P and N co-limited. This is particularly
true from February to May when peak phytoplankton productivity and biomass formation
coincide with peak freshwater discharge and nutrient loading. Inshore primary
productivity shifts to an N limited mode during the drier (lower freshwater discharge)
summer and fall seasons, and there are likely to be periods when both N and P are
supplied at low levels and co-limit phytoplankton production during the spring to summer
transition.
vi. the implications of reduction of phosphorus or nitrogen without concomitant
reduction of the other.
As discussed in Section 2.1, the Panel finds ample evidence to conclude that N
loading from the Mississippi Atchafalaya River basin is the significant factor driving the
timing and extent of hypoxia in the northern Gulf of Mexico. However, P supplies also
play a significant role in controlling primary production. Therefore, as discussed in
Section 2.1.8, reducing the size of the hypoxic zone requires both N and P discharge
reductions.
B. Comment on the state of the science for characterizing the onset, volume, extent and
duration of the hypoxic zone.
Section 2.1.9 describes modeling approaches that have been used to characterize
the onset, volume, extend, and duration of the hypoxic zone. Simple linear and multiple
regression models that use nutrient loadings to predict hypoxic zone area have been
constructed. Other models have included some consideration of processes and
mechanisms.
218
-------
5.2. Charge Questions on Nutrient Fate, Transport and Sources
II. Characterization of Nutrient Fate, Transport and Sources: Nutrient loads,
concentrations, speciation, seasonality and biogeochemical recycling processes have
been suggested as important causal factors in the development and persistence of
hypoxia in the Gulf. The Integrated Assessment (CENR 2000) presented information on
the geographic locations of nutrient loads to the Gulf and the human and natural
activities that contribute nutrient loadings.
A. Given the available literature and information (especially since 2000), data and
models on the loads, fate and transport and effects of nutrients, evaluate the importance
of various processes in nutrient delivery and effects. These may include:
i. The pertinent temporal (annual and seasonal) characteristics of nutrient
loads/fluxes throughout the Mississippi River basin and, ultimately, to the Gulf of
Mexico.
Total annual N flux discharged to the Gulf of Mexico, primarily nitrate-N and
particulate/organic N, has decreased during the past 25 years, as has the spring (April-
June) flux. Neither total P nor SRP fluxes show major annual or seasonal trends during
the same period.
As discussed in Section 3.1, the upper Mississippi and Ohio-Tennessee River
subbasins contribute about 82% of the annual nitrate-N flux, 69% of the TKN flux, and
58% of the total P flux to the Gulf of Mexico while representing only 31% of the
drainage area of the MARB. When the upper Mississippi River basin is further divided,
the subbasin contributing to the upper Mississippi River between Clinton, IA and
Grafton, IL (only 7% of the drainage area) contributes about 29% of the total annual
nitrate-N flux to the Gulf. Perhaps more importantly, the upper Mississippi and Ohio-
Tennessee River subbasins currently contribute nearly all the spring N flux to the Gulf.
These subbasins represent the tile-drained, corn-soybean landscape of Iowa, Illinois,
Indiana, and Ohio and illustrate that corn-soybean agriculture with tile drainage leaks
considerable N under the current management system. The source of riverine P is more
diffuse, although these subbasins are also the largest sources of P.
/'/'. The ability to determine an accurate mass balance of the nutrient loads
throughout the basin.
Estimates of mass balances for nutrient inputs during the period since the
Integrated Assessment have been recalculated and are discussed in Section 3.2, but the
research needs described in the Integrated Assessment remain unresolved. Therefore, the
Panel's ability to determine an accurate mass balance of nutrient inputs to the MARB is
limited by the available information and understanding. For example, some components
of the N mass balance (e.g., denitrification, N2 fixation, manure N, soil N pool processes
such as mineralization and immobilization) are not measured each year. N2 fixation and
manure N are the only two of these components that can be estimated. There are too few
data available for the remaining processes to allow calculations. There also is still a
disconnect between estimates of inputs to the land (i.e. fertilizer and manure use) and
219
-------
estimates of the proportion of N and P from those inputs that reach the riverine system
and contribute to the nutrient flux. Point sources discharge N and P directly to rivers, and
are estimated by this Panel to contribute about 22% and 34% of the annual riverine N and
P flux respectively, yet their contributions continue to be estimated from permit limits
and are not actually measured. Better point-source data are needed to improve mass
balance estimates of nutrient loads.
Hi. Nutrient transport processes (fate/transport, sources/sinks, transformations,
etc.) through the basin, the deltaic zone, and into the Gulf.
As discussed in Section 3.3, the percentage of annual N and P inputs removed by
in-stream processes varies by MARB subbasin and ranges from 20 to 55% for N and 20
to 75% for P based on model estimates. Denitrification can be a significant pathway for
N removal in small streams during low flow, warm periods, thereby enhancing local
water quality. However, most nitrate-N is exported to the Gulf during high flows in the
period from January to June, when denitrification is not an effective removal process.
Although current estimates of denitrification rates in coastal wetlands are higher than the
estimates used in the Integrated Assessment, current studies still conclude that river
diversions to coastal wetlands would remove only small amounts of nutrients relative to
the total fluxes. However, better estimates of nutrient and organic matter loss rates
(denitrification; long-term burial of C, N, and P; and plant uptake) are needed to better
understand observed differences between wetland inputs and outputs in coastal areas.
B. Given the available literature and information (especially since 2000) on nutrient
sources and delivery within and from the basin, evaluate capabilities to:
i. Predict nutrient delivery to the Gulf, using currently available scientific tools
and models; and
ii. route nutrients from their various sources and account for the transport
processes throughout the basin and deltaic zone, using currently available
scientific tools and models.
In Section 3.4, the SAB Panel singled out three models for discussion:
SPARROW, SWAT, and IBIS/THMB. Each is capable of N and P load estimation on
the scale of the MARB, yet each has strengths and weaknesses requiring further
development. The uncertainty of results from each model reflects the uncertainty of the
model structure and algorithms, as well as that propagated by the input data, user
parameterization, the calibration process, other user-defined conditions, and the skill of
the model user. Even though the capability to predict and route nutrients throughout the
MARB has improved since the Integrated Assessment, future adaptive management will
require a smooth interface between watershed, economic, and Gulf of Mexico hypoxia
models that will allow resource managers the capability to assess the effects of policy
decisions and management practices on the sources, fate, and transport of nutrients from
the MARB to the Gulf of Mexico.
220
-------
5.3. Charge Questions on Goals and Management Options
///. Scientific Basis for Goals and Management Options. The Task Force has stated
goals of reducing the 5-year running average areal extent of the Gulf of Mexico hypoxic
zone to less than 5,000 square kilometers by the year 2015, improving water quality
within the basin and protecting the communities and economic conditions within the
basin. Additionally, nutrient loads from various sources in the Mississippi River basin
have been suggested as the major driver for the formation, extent and duration of the
Gulf hypoxic zone.
A. Are these goals supported by present scientific knowledge and understanding of the
hypoxic zone, nutrient loads, fate and transport, sources and control options?
The SAB Panel affirms the major findings of the Integrated Assessment.
Although the 5,000 km2 target remains a reasonable endpoint for continued use in an
adaptive management context; it may no longer be possible to achieve this goal by 2015.
Accordingly, it is even more important to proceed in a directionally correct fashion to
manage factors affecting hypoxia than to wait for greater precision in setting the goal for
the size of the zone.
/'. Based on the current state-of- the-science, should the reduction goal for the size
of the hypoxia zone be revised?
No. As discussed in the Executive Summary, it is more important to begin to
move in a directionally correct fashion than to refine the goal for the exact size of the
hypoxic zone.
/'/'. Based on the current state-of-the-science, can the areal extent of Gulf hypoxia
be reducedwhile also protecting water quality and social welfare in the basin?
Social welfare can be protected by choosing policies that incorporate targeting,
provide economic incentives and maximize co-benefits. As discussed in Section 4.3,
improvements in large-scale integrated economic and bio-physical models are needed to
better capture system-wide response and effects.
B. Based on the current state-of- the-science, what level of reduction in causal agents
(nutrients/discharge) will be needed to achieve the current reduction goal for the size of
the hypoxic zone?
As discussed in Section 4.2, to reduce the size of the hypoxic zone, the SAB Panel
recommends an adaptive management approach targeting at least a 45% reduction in
discharges of total N and total P from the 1980 - 1996 fluxes.
C. Given the available literature and information (especially since 2000) on
technologies and practices to reduce nutrient loss from agriculture, runoff from other
221
-------
non-point sources and point source discharges, discuss options (and combinations of
options) for reducing nutrient flux in terms of cost, feasibility and any other social
welfare considerations.
In general, the social costs of reducing nutrients will vary widely with the policy
chosen, hence overall cost-effectiveness is largely a function of policy. Policies that
target and provide economic incentives are essential to minimize costs. A wide range of
policy options are discussed in Section 4.4, while management options are covered
extensively in Section 4.5.
These options may include:
i. the most effective agricultural practices, considering maintenance of soil
sustainability and avoiding unintended negative environmental consequences.
The cost and reduction efficency rankings of agricultural management practices
will vary by site and region, historic land use and management, depending on crops
grown, local soil conditions, distance to waterway, field slopes and configuration,
presence of buffers, drainage structures and so forth. Table 16 in Section 4.5.10 provides
the SAB Panel's summary of the evidence comparing the relative effectiveness of
nutrient (N and P) reduction options in agriculture. Section 4.5.6 discusses management
options for in-field nutrients. A targeted and adaptive management framework will
maximize local and regional water quality benefits in the MARB and Gulf.
ii. the most effective actions for other non-point sources
As discussed in Section 4.5.7, there are significant policy opportunities to reduce
atmospheric deposition of N, however a detailed examination of air pollution control
policy options was beyond the SAB Panel's scope. Nonetheless, the Panel strenuously
recommends incorporating water quality benefits and effects on hypoxia in air pollution
control decisions.
in. the most effective technologies for industrial and municipal point sources.
As discussed in Section 4.5.8, a targeted permit by permit approach to industrial
point source discharges could yield significant opportunities for nutrient (N and P)
reduction since frequently a limited number of permitted facilities are responsible for a
large part of the N and P loads. Municipal point sources are also discussed in Section
4.5.8 where the SAB Panel recommends an analysis to assess the cost and feasibility of
tightening limits on N and P concentrations in discharges for large sewage treatment
plants.
In all three areas, please address research and information gaps (expanded monitoring,
documentation of sources and management practices, effects of practices, further model
development and validation, etc.) that should be addressed prior to the next 5-year
review.
222
-------
Recommendations for monitoring and research are found in nearly every section
of the report and are included below in the summary of the SAB Panel's
recommendations.
5.4. Conclusion
This report constitutes the SAB Panel's response to charge questions posed by the
EPA Office of Water. This Advisory reaffirms the major findings of the Integrated
Assessment, while pointing out the need for economic incentives to encourage
conservation in the Mississippi Atchafalaya River basin. Although the science has
grown, actions to control hypoxia have lagged. The SAB urges the EPA and other
agencies to utilize the recommendations of this Advisory and move ahead with
implementing programs, strategies and policies to reduce the size of the hypoxic zone and
improve water quality in the Mississippi Atchafalaya River basin.
Most of the research and monitoring needs identified in the Integrated Assessment
have not been met, and fewer rivers and streams are monitored today than in 2000. The
majority of monitoring recommendations in the Integrated Assessment remain relevant
and should be heeded, specifically the CENR's call to improve and expand monitoring of
the temporal and spatial extent of hypoxia and the processes controlling its formation; the
flux of nutrients, carbon, and other constituents from non-point sources throughout the
MARB and to the NGOM; and measured (rather than estimated) nitrogen and phosphorus
fluxes from municipal and industrial point sources. Echoing the CENR, the SAB Panel
affirms the need for research on the ecological effects of hypoxia; watershed nutrient
dynamics; effects of different agricultural practices on nutrient losses from land,
particularly at the small watershed scale; nutrient cycling and carbon dynamics; long-
term changes in hydrology and climate; and economic and social impacts of hypoxia. A
suite of models is needed to simulate the processes and linkages that regulate the onset,
duration and extent of hypoxia. Emerging coastal ocean observation and prediction
systems should be encouraged to monitor dissolved oxygen and other physical and
biogeochemical parameters needed to continue improving hypoxia models.
Although there are over 90 recommendations in this report, the following major
recommendations reflect the SAB Panel's consideration of the new science that has
emerged since the Integrated Assessment.
To advance the science characterizing hypoxia and its causes, the SAB Panel
finds that research is needed to:
collect and analyze additional sediment core data needed to develop a better
understanding of spatial and temporal trends in hypoxia;
investigate freshwater plume dispersal, vertical mixing processes and
stratification over the Louisiana-Texas continental shelf and Mississippi Sound,
223
-------
and use three-dimensional hydrodynamic models to study the consequences of
past and future flow diversions to NGOM distributaries;
advance the understanding of biogeochemical and transport processes affecting
the load of biologically available nutrients and organic matter to the Gulf of
Mexico, and develop a suite of models that integrate physics and
biogeochemistry;
elucidate the role of P relative to N in regulating phytoplankton production in
various zones and seasons, and investigate the linkages between inshore primary
production, offshore production, and the fate of carbon produced in each zone;
improve models that characterize the onset, volume, extent, and duration of the
hypoxic zone, and develop modeling capability to capture the importance of P, N,
and P-N interactions in hypoxia formation.
With respect to advancing the science on sources, fate and transport of nutrients,
the SAB Panel finds that research is needed to:
develop models to simulate fluvial processes and estimate N and P transfer to
stream channels under different management scenarios;
improve the understanding of temporal and seasonal nutrient fluxes and develop
nutrient, sediment, and organic matter budgets within the MARB;
To enhance the scientific basis for implementation of management options, the
SAB Panel finds that research is needed to:
examine the efficacy of dual nutrient control practices;
determine the extent, pattern, and intensity of agricultural drainage as well as
opportunities to reduce nutrient discharge by improving drainage management;
integrate monitoring, modeling, experimental results, and ongoing management
into an improved conceptual understanding of how the forces at key management
scales influence the formation of the hypoxia zone; and
develop integrated economic and watershed models to fully assess costs and
benefits, including co-benefits, of various management options.
To reduce the size of the hypoxic zone, the SAB Panel recommends at least a
45% reduction in N accompanied by a comparable reduction in P. The Panel found five
areas that offer the most significant opportunities for N and P reductions:
promotion of environmentally sustainable approaches to biofuel production and
associated cropping systems (e.g. perennials).
224
-------
improved management of nutrients by emphasizing infield nutrient management
efficiency and effectiveness to reduce losses;
construction and restoration of wetlands, as well as criteria for targeting those
wetlands that may have a higher priority for reducing nutrient losses;
introduction of tighter N and P limits on municipal point sources; and
improved targeting of conservation buffers, including riparian buffers, filter strips
and grassed waterways, to control surface-borne nutrients.
225
-------
References
Ackerman, F., Biewald, B., White, D., Woolf, T., and Moomaw, W., 1999, Grand-
fathering and coal plant emissionsThe cost of cleaning up the Clean Air Act:
Energy Policy, v. 27, p. 929-940.
Adviento-Borbe, M.A.A., Haddix, M.L., Binder, D.L., Walters, D.T., and Dobermann,
A., 2007, Soil greenhouse gas fluxes and global warming potential in four high-
yielding maize systems: Global Change Biology, in press, listed online as Accepted
Articles at: http://www.blackwell-synergy.com/doi/abs/10.Ill 1/j.1365-
2486.2007.01421.x. last accessed August 15, 2007.
Aillery, M., Gollehon, N., Johansson, R., Kaplan, J., Key, N., and Ribaudo, M., 2005,
Managing manure to improve air and water quality: Washington, D.C., U.S.
Government Printing Office, U.S. Department of Agriculture, Economic Research
Service, Economic Research Report 9, 65 p., available online at:
http://www.ers.usda.gov/publications/err9/err9.pdf
Alexander, R.B., Boyer, E.W., Smith, R.A., Schwarz, G.E., and Moore, R.B., 2007a, The
role of headwater streams in downstream water quality: Journal of the American
Water Resources Association, v. 43, no. 1, p. 41-59.
Alexander, R.B., Elliott, A.H., Shankar, U., and McBride, G.B., 2002a, Estimating the
sources and transport of nutrients in the Waikato River basin, New Zealand: Water
Resources Research, v. 38, p. 1268-1290.
Alexander, R.B., Johnes, P.J., Boyer, E.W., and Smith, R.A., 2002b, A comparison of
models for estimating the riverine export of nitrogen from large watersheds:
Biogeochemistry, v. 57/58, p. 295-339.
Alexander, R.B., Slack, J.R., Ludtke, A.S., Fitzgerald, K.K., and Schertz, T.L., 1998,
Data from selected U.S. Geological Survey national stream water-quality monitoring
networks: Water Resources Research, v. 34, no. 9, p. 2401-2405.
Alexander, R.B., and Smith, R.A., 2006, Trends in the nutrient enrichment of U.S. rivers
during the late 20th century and their relation to changes in probable stream trophic
conditions: Limnology and Oceanography, v. 51, p. 639-654.
Alexander, R.B., Smith, R.A., and Schwarz, G.E., 2000, Effect of stream channel size on
the delivery of nitrogen to the Gulf of Mexico: Nature, v. 403, p. 758-761.
Alexander, R.B., Smith, R.A., and Schwarz, G.E., 2004, Estimates of diffuse phosphorus
sources in surface waters of the United States using a spatially referenced watershed
model: Water Science and Technology, v. 49, no. 3, p. 1-10.
Alexander, R.B., Smith, R.A., Schwarz, G.E., Boyer, E.W., Nolan, J.V., and Brakebill,
J.W., in press, Differences in phosphorus and nitrogen delivery to the Gulf of
Mexico from the Mississippi River basin: Environmental Science and Technology,
(In press), (accepted for publication 10/30/2007).
Alexander, R.B., Smith, R.A. Schwarz, G.E., Preston, S.D., Brakebill, J.W., Srinivasan,
R., and Pacheco, P.A., 2001, Atmospheric nitrogen flux from the watersheds of
major estuaries of the United States: An application of the SPARROW watershed
226
-------
model, in Valigura, R., Alexander, R., Castro, M., Meyers, T., Paerl, H., Stacey, P.,
and Turner, R.E., eds., Nitrogen loading in coastal water bodiesAn atmospheric
perspective: American Geophysical Union Monograph 57, p. 119-170.
Aller, R.C., 1998, Mobile deltaic and continental shelf muds as sub-oxic, fluidized bed
reactors: Marine Chemistry, v. 61, p. 143-155.
Aller, R.C., Heilbrun, C., Panzeca, C., Zhu, Z.-B., and Baltzer, F., 2004. Coupling
between sedimentary dynamics, early diagenetic processes, and biogeochemical
cycling in the Amazon-Guianas mobile mud belt: Coastal French Guiana: Marine
Geology, v. 208, p. 331-360.
Ammerman, J.W., and Sylvan, J.B., 2004, Phosphorus limitation of phytoplankton
growth in the Mississippi River plumeA case for dual nutrient control?: EOS
Transactions AGU, v. 85, no. 47, Fall Meeting Supplement, Abstract OS11B-07.
Anand, S., Mankin, K.R., McVay, K.A., Janssen, K.A., Barnes, P.L., and Pierzynski,
G.M., 2007, Calibration and validation of ADAPT and SWAT for field-scale runoff
prediction: Journal of the American Water Resources Association, v. 43, no. 4, p.
899-910.
Anderson, C.J., Nairn, R.W., and Mitsch, W.J., 2005, Temporal and spatial development
of surface soil conditions at two created riverine marshes: Journal of Environmental
Quality, v. 34, p. 2072-2081.
Anderson, D.M., and Garrison, D.J., eds., 1997, The ecology and oceanography of
harmful algal blooms: American Society of Limnology and Oceanography Special
Issue, v. 42, no. 5, p. 1009-1305.
Andraski, T.W., and Bundy, L.G., 2003, Relationships between phosphorus levels in soil
and in runoff from corn production systems: Journal of Environmental Quality, v. 32,
p. 310-316.
Arnold, J.G., Srinivasan, R., Muttiah, R.S., and Allen, P.M., 1999, Continental scale
simulation of the hydrologic balance: Journal of the American Water Resources
Association, v. 35, no. 5, p. 1037-1051.
Arnold, J.G., Srinivasan, R., Muttiah, R.S., and Williams, J.R., 1998, Large area
hydrologic modeling and assessmentPart I, Model development: Journal of the
American Water Resources Association, v. 34, no. 1, p. 73-89.
Atwood, J.D., Benson, V.W., Srinivasan, R., Walker, C., and Schmid, E., 2001,
Simulated nitrogen loading from corn, sorghum, and soybean production in the
Upper Mississippi Valley, in Stott, D.E., Mohtar, R.H., and Steinhardt, G.C., eds.,
Sustaining the Global Farm, 10th International Soil Conservation Organization
Meeting, Purdue University, IN, May 24-29, 1999, p. 344-348.
Aulenbach, B.T., and Hooper, R.P., 2006, The composite methodAn improved method
for stream-water solute load estimation: Hydrological Processes, v. 20, p. 3029-
3047.
Aulenbach, B.T., Buxton, H.T., Battaglin, W.A., and Coupe, R.H., 2007, Streamflow and
nutrient fluxes of the Mississippi-Atchafalaya River basin and subbasins for the
227
-------
period of record through 2005: U.S. Geological Survey Open-File Report 2007-1080,
available online at: http://toxics.usgs.gov/pubs/of-2007-1080/index.html.
Babcock, B.A., Gassman, P.W., Jha, M., and Kling, C.L., 2007, Adoption subsidies and
environmental impacts of alternative energy crops: Iowa State University, Center for
Agricultural and Rural Development (CARD) Briefing Paper 07-BP 50, 15 p,
available on line at:
http://www.card.iastate.edu/publications/DBS/PDFFiles/07bp50.pdf.
Baker, J.L., David, M.B., and Lemke, D.W., in press, Understanding nutrient fate and
transport, including the importance of hydrology in determining losses, and potential
implications on management systems to reduce those losses, in Proceedings of Gulf
Hypoxia and Local Water quality Concerns Workshop, Ames, IA, September 26-28,
2005. (in press)
Baker, J.L., Melvin, S.W., Lemke, D.W., Lawlor, P.A., Crumpton, W.G., and Helmers,
M.J., 2004, Subsurface drainage in Iowa and the water quality benefits and problem,
in Cooke, R., ed., Proceedings of the Eighth International Drainage Symposium,
Sacramento, CA, March 21, 2004, ASAE Pub #701P0304, p. 39-50.
Baker, J.L., Mickelson, S.K., and Crumpton, W.G., 1997, Integrated crop management
and off-site movement of nutrients and pesticides, in Hatfield, J.C., Buhler, D.B., and
Stewart, B.A., eds., Weed biology, soil management, and weed management
Advances in soil science: Boca Raton, CA, CRC Press, p. 135-160.
Baker, D.B., and Richards, P.R., 2002, Phosphorus budgets and riverine phosphorus
export in northwestern Ohio watersheds: Journal of Environmental Quality, v. 31,
p. 96-108.
Baltz, D.M., Hiram, W.L., Rossignol, P.A., Chesney, E.J., and Switzer, T.S., 2006, A
qualitative assessment of the relative effects of by catch reduction of fisheries and
hypoxia on coastal nekton communities in the Gulf of Mexico: Paper presented at
Hypoxia Effects on Living Resources in the Gulf of Mexico, September 25-26,
2006: New Orleans, Louisiana, Tulane University, sponsored by National Oceanic
and Atmospheric Administration Center for Sponsored Coastal Ocean Research.
Barker, D.W., Sawyer, I.E., and Al-Kaisi, M.M., 2006a, Assessment of the amino sugar-
nitrogen test on Iowa soilsI. Evaluation of soil sampling and corn management
practices: Agronomy Journal, v. 98, p. 1345-1351.
Barker, D.W., Sawyer, I.E., and Al-Kaisi, M.M., 2006b, Assessment of the amino sugar-
nitrogen test on Iowa soilsII. Field correlation and calibration: Agronomy Journal,
v. 98, p. 1352-1358.
Batie, S.S., Gilliam, J.W., Groffman, P.M., Hallberg, G.R., Hamilton, N.D., Larson,
W.E., Lee, L.K., Nowak, P.J., Renard, K.G., Rominger, R.E., Stewart, B.A., Tanji,
K.K., Van Schilfgaarde, J., Wagenet, R.J., and Young, D.L., 1993, Soil and water
quality: An agenda for agriculture: National Academy of Sciences, Board of
Agriculture, 278 p.
Battaglin, W., 2006, Streamflow and nitrogen, phosphorus, and silica flux at selected
sites in the Mississippi River basin, 1980-2005, presented at Science Symposium:
228
-------
Sources, Transport and Fate of Nutrients in the Mississippi and Atchafalaya River
basins, November 7-9, 2006, Minneapolis, MN.
Baumol, W., and Gates, W., 1988, The Theory of Environmental Policy, 2nd edition:
Cambridge University Press, Cambridge, UK, 299 p.
Beegle, D.B., 2005, Assessing soil phosphorus for crop production by soil testing, in
Sims, J.T., and Sharpley, A.N., eds., Phosphorus: Agriculture and the Environment,
Madison, WI, American Society of Agronomy Monograph Series No. 46, p. 123-
144.
Belabbassi, L., 2006, Examination of the relationship of river water to occurrences of
bottom water with reduced oxygen concentrations in the northern Gulf of Mexico:
College Station, Texas, Texas A&M University, Ph.D. thesis, xii + 119 p.
Benner, R., and Opsahl, S., 2001, Molecular indicators of the sources and transformations
of dissolved organic matter in the Mississippi River plume: Organic Geochemistry,
v. 32, p. 597-611.
Bermudez, M., and Mallarino, A.P., 2007, Impacts of varable-rate phosphorus
fertilization based on dense grid soil sampling on soil-test phosphorus and grain
yield of corn and soybean: Agronomy Journal, v. 99, p. 822-832.
Bernot, M.J., and Dodds, W.K., 2005, Nitrogen retention, removal, and saturation in lotic
ecosystems: Ecosystems, v. 8, p. 442-453.
Besiktepe, S.T., Lermusiaux, P.F.J., and Robinson, A.R., 2003, Coupled physical and
biogeochemical data-driven simulations of Massachusetts Bay in late summer: Real-
time and postcruise data assimilation: Journal of Marine Systems, v. 40-41, p. 171-
212.
Beven, K.J., 2001, Rainfall-runoff modeling: The primer: Wiley, Chichester, UK, 360 p.
Bharati, L., Lee, K.H., Isenhart, T.M., and Schultz, R.C., 2002, Soil-water infiltration
under crops, pasture and established riparian buffer in Midwestern USA:
Agroforestry Systems, v. 56, p. 249-257.
Bianchi, T..S., Allison, M.A., Canuel, E.A., Corbett, D.R., McKee, B.A., Sampere, T.P.,
Wakeham, S.G., Waterson, E., 2006, Rapid export of organic matter to the
Mississippi canyon, Mississippi: EOS, Transactions of the American Geophysical
Union, v. 87, no. 50, p. 572-574.
Bianchi, T.S., Filley, T., Dria, K., and Hatcher, P.G., 2004, Temporal variability in
sources of dissolved organic carbon in the lower Mississippi River: Geochimica et
Cosmochimica Acta, v.68, no. 5, p. 959-967.
Bianchi, T.S., Galler, J.J., and Allison, M.A.,2007, Hydrodynamic sorting and transport
of terrestrially derived organic carbon in sediments of the Mississippi and
Atchafalaya Rivers: Estuarine, Coastal and Shelf Science, v. 73, nos. 1-2, p 211-
222.
Bianchi, T.S., Mitra, S., and McKee, B., 2002, Sources of terrestrially-derived carbon in
the lower Mississippi River and Louisiana shelfImplications for differential
229
-------
sedimentation and transport at the coastal margin: Marine Chemistry, v. 77, p. 211-
223.
Bierman, V.J., Jr., Hinz, S.C., Zhu, D.-W., Wiseman, W.J., Jr., Rabalais, N.N., and
Turner, R.E., 1994, A preliminary mass balance model of primary productivity and
dissolved oxygen in the Mississippi River Plume/Inner Gulf Shelf region: Estuaries,
v. 17, no. 4, p. 886-899.
Blaylock, A.D., 2006, Review of enhanced-efficiency nitrogen fertilizers: in Proceedings
of Southern Plant Nutrient Management Conference, Olive Branch, MS, October 3-
4, 2006, p. 4-10.
Blomqvist, S., Gunnars, A., and Elmgren, R., 2004, Why the limiting nutrient differs
between temperate coastal seas and freshwater lakesA matter of salt: Limnology &
Oceanography, v. 49, p. 2236-2241.
Bode, A., and Dortch, Q., 1996, Uptake and regeneration of inorganic nitrogen in coastal
waters influenced by the Mississippi RiverSpatial and seasonal variations: Journal
of Plankton Research, v. 18, no. 12, p. 2251-2268.
Boesch, D.F., 2002, Challenges and opportunities for science in reducing nutrient over-
enrichment of coastal ecosystems: Estuaries, v. 25, p. 744-758.
Boesch, D.F., 2003, Continental shelf hypoxia: Some compelling answers: Comments on
"Continental shelf hypoxia: Some nagging questions": Gulf of Mexico Science, v.
21, no. 2, p. 202-205. Available on line at:
http://goms.disl.org/tocpages/december2003vol21no2.htm
Boicourt, W.C., 1992, Influences of circulation processes on dissolved oxygen in the
Chesapeake Bay, in Smith, D.E., Leffler, M., and Mackiernan, G., eds., Oxygen
dynamics in the Chesapeake BayA synthesis of recent research: College Park,
MD, Maryland Sea Grant College, p. 7-79.
Booth, M.S., and Campbell, C., 2007, Spring nitrate flux in the Mississippi River basin:
A landscape model with conservation applications: Environmental Science and
Technology, v. 41, no. 15, p. 5410-5418.
Boyer, E.W.,Goodale, C.L., Jaworski, N.A., and Howarth, R.W., 2002, Anthropogenic
nitrogen sources and relationships to riverine nitrogen export in the northeastern
U.S.A.: Biogeochemistry, v. 57/58, p. 137-169.
Boyer, E.W., and Howarth, R.W., in press, Nitrogen fluxes from rivers to the coastal
oceans, in Capone, D., Mulholland, M., and Carpenter, E., eds., Nitrogen in the
marine environment, 2nd ed.: New York, NY, Academic Press, (in press).
Boynton, W.R., and Kemp, W.M., 2000, Influence of river flow and nutrient loads on
selected ecosystem processes: A synthesis of Chesapeake Bay data, in Hobbie, J.E.,
ed., Estuarine scienceA synthetic approach to research and practice: Washington,
D.C., Island Press, p. 269-298.
Bradley, M.J., and Jones, B.M., 2002, EmissionsDeveloping advanced energy and
transportation technologies: AMBIO, A Journal of the Human Environment, v. 31,
no. 2, p. 141-149.
230
-------
Bratkovich, A., Dinnel, S.P., and Goolsby, D.A., 1994, Variability and prediction of
freshwater and nitrate fluxes for the Louisiana-Texas shelfMississippi and
Atchafalaya river source functions: Estuaries, v. 17, p. 766-778.
Breetz, H., Fisher-Vander, K., Garzon, L., Jacobs, H., Droetz, K., and Terry, R., 2004,
Water quality trading and offset initiatives in the U.S.A comprehensive survey:
Dartmouth College, Hanover, NH, 337 p. Available on line at:
http://www.dartmouth.edu/~kfv/waterqualitytradingdatabase.pdf.
Brezonik, P.L., Bierman, V.J., Jr., Alexander, R., Anderson, J., Barko, J., Dortch, M.,
Hatch, L., Hitchcock, G.L., Keeney, D., Mulla, D., Smith, B., Walker, C., Whitledge,
T., and Wiseman, W.J., Jr., 1999, Effects of reducing nutrient loads to surface waters
within the Mississippi River basin and the Gulf of Mexico: Topic 4 report for the
integrated assessment of hypoxia in the Gulf of Mexico: Silver Spring, MD, National
Oceanic and Atmospheric Administration Coastal Ocean Program Decision Analysis
Series No. 18, 158 p. Available on line at:
http://oceanservice.noaa. gov/products/hypox_t4final.pdf.
Bridgham, S.D., Johnston, C.A., Schubauer-Berigan, J.P., and Wesihampel, P., 2001,
Phosphorus sorption dynamics in soils and coupling with surface and pore water in
riverine wetlands: Soil Sciences Society of America Journal, v. 65, p. 577-588.
Broshears, R.E., Clark, G.M., and Jobson, H., 2001, Simulation of stream discharge and
transport of nitrate and selected herbicides in the Mississippi River Basin:
Hydrological Processes, v. 15, p. 1157-1167.
Brouwer, M., 2006, Changes in gene and protein expression and reproduction in grass
shrimp, Palaemonetespugio, exposed to chronic hypoxia: Paper presented at
Hypoxia Effects on Living Resources in the Gulf of Mexico, September 25-26,
2006, Tulane University, New Orleans, Louisiana, sponsored by National Oceanic
and Atmospheric Administration Center for Sponsored Coastal Ocean Research.
Bruland, G.L., and Richardson, C.J., 2006, An assessment of the phosphorus retention
capacity of wetlands in the Painter Creek Watershed, Minnesota, USA: Water, Air,
and Soil Pollution, v. 171, p. 169-184.
Bundy, L.G., 1998, Corn fertilization: University of Wisconsin Cooperative Extension
Service Publication AA3340.
Cai, W.J., and Lohrenz, S.E., 2005, Carbon, nitrogen, and phosphorus fluxes from the
Mississippi River and the transformation and fate of biological elements in the river
plume and the adjacent margin, in Liu, K.K., Atkinson, L., Quinones, R., Talaue-
McManus, L., eds., Carbon and nutrient fluxes in continental marginsA global
synthesis: NY, Springer-Verlag.
Cai, W.J., 2003, Riverine inorganic carbon flux and rate of biological uptake in the
Mississippi River plume: Geophysical Research Letters, v. 30, no. 2,
doi:10.1029/2002GL016312.
Caraco, N., Cole, J.J., and Likens, G.E., 1990, A comparison of phosphorus
immobilization in sediments of freshwater and coastal marine systems:
Biogeochemistry, v. 9, p. 277-290.
231
-------
Caraco, N.F., Cole, J.J., and Likens, G.E., 1989, Evidence for sulfate controlled
phosphorus release from sediments of aquatic systems: Nature, v. 341, p. 316-318.
Carter, G.S., Gregg, M.C., and Lien, R.C., 2005, Internal waves, solitary like waves, and
mixing on the Monterey Bay shelf: Continental Shelf Research, v. 25, p. 1499-1520.
CENR, 2000, Integrated assessment of hypoxia in the northern Gulf of Mexico: National
Science and Technology Council, Committee on Environmental and Natural
Resources, May 2000, 66 p. Available on line at:
http://oceanservice.noaa.gov/products/hypox final.pdf.
Cerco, C.F., and Cole, T., 1993, Three-dimensional eutrophication model of Chesapeake
Bay: Journal of Environmental Engineering, v. 119, no. 6, p. 1006-1025.
Chen, N., Bianchi, T.S., and McKee, B.A., 2005, Early diagnensis of chloropigment
biomarkers in the lower Mississippi River and Louisiana shelfImplications for
carbon cycling in a river-dominated margin: Marine Chemistry, v. 93, p. 159-177.
Chen, N., Bianchi, T.S., McKee, B.A., and Bland, J.M., 2001, Historical trends of
hypoxia on the Louisiana shelfApplications of pigments as biomarkers: Organic
Geochemistry, v. 32, p. 543-561.
Chen, R.F., and Gardner, G.B., 2004, High resolution measurements of chromophoric
dissolved organic matter in the Mississippi and Atchafalaya river plume regions:
Marine Chemistry, v. 89, p. 103-125.
Chen, X., Lohrenz, S.E., and Wiesenburg, D.A., 2000, Distribution and controlling
mechanisms of primary production over the Louisiana-Texas continental shelf:
Journal of Marine Systems, v. 25, p. 179-207.
Chesapeake Bay Commission, 2004, Cost effective strategies for the Bay: Annapolis,
MD, 14 p.
Chesapeake Bay Program, 2006, Watershed model progress scenario results for 2005:
Available on line at: http://www.chesapeakebav.net/tribtools.htm.
Chesney, E.J., and Baltz, D.M., 2001, The effects of hypoxia on the northern Gulf of
Mexico coastal ecosystemA fisheries perspective, in Rabalais, N.N., and Turner,
R.E., eds., Coastal hypoxiaConsequences for living resources and ecosystems:
Washington, D.C., Coastal and Estuarine Studies 58, American Geophysical Union,
p. 321-354.
Childs, C.R., Rabalais, N.N., Turner, R.E., and Proctor, L.M., 2002, Sediment
denitrification in the Gulf of Mexico zone of hypoxia: Marine Ecology Progress
Series, v. 240, p. 285-290.
Childs, C.R., Rabalais, N.N., Turner, R.E., and Proctor, L.M., 2003, ErratumSediment
denitrification in the Gulf of Mexico zone of hypoxia: Marine Ecology Progress
Series, v. 247, p. 310.
Chow, T.L., Rees, H.W., and Daigle, J.L., 1999, Effectiveness of terraces/grassed
waterway systems for soil and water conservationA field evaluation: Journal of
Soil and Water conservation, v. 54, no. 3, p. 577-583.
232
-------
Chua, T.T., Bronson, K.F., Booker, J.D., Keeling, J.W., Mosier, A.R., Bordovsky, J.P.,
Lascano, R.J., Green, C.J., and Segarra, E., 2003, In-season nitrogen status sensing
in irrigated cotton I. Yields and nitrogen-15 recovery: Soil Science Society of
America Journal, v. 67, p. 1428-1438.
Chung, S.W., Gassman, P.W., Gu, R., and Kanwar, R.S., 2002, Evaluation of EPIC for
assessing tile flow and nitrogen losses for alternative agricultural management
systems: Transactions of the American Society of Agricultural Engineers, v. 45,
no. 4, p. 1135-1146.
Claassen, R., 2000, Agricultural resources and environmental indicatorsCompliance
provisions for soil and wetland conservation: U.S. Department of Agriculture,
Economic Research Service, Agricultural Resources and Environmental Indicators,
chap. 6.3, 20 p..
Claassen, R., Breneman, V., Bucholtz, S., Cattaneo, A., Johansson, R., and Morehart, M.,
2004, Environmental compliance in U.S. agricultural policyPast performance and
future potential: U.S. Department of Agriculture, Economic Research Service,
Agricultural Economic Report No. 832, p. 52.
Clapp, C.E., Allmaras, R.R., Layese, M.F., Linden, D.R., and Dowdy, R.H., 2000, Soil
organic carbon and 13C abundance as related to tillage, crop residue, and nitrogen
fertilization under continuous corn management in Minnesota: Soil & Tillage
Research, v. 55, no. 3, p. 127-142.
Clark, C., and Russell, C., 2005, Public information provision as a tool of environmental
policy?, in Krarup, S., and Russell, C., eds., Environment, information, and
consumer behaviour, chap. 6: Cheltenham, United Kingdom, Edward Elgar
Publishing.
Cogeneration Technologies, 2006, Clean coal technology & the President's clean coal
power initiative: Available on line at
http://www.cogeneration.net/IntegratedGasificationCombinedCycle.htm.
Cohn, T.A., Caulder, D.L., Gilroy, E.J., Zynjuk, L.D., and Sommers, R.M., 1992, The
validity of a simple statistical model for estimating fluvial constituent loads: An
empirical study involving nutrient loads entering Chesapeake Bay: Water Resources
Research, v. 28, p. 2352-2363.
Collie, J.S., Richardson, K., and Steele, J.H., 2004, Regime shiftsCan ecological theory
illuminate the mechanisms?: Progress in Oceanography, v. 60, p. 281-302.
Conley, D.J., 2002, Terrestrial ecosystems and the global biogeochemical silica cycle:
Global Biogeochemical Cycles, v. 16, p. 1121, doi:10.1029/2002GV001894.
Conley, D. J., Carstensen, J., ^Ertebjerg, G., Christensen, P.B., Dalsgaard, T., Hansen,
J.L.S., and Josefson, A.B., 2007 Long-term changes and impacts of hypoxia in
Danish coastal waters: Ecological Applications, Supplement, v. 17, no. 5, p. SI 65-
S184.
Conley, D.J., Humborg, C., Rahm, L., Savchuk, O.P., and Wulff, F., 2002a, Hypoxia in
the Baltic Sea and basin-scale changes in phosphorus biogeochemistry:
Environmental Science and Technology, v. 36, p. 5315-5320.
233
-------
Conley, D.J., Markager, S., Andersen, J., Ellermann, T., and Svendsen, L.M., 2002b,
Coastal eutrophication and the Danish National Aquatic Monitoring and Assessment
Program: Estuaries, v. 25, p. 706-719.
Cooke, R.A., Sands, G.R., and Brown, L.C., in press, Drainage water management: a
practice for reducing nitrate loads from subsurface drainage systems, in Proceedings
of Gulf Hypoxia and Local Water Quality Concerns Workshop, Iowa State
University, Ames, IA, September 26-28, 2005. (in press)
Corbett, D.R., McKee, B.A., and Allison, M.A., 2006, Nature of decadal-scale sediment
accumulation in the Mississippi river deltaic region: Continental Shelf Research,
v. 26, p. 2125-2140.
Corbett, D.R., McKee, B.A., and Duncan, D., 2004, An evaluation of mobile mud
dynamics in the Mississippi River deltaic region: Marine Geology, v. 209, p. 91-112.
CPRA, 2007, Integrated ecosystem restoration and hurricane protection: Louisiana's
comprehensive master plan for a sustainable coast: Coastal Protection and
Restoration Authority (CPRA) of Louisiana, Office of the Governor (Louisiana), 140
p. Available on line at: http://www.lacpra.org/assets/docs/cprafmalreport5-2-07.pdf
Craig, J.K., and Crowder, L.B., 2005, Hypoxia-induced habitat shifts and energetic
consequences in Atlantic croaker and brown shrimp on the Gulf of Mexico shelf:
Marine Ecology Progressive Series, v. 294, p. 79-94.
Craig, J.K., Crowder, L.B., and Kenwood, T.L., 2005, Spatial distribution of brown
shrimp (Farfantepenaeus aztecus) on the northwestern Gulf of Mexico shelf
Effects of abundance and hypoxia: Canadian Journal of Fisheries and Aquatic
Sciences, v. 62, p. 1295-1308.
Craig, J.K., Gray, C.D., McDaniel, C.M., Kenwood, T.L., andHanifen, J.G., 2001,
Ecological effects of hypoxia on fish, sea turtles, and marine mammals in the
northwestern Gulf of Mexico, in Rabalais, N.N., and Turner, R.E., eds., Coastal
hypoxiaConsequences for living resources and ecosystems: Washington, D.C.,
Coastal and Estuarine Studies 58, American Geophysical Union, p. 269-291.
Crespi, J., and Marette, S., 2005, Eco-labeling economicsIs public involvement
necessary?, in Krarup, S., and Russell, C.S., eds., Environment, information, and
consumer behaviour, chap. 5: Northhampton, Edward Elgar Publishing.
Crumpton, W.G., Kovacic, D., Hey, D., and Kostel, J., in press, Potential of wetlands to
reduce agricultural nutrient export to water resources in the corn belt, in Proceedings
of Gulf Hypoxia and Local Water Quality Concerns Workshop, Iowa State
University, Ames, IA, September 2005. (in press)
Crumpton, W.G., Stenback, G.A., Miller, B.A., and Helmers, M.J., 2006, Potential
benefits of wetland filters for tile drainage systemsImpact on nitrate loads to
Mississippi river subbasins: U.S. Department of Agriculture, CSREES Project
Completion Report, (release pending).
Czapar, G.G., Payne, J., and Tate, J., 2007, An educational program on the proper timing
of fall-applied nitrogen fertilizer: On-line Crop Management, doi: 10.194/CM-2007-
1510-01-RS.
234
-------
D'Sa, E.J., and Miller, R.L., 2003, Bio-optical properties in waters influenced by the
Mississippi River during low flow conditions: Remote Sensing of Environment,
v. 84, p. 538-549.
Dagg, M., Benner, R., Lohrenz, S., and Lowrence, D., 2004, Transformation of dissolved
and particulate materials on continental shelves influenced by large riversPlume
processes: Continental Shelf Research, v. 24, p. 833-858.
Dagg, M.J., 1995, Copepod grazing and the fate of phytoplankton in the northern Gulf of
Mexico: Continental Shelf Research, v. 15, nos. 11-12, p. 1303-1317.
Dagg, M.J., Ammerman, J.V., Amon, R., Gardner, W., Green, R., and Lohrenz, S., in
press, Water column processes influencing hypoxia in the northern Gulf of Mexico:
Estuaries and Coasts, v. 30, no. 4, p. - , in press. Accepted for publication notice
last accessed August 16, 2007 at:
http://estuariesandcoasts.org/contents/upcoming.html. (Draft 46 page manuscript
available online at: http://www.epa.gov/msbasin/taskforce/pdf/session4dagg.pdf).
Dagg, M.J., and Breed, G.A., 2003, Biological effects of Mississippi River nitrogen on
the northern Gulf of MexicoA review and synthesis: Journal of Marine Systems,
v. 43, p. 133-152.
Dagg, M.J., and Brown, S.L., 2005, The potential contribution of fecal pellets from the
larvacean Oikopleura dioica to vertical flux of carbon in a river dominated coastal
margin, in Gorsky, G., Youngbluth, M.J., and Deibel, D., eds., Response of marine
ecosystems to global change: Ecological Impact of Append!cularians, Gordon and
Breach, p. 293-307.
Dahl, T.E., 1990, Wetlands losses in the United States1780s to 1980s: Washington,
D.C., U.S. Department of Interior, Fish and Wildlife Service, 21 p.
Dale, V.H., Brown, S., Haeuber, R.A., Hobbs, N.T., Huntly, N., Naiman, R.J., Riebsame,
W.E., Turner, M.G., and Valone, T.J., 2000, Ecological principles and guidelines for
managing the use of land: Ecological Applications, v. 10, p. 639-670.
Dalsgaard, T., Canfield, D.E., Petersen, J., Thamdrup, B., and Acuha-Gonzalez, J., 2003,
N2 projection by the annamox reaction in the anoxic water column of Golfo Dulce,
Costa Rica: Nature, v. 422, p. 606-608.
Dalzell, B.J., Gowda, P.H., and Mulla, D.J., 2004, Modeling sediment and phosphorus
losses in an agricultural watershed to meet TMDLs: Journal of American Water
Resources Association, v. 40, p. 533-543.
Dampney, P.M.R., Lord, E.I., and Chambers, B.J., 2000, Development of advice for
farmers and advisors: Soil Use and Management, v. 16, p. 162-166.
Darrow, B.P., Walsh, J.J., Vargo, G.A., Masserini, R.T., Jr., Fanninga, K.A., and Zhang,
J.-Z., 2003, A simulation study of the growth of benthic microalgae following the
decline of a surface phytoplankton bloom: Continental Shelf Research, v. 23,
p. 1265-1283.
235
-------
David, M.B., and Gentry, L.E., 2000, Anthropogenic inputs of nitrogen and phosphorus
and riverine export for Illinois, USA: Journal of Environmental Quality, v. 29,
p. 494-508.
David, M.B., Gentry, L.E., Kovacic, D.A., and Smith, K.M., 1997, Nitrogen balance in
and export from an agricultural watershed: Journal of Environmental Quality, v. 26,
p. 1038-1048.
David, M.B., Mclsaac, G.F., Royer, T.V., Darmody, R.G., and Gentry, L.E., 2001,
Estimated historical and current nitrogen balances for Illinois: The Scientific World,
v. 1, p. 597-604.
David, M.B., Wall, L.G., Royer, T.V., and Tank, J.L., 2006, Denitrification and the
nitrogen budget of a reservoir in an agricultural landscape: Ecological Applications,
v. 16, p. 2177-2190.
Davis, C.B., Baker, J.L., van der Valk, A.G., and Beer, C.E., 1981, Prairie pothole
marshes as traps for nitrogen and phosphorus in agricultural runoff, in Richardson,
B., ed., Proceedings of the Midwestern Conference on Wetland Values and
Management, St. Paul, MN, 17-19 June 1981: Navarre, MN, Fresh Water Society,
p. 153-163.
Davis, J.G., Kitchen, N.R., Sudduth, K.A., and Drummond, S.T., 1997, Using
electromagnetic induction to characterize soils: Potash & Phosphate Institute, Better
Crops, v. 81, no. 4, p. 6-8.
Day, J.W., Jr., Yanez Arancibia, A., Mitsch, W.J., Lara-Dominquez, A.L., Day, J.N., Ko,
J-Y., Lane, R., and Lindsey, J., 2003, Using ecotechnology to address water quality
and wetland habitat loss problems in the Mississippi basinA hierarchical approach:
Biotechnology Advances, v. 22, p. 135-159.
Denbaly, M., and Vrooman, H., 1993, Dynamic fertilizer nutrient demands for corn: A
co-integrated and error-correcting system: American Journal of Agricultural
Economics, v. 75, p. 203-209.
Deutsch, C., Sarmiento, J.L., Sigman, D.M., Gruber, N., and Dunne, J.P., 2007, Spatial
coupling of nitrogen inputs and losses in the ocean: Nature, v. 445, p. 163-167,
doi:10.1038/nature05392.
Devlin, D., Dhuyvetter, K., McVay, K., Kastens, T., Rice, C., Janssen, K., and Pierznski,
G., 2003, Water quality best management practices, effectiveness, and cost for
reducing contaminant losses from cropland: Kansas State University Agricultural
Experiment Station and Cooperative Extension Service MF-2572, 4 p.
Diaz, R.J., 2001, Overview of hypoxia around the world: Journal of Environmental
Quality, v. 30, no. 2, p. 275-281.
Diaz, R.J., Nestlerode, J., and Diaz, M.L., 2003, A global perspective on the effects of
eutrophication and hypoxia on aquatic biota, in Proceedings of the Seventh
International Symposium, Fish Physiology, Toxicology, and Water Quality, May 12-
15,2003: Tallinn, Estonia.
236
-------
Diaz, R.J., and Rosenberg, R., 1995, Marine benthic hypoxiaA review of its ecological
effects and the behavioral responses of benthic macrofauna: Oceanography &
Marine Biology, An Annual Review, v. 33, p. 245-303.
Diaz, R.J., and Solow, A., 1999, Ecological and economic consequences of hypoxia
Topic 2, Gulf of Mexico hypoxia assessment: Silver Springs, MD, National Ocean
and Atmospheric Administration Coastal Ocean Program Decision Analysis Series,
86 p., available online at: http://oceanservice.noaa.gov/products/hypox_t2final.pdf.
DiMarco, S., Hetland, R., Howden, S., Murray, S., Walker, N., and Wiseman, W., 2007,
Influence of physical oceanographic processes on the distribution and extent of the
hypoxic zone: (In review)
DiMarco, S.F., Walker, N., Wiseman, W.J., Jr., Murray, S.P., and Howden, S.D., 2006,
Physical processes of the northern Gulf of Mexico and their influence on hypoxia of
the Texas-Louisiana shelf:
Dinnel, S.P., and Wiseman, W.J., 1986, Fresh-water on the Louisiana and Texas shelf:
Continental Shelf Research, v. 6, p. 765-784.
Dinnes, D.L., 2004, Assessments of practices to reduce nitrogen and phosphorus
nonpoint source pollution of Iowa's surface waters: Ames, IA, U.S. Department of
Agriculture, Agricultural Research Service, National Soil Tilth Laboratory, 376 p.
Djodjic, F., Ulen, B., and Bergstrom, L., 2000, Temporal and spatial variations of
phosphorus losses and drainage in a structured clay soil: Water Research, v. 34,
p.1687-1695.
Dobermann, A., Walters, D.T., and Adviento-Borbe, M.A.A., 2007, Global warming
potential of high-yielding continuous corn and corn-soybean systems: Better Crops,
v. 91, no. 3, p. 16-19.
Dodds, W.K., Marti, E., Tank, J.L., Pontius, J., Hamilton, S.K., Grimm, N.B., Bowden,
W.B., McDowell, W.H., Peterson, B.J., Valett, H.M., Webster, J.R., and Gregory, S.,
2004, Carbon and nitrogen stoichiometry and nitrogen cycling rates in streams:
Oceologia, v. 140, p. 458-467.
Doering, O., Diaz-Hermelo, F., Howard, C., Heimlich, R., Hitzhusen, F., Kazmierczak,
R., Lee, J., Libby, L., Milon, W., Prato, T., and Ribaudo, M., 1999, Evaluation of the
economic costs and benefits of methods for reducing nutrient loads to the Gulf of
Mexico: Topic 6 report for the integrated assessment of hypoxia in the Gulf of
Mexico: Silver Springs, MD, National Oceanic and Atmospheric Administration
Coastal Ocean Program, Decision Analysis Series No. 20, 137 p. Available on line
at: http://oceanservice.noaa.gov/products/hypox_t6fmal.pdf
Doering, O.C., 2002, Economic linkages driving the potential response to nitrogen over-
enrichment: Estuaries, v. 25, no. 4B, p. 809- 818.
Donner, S.D., 2006, Surf or turfA shift from feed to food cultivation could reduce
nutrient flux to the Gulf of Mexico: Global Environmental Change, v. 17, no. 1, p.
105-113.
237
-------
Donner, S.D., Coe, M.T., Lenters, J.D., Twine, I.E., and Foley, J.A., 2002, Modeling the
impact of hydrological changes on nitrate transport in the Mississippi River Basin
from 1955 to 1994: Global Biogeochemical Cycles, v. 16,
doi:10.1029/2001GB001396.
Donner, S.D., and Kucharik, C.J., 2003a, Evaluating the impacts of land management and
climate variability on crop production and nitrogen export across the Upper
Mississippi Basin: Global Biogeochemical Cycles, v. 17, doi:10.1028/2001GB1808.
Donner, S.D., and Kucharik, C.J., 2003b, The distribution of the primary crops in the
U.S. since 1950 and the relationship to river nutrient levels: Global Biogeochemical
Cycles, v. 17, doi:10.1029/2001GB1808.
Donner, S.D., Kucharik, C.J., and Foley, J.A., 2004, Impact of changing land use
practices on nitrate export by the Mississippi River: Global Biogeochemical Cycles,
v.18, no. GB1028.
Donner, S.D., and Scavia, D., 2007, How climate controls the flux of nitrogen by the
Mississippi River and the development of hypoxia in the Gulf of Mexico: Limnology
and Oceanography, v. 52, no. 2, p. 856-861.
Dortch, Q., Rabalais, N.N., Turner, R.E., and Qureshi, N.A., 2001, Impacts of changing
Si/N ratios and phytoplankton species composition, in Rabalais, N.N., and Turner,
R.E., eds., Coastal hypoxiaConsequences for living resources and ecosystems:
Washington, D.C., Coastal and Estuarine Studies 58, American Geophysical Union,
p. 37-48.
Dortch, Q., and Whitledge, T.E., 1992, Does nitrogen or silicon limit phytoplankton
production in the Mississippi River plume and nearby regions?: Continental Shelf
Research, v. 12, p. 1293-1309.
Dosskey, M.G., 2001, Toward quantifying water pollution abatement in response to
installing buffers on crop land: Environmental Management, v. 28, no. 5, p. 577-
598.
Dou, Z., Lanyon, L.E., Ferguson, J.D., Kohn, R.A., Boston, R.C., and Chalupa, W., 1998,
An integrated approach to managing nitrogen on dairy farmsEvaluating farm
performance using dairy nitrogen planner: Agronomy Journal, v. 90, p. 573-581.
Du, B., Saleh, A., Jaynes, D.B., and Arnold, J.G., 2006, Evaluation of SWAT in
simulating nitrate nitrogen and atrazine fates in a watershed with tiles and potholes:
Transactions of the American Society of Agricultural and Biological Engineers,
v. 49, no. 4, p. 949-959.
Duan, S., and Bianchi, T.S., 2006, Seasonal changes in the abundance and composition of
plant pigments in particulate organic carbon in the lower Mississippi and Pearl
Rivers: Estuaries and Coasts, v. 29, no. 3, p. 427-442.
Duan, S., Bianchi, T.S., and Sampere, T.P., 2007, Temporal variability in the
composition and abundance of terrestrially-derived dissolved organic matter in the
lower Mississippi and Pearl Rivers: Marine Chemistry, v. 103, p. 172-184.
238
-------
Edwards, D.R., and Daniel, T.C., 1993a, Drying-interval effects on runoff from fescue
plots receiving swine manure: Transactions of the American Society of Agricultural
Engineers, v. 36, p. 1673-1678.
Edwards, D.R., and Daniel, T.C., 1993b, Runoff quality impacts of swine manure applied
to fescue plots: Transactions of the American Society of Agricultural Engineers,
v. 36, p. 81-80.
Eldridge, P.M., and Morse, J.W., in press, Origins and temporal scales of hypoxia on the
Louisiana shelfImportance of benthic and sub-pycnocline water metabolism: (in
press).
Elobeid, A., Tokgoz, S., Hayes, D.J., Babcock, B.A., and Hart, C.E., 2006, The long-run
impact of corn-based ethanol on the grain, oilseed and livestock sectors: A
preliminary assessment: Ames, IA, Center of Agriculture and Rural Development
Briefing Paper 06-BP 49.
Engelhaupt, E., and Bianchi, T.S., 2001, Sources and composition of high-molecular-
weight dissolved organic carbon in a southern Louisiana tidal stream (Bayou
Trepagnier): Limnology and Oceanography, v. 46, p. 917-926.
English, B.C., De la Torre Ugarte, D.G., Jensen, K., Hellwinckel, C., Menard, J., Wilson,
B., Roberts, R., and Walsh, M., 2006, 25% renewable energy for the Unites States by
2025: Agricultural and economic impacts: Knoxville, TN, University of Tennessee
Technical Report.
Etter, P.C., Howard, M.K., and Cochrane, J.D., 2004, Heat and freshwater budgets of the
Texas-Louisiana shelf: Journal of Geophysical Research, v. 109, C02024,
doi: 10.1029/2003 JCOO1820.
Farrell, A.E., Plevin, R.J., Turner, B.T., Jones, A.D., O'Hare, M., and Kammen, D.M.,
2006, Ethanol can contribute to energy and environmental goals: Science, v. 311, no.
5760, p. 506-508.
Feather, P., Hellerstein, D., and Hansen, L., 1999, Economic valuation of environmental
benefits and the targeting of conservation programsThe case of the CRP: U.S.
Department of Agriculture, Economic Research Service, Agricultural Economics
Report No. 778, 64 p.
Feng, H., Kurkalova, L.A., Kling, C.L., and Gassman, P.W., 2005, Economic and
environmental co-benefits of carbon sequestration in agricultural soilsRetiring
agricultural land in the upper Mississippi River basin: Iowa State University Center
for Agricultural and rural Development (CARD) Publication 05-WP384, 23 p.,
available online at:
http://www.card.iastate.edu/publications/DBS/PDFFiles/05wp384.pdf
Feng, H., Kurkalova, L.A., Kling, C.L., and Gassman, P.W., 2006, Environmental
conservation in agriculture: Land retirement vs. changing practices on working land:
Journal of Environmental Economics and Management, v. 52, p. 600-614.
Ferguson, R.B., Lark, R.M., and Slater, G.P., 2003, Approaches to management zone
definition for use of nitrification inhibitors: Soil Science Society of America Journal,
v. 67, p. 937-947.
239
-------
Fiener, P., and Auerswald, K., 2003, Effectiveness of grassed waterways in reducing
runoff and sediment delivery from agricultural watersheds: Journal of Environmental
Quality, v. 32, no. 3, p. 927-936.
Fisher, T.R., and Gustafson, A.B., 2004, Progress ReportAug. 1990-Dec. 2003,
Nutrient-addition bioassays in Chesapeake Bay to assess resources limiting
phytoplankton growth: Annapolis, MD, Maryland Department of Natural Resources,
50 p.
Fisher, T.R., Peele, E.R., Ammerman, J.W., and Harding, L.W., 1992, Nutrient limitation
of phytoplankton in Chesapeake Bay: Marine Ecology Progress Series, v. 82, p. 51-
63.
Forman, R.T.T., 1995, Land mosaicsThe ecology of landscapes and regions:
Cambridge, England, Cambridge University Press, 652 p.
Fox, L., Sager, S.L., and Wofsy, S.C., 1985, Factors controlling the concentrations of
soluble phosphorus in the Mississippi estuary: Limnology & Oceanography, v. 30,
p. 826-832.
Frankenberger, J., Kladivko, E., Sands, G., Jaynes, D.B., Fausey, N.R., Helmers, M.,
Cooke, R., Strock, J., Nelson, K., and Brown, L., 2006, Drainage water management
for the Midwest: Purdue Extension: Knowledge to Go, WQ-44, 8 p., available online
at: http://www.ces.purdue.edu/extmediaAVQAVO-44.pdf
Froelich, P.N., 1988, Kinetic control of dissolved phosphate in natural rivers and
estuariesA primer on the phosphate buffer mechanism: Limnology &
Oceanography, v. 33, p. 649-668.
Galler, J.J., Bianchi, T.S., Allison, M.A., Campanella, R., and Wysocki, L., 2003,
Sources of aged terrestrial organic carbon to the Gulf of Mexico from relict strata in
the Mississippi River: EOS, Transactions of the American Geophysical Union, v. 84,
p. 469-476.
Gardner, G., 1998, Recycling organic wastes, in Brown, L., Flavin, C., and French, H.,
eds., State of the world: New York, NY, W.W. Norton, p. 96-112.
Gardner, W.S., Benner, R., Chin-Leo, G., Cotner, J.B., Eadie, B.J., Cavaletto, J.F.,
Lansing, M.B., 1994, Mineralization of organic material and bacterial dynamics in
Mississippi River plume water: Estuaries, v. 17, no. 4, p. 816-828.
Gamier, J., Leporcq, B., Sanches, N., and Philippon, X., 1999, Biogeochemical mass
balances (C, N, P, Si) in three large reservoirs of the Seine Basin (France):
Biogeochemistry, v. 47, p. 119-146.
Gamier, J., Sferratore, A., Meybeck, M., Billen, G., and Durr, H., 2006, Modeling silica
transfer process in watersheds, in Ittekot, V., Unger, D., Humborg, C., and An, N.T.,
eds., The silicon cycleHuman perturbations and impacts on aquatic systems:
Washington, D.C., Island Press, p. 139-162.
Gassman, P.W., Reyes, M.R., Green, C.H., and Arnold, J.G., 2007, The soil and water
assessment toolHistorical development applications, and future research
240
-------
directions: Transactions of the American Society of Agricultural and Biological
Engineers, v.50, no. 4, p. 1211-1240.
Gehl, R.J., Schmidt, J.P., Maddux, L.D., and Gordon, W.B., 2005, Corn yield response to
nitrogen rate and timing in sandy irrigated soils: Agronomy Journal, v. 97, p. 1230-
1238.
Gentry, L.E., Below, F.E., David, M.B., and Bergerou, J.A., 2001, Source of the soybean
N credit in maize production: Plant and Soil, v. 236, p. 175-184.
Gentry, L.E., David, M.B., Royer, T.V., Mitchell, C.A., and Starks, K.M., 2007,
Phosphorus transport pathways to streams in tile-drained agricultural watersheds:
Journal of Environmental Quality, v. 36, p. 408-415.
Gilroy, E.J., Hirsh, R.M., and Cohn, T.A., 1990, Mean square error of regression-based
constituent transport estimates: Water Resources Research, v. 26, p. 2069-2077.
Gitau, M.W., Gburek, W. J., and Jarrett, A.R., 2005, A tool for estimating best
management practice effectiveness for phosphorus pollution control: Journal of Soil
and Water Conservation, v. 60, no. 1, p. 1-10.
Gollehon, N., Caswell, M., Ribaudo, M., Kellogg, R., Lander, C., and Letson, D., 2001,
Confined animal production and manure nutrients: U.S. Department of Agriculture,
Economic Research Service, Resource Economics Division, Agriculture Information
Bulletin No. 771.
Goni, M.A., Ruttenberg, K.C., and Eglinton, T.I., 1998, A reassessment of the sources
and importance of land-derived organic matter in surface sediments from the Gulf of
Mexico: Geochimica et Cosmochimica Acta, v. 62, no. 18, p. 3055-3075.
Goolsby, D.A., Battaglin, W.A., Aulenbach, B.T., and Hooper, R.P., 2001, Nitrogen
input to the Gulf of Mexico: Journal of Environmental Quality, v. 30, no. 2, p. 329-
336.
Goolsby, D.A., Battaglin, W.A., Lawrence, G.B., Artz, R.S., Aulenbach, B.T., Hooper,
R.P., Keeney, D.R., and Stensland, G.S., 1999, Flux and sources of nutrients in the
Mississippi-Atchafalaya River basinTopic 3, Report for the integrated assessment
of hypoxia in the Gulf of Mexico: National Oceanic and Atmospheric Administration
Coastal Ocean Program Decision Analysis Series 17, 128 p.
Gordon, E.S., Goni, M.A., Roberts, Q.N., Kineke, G.C., and Allison, M.A., 2001,
Organic matter distribution and accumulation on the inner Louisiana shelf west of
the Atchafalaya River: Continental Shelf Research, v. 21, p. 1691-1721.
Gowda, P.H., Dalzell, B.J., and Mulla, D.A., 2007, Model based nitrate TMDLs for two
agricultural watersheds of southeastern Minnesota: Journal of the American Water
Resources Association, v. 43, no. 1, p. 254-263.
Gowda, P.H., and Mulla, D.J., 2006, Modeling alternative agricultural management
practices for High Island Creek watershed in South-Central Minnesota: Journal of
Environmental Hydrology, v. 14, no. 13, p. 1-15.
Graco, M., Farias, L., Molina, V., Gutierrez, D., and Nielsen, L.P., 2001, Massive
developments of microbial mats following phytoplankton blooms in a naturally
241
-------
eutrophic bayImplications for nitrogen cycling: Limnology & Oceanography,
v. 46, p. 821-832.
Graff, C.D., Sadeghi, A.M., Lowrance, R.R., and Williams, R.G., 2005, Quantifying the
sensitivity of the riparian ecosystem management model (REMM) to changes in
climate and buffer characteristics common to conservation practices: Transactions of
the American Society of Agricultural Engineers, v. 48, no. 4, p. 1377-1387.
Grandy, A.S., Loecke, T.D., Parr, S., and Robertson, G.P., 2006, Long-term trends in
nitrous oxide emissions, soil nitrogen, and crop yields of till and no-till cropping
systems: Journal of Environmental Quality, v. 35, p. 1487-1495.
Graham, R.L., Nelson, R., Sheehan, I, Perlack, R.D., and Wright, L.L., 2007, Current
and potential U.S. corn stover supplies: Agronomy Journal, v. 99, p. 1-11.
Green, C.H., Tomer, M.D., DiLuzio, M., and Arnold, J.G., 2006a, Hydrologic evaluation
of the soil and water assessment tool for a large tile-drained watershed in Iowa:
Transactions of the American Society of Agricultural and Biological Engineers,
v. 49, no. 2, p. 413-422.
Green, R.E., Bianchi, T.S., Dagg, M.J., Walker, N.D., and Breed, G.A., 2006b, An
organic carbon budget for the Mississippi River turbidity plume and plume
contributions to air-sea CO2 fluxes and bottom water hypoxia: Estuaries, v. 29, no. 4,
p. 579-597.
Green, R.E., Breed, G.A., Dagg, M.J., and Lohrenz, S.E., in press, Modeling the response
of primary production and sedimentation to variable nitrate loading in the
Mississippi River plume: (In review).
Green, W.R., and Haggard, B.E., 2001, Phosphorus and nitrogen concentrations and
loads at Illinois River south of Siloam Springs, Arkansas, 1997-1999: U.S.
Geological Survey Water Resources Investigations Report 01-4217, 12 p., available
online at http://pubs.er.usgs.gov/usgspubs/wri/wri014217
Greenhalgh, S., and Sauer, A., 2003, Awakening the dead zone: An investment for
agriculture, water quality, and climate changes: World Resources Institute, WRI
Issue Brief, February, 24 p.
Gregg, M.C., 1999, Uncertainties and limitations in measuring & and XT: Journal of
Atmospheric and Oceanic Technology, v. 16, p. 1483-1490.
Grizzetti, B., Bouraoui, F., and De Marsily, G., 2005, Modeling nitrogen pressure in river
basinsA comparison between a statistical approach and the physically-based
SWAT model: Physics and Chemistry of the Earth, Parts A/B/C, v. 30, nos. 8-10,
p. 508-517.
Groffman, P., Altabet, M.A., Bohlke, J.K., Butterbach-Bahl, K., David, M.B., Firestone,
M.K., Giblin, A.E., Kana, T.M., Nielsen, L.P., and Voytek, M.A., 2006, Methods for
measuring denitrificationDiverse approaches to a difficult problem: Ecological
Applications, v. 16, p. 2091-2122, available online at:
https ://darchive. mbl whoilibrary. org/bitstream/191271425/1 /Groffman%20et%20al. %
20%282006%29%20-%20denitrification%20methods%20paper.pdf.
242
-------
Hagy, J.D., Boynton, W.R., Keefe, C.W., and Wood, K.V., 2004, Hypoxia in Chesapeake
Bay, 1950-2001Long-term change in relation to nutrient loading and river flow:
Estuaries, v. 27, p. 634-658.
Harding, L.W., Jr., 1994, Long-term trends in the distribution of phytoplankton in
Chesapeake BayRoles of light, nutrients, and streamflow: Marine Ecology
Progress Series, v. 104, p. 267-291.
Harding, L.W., Jr., Mallonee, M.E., and Perry, E.S., 2002, Toward a predictive
understanding of primary productivity in a temperate, partially stratified estuary:
Estuarine Coastal & Shelf Science, v. 55, p. 437-463.
Haufler, J., ed., 2005, Fish and wildlife benefits of Farm Bill Conservation Programs
2000-2005 update: The Wildlife Society, Technical Review 05-2.
Haygarth, P.M., Chapman, P.J., Jarvis, S.C., and Smith, R.V., 1998, Phosphorus budgets
for two contrasting grassland farming systems in the UK: Soil Use and Management,
v. 14, p. 160-167.
Heckrath, G., Brookes, P.C., Paulton, P.R., and Goulding, K.W.T., 1995, Phosphorus
leaching from soils containing different phosphorus concentrations in the Broadbalk
experiment: Journal of Environmental Quality, v. 24, p. 904-910.
Helmers, M., 2007, Iowa State University agricultural drainage: Available at
http://www3.abe.iastate.edu/agdrainage/.
Helsel, D. R., and Hirsch, R.M., 2002, Statistical methods in water resources: U.S.
Geological Survey Techniques of Water-Resources Investigations, Book 4, Chapter
A3, available at http://water.usgs.gov/pubs/twri/twri4a3/.
Helton, A.M., 2006, An inter-biome comparison of stream network nitrate dynamics:
Athens, GA, University of Georgia, M.S. thesis, 143 p.
Hendon, L.A., Carlson, E.A., Manning, S., and Brouwer, M., 2006, Cross-talk between
pyrene and hypoxia signaling pathways in embryonic Cyprinodon variegates: Paper
presented at Hypoxia Effects on Living Resources in the Gulf of Mexico, September
25-26, 2006: New Orleans, Louisiana, Tulane University, sponsored by National
Oceanic and Atmospheric Administration Center for Sponsored Coastal Ocean
Research.
Herbert, R.A., 1999, Nitrogen cycling in coastal marine systems: FEMS Microbiology
Reviews, v. 23, p. 563-590.
Hernandez, M.E., and Mitsch, W.J., 2006, Influence of hydrologic pulses, flooding
frequency, and vegetation on nitrous oxide emissions from created riparian marshes:
Wetlands, v. 26, no. 3, p. 862-877.
Hernes, P.J., and Benner, R., 2003, Photochemical and microbial degradation of
dissolved lignin phenolsImplications for the fate of terrigenous dissolved organic
matter in marine environments: Journal of Geophysical ResearchOceans,
v. 108(C9).
243
-------
Hetland, R.D., and DiMarco, S.F., 2007, How does the character of oxygen demand
control the structure of hypoxia on the Texas-Louisiana continental shelf?: Journal of
Marine Systems, doi:10.1016/j.jmarsys.2007.03.002.
Hey, D.L., Kenimer, A.L., and Barrett, K.R., 1994, Water quality improvement by four
experimental wetlands: Ecological Engineering, v. 3, p. 381-397.
Hey, D.L., Montgomery, D.L., Urban, L.S., Prato, T., Zarwell, R., Forbes, A., Martell,
M., Pollack, J., and Steele, Y., 2004, Flood damage reduction in the Upper
Mississippi River BasinAn ecological alternative: The Wetlands Initiative,
Chicago, IL, 44 p., available online at: http://www.wetlands-
initiative.org/images/UMRBFinalReport.pdf.
Hoffman, B.S., Brouder, S.M., and Turco, R.F., 2004, Tile spacing impacts on Zea mays
L. yield and drainage water nitrate load: Ecological Engineering, v. 23, p. 251-267.
Holland, E.F., Braswell, B.H., Sulzman, J., and Lamarque, J., 2005, Nitrogen deposition
to the United States and Western EuropeSynthesis of observations and models:
Ecological Applications, v. 15, p. 38-57.
Holland, E.F., Dentener, B., Braswell, B.H., and Sulzman, J., 1999, Contemporary and
pre-industrial global reactive nitrogen budgets: Biogeochemistry, v. 4, p. 7-43.
Hong, N., White, J.G., Weisz, R., Crozier, C.R., Gumpertz, M.L., and Cassel, O.K., 2006,
Remote sensing-informed variable-rate nitrogen management of wheat and corn
Agronomic and groundwater outcomes: Agronomy Journal, v. 98, p. 327-338.
Howarth, R.W., 1984, The ecological significance of sulfur in the energy dynamics of
salt marsh and marine sediments: Biogeochemistry, v. 1, p. 5-27.
Howarth, R.W., 1998, An assessment of human influences on inputs of nitrogen to the
estuaries and continental shelves of the North Atlantic Ocean: Nutrient Cycling in
Agroecosystems, v. 52, p. 213-223.
Howarth, R.W., 2006, Atmospheric deposition and nitrogen pollution in coastal marine
ecosystems, in Visgilio, G.R., and Whitelaw, D.M., eds., Acid in the environment
Lessons learned and future prospects: NY, Springer, p. 97-116.
Howarth, R.W., Billen, G., Swaney, D., Townsend, A., Jarworski, N., Lajtha, K.,
Downing, J.A., Elmgren, R., Caraco, N., Jordan, T., Berendse, F., Freney, J.,
Kueyarov, V., Murdoch, P., and Zhao-Hang, Zhu., 1996, Riverine inputs of nitrogen
to the North Atlantic OceanFluxes and human influences: Biogeochemistry, v. 35,
p. 75-139.
Howarth, R.W., Boyer, E.W., Pabich, W.J., and Galloway, J.N., 2002, Nitrogen use in the
United States from 1961-2000 and potential future trends: Ambio, v. 31, p. 88-96.
Howarth, R.W., Jensen, H.S., Marino, R., and Postma, H., 1995, Transport to and
processing of P in near-shore and oceanic waters, in Tiessen, H., ed., Phosphorus in
the global environment: Wiley, p. 323-345.
Howarth, R.W., and Marino, R., 2006, Nitrogen as the limiting nutrient for eutrophication
in coastal marine ecosystemsEvolving views over 3 decades: Limnology &
Oceanography, v. 51, p. 364-376.
244
-------
Howarth, R.W., Marino, R., Swaney, D.P., and Boyer, E.W., 2006a, Wastewater and
watershed influences on primary productivity and oxygen dynamics in the lower
Hudson River estuary, in Levinton, J.S., and Waldman, J.R., eds., The Hudson River
Estuary: Cambridge University Press, p. 121-139.
Howarth, R.W., Ramakrishna, K., Choi, E., Elmgren, R., Martinelli, L., Mendoza, A.,
Moomaw, W., Palm, C., Boy, R., Scholes, M., and Zhao-Liang, Zhu, 2005, Nutrient
management, responses assessment, in Ecosystems and human well-beingVolume
3, Policy responses, The millennium ecosystem assessment: Washington, D.C.,
Island Press, chap. 9, p. 295-311.
Howarth, R.W., Swaney, D.P., Boyer, E.W., Marino, R., Jaworski, N. and Goodale, C.,
2006b, The influence of climate on average nitrogen export from large watersheds in
the Northeastern United States: Biogeochemistry, v. 79, p 163-186.
Hu, X., Mclssac, G.F., David, M.B., and Louwers, C.A.L., 2007, Modeling riverine
nitrate export from an east-central Illinois watershed using SWAT: Journal of
Environmental Quality, v. 36, p. 996-1005.
Humborg, C., Conley, D.J., Rahm, L., Wulff, F., Cociasu, A., and Ittekkot, V., 2000,
Silicon retention in river basinsFar-reaching effects on biogeochemistry and
aquatic food webs in coastal marine environments: Ambio, v. 29, p. 45-50.
IATP, 2006, Staying home: How ethanol will change U.S. corn exports: Minneapolis,
MN, Institute for Agriculture and Trade Policy, 26 p.
Inwood, S.E., Tank, J.L., and Bernot, M.J., 2005, Patterns of denitrification associated
with land use in 9 mid-western headwater streams: Journal of the North American
Benthological Society, v. 24, p. 227-245.
Isik, M., and Khanna, M., 2002, Variable rate nitrogen application under uncertainty:
Implications for profitability and nitrogen use: Journal of Agricultural and Resource
Economics, v. 27, no. 1, p. 61-76.
Isik, M., and Khanna, M., 2003, Stochastic technology, risk preferences and adoption of
site-specific technologies: American Journal of Agricultural Economics, v. 85, no. 2,
p. 305-317.
James, E.E., 2005, Factors influencing the adoption and non-adoption of the conservation
reserve enhancement program in the Cannonsville Watershed, New York: University
Park, PA, The Pennsylvania State University, Department of Agricultural Economics
and Rural Sociology, M.Sc. dissertation, 174 p.
James, E.E., Kleinman, P.J.A., Veith, T.L., Stedman, R., and Sharpley, A.N., 2007,
Phosphorus contributions from pastured dairy cattle to streams in the Cannonsville
watershed: Journal of Soil and Water Conservation, v. 62, p. 40-47.
Jarosz, E., and Murray, S.P., 2005, Velocity and transport characteristics of the
Louisiana-Texas coastal current: Geophysical Monograph, American Geophysical
Union, Circulation in the Gulf of Mexico Observations and Models, v. 161, p. 143-
156.
245
-------
Jaynes, D.B., Colvin, T.S., Karlen, D.L., Cambardella, C.A., and Meek, D.W., 2001,
Nitrate loss in subsurface drainage as affected by nitrogen fertilizer rate: Journal of
Environmental Quality, v. 30, p. 1305-1314.
Jaynes, D.B., Dinnes, D.L., Meek, D.W., Karlen, D.L., Cambardella, C.A., and Colvin,
T.S., 2004, Using the late spring nitrate test to reduce nitrate loss within a watershed:
Journal of Environmental Quality, v. 33, p. 669-677'.
Jaynes, D.B., and James, D.E., 2007, The extent of farm drainage in the United States:
Poster presentation at Soil and Water Conservation Society 2007 Annual
Conference, July 21-25, 2007, Tampa, Florida, online at:
http://www.ars.usda.gov/Research/Researchl trm?modecode=36-25-15-00.
Jaynes, D.B., and Karlen, D.L., 2005, Sustaining soil resources while managing nutrients,
in Proceedings of the Upper Mississippi River Sub-Basin Hypoxia Nutrient
Committee Workshop, September 26-28, 2005, Ames, Iowa, p. 141-150, accessed
June 28, 2007, at www.umrshnc.org/files/Hypwebversion.pdf
Jha, M., Arnold, J.G., Gassman, P.W., Giorgi, F., and Gu, R.R., 2006, Climate change
sensitivity assessment on Upper Mississippi River Basin streamflows using SWAT:
Journal of American Water Resources Association, v. 42, no. 4, p. 997-1016.
Jochem, F.J., McCarthy, M.J., and Gardner, W.S., 2004, Microbial ammonium recycling
in the Mississippi River plume during the drought spring of 2000: Journal of
Plankton Research, v. 26, p. 1265-1275.
Joye, S.B., and Hollibaugh, J.T., 1995, Influence of sulfide inhibition on nitrification and
nitrogen regeneration in sediments: Science, v. 270, p. 623-625.
Justic, D., Bierman, V.J., Jr., Scavia, D., and Hetland, R., in press, Forecasting Gulfs
hypoxia: The next 50 years?: (in press).
Justic, D., Rabalais, N.N., and Turner, R.E., 1996, Effects of climate change on hypoxia
in coastal watersA doubled CC>2 scenario for the northern Gulf of Mexico:
Limnology & Oceanography, v. 41, p. 992-1003.
Justic, D., Rabalais, N.N., and Turner, R.E., 2002, Modeling the impacts of decadal
changes in riverine nutrient fluxes on coastal eutrophication near the Mississippi
River delta: Ecological Modeling, v. 152, p. 33-46.
Justic, D., Rabalais, N.R., and Turner, R.E., 2003a, Simulated responses of the Gulf of
Mexico hypoxia to variations in climate and anthropogenic nutrient loading: Journal
of Marine Systems, v. 42, p. 115-126.
Justic, D., Rabalais, N.N., Turner, R.E., and Dortch, Q., 1995, Changes in nutrient
structure of river-dominated coastal watersStoichiometeric nutrient balance and its
consequences: Estuarine, Coastal Shelf Science, v. 40, p. 339-356.
Justic, D., Rabalais, N.N., Turner, R.E., and Wiseman, W.J., Jr., 1993, Seasonal coupling
between riverborne nutrients, net productivity and hypoxia: Marine Pollution
Bulletin 26, p. 184-189.
246
-------
Justic, D., Turner, R.E., and Rabalais, N.N., 2003b, Climate influences on riverine nitrate
fluxImplications for coastal marine eutrophication and hypoxia: Estuaries, v. 26,
no. l,p. 1-11.
Kantha, L.H., 2005, Barotropic tides in the Gulf of Mexico, in Sturges, W., and Lugo-
Fernandez, A., eds., Circulation in the Gulf of MexicoObservations and models:
American Geophysical Union, Geophysical Monograph, v. 161, p. 159-163.
Kantha, L.H., and Clayson, C.A., 2000, Small scale processes in geophysical fluid flows:
San Diego, CA, Academic Press, 668p.
Kanwar, R.S., Colvin, T.S., and Karlin, D.L., 1997, Ridge, moldboard, chisel and no-till
effects on tile water quality beneath two cropping systems: Journal of Production
Agriculture, v. 10, p. 227-234.
Karlen, D.L., Dinnes, D.L., Jaynes, D.B., Hurburgh, C.R., Cambardella, C.A., Colvin,
T.S., and Rippke, G.R., 2005, Corn response to late-spring nitrogen management in
the Walnut Creek watershed: Agronomy Journal, v. 97, p. 1054-1061.
Karlen, D.L., Kramer, L.A., and Logsdon, S.D., 1998a, Field-scale nitrogen balances
associated with long-term continuous corn production: Agronomy Journal, v. 90,
p. 644-650.
Karlen, D.L., Kumar, A., Kanwar, R.S., Cambardella, C.A., and Colvin, T.S., 1998b,
Tillage system effects on 15 year carbon-based and simulated N budgets in a tile-
drained Iowa field: Soil & Tillage Research, v. 48, p. 155-165.
Karlson, K., Rosenberg, R., and Bonsdorff, E., 2002, Temporal and spatial large-scale
effects of eutrophi cation and oxygen deficiency on benthic fauna in Scandinavian
and Baltic watersA review: Oceanography & Marine Biology, An Annual Review,
v. 40, p. 427-489.
Keene, W.C., Montag, J.A., Maben, J.A., Southwell, M., Leonard, J., Church, T.M.,
Moody, J.L., and Galloway, J.N., 2002, Organic nitrogen in precipitation over
eastern North America: Atmospheric Environment, v. 36, p. 4529-4540.
Kellogg, L.E., and Bridgham, S.D., 2003, Phosphorus retention and movement across an
ombrotrophic-minerotrophic peatland gradient: Biogeochemistry, v. 63, p. 299-315.
Kellogg, R.L., Lander, C.H., Moffitt, D.C., and Gollehon, N., 2000, Manure nutrients
relative to the capacity of cropland and pastureland to assimilate nutrientsSpatial
and temporal trends for the United States: U.S. Department of Agriculture, Natural
Resources Conservation Service and Economic Research Service, Resource
Assessment and Strategic Planning Working Paper 98-1, 140 p., available online at
http://www.nrcs.usda.gov/technical/land/pubs/manntr.pdf
Kemp, W.M., Sampou, P., Caffrey, J., and Mayer, M., 1990, Ammonium recycling
versus denitrification in Chesapeake Bay sediments: Limnology & Oceanography,
v. 351, p. 545-563.
Khan, S.A., Mulvaney, R., and Hoeft, R.G., 2001, A simple soil test for detecting sites
that are nonresponsive to nitrogen fertilization: Soil Science Society of America
Journal, v. 65, p. 1751-1760.
247
-------
Khanna, M., 2001, Sequential adoption of site-specific technologies and its implications
for nitrogen productivity: A double selectivity model: American Journal of
Agricultural Economics, v. 83, p. 35-51.
Khanna, M., Isik, M., and Winter-Nelson, A., 2000, Investment in site-specific crop
management under uncertainty: Implications for nitrate pollution control and
environmental policy: Agricultural Economics, v. 24, no. 1, p. 9-21.
Khanna, M., Yang, W., Farnsworth, R., and Onal, H., 2003, Cost effective targeting of
CREP to improve water quality with endogenous sediment deposition coefficients:
American Journal of Agricultural Economics, v. 85, p. 538-553.
Khoshmanesh, A., Hart, B.T., Duncan, A., and Beckett, R., 1999, Biotic uptake and
release of phosphorus by a wetland sediment: Environmental Technology, v. 29,
p. 85-91.
King, D., 2005, Crunch time for water quality trading: Choices, v. 20, p. 71-75.
Kirsch, K., Kirsch, A., and Arnold, J.G., 2002, Predicting sediment and phosphorus loads
in the Rock River Basin using SWAT: Transactions of the American Society of
Agricultural and Biological Engineers, v. 45, no. 6, p. 1757-1769.
Kitchen, N.R., Sudduth, K.A., andDrummond, S.T., 1999, Soil electrical conductivity as
a crop productivity measure for claypan soils: Journal of Production Agriculture,
v. 12, p. 607-617.
Kladivko, E.J., Frankenberger, J.R., Jaynes, D.B., Meek, D.W., Jenkinson, B.J., and
Fausey, N.R., 2004, Nitrate leaching to subsurface drains as affected by drain
spacing and changes in crop production system: Journal of Environmental Quality,
v. 33, p. 1803-1813.
Kling, C., Secchi, S., Jha, M., Feng, H., Gassman, P., and Kurkalova, L., 2006, Upper
Mississippi River basin modelling system, Part 3Conservation practice scenario
results, in Singh, V., and Xu. Y., eds., Water Resources Publication, Coastal
Hydrology and Processes, p. 127-134.
Koch, B., Khosla, R., Frasier, W.M., Westfall, D.G., and Inman, D., 2004, Economic
feasibility of variable-rate nitrogen application utilizing site-specific management
zones: Agronomy Journal, v. 96, p. 1572-1580.
Kovacic, D.A., David, M.B., Gentry, L.E., Starks, K.M., and Cooke, R.A., 2000,
Effectiveness of constructed wetlands in reducing nitrogen and phosphorus export
from agricultural tile drainage: Journal of Environmental Quality, v. 29, p. 1262-
1274.
Krom, M.D., and Berner, R.A., 1980, Adsorption of phosphate in anoxic marine
sediments: Limnology & Oceanography, v. 25, p. 797-806.
Laboski, C.A.M., Sawyer, J.E., Walters, D.T., Bundy, L.G, Hoeft, R.G, Randall, G.W.,
and Andraski, T.W., 2006, Evaluation of the Illinois soil nitrogen test in the north
central region, in Proceedings of the North Central Extension-Industry Soil Fertility
Conference, Des Moines, IA, v. 22, p. 86-93.
248
-------
Lambert, D.M., Lowenberg-DeBoer, J., and Malzer, G.L., 2006, Economic analysis of
spatial-temporal patterns in corn and soybean response to nitrogen and phosphorus:
Agronomy Journal, v. 98, p. 43-54.
Landeck-Miller, R.E., and St. John, J.P., 2006, Modeling primary production in the
Lower Hudson River Estuary, in Levington, J.S., and Waldman, J.R., eds., The
Hudson River Estuary, New York, Cambridge University Press, p. 140-153.
Lane, R.R., Day, J.W., Marx, B., Reyes, E., and Kemp, G.P., 2002, Seasonal and spatial
water quality changes in the outflow plume of the Atchafalaya River, Louisiana,
USA: Estuaries, v. 25, no. 1, p. 30-42.
Lanyon, L.E., 2005, Phosphorus, animal nutrition and feeding: Overview, in Sims, J.T.,
and Sharpley, A.N., eds., Phosphorus: Agriculture and the Environment, Madison,
WI, American Society of Agronomy Monograph Series No. 46, p. 561-586.
Larson, R.S., 2001, Water quality trends of the Illinois waterway system upstream of
Peoria including the Chicago metropolitan area: Champaign, IL, Illinois State Water
Survey, Contract Report 2001-03.
Leavitt, P.R., and Hodgson, D.A., 2001, Sedimentary pigments, in Smol, J.P., Birks,
H.J.B, and Last, W.M., eds., Tracking environmental change using lake sediments,
v. 3, Terrestrial, algal, and siliceous indicators: Kluwer Academic Publishers,
p. 295-325.
Lee, K.H., Isenhart, T.M., and Schultz, R.C., 2003, Sediment and nutrient removal in an
established multi-species riparian buffer: Journal of Soil and Water Conservation,
v. 58, no. 1, p. 1-7.
Lee, K.H., Isenhart, T.M., Schultz, R.C., and Mickelson, S.K., 2000, Multispecies
riparian buffers trap sediment and nutrients during rainfall simulations: Journal of
Environmental Quality, v. 29, no. 4, p. 1200-1205.
Lemunyon, J.L., and Gilbert, R.G., 1993, The concept and need for a phosphorus
assessment tool: Journal of Production Agriculture, v. 6, p. 483-496.
Lewandrowski, J., Peters, M., Jones, C., House, R., Sperow, M., Eve, M., and Paustian,
K., 2004, Economics of sequestering carbon in the U.S. agricultural sector: U.S.
Department of Agriculture, Economic Research Service, Technical Bulletin
No. 1909, 69 p.
Liu, H.B., Dagg, J.M. Campbell, L., and Urban-Rich, J., 2004, Picophytoplankton and
bacterioplankton in the Mississippi River plume and its adjacent waters: Estuaries,
v. 27, no. l,p. 147-156.
Lohrenz, S.E., Dagg, M.L., and Whitledge, T.E., 1990, Enhanced primary production at
the plume/oceanic interface of the Mississippi River: Continental Shelf Research,
v. 10, no. 7, p. 639-664.
Lohrenz, S.E., Fahnenstiel, G.L., and Redalje, D.G., 1994, Spatial and temporal
variations of photosynthetic parameters in relation to environmental conditions in
northern Gulf of Mexico coastal waters: Estuaries, v. 17, p. 779-795.
249
-------
Lohrenz, S.E., Fahnenstiel, G.L., Redalje, D.G., and Lang, G.A., 1992, Regulation and
distribution of primary production in the northern Gulf of Mexico, in Program,
N.C.O., ed., Nutrient Enhanced Coastal Ocean Productivity, NECOP Workshop
Proceedings, October 1991: College Station, TX, Texas Sea Grant Publications,
p. 95-104.
Lohrenz, S.E., Fahnenstiel, G.L., Redalje, D.G., Lang, G.A., Chen, X.G., and Dagg, M.J.,
1997, Variations in primary production of northern Gulf of Mexico continental shelf
waters linked to nutrient inputs from the Mississippi River: Marine Ecology-Progress
Series, v. 155, p. 45-54.
Lohrenz, S.E., Fahnenstiel, G.L., Redalje, D.G., Lang, G.A., Dagg, M.J., Whitledge,
T.E., and Dortch, Q., 1999a, Nutrients, irradiance, and mixing as factors regulating
primary production in coastal waters impacted by the Mississippi River plume:
Continental Shelf Research, v. 19, p. 1113-1141.
Lohrenz, S.E., Wiesenburg, D.A., Arnone, R.A., and Chen, X.G., 1999b, What controls
primary production in the Gulf of Mexico?, in Kumpf, H., Steidinger, K., and
Sherman, K., eds., The Gulf of Mexico large marine ecosystem: Maiden, MA,
Assessment, Sustainability and Management, Blackwell Science, Inc., p. 151-170.
Lory, J.A., and Scharf, P.C., 2003, Yield goal versus delta yield for predictiong fertilizer
nitrogen need in corn: Agronomy Journal, v. 95, p. 994-999.
Loureiro, M.L., McCluskey, J.J., and Mittelhammer, R.C., 2001, Assessing consumers
preferences for organic, eco-labeled and regular apples: Journal of Agricultural and
Resource Economics, v. 26, no. 2, p. 404-416.
Lubowski, R.N., Bucholtz, S., Claassen, R., Roberts, M.J., Cooper, J.C., Gueorguieva,
A., and Johansson, R., 2006, Environmental effects of agricultural land-use change:
The role of Economics and policy: U.S. Department of Agriculture, Economic
Research Service Report Number ERR-25, 82 p., available online at:
http://www.ers.usda.gov/Publications/ERR25/.
LUMCON, 2007, Dead zone size near top end: LUMCON News, available on line at:
http://www.lumcon.edu/Informati on/news/default. asp?XMLFilename=20070731164
S.xml, last accessed August 7, 2007.
MacKinnon, J.A., and Gregg, M.C., 2005, Spring mixing: Turbulence and internal waves
during restratification on the New England shelf: Journal of Physical Oceanography,
v. 35, p. 2425-2443.
Mallarino, A.P., and Schepers, J.S., 2005, Role of precision agriculture in phosphorus
management practices, in Sims, J.T., and Sharpley, A.N., eds., Phosphorus: Madison,
WI, Agriculture and the Environment, American Society of Agronomy Monograph
Series No. 46, Crop Science Society of America and Soil Science Society of
America, p. 881-908.
Maloney, M., and Brady, G., 1988, Capital turnover and marketable emission rights:
Journal of Law and Economics, v. 31, p. 203-226.
250
-------
Mamo, M., Malzer, G.L., Mulla, D.J., Huggins, D.R., and Strock, 1, 2003, Spatial and
temporal variation in economically optimum nitrogen rate for corn: Agronomy
Journal, v. 95, p. 958-964.
Mann, L., and Tolbert, V., 2000, Soil sustainability in renewable biomass plantings:
Ambio, v. 29, no. 8, p. 492-498.
Marquez, C.O., Cambardella, C.A., Isenhart, T.M., and Schultz, R.C., 1999, Assessing
soil quality in a riparian buffer strip system by testing organic matter fractions:
Agroforestry Systems, v. 44, p. 133-140.
MART, 2006a, Management Action Review Team Report: U.S. Environmental
Protection Agency, Mississippi River/Gulf of Mexico Watershed Nutrient Task
Force, Management Action Reassessment Team, 31 p., available online at
http://www.epa.gov/msbasin/taskforce/MART.pdf
MART, 2006b, Reassessment of point source nutrient mass loadings to the Mississippi
River basin: U.S. Environmental Protection Agency, Mississippi River/Gulf of
Mexico Watershed Nutrient Task Force, Management Action Reassessment Team,
31 p., available online at
http://www.epa.gov/msbasin/taskforce/Point_Source_Mass_Loading.pdf
Maryland Department of the Environment, 2005, Chesapeake Bay Restoration Program:
Baltimore, MD.
Mayer, P.M., Reynolds, S.K., McCutchen, M.D., and Canfield, T.J., 2006, Riparian
buffer width, vegetative cover, and nitrogen removal effectivenessA review of
current science and regulations: Cincinnati, OH, U.S. Environmental Protection
Agency, EPA/600/R-05/118.
Mazurek, J., 2002, Government-sponsored voluntary programs for firmsAn initial
survey, in Dietz, T., and Stern, P.C., eds., New Tools for Environmental Protection:
Education, Information, and Voluntary Measures, The National Academies Press,
National Academy of Sciences, p. 219-234.
McDowell, L.L., and McGregor, K.C., 1984, Plant nutrient losses in runoff from
conservation tillage corn: Soil Tillage Research, v. 4, p. 79-91.
McDowell, R.W., and Sharpley, A.N., 2001, A comparison of fluvial sediment
phosphorus (P) chemistry in relation to location and potential to influence stream P
concentrations: Aquatic Geochemistry, v. 7, p. 255-265.
McDowell, R.W., and Sharpley, A.N., 2003, Uptake and release of phosphorus from
overland flow in a stream environment: Journal of Environmental Quality, v. 32,
p. 937-948.
McDowell, R.W., Sharpley, A.N., and Kleinman, P.J.A., 2002, Integrating phosphorus
and nitrogen decision management at watershed scales: Journal of American Water
Resources Association, v. 38, no. 2, p. 479-491.
McGraw, T., and Hemb, R., 1995, Fertility variability in the Minnesota River Valley
watershed in 1993 as determined from grid soil testing results on 52,000 acres on
commercial fields, in Robert, P.C., Rust, R.H., and Larson, W.E., eds., Site-specific
251
-------
management for agricultural systems: Minneapolis, MN, American Society of
Agronomy, Crop Science Society of America and Soil Science Society of America,
2nd International Conference.
Mclsaac, G.F., 2006, Net anthropogenic nitrogen inputs (NANI) to the Mississippi River
basin: Presented at Science SymposiumSources, transport, and fate of nutrients in
the Mississippi and Atchafalaya River basins, November 7-9, 2006, Minneapolis,
MN.
Mclsaac, G.F., David, M.B., Gertner, G.Z., and Goolsby, D.A., 2001, Nitrate flux in the
Mississippi River: Nature, v. 414, p. 166-167.
Mclsaac, G.F., David, M.B., Gertner, G.Z., and Goolsby, D.A., 2002, Relating net
nitrogen input in the Mississippi River basin to nitrate flux in the lower Mississippi
RiverA comparison of approaches: Journal of Environmental Quality, v. 31,
p.1610-1622.
Mclsaac, G.F., and Hu, X., 2004, Net N input and riverine N export from Illinois
agricultural watersheds with and without extensive tile drainage: Biogeochemistry,
v. 70, p. 251-271.
McKee, B.A., Aller, R.C., Allison, M.A., Bianchi, T.S., and Kineke, G.C., 2004.
Transport and transformation of dissolved and particulate materials on continental
margins influenced by major riversBenthic boundary layer and seabed processes:
Continental Shelf Research, v. 24, p. 899-926.
McLaughlin, S.B., and Kszos, L.A., 2005, Biomass and bioenergy development of
switchgrass (Panicum virgatum) as a bioenergy feedstock in the United States:
Biomass & Bioenergy, v. 28, p. 515-535.
McLaughlin, S., and Walsh, M., 1998, Evaluating environmental consequences of
producing herbaceous crops for bioenergy: Biomass & Bioenergy, v. 14, no. 4,
p. 317-324.
McMahon, G., Alexander, R.B., and Qian, S., 2003, Support of total maximum daily load
programs using spatially referenced regression models: Journal of Water Resources
Planning and Management, v. 129, p. 315-329.
Mee, L.D., 2006, Reviving dead zones: Scientific American, v. 295, p. 78-85.
Meisinger, J.J., and Delgado, J.A., 2002, Priciples for managing nitrogen leaching:
Journal of Soil and Water Conservation, v. 57, no. 6, p. 485-499.
Melillo, J.M., and Cowling, E.B., 2002, Reactive nitrogen and public policies for
environmental protection: AMBIO, A Journal of the Human Environment, v. 31,
no. 2, p. 141-149.
Mellor, G.L., and Yamada, T., 1982, Development of a turbulence closure model for
geophysical fluid problems: Reviews of Geophysics and Space Physics, v. 20,
p. 851-875.
Meretsky, V.J., Wegner, D.L., and Stevens, L.E., 2000, Balancing endangered species
and ecosystemsA case study of adaptive management in Grand Canyon:
Environmental Management, v. 25, no. 6, p. 579-586.
252
-------
Metropolitan Council, 2004, Regional progress in water qualityAnalysis of water
quality data from 1976 to 2002 for the major rivers in the Twin Cities: St. Paul, MN,
Metropolitan Council, Publication 32-04-045.
Mitsch, W.J., Day, J.W., Jr., Gilliam, W., Groffman, P.M., Hey, D.L., Randall, G.W., and
Wang, N., 2001, Reducing nitrogen loading to the Gulf of Mexico from the
Mississippi River basinStrategies to counter a persistent ecological problem:
Bioscience, v. 51, p. 373-388.
Mitsch, W.J., Day, J.W., Jr., Gilliam, J.W., Groffman, P.M., Hey, D.L., Randall, G.W.,
and Wang, N., 1999, Reducing nutrient loads, especially nitrate-nitrogen, to surface
water, ground water, and the Gulf of Mexico: Topic 5 report for the integrated
assessment on hypoxia in the Gulf of Mexico: Silver Spring, MD, National Oceanic
and Atmospheric Administration Coastal Ocean Program Decision Analysis Series
No. 19, 111 p. Available on line at:
http://oceanservice.noaa.gov/products/hypox t5final.pdf.
Mitsch, W.J., Day, J.W., Jr., Zhang, L., and Lane, R.R., 2005a, Nitrate-nitrogen retention
in wetlands in the Mississippi River Basin: Ecological Engineering, v. 24, p. 267-
278.
Mitsch, W.J., Zhang, L., Anderson, C.J., Altor, A.E., and Hernandez, M.E., 2005b,
Creating riverine wetlandsEcological succession, nutrient retention, and pulsing
effects: Ecological Engineering, v. 25, p. 510-527.
Montagna, P., Hodges, B., Maidment, D., and Minsker, B., 2006, Long-term studies of
hypoxia in Corpus Christi Bay: The cybercollaboratory testbed: Paper presented at
Hypoxia Effects on Living Resources in the Gulf of Mexico, September 25-26,
2006: New Orleans, Louisiana, Tulane University, sponsored by National Oceanic
and Atmospheric Administration Center for Sponsored Coastal Ocean Research.
Moomaw, W.R., 2002, Energy, industry and nitrogenStrategies for decreasing reactive
nitrogen emissions: AMBIO, A Journal of the Human Environment, v. 31, no. 2,
p. 184-189.
Morey, S.L., Martin, P.J., O'Brien, J.J., Wallcraft, A.A., Zavala-Hidalgo, J., 2003a,
Export pathways for river discharged fresh water in the northern Gulf of Mexico:
Journal of Geophysical Research, v. 108, CIO, 3303, doi:1029/2002JC001674.
Morey, S.L., Schroeder, W.W., O'Brien, J.J., and Zavala-Hidalgo, J., 2003b, The annual
cycle of riverine influence in the eastern Gulf of Mexico basin: Geophysical
Research Letters, v. 30, 1867, doi:10.1029/2003GL017348.
Morgenstern, R., and Pizer, W., eds., 2007, Reality checkThe nature and performance
of voluntary environmental programs in the United States, Europe, and Japan:
Washington, D.C., Resources for the Future Press, 200 p.
Morse, J.W., and Eldridge, P.M., in press, A non-steady state diagenetic model for
changes in sediment biogeochemistry in response to seasonally hypoxic/anoxic
conditions in the "dead zone" of the Louisiana shelf: Marine Chemistry, v. , p. -
, Accepted for publication, draft available online at:
253
-------
http://vosetnite.epa.gov/sab/sabhap.nsf/2a890dc663b46bc685256d63006ac3aa/6ca36
cf055bc5b2d8525727c004bc993/$FILE/Morse%20and%20Eldridge%202007.pdf.
Morse, J.W., and Rowe, G.T., 1999, Benthic biogeochemistry beneath the Mississippi
River plume: Estuaries, v. 22, p. 206-214.
Mortimer, C.H.J., 1941, The exchange of dissolved substances between mud and water in
lakesI and II: Journal of Ecology, v. 29, p. 280-329.
Mosier, A., Kroeze, C., Nevison, C., Oenema, O., Seitzinger, S., and van Cleemput, O.,
1998, Closing the global N2O budget: Nitrous oxide emissions through the
agricultural nitrogen cycle: Nutrient Cycling in Agroecosystems, v. 52, no. 2-3, p.
225-248, available on-line at:
http://www.springerlink.com/content/lu04703848m261w5/.
MR/GMWNTF, 2001, Action plan for reducing, mitigating, and controlling hypoxia in
the northern Gulf of Mexico: Washington, D.C., Mississippi river/Gulf of Mexico
Watershed Nutrient Task Force, 36 p. Available on line at:
http://www.epa.gov/msbasin/taskforce/pdf/actionplan.pdf.
Mueller, D.H., Wendt, R.C., and Daniel, T.C., 1984, Phosphorus losses as affected by
tillage and manure application: Soil Science Society of America Journal, v. 48,
p. 901-905.
Mulholland, P.J., Steinman, A.D., Marzolf, E.R., Hart, D.R., and DeAngelis, D.L., 1994,
Effect of periphyton biomass on hydraulic characteristics and nutrient cycling in
streams: Oecologia, v. 98, p. 40-47.
Mulvaney, R.L., Khan, S.A., Hoeft, R.G., and Brown, H.M., 2001, A soil organic
nitrogen fraction that reduces the need for nitrogen fertilization: Soil Science Society
of America Journal, v. 65, p. 1164-1172.
Murdock, L.W., Howe, P.L., and Schwab, G.J., 2002, Variable rate nitrogen fertilizer for
corn grown in Kentucky, in Proceedings of the North-Central Extension-Industry
Soil Fertility Conference: Brookings, SD, Potash and Phosphate Institute, v. 18,
p. 81-85.
NASA-SeaWiFS, 2007, Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Project:
National Aeronautics and Space Administration, available on line at:
http://oceancolor.gsfc.nasa.gov/SeaWiFS/.
National Academy of Sciences, 2007, Mississippi River water quality and the Clean
Water Act: Progress, challenges, and opportunities: National Research Council,
National Academy of Sciences, Committee on the Mississippi River and the Clean
Water Act, 215 p., available on line at:
http://www.nap.edu/catalog.php7record id=12051.
National Research Council, 2000, Clean coastal watersUnderstanding and reducing the
problems from nutrient pollution: Washington, D.C., National Academy of Sciences
Press, 405 p.
National Research Council, 2004, Adaptive management for water resource planning:
Washington, D.C., National Academy of Science Press, 123 p.
254
-------
Neal, C., 2001, The potential for phosphorus pollution mediation by calcite precipitation
in UK freshwaters: Hydrology and Earth System Sciences, v. 5, p. 119-131.
Neitsch, S.L., Arnold, J.G., Kiniry, J.R., Srinivasan, R., and Williams, J.R., 2004, Soil
and water assessment tool input/output file documentation, version 2005: Temple,
TX, U.S. Department of Agriculture, Agricultural Research Service, Grassland, Soil
and Water Research Laboratory, available online at:
ftp://ftp.brc.tamus.edu.pub/outgoing/sammons/swat2005 (accessed 11/28/06).
Nelson, R., Tietenberg, T., and Donihue, M.R., 1993, Differential environmental
regulation: effects on electric utility capital turnover and emissions: Review of
Economics and Statistics, v. 75, p. 368-373.
Nielsen, K., Nielsen, L.P., and Rasmussen, P., 1995, Estuarine nitrogen retention
independently estimated by the denitrification rate and mass balance methodsA
study of Norsminde Fjord, Denmark: Marine Ecology Progress Series, v. 119,
p. 273-283.
Nixon, S.W., 1992, Quantifying the relationship between nitrogen input and the
productivity of marine ecosystems: Tokyo, Japan, Advanced Marine Technology
Conference, no. 5.
Nixon, S.W., Ammerman, J., Atkinson, L., Berounsky, V., Billen, G., Boicourt, W.,
Boynton, W., Church, T., DiToro, D., Elmgren, R., Garber, J., Giblin, A., Jahnke, R.,
Owens, N., Pilson, M.E.Q., and Seitzinger, S., 1996, The fate of nitrogen and
phosphorus at the land-sea margin of the North Atlantic Ocean: Biogeochemistry,
v. 35, p. 141-180.
Nixon, S.W., Kelly, J.R., Furnas, B.N., Oviatt, C.A., and Hale, S.S., 1980, Phosphorus
regeneration and the metabolism of coastal marine bottom communities, in Tenore,
K.R., and Coull, B.C., eds., Marine Benthic Dynamics: University of South Carolina
Press, p. 219-242.
NOAA, 2007, Nutrient Enhanced Coastal Ocean Productivity (NECOP) Program:
National Oceanic and Atmospheric Administration, Washington, DC, available
online at: http://www.aoml.noaa.gov/ocd/necopA accessed April, 2007.
Northcott, W.J., Cooke, R.A., Walker, S.E., Mitchell, J.K., and Hirschi, M.C., 2001,
Application of DRAINMOD-N to fields with irregular drainage systems:
Transactions of the American Society of Agricultural Engineers, v. 44, no. 2, p. 241-
249.
Novak, J.M., Stone, K.C., Watts, D.W., and Johnson, M.H., 2003, Dissolved phosphorus
transport during storm and base flow conditions from an agriculturally intensive
Southeastern Coastal Plain watershed: Transactions of the American Society of
Agricultural Engineers, v. 46, no. 5, p. 1355-1363.
Nowlin, W.D., Jochems, A.E., DiMarco, S.F., Reid, R.O., and Howard, M.K., 2005,
Low-frequency circulation over the Texas-Louisiana continental shelf, in Sturges,
W., and Lugo-Fernandez, A., eds., Circulation in the Gulf of MexicoObservations
and models: American Geophysical Union, Geophysical Monograph, v. 161, p. 219-
240.
255
-------
Oberle, S.L., and Keeney, D.R., 1990, Soil type, precipitation, and fertilizer N effects on
corn yields: Journal of Production Agriculture, v. 3, p. 522-527.
O'Connor, T., and Whitall, D., 2007, Linking hypoxia to shrimp catch in the northern
Gulf of Mexico: Marine Pollution Bulletin, v. 54, no. 4, p. 460-463.
O'Donnell, T.K., and Galant, D.L., in press, Evaluating success criteria and project
monitoring in river enhancement within an adaptive management framework:
Environmental Management, (in press).
Oey, L.-Y., Ezer, T., Forristall, G., Cooper, C., DiMarco, S., and Fan, S., 2005a, An
exercise in forecasting loop current and eddy frontal positions in the Gulf of Mexico:
Geophysical Research Letters, v. 32, L12611, doi:10.1029/2005GL023253.
Oey, L.-Y., Ezer, T., and Lee, H.-C., 2005b, Loop current rings and related circulation in
the Gulf of MexicoA review of numerical models and future challenges, in
Sturges, W., and Fernandez, A., eds., Circulation in the Gulf of Mexico
Observations and models: American Geophysical Union, p. 31-56.
Ogden, J.C., Davis, S.M., Jacobs, K.J., Barnes, T., and Fling, H.E., 2005, The use of
conceptual ecological models to guide ecosystem restoration in south Florida:
Wetlands, v. 25, no. 4, p. 795-809.
Oguz, T., and Gilbert, D., 2007, Abrupt transitions of the top-down controlled Black Sea
pelagic ecosystem during 1960-2000Evidence for regime-shifts under strong
fishery exploitation and nutrient enrichment modulated by climate-induced
variations: Deep-Sea Research, v. 54, p. 220-242.
Ohlmann, J.C., and Niiler, P.P., 2005, Circulation over the continental shelf in the
northern Gulf of Mexico: Progress in Oceanography, v. 64, p. 45-81.
Olness, A.E., Smith, S.J., Rhoades, E.D., and Menzel, R.G., 1975, Nutrient and sediment
discharge from agricultural watersheds in Oklahoma: Journal of Environmental
Quality, v. 4, p. 331-336.
Olson, R.J., Hensler, R.F., Attoe, O.J., Witzel, S.A., and Peterson, L.A., 1970, Fertilizer
nitrogen and crop rotation in relation to movement of nitrate nitrogen through soil
profiles: Soil Science Society of America Proceedings, v. 34, p. 448-452.
Omay, A.B., Rice, C.W., Maddux, L.D., and Gordon, W.B., 1997, Changes in soil
microbial and chemical properties under long-term crop rotation and fertilization:
Soil Science Society of America Journal, v. 61, p. 1672-1678.
Opdyke, M.R., David, M.B., and Rhoads, B.L., 2006, The influence of geomorphological
variability in channel characteristics on sediment denitrification in agricultural
streams: Journal of Environmental Quality, v. 35, p. 2103-2112.
Osborne, S.L., Schepers, J.S., Francis, D.D., and Schlemmer, M.R., 2002, Detection of
phosphorus and nitrogen deficiencies in corn using spectral radiance measurements:
Agronomy Journal, v. 94, p. 1215-1221.
Osterman, L.E., Poore, R.Z., Swarzenski, P.W., and Turner, R.E., 2005, Reconstructing a
180-yr record of natural and anthropogenic induced hypoxia from the sediments of
the Louisiana continental shelf: Geology, v. 33, no. 4, p. 329-332.
256
-------
Owens, L.B., Edwards, W.M., and Van Keuren, R.W., 1997, Runoff and sediment losses
resulting from winter feeding on pastures: Journal of Soil and Water Conservation,
v. 52, p. 194-197.
Owens, L.B., Malone, R.W., Shiptalo, M.J., Edwards, W.M., and Bonta, J.V., 2000,
Lysimeter study of nitrate leaching from a corn-soybean rotation: Journal of
Environmental Quality, v. 29, p. 467-474.
Paerl, H.W., and Fulton, R.S., III, 2006, Ecology of harmful cyanobacteria, in Graneli,
E., and Turner, J., eds., Ecology of harmful marine algae: Berlin, Springer-Verlag,
p. 69-78.
Paerl, H.W., Valdes, L.M., Piehler, M.F., and Stow, C.A., 2006b, Assessing the effects of
nutrient management in an estuary experiencing climatic changeThe Neuse River
Estuary, NC, USA: Environmental Management, v. 37, p. 422-436.
Pakulski, J.D., Benner, R., Whitledge, T., Amon, R., Eadie, B., Cifuentes, L.,
Ammerman, J., and Stockwell, D., 2000, Microbial metabolism and nutrient cycling
in the Mississippi and Atchafalaya River plumes: Estuarine and Coastal Shelf
Science, v. 50, no. 2, p. 173-184.
Paludan, C., and Blicher-Mathiesen, G., 1996, Losses of inorganic carbon and nitrous
oxide from a temperate freshwater wetland in relation to nitrate loading:
Biogeochemistry, v. 35, no. 2, p. 305-326.
Parkin, T.B., and Kaspar, T.C., 2006, Nitrous oxide emissions from corn-soybean
systems in the Midwest: Journal of Environmental Quality, v. 35, p. 1496-1506.
Patrick, W.H., Jr., Gotoh, S., and Williams, E.G., 1973, Strengite dissolution in flooded
soils and sediments: Science, v. 179, p. 564-565.
Patterson, B.D., 1987, The principle of nested subsets and its implications for biological
conservation: Conservation Biology, v. 1, p. 323-334.
Pavelis, G.A., ed., 1987, Farm drainage in the United StatesHistory, status, and
prospects: U.S. Department of Agriculture, Economic Research Service, Misc. Pub.
1455, 170 p.
Perez, A.N., Oehlers, L., and Walter, R.B., 2006, Detection of hypoxia-related proteins in
Medaka (Oryzias latipes) by difference gel electrophoresis and identification by
sequencing of peptides using MALDI-TOF mass spectrometry: Paper presented at
Hypoxia Effects on Living Resources in the Gulf of Mexico, September 25-26,
2006, Tulane University, New Orleans, Louisiana, sponsored by National Oceanic
and Atmospheric Administration Center for Sponsored Coastal Ocean Research.
Perlack, R.D., and Turhollow, A.F., 2003, Feedstock cost analysis of corn stover residues
for further processing: Energy, v. 28, no. 14, p. 1395-1403.
Perlack, R.D., Wright, L.L., Turhollow, A.F., Graham, R.L., Stokes, B.J., Erbach, D.C.,
2005, Biomass as feedstock for a bioenergy and bioproducts industry: The technical
feasibility of a billion-ton annual supply: Oak ridge National Laboratory, 78 p.,
available on line at: http://feedstockreview.ornl.gov/pdf/billion ton vision.pdf
257
-------
Peters, R.H., 1986, The role of prediction in limnology: Limnology and Oceanography,
v. 31, p. 1143-1159.
Phipps, R.G., 1997, Nitrate removal capacity of constructed wetlands: Ames, IA, Iowa
State University, Ph.D. dissertation, 68 p.
Phipps, R.G., and Crumpton, W.G., 1994, Factors affecting nitrogen loss in experimental
wetlands with different hydrologic loads: Ecological Engineering, v. 3, p. 399-408.
Pierson, S.T., Cabrere, M.L., Evanylo, O.K., Kuykendall, H.A., Hoveland, C.S.,
McCann, M.A., and West, L.T., 2001, Phosphorus and ammonium concentrations in
surface runoff from grasslands fertilized with broiler litter: Journal of Environmental
Quality, v. 30, p. 1784-1789.
Platon, E., and Sen Gupta, B.K., 2001, Benthic foraminiferal communities in oxygen
depleted environments of the Louisiana Continental Shelf, in Rabalais, N.N., and
Turner, R.E., eds., Coastal hypoxiaConsequences for living resources and
ecosystems: American Geophysical Union, Coastal andEstuarine Study Series,
v. 58, p. 147-163.
Platon, E., Sen Gupta, B.K., Rabalais, N.N., and Turner, R.E., 2005, Effect of seasonal
hypoxia on the benthic forminiferal community of the Louisiana inner continental
shelfThe 20th century record: Marine Micropaleontology, v. 54, p. 263-283.
Popova, Y.A., Keyworth, V.G., Haggard, B.E., Storm, D.E., Lynch, R.A., and Payton,
M.E., 2006, Stream nutrient limitation and sediment interactions in the Eucha-
Spavinaw basin, USA: Journal of Soil and Water Conservation, v. 61, p. 105-115.
Pote, D.H., Daniel, T.C., Nichols, D.J., Sharpley, A.N., Moore, P.A., Jr., Miller, D.M.,
and Edwards, D.R., 1999, Relationship between phosphorus levels in three Ultisols
and phosphorus concentrations in runoff: Journal of Environmental Quality, v. 28,
p. 170-175.
Pote, D.H., Daniel, T.C., Sharpley, A.N., Moore, P. A., Jr., Edwards, D.R., and Nichols,
D.J., 1996, Relating extractable soil phosphorus to phosphorus losses in runoff: Soil
Science Society of America Journal, v. 60, p. 855-859.
Power, J.F., Wiese, R., and Flowerday, D., 2000, Managing nitrogen for water quality
Lessons from management systems evaluation area: Journal of Environmental
Quality, v. 29, p. 355-366.
PPI/PPIC/FAR, 2005, Soil test levels in North America Summary update: Norcross,
GA, Potash & Phosphate Institute, PPI/PPIC/FAR Technical Bulletin 2005-1.
PPI/PPIC/FAR, 2002, Plant nutrient use in North American agricultureProducing food
and fiber, preserving the environment and integrating organic and inorganic sources:
Norcross, GA, Potash & Phosphate Institute, PPI/PPIC/FAR Technical Bulletin
2002-1.
Preston, S.D., and Brakebill, J.W., 1999, Application of spatially referenced regression
modeling for the evaluation of total nitrogen loading in the Chesapeake Bay
watershed: U.S. Geological Survey Water-Resources Investigations Report 99-4054,
258
-------
12 p., available online at http://md.water.usgs.gov/publications/wrir-99-
4054/html/index.htm
Prospero, J.M., Barrett, K., Church, T., Dentener, F., Duce, R.A., Galloway, J.N., Levy,
H., Moody, J., and Quinn, P., 1996, Atmospheric deposition of nutrients to the North
Atlantic Basin: Biogeochemistry, v. 35, p. 27-73.
Rabalais, N.N., 2006, Benthic communities and the effects of hypoxia in Louisiana
coastal waters: Paper presented at Hypoxia Effects on Living Resources in the Gulf
of Mexico, September 25-26, 2006, Tulane University, New Orleans, Louisiana,
sponsored by National Oceanic and Atmospheric Administration Center for
Sponsored Coastal Ocean Research.
Rabalais, N.N., Atilla, N., Normandeau, C., and Turner, R.E., 2004, Ecosystem history of
Mississippi River-influenced continental shelf revealed through preserved
phytoplankton pigments: Marine Pollution Bulletin, v. 49, p. 537-547.
Rabalais, N.N., Lohrenz, S.E., Redalje, D.G., Dortch, Q., Justic, D., Turner, R.E.,
Qureshi, N.A., Dagg, M.J., Eadie, B.J., and Fahnensteil, G.L., 1999b, Nutrient-
enhanced coastal productivity and ecosystem responses, in Wiseman, W.J., Jr.,
Rabalais, N.N., Dagg, M.J., and Whitledge, T.E., eds., chap. 4, Nutrient enhanced
coastal ocean productivity in the northern Gulf of Mexico: Silver Spring, MD,
National Oceanic and Atmospheric Administration Coastal Ocean Program, Decision
Analysis Series No. 14, U.S. Department of Commerce, National Ocean Service,
Center for Sponsored Coastal Research, p. 51-78.
Rabalais, N.N., and Turner, R.E., 2001, Hypoxia in the northern Gulf of Mexico
Description, causes, and change, in Rabalais, N.N., and Turner, R.E., eds., Coastal
hypoxia Consequences for living resources and ecosystems: Washington, D.C.,
American Geophysical Union, Coastal and Estuarine Studies, v. 58, 454 p.
Rabalais, N.N., and Turner, R.E., 2006, Oxygen depletion in the Gulf of Mexico adjacent
to the Mississippi River, in Neretin, L.N., ed., Past and present marine water column
anoxia: NATO Science Series, IV-Earth and Environmental Sciences, Kluwer,
p. 225-245.
Rabalais, N.N., Turner, R.E., Dortch, Q., Justic, D., Bierman, V.J., and Wiseman, W.J.,
2002, Nutrient-enhanced productivity in the northern Gulf of MexicoPast, present
and future: Hydrobiologia, v. 475, p. 39-63.
Rabalais, N.N., Turner, R.E., Justic, D., Dortch, Q., and Wiseman, W.J., 1999a,
Characterization of hypoxia: Topic 1 report for the integrated assessment of hypoxia
in the Gulf of Mexico: Silver Spring, MD, National Oceanic and Atmospheric
Administration Coastal Ocean Program Decision Analysis Series No. 15, 203 p.
Available on line at: http://oceanservice.noaa.gov/products/hypox_tlfmal.pdf
Rabalais, N.N., Turner, R.E., Sen Gupta, B.K., Platon, E., and Parsons, M.L., 2007,
Sediments tell the history of eutrophication and hypoxia in the northern Gulf of
Mexico: Ecological Applications, Supplement, v. 17, no. 5, p. 129-143.
Ragauskas, A.J., Williams, C.K., Davison, B.H., Britovsek, G., Cairney, J., Eckert, C.A.,
Fredrick, W.J., Jr., Hallet, J.P., Leak, D.J., Liotta, C.L., Mielenz, J.R., Murphy, R.,
259
-------
Templer, R., and Tschaplinski, T., 2006, The path forward for biofuels and
biomaterials: Science, v. 311, p. 484-489.
Ragueneau, O., Conley, D.J., Leynaert, A., Longphuirt, S.N., and Slomp, C.P., 2006a,
Responses of coastal ecosystems to anthropogenic perturbations of silicon cycling, in
Ittekot, V., Unger, D., and Humborg, C., eds., The silicon cycle: Human
perturbations and impacts on aquatic systems: Washington, D.C., Island Press,
p. 197-213.
Ragueneau, O., Conley, D.J., Leynaert, A., Longphuirt, S.N., and Slomp, C.P., 2006b,
Role of diatoms in silica cycling and coastal marine food webs, in Ittekot, V., Unger,
D., and Humborg, C., eds., The silicon cycleHuman perturbations and impacts on
aquatic systems: Washington, D.C., Island Press, p. 163-195.
Raloff, Janet, 2004, Dead watersMassive oxygen-starved zones are developing along
the world's coasts: Science News Online, June 5, 2004, v. 165, no. 23, 8 p., accessed
June 28, 2007, at http://sciencenews.org/articles/20040605/bob9.asp.
Randall, G.W., Huggins, D.R., Russelle, M.P., Fuchs, D.J., Nelson, W.W., and Anderson,
J.L., 1997, Nitrate losses through subsurface tile drainage in conservation reserve
program, alfalfa, and row crop systems: Journal of Environmental Quality, v. 26,
p. 1240-1247.
Randall, G.W., and Sawyer, J., 2005, Nitrogen application timing, forms and additives, in
Gulf Hypoxia and Local Water Quality Concerns Workshop, Ames, IA, September
27-28, 2005, p. 73-84.
Randall, G.W., and Vetsch, J.A., 2005, Nitrate losses in subsurface drainage from a corn-
soybean rotation as affected by fall vs. spring application of nitrogen and nitrapyrin:
Journal of Environmental Quality, v. 34, no. 2, p. 590-597.
Randall, G.W., Vetsch, J.A., and Huffman, J.R., 2003, Nitrate losses in subsurface
drainage from a corn-soybean rotation as affected by time of nitrogen application
and use of nitrapyrin: Journal of Environmental Quality, v. 32, p. 1764-1772.
Raun, W.R., Solie, J.B., Stone, M.L., Martin, K.L., Freeman, K.W., Mullen, R.W.,
Zhang, H., Schepers, J.S., and Johnson, G.V., 2005, Optical sensor based algorithm
for crop nitrogen fertilization: Communications in Soil Science and Plant Analysis,
v. 36, nos. 19-20, p. 2759-2781.
Redalje, D.G., Lohrenz, S.E., and Fahnenstiel, G.L., 1992, Phytoplankton dynamics and
the vertical flux of organic carbon in the Mississippi River Plume and inner Gulf of
Mexico shelf region, in Falkowski, P.G., and Woodhead, A.D., eds., Primary
productivity and biogeochemical cycles in the sea: New York, Plenum Press, 526 p.
Redalje, D.G., Lohrenz, S.E., and Fahnenstiel, G.L., 1994b, The relationship between
primary production and the vertical export of particulate organic matter in a river
impacted coastal ecosystem: Estuaries, v. 17, p. 829-838.
Redalje, D.G., Lohrenz, S.E., and Fahnenstiel, G.L., 1994a, The vertical export of
parti culate and dissolved organic carbon from the surface waters of the northern Gulf
of Mexico shelf: Journal of Mississippi Academy of Sciences, v. 39, p. 63.
260
-------
Reddy, K.R., Kadlec, R.H., Flag, E., and Gale, P.M., 1999, Phosphorous retention in
streams and wetlandsA review: Environmental Science and Technology, Critical
Review, v. 29, p. 83-146.
Reddy, K.R., Wetzel, R.G., and Kadlec, R.H., 2005, Biogeochemistry of phosphorous in
wetlands, in PhosphorousAgriculture and the environment: Madison, WI,
Agronomy Monograph no. 46, American Society of Agronomy, Crop Science
Society of America, Soil Science Society of America, p. 263-316.
Redfield, A.C., 1958, The biological control of chemical factors in the environment:
American Scientist, v. 46, p. 205-222.
Reetz, H.F., Jr., Murrell, T.S., and Murrell, L.J., 2001, Site-specific nutrient
managementProduction examples: Norcross, GA, Potash & Phosphate Institute,
Better Crops, v. 85, no. 1, p. 12-13, 17.
Reichelderfer, K., 1985, Do USDA program participants contribute to soil erosion: U.S.
Department of Agriculture, Economic Research Service, Agricultural Economic
Report No. 532, 83 p.
Rhoads, D.C., Boesch, D.F., Tang, Z., Xu, F., Huang, L., and Nilsen, K.J., 1985,
Macrobenthos and sedimentary facies on the Changjiang delta platform and adjacent
continental shelf, East China Sea: Continental Shelf Research, v. 4, p. 189-213.
Ribaudo, M., 1989, Targeting the Conservation Reserve Program to maximize water
quality benefits: Land Economics, v. 65, p. 320-332.
Ribaudo, M., Heimlich, R., Claassen, R., and Peters, M., 2001, Least-cost management of
non-point source pollutionSource reduction versus interception strategies for
controlling nitrogen loss in the Mississippi basin: Ecological Economics, v. 37,
p. 183-197.
Ribaudo, M., Heimlich, R., and Peters, M., 2005, Nitrogen sources and Gulf hypoxia
Potential for environmental credit trading: Ecological Economics, v. 52, p. 159-168.
Richards, R.P., and Baker, D.B., 2002, Trends in water quality in LEASEQ rivers and
streams (northwestern Ohio), 1975-1995: Journal of Environmental Quality, v. 31,
p. 90-96.
Richards, R.P., Baker, D.B ., Kramer, J.W., Ewing, D.E., Merryfield, B.J., and Miller,
N.L., 2001, Storm discharge, loads, and average concentrations in northwest Ohio
rivers, 1975-1995: Journal of the American Water Resources Association, v.37,
p. 423-438.
Richardson, C.J., 1999, The role of wetlands in storage, release, and cycling of
phosphorus on the landscapeA 25-year retrospective, in Reddy, K.R., ed.,
Phosphorus biogeochemistry in sub-tropical ecosystems, CRS Press/Lewis
Publishers, Boca Raton, FL, p. 47-68.
Richardson, W.B., Strauss, E.A., Bartsch, L.A., Monroe, E.M., Cavanaugh, J.C.,
Vingum, L., and Soballe, D.M., 2004, Denitrification in the upper Mississippi
RiverRates, controls, and contribution to nitrate flux: Canadian Journal of
Fisheries and Aquatic Science, v. 61, p. 1102-1112.
261
-------
Risser, P.O., 1985, Toward a holistic management perspective: BioScience, v. 35,
p. 414-418.
Rizzo, W.M., Lackey, G.L., and Christian, R.R., 1992, Significance of euphotic, subtidal
sediments to oxygen and nutrient cycling in a temperate estuary: Marine Ecology
Progress Series, v. 86, p. 51-61.
Robertson, W.D., Blowes, D.W., Placek, C.J., and Cherry, J.A., 2000, Long-term
performance of in situ reactive barriers for nitrate remediation: Ground Water, v. 38,
p. 689-695.
Robinson, C.A., Cruse, R.M., and Ghaffarzadeh, M., 1996, Cropping system and nitrogen
effects on mollisol organic carbon: Soil Science Society of America Journal, v. 60,
p. 264-269.
Rowe, G.T. and Chapman, P., 2002, Continental shelf hypoxiaSome nagging
questions: Gulf of Mexico Science, v. 20, p. 155-160.
Rowe, G.T., Cruz-Kaegi, M.L., Morse, J.W., Boland, G.S., and Escobar Briones, E.G.,
2002, Sediment community metabolism associated with continental shelf hypoxia,
northern Gulf of Mexico: Estuaries, v. 25, no. 6, p. 1097-1106.
Royer, T.V., David, M.B., and Gentry, L.E., 2006, Timing of riverine export of nitrate
and phosphorus from agricultural watersheds in IllinoisImplications for reducing
nutrient loading to the Mississippi River: Environmental Science and Technology,
v. 40, p. 4126-4131.
Royer, T.V., Tank, J.L., and David, M.B., 2004, The transport and fate of nitrate in
headwater, agricultural streams in Illinois: Journal of Environmental Quality, v. 33,
p.1296-1304.
Runkel, R.L., Crawford, C.G., and Cohn, T.A., 2004, Load estimator (LOADEST)A
FORTRAN program for estimating constituent loads in streams and rivers: U.S.
Geological Survey Techniques and Methods, book 4, chap. A5, 69 p., available
online at http://pubs.usgs.gov/tm/2005/tm4A5/.
Russell, A.E., Laird, D.A., Parkin, T.B., and Mallarino, A.P., 2005, Impact of nitrogen
fertilization and cropping system on carbon sequestration in Midwestern mollisols:
Soil Science Society of America Journal, v. 69, p. 413-422.
Russelle, M.P., Lamb, J.F.S., Montgomery, B.R., Elsenheimer, D.W., Miller, B.S., and
Vance, C.P., 2001, Alfalfa rapidly remediates excess inorganic nitrogen at a fertilizer
spill site: Journal of Environmental Quality, v. 30, p. 30-36.
Santhi, C., Arnold, J.G., Williams, J.R., Dugas, W.A., Srinivasan, R., and Hauck, L.M.,
2001, Validation of the SWAT model on a large river basin with point and nonpoint
sources: Journal of the American Water Resources Association, v. 37, no. 5,
p. 1169-1188.
Saunders, D.C., and Kalff, J., 2001, Nitrogen retention in wetlands, lakes and rivers:
Hydrobiologia, v. 443, p. 205-212.
262
-------
Sawyer, I.E., and Nafziger, E.D., 2005, Regional approach to making nitrogen fertilizer
rate decisions for corn: Proceedings of North Central Extension-Industry Soil
Fertility Conference, v. 21, p. 16-24.
Sawyer, I.E., and Randal, G.W., in press, Nitrogen rates, in Proceedings of Gulf Hypoxia
and Local Water quality Concerns Workshop, Ames, IA, September 26-28, 2005. (in
press)
Scavia, D., and Donnelly, K.A., in press, Reassessing hypoxia forecasts for the Gulf of
Mexico: Environmental Science and Technology, (accepted for publication
09/24/2007), available on line at: http://pubs.acs.org/cgi-
bin/ab stract. cgi/esthag/asap/ab s/es071423 5. html.
Scavia, D., Justic, D., and Bierman, V.J., Jr., 2004, Reducing hypoxia in the Gulf of
MexicoAdvice from three models: Estuaries, v. 27, no. 3, p. 419-425.
Scavia, D., Kelly, E.L.A., and Hagy, J.D., III, 2006, A simple model for forecasting the
effects of nitrogen loads on Chesapeake Bay hypoxia: Estuaries and Coasts, v. 29,
no. 4, p. 674-684.
Scavia, D., Rabalais, N.N., Turner, R.E., Justic, D., and Wiseman, W.J., Jr., 2003,
Predicting the response of Gulf of Mexico hypoxia to variations in Mississippi River
nitrogen load: Limnology and Oceanography, v. 48, no. 3, p. 951-956.
Schaller, J.L, Royer, T.V., David, M.B., and Tank, J.L., 2004, Denitrification associated
with plants and sediments in an agricultural stream: Journal of the North American
Benthological Society, v. 23, no. 4, p. 667-676.
Scharf, P.C., Brouder, S.M., and Hoeft, R.G., 2006a, Chlorophyll meter readings can
predict nitrogen need and yield response of corn in the North-Central USA:
Agronomy Journal, v. 98, p. 655-665.
Scharf, P.C., Kitchen, N.R., Sudduth, K.A., Davis, J.G., 2006b, Spatially variable corn
yield is a weak predictor of optimal nitrogen rate: Soil Science Society of America
Journal, v. 70, p. 2154-2160.
Scharf, P.C., Kitchen, N.R., Sudduth, K.A., Davis, J.G., Hubbard, V.C., and Lory, J.A.,
2005, Field-scale variability in optimal nitrogen fertilizer rate for corn: Agronomy
Journal, v. 97, p. 452-461.
Scheffer, M., Carpenter, S., Folley, J.A., Folke, C., and Walker, B., 2001, Catastrophic
shifts in ecosystems: Nature, v. 413, p. 591-596.
Schipper, L.A., Barkle, G.F., and Vojvodic-Vukovic, M., 2005, Maximum rates of nitrate
removal in a denitrification wall: Journal of Environmental Quality, v. 34, p. 1270-
1276.
Schipper, L.A., Barkle, G.F., Hadfield, J.C., Vojvodic-Vukovic, M., and Burgess, C.P.,
2004, Hydraulic constraints on the performance of a groundwater denitrification wall
for nitrate removal from shallow groundwater: Journal of Contaminant Hydrology,
v. 69, p. 263-279.
263
-------
Schipper, L., and Vojvodic-Vukovic, M., 1998, Nitrate removal from groundwater using
a denitrification wall amended with sawdustField trial: Journal of Environmental
Quality, v. 27, p. 664-668.
Schipper, L.A., and Vojvodic-Vukovic, M., 2001, Five years of nitrate removal,
denitrification and carbon dynamics in a denitrification wall: Water Research, v. 35,
p. 3473-3477.
Schlegel, A.J., Grant, C.A., and Havlin, J.L., 2005, Challenging approaches to nitrogen
fertilizer recommendations in continuous cropping systems in the Great Plains:
Agronomy Journal, v. 97, p. 391-398.
Schlesinger, W.H., and Melack, J.M., 1981, Transport of organic carbon in the world's
rivers: Tellus, v. 33, p. 171-187.
Schmidt, J.P., Schmitt, M.A., Randall, G.W., Lamb, J.A., Orf, J.H., and Gollany, H.,
2000, Swine manure application to nodulating and non-nodulating soybean:
Agronomy Journal, v. 92, p. 987-992.
Schmitt, M.A., Schmidt, D.R., and Jacobson, L.D., 1996, A manure management survey
of Minnesota swine producersEffect of farm size on manure application: Applied
Engineering in Agriculture, v. 12, no. 5, p. 595-599.
Schultz, R.C., Isenhart, T.M., Simpkins, W.W., and Colletti, J.P., 2004, Riparian forest
buffers in agroecosystemsLessons learned from the Bear Creek Watershed, central
Iowa, USA: Agroforestry Systems, v. 61, p. 35-50.
Scott, C.A., Walter, M.F., Brooks, E.S., Boll, J., Hes, M.B., and Merrill, M.D., 1998,
Impacts of historical changes in land use and dairy herds in water quality in the
Catskills Mountains: Journal of Environmental Quality, v. 27, p. 1410-1417.
Segerson, K., and Miceli, T., 1998, Voluntary environmental agreements: Good or bad
news for environmental protection?: Journal of Environmental Economics and
Management, v. 36, p. 109-130.
Seitzinger, S.P., and Giblin, A.E., 1996, Estimating denitrification in North Atlantic
continental shelf sediments: Biogeochemistry, v. 35, p. 235-260.
Seitzinger, S.P., Styles, R.V., Boyer, E.W., Alexander, R.B., Billen, G., Howarth, R.W.,
Mayer, B., and van Breemen, N., 2002, Nitrogen retention in riversModel
development and application to watersheds in the northeastern U.S.A.:
Biogeochemistry, v. 57-58, p. 199-237.
Sferratore, A., Gamier, J., Billen, G., Conley, D.J., and Pinault, S., 2006, Diffuse and
point sources of silica in the Seine River watershed: Environmental Science &
Technology, v. 40, p. 6630-6635.
Shapiro, C.A., Ferguson, R.B., Hergert, G.W., Dobermann, A.R., and Wortmann, C.S.,
2003, Fertilizer suggestions for corn: University of Nebraska-Lincoln Extension,
G174, available on line at: http://elkhorn.unl.edu/epublic/live/gl74/build/tftarget2.
Sharpley, A.N., 1997, Rainfall frequency and nitrogen and phosphorus in runoff from soil
amended with poultry litter: Journal of Environmental Quality, v. 26, p. 1127-1132.
264
-------
Sharpley, A.N., Daniel, T., Gibson, G., Bundy, L., Cabrera, M., Sims, T., Stevens, R.,
Lemunyon, J., Kleinman, P.J., and Parry, R., 2006a, Best management practices to
minimize agricultural phosphorus impacts on water quality: U.S. Department of
Agriculture, Agricultural Research Service ARS-163, 52 p.
Sharpley, A.N., Lleinman, P.J.A., and McDowell, R.W., 2001, Innovative management
of agricultural phosphorus to protect soil and water resources: Communications in
Soil Science and Plant Analysis, v. 32, p. 1071-1100.
Sharpley, A.N., Meisinger, J.J., Breeuwsma, A., Sims, J.T., Daniel, T.C., and Schepers,
J.S., 1998, Impacts of animal manure management on ground and surface water
quality, in Hatfield, J.L., and Stewart, B.A., eds., Animal waste utilization
Effective use of manure as a soil resource: Boca Raton, FL, Ann Arbor Press,
p. 173-242.
Sharpley, A.N., Schmidt, J.P., and Hergert, L., 2006b, Nutrient management practices, in
Schnepf, M., and Cox, C., eds., Environmental benefits of conservation on
croplandThe status of our knowledge: Ankeny, IA, Society of Soil and Water
Conservation, p. 149-193.
Sharpley, A.N., and Smith, S.J., 1994, Wheat tillage and water quality in the Southern
Plains: Soil Tillage Research, v. 30, p. 33-38.
Sharpley, A.N., Smith, S.J., and Bain, R., 1993, Effect of poultry litter application on the
nitrogen and phosphorus content of Oklahoma soils: Soil Science Society of America
Journal, v. 57, p. 1131-1137.
Sharpley, A.N., Smith, S.J., Zollweg, J.A., and Coleman, G.A., 1996, Gully treatment
and water quality in the Southern Plains: Journal of Soil and Water Conservation,
v. 51, p. 512-517.
Sharpley, A.N., Weld, J.L., Beegle, D.B., Kleinman, P.J.A., Gburek, W.J., Moore, P.A.,
Jr., and Mullins, G., 2003, Development of phosphorus indices for nutrient
management planning strategies in the United States: Journal of Soil and Water
Conservation, v. 58, no. 3, p. 137-152.
Sharpley, A.N., Withers, P.J.A., Abdalla, C., and Dodd, A., 2005, Strategies for the
sustainable management of phosphorus, in Sims, J.T., and Sharpley, A.N., eds.,
PhosphorusAgriculture and the environment: American Society of Agronomy
Monograph, p. 1069-1101.
Shehandeh, H., Wright, A.L., Hons, P.M., and Lascano, R.J., 2005, Spatial and temporal
variation of soil nitrogen parameters related to soil texture and corn yield: Agronomy
Journal, v. 97, p. 772-782.
Shepard, R., 2005, Nutrient management planningIs it the answer to better
management?: Journal of Soil and Water Conservation, v. 60, p. 171-176.
Shibles, R.M., 1998, Soybean nitrogen acquisition and utilization, in Proceedings of the
28th North Central Extension-Industry Soil Fertility Conference, St. Louis, MO, 11-
12 Nov. 1998: Brookings, SD, Potash & Phosphate Institute, p. 5-11.
265
-------
Shirmohammadi, A., Chaubey, I, Bosch, D.D., Mufioz-Carpena, R., Dharmasri, C.,
Arabi, M., Wolfe, M.L,. Frankenberger, I, Graff, C., Sohrabi, T.M.,
Shirmohammadi, A., 2006, Uncertainty in TMDL models: Transactions of the
American Society of Agricultural and Biological Engineers, v. 49, no. 4, p. 1033-
1049.
Simpson, J.H., and Hunter, J.R., 1974, Fronts in the Irish Sea: Nature, v. 250, p. 404-406.
Sims, J.T., 1997, Agricultural and environmental issues in the management of poultry
wastes Recent innovations and long-term challenges, in Rechcigl, I.E., and
MacKinnon, H.C., eds., Uses of by-products and wastes in agriculture: Washington,
D.C., American Chemical Society, p. 72-90.
Sims, J.T., Joern, B.C., and Simard, R.R., 1998, Phosphorus losses in agricultural
drainageHistorical perspective and current research: Journal of Environmental
Quality, v. 27, p. 277-293.
Sims, J.T., and Kleinman, P.J.A., 2005, Managing agricultural phosphorus for
environmental protection, in Sims, J.T., and Sharpley, A.N., eds., Phosphorus
Agriculture and the environment: Madison, WI, American Society of Agronomy
Monograph, American Society of Agronomy, p. 1021-1068.
Singh, R., and Helmers, M.J., 2006, Subsurface drainage and its management in the
upper Midwest tile landscape, in Proceedings of the 2006 EWRI Congress, American
Society of Civil Engineers, Omaha, NE, May 21-25, 2006.
Skaggs, R.W., Breve, M.A., and Gilliam, J.W., 1995, Predicting effects of water table
management on loss of nitrogen from poorly drained soils: European Journal of
Agronomy, v. 4, no. 4, p. 441-451.
Skaggs, R.W., Youssef, M.A., and Chescheir, G.M., 2003, Effect of subsurface drain
depth on nitrogen losses from drained lands: Transactions of the American Society
of Agricultural Engineering, v. 46, p. 237-244.
Skaggs, R.W., Youssef, M.A., Chescheir, G.M., Gilliam, J.W., 2005, Effect of drainage
intensity on nitrogen losses from drained lands: Transactions of the American
Society of Agricultural Engineering, v. 48, p. 2169-2177.
Sklar, F.H., Chimney, M.J., Newman, S., McCormick, P., Gawlick, D., Miao, S.L.,
McVoy, C., Said, W., Newman, J., Coronado, C., Crozier, G, Korvela, M., and
Rutchey, K., 2005, The ecological-societal underpinnings of Everglades restoration:
Frontiers in Ecology and the Environment, v. 3, no. 3, p. 161-169.
Sloth, N.P., Blackburn, H., Hansen, L.S., Risgaard-Petersen, N., and Lomstein, B.A.,
1995, Nitrogen cycling in sediments with different organic loading: Marine Ecology
Progress Series, v. 116, p. 163-170.
Smith, R.A., and Alexander, R.B., 2000, Sources of nutrients in the nation's watersheds,
in Managing nutrients and pathogens from animal agriculture, Proceedings from the
Natural Resource, Agriculture, and Engineering Service Conference for Nutrient
Management Consultants, Extension Educators, and Producer Advisors March 28-
30, 2000, Camp Hill, PA.
266
-------
Smith, R.A., Schwarz, G.E., and Alexander, R.B., 1997, Regional interpretation of water
quality monitoring data: Water Resources Research, v. 33, p. 2781-2798.
Smith, S.J., Sharpley, A.N., Berg, W.A., Naney, J.W., and Coleman, G.A., 1992, Water
quality characteristics associated with Southern Plains grasslands: Journal of
Environmental Quality, v. 21, p. 595-601.
Smith, S.V., and Hollibaugh, J.T., 1989, Carbon-controlled nitrogen cycling in a marine
'macrocosm'An ecosystem-scale model for managing cultural eutrophication:
Marine Ecology Progress Series, v. 52, p. 103-109.
Snyder, C.S., 2006, Phosphorus and potassium budgets and soil test levels in the
Mississippi-Atchafalaya River Basin: Better Crops, v. 90, no. 1, p. 19-21, available
online at
http://www.ipni.net/ppiweb/bcrops.nsf/$webindex/D62FD5F4335D283E852571100
01556B9/$file/06-lpl9.pdf.
Snyder, C.S., Bruulsema, T.W., Sharpley, A.N., and Beegle, D.B., 1999, Site-specific use
of the environmental phosphorus index tool: Norcross, GA, Potash & Phosphate
Institute, SSMG-1, 4 p., available online at: http://www.ppi-
Ppic.org/ppiweb/ppibase.nsf/b369c6dbe705ddl3852568e3000de93d/la2c31b028f94
9238525695300581e03/$FILE/SSMGl.pdf.
Snyder, C.S., and Leep, R.H., 2007, Fertilization, in Barnes, R.F., Nelson, C.J., Moore,
K.J., and Collins, M., eds., ForagesVolume II, The science of grassland
agriculture, 6th ed.: Ames, Iowa, Blackwell Publishing, chap. 24, p. 355-378.
Snyder, C.S., Randall, G.W., Almond, R.E., and Hoeft, R.G., 2001, Fall nitrogen
management for agronomic response and environmental protection: Fall fertilization
factsOpportunities and considerations: Norcross, GA, Potash & Phosphate
Institute, available online at http://www.ppi-
ppic.org/ppiweb/ppibase.nsf/$webindex/BFA77B79E6C8028C8525694E002D096D
! opendocument
Sogbedji, M.J., and Mclsaac, G.F., 2006, Evaluation of the ADAPT model for simulating
nitrogen dynamics in a tile drained agricultural watershed in central Illinois: Journal
of Environmental Quality, v. 35, p. 1914-1923.
S0rensen, J., Rasmussen, L.K., and Koike, I, 1987, Micromolar sulfide concentrations
alleviate acetylene blockage of nitrous oxide reduction by denitrifying Pseudomonas
fluorescens: Canadian Journal of Microbiology, v. 33, p. 1001-1005.
Spieles, D.J., and Mitsch, W.J., 2000, The effects of season and hydrologic and chemical
loading on nitrate retention in constructed wetlandsA comparison of low and high
nutrient riverine systems: Ecological Engineering, v. 14, p. 77-91.
Sprague, L.A., Clark, M.L., Rus, D.L., Zelt, R.B., Flynn, J.L., and Davis, J.W., 2006,
Nutrient and suspended-sediment trends in the Missouri River Basin, 1993-2003:
U.S. Geological Survey Scientific Investigations Report 2006-5231, 80 p., available
online at http://pubs.usgs.gov/sir/2006/5231/
267
-------
St. John, J.P., Fitzpatrick, J.J., and Landeck Miller, R.E., in press, TMDL modeling for
Long Island Sound: New York Water Environment Association, Clearwaters, v. 37,
no. 3, (in press).
Stadmark, J., and Leonardson, L., 2005, Emissions of greenhouse gases from ponds
constructed for nitrogen removal: Ecological Engineering, v. 25, p. 542-551.
Stevens, R.J., and Laughlin, R.J., 1998, Measurement of nitrous oxide and di-nitrogen
emissions from agricultural soils: Nutrient Cycling in Agroecosystems, v. 52, p. 131-
139, available on-line at: http://www. springerlink.com/content/g61700k53q4152147.
Stone, M.C., Hotchkiss, R.C., Hubbard, C.M., Fontaine, T.A., Mearnes, L.O., and
Arnold, J.G., 2001, Impacts of climate change on Missouri River basin water yield:
Journal of the American Water Resources Association, v. 37, no. 5, p. 1119-1130.
Stow, C.A., Qian, S.S., and Craig, J.K., 2005, Declining threshold for hypoxia in the Gulf
of Mexico: Environmental Science and Technology, v. 39, p. 716-723.
Sugg, Zachary, 2007, Assessing U.S. farm drainage: Can GIS lead to better estimates of
subsurface drainage extent?: Water Resources Institute, 8 p., available online at:
http://www.wri.org/biodiv/pubs description.cfm?pid=4324#pdf files.
Sutula, M., Bianchi, T.S., and McKee, B.A., 2004, Effect of seasonal sediment storage in
the lower Mississippi River on the flux of reactive particulate phosphorus to the Gulf
of Mexico: Limnology & Oceanography, v. 49, p. 2223-2235.
Swaney, D.P., Shermana, D.M., and Howarth, R.W., 1996, Modeling water, sediment,
and organic carbon discharges in the Hudson/Mohawk basinCoupling two
terrestrial sources: Estuaries, v. 19, no. 4, p. 833-847.
Switzer, T.S., Chesney, E., and Baltz, D.M., 2006, Habitat selection by flatfishes along
gradients of environmental variabilityImplications for susceptibility to hypoxia in
the northern Gulf of Mexico: Paper presented at Hypoxia Effects on Living
Resources in the Gulf of Mexico, September 25-26, 2006, Tulane University, New
Orleans, Louisiana, sponsored by National Oceanic and Atmospheric Administration
Center for Sponsored Coastal Ocean Research.
Sylvan, J.B., Dortch, Q., Nelson, D.M., Maier Brown, A.F., Morrison, W., and
Ammerman, J.W., 2006, Phosphorus limits phytoplankton growth on the Louisiana
shelf during the period of hypoxia formation: Environmental Science and
Technology, v. 40, no. 24, p. 7548-7553, available on line at:
http://pubs3.acs.org/acs/journals/supporting_information.page?in_manuscript=es061
417t.
Taheripour, P., Khanna, M., and Nelson, C., 2005, Welfare impacts of alternative public
policies for agricultural pollution control in an open economy: A general equilibrium
framework: Presentation at the American Economic Association Annual Meeting,
Province, Rhode Island, available online http://ageecon.lib.umn.edu/cgi-
bin/pdf_view.pl?paperid=16472.
Teisl, M., Roe, B., and Hicks, R., 2002, Can eco-labels tune a market?Evidence from
dolphin-safe labeling: Journal of Environmental Economics and Management, v. 43,
p. 339-359.
268
-------
Terry, D., 2006, Fertilizer tonnage reporting in the U.S.Basis and current need: Better
Crops, v. 90, no. 4, p. 15-17, available online at
http://www.ipni.net/ppiweb/bcrops.nsf/$webindex/FOBE4489F424A3FE8525721400
27 lDCB/$file/06-4p 15 .pdf.
Tetra Tech, Inc., 1998, Documentation of phase I and phase II activities in support of
point source nutrient loading analysis in the Mississippi River system: Prepared for
USEPA Nonpoint-source Control Branch, contract no. 68-C7-0014, Washington,
D.C.
Thogersen, J., 2002, Promoting 'green' consumer behavior with eco-labels, /'wDietz, T.,
and Stern, P.C., eds., New Tools for Environmental Protection: Education,
Information, and Voluntary Measures, The National Academies Press, National
Academy of Sciences, p.83-104.
Thorn, W.O., and Sabbe, W.E., 1994, Soil sampling procedures for the southern region of
the United States: Lexington, KY, Kentucky Agricultural Experiment Station,
Southern Cooperative Series Bulletin 377.
Thorpe, S.A., 2004, Langmuir circulation: Annual Review of Fluid Mechanics, v. 36,
p. 55-79.
Tiwari, A.K., Risse, L.M., and Nearing, M.A., 2000, Evaluation of WEEP and its
comparison with USLE and RUSLE: Transactions of the American Society of
Agricultural Engineers, v. 43, no. 5, p. 1129-1135.
Tolbert, V., 1998, Guest editorial: Biomass & Bioenergy, v. 14, no. 4, p. 301-306.
Tufekcioglu, A., Raich, J.W., Isenhart, T.M., and Schultz, R.C., 2003, Biomass, carbon
and nitrogen dynamics of multi-species riparian buffers within an agricultural
watershed in Iowa, USA: Agroforestry Systems, v. 57, p. 187-198.
Turner, R.E., 1999, A comparative mass balance budget (C, N, P and suspended solids)
for a natural swamp and overland flow systems, in Vymazal, J., ed., Nutrient cycling
and retention in natural and constructed wetlands: Leiden, The Netherlands,
Backhuys Publishing, p. 61-71.
Turner, R.E., 2005, Nitrogen and phosphorus concentration and retention in water
flowing over freshwater wetlands, in Fredrickson, L., King, S.L., and Kaminski,
R.M., eds., Ecology and management of bottomland hardwood systems: Columbia,
MO, The State of Our Understanding, University of Missouri Press, p. 57-66.
Turner, R.E., Milan, C.S., and Rabalais, N.N., 2004, A retrospective analysis of trace
metals, C, N and diatom remnants in the Mississippi River delta shelf: Marine
Pollution Bulletin, v. 49, p. 548-556.
Turner, R.E., Qureshi, N., Rabalais, N.N., Dortch, Q., Justic, D., Shaw, R.F., and Cope,
J., 1998, Fluctuating silicateNitrate ratios and coastal plankton food webs:
Proceedings of the National Academy of Sciences, v. 95, p. 13,048-13,051.
Turner, R.E., and Rabalais, N.N., 1991, Changes in Mississippi River water quality this
centuryImplications for coastal food webs: BioScience, v. 41, p. 140-148.
269
-------
Turner, R.E., and Rabalais, N.N., 1994, Coastal eutrophication near the Mississippi River
delta: Nature, v. 368, p. 619-621.
Turner, R.E., Rabalais, N.N., Alexander, R.B., Mclsaac, G., and Howarth, R.W., in press,
Causes of Gulf of Mexico hypoxia 1Characterization of nutrient and organic
matter loads: Estuaries and Coasts, (in press).
Turner, R.E., Rabalais, N.N., and Justic, D., 1999, Long-term watershed and water
quality changes in the Mississippi River system, in Wiseman, W.J., Jr., Rabalais,
N.N., Dagg, M.J., and Whitledge, T.E., eds., Nutrient enhanced coastal ocean
productivity in the northern Gulf of Mexico: Silver Spring, Maryland, National
Oceanic and Atmospheric Administration Coastal Ocean Program, Decision
Analysis Series No. 14, U.S. Department of Commerce, National Ocean Service,
Center for Sponsored Coastal Research, chap. 3, p. 37-50.
Turner, R.E., Rabalais, N.N., and Justic, D., 2006, Predicting summer hypoxia in the
northern Gulf of MexicoRiverine N, P, and Si loading: Marine Pollution Bulletin,
v. 52, p. 139-148.
Twilley, R.R., Cowan, J., Miller-Way, T., Montagna, P.A., and Mortazavi, B., 1999,
Benthic nutrient fluxes in selected estuaries in the Gulf of Mexico, in Bianchi, T.S.,
Pennock, J.R., and Twilley, R.R., eds., Biogeochemistry of Gulf of Mexico
Estuaries: New York, Wiley, p. 163-209.
Udawatta, R.P., Motavalli, P.P., and Garrett, H.E., 2004, Phosphorus loss and runoff
characteristics in three adjacent agricultural watersheds with claypan soils: Journal of
Environmental Quality, v. 33, p. 1709-1719.
UMRSHNC, 2006, Gulf hypoxia and local water quality concerns workshopA
workshop assessing tools to reduce agricultural nutrient losses to water resources in
the corn belt, in Workshop Proceedings, September 26-28, 2005, Iowa State
University, Ames, Iowa, 205 p., available online at:
http://www.umrshnc.org/index.php?option=com_content&task=view&id=19&Itemi
d=34
U.S. Department of Agriculture, 2003, Cost associated with development and
implementation of Comprehensive Nutrient Management PlansPart 1 - Nutrient
management, land treatment, manure and wastewater handling and storage, and
recordkeeping: U.S. Department of Agriculture, Natural Resource Conservation
Service, 220 p., available online at
http://www.nrcs.usda.gov/technical/land/pubs/cnmpl.html
U.S. Department of Energy, 2006, Clean coal and natural gas power systems: Available
online at http://www.fe.doe.gov/programs/powersystems/index.html.
U.S. Environmental Protection Agency, 2000a, State compendiumPrograms and
regulatory activities related to animal feeding operations: Washington, DC, USEPA
Office of Water, Office of Waste Management.
U.S. Environmental Protection Agency, 2000b, National air pollution trends, 1900-1998:
Available online at: http://www.epa.gov/ttn/chief/trends/trends98/trends98.pdf.
270
-------
U.S. Environmental Protection Agency, 2000c, Ambient water quality criteria
recommendations: Information supporting the development of state and tribal
nutrient criteria: Rivers and streams in Ecoregion VI: Washington, DC, USEPA,
Office of Water, EPA 822-B-00-017, 91 p., available online at:
http://www.epa.gov/watersci ence/criteria/nutrient/ecoregions/rivers/rivers_6.pdf.
U.S. Environmental Protection Agency, 2000d, Nutrient criteria technical guidance
manual: Rivers and Streams: Chapter 1 - Introduction: Washington, DC, USEPA
Office of Water, Office of Science and Technology, EPA-822-B-00-002, 16 p.,
available online at:
http: //www. epa. gov/water sci ence/criteri a/nutri ent/gui dance/rivers/chapter 1. pdf.
U.S. Environmental Protection Agency, 2002, National water quality inventory2000
Report to Congress: Washington, DC, USEPA, Office of Water, EPA-841-R-02-001,
available online at: http://www.epa.gov/305b/2000report/.
U.S. Environmental Protection Agency, 2003b, Economic analyses of nutrients and
sediment reduction actions to restore Chesapeake Bay water quality: Annapolis, MD,
EPA Chesapeake Bay Program Office, 162 p., available online at
http://www.chesapeakebay.net/tribtools.htm
U.S. Environmental Protection Agency, 2003a, Nutrient reduction technology cost
estimations for point sources in the Chesapeake Bay watershed: Annapolis, MD,
EPA Chesapeake Bay Program Office, 132 p., available online at
http://www.chesapeakebay.net/tribtools.htm
U.S. Environmental Protection Agency, 2004, Managing manure guidance for
concentration animal feeding operations (CAFOs): Washington, D.C., USEPA,
Office of Water, EPA-821-B-04-009, U.S. Government Printing Office, available at:
http ://cfpub. epa. gov/npdes/afo/info.cfm#manure
U.S. Environmental Protection Agency, 2004a, Water quality trading assessment
handbookCan water quality trading advance your watershed's goals?: U.S.
Environmental Protection Agency, Office of Water, Office of Wetlands, Oceans, and
Watersheds, EPA 841-B-04-001, 120 p., available online at:
http://www.epa.gov/owow/watershed/trading/handbook/docs/NationalWOTHandboo
kFINAL.pdf
U.S. Environmental Protection Agency, 2004b, What is the status of point source
nitrogen reduction in the Chesapeake Bay watershed?: Annapolis, MD, EPA
Chesapeake Bay Program Office Fact Sheet, 2 p.
U.S. Environmental Protection Agency, 2005, National management measures to control
nonpoint source pollution from, urban areas: Washington, D.C., U.S. Environmental
Protection Agency, Office of Water, EPA-841-B-05-004.
U.S. Environmental Protection Agency, 2006b, Ecoregional nutrient criteria documents
for rivers & streams: Available online at:
http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/rivers/.
271
-------
U.S. Environmental Protection Agency, 2006a, National Emissions Inventory (NET) Air
Pollutant Emissions Trends Data: Available online at
http://www.epa.gov/ttn/chief/trends/index.html.
U.S. Environmental Protection Agency, 2007, The Long Island Sound study at EPA New
England: available on line at http://www.epa.gov/boston/eco/lis/epane.html.
U.S. Government Accounting Office, 1997, Global warmingInformation on the results
of four of EPA's voluntary climate change programs: Washington, D.C., U.S.
Government Printing Office, GAO:RCED-97-163, 32 p. Available on line at:
http://www.gao.gOv/archive/l 997/rc97163 .pdf.
Vache, K.B., Eilers, J.M., and Santelman, M.V., 2002, Water quality modeling of
alternative agricultural scenarios in the U.S. corn belt: Journal of the American
Water Resources Association, v. 38, no. 2, p. 773-787.
Vahtera, E., Conley, D., Gustufsson, E.G., Kuosa, H., Pitkanen, H., Savchuk, O.P.,
Tamminen, T., Vitasalo, M., Voss, M., Wasmund, N., and Wulff, F., 2007, Internal
ecosystem feedbacks enhance nitrogen-fixing cyanobacteria blooms and complicate
management in the Baltic Sea: Ambio, v. 36, no. 12, p. 186-194.
Van Driel, P.W., Merkley, L.C., and Robertson, W.D., 2006, Denitrification of
agricultural drainage using wood-based reactors: Transactions of the AS ABE, v. 49,
p. 565-573.
Van Liew, M.W., Veith, T.L., Bosch, D.D., and Arnold, J.G, 2006, Suitability of SWAT
for the Conservation Effects Assessment ProjectA comparison on USDA ARS
watersheds: Journal of Hydrologic Engineering, v.12, no. 2, p. 173-183.
Varvel, G.E., 2006, Soil organic carbon changes in diversified rotations of the western
cornbelt: Soil Science Society of America Journal, v. 70, p. 426-433.
Varvel, G.E., Schepers, J.S., and Francis, D.D., 1997, Ability for in-season correction of
nitrogen deficiency in corn using chlorophyll meters: Soil Science Society of
America Journal, v. 61, p. 1233-1239.
Vollenweider, R.A., 1976, Advances in defining critical loading levels of phosphorus in
lake eutrophication: Memorie dell' Istituto Italiano di Idrobiologia, v. 33, p. 53-83.
Von Holle, C.K., 2005, Agricultural nitrogen use and producer attitudes in tile-drained
watersheds of east-central Illinois: University of Illinois atUrbana-ChampaignM.S.
Thesis, Urbana, IL, 72 p.
Walker, N.D., and Rabalais, N.N., 2006, Relationships among satellite chlorophyll a,
river inputs, and hypoxia on the Louisiana continental shelf, Gulf of Mexico:
Estuaries and Coasts, v. 29, no. 6B, p. 1081-1093.
Wang, X., and Melesse, A.M., 2005, Evaluation of the SWAT model's snowmelt
hydrology in a northwestern Minnesota watershed: Transactions of the American
Society of Agricultural and Biological Engineers, v. 48, no. 4, p. 1359-1376.
Watershed Agriculture Council, 2004, History of the Watershed Agriculture Council:
Available online at http://www.nycwatershed.org/index wachistory.html
272
-------
Wawrik, B., Paul, J.H., Bronk, D.A., and Gray, J.D., 2004, High rates of ammonium
recycling drive phytoplankton productivity in the offshore Mississippi River plume:
Aquatic Microbial Ecology, v. 35, p. 175-184.
Wedwick, S., Lakhani, B., Stone, J., Waller, P., and Artiola, J., 2001, Development and
sensitivity analysis of the GLEAMS-IR model: Transactions of the American
Society of Agricultural Engineers, v. 44, no. 5, p. 1095-1104.
Weed, D.A.J., and Kanwar, R.S., 1996, Nitrate and water present in and flowing from
root zone soil: Journal of Environmental Quality, v. 25, p. 709-719.
Wells, M.C., Ju, Z., Heater, S.J., and Walter, R.B., 2006, Microarray gene expression
analyses in Medaka (Oryzias latipes) exposed to hypoxia: Paper presented at
Hypoxia Effects on Living Resources in the Gulf of Mexico, September 25-26, 2006
Tulane University, New Orleans, Louisiana, sponsored by National Oceanic and
Atmospheric Administration Center for Sponsored Coastal Ocean Research.
Westerman, P.W., Donnely, T.L., and Overcash, M.R., 1983, Erosion of soil and poultry
manureA laboratory study: Transactions of the American Society of Agricultural
Engineers, v. 26, p. 1070-1078, 1084.
Wilhelm, W.W., Johnson, J.M.F., Hatfield, J.L., Voorhees, W.B., and Linden, D.R.,
2004, Crop and soil productivity response to corn residue removalA literature
review: Agronomy Journal, v. 96, no. 1, p. 1-17.
Wiseman, W.J., Rabalais, N.N., Turner, R.E., Dinnel, S.P., and MacNaughton, A., 1997,
Seasonal and interannual variability within the Louisiana coastal current:
stratification and hypoxia: Journal of Marine Systems, v.12, p. 237-248.
Wiseman, Jr., W.J., Rabalais, N.N., Turner, R.E., and Justic, D., 2004, Hypoxia and the
physics of the Louisiana coastal current, in Nihoul, J.C.J., Zavialov, P.O., and
Micklin, P.P., eds., Dying and Dead Seas - Climate Versus Anthropogenic Causes,
Kluwer Academic Publishers, Dordrecht, p. 359-372.
Wisner, Robert, 2007, Iowa Farm Outlook: Presentation at Tel Aviv University, May 15,
2007, available online at:
http://www.econ.iastate.edu/facultywisner/documents/telavivethanolpresentation-
wisner07.pdf
Wittry, D.J., and Mallarino, A.P., 2002, Use of variable-rate technology for agronomic
and environmental phosphorus-based liquid swine manure management, in Robert,
P.C., Rust, R.H., and Larson, W.E., eds., Sixth International Conference on Site-
Specific Management for Agricultural Systems Proceedings, American Society of
Agronomy, July 14-17, 2002, Minneapolis, MN.
Wittry, D.J., and Mallarino, A.P., 2004, Comparison of uniform- and variable-rate
phosphorus fertilization for corn-soybean rotations: Agronomics Journal, v. 96,
p. 26-33.
Wollheim, W.M., Vorosmarty, C.J., Peterson, B.J., Seitzinger, S.P., and Hopkinson, C.S.,
2006, Relationship between river size and nutrient removal: Geophysical Research
Letters, v. 33, (L06410): doi:10.1029/2006GL025845.
273
-------
Wortmann, C., Helmers, M., Mallarino, A.P., Barden, C., Devlin, D., Pierzynski, G.,
Lory, J., Massey, R., Holz, J., Shapiro, C., and Kovar, J., 2005, Agricultural
phosphorus management and water quality protection in the Midwest: Lincoln, NE,
University of Nebraska-Lincoln Extension and CSREES-USDA, Heartland Regional
Water Coordination Initiative, Regional Publication 187, 24 p., available online at:
http://www.ianrpubs.unl.edu/epublic/live/rpl87/build/rpl87.pdf.
Wu, J., Adams, R., Kling, C., and Tanaka, K., 2004, Assessing the costs and
environmental consequences of agricultural land use changesA site-specific,
policy-scale modeling approach: American Journal of Agricultural Economics, v. 86,
p. 26-41.
Wu, J., and Babcock, B., 1999, Metamodeling potential nitrate water pollution in the
central United States: Journal of Environmental Economics and Management, v. 28,
p. 1916-1928.
Wu, J., and Tanaka, K., 2005, Reducing nitrogen runoff from the Upper Mississippi
River basin to control hypoxia in the Gulf of MexicoEasements or taxes?: Marine
Resource Economics, v. 20, p. 121-144.
Wysocki, L.A, Bianchi, T.S., Powell, R., and Reuss, N., 2006, Spatial variability in the
coupling of organic carbon, nutrients, and phytoplankton pigments in surface waters
and sediments of the Mississippi River plume: Estuarine, Coastal and Shelf Science,
v. 69, p. 47-63.
Xu, Y. Jun, 2006, Total nitrogen inflow and outflow from a large river swamp basin to
the Gulf of Mexico: Hydrological Sciences - Journal - des Sciences Hydrologiques,
v. 51, no. 3, p. 531-542.
Xue, Y., David, M.B., Gentry, L.E., and Kovacic, D.A., 1998, Kinetics and modeling of
dissolved phosphorus export from a tile-drained agricultural watershed: Journal of
Environmental Quality, v. 27, p. 917-922.
Yang, W., Khanna, M., and Farnsworth, R., 2005, Effectiveness of conservation
programs in Illinois and gains from targeting: American Journal of Agricultural
Economics, v. 5, p. 1248-1255.
Yang, W., Khanna, M., Farnsworth, R., and Onal, H., 2004, Is geographical targeting
cost-effective: The case of the Conservation Reserve Enhancement Program in
Illinois: Review of Agricultural Economics, v. 27, p. 70-88.
Yang, W., Khanna, M., Farnsworth, R., and Onal, H., 2003: Integrating economics,
environmental and GIS modeling to target cost effective land retirement in multiple
watersheds: Ecological Economics, v. 46, p. 249-267.
Yuan, Y., Bingner, R.L., and Rebich, R.A., 2001, Evaluation of AnnAGNPS on
Mississippi Delta MSEA watersheds: Transactions of the ASAE, v. 44, no. 5,
p. 1183-1190.
Yuan, T., Bingner, R.L., Theurer, F.D., 2006, Subsurface flow component for
AnnAGNPS: Applied Engineering in Agriculture, v. 22, no. 2, p. 231-241.
274
-------
Yuan, Y., Bingner, F.D., Theurer, F.D., Ribich, R.A., and Moore, P.A., 2005, Phosphorus
component in AnnAGNPS: Transactions of the American Society of Agricultural
Engineers, v. 48, no. 6, p. 2145-2154.
Zavala-Hidalgo, J., Morey, S.L., and O'Brien, J.J., 2003, Seasonal circulation on the
western shelf of the Gulf of Mexico using a high-resolution numerical model:
Journal of Geophysical Research, v. 108, C12, 3389, doi: 10.1029/2003 JCOO1879.
Zhang, L., and Mitsch, W.J., 2000, Hydrologic budgets of the two Olentangy River
experimental wetlands, 1994-99, in Mitsch, W.J., and Zhang, L., eds., Olentangy
River Wetland Research Park at the Ohio State University, Annual Report 1999,
p. 41-46.
Zhang, L., and Mitsch, W.J., 2001, Water budgets of the two Olentangy River
experimental wetlands in 2000, in Mitsch, W.J., and Zhang, L., eds., Olentangy
River Wetland Research Park at the Ohio State University, Annual Report 2000,
p. 17-28.
Zhang, L., and Mitsch, W.J., 2002, Water budgets of the two Olentangy River
experimental wetlands in 2001, in Mitsch, W.J., and Zhang, L., eds., Olentangy
River Wetland Research Park at the Ohio State University, Annual Report 2001,
p. 23-34.
Zhang, L., and Mitsch, W.J., 2004, Water budgets of the two Olentangy River
experimental wetlands in 2003, in Mitsch, W.J., Zhang, L., and Tuttle, C., eds.,
Olentangy River Wetland Research Park at the Ohio State University, Annual Report
2003, p. 39-52.
Zimmerman, A.R., and Nance, J.M. 2001, Effects of hypoxia on the shrimp industry of
Louisiana and Texas, in Rabalais, N.N., and Turner, R.E., eds., Coastal hypoxia
Consequences for living resources coastal and estuarine studies: Washington, D.C.,
American Geophysical Uniton, chap. 15, v. 58, p. 293-310.
Zou, E., 2006, Impacts of hypoxia on physiology and toxicology of the brown shrimp
Penaeus aztecus: Paper presented at Hypoxia Effects on Living Resources in the
Gulf of Mexico, September 25-26, 2006, Tulane University, new Orleans, Louisiana,
sponsored by National Oceanic and Atmospheric Administration Center for
Sponsored Coastal Ocean Research.
275
-------
Appendix A
Appendices
A. Appendix A: Studies on the Effects of Hypoxia on Living Resources
The abstracts in this appendix all came from a workshop sponsored by the NOAA
Center for Sponsored Coastal Ocean Research, held at Tulane University, New Orleans,
LA held September 25-26, 2006.
Brouwer, Marius, 2006. "Changes in Gene and Protein Expression and Reproduction in
Grass Shrimp, Palaemonetes pugio, Exposed to Chronic Hypoxia" Presentation at
"Hypoxia Effects on Living Resource in the Gulf of Mexico" NOAA Center for
Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA. September 25
- 26, 2006.
Abstract: Hypoxic conditions in estuaries are one of the major factors responsible for declines in
habitat quality. Previous studies examining the effects of hypoxia on Crustacea have focused on
individual/population-level, physiological or molecular responses but have not considered more
than one type of response in the same study. The objective of this study was to integrate
disciplines by examining the responses of grass shrimp to chronic hypoxia both at the molecular
and whole animal level. Hypoxia-induced alterations in gene expression were screened using
custom cDNA macroarrays containing 78 clones from a hypoxia-responsive suppression
subtractive hybridization (SSH) cDNA library. Grass shrimp respond differently to moderate (2.5
ppm DO) versus severe (1.5 ppm DO) chronic hypoxia. The initial response to moderate hypoxia
was down-regulation of genes coding for ribosomal proteins, HSP 70 and MnSOD. The initial
response after short-term (3 d) exposure to severe hypoxia was upregulation of genes involved in
oxygen uptake/transport and energy production, such as hemocyanin and ATP synthases. The
major response by day 7 was an increase of transcription of genes present in the mitochondrial
genome, together with upregulation of a putative heme binding protein and the iron storage
protein, ferritin. By day 14 a dramatic reversal was seen, with a significant downregulation of
transcription of genes in the mitochondrial genome. Both ferritin and the heme binding protein
were downregulated as well. Levels of Hypoxia Inducible Factor (HIFl-alpha) remained
unchanged. The macroarray data were validated using real-time qPCR. Changes in mitochondrial
proteins were examined by separating proteins in 2 dimensions (IEF and reverse phase) followed
by MS. At the organismal level, hypoxia exposure resulted in marked effects on shrimp egg
production and larval survival, suggesting population-level implications of long-term hypoxia.
Baltz, Donald M., Hiram W. Li, Philippe A. Rossignol, Edward J. Chesney and
Theodore S. Switzer, 2006. "A Qualitative Assessment of the Relative Effects of
Bycatch Reduction,Fisheries and Hypoxia on Coastal Nekton Communities in the Gulf of
Mexico", Presentation at "Hypoxia Effects on Living Resource in the Gulf of Mexico"
NOAA Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans,
LA. September 25 - 26, 2006.
Abstract: We applied qualitative mathematical models to develop an understanding of linkages
that influence shrimp, fishes, and fisheries in coastal Louisiana where biotic communities face
many natural and anthropogenic stressors, one of which is fishing activities related to the harvest
of shrimp. Shrimp trawling ranks high in terms of impact on nekton and their habitats, and like
most fishing gears catches non-target species or sizes that are not marketed. These individuals,
termed 'bycatch', are often returned to the water in dead or dying condition. Numerous other
A-l
-------
Appendix A
individuals are not 'caught' per se but also suffer the 'effects of fishing', that can degrade habitats
or cause injuries leading to mortality. Modeling was used to examine the effects of fishing and
bycatch mortality on community structure in the 'Fertile Fisheries Crescent' and how major
stressors interact with hypoxia to influence fisheries. We explored direct and indirect interactions
between shrimp, their predators, bycatch species, and shrimp landings. A major finding was that
bycatch reduction efforts may feedback on fisheries and shrimp populations in an unexpectedly
negative manner. Another was that changes in community structure that might be attributed to
hypoxia are also possible from fishing alone. To corroborate our models, we analyzed 15 years of
quantitative data on National Marine Fisheries Service shrimp landings, Louisiana Department of
Wildlife and Fisheries (LDWF) gillnet surveys, and LDWF shrimp trawl surveys from central
Louisiana. Abundant bycatch and other species were summarized into several functional groups
including small and large shrimp predators, non-shrimp predators, major bycatch consumers,
minor bycatch consumers, and non-bycatch consumers. Factor and correlation analyses of
quantitative data for functional groups on a bimonthly basis corroborated results from the
qualitative models, and combined indicated that shrimp abundance and shrimp landings would
likely suffer from increased natural mortality if the shrimp-fishery bycatch was substantially
reduced.
Craig, J. Kevin and Larry B. Crowder, 2005. "Hypoxia-induced habitat shifts and
energetic consequences in Atlantic croaker and brown shrimp on the Gulf of Mexico
shelf Marine Ecology Progress Series, Vol. 294, pp 79-94.
Abstract: This paper evaluates the effects of hypoxia-induced habitat loss on Atlantic croaker and
brown shrimp. The compare spatial distributions and the relationship to abiotic factors, including
temperature, dissolved oxygen and salinity across years with differing levels of hypoxia using 14
years of fishery-independent trawl data. They find that hypoxia results in considerable shifts in
temperature and oxygen conditions that croaker and brown shrimp experience. Croaker typically
occupy relative warm, inshore waters. During periods of hypoxia, croaker remain in the warmest
inshore waters, but are also displaced to cooler offshore waters. Brown shrimp typically are
distributed more broadly and further offshore. During periods of hypoxia, brown shrimp shift to
warm inshore waters and cooler waters near the offshore edge of the hypoxic zone. The shifts in
spatial distribution are reflected in decreases in water temperature for croaker that are displaced
offshore the hypoxic region, and increases in water temperature for brown shrimp that are displace
inshore of the hypoxic zone. Both species also face increased variance in water temperatures due
to hypoxia-induced habitat displacement. Despite avoidance of the lowest oxygen waters, high
densities of croaker and brown shrimp occur in areas of 1.6 to 3.7 mg/1 near the offshore hypoxic
edge. Shifts in spatial distribution during severe hypoxia may impact organism energy budgets.
For example, laboratory studies indicate low oxygen impacts individual movement, growth, and
mortality (Wannamaker & Rice 2000, Taylor & Miller 2001, Wu 2002). High croaker and shrimp
densities near the hypoxic edge likely have implications for trophic interactions as well as the
harvest of both target (brown shrimp) and nontarget (croaker) species by the commercial shrimp
fishery. Croaker may benefit from high concentrations of brown shrimp at the edge of the hypoxic
zone, while brown shrimp may become more susceptible to predation by croaker.
Craig, J. Kevin, Larry B. Crowder, and Tyrrell A. Kenwood, 2005. "Spatial distribution
of brown shrimp (Farfantepenaeus aztecus) on the northwestern Gulf of Mexico shelf:
effects of abundance and hypoxia" Canadian Journal of Fisheries and Aquatic Science.
Vol. 62 pp 1295-1308.
Abstract: This paper uses fishery-independent hydrographic and bottom trawl surveys from 1983-
2000 used to test for density dependence and effects of hypoxia on spatial distribution of brown
shrimp. The spatial distribution of shrimp was found to be positively related to abundance on the
Texas shelf, but negatively related to abundance on the Louisiana shelf. Density dependence was
weak, and may have been due to factors other than habitat selection. Large-scale hypoxia (up to
A-2
-------
Appendix A
~20 000 km2) on the Louisiana shelf occurs in regions of typically high shrimp density, resulting
in loss of up to 25% of shrimp habitat on the Louisiana shelf. They also find shifts in distribution
and densities both inshore and offshore of the hypoxic region. Results placed in terms of the
generality of density-dependent spatial distributions in marine populations. Potential consequences
of habitat loss and associated shifts in distribution due to low dissolved oxygen. They note that
shifts in spatial distribution may precede major stock declines, and thus could potentially serve as
an early warning sign of future declines in abundance (Gomes et al. 1995; Rose et al. 2000;
Overholtz 2002).
Diaz, Robert, 2001. "Overview of Hypoxia around the World" Journal of Environmental
Quality Vol. 30, No. 2. (March-April) 275-281.
Abstract: This paper summarizes effects of hypoxia in various locations around the world, which
provides lessons for potential consequences of hypoxia in the Gulf of Mexico. They note that
hypoxia was probably not a prominent feature of the shallow continental shelf in the Northern
Gulf of Mexico prior to the 1920's through 1950's based on geo-chronology of sediment cores. A
longer, 2000-year chronology in the Chesapeake indicates that early European settlement of the
watershed was a key feature that set the stage for current oxygen problems. Improved water
quality in Lake Erie is the best example in the US that large ecosystems can respond positively to
nutrient regulation, but the time interval for recovery can be long. In Lake Erie, the extent of
hypoxia was similar between 1970 and 1990 despite reduced nutrient loads. Delayed
improvements in oxygen levels are argued to be consistent with mechanisms and processes that
contribute to ecosystem's resilience (Charlton et al, 1993), and as a consequence improvements in
oxygen may not be noticed for decades following implementation of management actions.
Hendon, Laura A. Erik A. Carlson, Steve Manning, and Marius Brouwer, 2006. "Cross-
talk between Pyrene and Hypoxia Signaling Pathways in Embryonic Cyprinodon
variegates" Presentation at "Hypoxia Effects on Living Resource in the Gulf of Mexico"
NOAA Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans,
LA. September 25 - 26, 2006.
Abstract: The aryl hydrocarbon nuclear translocator (ARNT) is a general dimeric partner for the
aryl hydrocarbon receptor (AhR) and hypoxia-inducible factor one alpha (HIFl-a). The
AhR/ARNT complex binds to promoters in target genes, such as CYP1 Al, resulting in alterations
in gene expression, while the HIFl-o/ARNT heterodimer binds to hypoxia response elements in
target genes, such as VEGF. While AhR is activated by PAHs, such as pyrene, HIFl-a is
activated by hypoxia. Since ARNT is a general dimeric partner for both AhR and HIFl-a,
possible cross-talk may exist between the two pathways in which the activation of one results in
inhibition of the other. The objective of this study was to determine if pyrene-activation of AhR2,
or hypoxia-activation of HIFl-a could sequester the ARNT protein away from HIFl-a and AhR2,
respectively, resulting in reduced developmental toxicity associated with hypoxia or pyrene alone
in embryonic Cyprinodon variegatus. As a first step to examine this hypothesis, we cloned AhR2,
CYP1A1 (PAH-activated gene) and VEGF (HIF-activated gene). Next, pyrene (20, 60, and 150
ppb) and hypoxia's (1-2 ppm) individual developmental toxicity endpoints were determined,
together with CYP1 Al and VEGF expression levels using real-time quantitative RT-PCR.
Combined treatments of pyrene and hypoxia were examined in order to determine sequestration of
the ARNT protein and developmental toxicity endpoints. Results demonstrate that pyrene-treated
embryos alone develop toxicity endpoints such as pericardial edema and dorsal body curvature.
Hypoxia-treated embryos alone display delayed hatching and less-developed characteristics in
comparison to normoxic treatments. Under hypoxic conditions alone, real-time quantitative RT-
PCR determined that VEGF was down-regulated significantly at 24 hpf, while at 14 dph, the HIF-
activated gene was significantly up-regulated. Pyrene-treated embryos showed a dose-dependent
and time-dependent response in CYP1 Al regulation with increasing expression over time of
A-3
-------
Appendix A
exposure. The combined effects of pyrene and hypoxia appeared to alter VEGF expression, while
CYP1A1 remained unaffected in C. variegatus.
Montagna, Paul, Ben Hodges, David Maidment and Barbara Minsker, 2006. "Long-
Term Studies of Hypoxia in Corpus Christi Bay: The Cybercollaboratory Testbed"
Presentation at "Hypoxia Effects on Living Resource in the Gulf of Mexico" NOAA
Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA.
September 25 - 26, 2006.
Abstract: Corpus Christi Bay is a shallow (~3.2 m) enclosed bay with a level bottom. It
experiences high wind speeds, temperatures, and receives a low amount of fresh water inflow.
Hypoxia has been documented in the southeastern region of Corpus Christi Bay every summer
since 1988. Hypoxia found in bottom waters, usually within 1 m from bottom, when the bay is
stratified. Over the last 20 years, there has been increased surface water temperatures, but no
change in nutrient concentrations, which are low. Ecosystem processes during salinity
stratification likely drive the hypoxia, because respiration is stimulated and the surface and bottom
water masses are not mixing. Hypoxia causes reduced benthos abundance, biomass, and diversity.
The reduction is due to loss of deeper-dwelling organisms, and is likely a direct effect (stress or
death), and not an indirect effect (increased predation by exposure to the surface). There is
increased interest in developing real-time environmental forecasting and management to better
monitor and understand large-scale, event-based environmental phenomena, e.g., hypoxia and
flooding. A new project focuses on creating a new Corpus Christi Bay Observatory Testbed
Project to demonstrate how cyberinfrastructure can enable real-time forecasting from a
hydrographic information system. Although only a few months old, the testbed project has
already created a few simple models and visualization tools that improved sampling designs to
better identify hypoxic events, extent, and intensity.
O'Connor, Thomas and David Whitall, 2007. "Linking Hypoxia to Shrimp Catch in the
Northern Gulf of Mexico", Marine Pollution Bulletin Vol. 54, no. 4 (April), Pp 460-463.
Abstract: This study carries out updates the statistical analysis of Zimmerman and Nance () of the
effect of hypoxia on commercial shrimp landings data for 1985 through 2004. This study uses
commercial landings data, not the interview data, and is therefore does not use spatial data on the
location of catch. The paper confirms the results of Zimmerman and Nance that there is no
correlation of hypoxic area with landings of white shrimp or with landings of brown shrimp in
Louisiana, but there is a significant correlation with the total combined landings in Texas and
Louisiana. Unlike Zimmermann and Nance, they find a significant relationship between the
hypoxic area and brown shrimp landings in Texas alone. Hypoxia explains about 32% of the
variance in catch using data for catch in July and August, and about 27% of the variance in catch
using annual data.
Perez, Amy N., Leon Oehlers and Ronald B. Walter, 2006. "Detection of Hypoxia-
related Proteins in Medaka (Oryzias latipes) by Difference Gel Electrophoresis and
Identification by Sequencing of Peptides using MALDI-TOF Mass Spectrometry"
Presentation at "Hypoxia Effects on Living Resource in the Gulf of Mexico" NOAA
Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA.
September 25 - 26, 2006.
Abstract: Multidimensional separation techniques combined with matrix-assisted laser
desorption/ionization tandem time-of-flight mass spectrometry (MALDI-TOF/TOF-MS) were
used to identify hypoxia-related biomarker proteins in tissues of medaka fish (Oryzias latipes) and
medaka cultured cells. The multidimensional protein/peptide separation methods used included
A-4
-------
Appendix A
two-dimensional difference gel electrophoresis (2D-DIGE) using fluorescent cyanine dyes, and
gel electrophoresis combined with reversed phase liquid chromatrography of tryptic peptides
isotopically labeled with 16O or ISO (geLC-MS). In both methods, control and hypoxia-treated
tissue or cell protein extracts were differentially labeled, combined in 1:1 mass ratios, and
subjected to separation and MALDI-TOF/TOF-MS analysis of tryptic peptides derived from
proteins exhibiting significant changes in expression upon hypoxia exposure. Prior to MALDI-
TOF/TOF-MS analysis, the peptides were N-terminally sulfonated using the derivatizing reagent
4-sulfophenyl isothiocyanate (SPITC) to enhance the post-source decay (PSD) fragmentation
spectra of the peptides in MALDI-TOF/TOF-MS, which was shown to dramatically improve de
novo sequencing of labeled peptides. The methods described here were used to monitor and
analyze the changes in protein resulting from exposures of both cultured medaka cells and medaka
fish to hypoxic conditions (0.8-1.0 mg/L dissolved oxygen) for periods up to 120 hours. We have
identified a number of potential candidate biomarker proteins differentially-regulated upon
exposure to hypoxia, including carbonic anhydrase, hemoglobin, calbindin, aldolase, glutathione-
S-transferase, succinate dehydrogenase, and lactate dehydrogenase.
Rabalais, Nancy N. 2006. "Benthic Communities and the Effects of Hypoxia in
Louisiana Coastal Waters" Presentation at "Hypoxia Effects on Living Resource in the
Gulf of Mexico" NOAA Center for Sponsored Coastal Ocean Research, Tulane
University, New Orleans, LA. September 25 - 26, 2006.
Abstract: The responses of the benthic fauna to decreasing concentration of dissolved oxygen
follow a fairly consistent pattern of progressive stress and mortality as the oxygen concentration
decreases from 2 mg 1-1 to anoxia (0 mg 1-1). Motile organisms (fish, portunid crabs,
stomatopods, penaeid shrimp and squid) are seldom found in bottom waters with oxygen
concentrations less than 2 mg 1-1. Below 1.5 to 1 mg 1-1 oxygen concentration, less motile and
burrowing invertebrates exhibit stress behavior, such as emergence from the sediments, and
eventually die if the oxygen remains low for an extended period. At minimal concentrations just
above anoxia, sulfur-oxidizing bacteria form white mats on the sediment surface, and at 0 mg 1-1,
there is no sign of aerobic life, just black anoxic sediments. The composition of the benthic
communities reflects differences in sedimentary regime, seasonal input of organic material and
seasonally severe hypoxia/anoxia. Decreases in species richness, abundance and biomass of
organisms are dramatic when bottom-waters are affected by severe hypoxia/anoxia. Some
macroinfauna, the polychaetes Ampharete and Magelona and a sipuculan Aspidosiphon, are
capable of surviving extremely low dissolved oxygen concentrations and/or high hydrogen sulfide
concentrations. Macroinfauna, primarily opportunistic polychaetes, increase in the spring
following flux of primary produced carbon, and increase to a lesser extent in the fall following the
dissipation of hypoxia. Fewer taxonomic groups characterize the severely affected benthos, and
long-lived, higher biomass and direct-developing species are mostly excluded. Suitable feeding
habitats (in terms of severely reduced populations of macroinfauna that may characterize
substantial areas of the seabed) are frequently removed from the foraging base of demersal
organisms, including the commercially important penaeid shrimps.
Switzer, Theodore S., Edward J. Chesney, and Donald M. Baltz, 2006. "Habitat Selection
by Flatfishes along Gradients of Environmental Variability: Implications for
Susceptibility to Hypoxia in the Northern Gulf of Mexico" Presentation at "Hypoxia
Effects on Living Resource in the Gulf of Mexico" NOAA Center for Sponsored Coastal
Ocean Research, Tulane University, New Orleans, LA. September 25 - 26, 2006.
Abstract: Although eutrophication in the northern Gulf of Mexico contributes to the high fisheries
productivity characteristic of the region, nutrient over-enrichment leads to the seasonal formation
of hypoxic (< 2 mg L-l O2) bottom water along the Louisiana-Texas continental shelf. Despite an
increase in the magnitude and duration of hypoxic episodes in recent decades, fisheries landings
A-5
-------
Appendix A
have remained high; nevertheless, hypoxia remains a persistent threat to the long-term
sustainability of regional fisheries production. The greatest threat to mobile nekton is likely the
influence of reduced dissolved oxygen concentrations on habitat quality, potentially forcing the
movement of individuals and/or prey from generally favorable habitats. At the population level,
these movements may result in altered spatial distributions that reflect selection of resources along
gradients of environmental variability. To unravel the potential influence of hypoxia on the
distribution of nekton, we examined patterns of habitat use by several abundant flatfishes based on
data collected during summer SEAMAP groundfish surveys from 1987 to 2000. Results from
habitat suitability analyses indicated that most flatfishes selected a restricted range of suitable
depths, temperatures, and salinities. Although most flatfishes were tolerant of moderately-low
dissolved oxygen concentrations, hypoxic environments were generally avoided, indicating that
hypoxia likely renders large areas of the Gulf of Mexico unsuitable. In comparisons of spatial
habitat suitabilities between years of moderate (< 15,000 km2) and severe hypoxia (>15,000 km2),
all flatfishes exhibited a reduction in the suitability of areas immediately west of the Mississippi
River and a concomitant increase in suitability within adjacent areas. Altered spatial distributions
corresponded to species-specific suitabilities along depth, temperature, and salinity gradients,
indicating that habitat suitability analyses may be effective in predicting population-level
responses to hypoxic episodes.
Wells, Melissa C., Zhenlin Ju, Sheila J. Heater and Ronald B. Walter, 2006. "Microarray
Gene Expression Analyses in Medaka (Oryzias latipes) Exposed to Hypoxia"
Presentation at "Hypoxia Effects on Living Resource in the Gulf of Mexico" NOAA
Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA.
September 25 - 26, 2006.
Abstract: We are investigating the genomic and proteomic effects of hypoxia exposure using the
Japanese medaka (Oryzias latipes) aquaria fish model as a tool for biomarker discovery. We have
developed a hypoxia exposure system allowing programmable exposure scenarios and have
initiated experimental assessment of changes in gene expression and protein abundance using
microarray and 2D-DIGE gel analyses of hypoxia exposed fish. We present the design,
construction, validation, and subsequent use of a medaka 8,046 (8K) unigene oligonucleotide
microarray to begin the study of hypoxia exposure. Array performance was validated via serf-serf
hybridization. Optimization of sample size needed for robust array data, based upon the number
features detected and the signal intensity, suggest 2 ug total RNA as a starting template for
amplification is sufficient. For treatment, adult medaka are exposed to a hypoxic environment of
4% dissolved oxygen (DO) for 2 days and then the DO lowered to 2% for an additional 5 days.
Upon sacrifice, changes in gene expression in brain, liver, skin, and gill tissues of these fish were
assessed in conjunction with matched control fish exposed similarly to 18% DO. Analyses of
array results identified 501 features from brain, 442 from gill, and 715 features from liver that
exhibit statistically significant changes in transcript abundance upon hypoxia exposure. Nine
features were found to exhibit common expression patterns between all three tissues. Data mining
of the array results suggest hypoxic exposure results in a general slowdown of metabolic function.
Real-time PCR was then employed to support the microarray results and this independent
validation agreed well with the microarray findings. Overall these results indicate the medaka
microarray will be a sound diagnostic tool for changes in gene expression due to hypoxia
exposure.
Zimmerman, Roger J. and James M. Nance, 2001. "Effects of Hypoxia on the Shrimp
Industry of Louisiana and Texas" Chapter 15 in Rabalais, N.N. and R.E. Turner, Coastal
Hypoxia: Consequences for Living Resources Coastal and Estuarine Studies, 58 pp 293-
310.
A-6
-------
Appendix A
Abstract: This study carries out a statistical test for effects of hypoxia on commercial catch of
shrimp in the Gulf of Mexico for 1985-97. The analysis combines landings data and interview
data on fishing effort, catch and location of each trip. The analysis is spatially explicit, based on
catch in 9 statistical subareas in Louisiana and Texas, with each subarea divided into 10 depth
zones. Zimmerman and Nance found no correlation of hypoxic area with landings of white shrimp
or with landings of brown shrimp in Louisiana, but they found a statistically significant
relationship between hypoxia and combined landings in Texas and Louisiana. The finding of no
relationship for white shrimp is consistent with prior expectations, because white shrimp are less
sensitive to hypoxia (Renaud, 1986), and because white shrimp habitat is mostly in-shore the
hypoxic region. In comparison, brown shrimp travel from inshore areas to offshore in order to
spawn. Since brown shrimp migrate through the hypoxic region, they are more likely to be
effected by hypoxia. The absence of a significant relationship between the size of the hypoxic
region and catch of brown shrimp in Louisiana may be explained by the fact that much of the
catch in Louisiana occurs in-shore of the hypoxic region, while catch in Texas occurs offshore.
Zou, Enmin, 2006. "Impacts of Hypoxia on Physiology and Toxicology of the Brown
Shrimp Penaeus aztecus" Presentation at "Hypoxia Effects on Living Resource in the
Gulf of Mexico" NOAA Center for Sponsored Coastal Ocean Research, Tulane
University, New Orleans, LA. September 25 - 26, 2006.
Abstract: The brown shrimp, Penaeus aztecus, in the northern Gulf of Mexico is faced with dual
stresses of environmental hypoxia, which occurs as a result of oxygen depletion from microbial
decomposition of organic materials from algal blooms, and pollution from polycyclic aromatic
hydrocarbons (PAHs) from petroleum and gas production on the continental shelf of the northern
Gulf of Mexico. This study aimed to address the questions of 1) whether the presence of PAH
contamination makes penaeid shrimps more susceptible to hypoxia and 2) whether hypoxia can
promote PAH bioaccumulation in penaeid shrimps. The susceptibility of shrimps to hypoxia was
represented by the oxyregulating capacity, a physiological parameter that describes how well an
animal regulates its oxygen consumption when subjected to hypoxia. It was found that acute
exposure to naphthalene significantly reduced the oxyregulating capacity of Penaeus aztecus. An
ensuing consequence of a decrease in oxyregulating ability is that the stress from the lack of
oxygen would set in sooner in the presence of PAH contamination than when shrimps are in the
clean environment. Hypoxia was found to have no significant effect on naphthalene
bioaccumulation in Penaeus aztecus. The absence of a significant effect was attributed to increased
naphthalene metabolism in the brown shrimp subjected to hypoxia.
A-7
-------
Appendix B
B. Appendix B: Flow Diagrams and Mass Balance of Nutrients
Global Material Cycles
For the reader's information, the following flow diagrams of the global nitrogen,
phosphorus and silicon cycles are taken from the Encyclopedia of Earth, a new electronic
reference about the Earth, its natural environments, and their interaction with society. The
Encyclopedia is a free, fully searchable collection of articles written by scholars,
professionals, educators, and experts (http://www.eoearth.org/eoe/about). Its contents
may be freely copied and distributed with proper attribution. These diagrams are not the
deliberative products of the SAB Panel but are provided to illustrate important processes
discussed in this report including: fertilization, nitrogen fixation, nitrification,
denitrification, ammonification, nutrient assimilation, sedimentation, recycling from
sediment, and weathering of rocks. Chemical equations representing processes depicted
in the flow diagrams are available from many sources in the published literature
including standard textbooks on biogeochemistry, limnology, and oceanography.
-------
Appendix B
1<3
LU
flows dominated by human actions
flows dominated by bacteria
other flows
Figure 54: Nitrogen Cycle Flow Diagram. Taken from Encyclopedia of Earth (2007) at
http:www.eoearth.org/global_material_cycles.
B-9
-------
Appendix B
HUMAN ACTIVITY
flows dominated by human actions
other flows
Figure 55: Phosphorus Cycle Flow Diagram. Taken from Encyclopedia of Earth (2007) at
http:www.eoearth.org/global_material_cycles.
B-10
-------
recycled in
surface layer
upwelling
recycled in the
deep
Appendix B
biogenic silica
production
recycled at-the
exported toward
the deep ocean
biogenk silica
to sediments
hydro thermal
T3
^X
n
wind
I
Figure 56: Silicon Cycle Flow Diagram. Taken from Encyclopedia of Earth (2007) at
http:www.eoearth.org/global_material_cycles.
B-ll
-------
Appendix B
Atmospheric deposition
The Integrated Assessment concluded that atmospheric deposition as a new
nitrogen input to the Mississippi River basin was not as important as agricultural sources
but that deposition nonetheless was a significant source (Goolsby et al., 1999).
Atmospheric deposition of nitrogen generally shows a trend of increasing from west to
east in the Mississippi basin, and deposition was a particularly important source of
nitrogen in the Ohio River basin (Goolsby et al., 1999). The Integrated Assessment
followed the net anthropogenic nitrogen input (NAM) budgeting approach established by
the International SCOPE Nitrogen Project in assuming that deposition of oxidized
nitrogen (NOy) is a new input of nitrogen while the deposition of ammonium is not but
rather is a recycling of nitrogen emitted to the atmosphere from agricultural sources
within the basin (Howarth et al., 1996). The oxidized nitrogen is presumed to come
largely from fossil-fuel combustion and, thus, is not accounted for in any other input to
the budget (Howarth et al., 1996; Goolsby et al., 1999). The Integrated Assessment
further considered that the deposition of organic nitrogen was a new input of nitrogen
(Goolsby et al., 1999).
The Integrated Assessment used monitoring data to estimate NOy deposition and
made a very rough guestimate for the magnitude of deposition of organic nitrogen. They
used data from the NADP for wet deposition and from CASTnet for dry deposition. This
yielded an average estimate of NOy deposition for the Mississippi River basin for the
time period 1988 to 1994 of 3.4 kg N/ha/yr (3 Ib N/ac/yr), of which 2 kg N/ha/yr (1.8 Ib
N/ac/yr) was nitrate in wet deposition and 1.4 kg N/ha/yr (1.25 Ib N/ac/yr) was NOy dry
deposition (Goolsby et al., 1999). The assessment estimated the deposition of organic
nitrogen as 1 kg N/ha/yr (0.89 Ib N/ac/yr), yielding a total estimate for new nitrogen
deposition of 4.4 kg N/ha/yr (3.9 Ib N/ac/yr) (Goolsby et al., 1999). This can be
compared with an estimate for NOy deposition derived from the GCTM model, which
estimates deposition rates from data on emissions to the atmosphere and on rates of
reaction and advection within the atmosphere (Prospero et al., 1996). For the Mississippi
River basin for essentially the same time period used in the Integrated Assessment, the
GCTM model suggested a total NOy deposition of 6.6 kg N/ha/yr (5.9 Ib N/ha/yr), with
6.2 kg N/ha/yr (5.5 Ib N/ac/yr) of this input being attributable to new inputs from fossil-
fuel burning and 0.4 kg N/ha/yr (0.36 Ib N/ac/yr) originating from natural sources
(Howarth et al., 1996).
Holland et al. (1999, 2005) noted that deposition estimates based on monitoring
data are typically lower than those from emission-based models across most of the United
States. For the northeastern United States from Maine through Virginia, the estimates
from the GCTM model (Howarth et al., 1996) are again almost twice as high as are
estimates from NADP and CASTnet monitoring data (Boyer et al., 2002). There are
many possible reasons for this discrepancy, but probably at least part of the problem lies
with an underestimation of dry deposition by the CASTnet program (Holland et al., 1999;
Howarth et al., 2006b; Howarth, 2006). Most CASTnet monitoring stations are
purposefully located away from emission sources, and deposition is likely to be higher
near these emission sources, creating a bias in the network. Further, the CASTnet
B-12
-------
Appendix B
program only estimates deposition of nitrogen in particles and deposition of nitric acid
vapor. The deposition of several other gases (including NO, NC>2, and nitrous acid vapor)
is not measured. Deposition of these gases, which would be included in the estimates
from the emission-based models, is likely to be particularly high near emission sources
(Howarth, 2006). Both the GCTM and TM3 models only estimate deposition at coarse
spatial scales, but a new emission-based model (CMAQ) shows promise for estimation at
relatively fine spatial scales (Robin Dennis, NOAA, personal communication). Note that
this model suggests very high NOy deposition rates near urban centers in the eastern US
and associated with power plant emissions in the Ohio River basin (Figure 57).
B-13
-------
Appendix B
TOTAL OXIDIZED NITROGEN
2001 BASE (J4f)-ANNUAL
(wet ox-n bias adjusted)
0.00 1
kg/hectare 1
148
PAUE
by
MCNC
January 1,2001 1:00:00
Min= 0.11 at (144,4), Max= 17.32 at (94,28)
Figure 57: Annual average deposition of NOy across the United States (kg N /hectare-year)
based on beta-testing runs of the CMAQ model. Note the very high rates of deposition in the
Ohio River basin. Courtesy of Robin Dennis. NOAA.
In the mass balance presented in Section 3.2, deposition was estimated as in Goolsby et
al. (1999). Organic N was not included, however, as there it is unclear what the
importance is of this form of N, or what an appropriate estimate would be (Keene et al.,
2002). A comparison was made of deposition inputs by region of the NOy estimate used
in the mass balance and deposition from the CMAQ model for 2001 . For the upper
Mississippi basin, NOy deposition was 4.2 kg N/ha/yr (3.8 Ib N/ac/yr), the same as the
CMAQ model1. For the Missouri basin both methods again gave similar estimates, with
NOy deposition of 2.2 N/ha/yr (2 Ib N/ac/yr), and CMAQ modeled deposition 2.1 kg
N/ha/yr (1.9 Ib N/ac/yr). For regions with more fuel combustion, the pattern was
different, with an Ohio basin NOy estimate of 5.0 N/ha/yr (4.5 Ib N/ac/yr), and the
CMAQ model estimate of 8.8 kg N/ha/yr (7.8 Ib N/ac/yr). For the lower Mississippi
River basin, NOy was 3.7 kg N/ha/yr (3.3 Ib N/ac/yr), and the CMAQ estimate 5. 1 kg
N/ha/yr (4.6 Ib N/ac/yr). Overall, this supports mass balance analysis that for the upper
Mississippi basin, atmospheric deposition is a small component of N inputs (about 8% of
N inputs) and is more important in the Ohio region (about 16% of N inputs using the
CMAQ model for 2001).
model unpublished results courtesy of Robin Dennis, NOAA, with analysis by states provided
by Dennis Swaney, Cornell University; unpublished.
B-14
-------
Appendix C
C. Appendix C: EPA's Guidance on Nutrient Criteria
In 2000, EPA recommended criteria to States and Tribes for use in establishing
their water quality standards consistent with section 303(c) of the Clean Water Act
(CWA). Under section 303(c) of the CWA, States and authorized Tribes have the
primary responsibility for adopting water quality standards as State or Tribal law or
regulation. The standards must contain scientifically defensible water quality criteria that
are protective of designated uses. On its website at
http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/, EPA provides
recommended criteria for nutrients in four major types of waterbodies - lakes and
reservoirs, rivers and streams (U.S. EPA, 2006b), estuarine and coastal areas, and
wetlands - across fourteen major ecoregions of the United States. The SAB Panel asked
EPA for a comparison of the SAB Panel's proposed 45% reductions for TN and TP flux
to the nutrient levels that would correspond to EPA's recommended ecoregional criteria.
Before presenting that preliminary analysis, the following caveats are stressed.
EPA's recommended ecoregional nutrient criteria are not laws or
regulations; they are guidance that States and Tribes may use as a starting
point for developing criteria for their water quality standards. Final
criteria developed by States and Tribes may have concentrations higher or
lower than EPA ecoregional recommendations, or, if scientifically
defensible, not include a nutrient if an impact on "designated use" was not
found.
EPA's recommended ecoregional nutrient criteria do not take into account
local site-specific conditions and "designated uses" for particular water
bodies (e.g., recreation, water supply, aquatic life, agriculture).
EPA's guidance for ecoregional nutrient criteria are based on ambient
concentrations of nutrients (expressed in mg/L or ug/L) in various
ecoregions. By contrast, the SAB Panel's recommended reductions of TN
and TP are based on flux (expressed in million metric tons of TN and TP
discharged at the mouth of the Mississippi River). A direct comparison of
concentrations to flux necessitates the simplifying assumption that
percentage reductions in concentrations have a one-to-one correspondence
with percentage reductions in flux.
EPA's guidance for ecoregional criteria is based on estimated "reference
conditions" i.e., reference sites chosen to represent the least culturally
impacted waters of the class existing at the present time. The estimated
reference conditions are based on the 25th percentile of the frequency
distribution of nutrient concentration data available for each ecoregion.
This assumption lends uncertainty to EPA's guidance for ecoregional
nutrient criteria.
C-15
-------
Appendix C
Given these caveats, the following analysis by EPA Office of Water's Office of
Science and Technology and EPA's Office of Research and Development allows some
comparison between EPA's guidance for ecoregional nutrient criteria and the SAB
Panel's proposed 45% nutrient reductions.
Comparison of SAB Nitrogen and Phosphorus Recommendations with EPA Nitrogen
and Phosphorus Criteria Recommended Reference Conditions - Submitted by EPA 's
Office of Water, 8-24-07.
Question: How do the 45% recommended reductions in nitrogen (N) and phosphorus
(P) at the mouths of the Mississippi and Atchafalaya Rivers compare with the 25th
percentile of TN and TP concentration data from ecoregions draining the Mississippi-
Atchafalaya River Basin (MARB)?
Answer: This question is addressed with a preliminary approach. A more thorough
approach is needed, but this would require a longer period of time.
The preliminary approach was developed by staff from the EPA Office of
Research and Development's Gulf Breeze Lab and the EPA Office of Water's Office
of Science and Technology using USGS loading estimates from the lower Mississippi
River at St. Francisville, LA and the Atchafalaya River at Melville, LA over the past
20 years. This approach compares the 45% reduction in nitrogen and phosphorus
recommended by the SAB, to the 25th percentiles of the distribution of data in EPA's
National Nutrient Database for total nitrogen (TN) and total phosphorus (TP) in each
aggregate nutrient ecoregion of the MARB. These 25th percentiles represent EPA's
approximated reference conditions for those ecoregions.
It is important to note that these 25th percentile values are not intended to be
implemented or promulgated directly as criteria. Rather, EPA developed the nutrient
criteria recommendations with the intent that they serve as a starting point for States
and Tribes to develop more refined criteria, as appropriate, to reflect local conditions.
States and Tribes may adopt criteria that are higher or lower than these 25th
percentiles. Text in two EPA documents help clarify the use of the ecoregional
reference condition values. See introductions to the ecoregional criteria documents at
http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/rivers/index.html and
EPA's Nutrient Criteria Technical Guidance Manual for Rivers and Streams
(http ://www. epa. gov/waterscience/criteria/nutrient/guidance/rivers/chapter_l .pdf).
Given this description, one can compare a 45% reduction in N and P measured
in two locations to the estimated reference conditions in each of the MARB
ecoregions to obtain a rough estimate of whether a 45% reduction could be more or
less stringent than what could result if EPA's recommended reference conditions
were adopted without further modification, as state water quality standards.
Data Sources: River flow and nutrient flux are monitored and computed by the U.S.
Geological Survey's (USGS) National Stream Quality Accounting Network
C-16
-------
Appendix C
(NASQAN) program at numerous river gauge stations in the Mississippi River Basin.
A description of the USGS NASQAN program, the flux estimation methodology and
the downloadable data records are available at http://toxics.usgs.gov/hypoxia/.
Monthly average nutrient concentrations were calculated from the USGS data as
The Monthly Average Nutrient Concentration = USGS Monthly Load/ Monthly
Average discharge rate where monthly average discharge rates for the mainstem
Mississippi River were calculated from daily discharge rates obtained at the Tarbert
Landing, MS gauge (ID = 01100).
Nitrogen. The median monthly nitrate concentration for the combined Mississippi
River at St. Francisville and Atchafalaya River at Melville over the period 1979 -
2007 is 1.24 mg/L. In comparison, historical data from the Mississippi River at St.
Francisville indicate that the median nitrate concentrations during the period 1955-
1970 was 0.6 mg/L.
Nitrate, as a component of TN is about 60% on average (based on USGS nutrient load
data); thus 1.24 mg/L nitrate would extrapolate to 2.07 mg/L TN.
A proposed 45% reduction of 2.07 mg/L TN would yield a concentration of 1.14
mg/LTN.
The relevant EPA recommended ecoregional reference conditions for TN are:
Ecoregion IV - 0.56 mg/L
Ecoregion V - 0.88 mg/L
Ecoregion VI - 2.18 mg/1
Ecoregion VII - 0.54 mg/L
Ecoregion IX - 0.69 mg/L
Ecoregion X - 0.76 mg/L
Ecoregion XI - 0.31 mg/L
These values range from 27% to 191% of the estimated 1.14 mg/L TN that would
result from a 45% reduction, with all but one value below 100% (the Corn Belt and
northern Great Plains ecoregion VI). This suggests that a 45% reduction of estimated
median monthly TN concentrations to 1.14 mg/L would likely be less stringent than
could be obtained if states adopted EPA's recommended reference condition values
into state water quality standards for TN.
Phosphorus. Using the same data (Mississippi River at St. Francisville and
Atchafalaya River at Melville, 1979-2007, monthly means), the median monthly
concentration of TP is 202 ug/L. Thus a 45% reduction of 202 ug/L TP would yield a
concentration of 111 ug/L.
The relevant EPA recommended ecoregional reference conditions for TP are:
C-17
-------
Appendix C
Ecoregion IV - 23.00 ug/L
Ecoregion V - 67.00 ug/L
Ecoregion VI - 76.00 ug/L
Ecoregion VII - 33.00 ug/L
Ecoregion IX - 36.00 ug/L
Ecoregion X - 128.00 ug/L
Ecoregion XI - 10.00 ug/L
These values range from 9% to 115% of the estimated 111 ug/L TP that would result
from a 45% reduction, with all but one value below 100% (the Texas-Louisiana
Coastal and Mississippi Alluvial Plains ecoregion X). This also suggests that a 45%
reduction of estimated median monthly TP concentrations tol 11 ug/L would likely be
less stringent than could be obtained if states adopted EPA's recommended reference
condition values into state water quality standards for TP.
A More Comprehensive Approach
A thorough comparison of the distribution approach to reference condition
estimation and the 45% reduction in TN and TP could be made by calculating the
nutrient concentrations from the USGS loading estimates at river gauge stations at
each of the nine subbasins. The USGS provides monthly or annual nutrient flux
estimates and river flow data from which nutrient concentration data can be derived
(http://toxics.usgs.gov/hypoxia/.) These data provide values over many years for 9
subbasins located within the MARB. The data could be used in the following steps to
compare the two sets of values:
Use the USGS nutrient loading data to compile a TN and TP concentration
dataset for each subbasin;
Calculate the median TN and TP concentrations at each of the nine subbasin
river gauge stations;
Overlay nutrient ecoregions on subbasins and extract nutrient ecoregional data
from subbasins. From this refined data set, calculate the median value of the
seasonal 25th percentiles of TN and TP for the ecoregion-subbasin.
These data can be used for the following comparisons:
1) Calculate the concentrations resulting from a 45% reduction in the median
concentration for each subbasin.
2) Compare these to the EPA 25th percentiles (ecoregional reference conditions) in
each subbasin, or specific subbasins of interest.
Submitted by EPA 's Office of Water, 8-24-07.
C-18
-------
Appendix D
D. Appendix D: Calculation of Point Source Inputs of N and P
As discussed in Sections 3.2 and 4.5.8, estimates of N and P fluxes from sewage
treatment plants from the MART (2006b) report were much lower for both total N and
total P than in the Integrated Assessment. As pointed out in the MART report, much of
this decline is thought to be due to the values assigned for total N and P concentrations in
sewage treatment plants effluent. Few measured data were used, but rather estimated
values were applied to most. Most estimates were made using a "typical pollutant
concentration" (TPC) for N or P based on the level of treatment. These TPC's were from
an update of values compiled in a report by Tetra Tech (1998). The 2006 MART report
assumed that sewage treatment plants with advanced wastewater treatment had TPC
values of 5.6 and 0.82 mg/L for total N and total P, respectively. The MART report then
applied these assumed values to estimated daily discharges to calculate an estimated daily
flux. The MART report further assumed that plants that had less than advanced
wastewater treatment had TPC values of 11.2 and 2.02 mg/L for total N and total P,
respectively, applied to estimated daily discharges to calculate an estimated daily flux.
The Panel is not comfortable with these assumptions and instead believes that most
wastewater treatment plants in the MARB had TPC's applied that were too low. The
Panel, therefore, adjusted the database, by using TPC's of 11.2 and 2.02 mg/L for total N
and total P, respectively, for plants with advanced wastewater treatment, and TPC's of 15
and 4 mg/L for total N and total P, respectively, for plants with less than advanced
treatment.
As an example of how these adjustments changed estimates, the Panel examined
seven Chicago plants (Stickney is the largest sewage treatment plant in the basin) and one
in Champaign-Urbana, IL, where measured flux data were available (daily to weekly
measurements of total N and total P and flow were made at each plant). From this
analysis, it is clear that the TPCs used in the MART report were not appropriate and gave
substantially lower flux estimates (Table 20) than the actual measured values. The
MART report indicated that each of these plants had advanced treatment, and therefore
applied their estimated TPCs of 5.6 mg/L total N and 0.82 mg/L total P respectively.
Most plants in the MARB do not have treatment processes (either biological or chemical)
to remove P, and much of the advanced treatment is to nitrify ammonium to nitrate,
because most are permitted for ammonia in effluent.
D-19
-------
Appendix D
Table 20: Comparison of MART estimated sewage treatment plant annual effluent loads of total N and P
and values from measurements at each plant for 2004.
Plant MART Measured % Diff MART Measure % Diff
d
tons N/yr tons P/yr
Stickney
Calumet
Lemont
Northside
Egan
Hanover Park
Kirie
Champaign-Urbana
6,282
1,799 (3,599)
13 (26)
2,259(4,518)
209 (418)
70(139)
201 (402)
77(155)
9,850
3,243
51
3,161
386
144
331
310
64
55
25
71
54
48
61
25
921
264 (650)
2(5)
331 (207)
31(75)
10 (73)
29 (73)
11 (28)
1,105
1,065
8
441
99
33
44
58
83
25
23
75
31
31
67
20
* All plants are in Illinois. Also shown in red is the recalculated MART value as described below, except
for Stickney, where actual values were used because of plant size and concentration considerations.
This analysis supports the Panel's use of increased TPC's when estimating point
source loads. Therefore, all plants that were labeled as advanced treatment and used the
Clean Water Needs Survey data for load estimates were recalculated using total N and P
concentrations of 11.2 and 2.02 mg/L, respectively (this included most plants, and some
were estimated using the permit compliance system data and were not recalculated).
These concentrations were much closer to the values reported by the plants in Table 20
(red values in table), although there still was considerable variability, and included 2,080
point sources (the total database has 33,302 point sources of all types). For plants
identified as receiving secondary treatment, total N and P concentrations of 15 and 4
mg/L, respectively, were applied (there were 4,480 plants of this type). For the seven
plants in Table 20 recalculated this way, total N and P fluxes were 113 and 91% of
measured values, respectively, much closer to the measured values than the original
MART values. The Panel's discussion of point sources in the MARB utilizes these
adjustments to the MART values. Finally, the Panel again emphasizes that measured
data are generally not available in these large databases, so that many assumptions need
to be made.
D-20
-------
Appendix E
E. Appendix E: Animal Production Systems
Intensification of animal feeding operations
Current census information shows that there has been an 18% increase in the
number of pigs in the U.S. over the last 10 years along with a 72% decrease in the
number of farms. Over the same 10 years, the number of dairies has decreased by 40%,
but herd size has increased by 50%. A similar trend in the poultry and beef industries has
also occurred, with 97% of poultry production in the U.S. coming from operations with
more than 100,000 birds and over a third of beef production from <2% of the feedlots
(Gardner, 1998). Fattened cattle numbers remained fairly constant from 1982 to 1997 but
the number of fattening operations decreased over 50 percent (Kellogg et al., 2000).
Overall, cattle, pig, and poultry numbers have increased 10% to 30%, while the number
of farms on which they were reared has decreased 40% to 70% over the last 10 years
(Gardner 1998).
Nutrient budgets
The large-scale consolidation has created much larger animal feeding operations,
which makes economical utilization and re-distribution of manure to croplands difficult
and has profound consequences for farm and regional nutrient transfer and management
within the MARB. For example, the accumulation of nutrients is first evident at the farm
scale, where N and P management is affected by daily operation decisions and the long-
term goals of each farmer. For example, the potential for P and N surplus on farms with
AFOs can be much greater than in cropping systems where nutrient inputs become
dominated by feed rather than fertilizer (Table 21). With a greater reliance on imported
feeds, only 30% of N and 29% of P in purchased feed for a 1280-hog operation on a 30-
ha farm could be accounted for in farm outputs. These nutrient budgets clearly show that
animal feed is the largest input of nutrients to farms with AFOs, and thus is the primary
source of on-farm nutrient excess, for which a resolution will require innovative
management. Current animal number and estimated manure N and P production within
the MARB is given in Table 22.
E-21
-------
Appendix E
Table 21: Farming System and Nutrient Budget.
Nutrient input in
Farming system
Feed Fertilizer
Nitrogen budget
Cash crop a - 95
Dairy b 155 40
Hogc 390 10
Poultry d 5800
Phosphorus budget
Cash crop a - 22
Dairy b 30 11
Hogc 105
Poultry d 1560
Output in
produce
92
75
120
1990
20
15
30
440
, Nutrient
Surplus ... .
r utilization
%
3 97
120 38
280 30
3810 34
2 91
26 37
75 29
1120 28
a 30 hectare cash crop farm growing corn and alfalfa.
b 40 hectare farm with 65 dairy Holsteins averaging 6600 kg milk cow"1 yr"1, 5 dry cows and 35 heifers.
Crops were corn for silage and grain and alfalfa and rye for forage.
0 30 hectare farm with 1280 hogs; surplus includes 36 kg P and 140 kg N ha"1 yr"1 manure exported from
the farm.
d 12 hectare farm with 74,000 poultry layers; surplus includes 180 kg P and 720 kg N ha"1 yr"1 manure
exported from the farm.
E-22
-------
Appendix E
Table 22: Number of animals and amount of manure produced and N and P excreted within the MARB
states based on information from the 1997 U.S. Census of Agriculture (data obtained from USDA-ERS,
http://ers.usda.gov/data/MANURE/).
Animal
type
M-T A i TT v Manure
# Farms Animal Units . t
excreted '
IVTa
Manure N
excreted
millir
Manure P
excreted
^n Vo
Beef
Dairy
Poultry
Swine
837,972
77,363
45,870
84,717
52,627,536
5,944,742
3,044,000
6,591,998
71,354,009
13,287,687
8,743,736
7,310,054
2,712
439
433
419
864
79
149
124
f Manure in dry state, as excreted adjusted for water content.
Nutrient Surpluses
USDA (2003) estimated the amount of manure produced from animal distribution
numbers from the 1997 U.S. Census of Agriculture, using standard values of manure
production and nutrient concentration for each animal type. Estimates of excess N and P
were calculated based on crop N and P removal and the assumption that all suitable crop
and pasture land was available for manure application (Figure 43 and Figure 44). Most
areas with CAFOs have some excess N (Figure 43) and P (Figure 44). These
distributions demonstrate that within the MARB, regional excesses were similar for N
and P.
Targeting Remedial Strategies Within the MARB
The importance of targeting nutrient management within a watershed is shown by
several MARB studies. In the early 1980's conservation practices were installed on
about 50% of the Little Washita River watershed (54,000 ha or 133,000 ac) in central
Oklahoma. Practices included construction of flood control impoundments, eroding gully
treatment, and conservation tillage (Sharpley and Smith, 1994; Sharpley et al., 1996).
Although conservation measures decreased N and P export 5 to 13 fold, there was no
effect on P concentration in flow at the outlet of the main Little Washita River watershed.
Thus, a lack of effective targeting of nutrient management and control of major sources
of nutrient export contributed to field or subwatershed scale responses not being
translated to reductions in nutrient export from the main Little Washita River watershed.
E-23
-------
Appendix E
Managing Manures
Manure application timing and method relative to rainfall influences the
concentration of N and P in runoff (Dampney et al., 2000; Sims and Kleinman, 2005).
For example, several studies have shown a decrease in N and P loss with an increase in
the length of time between manure application and surface runoff (Djodjic et al., 2000;
Edwards and Daniel, 1993a; Sharpley, 1997; Westerman et al., 1983). This decrease can
be attributed to the reaction of added P with soil and dilution of applied P by infiltrating
water from rainfall that did not cause surface runoff.
The incorporation of manure into the soil profile either by tillage or subsurface
placement decreases the potential for P loss in surface runoff. Rapid incorporation of
manure also reduces NH3 volatilization and potential loss in runoff as well as improving
the N:P ratio for crop growth. Mueller et al. (1984) showed that incorporation of dairy
manure by chisel plowing reduced total P loss in runoff from corn 20-fold, compared to
no-till areas receiving surface applications. In fact, P loss in runoff was decreased by a
lower concentration of P at the soil surface and a reduction in runoff with incorporation
of manure (Mueller et al., 1984; Pote et al., 1996). As with fertilizer application
methods, other factors are important in selecting or recommending the most appropriate
application method. Equipment availability, whether the soil is sufficiently free of rocks
to allow subsurface application, labor requirements, product availability, and availability
of operating capital all affect the application method decision.
Crop selected to receive manure application
Manure has traditionally been applied for corn or other grass production.
However, corn acreage to which manure is applied has not expanded proportionally to
animal operation expansions; thus the risks increase for applying manure in excess of the
amount necessary to meet crop nutrient requirements (Schmitt et al., 1996; Dou et al.,
1998). One solution to minimize these risks, and the subsequent potential risk of NOs
leaching to ground water, is to select alternative crops to receive manure applications.
Although legumes are not usually considered for manure application, soybean can
annually remove as much as 385 kg N/ha (344 Ib N/ac) (Shibles, 1998) and alfalfa as
much as 500 kg N/ha (446 Ib N/ac) (Russelle et al., 2001), compared to less than 200 kg
N/ha (179 Ib N/ac) for corn. Schmidt et al. (2000) demonstrated that nodulation in
soybean effectively compensated with additional N when manure N was insufficient to
meet crop demands; so if necessary, manure could be applied conservatively without risk
of applying too little to meet crop needs.
Rate and frequency of application
As might be expected, N and P loss in runoff increases with greater frequency and
rates of applied manure (Edwards and Daniel, 1993b; McDowell and McGregor, 1984).
Although rainfall intensity and duration, as well as when rainfall occurs relative to
applied manure, influence the concentration and overall loss of manure N and P in runoff,
the relationship between potential loss and application rate is critical to establishing
E-24
-------
Appendix E
environmentally sound nutrient management guidelines. Also evident is that the effect of
applied manure on increasing the concentration of P in surface runoff can be long lasting.
For instance, Pierson et al., 2001 found that a poultry litter application tailored to meet
pasture N demands elevated surface runoff P for up to 19 months after application.
Although few studies have evaluated the loss of P in surface runoff as a function of
application frequency, more frequent manure applications can be expected to rapidly
increases soil P (Haygarth et al., 1998; Sharpley et al., 1993; 2005; Sims et al., 1998),
with a concomitant increases in runoff P loss.
Intensity and duration of grazing
As beef grazing of pastures is an important component of animal production in
many regions of the MARB, careful management of grazing is needed to minimize P loss
and water quality impacts. The localized accumulations of P where manure is deposited
can saturate the P sorption capacity of a soil, increasing the potential for P loss from
grazed pastures in runoff or drainage waters. However, at a field and watershed scale, it
is likely that critical stocking factors, such as density and duration, will influence both
hydrologic and chemical factors controlling P transport. For example, Owens et al.
(1997) found that decreasing grazing density and duration dramatically reduced runoff
and erosion from a pastured watershed in Ohio. Clearly, increased runoff and erosion
with grazing will enhance the potential for P loss. In Oklahoma, Olness et al. (1975)
found that P losses were greater from continuously (4.6 kg P/ha/yr or 4.1 Ib P/ha/yr) than
rotationally grazed pastures (1.3 kg P/ha/yr or 1.2 Ib P/ha/yr). In fact, P losses with
continuous grazing were greater than from alfalfa or wheat (2.7 kg P/ha/yr (2.4 Ib
P/ha/yr); Olness et al., 1975). However, the work of Owens et al. (1997) does show that
when management is changed, the impacts of the previous grazing impacts were not long
lasting, changing within a year. Even so, there is a need to determine critical stocking
densities and durations as a function of grazing management.
Stream-bank fencing
By observing four pastured dairy herds with stream access over four intervals
during the spring and summer of 2003 in the Cannonsville Watershed south central, New
York, James et al. (2007) were able to estimate fecal P contributions to streams. In the
herds observed, on a per cow basis, cattle were especially likely to defecate in the stream,
although they spent a small proportion of their time there. On average, approximately
30% of all fecal deposits expected from a herd were observed to fall on land within 40-m
of a stream, and 7% fell directly into streams. Although amenities in pasture (such as
water troughs, feeders, salt, and shade located away from the stream) did affect where
cattle congregated, the stream demonstrated a consistent draw.
Using spatial databases of streams, pasture boundaries, and animal characteristics
(i.e., number of cattle, time in pasture, and type of cattle [heifers versus milk cows]) for
90% of the dairy farms in the Cannonsville watershed, approximately 3,600 kg (7,940 Ib)
of manure P are estimated as deposited directly into streams with 7,650 kg (16,900 Ib)
deposited in pasture near streams (<10 m) from the 11,000 dairy cattle in the watershed.
E-25
-------
Appendix E
At this magnitude, P loadings represent a significant environmental concern, with in-
stream deposits equivalent to approximately 12% of watershed-level P loadings attributed
to agriculture (Scott et al., 1998). Riparian shade can also attract grazing cattle and
influence P loss in stream flow.
E-26
------- |