United States Region III Region III EPA 903-R-03-002
Environmental Protection Chesapeake Bay Water Protection April 2003
Agency Program Office Division
In coordination with the Office of Water/Office of Science and Technology, Washington, DC
f \ Ambient Water Quality
'\Zj Criteria for Dissolved
PRO**-
Oxygen, Water Clarity and
Chlorophyll a for the
Chesapeake Bay and Its
Tidal Tributaries
April 2003
-------
Ambient Water Quality Criteria
for Dissolved Oxygen, Water Clarity
and Chlorophyll a for the Chesapeake Bay
and Its Tidal Tributaries
April 2003
U.S. Environmental Protection Agency
Region III
Chesapeake Bay Program Office
Annapolis, Maryland
and
Region III
Water Protection Division
Philadelphia, Pennsylvania
in coordination with
Office of Water
Office of Science and Technology
Washington, D.C.
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vii
Foreword
In order to achieve and maintain the water quality conditions necessary to protect
the aquatic living resources of the Chesapeake Bay and its tidal tributaries, the U.S.
Environmental Protection Agency (EPA) Region III has developed this guidance
document, entitled Ambient Water Quality Criteria for Dissolved Oxygen, Water
Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries (Regional
Criteria Guidance). This document presents the EPA's regionally-based nutrient and
sediment enrichment criteria expressed as dissolved oxygen, water clarity and chloro-
phyll a criteria, applicable to the Chesapeake Bay and its tidal tributaries. EPA is
issuing this guidance pursuant to Section 117(b) of the Clean Water Act and in accor-
dance with the water quality standards regulations (40 CFR Part 131).
This Regional Criteria Guidance provides EPA's recommendations to the Chesa-
peake Bay states for use in establishing their water quality standards consistent with
Section 303(c) of the Clean Water Act. Under Section 303(c), 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 and existing uses. EPA's water
quality standards regulations suggest three possible sources for establishing protec-
tive criteria: 1) guidance for water quality criteria recommendations published under
the authority of Section 304(a) of the Clean Water Act, 2) Section 304(a) guidance
modified to reflect site-specific conditions, or 3) other scientifically defensible
methods (see 40 CFR 131.11). Section 117 of the Clean Water Act authorizes a
Chesapeake Bay programs office to publish information pertaining to the environ-
mental quality of the Chesapeake Bay, as well as to coordinate Federal and state
efforts to improve the quality of the Bay.
Quantified water quality criteria contained within state or tribal water quality stan-
dards are essential to a water quality-based approach to pollution control. Whether
expressed as numeric criteria or quantified translations of narrative criteria within
state or tribal water quality standards, quantified criteria serve as a critical basis for
assessing the attainment of designated uses and measuring progress toward meeting
Foreword
-------
the water quality goals of the Clean Water Act and the Chesapeake 2000 agreement.
This Regional Criteria Guidance presents scientifically defensible methods and
serves as guidance for the states to use in developing appropriate and protective
Section 303 criteria and standards for the Chesapeake Bay EPA's Regional Criteria
Guidance is not law or regulation; it is guidance that states in the Chesapeake Bay
watershed may consider in the development and/or modification of appropriate
criteria for their water quality standards.
REBECCA W. HANMER, Director
Region III Chesapeake Bay Program Office
JON M. CAPACASA, Acting Director
Region III Water Protection Division
GEOFFREY H. GRUBBS, Director
Office of Science and Technology
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ix
Executive Summary
In order to achieve and maintain water quality conditions necessary to protect
aquatic living resources of the Chesapeake Bay and its tidal tributaries, the U.S.
Environmental Protection Agency (EPA) Region III has developed guidance, entitled
Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chloro-
phyll a for the Chesapeake Bay and Its Tidal Tributaries (Regional Criteria
Guidance). This final guidance is intended to assist the Chesapeake Bay states,
Maryland, Virginia and Delaware, and the District of Columbia, in adopting revised
water quality standards to address nutrient and sediment-based pollution in the
Chesapeake Bay and its tidal tributaries.
EPA Region III developed this guidance to promote the overall goals of the Clean
Water Act and specifically in accordance with the EPA National Strategy for the
Development of Regional Nutrient Criteria, announced in June 1998. This national
nutrient strategy laid out the EPA's intentions to develop technical guidance manuals
for four types of waters (lakes and reservoirs, rivers and streams, estuaries and
coastal waters and wetlands) and to produce criteria for specific nutrient eco-regions
(www.epa.gov/ost/standards/nutrient.html). In addition, the EPA is committed to
working with states and tribes to develop more refined and localized nutrient and
nutrient enrichment-related criteria based on approaches described in the water body
guidance manuals. The Regional Criteria Guidance provides the regional nutrient
guidance applicable to the Chesapeake Bay and its tidal tributaries.
EPA Region III developed the Regional Criteria Guidance in accordance with
Section 117(b) of the Clean Water Act using the multi-stakeholder approach to
implementing the Chesapeake 2000 agreement. Chesapeake 2000 was signed on
June 28, 2000, by the governors of Maryland, Pennsylvania and Virginia, the mayor
of the District of Columbia, the chair of the Chesapeake Bay Commission and the
Administrator of the U.S. EPA. Subsequently, the governors of Delaware, New York
and West Virginia signed a Memorandum of Understanding committing to im-
plement the Water Quality Protection and Restoration section of the agreement.
The water quality criteria and tidal-water designated uses presented in this document
are the product of a collaborative effort among the Chesapeake Bay Program
partners. They represent a scientific consensus based on the best available scientific
Executive Summary
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X
findings and technical information defining the water quality conditions necessary to
protect Chesapeake Bay aquatic living resources from effects due to nutrient and
sediment over-enrichment. Various stakeholder groups have been involved in their
development, with contributions from staff of federal and state governments, local
agencies, scientific institutions, citizen conservation groups, business and industry.
In the Regional Criteria Guidance the EPA recommends and expects that the numer-
ical criteria and refined designated uses will be considered by and appropriately
incorporated into the water quality standards of the Chesapeake Bay jurisdictions
with tidal waters—Maryland, Virginia, Delaware and the District of Columbia.
Using existing state authority and public process, each jurisdiction is expected to
consider and propose criteria and appropriate designated uses, subject to review and
approval by the EPA, that are consistent with the requirements of the Clean Water
Act. The EPA will consider the Regional Criteria Guidance in reviewing any state
submission regarding this issue. The guidance contained in this document is subject
to change with the synthesis and interpretation of future scientific findings.
REFINED DESIGNATED USES:
ESSENTIAL AQUATIC LIFE COMMUNITIES
EPA Region III has identified and described five habitats (or designated uses) that,
when adequately protected, will ensure the protection of the living resources of the
Chesapeake Bay and its tidal tributaries. Those five uses (see Figure 1) provide the
context in which EPA Region III derived adequately protective Chesapeake Bay
water quality criteria for dissolved oxygen, water clarity and chlorophyll a, which
are the subject of this Regional Criteria Guidance. Accurate delineation of where to
apply these tidal-water designated uses is critical to the Chesapeake Bay water
quality criteria. EPA Region III is publishing a Technical Support Document for the
Identification of Chesapeake Bay Designated Uses and Attainability, which provides
further information on the development and geographical extent of the designated
uses to which the criteria may apply.
The migratory flsh spawning and nursery designated use protects migratory and
resident tidal freshwater fish during the late winter to late spring spawning and
nursery season in tidal freshwater to low-salinity habitats. Located primarily in the
upper reaches of many Bay tidal rivers and creeks and the upper mainstem Chesa-
peake Bay, this use will benefit several species including striped bass, perch, shad,
herring, sturgeon and largemouth bass.
The shallow-water bay grass designated use protects underwater bay grasses and the
many fish and crab species that depend on the vegetated shallow-water habitat
provided by underwater grass beds.
The open-water fish and shellfish designated use focuses on surface water habitats
in tidal creeks, rivers, embayments and the mainstem Chesapeake Bay, and protects
diverse populations of sport fish, including striped bass, bluefish, mackerel and sea
trout, as well as important bait fish such as menhaden and silversides.
Executive Summary
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xi
The deep-water seasonal fish and shellfish designated use protects animals inhabiting
the deeper transitional water-column and bottom habitats between the well-mixed
surface waters and the very deep channels. This use protects many bottom-feeding fish,
crabs and oysters, and other important species such as the bay anchovy.
The deep-channel seasonal refuge designated use protects bottom sediment-
dwelling worms and small clams that bottom-feeding fish and crabs consume
naturally. Low to occasional no dissolved oxygen conditions occur in this habitat
zone during the summer.
Migratory Fish
Spawning and
, Nursery Use
Shallow-Water
Bay Grass Use
Open-Water
Fish and Shellfish Use
A. Cross-Section of Chesapeake Bay or Tidal Tributary
Deep-Water
Seasonal Fish and
Shellfish Use
X;
Deep-Channel
Seasonal Refuge Use
B. Oblique View of the Chesapeake Bay and its Tidal Tributaries
Shallow-Water
Bay Grass Use
Deep-Water
Seasonal Fish and
Shellfish Use
Open-Water
Fish and Shellfish Use
Deep-Channel
Seasonal Refuge Use
Figure 1. Conceptual illustration of the five Chesapeake Bay tidal water designated use zones.
Executive Summary
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xii
The Chesapeake Bay watershed states with tidally influenced Bay waters—Mary-
land, Virginia, Delaware and the District of Columbia—are ultimately responsible
for defining and formally adopting a refined set of designated uses into their respec-
tive water quality standards.
DISSOLVED OXYGEN CRITERIA
Oxygen is one of the most essential environmental constituents supporting life. In
the Chesapeake Bay's deeper waters, there is a natural tendency toward reduced
dissolved oxygen conditions because of the Bay's physical morphology and estu-
arine circulation. The Chesapeake Bay's highly productive shallow waters, coupled
with strong density stratification, long residence times (weeks to months), low tidal
energy and its tendency to retain, recycle and regenerate nutrients from the
surrounding watershed, all set the stage for low dissolved oxygen conditions.
Against this backdrop, EPA Region III has derived a set of dissolved oxygen criteria
to protect specific aquatic life communities (outlined above) and reflect the Chesa-
peake Bay's natural processes that define distinct habitats (Figure 1).
The derivation of these criteria followed the EPA's national guidelines; the EPA,
National Marine Fisheries Science and U.S. Fish and Wildlife Service's joint
national endangered species consultation guidelines; and the risk-based approach
used in developing the EPA's Virginian Province saltwater dissolved oxygen criteria
(for estuarine and coastal waters from Cape Cod, Massachusetts to Cape Hatteras,
North Carolina). The resulting criteria reflect the needs and habitats of Chesapeake
Bay estuarine living resources and are structured to protect five tidal-water desig-
nated uses (Figure 2).
Criteria for the migratory fish spawning and nursery, shallow-water bay grass and
open-water fish and shellfish designated uses were set at levels to prevent impairment
of growth, and to protect the reproduction and survival of all organisms (Table 1).
Criteria for deep-water seasonal fish and shellfish designated use habitats during
seasons when the water column is significantly stratified were set at levels to protect
juvenile and adult fish, shellfish and the recruitment success of the bay anchovy.
Criteria for deep-channel seasonal refuge designated use habitats in summer were set
to protect the survival of bottom sediment-dwelling worms and clams.
WATER CLARITY CRITERIA
Underwater bay grass beds in the Chesapeake Bay create rich animal habitats that
support the growth of diverse fish and invertebrate populations. Underwater bay
grasses, also referred to as submerged aquatic vegetation or SAV, help improve tidal
water quality by retaining nutrients as plant material, stabilizing bottom sediments
(preventing their resuspension) and reducing shoreline erosion. The health and
survival of these underwater plant communities in the Chesapeake Bay and its tidal
Executive Summary
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6
Migratory Spawning and
Nursery Habitats
Shallow-Water and
Open-Water Habitats
5
V*" Striped Bass: 5-6
American Shad: 5
White Perch: 5
4
Yellow Perch:' 5
Hard Clams: 5 ^1|
Deep-Water Habitats
3
„~ Alewife: 3.6
2
Crabs: 3 Bay Anchovy: 3
Deep-Channel Habitats
1
Spot: 2 1
0
Worms: "f
Figure 2. Dissolved oxygen (mg liter1) concentrations required by different Chesapeake Bay species
and communities.
tributaries depend on suitable environmental conditions. The loss of underwater bay
grasses from the shallow waters of the Chesapeake Bay, which was first noted in the
early 1960s, is a widespread, well-documented problem. The primary causes of the
decline of these underwater bay grasses are nutrient over-enrichment and increased
suspended sediments in the water, and associated reductions in light availability
( Figure 3). Other factors such as climatic events and herbicide toxicity may also have
contributed to the loss of bay grasses. In order to restore these critical habitats and
food sources, enough light must penetrate the shallow waters to support the survival,
growth and repropagation of diverse, healthy underwater bay grass communities.
EPA Region III has identified Chesapeake Bay water clarity criteria to establish the
minimum level of light penetration required to support the survival, growth and
continued propagation of underwater bay grasses. Using a worldwide literature
synthesis, an evaluation of Chesapeake Bay-specific field study findings, as well as
model simulation and diagnostic tools, the EPA derived Chesapeake Bay-specific
water clarity criteria for low and higher salinity habitats (Table 2).
The water clarity criteria, applied only during the bay grass growing seasons, are
presented in terms of the percent ambient light at the water surface extending
through the water column and the equivalent Secchi depth by application depth. The
Executive Summary
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XIV
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XV
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Light Transmission
Plankton
Total
Chlorophyll a
Suspended
Solids
•Water
• Particles
•Color
Light-through-Water
•Algae
•Detritus
Light-at-Leaf 'Sediments
Epiphytes
Light Attenuation
Surface
Reflection
Water-Column
Light Attenuation
(Kd)
Epiphyte
Light Attenuation
(Ke)
Underwater Bay Grasses
Figure 3. Availability of light for underwater bay grasses is influenced by water-column
and at-the-leaf surface light attenuation processes. DIN = dissolved inorganic nitrogen and
DIP = dissolved inorganic phosphorus.
Table 2. Summary of Chesapeake Bay water clarity criteria for application to shallow-water bay grass
designated use habitats.
Salinity
Regime
Water Clarity
Criteria as
Percent Light-
through-Water
Water Clarity Criteria as Secchi Depth
Temporal
Application
Water Clarity Criteria Application Depths
0.25
0.5
0.75
1.0
1.25
1.5
1.75
2.0
Secchi Depth (meters) for above Criteria Application Depth
Tidal-fresh
13%
0.2
0.4
0.5
0.7
0.9
1.1
1.2
1.4
April 1 - October 31
Oligohaline
13%
0.2
0.4
0.5
0.7
0.9
1.1
1.2
1.4
April 1 - October 31
Mesohaline
22%
0.2
0.5
0.7
1.0
1.2
1.4
1.7
1.9
April 1 - October 31
Polyhaline
22%
0.2
0.5
0.7
1.0
1.2
1.4
1.7
1.9
March 1 - May 31,
September 1 - November 30
Executive Summary
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xvi
recommended percent light-through-water criteria can be directly measured using a
Secchi disk or a light meter. A specific application depth is required in order to apply
and determine attainment of the water clarity criteria.
CHLOROPHYLL A CRITERIA
Phytoplankton are small, often microscopic plants floating in the water. These organ-
isms form the base of the Chesapeake Bay's food web, linking nutrients and sunlight
energy with forage fish such as menhaden and bay anchovy, and with bottom-
dwelling invertebrates such as oysters, clams and worms. The majority of the Bay's
animals feed directly on phytoplankton or on organisms that consume the phyto-
plankton. Therefore, the Bay's "carrying capacity," or its ability to produce and
maintain a diversity of species, depends in large part on how well phytoplankton
meet the nutritional needs of their consumers.
A primary characteristic of algae is the presence of photosynthetic pigments. Chloro-
phyll a is the primary photosynthetic pigment in algae and cyanobacteria (blue-green
algae). Since chlorophyll a is a measure of photosynthesis, it is thus also a measure
of the primary food source of aquatic food webs.
Chlorophyll a also plays a direct role in reducing light penetration in shallow-water
habitats, which has a direct impact on underwater bay grasses. Uneaten by
zooplankton and filter-feeding fish or shellfish, excess dead algae are consumed by
bacteria, and in the process, remove oxygen from the water column. Phytoplankton
assemblages can become dominated by single species which represent poor food
quality or even produce toxins that impair the animals that feed directly on them.
From a water quality perspective, chlorophyll a is the best available, most direct
measure of the amount and quality of phytoplankton and the potential to lead to
reduced water clarity and low dissolved oxygen impairments.
The EPA is providing the states with a recommended narrative chlorophyll a criteria
applicable to all Chesapeake Bay and tidal tributary waters (Table 3). The EPA
encourages states to adopt numerical chlorophyll a criteria for application to tidal
waters in which algal-related designated use impairments are likely to persist even
after attainment of the applicable dissolved oxygen and water clarity criteria. The
technical information supporting states' quantitative interpretation of the narrative
chlorophyll a criteria is published in the body of the Chesapeake Bay water quality
criteria document.
The three Chesapeake Bay criteria-dissolved oxygen, water clarity and chlorophyll
a-should be viewed as an integrated set of criteria applied to their respective sets of
designated use habitats and addressing similar and varied ecological conditions and
water quality impairments. They provide the basis for defining the water quality
conditions necessary to protect the five essential Chesapeake Bay tidal-water desig-
nated uses.
Executive Summary
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xvii
Table 3. Chesapeake Bay narrative chlorophyll a criteria.
Concentrations of chlorophyll a in free-floating microscopic aquatic plants (algae) shall not exceed levels that
result in ecologically undesirable consequences—such as reduced water clarity, low dissolved oxygen, food
supply imbalances, proliferation of species deemed potentially harmful to aquatic life or humans or aesthetically
objectionable conditions—or otherwise render tidal waters unsuitable for designated uses.
CRITERIA IMPLEMENTATION
EPA Region III also is presenting Chesapeake Bay criteria implementation proce-
dures as additional regional guidance in accordance with Section 117(b)(2) of the
Clean Water Act to the Chesapeake Bay watershed states and other agencies, institu-
tions, groups or individuals considering how to apply the criteria to determine the
degree of attainment. The EPA expects that these procedures will promote consis-
tent, baywide application of the criteria across jurisdictional boundaries.
The criteria were derived specifically to protect species and communities in the five
tidal-water designated uses during specific time periods. For example, dissolved
oxygen criteria have been derived for application to each of the five designated uses,
whereas the chlorophyll a criteria apply only to open-water fish and shellfish desig-
nated use habitats and the water clarity criteria only to the shallow-water bay grass
designated use habitats.
In defining what it means for the criteria to be attained, stressor magnitude, duration,
return frequency, spatial extent and temporal assessment period must be accounted
for. Stressor magnitude refers to how much of the pollutant or condition can be
allowed (e.g., 5 mg liter1) while still achieving the designated uses. Duration refers
to the period of time over which measurements of the pollutant or water quality
parameter is to be averaged (e.g., the 30-day mean). The allowable return frequency
at which the criterion can be violated without a loss of the designated use also must
be considered. Attainment of all three Chesapeake Bay criteria within the respective
designated use habitats should be assessed at the spatial scale of the 78 Chesapeake
Bay segments (spatial extent) using the most recent three consecutive years of appli-
cable tidal water quality monitoring data (temporal assessment period).
As the estuarine habitats gradually attain the three Chesapeake Bay criteria, not only
will the concentrations and values increase (i.e., dissolved oxygen and water clarity)
or decrease (chlorophyll a), but also occurrences of extreme changes in concentra-
tions over a short period of time (e.g., dissolved oxygen concentration changes from
6 mg liter1 to 2 mg liter1 in a matter of hours) will be greatly reduced. Even if the
Chesapeake Bay ecosystem is fully restored, it is unlikely that a circumstance of
'zero violation' of these criteria will ever be observed, given natural Bay processes
and extreme weather events. As these criteria were developed with conservative
(protective) assumptions, allowing a small percentage of circumstances in which the
criteria may be exceeded will still fully protect the tidal-water designated uses.
Executive Summary
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xviii
The cumulative frequency distribution methodology for defining criteria attainment
addresses the circumstances under which the criteria may be exceeded in a small per-
centage of instances, by integrating the five elements of criteria definition and
attainment: magnitude, duration, return frequency, space and time. The methodology
summarizes the frequency of instances in which the water quality threshold
(e.g., dissolved oxygen concentration) is exceeded, as a function of the area or
volume affected at a given place and over a defined period of time. Acceptable and
protective combinations of the frequency and spatial extent of such instances are
defined using a biologically based reference curve.
Using this approach to define criteria attainment, the EPA recommends a procedure
to quantify the spatial extent (area or volume) to which the water quality criterion
has been achieved or exceeded for each monitoring event. For example, under a
monthly monitoring program, the spatial extent to which the criterion has been
achieved or exceeded would be estimated for each month. This could be accom-
plished through interpolation of the available point, transect and remote-sensing
data. The criteria measure could thus be estimated at all locations in a given spatial
unit. The spatial extent to which a water quality criterion had been exceeded for a
given monitoring event would be defined as the fraction of the total area or volume
(expressed as a percent) that exceeds the criterion.
Through the integrated application of coupled airshed, watershed and tidal-water
quality Chesapeake Bay models and long-term tidal water quality monitoring data
records, the reductions in air, land and water-based loadings of nitrogen, phosphorus
and sediments required to attain the criteria-defined ambient tidal-water concentra-
tions of dissolved oxygen, water clarity and chlorophyll a can be directly
determined. In effect, the conditions necessary for attaining the three sets of Chesa-
peake Bay water quality criteria can be translated into watershed-based caps on
nutrient and sediment loadings and further allocated to specific sources and locations
within those watersheds.
Executive Summary
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Notices
This document has been developed by the U.S. Environmental Protection Agency
(EPA) Region III Chesapeake Bay Program Office, with the assistance and
support of the EPA Region III Water Protection Division, EPA Region II, EPA Head-
quarters Office of Water and Office of Research and Development, the states of
Maryland, Virginia, Delaware, Pennsylvania, New York and West Virginia and the
District of Columbia.
The goal of the Clean Water Act is to restore and maintain the chemical, physical and
biological integrity of the nation's waters and, where attainable, to achieve water
quality that provides for the protection and propagation of fish, shellfish and wildlife
and recreation in and on the water. As a means of meeting this goal, the Clean Water
Act requires states and authorized tribes to establish water quality criteria to protect
designated uses. This document provides regional technical guidance and recom-
mendations to states, authorized tribes and other authorized jurisdictions to develop
water quality criteria and water quality standards under the Clean Water Act to
protect against the adverse effects of nutrient and sediment over-enrichment in the
Chesapeake Bay and its tidal tributary waters.
States and tribal decision-makers retain the discretion to adopt approaches that differ
from this regional guidance on a case-by-case basis when appropriate and scientifi-
cally defensible, consistent with the Clean Water Act. While this document contains
the EPA's scientific findings and policy recommendations regarding ambient concen-
trations of dissolved oxygen, water clarity and chlorophyll a that protect Chesapeake
Bay estuarine aquatic resources, it is not a substitute for the Clean Water Act or EPA
regulations; nor is it a regulation. Thus, it cannot impose legally binding require-
ments on the EPA, states, authorized tribes or the regulated community, and it may
not apply to particular situations or circumstances. The EPA may change this
regional guidance in the future.
This document is available to the public through the Internet at http://www.epa.gov/
waterscience/standards/nutrient.html or www.chesapeakebay.net/baycriteria.htm.
Requests for the document should be sent to the U.S. Environmental Protection
Agency, National Service Center for Environmental Publications, 11029 Kenwood
Road, Building 5, Cincinnati, Ohio 45242 (513-489-8190) or by email (waterpubs@
epamail.epa.gov). Please refer to EPA document number EPA 903-R-03-002.
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xxi
Acknowledgments
These Chesapeake Bay-specific water quality criteria were derived through the
collaborative efforts, collective knowledge and applied expertise of the following
four Chesapeake Bay criteria and standards coordinator teams.
Water Clarity Criteria Team
Richard Batiuk, U.S. EPA Chesapeake Bay Program Office; Peter Bergstrom,
U.S. Fish and Wildlife Service; Arthur Butt, Virginia Department of Environmental
Quality; Ifeyinwa Davis, U.S. EPA Office of Water; Frederick Hoffman, Virginia
Department of Environmental Quality; Charles Gallegos, Smithsonian
Environmental Research Center; Will Hunley, Hampton Roads Sanitation District;
Michael Kemp, University of Maryland Horn Point Laboratory; Ken Moore, Virginia
Institute of Marine Science; Michael Naylor, Maryland Department of Natural
Resources; and Nancy Rybicki, U.S. Geological Survey.
Without the efforts of the authors of the first and second Chesapeake Bay underwater
bay grass technical syntheses, the Bay-specific water clarity criteria could not have
been developed: Steve Ailstock, Anne Arundel Community College; Rick Bartleson,
University of Maryland Horn Point Laboratory; Richard Batiuk, U.S. EPA
Chesapeake Bay Program Office; Peter Bergstrom, U.S. Fish and Wildlife Service;
Steve Bieber, Maryland Department of the Environment; Virginia Carter, U.S.
Geological Survey; William Dennison, University of Maryland Center for
Environmental Studies; Charles Gallegos, Smithsonian Environmental Research
Center; Patsy Heasly, Chesapeake Research Consortium; Edward Hickman,
U.S. Geological Survey; Lee Karrh, Maryland Department of Natural Resources;
Michael Kemp, University of Maryland Horn Point Laboratory; Evamaria Koch,
University of Maryland Horn Point Laboratory; Stan Kollar, Harford Community
College; Jurate Landwehr, U.S. Geological Survey; Ken Moore, Virginia Institute of
Marine Science; Laura Murray, University of Maryland Horn Point Laboratory;
Michael Naylor, Maryland Department of Natural Resources; Robert Orth, Virginia
Institute of Marine Science; Nancy Rybicki, U.S. Geological Survey; Lori Staver,
University of Maryland; Court Stevenson, University of Maryland Horn Point
Acknowledgments
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xxii
Laboratory; Mirta Teichberg, Woods Hole Oceanographic Institution; and David
Wilcox, Virginia Institute of Marine Science.
Dissolved Oxygen Criteria Team
Richard Batiuk, U.S. EPA Chesapeake Bay Program Office; Denise Breitburg,
Academy of Natural Sciences; Arthur Butt, Virginia Department of Environmental
Quality; Thomas Cronin, U.S. Geological Survey; Ifeyinwa Davis, U.S. EPA Office
of Water; Robert Diaz, Virginia Institute of Marine Science; Frederick Hoffman,
Virginia Department of Environmental Quality; Steve Jordan, Maryland Department
of Natural Resources; James Keating, U.S. EPA Office of Water; Marcia Olson,
NOAA Chesapeake Bay Office; James Pletl, Hampton Roads Sanitation District;
David Secor, University of Maryland Chesapeake Biological Laboratory; Glen
Thursby, U.S. EPA Office of Research and Development; and Erik Winchester, U.S.
EPA Office of Research and Development.
Scientists from across the country, well-recognized for their work in the area of low
dissolved oxygen effects on individual species up to ecosystem trophic dynamics,
contributed their time, expertise, publications and preliminary data and findings to
support the derivation of Chesapeake Bay-specific criteria: Steve Brandt, NOAA
Great Lakes Environmental Research Laboratory; Walter Boynton, University of
Maryland Chesapeake Biological Laboratory; Ed Chesney, Louisiana Universities
Marine Consortium; Larry Crowder, Duke University Marine Laboratory; Peter
deFur, Virginia Commonwealth University; Ed Houde, University of Maryland
Chesapeake Biological Laboratory; Julie Keister, Oregon State University; Nancy
Marcus, Florida State University; John Miller, North Carolina State University; Ken
Paynter, University of Maryland; Sherry Poucher, SAIC; Nancy Rabalais, Louisiana
Universities Marine Consortium; Jim Rice, North Carolina State University; Mike
Roman, University of Maryland Horn Point Laboratory; Linda Schaffner, Virginia
Institute of Marine Science; Dave Simpson, Connecticut Department of
Environmental Protection; and Tim Target, University of Delaware.
Chlorophyll a Criteria Team
Richard Batiuk, U.S. EPA Chesapeake Bay Program Office; Claire Buchanan,
Interstate Commission on the Potomac River Basin; Arthur Butt, Virginia
Department of Environmental Quality; Ifeyinwa Davis, U.S. EPA Office of Water;
Tom Fisher, University of Maryland Horn Point Laboratory; David Flemer, U.S.
EPA Office of Water; Larry Haas, Virginia Institute of Marine Science; Larry
Harding, University of Maryland Horn Point Laboratory/Maryland Sea Grant;
Frederick Hoffman Virginia Department of Environmental Quality; Will Hunley,
Hampton Roads Sanitation District; Richard Lacouture, Academy of Natural
Sciences; Robert Magnien, Maryland Department of Natural Resources; Harold
Marshall, Old Dominion University; Robert Steidel, Hopewell Regional Wastewater
Facility; and Peter Tango, Maryland Department of Natural Resources.
Acknowledgments
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xxiii
Without the efforts of the Chesapeake Bay Phytoplankton Restoration Goals Team
forging connections between reference phytoplankton communities and resulting
chlorophyll a concentrations would not have been possible: Claire Buchanan,
Interstate Commission on the Potomac River Basin; Richard Lacouture, Academy of
Natural Sciences; Harold Marshall, Old Dominion University; Stella Sellner,
Academy of Natural Sciences; Jacqueline Johnson, Interstate Commission on the
Potomac River Basin/Chesapeake Bay Program Office; Jonathan Champion,
Chesapeake Research Consortium/Chesapeake Bay Program Office; Marcia Olson,
NOAA Chesapeake Bay Office; Fred Jacobs, AKRF, Inc.; John Seibel, PBS & J,
Inc.; and Elgin Perry.
Water Quality Standards Coordinators Team
Richard Batiuk, U.S. EPA Chesapeake Bay Program Office; Jerusalem Bekele,
District of Columbia Department of Health; Libby Chatfield, West Virginia
Environmental Quality Board; Joe Beaman, Maryland Department of the
Environment; Thomas Gardner, U.S. EPA Office of Water (Criteria); Jean Gregory,
Virginia Department of Environmental Quality; Denise Hakowski, U.S. EPA Region
III; Elaine Harbold, U.S. EPA Region III; Wayne Jackson, U.S. EPA Region II;
James Keating, U.S. EPA Office of Water (Standards); Larry Merrill, U.S. EPA
Region III; Garrison Miller, U.S. EPA Region III; Joel Salter, U.S. EPA Office of
Water (Permits); John Schneider, Delaware Department of Natural Resources and
Environmental Control; Mark Smith, U.S. EPA Region III; Scott Stoner, New York
State Department of Environmental Conservation; and Carol Young, Pennsylvania
Department of Environmental Protection.
Without the efforts of the Chesapeake Bay Tidal Monitoring Network Design Team,
the development of the criteria attainment procedures contained in this document
would not have been developed: Claire Buchanan, Interstate Commission on the
Potomac River Basin; Paul Jacobson; Marcia Olson, NOAA Chesapeake Bay Office;
Elgin Perry; Steve Preston, U.S. Geological Survey/Chesapeake Bay Program
Office; Walter Boynton, University of Maryland Chesapeake Biological Laboratory;
Larry Haas, Virginia Institute of Marine Science; Frederick Hoffman, Virginia
Department of Environmental Quality; Bruce Michael, Maryland Department of
Natural Resources; Jacqueline Johnson, Interstate Commission for the Potomac
River Basin; Kevin Summers, U.S. EPA Office of Research and Development; Dave
Jasinski, University of Maryland; Mary Ellen Ley, U.S. Geological Survey/
Chesapeake Bay Program Office; and Lewis Linker, U.S. EPA Chesapeake Bay
Program Office.
Acknowledgments
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xxiv
The contributions of the 12 independent scientific peer reviewers, selected based on
their recognized national expertise and drawn from institutions and agencies from
across the country, are hereby acknowledged.
Without the contributions of the more than 100 individuals listed as authors or
technical contributors to various syntheses of Chesapeake Bay living resource
habitat requirements over the past two decades, the scientific basis for a set of
designated uses tailored to Chesapeake Bay tidal habitats and species would not have
been forged. Without the efforts of the many individuals involved in all aspects of
collection, management and analysis of Chesapeake Bay Monitoring Program data
over the past two decades, these criteria could not have been derived. Their
collective contributions are hereby fully acknowledged.
The technical editing, document preparation and desk-top publication contributions
of Robin Bisland, Donna An and Susan Vianna are hereby acknowledged.
Acknowledgments
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Contents
Foreword vii
Executive Summary ix
Notices xix
Acknowledgments xxi
I. Introduction 1
National Criteria 2
Regional Nutrient Criteria 2
Chesapeake Bay Criteria 3
II. Chesapeake Bay Nutrient and Sediment Enrichment Criteria ... 5
III. Dissolved Oxygen Criteria 7
Background 7
Chesapeake Bay science 7
Natural dissolved oxygen processes 8
Chesapeake Bay oxygen dynamics 8
Low dissolved oxygen: historical and recent past 10
Approach to Deriving Dissolved Oxygen Criteria 12
Chesapeake Bay dissolved oxygen restoration goal framework .... 14
Regionalizing the EPA Virginian Province
saltwater dissolved oxygen criteria 15
Applying the EPA freshwater dissolved oxygen criteria 25
Species listed as threatened or endangered 27
Scientific literature findings 33
Instantaneous minimum versus daily mean 33
Strengths and limitations of the criteria derivation procedures 34
Chesapeake Bay Dissolved Oxygen Criteria Derivation 40
Migratory fish spawning and nursery designated use criteria 42
Open-water fish and shellfish designated use criteria 46
Deep-water seasonal fish and shellfish designated use criteria 52
Deep-channel seasonal refuge designated use criteria 60
Contents
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Chesapeake Bay Dissolved Oxygen Criteria 65
Literature Cited 67
IV. Water Clarity Criteria 81
Background 81
Approach 82
The relationships between water quality, light and
underwater bay grasses 82
Determining light requirements 84
Strengths and limitations of the criteria derivation procedures 85
Water Clarity Criteria Derivation 90
Minimum light requirements 90
Light-through-water requirements 95
Chesapeake Bay Water Clarity Criteria 96
Literature Cited 97
V. Chlorophyll a Criteria 101
Background 101
Scope and magnitude of nutrient enrichment in Chesapeake Bay . . 101
Chlorophyll a: key indicator of phytoplankton biomass 102
Chesapeake Bay Chlorophyll a Criteria 104
Supporting Technical Information and Methodologies 105
Context for the narrative Chesapeake Bay chlorophyll a criteria ... 105
Chlorophyll a concentrations characteristic of
various ecological conditions 107
Chlorophyll a concentrations characteristic of
trophic-based conditions 129
Chlorophyll a concentrations protective against
water quality impairments 132
Methodologies for deriving waterbody-specific
chlorophyll a criteria 134
Literature Cited 137
VI. Recommended Implementation Procedures 143
Defining Criteria Attainment 144
Dissolved oxygen criteria 144
Water clarity criteria 144
Chlorophyll a criteria 147
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V
Addressing Magnitude, Duration, Frequency, Space and Time 148
Developing the Cumulative Frequency Distribution 152
Step 1. Interpolation of water quality monitoring data 152
Step 2. Comparison of interpolated water quality
monitoring data to the appropriate criterion value 155
Step 3. Identification of interpolator cells that
exceed the criterion value" 156
Step 4. Calculation of the cumulative probability
of each spatial extent of exceedance 156
Step 5. Plot of spatial exceedance vs. the cumulative frequency . . . 159
Diagnosing the Magnitude of Criteria Exceedance 164
Defining the Reference Curve 166
Strengths and limitations 166
Approaches to defining reference curves 167
Reference curves for dissolved oxygen criteria 168
Reference curves for water clarity criteria 171
Reference curves for chlorophyll a criteria 174
Reference curve implementation 174
Monitoring to Support the Assessment of Criteria Attainment 176
Shallow-water monitoring 176
Dissolved oxygen criteria assessment 177
Water clarity criteria assessment 185
Chlorophyll a criteria assessment 191
Evaluation of Chesapeake Bay Water Quality Model Output 194
Chesapeake Bay Watershed Model 195
Chesapeake Bay Water Quality Model 196
Integration of Monitoring and Modeling for Criteria Assessment . . 196
Literature Cited 197
VII. Diagnostic Procedures for Natural Processes and
Criteria Nonattainment 201
Addressing Natural Exceedance of the Chesapeake Bay Criteria ... 201
Natural excursions of low dissolved oxygen conditions 202
Natural reductions in water clarity levels 206
Natural elevated chlorophyll a concentrations 209
Contents
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Diagnosing Causes of Criteria Nonattainment 210
Dissolved oxygen criteria 210
Water clarity criteria 211
Chlorophyll a criteria 218
Literature Cited 218
Glossary 221
Acronyms 229
Appendices
A. Refined Designated Uses for the Chesapeake Bay and
Tidal Tributaries A-l
B. Sensitivity to Low Dissolved Oxygen Concentrations for
Northern and Southern Atlantic Coast Populations of
Selected Test Species B-l
C. Summary of Literature on the Tolerance of Chesapeake Bay
Macrobenthic Species to Low Dissolved Oxygen Conditions C-l
D. Narrative, Numerical and Method-based Chlorophyll a Criteria
Adopted as Water Quality Standards by States Across the U.S D-l
E. 1950s-1990s Chesapeake Bay and Tidal Tributary Chlorophyll a
Concentrations by Chesapeake Bay Program Segment E-1
F. Phytoplankton Reference Community Data Analyses F-l
G. Data Supporting Determination of Adverse Affect Thresholds for
Potentially Harmful Algal Bloom Species G-l
H. Derivation of Cumulative Frequency Distribution Criteria
Attainment Reference Curves H-l
I. Analytical Approaches for Assessing Short-Duration
Dissolved Oxygen Criteria I-1
J. Development of Chesapeake Bay Percent Light-at-the-Leaf
Diagnostic Requirements J-1
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chapter |
Introduction
Nutrients are essential to the health and diversity of our surface waters. However,
excessive nutrients lead to low dissolved oxygen, fish kills, algal blooms and imbal-
ances in the aquatic food web. They also pose potential risks to human health, such
as those recently manifested in the harmful algal blooms of the Gulf and East coasts,
including the tidal tributaries of the Chesapeake Bay.
National water quality inventories have repeatedly shown that nutrients are a major
cause of ambient water quality impairments. The EPA's 1996 Section 305(b) report
identified excessive nutrients as the leading cause of impairments to lakes and the
second leading cause of impairments to rivers, after siltation. In addition, nutrients
were the second leading cause of impairments reported by the states in their 1998
Section 303(d) lists. Nutrients, along with sediment, were the primary causes of
impairments to the Chesapeake Bay and its tidal tributaries on the respective Mary-
land and Virginia Section 303(d) lists. To meet the objectives of the Clean Water Act,
the EPA's implementing regulations specify that states must adopt criteria that
contain sufficient parameters to protect existing and designated uses. Until 2000, the
EPA had not published recommended quantitative water quality criteria for nutrients
that states could adopt to protect uses.
In order to achieve and maintain water quality conditions necessary to protect the
aquatic living resources of the Chesapeake Bay and its tidal tributaries from the
effects of nutrient and sediment pollution, EPA Region III has developed Ambient
Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for
the Chesapeake Bay and Its Tidal Tributaries (Regional Criteria Guidance). EPA
Region III has also identified and described five habitats (or designated uses) that
when adequately protected will ensure the protection of the living resources of the
Bay and its tidal tributaries. Those five uses (described in Appendix A) provide the
context in which EPA Region III derived protective Chesapeake Bay water quality
criteria for dissolved oxygen, water clarity and chlorophyll a (see Figure 1 in the
Executive Summary), which are the subject of the Regional Criteria Guidance. EPA
Region III has also published the Technical Support Document for the Identification
of Chesapeake Bay Designated Uses and Attainability. This document provides
chapter i • Introduction
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farther information on the development and geographical extent of the designated
uses to which the criteria may apply.
NATIONAL CRITERIA
Under the Clean Water Act Section 304(a), the EPA issues national criteria recom-
mendations to states and tribes to assist them in developing their water quality
standards. When the EPA reviews a state or tribal water quality standard for approval
under 303(c) of the Clean Water Act, the agency must determine whether the adopted
designated uses are consistent with the Clean Water Act requirements and whether
the adopted criteria protect the designated use. The EPA's regulations encourage
states and tribes, when adopting water quality criteria as part of their water quality
standards, to employ the EPA's Section 304(a) guidance, to modify the EPA's 304(a)
guidance to reflect site-specific conditions or to use other scientifically defensible
methods to derive criteria to protect the designated uses.
REGIONAL NUTRIENT CRITERIA
In 1995 the EPA gathered a group of nationally recognized scientists and managers
to address the national nutrient problem. They recommended that the agency avoid
setting criteria for phosphorus or nitrogen that would apply to all water bodies and
regions of the country. Instead they suggested that the EPA develop guidance
(assessment tools and control measures) for specific bodies of water and ecological
regions across the country and use reference conditions, which reflect pristine or
minimally affected waters, as a basis for developing nutrient criteria.
Using these suggestions as starting points, the EPA published the National Strategy>
for the Development of Regional Nutrient Criteria in June 1998. The strategy artic-
ulated the EPA's intention to develop technical guidance manuals for four types of
waters (lakes and reservoirs, rivers and streams, estuaries and coastal waters, and
wetlands) and produce nutrient criteria for specific eco-regions. In addition, the EPA
is committed to working with states and tribes to develop more refined and localized
nutrient criteria based on approaches described in the water body guidance manuals.
The Regional Criteria Guidance provides EPA's recommendations to the Chesa-
peake Bay states for use in establishing their water quality standards consistent with
Section 303(c) of the Clean Water Act.
chapter i • Introduction
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3
CHESAPEAKE BAY CRITERIA
The EPA's current guidance for dissolved oxygen can be found in the 1986 fresh-
water dissolved oxygen criteria and 2000 Virginian Province saltwater criteria
documents. EPA Region III developed the criteria presented in this document by
integrating and supplementing the scientific findings and data to fully protect
specific Chesapeake Bay tidal-water habitats. The revised criteria are based on and
consistent with the existing EPA dissolved oxygen criteria.
There are no national 304(a) criteria specific to chlorophyll a or water clarity. In
accordance with sections 117(b) and 303 of the Clean Water Act, EPA Region III
derived the water quality criteria addressing these critical nutrient and sediment
enrichment parameters specifically to protect Chesapeake Bay living resources and
their tidal-water habitats.
The water quality criteria presented in this document are designed to apply to the
Chesapeake Bay and its tidal tributaries and embayments within the tidally influ-
enced waters of the states of Maryland, Virginia and Delaware and the District of
Columbia (Figure 1-1). These regional criteria may also apply to other estuarine and
coastal systems, with appropriate modifications.
The regional criteria and designated uses presented in this document and the Tech-
nical Support Document are the product of a collaborative effort among the
Chesapeake Bay Program partners. They represent a scientific consensus based on
the best available scientific findings and technical defining water quality conditions
necessary to protect Chesapeake Bay aquatic living resources from effects due to
nutrient and sediment over-enrichment. Various stakeholder groups have been
involved in their development, with contributions from the staffs of federal and state
governments, local agencies, scientific institutions, citizen conservation groups,
business and industry. In the Regional Criteria Guidance the EPA recommends and
expects that the numerical criteria and refined designated uses will be considered by
and appropriately incorporated into the water quality standards of the Chesapeake
Bay jurisdictions with tidal waters—Maryland, Virginia, Delaware and the District
of Columbia.
chapter i • Introduction
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New York
Pennsylvania
Maryland
West Virginia
Delaware
DC
Virginia
Figure 1-1. The Chesapeake Bay watershed crosses the boundaries of six states—Maryland, Virginia,
Delaware, Pennsylvania, New York and West Virginia—and the District of Columbia.
chapter i • Introduction
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chapter |
Chesapeake Bay Nutrient and
Sediment Enrichment Criteria
The Chesapeake 2000 agreement committed its signatories (the states of Pennsyl-
vania, Maryland and Virginia; the District of Columbia; the Chesapeake Bay
Commission and the EPA) to "define the water quality conditions necessary to
protect aquatic living resources" in the Chesapeake Bay and its tidal tributaries.
New York, Delaware and West Virginia agreed to the same commitment through a
separate six-state memorandum of understanding with the EPA.
EPA Region III has identified the water quality conditions that are necessary to
protect living resources through the Chesapeake Bay-specific water quality criteria
for dissolved oxygen, water clarity and chlorophyll a published in this document.
The Chesapeake Bay criteria have been derived to protect a series of five refined
tidal-water designated uses which, in turn, reflect important and unique habitats
throughout the Chesapeake Bay and its tidal tributaries (Appendix A). More detailed
descriptions of these refined subcategories of tidal-water designated uses and their
recommended boundaries can be found in the EPA Region III publication, Technical
Support Document for the Identification of Chesapeake Bay Designated Uses and
Attainability. Collectively, these three water quality conditions provide the best and
most direct measures of the effects of too much nutrient and sediment pollution on
the Chesapeake Bay's aquatic living resources-fish, crabs, oysters, their prey species
and underwater bay grasses.
Fish and other aquatic life require specific levels of dissolved oxygen to survive.
Seasonal algae blooms, when uneaten by fish and shellfish, deplete dissolved
oxygen, potentially rendering the deep waters of the Bay uninhabitable to certain
species during certain times of the year. The Chesapeake Bay dissolved oxygen
criteria were based on the oxygen levels required by different aquatic communities
inhabiting distinct habitats in the Bay's tidal waters during different times of the year
(Chapter III).
Underwater bay grasses are an essential component of the Chesapeake Bay's habitat
and an important food source for waterfowl. Decreased water clarity inhibits the
growth of underwater bay grasses. Building on decades of scientific research, the
chapter ii • Chesapeake Bay Nutrient and Sediment Enrichment Criteria
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Chesapeake Bay water clarity criteria were derived to protect the minimum light
required by both low and higher salinity underwater plant communities (Chapter IV).
Measurements of chlorophyll a indicate levels of phytoplankton or algal biomass in
the water column. Levels that are too high indicate algal blooms, which lead to a
proliferation of less desirable species, shade the light in shallow-water habitats and
cause low dissolved oxygen conditions, as uneaten algae die off and sink to the
bottom. Narrative Chesapeake Bay chlorophyll a criteria were derived to support
desired ecological conditions and protect against an array of water quality
impairments (Chapter V).
The EPA provides Chesapeake Bay criteria implementation procedures as additional
regional guidance to the Chesapeake Bay watershed states and other agencies, insti-
tutions, groups or individuals for consideration of how to apply the criteria in order
to determine the degree of attainment of those criteria (Chapter VI). These imple-
mentation procedures are published in this document to promote consistent, baywide
application of the criteria across jurisdictional boundaries.
A series of diagnostic procedures and tools designed to explain the reasons for non-
attainment of the water quality criteria are documented (Chapter VII). Approaches
for addressing natural exceedances of the criteria not already accounted for in the
implementation procedures are provided for consistent application across all tidal
water habitats.
The EPA is publishing this Regional Criteria Guidance to further to goals of the
Clean Water Act and, specifically, pursuant to Sections 117(b) and 303(c) of the the
Act.
chapter ii • Chesapeake Bay Nutrient and Sediment Enrichment Criteria
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chapter hi
Dissolved Oxygen Criteria
BACKGROUND
Of all life-supporting environmental constituents, oxygen is one of the most essen-
tial. In cells, oxygen stores and liberates the energy that drives vital processes of fish,
crabs and shellfish such as feeding, growth, swimming and reproduction. Low
dissolved oxygen concentrations can increase mortality, reduce growth rates and
alter the distribution and behavior of aquatic organisms, all of which can produce
significant changes in the overall estuarine food web (Breitburg 2002).
The Chesapeake Bay and its tidal tributaries harbor diverse and productive commu-
nities of aquatic organisms that are supported by a complex array of food webs. To
establish dissolved oxygen criteria for these living resources and the food webs they
depend upon, we must characterize the dissolved oxygen conditions that lead to
stressful conditions for the living resources of the Chesapeake Bay, ranging from
copepods to sturgeon.
CHESAPEAKE BAY SCIENCE
The development of the scientific underpinnings for Chesapeake Bay-specific
criteria has been under way for decades. The first documentation of seasonal occur-
rence, low dissolved oxygen conditions in the Chesapeake Bay took place in the
1930s (Newcombe and Home 1938; Newcombe et al. 1939), with low oxygen con-
ditions documented in the lower Potomac River in the early 1900s (Sale and Skinner
1917). Chesapeake Bay dissolved oxygen dynamics, which are critical to deriving
criteria that reflect the ecosystem process, first became understood during the
research cruises of the Johns Hopkins Chesapeake Bay Institute during the 1950s
through the late 1970s. A five-year, multidisciplinary research program established
in the late 1980s, funded and coordinated by the Maryland and Virginia Sea Grant
programs, yielded significant advances in the understanding of Chesapeake Bay
oxygen dynamics, effects and ecosystem implications (Smith et al. 1992). The coor-
dinated state-federal Chesapeake Bay Water Quality Monitoring Program, initiated
chapter iii • Dissolved Oxygen Criteria
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8
in 1984, provided decadal scale records of seasonal to interannual variability in
dissolved oxygen conditions throughout the tidal waters. Building on the long-term
baywide monitoring data record, a series of multi-investigator, multi-year National
Science Foundation, NOAA and EPA-funded research programs provided new
insights into Bay ecosystem processes and responses. These investigations laid the
groundwork for management application of the resulting science.
NATURAL DISSOLVED OXYGEN PROCESSES
Dissolved oxygen in any natural body of water is primarily a function of atmospheric
oxygen (which diffuses into the water at the surface), oxygen produced by plants
(microscopic free-floating plants or phytoplankton) during photosynthesis and
aquatic animals, plants and bacteria that consume dissolved oxygen through respira-
tion. Oxygen also is consumed by chemical processes such as sulfide oxidation and
nitrification. The reduction of dissolved oxygen stimulates sulfate reduction and
results in hydrogen sulfide, a more toxic form of sulfur. Oxygen depletion also can
inhibit nitrogen removal via coupled nitrification and denitrification and enhance the
recycling of ammonia and phosphates as well as the release of heavy metals from
bottom sediments into the overlying water column.
The amount of oxygen dissolved in the water changes as a function of temperature,
salinity, atmospheric pressure and biological and chemical processes. Gill and
integumentary respiration, which most Chesapeake Bay aquatic species use, is
accomplished by extracting dissolved oxygen across a pressure gradient (rather than
a concentration gradient). As the partial pressure of dissolved oxygen increases in
the water (e.g., increasing temperature and salinity), it can more readily be extracted
by an organism. Cold-blooded organisms, however, have much higher metabolic
rates and oxygen requirements at higher temperatures, which more than offsets the
oxygen gained at the higher temperature. The interactions among metabolism,
temperature and salinity clearly are complex, but they must be considered in deriving
Chesapeake Bay dissolved oxygen criteria.
Biological processes such as respiration and photosynthesis can affect the concentra-
tion of dissolved oxygen before a new equilibrium can be reached with the
atmosphere. As a result, for relatively short periods of time, or under sustained
conditions of reduced physical mixing (i.e., the stratification of the water column),
dissolved oxygen concentrations can be driven well below the point of saturation.
They can decrease to zero (a condition known as anoxia), especially in deep or strat-
ified bodies of water, or increase to a concentration of 20 mg liter1 (a condition
known as supersaturation) during dense algal blooms.
CHESAPEAKE BAY OXYGEN DYNAMICS
It is critical to take into account the natural processes that control oxygen dynamics
in order to establish criteria that reflect natural conditions and protect different habi-
tats. The Chesapeake Bay tends to have naturally reduced dissolved oxygen
chapter iii • Dissolved Oxygen Criteria
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9
conditions in its deeper waters because of its physical morphology and estuarine
circulation. As in other estuarine systems (e.g., Boynton et al. 1982; Nixon 1988;
Caddy 1993; Cloern 2001), the Chesapeake's highly productive waters, combined
with sustained stratification, long residence times, low tidal energy and its tendency
to retain and recycle nutrients, set the stage for lower dissolved oxygen conditions.
The mesohaline mainstem Chesapeake Bay and lower reaches of the major tidal
rivers have a stratified water column, which essentially prevents waters near the
bottom from mixing with oxygenated surface waters. The recycling of nutrients and
water-column stratification lead to severe reductions in dissolved oxygen concentra-
tions during the warmer months of the year in deeper waters within and below the
pycnocline layer.
This reduction in dissolved oxygen generally results from a host of additional
biological and physical factors (e.g., Kemp and Boynton 1980; Kemp et al. 1992;
Sanford et al. 1990; Boynton and Kemp 2000). The annual spring freshet delivers
large volumes of fresh water to the Bay. The contribution of significant quantities of
nutrients in the spring river flows, combined with increasing temperatures and light,
produces a large increase in phytoplankton biomass. Phytoplankton not consumed
by suspension feeders (such as zooplankton, oysters and menhaden) sink to the
subpycnocline waters, where they are broken down by bacteria over a period of days
or weeks (e.g., Malone et al. 1986; Tuttle et al. 1987; Malone et al. 1988). This loss
of oxygen due to bacterial metabolism is exacerbated by restricted mixing with
surface waters because of the onset of increased water-column stratification.
The Chesapeake Bay's nearshore shallow waters periodically experience episodes of
low to no dissolved oxygen, in part because bottom water has been forced into the
shallows by a combination of internal lateral tides and sustained winds (Carter et al.
1978; Tyler 1984; Seliger et al. 1985; Malone et al. 1986; Breitburg 1990; Sanford
et al. 1990). Low dissolved oxygen conditions in the shallow waters of tidal tribu-
taries are more often the result of local production and respiration than the incursion
of bottom waters. Climatic conditions such as calm winds and several continuous
cloudy days in a row can contribute to oxygen depletion in these shallow-water habi-
tats. They can be exposed to episodes of extreme and rapid fluctuations in dissolved
oxygen concentrations (Sanford et al. 1990). In depths as shallow as 4 meters,
dissolved oxygen concentrations may decline to 0.5 mg liter"1 for up to 10 hours
(Breitburg 1990).
Diel cycles of low dissolved oxygen conditions often occur in nonstratified shallow
waters where water-column respiration at night temporarily reduces dissolved
oxygen levels (D'Avanzo and Kremer 1994). In nearshore waters of the mesohaline
mainstem Chesapeake Bay, near-bottom dissolved oxygen concentrations are char-
acterized by large diel fluctuations and daily minima during the late night and early
morning hours of July and August (Breitburg 1990).
The timing and extent of reduced dissolved oxygen conditions in the Chesapeake
Bay vary from year to year, driven largely by local weather patterns, the timing and
chapter iii • Dissolved Oxygen Criteria
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10
magnitude of freshwater river flows, the concurrent delivery of nutrients and sedi-
ments into tidal waters and the corresponding springtime phytoplankton bloom
(Officer et al. 1984; Seliger et al. 1985; Boynton and Kemp 2000; Hagy 2002). In
the Chesapeake Bay's mesohaline mainstem, these conditions generally occur from
June through September but have been observed to occur as early as May They may
persist through early October, until the water column is fully mixed in the fall. The
deeper waters of several Chesapeake Bay major tidal tributaries also can exhibit
hypoxic and anoxic conditions (Hagy 2002).
Anoxia is the absence of oxygen. Because most field dissolved oxygen meters are only precise to
± 0.1 or 0.2 mg liter1, areas with measured oxygen concentrations of 0.2 mg liter1 or less are
sometimes classified as anoxic. There is no accurate consensus on the scientific definition of
hypoxia, but it is often defined as oxygen concentrations below 2 mg liter1 (U.S. scientific
literature) or 2 ml liter1 (European scientific literature). These specific concentration-based
definitions are problematic when applied in an effects context, because many species show reduced
growth and altered behavior at oxygen levels above 2 mg liter1, and sensitive species experience
mortality during prolonged exposure at these low concentrations. As an operational definition,
hypoxia should be considered to be oxygen concentrations reduced from full saturation that impair
living resources.
LOW DISSOLVED OXYGEN: HISTORICAL AND RECENT PAST
Dissolved oxygen levels vary naturally in lakes, estuaries and oceans over varying
temporal and spatial scales due to many biological, chemical and physical processes.
In estuaries such as the Chesapeake Bay, freshwater inflow that influences water-
column stratification; nutrient input and cycling; physical processes such as
density-driven circulation; and tides, winds, water temperature and bacterial activity
are among the most important factors. These processes can lead to large natural
seasonal and interannual variability in oxygen levels in many parts of the Chesa-
peake Bay and its tidal tributaries.
Superimposed on this natural dissolved oxygen variability is a progressive increase
in the intensity and frequency of hypoxia and anoxia over the past 100 to 150 years,
most notably since the 1960s. This human-induced eutrophication is evident both
from instrumental data and geochemical and faunal/floral 'proxies' of dissolved
oxygen conditions obtained from the sedimentary record.
The instrumental record, while incomplete prior to the inception of the multi-agency
Chesapeake Bay Monitoring Program in 1984, suggests that as early as the 1930s
(Newcombe and Home 1938) and especially since the 1960s (Taft et al. 1980),
summer oxygen depletion has been recorded in the Chesapeake Bay. Officer et al.
(1984), Malone (1992), Harding and Perry (1997) and Hagy (2002) provide useful
chapter iii • Dissolved Oxygen Criteria
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11
discussions of the instrumental record of dissolved oxygen and related parameters
such as chlorophyll a across this multi-decade data record.
At issue is whether, and to what degree, dissolved oxygen reductions are a naturally
occurring phenomenon in the Chesapeake Bay Long sediment core records (17
meters to greater than 21 meters in length) indicate that the Chesapeake Bay formed
about 7,500 years ago (Cronin et al. 2000; Colman et al. 2002) when the rising sea
level after the final stage of Pleistocene deglaciation flooded the Susquehanna
channel. The modern estuarine circulation and salinity regime probably began in the
mid- to late Holocene epoch, about 4,000-5,000 years ago (in the regional climate of
the early Holocene, Chesapeake Bay's salinity differed from that of the late
Holocene). This is based on the appearance of 'pre-coloniaP benthic foraminiferal,
ostracode and dinoflagellate assemblages. It is against this mid- to late Holocene
baseline that we can view the post-European settlement and modern dissolved
oxygen regime of the Chesapeake Bay.
During the past decade, studies of the Chesapeake Bay's late Holocene dissolved
oxygen record have been carried out using several proxies of past dissolved oxygen
conditions, which are preserved in sediment cores that have been dated using the most
advanced geochronological methods. These studies, using various indicators of past
dissolved oxygen conditions, are reviewed in Cronin and Vann (2003) and provide
information that puts the monitoring record of the modern Chesapeake Bay into a
long-term perspective and permits an evaluation of natural variability in the context
of restoration targets. The following types of measurements of oxygen-sensitive
chemical and biological indicators have been used: nitrogen isotopes (Bratton et al.
2003); biogenic silica and diatom communities (Cooper and Brush 1991; Cooper
1995; Colman and Bratton 2003); molybdenum and other metals (Adelson et al.
2000; Zheng et al., in press); lipid biomarkers; acid volatile sulfur (AVS)/chromium
reducible sulfur (CRS) ratios; total nitrogen and total organic carbon (Zimmerman
and Canuel 2000); elemental analyses (Cornwell et al. 1996) and paleo-ecological
reconstructions based on dinoflagellate cysts (Willard et al. 2003); and benthic
foraminiferal assemblages (Karlsen et al. 2000). Although space precludes a com-
prehensive review of these studies, and the time period studied and level of
quantification vary, several major themes emerge, which are summarized here.
First, the 20th century sedimentary record confirms the limited monitoring record of
dissolved oxygen, documenting that there has been a progressive decrease in
dissolved oxygen levels, including the periods of extensive anoxia in the deep-
channel region of the Chesapeake Bay that have been prominent during the last 40
years. Most studies provide strong evidence that there was a greater frequency or
duration of seasonal anoxia beginning in the late 1930s and 1940s and again around
1970, reaching unprecedented frequencies or duration in the past few decades in the
mesohaline Chesapeake Bay and the lower reaches of several tidal tributaries. Clear
evidence of these low dissolved oxygen conditions has been found in all geochem-
ical and paleo-ecological indicators studied principally through their great impact on
benthic and phytoplankton (both diatom and dinoflagellate) communities.
chapter iii • Dissolved Oxygen Criteria
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12
Second, extensive late 18th and 19th century land clearance also led to oxygen reduc-
tion and hypoxia, which exceeded levels characteristic of the previous 2,000 years.
Best estimates for deep-channel mid-bay seasonal oxygen minima from 1750 to
around 1950 are 0.3 to 1.4-2.8 mg liter1 and are based on a shift to dinoflagellate
cyst assemblages of species tolerant of low dissolved oxygen conditions. This shift
is characterized by a four- to fivefold increase in the flux of biogenic silica, a greater
than twofold (5-10 millileter1) increase in nitrogen isotope ratios (15N) and periods
of common (though not dominant) Ammonia parkinsoniana, a facultative anaerobic
foraminifer. These patterns are likely the result of increased sediment influx and
nitrogen and phosphorous runoff due to extensive land clearance and agriculture.
Third, before the 17th century, dissolved oxygen proxy data suggest that dissolved
oxygen levels in the deep channel of the Chesapeake Bay varied over decadal and
interannual time scales. Although it is difficult to quantify the extremes, dissolved
oxygen probably fell to 3 to 6 mg liter1, but rarely if ever fell below 1.4 to 2.8 mg
liter1. These paleo-dissolved oxygen reconstructions are consistent with the Chesa-
peake Bay's natural tendency to experience seasonal oxygen reductions due to its
bathymetry, freshwater-driven salinity stratification, high primary productivity and
organic matter and nutrient regeneration (Boicourt 1992; Malone 1992; Boynton et
al. 1995).
In summary, the main channel of the Chesapeake Bay most likely experienced reduc-
tions in dissolved oxygen before large-scale post-colonial land clearance took place,
due to natural factors such as climate-driven variability in freshwater inflow.
However, this progressive decline in summer oxygen minima, beginning in the 18th
century and accelerating during the second half of the 20th century, is superimposed
on interannual and decadal patterns of dissolved oxygen variability. Human activity
during the post-colonial period has caused the trend towards hypoxia and most
recently (especially after the 1960s) anoxia in the main channel of the Chesapeake
Bay and some of its larger tidal tributaries. The impact of these patterns has been
observed in large-scale changes in benthos and phytoplankton communities, which
are manifestations of habitat loss and degradation.
APPROACH FOR DERIVING
DISSOLVED OXYGEN CRITERIA
Against this backdrop, a set of dissolved oxygen criteria have been derived to protect
Chesapeake Bay estuarine species living in different habitats that are influenced by
the Bay's natural processes. The Chesapeake Bay dissolved oxygen criteria directly
reflect natural oxygen dynamics. For example, instantaneous minimum to daily
mean criterion values reflect short-term variations in oxygen concentrations, and
seasonal application of deep-water and deep-channel criteria account for the natural
effects of water-column stratification on oxygen concentrations. Oxygen dynamics
and natural low- to no-oxygen conditions also were taken into account in developing
chapter iii • Dissolved Oxygen Criteria
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13
Chesapeake Bay Dissolved Oxygen Criteria Team
member Dr. Thomas Cronin, of the U.S. Geolog-
ical Survey (USGS), (surveyed five scientists1 who
have studied the history of anoxia and hypoxia in
the Chesapeake Bay over decadal and centennial
time scales, using geochemical and biological
proxies from sediment cores and instrumental and
historical records. The consensus of the five scien-
tists is that the Chesapeake Bay was seasonally
anoxic between 1900 and 1960. The seasonal
anoxia was extensive in the deep channel and prob-
ably lasted several months. Similarly between
1600 and 1900, the near-unanimous consensus is
that the Bay was seasonally anoxic for probably
weeks to months in the deep channel. One
researcher had reservations about his group's
earlier conclusion on definitive evidence of anoxia
prior to 1900, but cannot exclude the possibility of
anoxia during this period. Anoxia during the
1900-1960 period was probably geographically
less extensive in the Bay and perhaps occurred less
frequently (i.e., not every year) than after the
1960s. In addition to the geochemical and faunal
proxies of past trends in oxygen depletion, experts
cite the Sale and Skinner (1917) instrumental docu-
mentation of hypoxia and probable anoxia in the
lower Potomac in 1912.
For the period prior to European colonization
(-1600 AD), the consensus is that the deep
channel of the Bay may have been briefly hypoxic
(< 2 mg liter1), especially during relatively wet
periods (which did occur, based on the paleocli-
mate record). Anoxia probably occurred only
during exceptional conditions. It should be noted
that the late 16th and much of the 17th century was
an extremely dry period which was not conducive
to oxygen depletion.
In sum, hypoxia, and probably periodic spatially-
limited anoxia, occurred in the Bay prior to the
large-scale application of fertilizer, but since the
1960s oxygen depletion has become much more
severe.
These experts also unanimously believe that
restoring the Bay to mid-20th century, pre-1960
conditions might be possible but very difficult
(one expert suggested an 80 percent nitrogen
reduction was necessary), in light of remnant
nutrients in sediment in the Bay and behind dams,
likely increased precipitation as the climate
changes, population growth and other factors.
Most researchers believe that restoring the Bay to
conditions prior to 1900 is either impossible, or
not realistic, simply due to the fact that the
temporal variability (year-to-year and decadal) in
'naturally occurring' hypoxia renders a single
target dissolved oxygen level impossible to define.
'T. M. Cronin (USGS, Reston, Virginia), S. Cooper (Bryn Athyn College), J. F. Bratton (USGS, Woods Hole, Massachusetts),
A. Zimmerman (Pennsylvania State University), G. Helz (University of Maryland, College Park).
the refined tidal-water designated uses (see Appendix A; U.S. EPA 2003a), which
factor in natural conditions leading to low dissolved oxygen concentrations.
The derivation of these regional criteria followed the methodologies outlined in the
EPA's Guidelines for Deriving Numerical National Water Quality for the Protection
of Aquatic Organisms and their Uses (U.S. EPA 1985), the risk-based approach used
in developing the Ambient Aquatic Life Water Quality Criteria for Dissolved Oxygen
chapter iii • Dissolved Oxygen Criteria
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14
(Saltwaterj: Cape Cod to Cape Hatteras (U.S. EPA 2000) and the Biological
Evaluation on the CWA 304(a) Aquatic Life Criteria as part of the National Consulta-
tions, Methods Manual (U.S. EPA, U.S. Fish and Wildlife Service and NOAA National
Marine Fisheries Service, in draft). The resulting criteria factored in the physiological
needs and habitats of the Chesapeake Bay's living resources and are designed to
protect five distinct tidal-water designated uses (Appendix A; U.S. EPA 2003a).
Criteria for protecting the migratory fish spawning and nursery, shallow-water bay
grass and open-water fish and shellfish designated uses were set at levels to protect
the growth, recruitment and survival ecologically, recreationally and commercially
important fish and shellfish species. Criteria applicable to deep-water seasonal fish
and shellfish designated uses were set at levels to protect shellfish and juvenile and
adult fish, and to foster the recruitment success of the bay anchovy. Criteria for deep-
channel seasonal refuge designated uses were set to protect the survival of bottom
sediment-dwelling worms and clams. These summer deep-water and deep-channel
designated uses take into account the natural historic presence of low oxygen in
these habitats and the likelihood that such conditions may persist (U.S. EPA 2003a).
CHESAPEAKE BAY DISSOLVED OXYGEN
RESTORATION GOAL FRAMEWORK
The Chesapeake Bay dissolved oxygen restoration goal was published in 1992 in
response to the Chesapeake Executive Council's commitment to "develop and adopt
guidelines for the protection of water quality and habitat conditions necessary to
support the living resources found in the Chesapeake Bay system and to use these
guidelines" (Chesapeake Executive Council 1987). The 1992 goal contained specific
target dissolved oxygen concentrations for application over specified averaging
periods and locations (Table III-l; Jordan et al. 1992).
Information on the effects of low dissolved oxygen concentrations was compiled for
14 target species of fish, mollusks and crustaceans, as well as for other benthic and
planktonic communities in the Bay food web. These species were selected from a
larger list of important species reported in Habitat Requirements for Chesapeake
Bay Living Resources, Second Edition (Funderburk et al. 1991). The selection of
target dissolved oxygen concentrations and their temporal and spatial applications
followed an analysis of dissolved oxygen concentrations that would provide the
levels of protection needed to achieve the restoration goal. Where data gaps existed,
best professional judgment was used.
The original Chesapeake Bay dissolved oxygen restoration goal and its supporting
framework made three significant breakthroughs for the derivation and management
application of the Bay-specific dissolved oxygen criteria. First, the 1992 dissolved
oxygen target concentrations varied with the vertical depth of the water column and
horizontally across the Chesapeake Bay and its tidal tributaries, reflecting variations
in the levels of water quality required for the protection of different habitats (see
chapter iii • Dissolved Oxygen Criteria
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15
Table 111-1. 1992 Chesapeake Bay dissolved oxygen goal for restoration of living resource habitats.
The Chesapeake Bay dissolved oxygen goal for the restoration of living resource habitats is to
provide for sufficient dissolved oxygen to support the survival, growth and reproduction of
anadromous, estuarine and marine fish and invertebrates in the Chesapeake Bay and its tidal
tributaries by achieving, to the greatest spatial and temporal extent possible, the following
target concentrations of dissolved oxygen, and by maintaining the existing minimum
concentration of dissolved oxygen in areas of the Chesapeake Bay and its tidal tributaries
where dissolved oxygen concentrations fall above the recommended targets.
Target Dissolved Oxygen Concentrations
Time and Location
Dissolved oxygen > 1 mg liter1
All times, everywhere.
1.0 mg liter"1 > dissolved oxygen < 3 mg liter"1
For no more than 12 hours, interval between
excursions at least 48 hours, everywhere.
Monthly mean dissolved oxygen > 5 mg liter1
All times, throughout above-pycnocline1 waters.
Dissolved oxygen > 5 mg liter1
All times, throughout above-pycnocline waters
in spawning reaches, spawning rivers, and
nursery areas.
'The pycnocline is the portion of water column where density changes rapidly becaue of salinity and temperature.
Source: Jordan et al. 1992
Appendix A; U.S. EPA 2003a). Second, the averaging period for each target concen-
tration was tailored to each habitat, understanding that short-term exposures to
concentrations below the target concentrations were tolerable and could still protect
living resources (see "Chesapeake Bay Dissolved Oxygen Criteria Derivation," page
40). Finally, the 1992 dissolved oxygen restoration goal contained a methodology
through which water quality monitoring data and model scenario outputs, collected
over varying time periods, could be assessed to calculate the percentage of time that
areas of bottom habitat or volumes of water-column habitat would meet or exceed
the applicable target dissolved oxygen concentrations (see Chapter VI).
REGIONALIZING THE EPA VIRGINIAN PROVINCE SALTWATER
DISSOLVED OXYGEN CRITERIA
The EPA's Ambient Water Quality Criteria for Dissolved Oxygen (Saltwater): Cape
Cod to Cape Hatteras (U.S. EPA 2000), here referred to as the Virginian Province
criteria document, involved the development of an extensive database on dissolved
chapter iii • Dissolved Oxygen Criteria
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16
oxygen effects (Miller et al. 2002) and a close evaluation and synthesis of earlier
data, published in peer-reviewed literature. Ultimately the criteria were derived using
both traditional methodologies and a new biological risk-assessment framework. A
mathematical model was used to integrate effects over time, replacing the concept of
an averaging period, and protection limits were established for different life stages
(i.e., larvae versus juveniles and adults). Where practical, data were selected and
analyzed to conform to Guidelines for Deriving Numerical National Water Quality
Criteria for the Protection of Aquatic Organisms and their Uses (or EPA Guidelines,
U.S. EPA 1985).
The Virginian Province criteria document addressed three areas of protection:
1) juvenile and adult survival, 2) growth effects and 3) larval recruitment effects. In
doing so, it segregated effects on juveniles and adults from those on larvae. To
address cumulative effects of low dissolved oxygen on larval recruitment to the juve-
nile life stage (i.e., larval survival time), a new biological approach using a
mathematical model was taken. The model evaluated the effects of dissolved oxygen
conditions on larvae by tracking the intensity and duration of low dissolved oxygen
effects across the larval recruitment season (U.S. EPA 2000). Criteria to protect
larvae were derived using data based on varying dissolved oxygen exposures for
larval stages of nine sensitive estuarine and coastal organisms.
The juvenile and adult survival and growth criteria presented in the Virginian
Province document set boundaries forjudging the dissolved oxygen status of a given
site. If dissolved oxygen concentrations are above the Virginian Province chronic
growth criterion (4.8 mg liter1), then the site meets the objectives for protection. If
the dissolved oxygen conditions remain above the Virginian Province juvenile/adult
survival criterion (2.3 mg liter1) over a 24-hour period, the site meets the objectives.
When the dissolved oxygen conditions fall between these two values, then the site
requires further evaluation.
The Virginian Province criteria document supported the derivation of region-specific
dissolved oxygen criteria tailored to the species, habitats and dissolved oxygen ex-
posure regimes of varying estuarine, coastal and marine waters. The segregation by
life stage allows the criteria to be tailored to protect the individual refined Chesa-
peake Bay tidal-water designated uses, which reflect the use of different habitats by
different life stages (Appendix A). This segregation by life stage differs significantly
in approach from traditional aquatic life water quality criteria. However, the
Virginian Province criteria were not designed to address natural variations in
dissolved oxygen concentrations from surface waters to greater water-column
depths. If Chesapeake Bay-specific dissolved oxygen criteria had been derived using
only a strict application of this criteria methodology, they would not be flexible
enough to tailor each set of criteria to the refined tidal-water designated uses
presented in Appendix A. The resulting criteria would be driven solely by larval
effects data, irrespective of depth and season.
chapter iii • Dissolved Oxygen Criteria
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17
Therefore, the dissolved oxygen criteria specific to the Chesapeake Bay were
derived through the regional application of the Virginian Province criteria and the
application of both EPA published traditional toxicological and new EPA biological-
based methodologies. Chesapeake Bay-specific science was factored into each step
of the process. The extensive Virginian Province data base was supplemented with
additional Chesapeake Bay-specific data from the scientific literature. The Virginian
Province larval recruitment model parameters were adjusted to better reflect Chesa-
peake Bay conditions, data and species. Finally, steps were taken to ensure
protection of species listed as threatened or endangered in Chesapeake Bay tidal
waters following both national EPA guidelines and joint U.S. EPA, U.S. Fish and
Wildlife and National Marine Fisheries Service national Endangered Species Act
consultation methodologies. The Chesapeake Bay-specific dissolved oxygen criteria
were derived with the fall support of and technical assistance from the U.S. EPA
Office of Research and Development's Atlantic Ecology Division and the U.S. EPA
Office of Water's Office of Science and Technology.
Chesapeake Bay Species
A total of 36 species of fish, crustaceans and mollusks were included in the Virginian
Province criteria data base (U.S. EPA 2000). Only four are not resident Chesapeake
Bay species (Table III-2, U.S. EPA 1998), including the green crab and the mysid
Americamysis bahia. Both the American lobster and Atlantic surf clam have been
observed in the Chesapeake Bay, but only near the Bay mouth, in high salinities.
American lobster larvae require relatively low temperatures (20°C) and high salini-
ties (30 ppt) for successful development, and these conditions do not normally occur
in the Chesapeake Bay.
The EPA guidelines on criteria recalculation, which allow regional and site-specific
criteria derivation, state that species should be deleted from the effects data base only
if the class is absent (U.S. EPA 1994). Emphasis is placed on deriving criteria using
an effects data base that represents the range of sensitivity of tested and untested
species from, in this case, the Chesapeake Bay and its tidal tributaries. As described
below, including these four non-Chesapeake Bay species in the effects data base
does not change the Bay-specific dissolved oxygen criteria. To ensure consistency
with national EPA guidelines, no species were dropped from the original Virginian
Province effects data base when deriving these Chesapeake Bay-specific criteria.
Juvenile and Adult Survival Criteria
The criterion minimum concentration, or CMC, provides a lower limit for a 24-hour
averaged concentration to protect juvenile and adult survival. The CMC for juvenile
and adult survival was recalculated using a Chesapeake Bay-specific effects data
base of 32 species of fish, crustaceans and mollusks (Table III-2). Dropping the four
non-Chesapeake Bay species from the original Virginian Province data base resulted
in a recalculated Chesapeake Bay-specific juvenile/adult survival CMC value of
chapter iii • Dissolved Oxygen Criteria
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18
Table III-2. U.S. EPA Virginian Province criteria data base species found in the Chesapeake Bay.
Common Name
Scientific Name
Found in the Chesapeake Bay
Notes
Species
Genus Only
American lobster
Homarus americanus
(Y es)
-
1
Amphipod
Ampelisca abdita
Yes
-
Atlantic menhaden
Brevoortia tyrannus
Yes
-
Atlantic rock crab
Cancer irroratus
Yes
-
Atlantic silverside
Menidia menidia
Yes
-
Atlantic surfclam
Spisula solidissima
(Yes)
-
2
Blue crab
Callinectes sapidus
Yes
-
Burry's octopus
Octopus burryi
No
Yes
4
Daggerblade grass shrimp
Palaemonetes pugio
Yes
-
Eastern oyster
Crassostrea virginica
Yes
-
Flatback mud crab
Eurypanopeus depressus
Yes
-
Fourspine stickleback
Apeltes quadracus
Yes
-
Green crab
Carcinus maenas
No
No
6
Hard clam
Mercenaria mercenaria
Yes
-
Harris mud crab
Rhithropanopeus harrisii
Yes
-
Inland silverside
Menidia beryllina
Yes
-
Longfm squid
Loligo pealeii
(Y es)
-
3
Longnose spider crab
Libinia dubia
Yes
-
Marsh grass shrimp
Palaemonetes vulgaris
Yes
-
Mysid shrimp
Americamysis bahia
No
No
7
Naked goby
Gobiosoma bosc
Yes
-
Northern sea robin
Prionotus carolinus
Yes
-
Pipe fish
Syngnathus fuse us
Yes
-
Rock crab
Cancer irroratus
Yes
-
Sand shrimp
Crangon septemspinosa
Yes
-
continued
chapter iii • Dissolved Oxygen Criteria
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19
Table III-2. U.S. EPA Virginian Province criteria data base species found in the Chesapeake Bay (continued).
Common Name
Scientific Name
Found in the Chesapeake Bay
Notes
Species
Genus Only
Say mud crab
Dyspanopeus sayi
Yes
-
5
Scup
Stenotomus chysops
Yes
-
Sheepshead minnow
Cyprinodon variegatus
Yes
-
Skillet fish
Gobiesox strumosus
Yes
-
Striped bass
Morone saxatilis
Yes
-
Striped blenny
Chasmodes bosquianus
Yes
-
Spot
Leiostomus xanthurus
Yes
-
Summer flounder
Paralichthys dentatus
Yes
-
Tautog
Tautoga onitis
Yes
-
Windowpane flounder
Scophthalmus aquosus
Yes
-
Winter flounder
Pleuronectes americanus
Yes
-
Notes:
1. Occasionally found in the Chesapeake Bay mouth region outside of the Bay Bridge/tunnel during blue
crab winter dredge surveys.
2. Found near the Chesapeake Bay mouth at high salinities.
3. Found in the region around the Chesapeake Bay mouth.
4. Octopus americanus is found in the higher salinity reaches of the Chesapeake Bay.
5. Genus Dyspanopeus supercedes genus Neopanope (See Weiss, H. 1995. Marine Animals of Southern
New England and New York, State Geological and Natural History Survey of Connecticut).
6. If found in the Chesapeake Bay, Carcinus maenas would be at the extreme southern edge of its range
(See Gosner, K. 1979. Field Guide to the Atlantic Seashore : Invertebrates and Seaweeds of the Atlantic
Coast, from the Bay of Fundy to Cape Hatteras, Houghton Mifflin. Boston. ). This species has not been
documented in the Comprehensive List of Chesapeake Bay Basin Species (U.S. EPA 1998 ).
7. Americamysis bahia supercedes Mysidopsis bahia. (See Price W. W., R. W. Heard, L. Stuck 1994.
Observations on the genus Mysidopsis Sars, 1864 with the designation of a new genus, Americamysis,
and the descriptions of Americamysis alleni and A. stucki (Peracarida: Mysidacea: Mysidae), from the
Gulf of Mexico. Proceedings of the Biological Society of Washington 107:680-698).
Sources: U.S. EPA 1998, 2000.
chapter iii • Dissolved Oxygen Criteria
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20
2.24 mg liter"1, very close to the EPA Virginian Province criterion value of 2.27 mg
liter1 (U.S. EPA 2000). To maintain consistency with EPA Virginian Province
criteria and national EPA guidelines, no changes were made to the Virginian
Province criteria value of 2.27 mg liter1 (rounded off to 2.3 mg liter1 for purposes
of this criteria document), applied as a 1-day mean concentration.
Larval and Juvenile Growth Criteria
The criterion value protecting against adverse effects on growth under continuous
exposures, called the criterion continuous concentration (or CCC), when recalcu-
lated for only Chesapeake Bay species, increased 0.2 mg liter1 to a Chesapeake
Bay-specific value of 5.0 mg liter1. To maintain consistency with EPA Virginian
Province criteria and the national EPA criteria derivation guidelines, no changes
were made to the Virginian Province criteria value of 4.8 mg liter1.
Larval Recruitment Model Application
The Virginian Province criteria larval recruitment model was used only to confirm
that the criterion values selected for the migratory fish spawning and nursery,
shallow-water and open-water criteria fully protected larval recruitment. Only in the
case of the deep-water criteria was application of the larval recruitment model
central to deriving Chesapeake Bay-specific dissolved oxygen criteria values.
Virginian Province Larval Recruitment Model. The recruitment model is a
discrete time, density-independent model consisting of several equations that allow
the cumulative impact of low dissolved oxygen to be expressed as a proportion of the
potential annual recruitment of a species. The model is run by inputting the neces-
sary bioassay and biological information, selecting dissolved oxygen durations to
model, and then, through an iterative process, assessing various dissolved oxygen
concentrations until the desired percent recruitment impairment is obtained. The
resulting pairs of duration and dissolved oxygen concentration become the recruit-
ment curve. The process has been incorporated in a spreadsheet for simplicity. The
model can be set up to handle unlimited and various life history stages. Its applica-
tion for dissolved oxygen effects is to model larval recruitment to the juvenile stage.
The model's equations and the major assumptions used in its application are
explained in Appendix E of the Virginian Province document (U.S. EPA 2000). The
life history parameters in the model include larval development time, larval season,
attrition rate and spatial distribution (e.g., vertical distribution). The magnitude of
effects on recruitment is influenced by each of the four life history parameters. For
instance, larval development time establishes the number of cohorts that entirely or
partially co-occur within the interval of low dissolved oxygen stress. The second
parameter, the length of the larval season, is a function of the spawning period, and
also influences the relative number of cohorts that fall within the window of hypoxic
chapter iii • Dissolved Oxygen Criteria
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21
stress. The third life history variable, natural attrition rate, gauges the impact, if any,
of slower growth and development of the larvae in response to low dissolved oxygen
by tracking the associated increase in natural mortality (e.g., predation). The model
assumes a constant rate of attrition, so increased residence time in the water column
due to delayed development translates directly to decreased recruitment. Finally, the
distribution of larvae in the water column determines the percentage of larvae from
each cohort that would be exposed to reduced dissolved oxygen under stratified
conditions.
The recruitment model assumes that the period of low dissolved oxygen occurs
within the larval season (hypoxic events always begin at the end of the development
time of the first larval cohort), and that hypoxic days are contiguous. Use of the
current model also assumes that a new cohort occurs every day of the spawning
season, and that each cohort is equal in size. Use of the model, however, does not
require that a fresh cohort be available every day. Successful calculation of recruit-
ment impairment only requires knowing the total number of cohorts available during
a recruitment season (i.e., it does not matter whether they were created daily, weekly,
monthly, etc.) and whether a cohort is exposed to hypoxia. The application of the
model is further simplified by assuming that none of the life history parameters
change in response to hypoxia.
Chesapeake Bay Larval Recruitment Model Refinements. A series of
refinements were made to the Virginian Province criteria parameters for length of
recruitment season and duration of larval development. These values were revised to
reflect Chesapeake Bay-specific conditions (Table III-3).
Crustaceans. The Virginian Province criteria document states that the larval model
for crustaceans includes all larval stages and the transition from larval to megalopal
(post-larval) stage, but not the megalopal stage in its entirety (U.S. EPA 2000).
Therefore, the duration used in the model was based on the duration of larval devel-
opment, plus one day for molting to the megalopal stage. The following Chesapeake
Bay-specific estimates of the duration of larval development are rounded to the
nearest whole day: rock crab—22 days; say mud crab—17 days; lobster—15 days;
spider crab—6 days; and grass shrimp—15 days. These estimates also are supported
by a wide array of literature (Anger et al. 1981a; Anger et al. 1981b; Broad 1957;
Chamberlain 1957; Costlow and Bookhout 1961; Johns 1981; Logan and Epifanio
1978; Maris 1986; Ryan 1956; Sandifer 1973; Sandifer and Van Engel 1971; Sasaki
et al. 1986; Sastry 1970; Sastry 1977; Sastry and McCarthy 1973; Sulkin and
Norman 1976; Wass 1972; Williams 1984).
The literature supports a larval release season (here termed the reproductive season)
of 120 days or more for rock crab, say mud crab and spider crab, based on the pres-
ence of gravid females and larvae in field collections (Anger et al. 1981a; Anger et
al. 1981b; Broad 1957; Chamberlain 1957; Costlow and Bookhout 1961; Johns
1981; Logan and Epifanio 1978; Maris 1986; Ryan 1956; Sandifer 1973; Sandifer
chapter iii • Dissolved Oxygen Criteria
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22
Table 111-3. Original U.S. EPA Virginian Province saltwater dissolved oxygen criteria larval
recruitment values and the revised recruitment season and larval development
values reflecting Chesapeake Bay-specific conditions.
Species
Length of
Recruitment
Season (days)1
Duration of Larval
Development
(days)1
Attrition
Rate (percent
per day)
Percentage
Population
Exposed to
Hypoxic Event
Rock crab
65/100
35/22
5%
20%
Say mud crab
66/90
21/17
5%
75%
Flatback mud crab
66/90
21/17
5%
75%
Lobster
95
35/15
5%
20%
Spider crab
66/80
21/6
5%
50%
Silverside
42/150
14
5%
50%
Striped bass
49/70
28
5%
50%
Grass shrimp
100/120
12/15
5%
50%
Red dram
49/140
21
5%
50%
1 First value is the original Virginian Province-wide value; the second value following the slash is the
Chesapeake Bay-specific value.
and Van Engel 1971; Sasaki et al. 1986; Sastry 1970; Sastry 1977; Sastry and
McCarthy 1973; Sulkin and Norman 1976; Wass 1972; Williams 1984). Lobster
larvae and adults are rarely found in the Chesapeake Bay, therefore, collection data
were not available.
Grass shrimp have an extremely long reproductive season that extends even longer
than the brachyurans. The Virginian Province criteria document implies that the
actual period over which most of these crustaceans release larvae is only 30 to 40
days (except for grass shrimp). This was not supported in the literature for the Chesa-
peake Bay. However, given the interest in capturing "the period of predominant
recruitment, rather than observance of the first and last dates for zoeal presence in
the water column" (U.S. EPA 2000), one could reasonably state that brachyuran
larvae are released over a 75-day period in the Chesapeake Bay. Grass shrimp larvae
are released over a period of at least 100 days due to their greater reproductive flex-
ibility. These reproductive season values, added to the duration of the larval
development, provided the following values for the length of the recruitment season
in the Chesapeake Bay: rock crab—100 days; mud crab—90 days; spider crab—
80 days; and grass shrimp—120 days (Table III-3).
chapter iii • Dissolved Oxygen Criteria
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23
Fishes. In the Chesapeake Bay, striped bass spawn over a 30- to 40-day period. By
adding in the duration of larval development of 28 to 50 days, a reasonable estimate
for the recruitment season is 70 days (Grant and Olney 1991; McGovern and Olney
1996; Olney et al. 1991; Rutherford and Houde 1995; Secor and Houde 1995;
Ulanowicz and Polgar 1980). It should be noted that most spawning in a given trib-
utary may occur over a much shorter period of 7 to 21 days (Rutherford 1992; Olney
et al. 1991). However, given the inability to predict which portion of the reproduc-
tive season will result in recruitment, it is important to provide water quality
conditions that support recruitment for the duration of spawning season (Secor 2000;
Secor and Houde 1995).
Silversides, along with other East Coast estuarine-dependent species, tend to show
differences in the date of initiation of spawning and spawning duration from north to
south (e.g., southern sites have longer durations). Silversides are serial batch
spawners that spawn over a less than two-month period in the northern regions of the
east coast, from two to three months around New York, and from three to four
months in the Maryland portion of the Chesapeake Bay (Conover and Present 1990;
Conover 1992; Gleason and Bengston 1996). A 140-day recruitment season factors
in a 90-day reproductive season and a 50-day duration of larval development.
Red drum also are serial batch spawners. Documentation of the red drum spawning
season is mostly for southern systems and varies between two months (Wilson and
Neiland 1994; Rooker and Holt 1997) and three months (McMichael and Peters
1987). The 140-day recruitment season applied here factors in a 90-day reproductive
season and a 50-day duration of larval development.
Impairment Percentage. Population growth of estuarine and coastal organisms
may be more affected by mortality of the juvenile and adult stages than the larval
stage. In nature only a small fraction of a season's larvae will make it to the juve-
nile/adult stage. Thus, removal of a single larva from exposure to low dissolved
oxygen (which has a high probability of being removed naturally) is not nearly as
important as the loss of a single juvenile (at each successive life stage—from egg to
larva to juvenile to adult—the probability of survival to the next stage increases).
Juveniles are much closer to the reproductive stage and represent the loss not only
of the individual, but also of the potential larvae from that individual for the next
season. In this regard, an individual larvae is not as important to the population as an
individual juvenile or adult. Therefore, populations can tolerate different levels of
impact at different stages of individual development (U.S. EPA 2000). At the same
time, the criteria need to protect members of a species at all life stages so they can
develop from an egg to an adult.
Protection against a greater than 5 percent cumulative reduction in larval seasonal
recruitment due to exposure to low oxygen conditions was applied in the Chesapeake
Bay-specific larval recruitment effects models, consistent with the level of protection
selected for the Virginian Province criteria (U.S. EPA 2000). The selection of a
5 percent impairment of early life stages accords the same level of protection as that
chapter iii • Dissolved Oxygen Criteria
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24
set for adult and juvenile life stages through the CMC criteria. The 5 percent impair-
ment also is consistent with EPA guidelines for deriving ambient aquatic life water
quality criteria (U.S. EPA 1985). The 5 percent impairment sets the potential reduction
in seasonal recruitment of affected species due to low dissolved oxygen exposure at a
low level, relative to the cumulative effects of other natural and anthropogenic factors.
The EPA's criteria derivation guidelines and technical support documents do not
state that the purpose of criteria is to prevent any losses; the purpose of the criteria
is to prevent "unacceptable" losses. The EPA has acknowledged throughout the
history of the criteria development process that criteria may allow some adverse
effects to occur, e.g., the use of 95th percentile means that there is the possibility that
5 percent of the communities' genera will experience some impact (U.S. EPA 1985).
The EPA recognizes that large losses of larval life stages occur naturally. Some
species may be able to withstand a greater than 5 percent loss of larvae from expo-
sure to low dissolved oxygen or other causes without an appreciable effect on
juvenile recruitment. However, this may not be the case for certain highly sensitive
species or populations that already are highly stressed, such as threatened/
endangered species where the 5 percent impairment is not applied.
In the absence of data showing how much impairment may be caused by low
dissolved oxygen conditions alone and still have a minimal effect on natural larval
recruitment to the juvenile stage for all species protected, a conservative level of
acceptable impairment has been applied. The goal is to provide a level of protection
from exposure to low dissolved oxygen that will not cause unacceptable loss to the
juvenile recruitment class above what is expected to occur naturally.
Regional Species Effects
The same species from different regions may react differently to low dissolved
oxygen conditions. For example, populations from traditionally warmer waters may
be less sensitive because they have adapted to lower concentrations of oxygen asso-
ciated with native warmer temperatures. Alternatively, higher temperatures may
cause warmer-water populations to need more dissolved oxygen and thereby make
them more sensitive to lower concentrations.
Most of the effects data in the EPA Virginian Province saltwater dissolved oxygen
criteria document were from EPA-sponsored laboratory tests conducted with species
collected in the northern portion of the province. To determine whether such
geographic differences exist, northern (Rhode Island) and southern populations
(Georgia or Florida) of two invertebrates, the mud crab and the grass shrimp, and one
fish, the inland silverside, were tested in the laboratory at non-stressful temperatures.
Exposure-response relationships were similar for northern and southern populations of
each species, supporting the use of data from one region to help develop safe dissolved
oxygen limits for other regions (Coiro et al., unpublished data; see Appendix B).
chapter iii • Dissolved Oxygen Criteria
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25
Temperature/Dissolved Oxygen Interactions
This document includes effects data collected at temperatures that are greater than
20°C and many greater than 25°C. Where there are data for the same species at
multiple temperatures, for example, grass shrimp larvae tested at temperatures
ranging from 20°C to 30°C (see Appendix B), there is no evidence for a temperature
effect on sensitivity to hypoxia over the range of temperatures tested.
The findings reported in detail in Appendix B indicate that the low dissolved oxygen
effects data were gathered over a range of different temperatures that did not influ-
ence the resulting effects findings. These findings further confirmed that test
organisms from the northern portion of the Virginian Province were no more or less
sensitive than organisms collected well south of the province boundaries. (See
"Strengths and Limitations of the Criteria Derivation Procedures," p. 34, for a de-
scription of the potential interaction between dissolved oxygen effects and stressful
temperatures.)
APPLYING THE EPA FRESHWATER DISSOLVED OXYGEN CRITERIA
The Virginian Province saltwater criteria were derived largely from laboratory-based
effects data using test conditions with salinities ranging from oligohaline to oceanic.
Although a majority of the tests were run at salinities of greater than 15 ppt, data
from the literature included tests whose estuarine species were exposed to salinities
as low as 5 ppt. Many of the estuarine species tested tolerate a wide range of salini-
ties, but the location of the U.S. EPA Office of Research and Development Atlantic
Ecology Division laboratory at Narragansett Bay, Rhode Island, dictated that the
tests be run at higher salinities. With extensive tidal-fresh (0-0.5 ppt) and oligohaline
(> 0.5-5ppt) habitats in the upper Chesapeake Bay and upper reaches of most tidal
tributaries, criteria established for these less saline habitats must protect resident
species. To bridge this gap, the applicable EPA freshwater dissolved oxygen criteria
were applied to ensure that the Chesapeake Bay-specific criteria protected fresh-
water species inhabiting tidal waters.
Freshwater Dissolved Oxygen Criteria
The EPA freshwater criteria document, published in 1986, stipulated five limits for
dissolved oxygen effects on warm-water species (Table III-4, U.S. EPA 1986). To
protect early life stages, the criteria include a 7-day mean of 6 mg liter1 and an
instantaneous minimum of 5 mg liter1. To protect other life stages, additional criteria
were derived. These are a 30-day mean of 5.5 mg liter1, a 7-day mean of 4 mg
liter1 and an instantaneous minimum of 3 mg liter1. Some of the most sensitive
survival and growth responses reported for warm-water species in the freshwater
criteria document were for early life stages of channel catfish and largemouth bass,
both of which are present in tidal-fresh habitats throughout the Chesapeake Bay and
its tidal tributaries (Murdy et al. 1997).
chapter iii • Dissolved Oxygen Criteria
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26
The freshwater dissolved oxygen criteria docu-
mentation contains data for effects on an
extensive array of fish species. In addition, the
freshwater document focuses on growth effects
to early life stages, which are the more sensi-
tive stages. Recognizing that the 1986
freshwater dissolved oxygen criteria were not
derived following the EPA's 1985 criteria deri-
vation guidelines, the EPA conducted a
preliminary survey of the literature since the
1986 freshwater document was published and
did find additional data that were consistent
with the 1985 EPA guidelines. However, the
effects data that were found (additional field
observations and short-term [several hours]
laboratory exposures), most of which focused
on respiratory effects, indicated that the 1986 freshwater criteria were protective.
Therefore, the EPA believes that its existing freshwater criteria accurately portray the
expected effects of low dissolved oxygen on freshwater aquatic species.
Early Life Stages
The EPA freshwater early life stage criteria were based on embryonic and larval data
for the following eight species: largemouth bass, black crappie, white sucker, white
bass, northern pike, channel catfish, walleye and smallmouth bass (U.S. EPA 1986).
Fishes of Chesapeake Bay (Murdy et al. 1997) documents smallmouth bass as "occa-
sional to common in Chesapeake Bay tributaries from Rappahannock northward,
rare to occasional south of the Rappahannock, and absent from Eastern Shore
streams and rivers." Regarding white suckers: "Found in all tributaries to Chesa-
peake Bay throughout the year, the white sucker occurs in nearly every kind of
habitat..." The largemouth bass is "common to abundant in all tributaries of Chesa-
peake Bay." Black crappie were reported to be "occasional to abundant inhabitants
in major tributaries of Chesapeake Bay." Finally, channel catfish were "common in
all tributaries of Chesapeake Bay." (All references Murdy et al. 1997.)
Given that five of these species—largemouth bass, black crappie, white sucker,
channel catfish and smallmouth bass—are resident in Bay tidal-fresh waters, the
freshwater early life stage criteria are fully applicable to Chesapeake Bay tidal-fresh
habitats. (See Figure 1 on page 14 and the text on pages 17-18 in the EPA's Ambient
Water Quality Criteria for Dissolved Oxygen [Freshwater] for more details; U.S.
EPA 1986.) No efforts were made to recalculate the national freshwater criteria using
only Chesapeake Bay species, given the limited number of species used in deriving
the 1986 criteria. Dropping any of the eight species would not provide an effects data
set meeting the EPA's guidelines for criteria recalculation to address site-specific
conditions (U.S. EPA 1994).
Table 111-4. U.S. EPA freshwater dissolved oxygen
water quality criteria (mg liter1)
for warm-water species.
Duration
Early
Life Stages1
Other
Life Stages
30-day mean
NA2
5.5
7-day mean
6
NA
7-day mean minimum NA
4
1-day minimum3
5
3
includes all embryonic and larval stages and all juvenile
forms to 30 days following hatching.
2Not applicable.
3A11 minima should be considered as instantaneous
concentrations to be achieved at all times.
Source: U.S. EPA 1986.
chapter iii • Dissolved Oxygen Criteria
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27
Other Life Stages
The warm-water freshwater criteria that protect other life stages were derived from
a much wider array of fish and invertebrate species, many of which occur in Chesa-
peake Bay tidal-fresh habitats (U.S. EPA 1998). These criteria apply to Chesapeake
Bay habitats with salinities of less than 0.5 ppt. The national EPA freshwater criteria
protecting other warm-water species life stages were not recalculated using only
Chesapeake Bay species, for the same reasons described above.
Given the differences in the available effects data, the methodologies followed in
deriving the freshwater dissolved oxygen criteria differed from those used in developing
the Virginian Province dissolved oxygen criteria. In-depth descriptions of both method-
ologies can be found in each respective criteria document (U.S. EPA 1986, 2000).
SPECIES LISTED AS THREATENED OR ENDANGERED
When a threatened or endangered species occurs at a site and sufficient data indicate
that it is sensitive at concentrations above the recommended criteria, site-specific
dissolved oxygen criteria may be derived (U.S. EPA 2000). Based on a review of all
federal and Chesapeake Bay tidal water state lists of threatened or endangered
species (U.S. Fish and Wildlife Service; National Oceanic and Atmospheric
Administration; the states of Maryland, Virginia and Delaware and the District of
Columbia), the only federally listed endangered species found to need protection
from the effects of low dissolved oxygen conditions was shortnose sturgeon
(U.S. EPA 2003b).
Shortnose sturgeon occur in the Chesapeake Bay and several tidal tributaries
(Skjeveland et al. 2000; Mangold 2003; Spells 2003). Genetic evidence suggests that
the shortnose captured in the Chesapeake Bay share the same gene pool with
Delaware Bay shortnose sturgeon, and movement has been documented between the
two bays through the C & D Canal (Welsh et al. 2002; Wirgin et al., in review).
Shortnose sturgeon have been federally protected since 1967 (National Marine Fish-
eries Service 1998). Chesapeake Bay shortnose sturgeon are listed as a Distinct
Population Segment in the National Oceanic and Atmospheric Administration's
National Marine Fisheries Service Shortnose Sturgeon Recovery Plan. Since 1996,
50 sub-adult and adult shortnose sturgeon have been captured in the upper Chesa-
peake Bay, Potomac River and Rappahannock River (Skjeveland et al. 2000).
Mitochondrial DNA analysis indicated that these were a subset of the Delaware
population's gene pool.
Currently two views are held on the status of shortnose sturgeon in the Chesapeake
Bay. One view holds that shortnose sturgeon may continue to reproduce in the Bay,
arguing that the genetic evidence is inconclusive or that the Delaware Bay and
Chesapeake Bay populations may share the same gene pool. The other opinion is
that the C & D Canal serves as an important migration corridor, and shortnose occur-
rences in the Chesapeake Bay result from immigration from the Delaware Bay.
chapter iii • Dissolved Oxygen Criteria
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28
Further, due to salinity preferences it is conceivable that their immigration (and
recent occurrences) has been favored by the recent series of wet years. Several stur-
geon population geneticists, ecologists and icthyologists favor this latter view (Secor
2003; Wirgin et al. in review; I. Wirgin, personal communication; J. Waldman,
personal communication; J. Musick, personal communication). Regardless of
whether shortnose sturgeon populations remain in the Chesapeake Bay, the
Chesapeake Bay dissolved oxygen criteria have been derived to be protective of all
life stages of both shortnose and Atlantic sturgeon.
Sturgeon Dissolved Oxygen Sensitivity
Sturgeon in Chesapeake Bay and elsewhere are more sensitive to low dissolved
oxygen conditions than most other fish. In comparison with other fishes, sturgeon
have a limited behavioral and physiological capacity to respond to hypoxia (multiple
references reviewed and cited by Secor and Niklitschek 2003). Sturgeon basal
metabolism, growth, consumption and survival are all very sensitive to changes in
oxygen levels, which may indicate their relatively poor ability to oxyregulate. In
summer, temperatures greater than 20°C amplify the effect of hypoxia on sturgeon
and other fishes due to a temperature-oxygen 'habitat squeeze' (Coutant 1987). Deep
waters with temperatures that sturgeon prefer tend to have dissolved oxygen concen-
trations below the minimum that they require. Sturgeon are therefore either forced to
occupy unsuitable habitats or have a reduction in habitat.
Several studies have directly addressed the lethal effects of hypoxia on sturgeon
species important to the Chesapeake Bay. Jenkins et al. (1993) examined the effects
of different salinities and dissolved oxygen levels on juveniles of the shortnose stur-
geon Acipenser brevirostrum. The dissolved oxygen tests were all conducted at a
mean temperature of 22.5°C. The authors state:
Due to various constraints including limitations of facilities and test
animals, strictly controlled and standardized methods could not be
followed in all tests. The findings reported should be considered as
preliminary until such time as more rigorous testing can be accomplished.
In addition, the authors report nominal2 oxygen levels rather than those specific
dissolved oxygen concentrations experienced during each replicate experiment. All
experiments were conducted in freshwater. Still, strong evidence was presented that
younger fish were differentially susceptible to low oxygen levels in comparison to
older juveniles. Fish older than 77 days experienced minimal mortality at nominal
levels 2.5 mg liter1, but at 2 mg liter1 experienced 24 to 38 percent mortality.
2The authors report that dissolved oxygen levels were monitored every 30 minutes throughout the
6-hour tests, and state that each parameter remained at "satisfactory levels." The dissolved oxygen
values reported are 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0 and 7.5 mg liter"1. Since up to five replicates were
used with as many as 12 measurements, it seems unlikely that these exact dissolved oxygen concen-
tration values were maintained consistently throughout all the tests.
chapter iii • Dissolved Oxygen Criteria
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Younger fish experienced 18 to 38 percent mortality in the 3 mg liter"1 treatment and
>80 percent mortality in the 2.5 mg liter1 treatment. Mortality of juveniles 77 days
or older at treatment levels >3.5 mg liter"1 was not significantly different than control
levels. Because only nominal dissolved oxygen concentrations were reported, the
EPA could not derive LC50 values for criteria derivation purposes based upon
responses reported by Jenkins et al. (1993).
Criterion Protective of Sturgeon
More rigorous tests with shortnose sturgeon were recently performed using young-
of-the-year fish 77 to 134 days old (Campbell and Goodman 2003). Campbell and
Goodman (2003) present four 24-hr LC50 values for shortnose sturgeon (Acipenser
breviwstmm). Three of these are from tests with non-stressful temperatures
(22-26°C) for this species. The fourth test was conducted at 29°C and was con-
sidered to be a stressful temperature by the authors (Larry Goodman, personal
communication). Fish from this fourth test also were exposed to temperatures as
high as 31°C during the acclimation period immediately preceding their exposure to
hypoxia. Since the data from the fourth test also include an effect due to temperature
stress, they should be considered separately from data from the other three tests.
The current draft (December 2002) of the "National Consultation" on threatened and
endangered species (being negotiated between the U.S. EPA, the U.S. Fish and
Wildlife Service and the NOAA National Marine Fisheries Service) states:
Where acute toxicity data are available for the species of interest, only
these data will be used for designating the LC50 for this species. If these
data include more than one test, the geometric mean of the LC50s of these
tests will be used in risk calculations. If only one toxicity test has been
conducted, the lower 95% confidence interval of the LC50 from this test
will be used.
Following this guidance the final LC50 for shortnose sturgeon under ambient con-
ditions of non-stressful temperatures would be the geometric mean of 2.2, 2.2 and
2.6 mg liter1, or 2.33 mg liter1. Under stressful temperatures, the LC50 value that
should be used would be 3.1 mg liter1 (this is the LC50 of the 29°C test, since the
3.1 mg liter1 treatment resulted in exactly 50 percent mortality there was no
95 percent confidence interval) (Campbell and Goodman 2003).
Long-term exposures (10 days) of Atlantic sturgeon, Acipenser oxyrinchus, young-
of-the-year (150 to 200 days old) to 2.8 to 3.3 mg liter1 at 26°C resulted in complete
mortality over a 10-day period in three of four replicates (Secor and Gunderson
1998). The fourth replicate experienced 50 percent mortality. At 19°C and 2.3 to
3.2 mg liter"1, only 12 to 25 percent mortality was recorded. There was insufficient
data to calculate an LC50 for 19°C (it was less than 2.70 mg liter1 3, but could not
determine how much less). However, based on survival data present in Secor and
3Based on daily dissolved oxygen data provided by the lead author, Dr. David Secor, University of
Maryland Center for Environmental Science, Chesapeake Biological Laboratory, Solomons, Maryland.
chapter iii • Dissolved Oxygen Criteria
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30
Gunderson (1998), a 96-hour LC50 of 2.89 mg liter1 3 was estimated for Atlantic
sturgeon at 26°C. This value is very similar to the "high temperature" value of
3.1 mg liter1 calculated for shortnose sturgeon by Campbell and Goodman (2003).
Data from Secor and Niklitschek (2001) show that shortnose sturgeon are more
tolerant of higher temperatures than Atlantic sturgeon, which could explain why
26°C is not a stressful temperature for shortnose sturgeon (Campbell and Goodman
2003), but is for Atlantic sturgeon (Secor and Gunderson 1998). Alternatively, the
temperature difference between the two species could be because the shortnose stur-
geon were from Savannah River progeny and were held at higher temperatures than
the Atlantic sturgeon, which came from Hudson River progeny.
Using the above data, the EPA calculated acute criteria for the protection of sturgeon
survival in the Chesapeake Bay under both non-stressful and stressful temperatures.
The only LC50 value available for non-stressful temperatures that meets the require-
ments for criteria derivation based on the EPA's 1985 guidelines (U.S. EPA 1985) is
the 24-hour 2.33 mg liter1 calculated above from Campbell and Goodman (2003).
To be consistent with EPA guidelines, this value was used with the original Virginian
Province criteria acute data set to recalculate the Final Acute Value (FAV). The new
FAV, 2.12 mg liter1, is more protective than the 1.64 mg liter1 from the Virginia
Province document, but still substantially lower than the 2.33-3.5 mg liter"1 derived
directly from the empirical study on shortnose sturgeon. Therefore, the EPA
defaulted to the 2.33 mg liter1 value, multiplying it by 1.384 to arrive at a new CMC,
3.2 mg liter1 (rounded to two significant figures). This value is expected to be
protective of sturgeon survival at non-stressful temperatures. Campbell and
Goodman (2003) indicate that most of the mortality for shortnose sturgeon occurs
within the first 2 to 4 hours of a test. Therefore, using this value as an instantaneous
value should protect sturgeon under most conditions.
A higher dissolved oxygen criterion would be needed in areas and times of the year
where sturgeon are to be protected and temperatures are likely to be considered
stressful (e.g., 29°C and above for shortnose sturgeon). The simplest approach is to
use the LC50 value of 3.1 mg liter1 from the fourth test of Campbell and Goodman
(2003). Multiplying this by 1.38 results in a high temperature CMC for shortnose
sturgeon of 4.3 mg liter1.
To determine a criterion value that would also protect sturgeon from nonlethal
effects, bioenergetic and behavioral responses were considered which had been
derived from laboratory studies conducted on juvenile Atlantic and shortnose stur-
geon (Niklitschek 2001; Secor and Niklitschek 2001). Growth was substantially
reduced at 40 percent oxygen saturation compared to normal oxygen saturation
4 This value is the geometric mean of the LC5/LC50 ratios from the Virginian Province document
(U.S. EPA 2000). The ratio for the shortnose sturgeon tests from Campbell and Goodman (2003) was
1.30 based on an analysis of raw data provided by the co-author, Larry Goodman, U.S. Environmental
Protection Agency, office of Research and Development, Gulf Ecology Division, Gulf Breeze,
Florida. To be consistent with the Virginian Province document, EPA applied the 1.38 ratio.
chapter iii • Dissolved Oxygen Criteria
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31
conditions (greater than or equal to 70 percent saturation) for both species at temper-
atures of 20°C and 27°C. Metabolic and feeding rates declined at oxygen levels
below 60 percent oxygen saturation at 20°C and 27°C. In behavior studies, juveniles
of both sturgeon species actively selected 70 percent or 100 percent oxygen satura-
tion levels over 40 percent oxygen saturation levels. Based on these findings, a
60 percent saturation level was deemed protective for sturgeon. This corresponds to
5 mg liter1 at 25°C. Therefore, a 5 mg liter1 Chesapeake Bay criterion protecting
against adverse growth effects would protect sturgeon growth as well.
In accordance with Section 7 of the Endangered Species Act, the EPA is continuing
consultation with the NOAA National Marine Fisheries Service to promote the
recovery and protection of the endangered shortnose sturgeon in the Chesapeake Bay
and its tidal tributaries.
Historical and Potential Sturgeon Tidal Habitats
in Chesapeake Bay
Atlantic and shortnose sturgeon probably most recently colonized the Chesapeake
Bay 5,000-8,000 years ago after the last glaciation, when climate and the water-
shed's hydraulic regime became more stable (Custer 1986; Miller 2001; also see
page 11). The Chesapeake Bay during this period already exhibited the two-layer
circulation pattern. Thus, we should expect that deep-channel habitats during periods
of strong stratification were hypoxic during the past 5,000 years, albeit not at the
same spatial extent or severity that has occurred over the past 50 years (Officer et al.
1984; Cooper and Brush 1991). Atlantic sturgeon in other estuarine and coastal
systems will use habitats greater than 15 meters in depth (see below), but these other
systems do not exhibit the same characteristics of estuarine circulation, watershed
areal extent and bathymetry that contribute to natural deep-water and deep-channel
hypoxia in the mesohaline Chesapeake Bay.
The geochemical, paleo-ecological and instrumental record of the 20th century indi-
cates that deep-channel regions have not served as potential habitats for sturgeon
because seasonal (summer) anoxia and hyopxia have occurred most years, reaching
levels below those required by sturgeon. Hypoxia, and probably periodic, spatially-
limited anoxia occurred in the Chesapeake Bay prior to the large-scale application of
fertilizer, but since the 1960s oxygen depletion has become much more severe (Hagy
2002), prohibiting sturgeon use of this habitat during summer months. Analysis of
recent U.S. Fish and Wildlife Service sturgeon capture location data showed absence
of sturgeon occurrences in deep-channel habitats during summer months (June 1
through September 30), but substantial numbers of occurrences in these same habi-
tats during other seasons (U.S. EPA 2003b). In summary, based upon the recent
relevant history of the Chesapeake Bay ecosystem, deep-channel regions in summer
are not considered sturgeon habitats.
Deeper water-column regions may continue to provide temperature refuges, migra-
tion corridors and foraging for sturgeon in the absence of strong water-column
chapter iii • Dissolved Oxygen Criteria
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32
stratification (which results in dissolved oxygen concentrations well below
saturation levels, due to restricted mixing with the well-oxygenated surface waters.)
Recent fisheries-dependent data did not show overlap during summer months
(June 1-September 30) between deep-water regions and sturgeon occurrences, but
most gear deployed were for shallow waters (i.e., pound nets). During other months
(October-May), deeper fishing gill nets captured sturgeon in both deep-channel and
deep-water regions (U.S. EPA 2003b). Fishery-independent gill netting in the upper
Chesapeake Bay above the Bay Bridge resulted in several Atlantic sturgeon captured
in June and July at one station in pycnocline waters.
In other systems where strong water-column stratification does not occur to the
degree observed in the Chesapeake Bay and its tidal tributaries, both sturgeon
species are known to use deep-water habitats in summer months as thermal refuges.
During the period of 1990-1999, very little summer deep-water habitat was
predicted to support sturgeon production based on a bioenergetics model, due prin-
cipally to pervasive hypoxia (Secor andNiklitschek, in press). Further, sturgeons are
able to respond behaviorally to favorable gradients in dissolved oxygen (Secor and
Niklitschek 2001).
Based on this evidence, pycnocline deep-water habitat does not comprise 'potential'
habitats for sturgeon during periods of strong water-column stratification limiting
exchange with overlying, more oxygenated waters. In the absence of strong water-
column stratification, these deeper water-column habitats are considered open-water
habitat and comprise 'potential' habitats for sturgeon.
Atlantic sturgeon occur at depths between 1 meter to more than 25 meters; shortnose
sturgeon occur at depths between 1 and 12 meters (Kieffer and Kynard 1993; Savoy
and Shake 2000: Welsh et al. 2000). In winter, Atlantic sturgeon select deeper
habitats occurring in the Chesapeake Bay's deep channel (Secor et al. 2000; Welsh
et al. 2000).
Distribution studies and laboratory experiments support the view that shortnose stur-
geon prefer riverine and estuarine habitats over marine ones (e.g., Secor 2003).
Shortnose adults have been reported occasionally in coastal waters up to 31 ppt, but
typically occur within several kilometers of their natal estuaries (Dadswell et al. 1984;
Kynard 1997). This contrasts with the sympatric Atlantic sturgeon, which are consid-
ered true anadromous fish that must migrate into coastal waters to complete their life
cycles (Kynard 1997). In general, shortnose sturgeon do not invade salinities greater
than 15 ppt, with centers of concentrations at less than 5 ppt for all life history stages
during summer months (Dadswell et al. 1984; Brundage and Meadows 1982; Dovel
et al. 1992; Geoghegan et al. 1992; Collins and Smith 1996; Bain 1997; Haley 1999).
Atlantic sturgeon older than one year fully tolerate marine salinities and are expected
to be distributed across all salinities, depending on season, reproduction and foraging
conditions after their first year of life (Dovel and Berggen 1983; Dovel et al. 1992;
Kieffer and Kynard 1993; Colligan et al. 1998; Secor et al. 2000).
chapter iii • Dissolved Oxygen Criteria
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33
Thus, Atlantic sturgeon are not limited by bathymetry and salinity in the Chesapeake
Bay and would be expected to inhabit all tidal waters, including pycnocline and sub-
pycnocline waters, if water quality conditions permitted. Shortnose sturgeon habitats
would overlap those of Atlantic sturgeon for salinities less than 15 ppt. But there is
strong evidence that both species historically have not used deep-water and deep-
channel designated use habitats during the summer months (U.S. EPA 2003b) due to
naturally pervasive low dissolved oxygen conditions (see above and the prior section
titled "Low Dissolved Oxygen: Historical and Recent Past").
SCIENTIFIC LITERATURE FINDINGS
For each tidal-water designated use-based set of Chesapeake Bay dissolved oxygen
criteria, a review was conducted of the relevant dissolved oxygen effects literature
beyond those data contained in the Virginian Province criteria document, to include
recent published findings and Chesapeake Bay-specific data. These findings were
used to confirm the derived criteria values and support the adoption of criteria with
instantaneous minimum durations. In the case of the deep-channel designated use,
the scientific literature formed the basis for the seasonal-based Chesapeake Bay
deep-channel criterion value.
INSTANTANEOUS MINIMUM VERSUS DAILY MEAN
The scientific literature provides clear evidence that mortality occurs rapidly from
short-term exposure (less than 6 to 12 hours) to low oxygen concentrations
(Magnusson et al. 1998; Breitburg 1992; Jenkins et al. 1993; Chesney and Houde
1989; Campbell and Goodman 2003). In a recent comprehensive review of the
effects of hypoxia on coastal fishes and fisheries, Breitburg (2002) stated:
Oxygen concentrations below those that result in the standardly calculated
50% mortality in 24 to 96 h exposure test can lead to mortality in minutes
to a few hours. For example, in the case of naked gobies, exposure to
dissolved oxygen concentrations of 0.25 mg liter1 leads to death in a
matter of a few minutes (Breitburg 1992). As exposure time increases, the
oxygen saturation that causes death approaches the saturation level that
results in reduced respiration—typically a saturation level 2 to 3 times
higher than found to be lethal in 24 h tests (Magnusson et al. 1998).
Temperature is often an important cofactor determining when lethal
conditions are reached because it can affect both the amount of oxygen
that can dissolve in water, and the metabolic requirements of fish. Studies
to date indicate that fish require higher oxygen saturations and higher
dissolved oxygen concentrations for survival at higher temperatures....
The effects of exposure duration and temperature are thus very important
to consider in setting water quality standards for dissolved oxygen
concentration, highlighting the need to set absolute minima, instead of
time-averaged minima, and the need to consider geographic variation in
maximum water temperatures.
chapter iii • Dissolved Oxygen Criteria
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34
Data on laboratory tests of asphyxia and field data on fish kills associated with intru-
sions of hypoxic bottom water indicate that mortality rapidly occurs from short-term
exposure to very low dissolved oxygen concentrations. Asphyxia occurs at about half
the dissolved oxygen concentration resulting in reductions in respiration
(Magnusson et al. 1998). For the species illustrated, respiration declines at dissolved
oxygen concentrations of about 85 percent of the LC50 concentration (see Figure 2
in Magnusson et al. 1998).
Asphyxia, as stated above, has been reported at dissolved oxygen concentrations
well below the reported LC50 concentrations. To ensure full protection of each of the
five designated uses, an instantaneous minimum criterion has been recommended. In
addition, a daily mean criterion value has been recommended for the deep-water use
to ensure fall protection of the open-water juvenile and adult fish that use deep-water
habitats for short periods in summer to forage for food.
STRENGTHS AND LIMITATIONS OF
THE CRITERIA DERIVATION PROCEDURES
As with any science-based set of criteria, the approach used in deriving these criteria
has its strengths and limitations. The dissolved oxygen criteria are designed to
protect the five proposed designated uses under the conditions in which the under-
lying effects data were generated. Elevated temperatures, for example, will stress
organisms regardless of the dissolved oxygen concentrations. The proposed condi-
tions will protect the designated uses along with the application of other appropriate
water quality criteria that protect against temperature, chemical contaminant and
other related stresses.
The EPA recognizes that interactions among other stressors and dissolved oxygen
exist. Conservative assumptions, documented in this chapter and associated appen-
dices, were made to reflect these remaining uncertainties with regard to interactions
with other stressors. Incorporation of arbitrary 'margins of safety' were not part of
the Chesapeake Bay criteria derivation process, consistent with national EPA guide-
lines (U.S. EPA 1985). The EPA believes that the criteria provided in this document
are protective under water quality conditions in which aquatic organism are not
otherwise unduly stressed by other factors.
Salinity Effects
The Virginian Province criteria document is geared toward >15 ppt salinities, with a
subset of tests run at much lower salinities (e.g., striped bass larvae). However, low
dissolved oxygen effects synthesized from the science literature used in deriving the
EPA criteria included tests run at salinities lower than 15 ppt salinity (e.g., Burton et
al. 1980, research on menhaden and spot). All tests were run at salinities found to be
nonstressfal to the respective organisms. These results and a review of the literature
indicated that nonstressfal salinity levels do not influence an organism's sensitivity
to low dissolved oxygen.
chapter iii • Dissolved Oxygen Criteria
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35
Temperature Effects
With the exception of the criterion derived to protect shortnose sturgeon, Chesapeake
Bay criteria do not explicitly address potential interactions between varying stressful
temperature levels and the effects of low dissolved oxygen. The amount of available
dissolved oxygen changes as temperature changes, and the metabolic rates of organ-
isms increase as temperature increases. In both cases, temperature directly affects
organisms and their responses to dissolved oxygen conditions.
High temperatures and low dissolved oxygen concentrations often appear together.
Generally, low dissolved oxygen concentrations would be more lethal at water
temperatures approaching the upper thermal limit for a species. Surface or shoal
regions of high temperature will cause fish to seek cooler habitats, yet these deeper
habitats are more likely to contain hypoxic waters. The resulting 'habitat squeeze'
(Coutant 1985) curtails summertime habitats and production (Brandt and Kirsch
1993; Secor and Niklitschek 2001). A number of species have shown heightened
sensitivity to low dissolved oxygen concentrations at higher, yet nonlethal, tempera-
tures (Breitburg et al. 2001). At this time sufficient data exist only for specific life
history stages of some species (i.e., juvenile shortnose and Atlantic sturgeon) to fully
quantify and build temperature and dissolved oxygen interactions into a set of
Chesapeake Bay-specific dissolved oxygen criteria. Clearly, given the well-
documented role of temperature and dissolved oxygen interactions in constraining
the potential habitats of striped bass, sturgeon and other Chesapeake Bay fishes,
more research and model development are needed.
The EPA does not think that a margin of safety for temperature effects is needed.
Although having more data specific to an issue is always desirable, the available data
are sufficient to derive dissolved oxygen criteria for the Chesapeake Bay that are
protective of most species most of the time (which was the original intent of the
EPA's 1985 national aquatic life criteria derivation guidelines). The data in Appendix
B show that high, but nonstressful temperatures will not alter the dissolved oxygen
criteria (some of these temperatures were as high as 30°C). The only rigorous data
that are available for a single Chesapeake Bay species using nonstressful and
stressful temperatures are for shortnose sturgeon (Campbell and Goodman 2003).
These data have been used in the revised the Chesapeake Bay open-water dissolved
oxygen criteria to derive protection limits specifically aimed at shortnose sturgeon in
higher, stressful temperature waters.
pH Effects
The interaction between pH levels and dissolved oxygen concentrations is more of an
issue in laboratory experimentation and the analysis of laboratory-based effects data
than in deriving and applying the dissolved oxygen criteria themselves. Given the great
buffering capacity of seawater, pH, although a potentially important factor, is unlikely
to change much in seawater. Existing pH water quality criteria, along with the applica-
tion of the appropriate dissolved oxygen criteria, should be protective of the use.
chapter iii • Dissolved Oxygen Criteria
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36
Behavioral Effects
As Breitburg (2002) concluded from a recent extensive review of the scientific liter-
ature, clear evidence exists of behavioral responses to low dissolved oxygen
conditions.
Field studies have repeatedly shown that as oxygen concentrations
decline, the abundance and diversity of demersal fishes decrease (e.g.,
Howell and Simpson 1994; Baden and Pihl 1996; Eby 2001; Breitburg et
al. 2001). Bottom waters below approximately 2 mg liter1 have ex-
tremely depauperate fish populations. Some individual species appear to
have threshold concentrations below which their densities decline precip-
itously (Howell and Simpson 1994; Baden and Pihl 1996; Eby 2001).
However, because fish species vary in both physiological tolerance and
behavior, total fish abundance and fish species richness tend to decline
gradually with declining oxygen concentrations.
Longer duration exposures to low oxygen and more severe hypoxia lead
to avoidance of and emigration from affected habitat. All larval, juvenile
and adult fishes that have been tested to date respond to oxygen gradients
by moving upwards or laterally away from waters with physiologically
stressful or potentially lethal dissolved oxygen towards higher oxygen
concentrations (e.g., Deubler and Posner 1963; Stott and Buckley 1979;
Breitburg 1994; Wannamaker and Rice 2000). Mortality from direct
exposure to hypoxic and anoxic conditions is less than might otherwise
occur because of this potential capacity for behavioral avoidance.
Habitat loss due to hypoxia in coastal waters is, however, far greater than
would be calculated based on the spatial extent of lethal conditions,
because most fish avoid not only lethal oxygen concentrations but also
those that would reduce growth and require greatly increased energy
expenditures for ventilation. Field and sampling and laboratory experi-
ments indicate that oxygen concentrations that are avoided tend to be 2 to
3 times higher than those that lead to 50 percent mortality in 24-to 96-
hour exposures, and approximately equal to concentrations that have been
shown to reduce growth rates in laboratory experiments.
The net result of emigration and mortality is reduced diversity, abundance
and production of fishes within the portion of the water column affected
by low dissolved oxygen. Emigration leading to reduced densities of
fishes even at oxygen concentrations approaching 40 to 50 percent satu-
ration (3 to 4 mg liter1) is supported by the pattern of increasing number
of species in trawl samples with increasing dissolved oxygen concentra-
tion in Long Island Sound (Howell and Simpson 1994) and Chesapeake
Bay (Breitburg et al. 2001) and the increasing number of finfish individ-
uals caught per trawl hour within increasing bottom dissolved oxygen off
the Louisiana Coast in the Gulf of Mexico (Diaz and Solow 1999).
Concentrations associated with avoidance are very similar to those observed to result
in adverse effects on growth (Breitburg 2002). Figure III-1 illustrates the relationship
chapter iii • Dissolved Oxygen Criteria
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37
03
~o
O
>
03
CO 1
(A)
(B)
I 3
D
"D
(D
O
5
0.0 0.5
O)
0
o
c
03
~o
O
>
03
C
03
C
Q)
W
r =0.71, p=0.002
y=0.36 + 1.98X
0.0 0.5 1.0 1.5 2.0 2.5
LC50 (mg liter1)
r =0.65, p=0.052
y=-1.65 + 3.29X
1.0 1.5 2.0
LC50 (mg liter1)
2.5
2 •
(C)
• /-
• / •
r2=0.82, p=0.035
y=0.31 + 0.89X
1 2 3 4 5
Growth reduction (mg liter-1)
Dissolved oxygen concentrations
causing avoidance = 2.25*LC50
Dissolved oxygen concentrations
causing growth reduction = 2.28*LC50
Dissolved oxygen concentrations
causing avoidance = 0.98 * dissolved oxygen
causing growth reduction
Figure 111-1. Relationship between lethal dissolved oxygen concentrations and those resulting in reduced
growth and behavioral avoidance of affected habitat, (a) LC50 vs. avoidance behavior, (b) LC50 vs. growth
reduction, and (c) growth vs. avoidance behavior. Two identical points in (c) are indicated by the number 2
next to the data point. Data sources are as follows. Avoidance vs. mortality: Burton et al. 1980; Coutant
1985; Petersen and Petersen 1990; Pihl et al. 1991; Scholz and Waller 1992; Schurmann and Steffensen
1992; Howell and Simpson 1994; Petersen and Pihl 1995; Poucher and Coiro 1997; Wannamaker and Rice
2000; U.S. EPA 2000; Eby 2001. Growth vs. mortality: Burton et al. 1980; Petersen and Petersen 1990; Pihl et
al. 1991; Scholz and Waller 1992; Schurmann and Steffensen 1992; Petersen and Pihl 1995; Chabot and Dutil
1999; U.S. EPA 2000; McNatt 2002. Avoidance vs. growth: Couton 1985; Pihl et al. 1991; Scholz and Waller
1992; Howell and Simpson 1994; Petersen and Pihl 1995; Poucher and Coiro 1997; U.S. EPA 2000; Eby 2001;
and McNatt 2002. Only studies utilizing a range of dissolved oxygen concentrations are included in figures.
Data from multiple studies on the same species were averaged. If responses were tested at several
temperatures, the temperature with the most dissolved oxygen effects tested was selected.
Source: Breitburg 2002.
chapter iii • Dissolved Oxygen Criteria
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38
between dissolved oxygen concentrations that are lethal and those resulting in
reduced growth and behavioral avoidance of the affected habitat. Regressions, calcu-
lated from data from a variety of sources, included LC50 versus avoidance behavior,
LC50 versus growth reduction and growth versus avoidance behavior. Dissolved
oxygen concentrations associated with avoidance were found to be 2.25 times the
LC50 concentration (Figure Ill-la). Dissolved oxygen concentrations causing growth
reduction were 2.28 times the LC50 concentration (Figure Ill-lb). Dissolved oxygen
concentrations causing avoidance were essentially the same as those concentrations
causing growth reduction (Figure III-lc). Reduced growth and avoidance by fish
occur at similar oxygen concentrations relative to lethal levels. Thus, protecting for
one factor should protect for the other, if appropriate time durations are used.
The relationship between the average number of species per trawl across a range of
dissolved oxygen concentrations provides additional evidence for a strong dissolved
oxygen/behavioral connection that transcends individual estuarine and coastal
systems (Figure III-2; Breitburg 2002). Using data from the Chesapeake Bay, Long
Island Sound and Kattegat Sea, the number of species collected per trawl was shown
to increase with increasing dissolved oxygen concentration in all three estuarine and
coastal systems.
Individual species habitat requirements and the characteristics of habitats both deter-
mine the extent to which an ecosystem's habitats are used and contribute to the
health and production of Chesapeake Bay living resources. Each species' behavioral
responses, their predators and their prey can also be considered in deriving dissolved
oxygen criteria. Based on the limited data on behavioral responses, we are not sure
of the actual adverse effects that behavioral responses such as avoidance have on
individuals, much less on whole populations. Although considerable data on behav-
ioral avoidance of low oxygen habitats exist, we are unable to predict individual or
population-level consequences of such avoidance.
Although it is true that we cannot directly evaluate the effects of avoidance in the
same way that we can with effects on growth and survival, the EPA does not believe
that a margin of safety for avoidance behavior is needed. The data reviewed by Breit-
burg (2002) clearly show that concentrations that have an effect on avoidance are
nearly identical to those that affect growth. Therefore, criteria that protect growth
should also be protective of habitat squeeze due to avoidance.
Larval Recruitment Model
The larval recruitment model was used only in the actual derivation of the deep-
water criteria when it was applied specifically to bay anchovy egg and larval life
stages. In deriving the migratory spawning and nursery and open-water criteria, the
larval recruitment model results for nine different species were used to ensure that
the criteria based on other effects data would be fully protective of larval life stages.
chapter iii • Dissolved Oxygen Criteria
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39
4
A. Chesapeake Bay
3
2
1
0
12
B. Long Island Sound
14
C. Kattegat
12
10
8
6
4
2
0
3
4
>5
0
1
2
Bottom layer dissolved oxygen (mg liter"1)
Figure 111-2. Average number of species per trawl at a range of dissolved oxygen concentrations along the
a) western shore of the Chesapeake Bay near the Calvert Cliffs Nuclear Power Plant (Breitberg and Kolesar,
unpublished data), b) Long Island Sound (redrawn from Howell and Simpson 1994, figures 3 and 4), and
c) Kattegat (Baden et al. 1990; Baden and Phil 1996). Data are averaged in approximately 0.5 mg liter1
intervals and for all data >0.5 mg liter1. Note variation in scale of vertical axes.
Source: Breitburg 2002.
chapter iii • Dissolved Oxygen Criteria
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40
Uncertainties remain with respect to the percent of the population exposed to low
dissolved oxygen, the length of the actual spawning period and the protection of
spawning events concentrated over short periods of time. In addition, the assumption
implicit in the larval recruitment model is that all spawning days are equal.
Due to meteorological, food web and other influences, eggs hatched at different
times during the spawning season are not expected to contribute equally to
successful survival to juvenile and adult stages, nor are eggs produced continuously
throughout the spawning season. In particular, species show spawning behaviors
and early survival rates that depend on lunar tidal patterns, weather-driven changes
to water quality (e.g., winds and temperature changes) and available forage for
young. For example, it is well-documented that most striped bass survival can come
from a relatively narrow period of time during the entire spawning period
(Ulanowicz and Polgar 1980; Secor and Houde 1995; Secor 2000). Since we cannot
predict when this smaller window may occur relative to specific hypoxic events,
conservative assumptions must be made. These include always assuming in the
recruitment model that hypoxia will occur during times of maximum offspring
production.
A number of reports exist on the consequences of slow growth in terms of increased
predation mortality. The model does not contain a variable for growth (it only deals
with larval survival), however, it does increase the mortality (i.e., changes the sensi-
tivity to hypoxia) with increasing exposure duration.
The EPA acknowledges uncertainties with the parameters in the larval recruitment
model. This is why specific parameters within the model were chosen to be conser-
vative. Specifically, spawning periods reflect when the bulk of spawning occurs, not
just the first and last possible occurrence of a given species larvae in the water
column. In addition, the model always assumes that a hypoxic event occurs during
the spawning season of each species modeled. The percentages of each cohort that
is exposed during a hypoxic event were also intended to be conservative.
CHESAPEAKE BAY DISSOLVED OXYGEN
CRITERIA DERIVATION
Chesapeake Bay dissolved oxygen criteria were established to protect estuarine
living resources inhabiting five principal habitats: migratory spawning and nursery,
shallow-water, open-water, deep-water and deep-channel. These five categories are
drawn from the refined designated uses for the Chesapeake Bay and its tidal tribu-
tary waters (Figure III-3). See Appendix A and U.S. EPA 2003a for more detailed
descriptions of the refined designated uses.
The EPA's Guidelines for Deriving Numerical National Water Quality Criteria for
the Protection of Aquatic Organisms and their Uses (U.S. EPA 1985) is the primary
source on how to establish numerical criteria. Consistent with the national guidelines
provided, scientific judgment took precedence over the specifics of the guidelines,
chapter iii • Dissolved Oxygen Criteria
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41
A. Cross-Section of Chesapeake Bay or Tidal Tributary
Shallow-Water
Bay Grass Use
Open-Water
Fish and Shellfish Use
Deep-Water
Seasonal Fish and
Shellfish Use
—- Deep-Channel
Seasonal Refuge Use
B. Oblique View of the Chesapeake Bay and its Tidal Tributaries
Migratory Fish
Spawning and
> Nursery Use
Shallow-Water
Bay Grass Use
Open-Water
Fish and Shellfish Use
Deep-Water
Seasonal Fish and
Deep-Channel
Seasonal Refuge Use
Shellfish Use
Figure 111 -3. Conceptual illustration of the five Chesapeake Bay designated use zones.
when warranted. A similar judgment was applied in the development of the 2000
EPA Virginian Province saltwater and the 1986 EPA freshwater dissolved oxygen
criteria documents (U.S. EPA 1986, 2000).
The Chesapeake Bay dissolved oxygen criteria were derived using methodologies
documented in the EPA Virginian Province saltwater criteria document and using
criteria originally published in the EPA freshwater criteria document. The scientific
rationale for modifications to the 1985 EPA guidelines for deriving the saltwater
dissolved oxygen criteria for the Virginian Province and the national freshwater
dissolved oxygen criteria are detailed in those peer-reviewed, EPA documents.
chapter iii • Dissolved Oxygen Criteria
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42
Criteria for migratory fish spawning and nursery, shallow-water bay grass and open-
water fish and shellfish designated use habitats were set at levels to protect the
survival, growth and reproduction of all species. Criteria that apply to deep-water
seasonal fish and shellfish habitats in summer were set at levels to protect shellfish,
the survival of juvenile and adult fish, and the recruitment success of the bay
anchovy. Criteria for deep-channel seasonal refuge designated use habitats in
summer were set to protect the survival of sediment-dwelling worms and clams.
MIGRATORY FISH SPAWNING AND
NURSERY DESIGNATED USE CRITERIA
Criteria that support the migratory fish spawning and nursery designated use must fully
protect the "survival, growth and propagation of balanced indigenous populations of
ecologically, recreationally and commercially important anadromous, semi-anadro-
mous and tidal-fresh resident fish species inhabiting spawning and nursery grounds
from February 1 through May 31" (Appendix A; U.S. EPA 2003a). This covers the
survival and growth of all life stages—eggs, larvae, juveniles and adults—for a given
number of species and their underlying food sources. As described below, the criteria
are based on establishing dissolved oxygen concentrations to protect against losses in
larval recruitment, growth effects on larvae and juveniles and the survival and growth
effects on the early life stages of resident tidal-fresh species.
Criteria Components
Protection against Larval Recruitment Effects. Applying the Virginian
Province criteria larval recruitment effects model generates a relationship illustrated
as a curve, projecting the cumulative loss of recruitment caused by exposure to low
dissolved oxygen (Figure III-4). The number of acceptable days of exposure to low
dissolved oxygen decreases as the severity of the low oxygen conditions increases.
The migratory fish spawning and nursery designated use criteria must ensure protec-
tion of larvae as they are recruited into the juvenile/adult population. The Virginian
Province criteria larval recruitment curve levels out at approximately 4.6 mg liter1
beyond 30 days of exposure (Figure III-4). By dropping non-Chesapeake Bay
species and applying Chesapeake Bay-specific modifications to the larval recruit-
ment model parameters, as described previously, a curve is generated that closely
follows the original Virginian Province criteria curve but levels off just above 4.6 mg
liter1. Dissolved oxygen concentrations and exposure durations falling above the
Chesapeake Bay-specific curve, e.g., above 4.6 mg liter1 for 30 days, 3.4-3.5 mg
liter1 for up to seven days and 2.7-2.8 liter1 at all times, would protect against larval
recruitment effects.
Protection for Early Life Stages for Resident Tidal-Fresh Species. The
EPA freshwater dissolved oxygen criteria set a 7-day mean of 6 mg liter"1 and an
instantaneous minimum of 5 mg liter1 to protect early life-stage, warm-water, fresh-
water species (Table III-4) (U.S. EPA 1986).
chapter iii • Dissolved Oxygen Criteria
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43
5.0
4.5
>4.6 mg liter" past 30 days
4.0
3.5
t 3.4-3.5 mg liter" at
7 days
Chesapeake Bay curve
Virginia Province curve
3.0
2./-2.8mg liter" at 0 day
2.5
2.0
0
10
20
30
40
50
60
70
Time (days)
Figure 111 -4. Comparison of the Virginian Province-wide (—) and Chesapeake Bay-wide
(—) larval recruitment effects.
Protection against Growth Effects. To ensure recruitment to the adult popula-
tion, the Chesapeake Bay criteria must ensure protection against growth effects on
rapidly developing larvae and juveniles. The Virginian Province criteria document
recommends 4.8 mg liter1 as the threshold above which long-term, continuous
exposures should not cause unacceptable growth effects (U.S. EPA 2000). As
described previously, if the non-Chesapeake Bay species were removed from the
Virginian Province criteria dissolved oxygen growth effects data base, a recalculated
Chesapeake Bay-specific criterion would be 5 mg liter1.
These values were derived by observing the effects of low dissolved oxygen on
larval and early juvenile life stages. Growth effects on these stages served as the
basis for the chronic criterion because: 1) growth is generally the more sensitive
endpoint measure upon exposure to low dissolved oxygen compared with survival;
2) results for other sublethal endpoints such as reproduction were limited; 3) the
limited data available indicated that thresholds protecting against growth effects are
likely to protect against reproductive effects; and 4) larval and juvenile life stages
were more sensitive to effects from low dissolved oxygen than were adults (U.S.
EPA 2000). In addition to higher dissolved oxygen requirements, fish eggs and
larvae also are more vulnerable to low dissolved oxygen because of limitations in
behavioral avoidance (Breitburg 2002).
Protection against Effects on Threatened/Endangered Listed Species.
As documented previously, short-term exposures to dissolved oxygen concentrations
chapter iii • Dissolved Oxygen Criteria
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44
of > 3.2 mg liter"1 on the order of several hours at nonstressfal temperatures and longer-
term exposures of 30 days or longer at > 5 mg liter1 will not impair the survival or
growth of Atlantic and shortnose sturgeon (Secor and Niklitschek 2001, 2003;
Niklitschek 2001; Secor and Gunderson 1998; Campbell and Goodman 2003). At
stressful temperatures above 29°C, short-term exposures to dissolved oxygen con-
centrations >4.3 mg liter1 will not impair the survival of shortnose sturgeon.
Additional Scientific Literature Findings. Results from Brandt et al. (1998)
indicate that striped bass food consumption and growth decline as oxygen levels
decline. Continuous exposure to dissolved oxygen concentrations of 4 mg liter1 or
less caused striped bass to lose weight, even though food was always unlimited.
Previous experiments on the effects of oxygen levels on striped bass also have shown
that dissolved oxygen concentrations of less than 3 to 4 mg liter1 adversely affect
feeding (Chittenden 1971).
Jordan et al. (1992) summarized the literature supporting the adoption of the Chesa-
peake Bay restoration goal target concentration protecting anadromous spawning
and nursery areas as follows.
This target DO concentration (>5 mg liter1 at all times) was selected to
protect the early life stages of striped bass, white perch, alewife, blueback
herring, American shad, hickory shad and yellow perch. This concentra-
tion of DO will allow eggs to hatch normally (Bradford et al. 1968;
O'Malley and Boone 1972; Marcy and Jacobson 1976; Harrell and
Bayless 1981; Jones et al. 1988), as well as allow survival and growth of
larval and juvenile stages of all anadromous target species (Tagatz 1961;
Bogdanov et al. 1967; Krouse 1968; Bowker et al. 1969; Chittenden 1969,
1972, 1973; Meldrim et al. 1974; Rogers et al. 1980; Miller et al. 1982;
Coutant 1985; ASMFC 1987; Jones et al. 1988). For example, concentra-
tions of DO below 5 mg liter1 for any duration will not support normal
hatching of striped bass eggs (O'Malley and Boone 1972). Although one
hatchery operation was able to maintain striped bass fingerlings at DO
concentrations of 3 to 4 mg liter1 (Churchill 1985). Bowker et al. (1969)
found DO >3.6 mg liter1 was required for survival of juveniles.
Across an array of temperatures (13-25°C) and salinities (5-25 ppt), Krouse (1968)
observed complete mortality of striped bass at 1 mg liter1, 'minimal mortality' at
5 mg liter"1 and 'intermediate survival' at 3 mg liter"1 upon exposure of 72 hours.
Some field observations have indicated that juveniles and adults of anadromous
species prefer dissolved oxygen concentrations > 6 mg liter1 (Hawkins 1979;
Christie et al. 1981; Rothschild 1990). However, no lethal or sublethal effects other
than possible avoidance have been documented for dissolved oxygen concentrations
between 5 and 6 mg liter1.
Rationale
The migratory spawning and nursery designated use criteria must ensure full protec-
tion for warm-water freshwater species' egg, larval and juvenile life stages, which
chapter iii • Dissolved Oxygen Criteria
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45
co-occur with the tidal-fresh and low-salinity migratory spawning and nursery habi-
tats (Table III-5). To ensure full protection for resident tidal-fresh warm-water
species' early life stages, a 7-day mean criterion of 6 mg liter1 and an instantaneous
minimum criterion of 5 mg liter1 were selected, consistent with the EPA freshwater
criteria (U.S. EPA 1986).
To ensure protection not only of survival and recruitment of larvae into the juvenile
population but also to eliminate any potential for adverse effects on growth during
the critical larvae and early juvenile life stages, an instantaneous minimum criterion
of 5 mg liter1 was selected. The Virginian Province saltwater criteria document
states that exposures to dissolved oxygen concentrations above this concentration
should not result in any adverse effects on growth (U.S. EPA 2000). Given the lack
of information on the population level consequences of short- versus long-term
reductions in growth on the survival of larvae and juveniles, a specific averaging
Table 111-5. Migratory fish spawning and nursery designated use dissolved oxygen criteria components.
Criteria Components
Concentration
Duration
Source
Protection against growth effects
>4.8 mg liter"1
-
U.S. EPA 2000
Protection against larval
recruitment effects
>4.6 mg liter"1
> 3.4-3.5 mg liter"1
> 2.7-2.8 mg liter"1
30 to 40 days
7 days
instantaneous
minimum
U.S. EPA 2000
Protection of early life stages for
resident tidal freshwater species
> 6 mg liter"1
> 5 mg liter"1
7-day mean
instantaneous
minimum
U.S. EPA 1986
Protection against effects on
threatened/endangered species
(shortnose sturgeon)
> 5 mg liter"1
>3.5 mg liter"1
>3.2 mg liter"1 1
>4.3 mg liter"1 2
30 days
6 hours
2 hours
2 hours
Secor and Niklitschek
2003; Niklitschek 2001;
Secor and Gunderson
1998; Jenkins et al. 1993;
Campbell and Goodman
2003
Additional published findings
- Growth effects on striped bass
- Protect early life stages
- Intermediate striped bass
survival
- Full survival
- Preferred concentrations
< 3 to 4 mg liter"1
> 5 mg liter"1
> 3mg liter"1
>5 mg liter"1
> 6 mg liter"1
72 hours
72 hours
Brandt et al. 1998;
references in text
Krouse 1968
Krouse1968
Hawkins 1979; Christie
et al. 1981; Rothschild
1990
1 Protective of survival at nonstressful temperatures.
2 Protective of shortnose sturgeon at stressful temperatures (>29°C).
chapter iii • Dissolved Oxygen Criteria
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46
period was not recommended in the Virginian Province saltwater criteria. In the case
of anadromous species, a narrow set of natural conditions (e.g., salinity, temperature)
is required and a narrow time window exists for a successful spawn. Natural mortal-
ities for larvae already are extremely high. As even short-term reductions in growth
could influence advancement to the next stage through the impairment of survival
and the ability to avoid predators, the criterion value that protects against growth
effects is applied as an instantaneous minimum.
Setting the criterion duration of exposure as an instantaneous minimum is consistent
with the instantaneous minimum duration for the 5 mg liter1 concentration criterion
value from the EPA freshwater dissolved oxygen criteria for ensuring fall protection
of warm-water freshwater species' early life stages against short-term exposures
(Table III-4; U.S. EPA 1986). The instantaneous minimum of the 5 mg liter1 crite-
rion value also protects the survival and growth of shortnose sturgeon (Table III-5).
Migratory Spawning and Nursery Criteria
The following dissolved oxygen criteria fally support the Chesapeake Bay migratory
fish spawning and nursery designated use when applied from February 1 through
May 31: a 7-day mean > 6 mg liter1 applied to tidal-fresh waters with long-term
averaged salinities up to 0.5 ppt; and an instantaneous minimum > 5 mg liter1
applied across all the migratory fish spawning and nursery designated use habitats,
regardless of salinity. See U.S. EPA 2003a for details on the selection of February 1
through May 31 as the time period for applying the migratory spawning and nursery
designated use.
OPEN-WATER FISH AND SHELLFISH DESIGNATED USE CRITERIA
Criteria that support the open-water designated use must fally protect the "survival,
growth and propagation and growth of balanced, indigenous populations of ecolog-
ically, recreationally and commercially important fish and shellfish inhabiting
open-water habitats" (Appendix A; U.S. EPA 2003a). The dissolved oxygen require-
ments for the species and communities inhabiting open- and shallow-water habitats
are similar enough to ensure protection of both the open-water and shallow-water
designated uses with a single set of criteria. The open-water criteria were based on
establishing dissolved oxygen concentrations to protect against losses in larval
recruitment, growth effects on larvae and juveniles and the survival of juveniles and
adults in tidal-fresh to high-salinity habitats.
Criteria Components
Protection against Larval Recruitment Effects. Applying the Virginian
Province criteria model generates a relationship illustrated as a curve that projects
the cumulative loss of recruitment caused by exposure to low dissolved oxygen. The
number of acceptable days of exposure to low dissolved oxygen decreases as the
chapter iii • Dissolved Oxygen Criteria
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47
severity of the low oxygen conditions increases. The open-water designated use
criteria must ensure protection of larvae as they recruit into the juvenile/adult
population.
The Virginian Province larval recruitment effects curve levels out at approximately
4.6 mg liter1 beyond 30 days' exposure (Figure III-5). Dropping the non-Chesa-
peake Bay resident species and then applying a series of Chesapeake Bay-specific
modifications to the larval recruitment model parameters, as described previously,
yields a curve that follows the original Virginian Province criteria curve and also
levels out around 4.6 mg liter1 beyond 30 days exposure (Figure III-5). Setting the
larval exposure level to 100 percent5 results in a curve that levels out at 4.8 mg liter
The effects curves illustrated in Figure III-5 reflect the combined dissolved oxygen
concentration and duration of exposure protective against a five percent or greater
impact, thereby protecting 95 percent or greater of the seasonally produced
offspring. Dissolved oxygen concentrations/exposure durations falling above the
curve, e.g., above 2.7-2.9 mg liter1 at all times, above 3.4-3.6 mg liter1 for up to
seven days, and above 4.6-4.8 mg liter1 for 30 days, would protect against larval
recruitment effects greater than five percent in open-water designated use habitats.
5.0
o
CO
0
o
c
o
a
c
0
CO
>-
x
O
"U
o
>
o
3.4-3.6 ma liter
2.9 ma liter at 0 da
2.5
2.0
>4.6-4.8 mg liter past 30 days
at 7 days
Chesapeake Bay curve
at 100% Exposure
Chesapeake Bay curve
Virginia Province curve
10 20 30 40
Time (days)
50 60
70
Figure 111-5. Comparison of the Virginian Province-wide (—) and Chesapeake Bay specific
larval recruitment effects at variable (—) and 100 percent (...) exposures.
5 The larval recruitment model has a parameter for what percentage of a given cohort is exposed to low
dissolved oxygen conditions.
chapter iii • Dissolved Oxygen Criteria
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48
Protection of Juvenile/Adult Survival. The Virginian Province criteria docu-
ment recommends 2.3 mg liter1 as the threshold above which long-term, continuous
exposures should not cause unacceptable lethal conditions for juvenile and adult fish
and shellfish. As described below, this value does not protect the survival of short-
nose sturgeon.
Protection against Growth Effects. To ensure recruitment to the adult popula-
tion, the open-water designated use dissolved oxygen criteria must protect against
growth effects on rapidly developing larvae and juveniles. The Virginian Province
document recommends 4.8 mg liter1 as the threshold above which long-term,
continuous exposures should not cause unacceptable growth effects. If the non-
Chesapeake Bay species were to be removed from the Virginian Province growth
effects data base, the recalculated Bay-specific criterion protective against growth
effects would be 5 mg liter1.
This chronic criterion value was derived from laboratory evaluations of the effects of
low dissolved oxygen on growth, principally with larval and early juvenile life
stages. Growth effects on these early life stages were used as the basis of the chronic
criterion because: 1) growth is generally the more sensitive endpoint measure upon
exposure to low dissolved oxygen compared with survival; 2) results for other non-
mortality related endpoints such as reproduction were very limited; 3) the limited
data indicated that thresholds protecting against growth effects are likely to protect
against negative reproductive effects; and 4) larval life stages were more sensitive to
effects from low dissolved oxygen than were juveniles/adults (U.S. EPA 2000). The
derivation of a dissolved oxygen criterion value of 5 mg liter"1 to protect against
growth effects is consistent with findings reported by Breitburg (2002) that dissolved
oxygen concentrations causing growth reductions were 2.28 times the LC50 concen-
tration (2.28 x 1.64 = 3.7 mg liter1, where 1.64 is the Final Acute Value from the
EPA Virginian Province document).
Protection of Resident Tidal-Fresh Species. The open-water fish and shellfish
designated use criteria must also fully protect warm-water freshwater species that
co-occur in tidal-fresh and low-salinity open- and shallow-water habitats. The EPA
freshwater dissolved oxygen criteria set a 30-day mean of 5.5 mg liter1; a 7-day
mean minimum of 4.0 mg liter1, and an instantaneous minimum of 3.0 mg liter1 to
protect life stages for warm-water species beyond early life stages (Table III-4) (U.S.
EPA 1986).
Protection against Effects on Threatened/Endangered Listed Species.
As documented previously, short-term exposures of several hours to dissolved
oxygen concentrations of > 3.2 mg liter1 at nonstressful temperatures and longer-
term exposures of 30 days or more at > 5 mg liter1 would protect the survival and
growth of Atlantic and shortnose sturgeon (Secor and Niklitschek 2001, 2003;
Niklitschek 2001; Secor and Gunderson 1998; Campbell and Goodman 2003). At
chapter iii • Dissolved Oxygen Criteria
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49
stressful temperatures above 29°C, short-term exposures to dissolved oxygen
concentration > 4.3 mg liter1 will not impair the survival of shortnose sturgeon.
Additional Scientific Literature Findings. As striped bass larvae begin their
metamorphoses to the juvenile stage, they move into shallow-water habitats near
shore and in shoal areas less than 2 meters deep (Boreman and Klauda 1988;
Boynton et al. 1981; Setzler-Hamilton et al. 1981). Nursery areas for juvenile striped
bass with dissolved oxygen concentrations greater than 5 mg liter1 are preferable,
given findings that concentrations below 4 mg liter1 can adversely affect juvenile
growth rates, feeding rates, habitat use and susceptibility to predation (e.g., Kramer
1987; Breitburg et al. 1994). Mortality of juvenile striped bass has been observed at
dissolved oxygen concentrations < 3 mg liter1 (Chittenden 1972; Coutant 1985;
Krouse 1968).
Results from trawls in Long Island Sound showed significant reductions in both
species diversity and abundance at sites with dissolved oxygen < 2 mg liter1 (Howell
and Simpson 1994). At open water-column sites with dissolved oxygen concentra-
tions > 3 mg liter1, 15 of the 18 target species caught occurred with greater frequency
compared with sites with concentrations < 2 mg liter1. Further research indicated that
the total abundance of fish was relatively insensitive to low dissolved oxygen condi-
tions, reaching normal levels at 1.5 mg liter1. However, total fish biomass and species
richness were particularly sensitive, declining at dissolved oxygen concentrations of
3.7 mg liter"1 and 3.5 mg liter"1, respectively (Simpson 1995).
Rationale
To ensure the fall protection of survival and recruitment of larvae into the juvenile
population, reduce the potential for adverse effects on growth and protect threatened
or endangered species across tidal-fresh to high-salinity habitats, dissolved oxygen
criteria values of a 30-day mean of 5.5 mg liter1 applied to tidal-fresh habitats with
long- term averaged salinities up to 0.5 ppt; a 30-day mean of 5 mg liter1 applied to
all other open-water habitats (> 0.5 ppt salinity); a 7-day mean of 4 mg liter1; and
an instantaneous minimum of 3.2 mg liter1 were selected (Table III-6). At tempera-
tures stressful to shortnose sturgeon (>29°C), a 4.3 mg liter1 instantaneous
minimum criteria should apply.
The 5 mg liter1 value is based on the Virginian Province criterion protecting against
growth effects (U.S. EPA 2000). The Virginian Province criteria document states
that exposures to dissolved oxygen concentrations above this concentration will not
result in any adverse effects on growth. However, the document recommended no
specific duration. The extensive open-water habitats provide better opportunities for
avoiding predators and seeking food than the more confined, geographically limited
migratory spawning and nursery habitats. The 30-day mean averaging period for the
5 mg liter1 criterion value was selected to reflect current uncertainties over how
much impact growth reduction has on juvenile and adult survival and reproduction
in the shallow- and open-water Chesapeake Bay habitats. The 30-day mean averaging
chapter iii • Dissolved Oxygen Criteria
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50
Table 111-6. Open-water fish and shellfish designated use dissolved oxygen criteria components.
Criteria Components
Concentration
Duration
Source
Protection against larval
recruitment effects
> 4.6-4.8 mg liter1
> 3.4-3.6 mg liter1
> 2.7-2.9 mg liter1
30 to 40 days
7 days
< 24 hours
U.S. EPA 2000
Protection against growth
effects
>4.8 mg liter1
-
U.S. EPA 2000
Protection of juvenile/adult
survival
>2.3 mg liter1
24 hours
U.S. EPA 2000
Protection for resident tidal
freshwater species
>5.5 mg liter1
> 4 mg liter1
> 3 mg liter1
30 days
7 days
instantaneous
minimum
U.S. EPA 1986
Protection against effects on
threatened/endangered
species (shortnose sturgeon)
> 5 mg liter1
>3.5 mg liter1
>3.2 mg liter1 1
>4.3 mg liter1 2
30 days
6 hours
2 hours
2 hours
Secor and Niklitschek
2003; Niklitschek 2001;
Secor and Gunderson
1998; Jenkins et al. 1994;
Campbell and Goodman
2003
Additional published
findings
Preferred striped
bass juvenile habitat
Juvenile striped
bass growth,
feeding effects
Juvenile striped
bass mortality
Total fish biomass
declining
Total fish species
richness
> 5 mg liter1
< 4 mg liter1
< 3 mg liter1
<3.7 mg liter1
<3.5 mg liter1
-
Kramer 1987; Breitburg et
al. 1994
Kramer 1987; Breitburg et
al. 1994
Chittenden 1972; Coutant
1985; Krouse 1968
Simpson 1995
Simpson 1995
Protective of survival at nonstressful temperatures.
Protective of shortnose sturgeon at stress temperatures (> 29°C).
chapter iii • Dissolved Oxygen Criteria
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51
period is consistent with and fully protects against effects on larval recruitment (see
Figure III-5 and text below) and is consistent with the duration protection of fresh-
water species.
The criterion values of a 30-day mean of 5 mg liter1, a 7-day mean of 4 mg liter1
and an instantaneous minimum of 3.2 mg liter1 fully protect larval recruitment.
Depending on an assumption of partial or 100 percent exposure to low dissolved
oxygen concentrations, larval recruitment would be protected at concentrations
ranging between 4.6 and 4.8 mg liter1 beyond 30 days of exposure (Figure III-5). At
seven days of exposure, concentrations between 3.4 and 3.6 mg liter1, extracted
from the range of larval recruitment curves protects against effects. The 7-day mean,
4 mg liter1 concentration criterion value, therefore, protects recruitment. The instan-
taneous minimum 3.2 mg liter1 criterion would protect larval recruitment, given that
the instantaneous minimum exposure level concentrations are between 2.7 to 2.9 mg
liter"1.
The instantaneous minimum 3.2 mg liter"1 criterion will also protect the survival of
juvenile and adult fish and shellfish species inhabiting shallow- and open-water habi-
tats, given it has a higher value than the Virginian Province value of 2.3 mg liter1
(U.S. EPA 2000).
The 30-day mean 5.5 mg liter1 criterion value is consistent with the EPA freshwater
dissolved oxygen criteria to protect warm-water freshwater species (U.S. EPA 1986).
The other two components of the proposed open-water criteria—7-day mean of 4 mg
liter1 and instantaneous minimum of 3.2 mg liter1—are also consistent with the
EPA warm-water freshwater criteria (Table III-4).
The instantaneous minimum 3.2 mg liter1 criterion protects against lethal effects
from short-term exposures to low dissolved oxygen for both Bay species of sturgeon.
A 30-day mean 5 mg liter"1 criterion protects against growth effects for longer-term
exposures (Secor and Niklitschek 2001, 2003; Niklitschek 2001; Secor and
Gunderson 1998). Application of the 3.2 mg liter1 criterion as an instantaneous
minimum concentration is justified on the basis that effects on shortnose sturgeon
were observed after just two hours' exposure (Campbell and Goodman 2003).
From October 1 through May 31, when the open-water fish and shellfish designated
use extends through the water column into the seasonally defined deep-water seasonal
fish and shellfish and deep-channel seasonal refuge designated use habitats, these
habitats are important both to blue crabs and larger finfish species seeking refuge in
deeper, warmer waters (e.g., striped bass, white perch, Atlantic croaker, shortnose
sturgeon and Atlantic sturgeon) during the cooler months of the year (see Appendix
A; U.S. EPA 2003a). The criterion values described above will provide the necessary
levels of protection for all of these species, for both juvenile and adult life stages.
Open-Water Criteria
The following criteria fully support both the Chesapeake Bay open-water fish and
shellfish and shallow-water bay grass designated uses when applied year-round: a
chapter iii • Dissolved Oxygen Criteria
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52
30-day mean >5.5 mg liter 1 applied to tidal-fresh habitats only with long-term
averaged salinities of up to 0.5 ppt; a 30-day mean > 5 mg liter1; a 7-day mean
> 4 mg liter1; and an instantaneous minimum >3.2 mg liter1. At temperatures
stressful to shortnose sturgeon (>29°C), a 4.3 mg liter1 instantaneous minimum
criteria should apply.
DEEP-WATER SEASONAL FISH AND SHELLFISH
DESIGNATED USE CRITERIA
In deep-water habitats, where the physical exchange of higher oxygenated waters in
the upper water-column habitats is much reduced by density stratification and pycn-
ocline waters are not reoxygenated by riverine or oceanic bottom waters, dissolved
oxygen concentrations will naturally be lower during the warmer months of the year.
Criteria to support the deep-water seasonal fish and shellfish designated use must
fully "protect the survival, growth and propagation of balanced, indigenous popula-
tions of ecologically, recreationally and commercially important fish and shellfish
species inhabiting deep-water habitats" (Appendix A; U.S. EPA 2003a).
In the Chesapeake Bay, the bay anchovy is an abundant, ecologically significant fish
likely to be affected by low dissolved oxygen conditions, given its life history.
Although it is not a commercial species, the bay anchovy is prey for bluefish, weak-
fish and striped bass (Hartman and Brandt 1995), forms a link between zooplankton
and predatory fish (Baird and Ulanowicz 1989) and represents from 60 to 90 percent
of piscivorus fish diets on a seasonal basis (Hartman 1993). Bay anchovy spawn
from May to September in the Chesapeake Bay, with a peak in June and July (Olney
1983; Dalton 1987) across a broad range of temperatures and salinities throughout
the Chesapeake Bay (Dovel 1971; Houde and Zastrow 1991). Their spawning and
nursery periods coincide with the presence of low dissolved oxygen conditions in the
Chesapeake Bay and its tidal tributaries.
The hatchability of fish eggs is known to be influenced by the oxygen concentrations
to which they are exposed during incubation (Rombough 1988). Chesney and Houde
(1989) conducted laboratory experiments to test the effects of low dissolved oxygen
conditions on the hatchability and survival of bay anchovy eggs and yolk-sac larvae.
Their findings demonstrated that survival rates of bay anchovy eggs and larvae are
likely to be affected when exposed to dissolved oxygen concentrations less than 3
mg liter1 and 2.5 mg liter1, respectively. Breitburg (1994) found very similar effects
for 3- to 13-day post-hatch bay anchovy larvae, where 50 percent survival was
observed at 2.1 mg liter1.
Bay anchovy routinely inhabit waters within the pycnocline region. Bay anchovy
eggs have been found throughout the water column regardless of bottom layer
oxygen concentrations in mesohaline areas of tributaries (Keister et al. 2000), but
were retained in surface and pycnocline waters in the mesohaline mainstem Bay
(North 2001; Breitburg et al., unpublished data; Figure III-6). MacGregor and Houde
(1996) found that most bay anchovy eggs were distributed in water above the
chapter iii • Dissolved Oxygen Criteria
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53
a.
CD
0
E
CO
CD
CD
CD
i—
CD
_Q
E
0
-------
54
pycnocline when below pycnocline waters had dissolved oxygen concentrations of
< 2 mg liter1. Rilling and Houde (1999) observed bay anchovy eggs and larvae
throughout the water column during June and July. Bay anchovy larvae are found
throughout the water column when bottom oxygen concentrations are above 2 mg
liter1 (Keister et al. 2000).
Environmental conditions present during the egg, larval or juvenile life stages
strongly influence fish population dynamics. Key among these are changes in food
supply for first-feeding larvae and factors that modify predation mortality for the
highly vulnerable larval life stages. The majority of the species for which larval
effects data are available within the Virginian Province criteria document do not
routinely inhabit waters in the pycnocline layer. To derive a criteria to protect deep
waters located within the pycnocline layer that are generally inhabited by bay
anchovy and their eggs and larvae, a Chesapeake Bay-specific larval recruitment
effects model was generated for the bay anchovy.
Criteria Components
Protection against Egg/Larval Recruitment Effects. Two larval recruitment
effect models were derived that are specific to the Chesapeake's bay anchovy, based
on the original Virginian Province larval recruitment effects model (U.S. EPA 2000).
The bay anchovy eggs effects model was based on a 5 percent impairment of eggs
hatching to yolk-sac larvae, assuming a 100-day recruitment period and 1-day devel-
opment period (Chesney and Houde 1989). The larvae-based recruitment effects
model, also based on a 5 percent impairment, assumed that yolk-sac larvae and post-
yolk larvae or feeding larvae had the same sensitivity.
A development period of 32 days was applied, based on Houde's work (1987), which
documented an egg-to-larval duration of 33 days. One day was subtracted to reflect
the egg stage (Chesney and Houde 1989), yielding the 32-day development period.
A 132-day recruitment period was calculated by adding the 32-day development
period to the 100-day recruitment period mentioned above.
A 50 percent exposure6 to low dissolved oxygen concentrations was built into both
the egg and larvae recruitment effects models. Field-based observations have indi-
cated widespread distributions of bay anchovy eggs and larvae across the Bay's
mainstem waters and throughout the water column except in subpycnocline waters
with extremely low dissolved oxygen concentrations (MacGregor and Houde 1996;
Rilling and Houde 1999; Keister et al. 2000; Breitburg et al. 2003).
The final separate survival curves for both the egg and larval recruitment effect
models, illustrated in Figure III-7, were based on comparing the effects data from
6 The larval recruitment model has a parameter for what percentage of a given cohort is exposed to
low dissolved oxygen conditions.
chapter iii • Dissolved Oxygen Criteria
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55
o>
E
o
"ro
o
c
o
O
c
O)
>.
X
O
"O
>
o
to
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-
3 mg liter at 30 days
¦ Bay anchovy eggs curve
1.7 mg liter1 at 0 days
* Bay anchovy larvae curve
Combined bay anchovy
egg/larval recruitment curve
10
20
30 40
Time (days)
50
60
70
Figure 111 -7. Chesapeake Bay bay anchovy egg and larval recruitment effects curves.
Chesney and Houde (1989) with the final survival curve from Figure 5 in the
Virginian Province saltwater criteria document (U.S. EPA 2000). A single combined
egg/larval recruitment effects curve, based on the midpoint between the two indi-
vidual effects curves, also is illustrated in Figure III-7. The effects curves illustrated
in Figure III-7 reflect the combined dissolved oxygen concentration and duration of
exposure protective against a 5 percent or greater impact, thereby protecting 95
percent of the seasonally produced offspring. Dissolved oxygen concentrations and
exposure durations falling above the combined bay anchovy egg/larval recruitment
curve—3 mg liter1 for 30 days and 1.7 mg liter1 at all times—would protect against
egg and larval recruitment effects greater than 5 percent.
Protection of Juvenile/Adult Survival. The Virginian Province document
recommends 2.3 mg liter1 as the threshold above which long-term, continuous expo-
sures should not cause lethal conditions for juvenile and adult fish and shellfish (U.S.
EPA 2000).
Additional Scientific Literature Findings. Breitburg et al. (2001) provide a
synthesis of the acute sensitivities of an array of species that may inhabit the water
column or near-bottom habitats in the deep-water designated use habitats.
Adults and juveniles of most Chesapeake Bay species that have been
tested have 24 hr LC50 values near 1 mg l1 (i.e., approximately 13% satu-
ration at 25°C and 18 psu). Acute toxicity tests have yielded 50%
mortality rates with 24-hr exposures at 0.5-1.0 mg l"1 for species such as
chapter iii • Dissolved Oxygen Criteria
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56
hogchoker (Trinectes maculatiis), northern sea robin (Prionotus
carolinus), spot (Leiostomus xanthurus; but LC50 reported as >1 mg l1 by
Phil et al. 1991) tautog (Tautoga onitis), windowpane flounder
(Scopthalmus aquosus), and fourspine stickleback (Apeltes quadra cos),
and 50% mortality rates between 1.1 and 1.6 mg l1 for Atlantic menhaden
(Brex'oortia tvrannus), scup (Stenotomus chtysops), summer flounder
(Paralichthvus dentatus), pipefish {Syngnathus fiiscus) and striped bass
(Morone saxatilis) (Pihl et al. 1991; Poucher and Coiro, 1997; Thursby et
al. 2000 [cited here as U.S. EPA]). Thus, for nearly all species tested, the
range of tolerances is quite low; only a 1.0 mg l1 difference separates the
most and least sensitive species described above.
Although fewer species have been tested during the larval stage, larvae of
species that occur in Chesapeake Bay appear to be somewhat more sensi-
tive to low oxygen exposure than are most adults and juvenile. For
example, 50% mortality with 24-h exposure occurs between 1.0 and
1.5 mg l1 for skilletfish (Gobiesox strumosus), naked goby (Gobiosoma
bosc), and inland silverside (Menidia betyllina) larvae, while 50%
mortality occurs at 1.8 to 2.5 mg l1 for larval red drum (Sciaenops ocel-
latiis), bay anchovy (Anchoa mitchilli), striped blenny (Chasmodes
bosquianus) and striped bass (Saksena and Joseph 1972; Breitburg 1994;
Poucher and Coiro 1997). Field and laboratory observations indicate that
lethal dissolved oxygen concentrations for skilletfish, naked goby and
striped blenny adults are 1.0 mg l1 (Breitburg, unpublished data).
Embryo tolerances vary inconsistently in relation to tolerances of later
stages; 50% mortality in 12-96 h occurs at a higher dissolved oxygen
concentration than that for larval mortality for bay anchovy (2.8 mg l"1),
at a similar oxygen concentration as for larvae for inland silverside (1.25
mg l"1), and at lower concentrations than that leading to larval mortality
for winter flounder (Pleuronectes americanus; 0.7 mg l1) and naked
goby (approximately 0.6 mg l1) (Chesney and Houde 1989; Poucher and
Coiro 1997).
Roman et al. (1993) examined the distribution of two species of zooplankton cope-
pods—Acartia tonsa and Oithona colcan'a—through the water column in the
Chesapeake Bay. Acartia tonsa, which regularly migrate from open water down to
subpycnocline waters, were not found in bottom waters when oxygen concentrations
were < 1 mg liter1. The highest concentration of zooplankton were found at the
pycnocline level.
In a recent review of zooplankton responses to and ecological consequences of
zooplankton exposure to low dissolved oxygen, Marcus (2001) synthesized the
following literature findings.
Vargo and Sastry (1977) reported that 2-h LD50 values for Acartia tonsa
and Eiuytemora affinis adults collected from the Pettaquamscutt River
Basin, Rhode Island ranged from dissolved oxygen concentrations of
0.36 to 1.40 mg l1 and 0.57 to 1.40 mg l1 respectively. Roman et al.
(1993) tested the oxygen tolerance of adults of Acartia tonsa and Oithona
chapter iii • Dissolved Oxygen Criteria
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57
colcatva from Chesapeake Bay. Survival was considerably less after 24 h
in < 2.0 mg l1 oxygenated water.
Stalder and Marcus (1997) examined the 24-h survival of three coastal
copepod species in response to low oxygen. Acartia tonsa showed excel-
lent survival at concentrations as low as 1.43 mg l1. Between 1.29 and
0.86 mg l"1 survival declined markedly and at 0.71 mg l1 mortality was
100%. Labidocera aestiva and Centwpages hamatus were more sensitive
to reduced dissolved oxygen concentrations. The survival of these species
was significantly lower at 1.43 mg l1. The survival of nauplii of Labido-
cera aestiva and Acartia tonsa at low dissolved oxygen concentrations
was generally better than adult survival.
Rationale
Protecting the recruitment of bay anchovy eggs and larvae into the juvenile and adult
population is crucial to the integrity of the Chesapeake Bay ecosystem. The bay
anchovy is a primary food source for many fish species. To protect bay anchovy
recruitment, criteria values of a 30-day mean of 3 mg liter1 and an instantaneous
minimum of 1.7 mg liter1 were selected to best reflect the shape of the final
combined bay anchovy egg and larval recruitment effects curve illustrated in
Figure III-7.
This approach to criteria derivation is consistent with the approach to derive criteria
protective against larval effects in open-water habitats. These approaches followed
the guidelines published by the EPA in the Virginian Province dissolved oxygen
criteria document (U.S. EPA 2000). The bay anchovy 12- to 24-hour post larvae
hatch values from Chesney and Houde (1989) place bay anchovy larvae within the
upper range of larval life stage sensitivities for all 17 fish and invertebrate species
documented in the Virginian Province document (see Figure 4 on page 13 and
Appendix D). The criteria derived to protect bay anchovy early life stages should be
protective of other species that routinely inhabit deeper, pycnocline habitats.
The 1.7 mg liter"1 criterion value was derived as the dissolved oxygen concentration
where the combined egg/larval recruitment effects curve intercepted the y-axis
(Figure III-7). Given that the y-axis intercept reflects 'time zero,' an instantaneous
minimum duration was applied to the 1.7 mg liter1 criterion value. The 3 mg liter1
criterion value was derived as the approximate point where the combined egg/larval
recruitment effects curve levels out. The flattening of the curve beyond this point
indicates that dissolved oxygen concentrations much greater than 3 mg liter1 should
not cause increased impairment of egg/larval recruitment over longer periods of
exposure. The 3 mg liter1 concentration corresponded with 30 days on the x-axis
(Figure III-7).
These criteria values and durations are supported by findings published in the scien-
tific literature. Chesney and Houde (1989) evaluated 12- to 14-hour-old yolk-sac bay
anchovy larvae over 12 hours, yielding the effects data used in running the bay
chapter iii • Dissolved Oxygen Criteria
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58
anchovy egg/larval recruitment models. In deep-water habitats, field observations
support the presence of effects at durations of less than 24 hours, which supports the
selection of the instantaneous minimum versus a daily average criterion concentra-
tion (Breitburg 1992). Magnusson et al. (1998) have indicated that asphyxia, as
described previously, has been reported at dissolved oxygen concentrations well
below (> 50 percent) the reported LC50 concentrations. Given that the reported LC50
values for bay anchovy larvae range from 2.1 to 2.8 mg liter1 (Chesney and Houde
1989; Breitburg 1994), an instantaneous minimum criteria value above 1.4 mg
liter1 (50 percent of 2.8 mg liter1) is required to prevent lethal conditions at expo-
sures of less than 24-hour averaged conditions. Given that the reported laboratory
and field effects were manifested in less than 12 hours, an instantaneous minimum
concentration is further justified as the temporal period for application of the 1.7 mg
liter1 criterion value.
In addition to early life stages of bay anchovy, the instantaneous minimum of 1.7 mg
liter1 protects juvenile and adult survival of those fish species commonly inhabiting
water-column and bottom habitats within the pycnocline (e.g., spot, summer
flounder and winter flounder; Table III-7). See also Table 1, page 8 in U.S. EPA
(2000) for additional supporting effects data. This criterion value also will protect
zooplankton, the principal prey of the bay anchovy and many other fish during their
early life stages (Table III-7; Marcus 2001; Roman et al. 1993). Application of the
Virginian Province saltwater criteria for juvenile/adult survival, 2.3 mg liter1 as a 1-
day mean, will provide the required level of protection to short-term exposures to
low dissolved oxygen in deep-water habitats (U.S. EPA 2000).
The open-water criteria that apply to the summer-only deep-water designated use
habitats from October 1 through May 31 will protect Atlantic and shortnose sturgeon
inhabiting these deep waters in the winter (Secor et al. 2000; Welsh et al. 2000).
From June 1 to September 30, the deep-water designated use criteria will not fully
protect Atlantic and shortnose sturgeon.
Historically, natural low dissolved oxygen conditions (< 3 mg liter1) in deep-water
and deep-channel regions would have curtailed sturgeon access. Over the past several
hundred years, sturgeon probably have not used deep-water and deep-channel desig-
nated use habitats during summer months due to 'naturally' pervasive hypoxia (see
the sections above titled, "Low Dissolved Oxygen: Historical and Recent Past" and
"Historical Potential Sturgeon Tidal Habitats"). Behavioral studies indicate that stur-
geon are capable of avoiding these hypoxic regions (Niklitschek 2001) and probably
have done so for centuries. On the other hand, deep-water and deep-channel desig-
nated use habitats do recover to nonnoxic conditions during the fall, winter and spring
months. During these periods evidence supports a past and recent role for habitats as
thermal refuge and migration corridors in the Chesapeake Bay.
This criterion also will protect open-water species with higher dissolved oxygen
sensitivities that search for prey within these pycnocline habitats for short periods of
time. Field data from other estuarine and coastal systems, such as Long Island Sound
chapter iii • Dissolved Oxygen Criteria
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59
Table 111-7. Deep-water seasonal fish and shellfish designated use criteria components.
Criteria Components
Concentration
Duration
Source
Protection against egg/larval
3 mg liter"1
30 days
Chesney and Houde 1989;
recruitment effects
1.7 mg liter"1
instantaneous
Breitburg 1994; U.S. EPA 2000
minimum
Protection of juvenile/adult
> 2.3 mg liter"1
24 hours
U.S. EPA 2000
survival
Additional literature findings
50 percent mortality
0.5-1 mg liter"1
24 hours
Reviewed in Breitburg et al.
for hogchoker,
2001
northern sea robin,
spot
50 percent mortality
> 1 mg liter"1
24 hours
Reviewed in Breitburg et al.
for tautog,
2001; Pihl et al. 1991;
windowpane
flounder adults
50 percent mortality
1.1-1.6 mg liter"1
24 hours
Reviewed in Breitburg et al.
for menhaden,
2001; Pihl et al. 1991; Poucher
summer flounder,
and Coiro 1997; U.S. EPA
pipefish, striped bass
2000
adults
50 percent mortality
1-1.5 mg liter"1
24 hours
Breitburg 1994; Poucher and
for skilletfish, naked
Coiro 1997
goby, silverside
larvae
50 percent mortality
1.8-2.5 mg liter"1
24 hours
Saksena and Joseph 1972;
for red drum, bay
Breitburg 1994; Poucher and
anchovy, striped
Coiro 1997
blenny larvae
Zooplankton habitat
< 1 mg liter"1
-
Roman et al. 1993
avoidance
Reduced copepod
< 1 mg liter"1
-
Qureshi and Rabalais 2001
nauplii abundance
50 percent mortality
0.36-1.4 mg liter"1
2 hours
Vargo and Sastry 1977
for Acartia tonsct and
Eurytemora affinis
Mortality for Acartia
< 2 mg liter"1
24 hours
Roman et al. 1993
tonsa and Oithona
colcarva
100 percent mortality
0.71 mg liter"1
24 hours
Stalder and Marcus 1997
for copepods
Reduced survival for
<.86-1.3 mg liter"1
24 hours
Stalder and Marcus 1997
copepods
Acartia tonsa
> 1.43 mg liter"1
24 hours
Stalder and Marcus 1997
survival
chapter iii • Dissolved Oxygen Criteria
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60
and Albemarle-Pamlico Sound, clearly indicate that open-water species will use
pycnocline region habitats if dissolved oxygen concentrations are above levels that
result in avoidance (e.g., Howell and Simpson 1994; Simpson 1995; Eby 2001).
Recommended Criteria
The following criteria fully support the seasonal-based Chesapeake Bay deep-water
designated use when applied from June 1 through September 30: a 30-day mean
3 mg liter1, a 1-day mean 2.3 mg liter1 and an instantaneous minimum 1.7 mg liter1.
DEEP-CHANNEL SEASONAL REFUGE DESIGNATED USE CRITERIA
Deep-channel habitats are defined as the very deep water-column and adjacent
bottom surficial sediment habitats located principally in the river channel at the
lower reaches of the major rivers (e.g., the Potomac River) and along the spine of the
middle mainstem Chesapeake Bay at depths below which seasonal anoxic (< 0.2 mg
liter1 dissolved oxygen) to severe hypoxic conditions (< 1 mg liter1 dissolved
oxygen) routinely set in and persist for extended periods of time under current condi-
tions (Appendix A; U.S. EPA 2003a). From late spring to early fall, many of these
deep-channel habitats are naturally exposed to very low dissolved oxygen concen-
tration conditions. Under low dissolved oxygen conditions of 1 to 2 mg liter1, these
habitats are suitable only for survival of benthic infaunal and epifaunal organisms.
Criteria that support the deep-channel designated use must fully protect the "survival
of balanced, indigenous populations of ecologically important benthic infaunal and
epifaunal worms and clams that provide food for bottom-feeding fish and crabs"
(Appendix A; U.S. EPA 2003a). The seasonal-based deep-channel criteria are based
on establishing dissolved oxygen concentrations to protect the survival of bottom
sediment-dwelling worms and clams.
Components
The infauna of the deep-channel habitat are the most tolerant of all infaunal benthic
organisms in the Chesapeake Bay. Even if there were no problems with low
dissolved oxygen conditions, the benthic organisms inhabiting unconsolidated mud
habitats in these deep-channel designated use habitats probably would not change.
Looking at benthos from deep-channel habitats in the Chesapeake Bay that are not
hypoxic or anoxic, one finds the same benthic community species. On an annual
basis, productivity is about the same for hypoxic versus non-hypoxic deep unconsol-
idated mud bottom sediment habitats in the mesohaline Chesapeake Bay (Diaz and
Schaffner 1990). The factors that control what is present in these mesohaline benthic
habitats are salinity and sediment type. Hypoxic conditions run a distant third
(Holland et al. 1977). Hypoxic conditions change the benthic community structure
periodically, but the pool from which these low oxygen habitats are recolonized after
chapter iii • Dissolved Oxygen Criteria
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61
a severe low-oxygen to no-oxygen event is still the limiting factor for a benthic
community
Benthic infauna have high tolerances to low dissolved oxygen conditions (~1 mg
liter1) and many macrofaunal species demonstrate behavioral reactions before they
eventually die (Diaz and Rosenberg 1995). For the mesohaline zone of estuaries, the
critical dissolved oxygen level appears to be around 0.6-1.0 mg liter1 (Diaz and
Rosenberg 1995; Table III-8). At the high end of this dissolved oxygen range, the
bottom-dwelling community starts to lose moderately tolerant species, with more
tolerant species dying off at the low end of the range. In estuaries and coastal systems
exposed to seasonally varying low dissolved oxygen, the critical dissolved oxygen
concentration is closer to 1 mg liter1 (Llanso 1992), with subtle reductions in
dissolved oxygen concentration from 1 to 0.5 mg liter1 causing a full range of
responses from behavioral to death (Llanso and Diaz 1994). In their synthesis of
dissolved oxygen concentrations causing acute and chronic effects on Chesapeake
Bay benthic infaunal organisms, Holland et al. (1989) found a similar range of oxygen
concentrations that cause mortality or severe behavioral effects (Appendix C).
Table 111-8. Deep-channel designated use criteria effects data.
Effects Observed
Concentration
Source
— Mesohaline community
1 mg liter1
Numerous references cited in Diaz
mortality of moderately
and Rosenberg 1995
tolerant species
— Mesohaline community
0.6 mg liter1
Numerous references cited in Diaz
mortality of more tolerant
and Rosenberg 1995
species
— Behavioral to lethal responses
0.5-1 mg liter1
Llanso 1992; Llanso and Diaz 1994;
observed
references cited in Holland et al. 1989
— Behavior, growth and
< 2 mg liter1
Diaz et al. 1992
production effects observed
— Epifaunal community survival
0.5-2 mg liter1
Sagasti et al. 2000
In the deep channel of the Chesapeake Bay, communities of mud-burrowing worms
and clams have a broad tolerance to a wide range of sediment types, salinities,
dissolved oxygen concentrations and organic loadings. Several keystone Bay
bottom-dwelling polychaete worm species—Paraprionospio pinnata, Streblospio
benedicti, Loimia medusa and Heteromastus filiformis—are resistant to dissolved
oxygen concentrations as low as 0.6 mg liter1 (Llanso and Diaz 1994; Diaz et al.
1992; Llanso 1991).
Extensive mortality is likely only under persistent exposure to very low dissolved
oxygen concentrations (< 1 mg liter1) at higher summer temperatures in the Chesa-
peake Bay (Holland et al. 1977). Similar findings have been reported for other
chapter iii • Dissolved Oxygen Criteria
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62
estuarine and coastal systems (Rosenberg 1977; Jorgensen 1980; Stachowitsch 1984;
Gaston 1985).
While the macrobenthic community itself often is found to be insensitive to low
dissolved oxygen concentrations around 2 mg liter1, exposure of these bottom habi-
tats to brief periods of dissolved oxygen concentrations < 2 mg liter1 affects
behavior (resulting in decreased burrowing depth and exposure at the sediment
surface), growth and production (Diaz et al. 1992). From a synthesis of 12 years of
diverse observations and 5 years of remotely operated vehicle videotapes, Rabalais
et al. (2001) reported stressed behavior, such as emergence from the sediments by
burrowing invertebrates, at dissolved oxygen concentrations below 1.5 to 1 mg
liter1. At dissolved oxygen concentrations of 1 to 1.5 mg liter1, they observed "even
the most tolerant burrowing organisms, principally polychaetes, emerge partially or
completely from their burrows and he motionless on the bottom." Demersal feeding
fish change their feeding habits quickly to take advantage of stressed macrobenthos
that come to the sediment surface (Stachowitsch 1984; Jorgensen 1980), where they
become more vulnerable to predation during or following a low dissolved oxygen
event (Pihl et al. 1991, 1992).
Epifaunal communities living along the surfaces of bottom sediments in the Chesa-
peake Bay can persist with minimal changes in species composition and abundance
under brief exposures to dissolved oxygen concentrations in the range of 0.5 to 2.0
mg liter1 (Sagasti et al. 2000).
For the unconsolidated mud benthic infaunal community of the mesohaline Chesa-
peake Bay where the deep-channel designated use habitats are located, 1 mg liter1
is protective of survival. The global scientific literature points towards 2 mg liter1 as
the protective dissolved oxygen value, but this is the oxygen tolerance for higher
salinity, more structured benthic communities and species. Between 2 and 3.5 mg
liter1 there are definite behavioral changes for many species and mortality for sensi-
tive species in these higher salinity habitats. For Chesapeake Bay species in similar
higher salinity (polyhaline) habitats, 2 mg liter1 would be the dissolved oxygen
minimum requirement. Benthic communities in these polyhaline habitats in the
Chesapeake Bay will be protected by applying the open-water dissolved oxygen
criteria year-round. However, for the mesohaline Chesapeake Bay where the hypoxic
and anoxic conditions are focused during the summer months, the scientific litera-
ture for unconsolidated mud mesohaline benthic communities supports 1 mg liter1
as the bottom-line requirement. Dissolved oxygen concentrations of less than 1 mg
liter1 lead to mortality for even tolerant species.
Rationale
To ensure protection of the survival of bottom-dwelling worms and clams, an instan-
taneous minimum criterion of 1 mg liter1 was selected (Table III-9). As documented
through the extensive scientific literature reported here, this value will protect
against lethal effects from exposure to low dissolved oxygen. However, behavioral
chapter iii • Dissolved Oxygen Criteria
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63
Table 111-9. Response patterns of Chesapeake Bay benthic organisms to declining dissolved oxygen
concentrations (mg liter1).
Response
Dissolved Oxygen
Species
Reference
Avoidance
Infaunal swimming
1.1
Paraprionospio pinnata
Diaz et al. 1992
0.5
Nereis succinea
Sagasti et al. 2001
Epifaunal off
bottom
0.5
Neopanope sayi
Sagasti et al. 2001
0.5
Callinectes sapidus
Sagasti et al. 2001
1
Stylochus eUipticus
Sagasti et al. 2001
1
Mitrella lunata
Sagasti et al. 2001
0.5
Dirodella obscura
Sagasti et al. 2001
1
Cratena kaoruae
Sagasti et al. 2001
Fauna, unable to leave or escape, initiate a series of sublethal responses
Cessation of
feeding
0.5
Balanus improvisus
Sagasti et al. 2001
0.6
Streblospio benedicti
Llanso 1991
1
Loimia medusa
Llanso and Diaz
1994
1.1
Capitella sp.
Warren 1977; Forbes
and Lopez 1990
Decreased
activities not
related to
respiration
0.5
Balanus improvisus
Sagasti et al. 2001
0.5
Conopeum tenuissimum
Sagasti et al. 2001
0.5
Membranipora tenuis
Sagasti et al. 2001
1
Cratena kaoruae
Sagasti et al. 2001
1
Stylochus eUipticus
Sagasti et al. 2001
1
Streblospio benedicti
Llanso 1991
Cessation of
burrowing
1.1
Capitella sp.
Warren 1977
continued
chapter iii • Dissolved Oxygen Criteria
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64
Table II1-9. Response patterns of Chesapeake Bay benthic organisms to declining dissolved oxygen
concentrations (mg liter1) (continued).
Response
Dissolved Oxygen
Species
Reference
Emergence from
tubes or burrows
0.1-1.3
Ceriathiopis americanus
Diaz, unpublished
data
0.5
Sabellaria vulgaris
Sagasti et al. 2001
0.5
Polydora cornuta
Sagasti et al. 2001
0.7
Micropholis atra
Diaz et al. 1992
1
Hydroides dianthus
Sagasti et al. 2001
10% saturation
Nereis diversicolor
Vismann 1990
Siphon stretching
into water column
0.1-1.0
Mya arenaria, Abra alba
Jorgensen 1980
Siphon or body
stretching
0.5
Molgula manhattensis
Sagasti et al. 2001
0.5
Diadumene leucolena
Sagasti et al. 2001
Floating on surface
of water
0.5
Diadumene leucolena
Sagasti et al. 2001
Formation of
resting stage
0.5
Membranipora tenuis
Sagasti et al. 2001
0.5
Conopeum tenuissimum
Sagasti et al. 2001
Sources: Diaz and Rosenberg 1995; Sagastiet al. 2001.
chapter iii • Dissolved Oxygen Criteria
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65
changes leading to increased opportunities for predation are not protected by this
criterion. These changes may benefit bottom-feeding fish and crabs, giving them
direct access to food, albeit under potentially stressful water quality conditions.
The deep-channel criteria protect survival but not necessarily the growth of benthic
infaunal and epifaunal species from June through September. However, Diaz and
Schaffner (1990) reported that their evaluation of annual secondary productivity of
hypoxic habitats in the Bay's deep-channel habitats indicated no significant reduc-
tion in productivity from low dissolved oxygen conditions. Therefore, the
deep-channel criteria's failure to provide fall protection against growth impairments
is counteracted by growth during the rest of the year, when dissolved oxygen
concentrations are naturally higher than 1 mg liter1, which leads to a net result of
protection against growth impairment on an annual basis.
The instantaneous minimum value of 1 mg liter1 is much more protective of benthic
infaunal organisms than a 1- or 7-day average. In the case of bottom-dwelling organ-
isms, it is not the average condition that is most detrimental to the organisms but the
absolute minimum dissolved oxygen. When dissolved oxygen drops significantly
below 1 mg liter1 for even short periods of time (on the order of hours) mortality
increases, even for tolerant species. Other deep-channel criteria with higher concen-
trations than 1 mg liter"1 and with 1-, 7- or 30-day averaging periods were not
derived for deep-channel designated use habitats, since dissolved oxygen concentra-
tions are not expected to exceed 2 mg liter1 from June through September due to
natural constraints.
Deep-Channel Criteria
The instantaneous minimum 1 mg liter"1 criterion fully supports the seasonal-based
Chesapeake Bay deep-water designated use when applied from June 1 through
September 30.
CHESAPEAKE BAY DISSOLVED OXYGEN CRITERIA
The Chesapeake Bay dissolved oxygen criteria are structured to protect the five tidal-
water designated uses and reflect the needs and habitats of Bay estuarine living
resources (Table 111-10). Criteria for the migratory fish spawning and nursery,
shallow-water bay grass and open-water fish and shellfish designated uses were set
at levels to protect the reproduction and survival of all organisms and against impair-
ments to their growth. Criteria for deep-water habitats during seasons when the water
column is significantly stratified were set at levels to protect juvenile and adult fish,
shellfish and the recruitment success of the bay anchovy. Criteria for deep-channel
habitats in summer were set to protect the survival of bottom sediment-dwelling
worms and clams.
chapter iii • Dissolved Oxygen Criteria
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66
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Secor, D. H. 2000. Spawning in the nick of time? Effect of adult demographics on spawning
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Quality- Proceedings of the Sixth International Symposium, La Paz, Mexico, January 22-26,
2001. U. S. EPA Office of Research and Development, Ecosystems Research Division,
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Secor, D. H. and E. J. Niklitschek. 2001. Hypoxia and Sturgeons: Report to the Chesapeake
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Secor, D. H., E. Niklitschek, J. T. Stevenson, T. E. Gunderson, S. Minkkinen, B. Florence,
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Seliger, H. H., J. A. Boggs and S. H. Biggley. 1985. Catastrophic anoxia in the Chesapeake
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chapter IV
Water Clarity Criteria
BACKGROUND
The loss of underwater bay grasses1 from the shallow waters of the Chesapeake Bay,
which was noted in the early 1960s, is a widespread, well-documented problem.
Although other factors, such as climatic events and herbicide toxicity, may have
contributed to the decline of underwater bay grasses in the Bay, the primary causes
are nutrient over-enrichment and increased suspended sediments in the water and the
associated reduction of light. The loss of underwater bay grass beds is of particular
concern because these plants create rich animal habitats that support the growth of
diverse fish and invertebrate populations. Similar declines in underwater bay grasses
have been occurring worldwide with increasing frequency in the past several
decades.
One of the major features contributing to the high productivity of the Chesapeake
Bay has been the historical abundance of underwater bay grasses. There are more
than 20 freshwater and marine species of rooted, submerged flowering plants in
Chesapeake Bay tidal waters. These underwater bay grasses provide food for water-
fowl and provide critical habitat for shellfish and fish. Underwater bay grasses also
positively affect nutrient cycling, sediment stability and water turbidity.
The health and survival of these plant communities in the Chesapeake Bay and its
tidal tributaries depend on suitable environmental conditions, which define the
quality of underwater bay grass habitat. The key to restoring these critical habitats
and food sources is to provide the necessary levels of light penetration in shallow
waters to support their survival, growth and repropagation.
'The term underwater bay grasses refers to submerged vascular plants often referenced in the
scientific literature as 'seagrasses' as well as submerged aquatic vegetation or SAV, not to be
confused with emergent wetland plants.
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APPROACH
The Chesapeake Bay's scientific and resource management communities collaborated
to produce two internationally recognized technical syntheses of information that
support the quantitative habitat requirements for Chesapeake Bay underwater bay
grasses (Batiuk et al. 1992; Batiuk et al. 2000). Key findings, the underlying light
requirements and management-oriented diagnostic tools and restoration targets have
been reported in the peer-reviewed scientific literature (Dennison et al. 1993;
Gallegos 2001; Koch 2001; Kemp et al., in review). The two technical syntheses,
along with Chesapeake Bay-specific research and field studies and recent model
simulation and data evaluation, provide the scientific foundation for the Chesapeake
Bay water clarity criteria described here. Readers are encouraged to consult these two
syntheses and the resulting published papers for further details and documentation.
The Chesapeake Bay-specific water clarity criteria were derived in four stages: first,
water column-based light requirements for underwater bay grass survival and growth
were determined; second, factors contributing to water-column light attenuation
were quantified; third, contributions from epiphytes to light attenuation at the leaf
surface were factored into methods for estimating and diagnosing the components of
total light attenuation; and fourth, a set of minimal requirements for light penetration
through the water and at the leaf surface were determined to give the water clarity
criteria values.
THE RELATIONSHIPS BETWEEN WATER QUALITY,
LIGHT AND UNDERWATER BAY GRASSES
The principal relationships between water quality conditions and light regimes for
the growth of underwater bay grasses are illustrated in Figure IV-1. Incident light,
which is partially reflected at the water surface, is attenuated through the water
column above the underwater bay grasses by particulate matter (chlorophyll a and
total suspended solids), by dissolved organic matter and by water itself. In most
estuarine environments, the water-column light attenuation coefficient (called Kd) is
dominated by contributions from chlorophyll a and total suspended solids.
Light that actually reaches the underwater bay grass leaves also is attenuated by the
epiphytic material (i.e., algae, bacteria, detritus and sediment) that accumulates on
the leaves. This epiphytic light attenuation coefficient (called Ke) increases exponen-
tially with epiphyte biomass, where the slope of this relationship depends on the
composition of the epiphytic material. Dissolved inorganic nitrogen (DIN) and phos-
phorous (DIP) in the water column stimulate the growth of epiphytic algae (as well
as water-column algae), and suspended solids also can settle onto underwater bay
grass leaves. Because epiphytic algae also require light to grow, water depth and
water-column light attenuation constrain epiphyte accumulation on underwater bay
chapter iv • Water Clarity Criteria
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Light Transmission
Light Attenuation
Surface
Reflection
DIN
DIP
Grazers
I
Plankton
Total
Chlorophyll a
Suspended
Solids
i
•Water
•Particles
•Color
Light-through-W ater
•Algae
EPW68 -Detritus
Light-at-Leaf 'Sediments
Water-Column
Light Attenuation
(Kd)
Epiphyte
Light Attenuation
(Ke)
Underwater Bay Grasses
Figure IV-1. Availability of light for underwater bay grasses is influenced by water-column and at-the-leaf
surface light attenuation processes. DIN = dissolved inorganic nitrogen and DIP = dissolved inorganic
phosphorus.
grass leaves, and light attenuation by epiphytic material depends on the mass of both
algae and total suspended solids settling on the leaves.
An algorithm was developed to compute the biomass of epiphytic algae and other
materials attached to bay grass leaves and to estimate the light attenuation associated
with these materials (Kemp et al., in review; Batiuk et al. 2000). The algorithm was
verified by applying it to Chesapeake Bay water quality monitoring data. The results
of these field verifications are documented in Chapter V, "Epiphyte Contribution to
Light Attenuation at the Leaf Surface," in Batiuk et al. (2000).
The algorithm uses monitoring data for the water-column light attenuation coeffi-
cient (or Secchi depth), total suspended solids, dissolved inorganic nitrogen and
dissolved inorganic phosphorus concentrations to calculate the potential contribution
of epiphytic materials to total light attenuation for bay grasses at a particular depth
(Figure IV-2). Using a set of commonly monitored water quality parameters,
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Percent Light-through-Water (PLW)
Percent Light-at the-Leaf (PLL)
100% Ambient Light of Water Surface
Water
/Water
Color
Inputs
Kd measured directly
or
Kd calculated from
Secchi depth
Total Suspended
Solids
f Algae
Total suspended solids
Dissolved inorganic nitrogen
Dissolved inorganic phosphorus
Inputs
PLW=100exp(-Kd Zj
Calculation
[Epiphyte .
/ Attenuation
P1W
PLL
PLL=100[exp(-KdZ)][exp(-KeBe)]
*Ke = Epiphyte attenuation
*Be = Epiphyte biomass
Calculation
Evaluation
Calculated PLW vs. PLW criteria
Evaluation
PLL vs. PLL Diagnostic Requirement
Figure IV-2. Illustration of the inputs, calculation and evaluation of the two percent-light parameters:
percent light-through-water (PLW) and percent light-at-the-leaf (PLL).
attainment of the percent light-through-water (PLW) water clarity criteria (this
chapter) and percent light-at-the-leaf (PLL) diagnostic parameter (Chapter IV) can
be readily determined for any established restoration depth.
Much of the published literature values for underwater bay grass PLW minimum
light requirements were derived from studies of underwater bay grass light require-
ments in which epiphyte accumulation on plant leaves was not controlled. Therefore,
light measurements in those studies did not account for light attenuation due to
epiphytes on the underwater bay grass leaves themselves. To determine the Chesa-
peake Bay water clarity criteria necessary to ensure that sufficient light reaches
underwater bay grass leaves at a defined restoration depth, three lines of evidence
were compared:
1. Applied the original 1992 underwater bay grasses habitat requirements param-
eter values to the new algorithm for calculating PLL (Figure IV-2), for each of
the four salinity regimes;
2. Evaluated the results of light requirement studies from areas with few or no
epiphytes; and
3. Compared median field measurements of the amount of light reaching plants'
leaves (estimated through the PLL algorithm) along gradients of underwater
bay grasses growth observed in the Chesapeake Bay and its tidal tributaries.
DETERMINING LIGHT REQUIREMENTS
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The derived minimum light requirements apply to the bottom sediment surface in
order to accommodate plants with a variety of heights and plants just emerging from
the bottom sediments.
STRENGTHS AND LIMITATIONS OF THE CRITERIA DERIVATION
PROCEDURES
Scientific Syntheses
The water clarity criteria are based on a solid scientific foundation, synthesizing
more than 20 years of Chesapeake Bay research and related worldwide findings. The
criteria address the minimum light requirements of underwater bay grasses through
the water column (this chapter) and a separate diagnostic tool addresses the plants'
minimum light requirements at the leaf surface (Chapter VII), both applied at the
depth of intended restoration necessary to support the designated use for shallow-
water habitats (see U.S. EPA 2003).
The methods for determining attainment of the water clarity criteria use the Chesa-
peake Bay Program's water quality monitoring data generated across all Bay tidal
waters (see Chapter VI). Management tools for diagnosing the relative contributions
of various sources of light reduction through the water column and at the leaf surface
have been developed in tandem with the PLW criteria values (see Chapter VII). The
scientific basis for the criteria, diagnostic tools and criteria-attainment methodolo-
gies have been through independent peer reviews and have been published in
peer-reviewed scientific journals (Dennison et al. 1993; Gallegos 2001; Koch 2001;
Kemp et al., in review).
Light Availability Studies
The minimum light requirements used in deriving the Chesapeake Bay water clarity
criteria were based, in part, on data and models of light availability from freshwater,
estuarine and marine environments. The EPA recognizes that relatively few studies
of underwater bay grass light requirements have been conducted in lower salinity
estuarine habitats. Most of the underwater plant species growing in the Chesapeake
Bay and its tidal tributaries are, however, the same species as those that have been
observed in light requirement studies of lakes, higher salinity estuarine and coastal
marine habitats (see Chapter III and Appendix A in Batiuk et al. 2000). The EPA is
confident that the findings of these lake, estuarine and marine studies are directly
applicable to deriving the Chesapeake Bay water clarity criteria.
Light Requirements for Sparse versus
Dense UnderwaterBay Grass Beds
The Chesapeake Bay water clarity criteria call for sufficient light to address the
collective minimum light requirements for all these underwater plants' growth and
reproductive stages. The minimum light requirements of underwater plants in new,
chapter iv • Water Clarity Criteria
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sparse grass beds would be similar to those of individual plants in well-established,
dense underwater bay grass beds. However, since the water clarity criteria were
based in large part on relationships between existing underwater bay grasses and
water quality conditions, the criteria are less likely to protect new or sparse grass
beds, since existing, dense grass beds can directly influence their local water quality
conditions. Water velocities, algal abundance and suspended sediment concentra-
tions decrease inside dense, established underwater grass beds, improving water
clarity compared with adjacent open-water habitats. Established underwater bay
grass beds also are less likely to be affected by yearly fluctuations in water clarity
(Moore et al. 1995; Moore 1996). Additionally, their capacity to produce more abun-
dant seeds and propagules would improve their chances for revegetation (Orth et al.
1994). Unvegetated areas do not have these advantages; therefore, the light require-
ments for establishing new underwater grass beds are likely going to be greater.
The effect of improved water clarity on the restoration of underwater bay grasses is
demonstrated by the resurgence of 12 underwater bay grass species to the upper tidal
Potomac River by 1983. In the late 1930s, underwater bay grasses had virtually
disappeared from the tidal-fresh Potomac. The decline coincided with nutrient
enrichment, increased algal concentrations and extreme storms (Carter et al. 1985;
Rybicki and Carter 1986). Through the 1970s, high nitrogen and phosphorus
concentrations from municipal wastewater treatment plants and loadings from other
point and nonpoint sources fueled frequent algal blooms and decreased water clarity.
Secchi depth measurements between 1978 and 1981 averaged < 0.6 meters over the
growing season (corresponding to less than 9 percent light at the 1-meter depth).
Beginning in the early 1980s, improved treatment plant technologies and a ban on
phosphate detergents led to a reduction of nutrients and suspended solids, which
resulted in a significant improvement to water clarity by 1988. When the growing
season average Secchi depth improved to > 0.9 meters (corresponding to 20 percent
light at 1-meter depth, a value much higher than the PLW criterion of 13 percent),
water clarity had improved enough to spark a resurgence of underwater bay grasses
in the Potomac River tidal-fresh zone (Carter and Rybicki 1994; Carter et al. 1994).
Effective Depth of Photosynthesis/Application
Depth Relationship
The 'effective depth' measures the water-column depth at which the active photosyn-
thetic plant structures are located. For most plants grown from seed or from
underground tubers or rhizomes, minimum light requirements are most crucial for
newly formed leaves shortly after plants emerge from the bottom sediments. There-
fore the 'effective depth' for newly emerging shoots is the total water depth.
Additionally, although plants in the inner, shallower sections of a bed may extend
toward the water surface, effectively reducing the 'effective depth' of water over the
photosynthetic tissue compared to the actual water depth there, plants at the deepest
colonizing edge of the beds are typically very short and sparse. At this point the
'effective depth' and the total water depth are again similar. Based on these two
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important examples of the process of new bed formation and bed colonization, the
application depth is defined as the total water depth.
Plant Morphology's Influence on
Determining Light Requirements
The size of a plant's reproductive structures and its morphology play key roles in
survival during periods when light levels fall below minimum requirements at water-
column depths of 1 meter or less. Species that produce large reproductive structures
tolerate periods of poor water clarity better than those with small reproductive struc-
tures. Underwater plants that sprout from large reproductive structures (large tubers,
for example) have greater stored energy reserves and, regardless of light levels, may
elongate several decimeters towards the surface where light levels are more
adequate. The reserves alone may provide enough energy to sustain survival for
several weeks (Rybicki and Carter 2002).
If light levels are inadequate for short periods and become adequate thereafter, plants
from large tubers may survive and grow to heights where their minimum light
requirements are met. On the other hand, plants originating from small reproductive
structures (such as small tubers or seeds) have smaller amounts of energy reserves
and little elongation potential, and are more likely to become weak and brittle and to
evanesce. Spring, therefore, is an especially critical period for plants with small
reproductive structures.
Similarly, mature plants that are canopy-formers are more tolerant of poor water
clarity than are meadow-forming species. If minimum light requirements are met at
0.5 meters but not at 1 meter, the taller canopy-formers are more likely to have their
light requirement met than are shorter, meadow-formers growing at the same depth.
The minimum light requirements used in deriving the water clarity criteria are meant
to allow species of all growth types to survive at the desired restoration depth.
Validation of Predicted versus Actual Bay Grass Distribution
Batiuk et al. (2000) documented their validation of the PLL diagnostic requirements
by relating calculated PLL values to field data on underwater bay grass presence
(over a 13-year record) in areas adjacent to water quality monitoring stations. Under-
water bay grass presence was categorized as: always abundant (AA), always some
(AS), sometimes none (SN), usually none (UN) and always none (AN). It was
assumed that PLL value would exceed the minimum requirement in the AA areas
and would be approximately equal to the requirement in the AS and SN areas. In
fact, in tidal-fresh and oligohaline waters, the median values of PLL at the 0.5-meter
and 1-meter depths were 5 to 8 percent and 1 to 3 percent in AS and SN areas,
respectively, well below the minimum PLL requirement of 9 percent. The validation
results were much closer in mesohaline and polyhaline waters.
Similar results were found in relating PLW to changes in underwater bay grass
coverage from year to year in tidal-fresh and oligohaline waters (Batiuk et al. 2000).
chapter iv • Water Clarity Criteria
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Positive increases in bay grass coverage occurred even when the median PLW was
considerably less than the minimum requirement at 1 meter (mean low water).
Finally the authors noted that, based on light requirements alone, underwater bay
grasses often were found at depths greater than the predicted maximum. Clearly, data
must continue to be collected to ensure consistency between predicted and actual
underwater bay grass distribution.
Natural Water Color
Color, listed as 'dissolved organic matter,' is one factor that attenuates light (see
Figure IV-1). The quantitative role of color, accounted for directly as a component
of light attenuation in both the PLW criteria and the PLL diagnostic requirement, is
not addressed separately as a criterion, for several reasons. Color data are not
collected in the Chesapeake Bay Water Quality Program. The only color data that
exist for the Chesapeake Bay have been collected by research institutions, with
sporadic spatial and temporal coverage. Color in the Chesapeake Bay's tidal waters
is largely of natural origin, including the few tributaries on the Eastern Shore in
which dissolved color concentrations are high, such as the Pocomoke River. Some
decline in color might accompany a reduction in chlorophyll a as nutrient inputs are
reduced, but currently there is no way to gauge the probable magnitude of such a
response.
Other Environmental Factors
Although light is the principal factor controlling the distribution of underwater bay
grasses throughout the Chesapeake Bay, other biological, physical, geological and
chemical factors may preclude their growth in particular sites even when minimum
light requirements are met (Livingston et al. 1998). These factors include the avail-
ability of propagules (e.g., seeds and vegetative reproductive structures), salinity,
temperature, water depth, tidal range, grazers, suitable sediment quality (organic
content and grain size), sediment nutrients, wave action, current velocity and chem-
ical contaminants (Koch 2001). Some of these factors operate directly on underwater
plants, while others inhibit the interaction of underwater plants and light or their
habitat.
Very high wave energy may prevent bay grasses from becoming established (due to
the drag exerted on the plants and the constant sediment motion), even when the
minimum light requirements are met (Clarke 1987). Waves and tides alter the light
climate by changing the depth of the water through which light passes, and by resus-
pending bottom sediments, thereby increasing total suspended solids and associated
light attenuation (Koch 2001).
Particle sinking and other sedimentological processes alter the texture, grain-size
distribution and organic content of bottom sediments. These alterations can affect
underwater bay grass growth by modifying the availability of nutrients in the sedi-
ments (Barko and Smart 1986) and by producing reduced sulfur compounds that are
toxic to underwater plants (Carlson et al. 1994). In addition, pesticides and other
chapter iv • Water Clarity Criteria
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anthropogenic chemical contaminants tend to inhibit underwater bay grass growth.
An extensive review of the literature has revealed that certain underwater bay grass
species appear to have limited tolerance of certain physical, sedimentological and
chemical variables (Koch 2001).
Attaining the water clarity criteria in a given underwater bay grass growing season
does not guarantee the presence or return of underwater bay grasses, given the envi-
ronmental factors described above. However, a wealth of scientific evidence
indicates that not attaining the water clarity criteria at the desired restoration depth
will prevent or severely reduce survival and propagation of underwater bay grasses,
regardless of the status of other environmental factors (Dennison et al. 1993).
Areas for Refinement
The process of deriving the water clarity criteria has brought areas requiring further
research and understanding into focus. Particular attention should be paid to the rela-
tionships between epiphyte biomass and nutrient concentrations and flux, and
between total suspended solids and the total mass of epiphytic material. Also, a
better understanding of the relationships between water clarity and abundance of
underwater bay grasses in lower salinity areas is needed. In addition, the published
diagnostic PLL algorithm (see Chapter VII) has been documented both to under- and
overestimate epiphyte biomass when compared with field observations.
Although the second technical synthesis (Batiuk et al. 2000) provided an initial
consideration of physical, geological and chemical requirements for bay grass
habitat, more work is needed to develop physical, geological and chemical measures
of bay grass habitat suitability.
Finally, there is a general need for a better understanding of the minimum light
requirements for the survival and growth of underwater grass species in various
Chesapeake Bay tidal habitats, as well as the influence of other environmental
factors on minimum light requirements. Detailed field and laboratory studies are
needed to develop estimates of the minimum light required by each species, both for
the survival of existing bay grass beds and reestablishment of underwater bay grasses
in unvegetated sites. The area that remains most problematic is minimum light
requirements for turbid, low-salinity habitats (particularly estuarine turbidity
maximum zones) inhabited by canopy-forming plant species. The short-term
temporal applications of the minimum light requirements need further study to deter-
mine the critical length of time required for underwater bay grasses to recover after
short periods of extremely low light levels at various stages of the growing season.
The EPA maintains that these water clarity criteria reflect the best available science
compiled and interpreted by recognized national and international scientific experts
in this field. The criteria document recognizes and clearly documents known certain-
ties and uncertainties, and where professional judgments have been exercised. In
cases where such judgments have been made, these judgments have led to the publi-
cation of water clarity criteria that protects the full array of underwater bay grass
species inhabiting Chesapeake Bay tidal waters.
chapter iv • Water Clarity Criteria
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WATER CLARITY CRITERIA DERIVATION
MINIMUM LIGHT REQUIREMENTS
Determining the PLW requirements for bay grass survival and growth involved an
extensive search of the pertinent literature and examination of results from research
and monitoring conducted in the Chesapeake Bay. A detailed documentation of this
process can be found in Chapter III, "Light Requirements for SAV Survival and
Growth" in Batiuk et al. 2000. The authors interpreted the information to determine
the range of light requirements for individuals and groups of species occurring in the
four major salinity zones of the Chesapeake Bay.
They found that the information fell into four general categories: (1) physiological
studies of photosynthesis/irradiance relationships; (2) results of field observations of
the maximum depth of underwater bay grass colonization and available light at that
depth; (3) experiments involving the artificial or natural manipulation of light levels
during long- or short-term growth studies; and (4) statistical models intended to
generalize light requirements. These four categories are discussed in the order of
their perceived utility for the purpose of determining minimum light requirements,
with physiological studies considered the least useful and models and light manipu-
lation experiments considered the most useful. The literature reviewed included lake,
estuary and coastal marine studies throughout the world.
Photosynthesis-lrradiance Measurements
Numerous studies have presented photosynthesis-irradiance curves for underwater
plants. Photosynthesis-irradiance curves are generated by exposing whole plants,
leaves or leaf or stem sections to varying light intensities and measuring the rate of
photosynthesis based on the generation of oxygen or consumption of carbon dioxide.
Most photosynthesis-irradiance measurements are made in the laboratory, although
some studies use ambient light and environmental conditions, with plants suspended
in bottles at different water depths. As suggested by Zimmerman et al. (1989), it is
questionable to use short-term photosynthesis-light experiments to estimate light-
growth relationships and depth penetration, particularly when plants are not
acclimated to experimental conditions. In addition to the balance between photosyn-
thesis and respiration, estimates of minimum light requirements must consider other
losses of plant organic carbon through herbivory, leaf sloughing and fragmentation
as well as reproductive requirements.
Field Observations of Maximum Depth and Available Light
Numerous studies around the world link observations of the maximum depth to
which an underwater grass species grows (Zmax) to the available light (Im) at that
depth (see Appendix A in Batiuk et al. 2000). Individual maximum-depth-of-
chapter iv • Water Clarity Criteria
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91
colonization studies were not particularly useful for setting up minimum light
requirements for Chesapeake Bay environments. Most studies were of freshwater
and oligohaline species in freshwater lakes, where the water was clear and the
percent of surface light in midsummer on a clear day was not indicative of the plant's
seasonal light environment. Determinations were based on the maximum depth at
which the plants were rooted, disregarding chance fragments or propagules that
might have established outlier populations and not survive an entire growing season
(e.g., Moore 1996). Measurement frequency is a significant problem that should be
considered in these studies. However, taken in the aggregate, these field observations
serve as a basis for models that predict maximum depths of colonization or minimum
light requirements (see section titled "Light Availability Models" below).
Light Manipulation Experiments
Light requirements for the growth and survival of underwater bay grasses have been
tested using short- to long-term studies under experimental light conditions. These
studies were done in situ, in mesocosms where plants receive a measured percentage
of ambient light, or in the laboratory where underwater plants are grown under
constant light and temperature regimes. Most field studies were done using polyha-
line and mesohaline species. In the case of prolonged field experiments, recovery of
the plants was sometimes monitored. Some studies did not involve the actual manip-
ulation of light levels; for example, Dunton (1994) involved natural shading by an
algal bloom and continuous monitoring of light in Texas coastal bays, whereas
Kimber et al. (1995) and Agami et al. (1984) suspended plants in buckets at specific
depths and observed survival rates. Laboratory and mesocosm experiments under
controlled light, temperature and flow conditions may substantially underestimate
natural light requirements because of the absence of natural light variability,
herbivory, fragmentation losses and tidal or riverine currents.
Light Availability Models
In recent years attempts have been made to develop statistical regression models to
quantify the relationship of light availability to the depth of underwater bay grass
growth, based on the maximum depth of colonization and water-column light atten-
uation (Canfield et al. 1985; Chambers and Kalff 1985; Vant et al. 1986; Duarte
1991; Middleboe and Markager 1997). Models also have been developed to relate
light availability to productivity, primarily in polyhaline species (Zimmerman et al.
1994), and to show the relationships of various factors affecting underwater bay
grass survival (Wetzel and Neckles 1986). Since the models relating depth of colo-
nization and water clarity tend to use large data sets from different habitats, they are
considered more robust than models based on single studies or sites, yet some of the
more robust models still depend on one-time observations at maximum depth or light
availability from the literature.
Figure IV-3 shows a good correspondence among models. For lake species in general,
a depth of 1 meter would be colonized when Secchi depth = 0.4 to 0.7 meters. The
chapter iv • Water Clarity Criteria
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92
0.4- to 0.7-meter range is compa-
rable with the light constraints
described by Carter and Rybicki in
Batiuk et al. (1992). They
suggested that when median
seasonal Secchi depths were
0.7 meters, underwater bay grass
beds would increase in size,
whereas at Secchi depths less than
0.5 meters, revegetation would not
occur. Between 0.5 and 0.7 meters,
other factors, such as epiphyte
loading, available sunshine, size
and the number of tubers set in the
previous year all play a role in
determining survival.
Models relating maximum depth of
colonization to Secchi depth or
light attenuation and percent of
surface irradiance for mesohaline
and polyhaline species are summa-
rized in Batiuk et al. (2000).
Relationships between maximum
depth of colonization and light
attenuation coefficients indicate
that for any specific light attenua-
tion coefficient the maximum depth
of colonization is greater for tidal-
fresh and oligohaline species than
for mesohaline and polyhaline
species (see Figure III-4 in Batiuk et al. 2000). These studies indicate that there is a
greater minimum light requirement for mesohaline and polyhaline species.
Examination of the four types of evidence for minimum light requirements discussed
above—photosynthesis-irradiance curves, field observations, light manipulation and
models—indicates that models were the best source of comparative information for
developing minimum light requirements for the Chesapeake Bay (Batiuk et al.
2000). The shading experiments, although they did not help to refine the minimum
light requirements, illuminated the complexity of plant success under reduced light
conditions. Although the published literature did not provide specific numbers for
Chesapeake Bay minimum light requirements, the information was used to guide
decisions and suggest limiting factors.
A considerable fraction of the total studies on light requirements for underwater bay
grasses were done in estuarine environments. Most of these were, however,
chapter iv • Water Clarity Criteria
¦ Canfield
Chambers and Kalff
Duarte and Kalff
¦ M&M--Caulescent angiosperms, nonlinear
M&M--Caulescent angiosperms, linear
Vant et al.
0 0.5 1 1.5 2 2.5 3
Secchi Depth (meters)
Figure IV-3. Relationship of maximum depth of colonization (Zmax)
to Secchi depth for freshwater SAV species as modeled by
Canfield et al. (1985), Chambers and Kaff (1985), Duarte
and Kalff (1987), Middleboe and Markager (1997) and
Vant et al. (1986).
-------
93
conducted in higher salinity mesohaline and polyhaline habitat areas, and virtually
none are from lower salinity oligohaline and tidal-fresh portions of estuaries. The
EPA recognizes that there is a need for continued research to improve the under-
standing of light requirements for underwater plants in these environments.
Although results of these studies would certainly help to refine detailed knowledge
of underwater bay grass light requirements and of how to apply these to predict plant
survival in nature, the EPA is confident they will not change the broad foundations
of the water clarity criteria.
The present criteria are based on studies involving virtually all of the important
underwater bay grass species found in the Chesapeake Bay. Healthy populations of
the two seagrasses found in the Bay, Zostera marina and Riippia maritima, have
been studied in environments of widely varying salinity. On the other hand, low-
salinity regions of the upper Chesapeake Bay and its tributaries have historically
provided habitat for many freshwater species that tolerate brackish conditions. There
is no evidence that the light requirements for these species would be radically
different in freshwater versus low-salinity estuarine habitats.
Chesapeake Bay Research and Monitoring Findings
Research and monitoring results from the Chesapeake Bay also contributed to the
derivation of the minimum light requirements, especially in tidal-fresh and oligoha-
line waters where limited scientific literature existed. Batiuk et al. (1992) established
PLW requirements by salinity regime for the restoration of underwater bay grasses
to a depth of 1 meter throughout the Chesapeake Bay: Kd = 2 m_1 in tidal-fresh and
oligohaline regimes and Kd= 1.5 in1 in mesohaline and polyhaline segments. Light
attenuation coefficients are calculated using Beer's Law Iz = I0exp(-KdZ), where I0
is light (photosynthetically active radiation [PAR]) measured just below the surface
and Iz is light measured at depth Z. Using the relationship
PLW = 100exp(-KdZ) (Equation IV-1)
where Z = depth in the water column, and setting Z = 1 meter, the Chesapeake Bay
minimum seasonal percent light requirement as published in Batiuk et al. (1992) was
13.5 percent of ambient surface light in tidal-fresh and oligohaline environments and
22.3 percent of ambient surface light in mesohaline and polyhaline environments.
More specific seasonal criteria were suggested by Carter and Rybicki in Batiuk et al.
(1992) for the tidal Potomac River and estuary: Kd = 2.2 nr1 in tidal-fresh regions
and Kd= 2.7 nr1 in oligohaline regions, which translated into PLW requirements of
11 percent in tidal-fresh and 7 percent in oligohaline habitats.
Tidal-Fresh/Oligohaline Potomac River Findings. From 1983 through 1996,
underwater bay grass coverage in the tidal Potomac River varied greatly in both the
tidal-fresh and oligohaline reaches. The change in underwater bay grass coverage
from the previous year and the median PLW calculated from growing season Secchi
chapter iv • Water Clarity Criteria
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94
depth varied greatly, but both exhibited a general downward trend during this period.
When the change in underwater bay grass coverage from the previous year is plotted
against the median PLW at 1 meter during the underwater bay grass growing season
(April 1 through October 31), underwater bay grasses increased with increases in the
PLW. When median PLW was greater than 13 percent, underwater bay grass coverage
showed only positive increases over three years. However, positive increases occurred
even in years when median percent light at 1 meter was considerably less than 13
percent, indicating that other factors besides light also influence changes in coverage, or
that underwater bay grasses were growing at depths < 1 meter.
A median growing season PLW of 13 percent at 1 meter is equivalent to a median
Secchi depth of 0.7 meters or median Kd=2.07, assuming Kd = 1.45/Secchi depth.
Secchi depth is only reported to 0.1 meters, so the error in the median measurements
is ± 0.05 meter, median seasonal Secchi depth ranges from 0.65 to 0.75 meters and,
therefore, Kd ranges from 1.93 to 2.23 meter1. Carter, Rybicki and Landwehr
reported in Batiuk et al. (2000) that for the tidal-fresh and oligohaline segments of the
Potomac River, a corresponding range of PLW of 11 percent to 14.5 percent presented
a boundary condition for a net increase in growth from year to year. It should also be
noted that if other habitat conditions are favorable, underwater bay grasses may
tolerate worse light conditions for a season, but not on a protracted basis.
Tidal-Fresh Patuxent River Findings. Between 1985 and 1996, light conditions
at the tidal-fresh Patuxent River monitoring station PXT0402 (or TFI.5) improved.
Kd dropped from 6 meter1 to about 4 meter1 (Naylor, unpublished data reported in
Batiuk et al. 2000) and average Secchi depth increased from 0.25 to 0.4 meters.
During the last four years of this period, colonization by underwater bay grasses also
increased, primarily in the shallow areas less than 0.5 meters deep. A Kd of 4 meter
1 results in 13.5 percent light at a depth of 0.5 meters. A second Patuxent River
tidal-fresh water quality monitoring station (PXT0456 or TFI.4) also showed a
significant increase in Secchi depth during the underwater bay grass growing season
of this same period.
It appears that when the seasonal Secchi depth at monitoring station PXT0456 was
greater than a threshold value of 0.35 meters, the underwater bay grass coverage
continued to increase, whereas a Secchi depth below 0.35 meters coincided with a
decrease in underwater bay grass coverage. A Secchi depth threshold of 0.35 meters
for plants colonizing a depth of less than 0.5 meters is equivalent to a 0.68-meter
Secchi depth threshold for plants colonizing a depth of less than 1 meter (as seen in
the Potomac). Thus, it appears that similar threshold light conditions are required for
successful recolonization in the tidal-fresh areas of both the Potomac and Patuxent
rivers (Batiuk et al. 2000).
Mesohaline Potomac River Findings. In the mesohaline segment of the
Potomac River, underwater bay grasses have continued to increase steadily since
1983, although the coverage remains relatively small compared to pre-1960 condi-
tions. Colonization by underwater bay grasses has taken place primarily in areas less
chapter iv • Water Clarity Criteria
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95
than 1 meter deep. Midchannel light conditions are better in the mesohaline segment
of the river compared to either the tidal-fresh or oligohaline segments, with the
median seasonal Secchi depth generally never dropping below 1 meter for the period
of 1983 through 1996. Secchi depth is only reported to 0.1 meters, so the error in the
median measurements is at least ±0.05 meters. If median Secchi depth is 1 meter,
then using a conversion factor of 1.45 to calculate Kd median light conditions of 23.5
percent at 1-meter depth, with a range of 21.7 percent to 25.1 percent (Batiuk et al.
2000). Thus, the Chesapeake Bay water-column light requirements published previ-
ously by Batiuk et al. (1992) for mesohaline and polyhaline segments are consistent
with those observed in the mesohaline region of the Potomac River where under-
water bay grasses are recovering.
Mesohaline/Polyhaline York River Findings. Strong positive relationships
between water clarity and the maximum depth of the growth of underwater plants have
been demonstrated (Dennison et al. 1993; Duarte 1991; Olesen 1996). Assuming that
a light requirement of approximately 22 percent of surface irradiance at the sediment
surface is necessary for the long-term growth and survival of underwater bay grasses
in high salinity regions of the Chesapeake Bay (Batiuk et al. 2000), the presence of
underwater bay grasses to a depth of 1 to 1.5 meters below mean low water in this
region would require light-attenuation coefficients of approximately 1 meter1 or 0.7
meter"1, respectively. In the high mesohaline and polyhaline reaches of the lower York
River, field measurements of Kd have yielded long-term median values of 1 meter1 in
the shallow littoral zone where underwater bay grasses have been consistently growing
down to depths of 1 meter (Moore 1996; Moore et al. 2001).
LIGHT-THROUGH-WATER REQUIREMENTS
Based on a thorough review of the results of shading experiments and model find-
ings published in the scientific literature, a PLW value of greater than 20 percent is
needed for the minimum light requirement of Chesapeake Bay polyhaline and meso-
haline species (Batiuk et al. 2000). Consistent with the value derived from the
scientific literature, the PLW requirement of 22 percent was determined for mesoha-
line and polyhaline regions of the Chesapeake Bay and its tidal tributaries by
applying the appropriate 1992 underwater bay grass habitat requirement for Kd of
1.5 meter1 to Equation IV-1 (Batiuk et al. 1992). This PLW requirement was
confirmed by almost two decades of field observations in the mesohaline Potomac
River and mesohaline/ polyhaline York River (Batiuk et al. 1992, 2000; Moore 1996;
Moore et al. 2001) as discussed above.
Based on published model findings reviewed in detail by Carter, Rybicki and
Landwehr in Batiuk et al. (2000) and confirmed by a review of the results of recent
tidal Potomac and Patuxent River research and monitoring studies (see above), a
PLW requirement of 13 percent was determined to apply to Chesapeake Bay tidal-
fresh and oligohaline species. This light requirement was calculated using Equation
IV-1 and the appropriate 1992 SAY habitat requirement for Kd of 2 meter1 (Batiuk
chapter iv • Water Clarity Criteria
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96
et al. 1992). The PLW requirement also is consistent with the 13.5 percent value
published by Dennison et al. (1993).
These PLW requirements were validated through a comprehensive analysis of 13
years (1985-1998) of Chesapeake Bay water quality monitoring data. The results
were published in Chapter VII of Batiuk et al. (2000).
Table IV-1. Summary of Chesapeake Bay water clarity criteria for application to shallow-water bay
grass designated use habitats (application depths given in 0.25 meter depth intervals.2
Salinity
Regime
Water Clarity
Criteria as
Percent Light-
through-Water
Water Clarity Criteria as Secchi Depth
Temporal
Application
Water Clarity Criteria Application Depths
0.25
0.5
0.75
1.0
1.25
1.5
1.75
2.0
Secchi Depth (meters) for above Criteria Application Depth
Tidal-fresh
13 %
0.2
0.4
0.5
0.7
0.9
1.1
1.2
1.4
April 1 - October 31
Oligohaline
13%
0.2
0.4
0.5
0.7
0.9
1.1
1.2
1.4
April 1 - October 31
Mesohaline
22%
0.2
0.5
0.7
1.0
1.2
1.4
1.7
1.9
April 1 - October 31
Polyhaline
22 %
0.2
0.5
0.7
1.0
1.2
1.4
1.7
1.9
March 1 - May 31,
September 1 - November 30
2Base on application of Equation IV-1, PLW = 100exp(-KdZ), the appropriate PLW criterion value and the selected
application depth are inserted and the equation is solved for Kj. The generated Kj value is then converted to Secchi depth
(in meters) using the conversion factor Kj = 1.45/Secchi depth.
CHESAPEAKE BAY WATER CLARITY CRITERIA
The Chesapeake Bay water clarity criteria are summarized in Table IV-1 as PLW and
Secchi depth equivalents over a range of application depths. They reflect a set of
minimum light requirements to protect underwater bay grass species found in the
two sets of salinity regimes, that have different growth and reproductive strategies
and individual light requirements. The water clarity criteria were derived to support
the propagation and growth of a wide variety of species, including meadow formers
and perennials, not just canopy formers and annuals. In tidal-fresh and oligohaline
habitats, the water clarity criteria call for sufficient light to address the minimum
requirements of meadow-forming species (e.g., Vallisneria americana, or wild
celery), which generally need more light, as well as canopy-forming species (e.g.,
Myriophyllum spicatum, or milfoil), which require less. Water clarity criteria appli-
cable to mesohaline and polyhaline habitats call for light conditions necessary for
the survival and growth of the two principal species—widgeon grass (Riippia
maritima) and eelgrass (Zostera marina)—inhabiting the more saline shallow-water
habitats of Chesapeake Bay and its tidal tributaries.
chapter iv • Water Clarity Criteria
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97
For these reasons, these Chesapeake Bay water clarity criteria, along with the appro-
priate dissolved oxygen and chlorophyll a criteria, fully support the "survival,
growth and propagation of rooted underwater bay grasses necessary for the propaga-
tion and growth of balanced, indigenous populations of ecologically, recreationally
and commercially important fish and shellfish inhabiting vegetated shallow-water
habitats" (Appendix A; U.S. EPA 2003).
When these water clarity criteria were derived, there was an insufficient scientific
basis for deriving a set of water clarity or related (e.g., total suspended solids) criteria
for protection of open-water designated use habitats. The EPA will derive and
publish criteria addressing water clarity-related impairments for open-water habitat
when the necessary scientific data becomes available.
LITERATURE CITED
Agami, M., S. Beer and Y. Waisel. 1984. Seasonal variations in the growth capacity of Najas
marina L. as a function of various water depths at the Yarkon Springs, Israel. Aquatic Botany
19:45-51.
Batiuk, R. A., P. Bergstrom, M. Kemp, E. Koch, L. Murray, J. C. Stevenson, R. Bartleson, V.
Carter, N. B. Rybicki, J. M. Landwehr, C. Gallegos, L. Karrh, M. Naylor, D. Wilcox, K. A.
Moore, S. Ailstock and M. Teichberg. 2000. Chesapeake Bay Submerged Aquatic Vegetation-
Water Quality and Habitat-Based Requirements and Restoration Targets: A Second Technical
Synthesis. CBP/TRS 245/00 EPA 903-R-00-014. U.S. EPA Chesapeake Bay Program,
Annapolis, Maryland.
Batiuk, R. A., R. Orth, K. Moore, J. C. Stevenson, W. Dennison, L. Staver, V. Carter, N. B.
Rybicki, R. Hickman, S. Kollar and S. Bieber. 1992. Chesapeake Bay Submerged Aquatic
Vegetation Habitat Requirements and Restoration Targets: A Technical Synthesis. CBP/TRS
83/92. U.S. EPA Chesapeake Bay Program, Annapolis, Maryland.
Barko, J. W. and R. M. Smart. 1986. Sediment-related mechanisms of growth limitation in
submersed macrophytes. Ecology 67:1328-1340.
Canfield, E. D. Jr., K. A. Langeland, S. B. Linda and W. T. Haller. 1985. Relations between
water transparency and maximum depth of macrophyte colonization in lakes. Journal of
Aquatic Plant Management 23:25-28.
Carlson, P. R., L. A. Yarbro and T. R. Barber. 1994. Relationship of sediment sulfide to
mortality of Thalassia testudinum in Florida Bay. Bulletin of Marine Science 54:733-746.
Carter, V., Rybicki, N. B., Landwehr, J. M., and Turtora, M.. 1994. Role of weather and water
quality in population dynamics of submersed macrophytes in the tidal Potomac River. Estu-
aries 17(2) :417-426.
Carter, V., Paschal, J. E., Jr., and Rybicki (Bartow), N. 1985. Distribution and abundance of
submersed aquatic vegetation in the tidal Potomac River and Estuary, Maryland and Virginia,
May 1978 to November 1981. U.S. Geological Survey Water Supply Paper 2234A. 46 pp.
Carter, V., and Rybicki, N. B. 1994. Invasions and declines of submersed macrophytes in the
tidal Potomac River and Estuary, the Currituck Sound-Back Bay system, and the Pamlico
River Estuary. Lake and Reservoir Management 10(l ):39-48.
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Chambers, P. A. and J. Kalff. 1985. Depth distribution and biomass of submersed aquatic
macrophyte communities in relation to Secchi depth. Canadian Journal of Fisheries and
Aquatic Science 42:701-709.
Clarke, S. M. 1987. Seagrass-sediment dynamics in Holdfast Bay: Summary. Saflsh 11:4-10.
Czerny, A. B. and K. H. Dunton. 1995. The effects of in situ light reduction on the growth of
two subtropical seagrasses, Thalassia testudinum and Halodule wrightii. Estuaries 18:418-
427.
Dennison, W. C., R. J. Orth, K. A. Moore, J. C. Stevenson, V. Carter, S. Kollar, P. W.
Bergstrom and R. A. Batiuk. 1993. Assessing water quality with submersed aquatic vegeta-
tion habitat requirements as barometers of Chesapeake Bay health. Bioscience 43:86-94.
Duarte, C. M. 1991. Seagrass depth limits. Aquatic Botany 40:363-377.
Dunton, K. H. 1994. Seasonal growth and biomass of the subtropical seagrass Halodule
wrightii in relation to continuous measurements of underwater irradiance. Marine Biology
120:479-489.
Gallegos, C. L. 2001. Calculating optical water quality targets to restore and protect
submersed aquatic vegetation: Overcoming problems in partitioning the diffuse attenuation
coefficient for photosynthetically active radiation. Estuaries 24:381-397.
Kemp, W. M., R. A. Batiuk, R. Bartleson, P. Bergstrom, V. Carter, C. L. Gallegos, W. Hunley,
L. Karrh, E. Koch, J. M. Landwehr, K. A. Moore, L. Murray, M. Naylor, N. B. Rybicki, J. C.
Stevenson, and D. J. Wilcox. In review. Habitat requirements for submerged aquatic vegeta-
tion in Chesapeake Bay: Water quality, light regime and physical-chemical factors. Estuaries.
Kimber, A., J. L. Owens and W. G. Crumpton. 1995. Light availability and growth of wild
celery (Vallisneria americana) in upper Mississippi River backwaters. Regulated Rivers:
Research and Management 11:167-174.
Koch, E. W. 2001. Beyond light: Physical, geological and geochemical parameters as
possible submersed aquatic vegetation habitat requirements. Estuaries 24:1-17.
Livingston, R. J., S. E. McGlynn and X. Niu. 1998. Factors controlling seagrass growth in a
gulf coastal system: Water and sediment quality and light. Aquatic Botany 60:135-159.
Middleboe, A. L. and S. Markager. 1997. Depth limits and minimum light requirements of
freshwater macrophytes. Freshwater Biology 37:553-568.
Moore, K, D. Wilcox and B. Anderson. 2001. Analysis of historical distribution of
submerged aquatic vegetation (SAV) in the York and Rappahannock rivers as evidence of
historical water quality conditions. Special Report No. 375 in Applied Marine Science and
Ocean Engineering Virginia Institute of Marine Science, School of Marine Science, College
of William and Mary, Gloucester Point, Virginia.
Moore K. A. 1996. Relationships between seagrass growth and survival and environmental
conditions in a lower Chesapeake Bay tributary. Ph.D. dissertation. University of Maryland.
College Park, Maryland. 188 pp.
Moore, K.A., J. L. Goodman, J. C. Stevenson, L. Murray and K. Sundberg. 1995. Chesa-
peake Bay nutrients, light and SAV: relations between variable water quality and SAV in field
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land. 106 pp.
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Olesen, B. 1996. Regulation of light attenuation and eelgrass Zostera marina depth distribu-
tion in a Danish embayment. Marine Ecology Progress Series 134:187-194.
Orth, R. J., M. Lukenback and K. A. Moore. 1994. Seed dispersal in a marine macrophyte:
Implications for colonization and restoration. Ecology 75(7): 1927-1939.
Rybicki, N. B. and V. Carter. 2002. Light and temperature effects on the growth of wild celery
and hydrilla. Journal of Aquatic Plant Management 40:92-99.
Rybicki, N. B. and V. Carter. 1986. Effects of sediment depth and sediment type on the
survival of Vallisneria americana grown from tubers. Aquatic Botany 26:307-323.
U.S. Environmental Protection Agency. 2003. Technical support document for identifying
Chesapeake Bay designated uses and attainability. EPA 903-R-03-004. Chesapeake Bay
Program Office, Annapolis, Maryland.
Vant, W. N., R. J. Davies-Colley, J. S. Clayton and B. T. Coffey. 1986. Macrophyte depth
limits in North Island (New Zealand) lakes of differing clarity. Hydrobiologia 137:55-60.
Wetzel, R. L. and H. A. Neckles. 1986. A model of Zostera marina L. photosynthesis and
growth: Simulated effects of selected physical-chemical variables and biological interactions.
Aquatic Botany 26:307-323.
Zimmerman, R. C., A. Cabello-Pasini and R. S. Alberte. 1994. Modeling daily production of
aquatic macrophytes from irradiance measurements: A comparative analysis. Marine
Ecology Progress Series 114:185-196.
Zimmerman, R. C., R. D. Smith and R. S. Alberte. 1989. Thermal acclimation and whole-
plant carbon balance in Zostera marina L. (eelgrass). Journal of Experimental Biology and
Ecology 130:93-109.
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chapte v
Chlorophyll a Criteria
BACKGROUND
Phytoplankton are small microscopic plants, or algae, drifting in the water column
with the currents. They constitute a diverse group that contributes importantly to the
base of the Chesapeake Bay's food web, linking nutrients and the energy of sunlight
with small planktonic animals or zooplankton, forage fish, filter feeders such as
oysters, bottom-dwelling worms and clams and fishes (Bay and Horowitz 1983;
Tuttle et al. 1987; Malone et al. 1986; Heck 1987; Malone et al. 1988). The majority
of the Chesapeake Bay's animals feed directly on phytoplankton or secondarily on the
products of phytoplankton that support the 'microbial loop' (such as nonphotosyn-
thetic flagellates, protozoa, bacteria and fungi), all of which support higher trophic
levels. The Chesapeake Bay's 'carrying capacity' or its ability to support productive
and diverse populations of flora and fauna, including highly valued species, depends
largely on how well phytoplankton meet the nutritional needs both in quantity and
quality of the various consumers.
SCOPE AND MAGNITUDE OF
NUTRIENT ENRICHMENT IN CHESAPEAKE BAY
Problems caused by nutrient over-enrichment are perhaps the longest-standing water
quality issues created by people (Vollenweider 1992). Early marine scientists consid-
ered nutrients as a resource, not a problem (Brandt 1901) and considered ways to
fertilize coastal seas to increase fisheries production. However, this was before
human populations and land use activities to support these bourgeoning populations
had reached today's levels, especially since about the 1960s. The problem is espe-
cially challenging in the Chesapeake Bay ecosystem because the Bay ecosystem's
variable dynamics produce large, natural fluctuations. Superimposed onto these
natural changes are those caused by human disturbance, and nutrient enrichment is
only one among many other pressures experienced by the Bay ecosystem (Breitburg
chapter v • Chlorophyll a Criteria
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102
et al. 1999). The scientific challenge persists because human disturbance often is
subtle, indirect and sometimes is confounded by natural changes (Cloern 1996) that
are not yet understood enough for predictive purposes. Anthropogenic nutrient
enrichment of rivers—which deliver much of their nutrient loads to estuaries and
shelf waters—has resulted, in the U.S. in nitrogen fluctuations 5 to 14 times greater
than natural rates (Jaworski et al. 1997). Phosphorus loading to estuarine systems has
increased two- to sixfold since 1900 (Conley 2000).
Nutrient over-enrichment can cause ecological symptoms in the Chesapeake Bay
that impair designated uses, as defined by the Clean Water Act. Nutrient enrichment
and changes in important grazer populations such as oysters, menhaden,
zooplankton and benthic macroinvertebrates have potentially altered the natural
equilibrium between phytoplankton production and consumption in the last century
(Kennedy and Breisch 1981; Boynton et al. 1982; Officer et al. 1984; Marshall and
Lacouture 1986; Nixon et al. 1986; Gerritsen et al. 1988; Newell 1988; Verity 1987;
Malone et al. 1991; Malone 1992; Gerritsen et al. 1994; Hartman and Brandt 1995;
and Kemp et al. 1997). Phytoplankton populations currently reach very high concen-
trations (Filardo and Dunstan 1985; Boynton et al. 1982; Sellner et al. 1986;
Magnien et al. 1992; Malone 1992; Haas and Wetzel 1993; Lacouture et al. 1993;
Harding 1994; Glibert et al. 1995) and high production rates during the spring and
summer (Sellner et al. 1986; Magnien et al. 1992; Lacouture et al. 1993; Marshall
and Nesius 1996; Sin et al. 1999). Phytoplankton communities also are capable of
supporting several potentially toxic taxa (Seliger et al. 1975; Ho and Zubkoff 1979;
Luckenbach et al. 1993; Lewitus et al. 1995; Marshall 1995; Glibert et al. 2001).
Excess, uneaten phytoplankton accumulate in the water column and contribute to
reduced water clarity and summer oxygen depletion in bottom waters, ultimately
stressing the food webs they support (Neilson and Cronin 1981; Boynton et al. 1982;
Harding et al. 1986; Seliger et al. 1985; Fisher et al. 1988; Malone 1992). Nutrient
enrichment had already affected underwater bay grass distributions throughout much
of the Chesapeake Bay by the early 1960s (Flemer et al. 1983; Orth and Moore
1983) and deep-channel hypoxia and anoxia has been confirmed to have been initi-
ated during the early 1970s (Hagy 2002). Local nutrient over-enrichment problems
occurred earlier in some Bay tidal tributaries; massive blue-green algae blooms in
the upper tidal freshwater Potomac River Estuary began during the 1950s (Jaworski
et al. 1972), and Baltimore Harbor experienced a widening hypoxia problem well-
established by the mid-1800s (Capper et al. 1983).
CHLOROPHYLLS: KEY INDICATOR OF PHYTOPLANKTON BIOMASS
Scientific interest and practical management needs required that the quantity of
phytoplankton biomass in aquatic ecosystems be simply measured as an indicator of
water quality and ecosystem health. It was discovered many decades ago that chloro-
phyll a, a ubiquitous photosynthetic pigment often associated with other pigments in
freshwater and coastal marine phytoplankton, would serve as a useful indicator for
chapter v • Chlorophyll a Criteria
-------
103
both the photosynthetic potential and biomass of phytoplankton (Flemer 1970).
Thus, over the years, chlorophyll a has become a principal measure of the amount of
phytoplankton present in a water body Chlorophyll a also plays a direct role in
reducing light penetration (Lorenzen 1972). Relatively rapid methods evolved to
measure the concentration of chlorophyll a in discrete water samples and in vivo
(Flemer 1969; U.S. EPA 1997). Methods have been developed to measure chloro-
phyll a using aerial surveillance techniques based on passive multispectral signals
associated with phytoplankton (Harding 1992). As Harding and Perry (1997) wrote,
"Chlorophyll a is a useful expression of phytoplankton biomass and is arguably the
single most responsive indicator of N [nitrogen] and P [phosphorus] enrichment in
this system [Chesapeake Bay]."
Compelling evidence indicates that reduced water clarity and low dissolved oxygen
conditions improve when excess phytoplankton or blooms, measured as chlorophyll
a, are substantially reduced (National Research Council 2001). Improvement in
water clarity is a major issue for the recovery of the Bay's shallow-water underwater
grasses (see Chapter IV); correcting the low dissolved oxygen problems that occur
in the deeper waters of the mesohaline mainstem Chesapeake Bay and lower tidal
tributaries has been a challenge to Chesapeake Bay restoration for decades (see
Chapter III). High algal biomass present in small embayments may be associated
with super-saturated dissolved oxygen conditions during the day and hypoxic to
anoxic conditions during the early morning hours (D'Avanzo and Kremer 1994).
Attaining the Chesapeake Bay dissolved oxygen and water clarity criteria will
require reductions in chlorophyll a concentrations by reducing nutrient (yielding
nutrient limitation) and sediment (resulting in light saturation) loadings.
In addition to the habitats described above that require chlorophyll a criteria, other
locations in Chesapeake Bay tidal waters experience phytoplankton blooms that may
not be directly associated with low dissolved oxygen and the shading of underwater
bay grasses due to phytoplankton. Numerous small shallow-water embayments
continue to experience inordinately high chlorophyll a concentrations. Some of these
habitats may experience early-morning hypoxia or anoxia, while others may not
have contained documented growth of underwater bay grasses before the baywide
decline. In some parts of the Chesapeake Bay and its tidal tributaries, even reducing
nutrient and sediment loadings to levels that would result in attaining the deep-water
and deep-channel dissolved oxygen and shallow-water clarity criteria will not
prevent harmful algal blooms or ensure the return of high quality food to open-water
habitats. These areas include, but are not limited to, those without low oxygen condi-
tions for hydrologic reasons (e.g., high mixing rates) and those in which reduced
water clarity conditions are driven more by suspended sediments than by water-
column algae. For these reasons, the EPA believes it is necessary to develop and
adopt chlorophyll a criteria in addition to water clarity and dissolved oxygen criteria
for the protection of Chesapeake Bay tidal waters.
chapter v • Chlorophyll a Criteria
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104
CHESAPEAKE BAY CHLOROPHYLL A CRITERIA
This chapter presents the EPA's recommended narrative chlorophyll a criteria, along
with supporting numeric concentrations and methodological approaches to
addressing nutrient-enrichment impairments related to the overabundance of algal
biomass measured as chlorophyll a. The EPA expects states to adopt narrative
chlorophyll a criteria into their water quality standards for all Chesapeake Bay and
tidal tributary waters. The EPA strongly encourages states to develop and adopt site-
specific numerical chlorophyll a criteria for tidal waters where algal-related
impairments are expected to persist even after the Chesapeake Bay dissolved oxygen
and water clarity criteria have been attained.
The narrative chlorophyll a criteria in Table V-l, derived in part through a review of
other states' chlorophyll a water quality standards (Appendix D), are recommended
for encompassing the full array of possible impairments, all of which may not mani-
fest themselves within a particular water body at any one time. The site-specific
nature of impairments caused by the overabundance of algal biomass supports state
adoption of the EPA-recommended narrative criteria, with application of site-specific
numeric criteria for localized waters addressing local algal-related impairments.
Because of the regional and site-specific nature of algal-related water quality impair-
ments, baywide numerical criteria have not been published here. Therefore, the
chlorophyll a concentrations tabulated in this document are not numerical EPA
criteria. Along with the documented methodologies, the tabulated chlorophyll a
concentrations are provided as a synthesis of the best available technical information
for the states consideration and use in their development and adoption of more
regional and site-specific numerical chlorophyll a criteria. States can use this
information in deriving numerical translators for their narrative criteria, and use
these for their narrative criteria, target chlorophyll a concentrations in concert with
narrative criteria.
Several different approaches were evaluated to develop relationships among chloro-
phyll a concentrations and tidal-water designated uses. The states also should
consider the strengths and limitations of each approach, as well as other available
scientific and technical information, when deriving site-specific numerical chloro-
phyll a criteria or numerical translators for their narrative criteria.
Table V-1. Recommended Chesapeake Bay chlorophyll a narrative criteria.
Concentrations of chlorophyll a in free-floating microscopic aquatic plants (algae) shall not exceed
levels that result in ecologically undesirable consequences—such as reduced water clarity, low
dissolved oxygen, food supply imbalances, proliferation of species deemed potentially harmful to
aquatic life or humans or aesthetically objectionable conditions—or otherwise render tidal waters
unsuitable for designated uses.
chapter v • Chlorophyll a Criteria
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105
SUPPORTING TECHNICAL INFORMATION
AND METHODOLOGIES
Algae play a unique role at the base of the aquatic food web. The size and composi-
tion of the phytoplankton community strike a delicate balance between supporting a
balanced, productive ecosystem and fueling severe impairments of water quality and
natural ecological relationships. Given that an overabundance or a shift in species
composition can yield diverse negative ecological consequences, the supporting
chlorophyll a concentrations and methodologies have been structured to characterize
an array of ecological conditions. They are based on decades of historical observa-
tions; scientific findings published in the international, peer-reviewed literature; field
and laboratory experiments; historic Chesapeake Bay water quality data; and exten-
sive Chesapeake Bay-specific research, monitoring and modeling.
CONTEXT FOR THE NARRATIVE
CHESAPEAKE BAY CHLOROPHYLL A CRITERIA
To interpret the narrative chlorophyll a criteria that will protect the designated uses
of the Chesapeake Bay and its tributaries, various ecological conditions must be
considered and different water quality impairments should be addressed. Table V-2
presents various water quality conditions along the continuum of trophic status or
ecological conditions, framing the connections between algal growth and produc-
tivity, the various ecological and water quality consequences and, ultimately,
designated uses for Chesapeake Bay tidal waters.
An oligotrophic status indicates conditions that are not signficantly affected by nutrient and
sediment enrichment, typically characterized with low nutrient/low organic matter input or
production. Under mesotrophic conditions, a water body is nutrient-enriched but still functions
adequately without the enhanced production of algae having an adverse impact on the aquatic
food web. When a water body reaches eutrophic conditions, excess production of algae can lead
to low dissolved oxygen conditions, reduced water clarity, harmful algal blooms and other
ecological impairments that reflect alterations of the aquatic food web. Aquatic systems that have
become so overloaded with nutrients that they are unable to assimulate available nutrients are
characterized as hyper- or highly eutrophic.
Estuarine scientists and managers have borrowed from the field of limnology such
terms as oligotrophic, mesotrophic, eutrophic and hypereutrophic to reflect a range
in symptoms of nutrient over-enrichment. The reality is that there is no scientific
consensus on exactly what these terms mean for nutrient enrichment in estuaries. In
the case of the Chesapeake Bay, Table V-2 establishes an ecosystem trophic status
classification scheme useful for setting the context for the narrative Chesapeake Bay
chlorophyll a criteria (see Table V-l) and supporting technical information and
methodologies.
chapter v • Chlorophyll a Criteria
-------
106
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chapter v
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107
The analogy equating oligotrophic with pristine is somewhat forced, because even
before European contact, the Chesapeake Bay probably was never poor in nutrients
(in the sense of an oligotrophic lake, for example, where likely a small watershed
and a relatively impervious geology supplied very low nutrient loads). Proximity to
terrestrial nutrient inputs, long residence times for nutrient recycling and generally
shallow (8 meters average depth) conditions allowing fairly significant benthic-
pelagic coupling are all factors that would prevent the Chesapeake Bay from ever
being truly oligotrophic.
So, in a relative sense, the Chesapeake Bay might have been considered mesotrophic
during these earlier times and became eutrophic as changes in land uses resulted in
increased nutrient supplies. This is based on a definition of eutrophic as having
excess algae, leading to the observed more frequent, persistent and intense periods
of low to no dissolved oxygen and substantial reductions in water clarity. Tidal
waters surrounded by intensely developed lands have become hyper-eutrophic. In a
reference condition context, if a majority of Chesapeake Bay tidal waters are consid-
ered eutrophic now, a management goal might be to reduce nutrient loadings and,
therefore, chlorophyll a concentrations, to achieve a more mesotrophic condition, in
contrast to the present eutrophic to hypereutrophic situations.
CHLOROPHYLLS CONCENTRATIONS CHARACTERISTIC OF
VARIOUS ECOLOGICAL CONDITIONS
Described and documented below are the chlorophyll a concentrations characteristic
of various ecological conditions within Chesapeake Bay tidal-water habitats.
Historical Chlorophyll a Concentrations
Chlorophyll a concentrations that historically reflected a more balanced Bay
ecosystem were quantified through reviews and evaluations of 1950s through 2000
data (Harding 1994; Harding and Perry 1997; Olson 2002). The chlorophyll a
concentrations derived through this detailed analysis of historically observed
concentrations are characteristic of a mesotrophic estuarine system.
1950s to 1990s Concentration Trends. Harding and Perry (1997) documented
significantly increasing trends in chlorophyll a concentrations during the past several
decades in the Chesapeake Bay mainstem. Surface mixed-layer concentrations
increased five- to tenfold in the higher salinity mesohaline and polyhaline regions,
with 1.5- to twofold increases observed in the tidal-fresh to oligohaline regions of
the Bay. During this 50-year period, they documented three major patterns in fresh-
water flow to the Chesapeake Bay: a long period of low river flows during the 1960s,
followed by a series of high flow years throughout most of the 1970s, with a mix of
river flow levels in the following two decades, and the extreme droughts (1989) and
near-record river flows (1993, 1994) reported toward the end of the data record.
Harding and Perry (1997) applied an autoregressive moving-average procedure to
chapter v • Chlorophyll a Criteria
-------
108
explain possible chlorophyll a concentrations over time strictly on the basis of
observed freshwater inflow, salinity and temperature. When compared with observed
concentration trends over decades, the significant increases in chlorophyll a could
not be accounted for strictly by the variability of freshwater flow, salinity and
temperature. The resulting trends could be explained by increased nutrient enrich-
ment of the estuarine ecosystem.
Taking into account the effects of variable annual river flows, chlorophyll a concen-
trations were shown to respond to changes in nutrient loadings over the period of
record. These historically observed chlorophyll a concentrations were more repre-
sentative of mesotrophic conditions.
In oligohaline to tidal-fresh reaches of the Chesapeake Bay mainstem (regions V and
VI, respectively), Harding and Perry (1997) documented an increasing trend in
chlorophyll a concentrations from the 1950s to the 1970s, followed closely by a
decreasing trend that has carried through into the 1990s (Table V-3; Figure V-l). The
decreasing trends are likely due to significant decreases in phosphorus loadings to
the Bay, resulting from widespread upgrades in wastewater treatment for phos-
phorus. Bans on phosphates in detergents also were enacted in states surrounding the
Bay during the mid- to late 1980s. The phytoplankton in lower salinity systems
where phosphorus has been limited have responded positively, and this has led to
lower chlorophyll a concentrations, whereas comparable reductions in nitrogen
loads have not yet been achieved, limiting opportunities for reduced phytoplankton
biomass in the higher salinity regions of the mainstem Bay.
In the 1950s, recognizing limitations in the temporal and spatial coverage of the
available data, regional mean chlorophyll a concentrations were 3.19 and 2.51 «g
liter1 in the tidal-fresh to low- salinity regions between the Susquehanna Flats and
the Bay Bridge and between the Bay Bridge and the South River, respectively
(regions VI and V, respectively, Harding and Perry 1997; see Figure V-l). Concen-
trations peaked at 15.59 fig liter1 (1960s) and 13.12 jug liter1 (1970s) in these two
regions, respectively, and were recorded as regional means of 5.57 fig liter1 and
10.86 fig liter'during the 1985-1994 period.
In the higher salinity mesohaline regions—Region IV-South River down to the
Patuxent River and Region III-Patuxent River south to the Rappahannock River—
chlorophyll a concentrations increased 1.5- to twofold from the 1950s through the
mid-1990s (Figure V-l; Harding and Perry 1997). Regional mean chlorophyll a
concentrations ranged from 4.33 fig liter1 in the 1950s up to 8.20 fig liter1 for the
period of 1985- 1994 in the mainstem Bay between the South and Patuxent rivers.
At the same time, regional mean chlorophyll a concentrations were 3.58 fig liter_1and
8.03 fig liter1, respectively, in the mainstem Bay between the Patuxent and Rappa-
hannock rivers.
Harding and Perry (1997) reported the largest trends in the polyhaline regions of the
mainstem Bay, where chlorophyll a concentrations increased five- to tenfold in
nearly 50 years. In the mainstem Bay from the Rappahannock River down to
chapter v • Chlorophyll a Criteria
-------
109
Table V-3. Chesapeake Bay mainstem surface chlorophyll a concentration (jjg liter1)
annual means for 1950 to 1994.
Time Period
Region
Chlorophyll a
Annual Mean
Number of
Observations
Percent
Difference1
1950-1959
I
0.46
41
-
II
1.21
18
-
III
3.58
108
-
IV
4.33
7
-
V
3.19
15
-
VI
2.51
18
-
1960-1969
I
1.89
8
310
II
2.61
9
115
III
7.09
28
98
IV
7.48
58
73
V
7.79
97
144
VI
15.59
295
521
1970-1979
I
4.39
101
853
II
6.89
31
468
III
7.95
100
122
IV
7.29
206
68
V
13.12
324
311
VI
12.90
845
414
1985-1994
I
5.49
1862
1093
II
7.40
2350
510
III
8.03
1261
124
IV
8.20
1022
89
V
10.86
1164
240
VI
5.57
1005
122
1 Percent differaice of annual mean chlorophyll a concentration for each region is based upon a
comparison with the corresponding chlorophyll a concentration in 1950-1959.
Source: Harding and Perry 1997.
Mobjack Bay (Region II; Figure V-l), regional chlorophyll a concentrations aver-
aged 1.21 fig liter1 in the 1950s, but increased to 7.40 fig liter1 from 1985 to 1994.
The regional mean chlorophyll a concentration of 0.46 jiig liter Observed in the
1950s increased tenfold through the 1990s to 5.57 fig liter_1in the mainstem Bay
from Mobjack Bay to the mouth of the Bay.
Benchmark Levels Derived from Analysis of the CBP Water-Quality
Database. Evaluating a similar time period of data using different methodologies,
chapter v • Chlorophyll a Criteria
-------
110
Chesapeake Bay >})
Baltimore -A" ^
WasfiinWon DC. Yy
Chester R.
39.0
Choptank R.
Mesohaline,
L. Choptank R. _
Nanfcoke R.
38.5
/a
0>
"O
3 38.0
aj
_j
37.5
i lyhalipe
37.0
Norfolk
Atlantic
Ocean
77.0
76.5
76.0
Longitude
Figure V-1. The Chesapeake Bay showing locations of the six regions chosen
to represent major salinity provinces of the estuary, the principal rivers drain-
ing into the Chesapeake Bay and major metropolitan areas.
Source: Harding and Perry 1997.
Olson (2002) reported a series of benchmark concentrations for chlorophyll a as well
as for nitrogen, phosphorus and total suspended solids. Benchmark concentrations,
derived from a 1985 to 1990 benchmark data set, were applied to the entire 1950s
through late 1990s data set (Table V-4). Tabular summaries of decadal spring,
summer and annual median chlorophyll a concentrations across five decades are
documented in Appendix E, tables E-l and E-2. Table V-5 summarizes the results of
these reviews and evaluations of the extensive historical and recent chlorophyll a
concentration data records.
Strengths and Limitations. Consideration of the historical chlorophyll a
concentrations reflecting a more balanced, mesotrophic Chesapeake Bay ecosystem
must be tempered by a recognition of the limited spatial and temporal coverage of
chapter v • Chlorophyll a Criteria
-------
111
Table V-4. Historical chlorophyll a concentrations (jjg liter1) derived through applying relative
status benchmark data.
Season
Salinity
Zone
Chlorophyll a
Median
Chlorophyll a
Mean
Chlorophyll a
90th Percentile
Number of
Observations
Annual
Tidal-Fresh
3.1
4.2
10.2
972
Oligohaline
4.7
6.0
10.8
910
Mesohaline
7.3
7.2
10.9
4192
Polyhaline
4.4
4.3
7.0
1132
Spring
Tidal-Fresh
3.1
3.7
4.2
488
Oligohaline
5.1
5.9
9.8
279
Mesohaline
6.9
7.2
11.0
708
Polyhaline
3.4
4.1
12.9
91
Summer
Tidal-Fresh
7.3
7.0
8.7
423
Oligohaline
8.0
7.6
10.8
566
Mesohaline
8.4
7.9
11.1
1677
Polyhaline
4.3
3.7
6.0
341
Sources: Olson 2002.
Table V-5. Summary of historical Chesapeake Bay chlorophyll a concentrations (jjg liter1).
Salinity
Regime
Harding and Perry
(1997)-1950s
Chesapeake Bay
Mainstem Annual
Mean
Concentrations
Olson (2002)-1950s
Chesapeake Bay and
Tidal Tributaries
Spring/Summer/
Annual Mean
Concentrations
Olson (2002)-Relative
Status Spring/Summer/
Annual Benchmark
Concentrations
Tidal-fresh
2.51
1.1 / 1.1 / -
3.7/7.0/4.2
Oligohaline
2.51-3.19
2.3/2.0/3.1
5.9/7.6/6.0
Mesohaline
3.58-4.33
3.7/4.4/3.1
7.2/7.2/7.9
Polyhaline
0.46-1.21
3.9/-/3.2
4.1/3.7/4.3
Sources: Harding and Perry 1997; Olson 2002.
the available data for the 1950s and 1960s, as well as the different living resource
communities present in the Bay's tidal habitats more than 50 years ago. The data
limitations of the 1950s and 1960s data are particularly of concern in the lower
portion of the Chesapeake Bay The large reduction in filter-feeder (e.g., oysters,
menhaden) populations has reduced the capacity of the Chesapeake Bay's living
resources to assimilate nutrient loads and to maintain lower chlorophyll a concentra-
tions. Thus, the changes in living resources may have affected chlorophyll a
chapter v • Chlorophyll a Criteria
-------
112
concentrations as much as or more than the reverse. It should be noted that temporal
trends alone do not demonstrate causal relations between chlorophyll a concentra-
tions and specific ecological conditions.
Literature Values Related to Trophic Status
Several influential scientific papers, synthesizing data from many different aquatic
systems, describe conditions that were judged to reflect the trophic status of different
water bodies (e.g., Wetzel 2001; Ryding and Rast 1989; Smith 1998). Chlorophyll a
is the principal parameter quantified in these literature reviews. The information is
drawn from a diversity of systems across the spectrum of healthy (oligotrophic) to
severely stressed (eutrophic) water bodies.
Several papers in the literature synthesize data from many aquatic systems and focus on
conditions that reflect different trophic states of water bodies. R. G. Wetzel's Limnology
presents a table of phytoplankton-related trophic states based on hundreds of studies in
freshwater systems (Wetzel 2001). His text defines eutrophic systems as having the same
four dominant phytoplankton species as those currently found in most of the Chesapeake
Bay system's tidal-fresh or oligohaline habitats and chlorophyll a concentrations greater
than 10 ug liter1. A system is defined as eutrophic when it has: 1) very high productivity
but mostly occurring in the lower trophic levels (e.g., algae, bacteria); 2) a simplified
structure of biological components; and 3) reduced ability to withstand severe stresses
and return to pre-stress conditions. In a eutrophic condition, "excessive inputs commonly
seem to exceed the capacity of the ecosystem to be balanced, but in reality the systems
are out of equilibrium only with respect to the freshwater chemical and biotic character-
istics desired by man for specific purposes" (Wetzel 2001). Mesotrophic freshwater
systems are defined by Wetzel (2001) as having chlorophyll a concentrations in the range
of 2-15 jug liter1 (Table V-6).
Table V-6. Summary of aquatic system trophic status as characterized by mean chlorophyll a
concentrations (jug liter1).
Aquatic
System
Trophic
Status
Wetzel
(2001)
Ryding
and Rast
(1989)
Smith
et al.
(1999)
Molvaer
et al.
(1997)
Novotny
and Olem
(1994)
Fresh-
water
Eutrophic
>10
6.7-31
9-25
-
>10
Mesotrophic
2-15
3-7.4
3.5-9
-
4-10
Oligotrophic
0.3-3
0.8-3.4
<3.5
-
<4
Marine
Eutrophic
-
-
3-5
>7
-
Mesotrophic
-
-
1-3
2-7
-
Oligotrophic
-
-
<1
<2
-
Sources: Molvaer et al. 1997, Novotny and Olem 1994, Ryding and Rast 1989, Smith et al 1999, Wetzel 2001.
chapter v • Chlorophyll a Criteria
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113
Ryding and Rast (1989) also deal with characteristics of eutrophication in lakes,
based on surveys of hundreds of temperate lakes globally In Table 4.2, they give the
following boundary values for mean and peak chlorophyll a values (fig liter"1), as
follows:
Mean range Peak Range
Oligotrophic 0.8-3.4 2.6-7.6
Mesotrophic 3.0-7.4 8.9-29
Eutrophic 6.7-31 16.9-107
The peak range is for occasional blooms, and the mean ranges are those for annual
geometric means, with outliers removed (see Table 4.2 in Ryding and Rast 1989).
The ranges overlap slightly, and in fact the authors recommend using multiple
parameters, including total phosphorus, total nitrogen, chlorophyll a and Secchi
depth to classify the lakes. Using their criteria, much of the Chesapeake Bay would
clearly be classified as 'eutrophic.'
In a review of lake and marine systems, Smith et al. (1999) equated mesotrophic
status in lake systems to mean chlorophyll a concentrations ranging from 3.5 to 9 fig
liter1. A chlorophyll a concentration range of 1 to 3 fig liter1 was equated with
mesotrophic status in marine systems (assumed here to be principally polyhaline in
terms of salinity). Smith et al. (1999) also published values characteristic of hyper-
eutrophic lake (>25 fig liter1) and marine systems (>5 fig liter1).
The Norwegian Environmental Protection Agency has constructed a system for clas-
sifying estuaries and coastal waters with respect to water quality and eutrophication
using five classes of water quality (Molvaer et al. 1997). For salinities above 20 ppt,
chlorophyll a concentrations below 2 fig liter1 are considered Class I or 'very good,'
whereas concentrations above 20 fig liter_1are classified as "very bad" or Class V
waters. Sweden has adopted similar chlorophyll a water quality standards for its
estuarine (1.3 to 2.0 fig liter1) and marine (1.0 to 1.5 //g liter1) waters that reflect the
lower end of these concentration ranges (Sweden Environmental Protection Agency
2002).
Strengths and Limitations. The trophic classifications should be used with
caution since the majority of the scientific literature-based values were developed for
lake, coastal or marine systems, not temperate, partially mixed estuaries such as the
Chesapeake Bay. In particular, marine ecosystems should not be considered directly
comparable to polyhaline estuarine areas. The polyhaline areas of the Chesapeake
Bay are in much closer proximity to land-based freshwater and nutrient inputs.
Therefore, they should be expected to have higher nutrient concentrations and asso-
ciated chlorophyll a concentrations than marine systems.
chapter v • Chlorophyll a Criteria
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114
Trophic classifications are useful, general ecological concepts. However waters clas-
sified strictly by chlorophyll a concentrations may or may not experience all or any
of the ecological conditions characteristic of that category (see Table V-2).
Phytoplankton Growth-Limiting Water Quality Conditions
and Related Chlorophyll a Concentrations
Biological communities found in pristine or minimally affected habitats provide
essential information on how restoration efforts might improve ecosystem structures
and functions. They also serve as references for measuring restoration progress.
Chesapeake Bay water quality and phytoplankton data collected at Chesapeake Bay
Program phytoplankton monitoring stations between 1984 and 2001 were analyzed
to identify reference phytoplankton communities for Chesapeake tidal waters. The
seasonal and salinity-specific reference communities were used to quantify chloro-
phyll a concentrations in the least-impaired water quality conditions currently found
in the Chesapeake Bay.
For the purposes of deriving the reference communities, least-impaired water quality
conditions were defined as the co-occurrence of high light penetration, low dissolved
inorganic nitrogen and low dissolved inorganic phosphorus concentrations. Low
dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (P04)
concentrations are below the threshold concentrations shown to limit phytoplankton
growth in Chesapeake Bay waters (Fisher et al. 1999), whereas high light penetra-
tions are the Secchi depth values identified by the Relative Status, or benchmark,
method as 'good' (Olson 2002). The high light penetration levels are approximately
the same as those necessary for restoring underwater bay grasses (Batiuk et at.
2000). Thresholds for DIN, P04 and Secchi depth for spring and summer across four
salinity zones (tidal-fresh, oligohaline, mesohaline and polyhaline) were applied to
the 1984 through 2001 Chesapeake Bay water quality monitoring database to bin the
data records into six water quality categories. Reference communities were derived
from the least impaired water quality categories found in each season-salinity
regime.
Water quality conditions that met all three reference criteria ('better'/'best') between
1984 and 2001 occurred in 1.6 percent (spring) and 5.8 percent (summer) of the
mesohaline biomonitoring samples, and 21.1 percent (spring) and 10.4 percent
(summer) of the polyhaline water quality monitoring samples, so reference commu-
nities could be characterized directly from the data. Water quality conditions that met
all the reference criteria rarely occurred in tidal-fresh and oligohaline salinities. In
these cases, the 'mixed better light' category (see Appendix F for definition) was
used as a surrogate, since values of most phytoplankton parameters (e.g., chlorophyll
a, biomass, pheophytin and species composition) in this category closely resembled
those in 'better'/'best' in mesohaline and polyhaline waters. For the spring mesoha-
line reference community, 'better'/'best' data were augmented with 'mixed better
light' data to increase the number of data records. Chlorophyll a concentrations
chapter v • Chlorophyll a Criteria
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115
observed in the phytoplankton reference communities are shown in Table V-7. The
water quality binning method and identification of the phytoplankton reference
communities are described in more detail in Appendix F.
It is important to realize that the chlorophyll a concentrations in Table V-7 reflect
phytoplankton reference communities in the absence of robust grazer populations.
There are no undisturbed sites in the Chesapeake Bay with a full complement of
natural grazers. Harvesting and disease have significantly decreased Chesapeake
oyster abundances (Newell 1988). Menhaden populations have declined to approxi-
mately 5 percent of 1970s levels (data from the Maryland Department of Natural
Resources). Comparisons of historic and contemporary populations of mesozoo-
plankton and benthos indicate that declines may also have occurred in these grazers.
Median chlorophyll a concentrations in the reference communities are significantly
lower than those in impaired waters, and algal blooms are absent. Reference commu-
nity chlorophyll a concentrations are slightly higher than historic Chesapeake Bay
concentrations and are typical of mesotrophic conditions. They indicate the chloro-
phyll a concentrations that could be attained in the present-day Chesapeake Bay with
significant nutrient and sediment reductions, in the absence of robust populations of
grazers. If key grazer populations are at least partially restored to historical levels, it
is possible that the phytoplankton reference community chlorophyll a concentrations
will approach 1950s levels (see Table V-3).
Table V-7. Chlorophyll a concentrations in the salinity- and season-based Chesapeake phytoplankton
reference communities (jjg liter1). The median and range (5%-95%) are shown.
Reference community values are derived from samples with the least improved water
quality conditions in the 1984-2001 Chesapeake Bay Program phytoplankton and water
quality monitoring station data.
Salinity Regime
Spring
Summer
Tidal-Fresh
4.3 (1.0- 13.5)
8.6 (3.2 - 15.9)
Oligohaline
9.6 (2.4-24.3)
6.0 (2.5 - 25.2)
Mesohaline
5.6 (2.2 - 24.6)
7.1 (4.4-14.0)
Polyhaline
2.9 (1.1 -6.7)
4.4 (1.7-8.7)
Source: Chesapeake Bay Water Quality and Phytoplakton Monitoring Programs Databases.
http://www.chesapeakebay.net/data
Strengths and Limitations. It is important to realize that these values were
selected from samples subject with least-improved water quality, and they came
from a larger data set obtained from generally nutrient- and sediment-enriched
Chesapeake Bay. Under better water quality conditions (lower annual nutrient load-
chapter v • Chlorophyll a Criteria
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116
ings, more zooplankton grazing and better trophic coupling), these chlorophyll a
values might be even lower than those obtained under low current nutrient loadings
due to the carryover of nutrients from previous high load conditions.
The phytoplankton reference community approach does not demonstrate any direct
relationship between chlorophyll a concentrations and designated use impairments.
However, this method does provide solid insights into how chlorophyll a concentra-
tions will likely respond in estuarine systems as water quality improves, leading to
more nutrient-limited, light saturated conditions.
Chlorophyll a concentrations do not always show a high correlation with algal
biomass because after a bloom, some species of nonchlorophyll-bearing phyto-
plankton can feed on organic material (Livingston 2001).
Research Needs. Further analysis of the Chesapeake Bay monitoring databases
could help determine if nitrogen, phosphorus or suspended sediment reductions or a
combination thereof will be most effective in minimizing the occurrence of harmful
algal blooms.
Chlorophyll a Concentrations Characteristic of
Potentially Harmful Algal Blooms
The scientific literature indicates that certain phytoplankton community taxonomic
groups produce poor quality food and even toxins that impair the animals that feed
on them (Roelke et al. 1999; Roelke 2000). Phytoplankton assemblages can become
dominated by poor quality food taxonomic groups to an extent that the overall food
quality of that phytoplankton assemblage becomes significantly reduced. Chloro-
phyll a concentrations were identified that corresponded to an increased probability
that potentially harmful algal taxa would exceed specific impairment thresholds.
Several of the more than 700 phytoplankton species in the Chesapeake Bay are
known to be harmful to consumers. Approximately 2 percent of these species have
shown evidence of producing toxins (Marshall 1996). Some species, however, form
blooms and can dominate the community at particular locations during specific
times of the year. Some of these species are even capable of producing toxins.
The dinoflagellates, Prorocentrum minimum and Cochlodinium heterolobatum,
which commonly bloom in spring and summer, respectively, in certain mesohaline
areas of the estuary, have been shown to harm various life stages of the Eastern
oyster, Crassostrea virginica (Ho and Zubkoff, 1979; Luckenbach et al. 1993; Wick-
fors and Smolowitz 1995). The dinoflagellate Karlodinium micriim has been
associated with numerous fish kills in the Chesapeake Bay (Goshorn et al. 2003). In
tidal-fresh regions, a colonial cyanophyte, Microcystis aeruginosa, forms surface
blooms that cover the upper reaches of certain Bay tributaries for miles during the
summer. This species has been documented to affect zooplankton communities
under bloom conditions (Lampert 1981; Fulton and Paerl 1988).
chapter v • Chlorophyll a Criteria
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117
The occurrence of harmful algal blooms is a complex, incompletely-understood
phenomenon. Many harmful blooms cannot effectively be predicted or modeled at
this time, and the physical, chemical and biological controls on many such blooms
are not known. Nutrient concentrations or loads are only one of many environmental
parameters that can potentially affect harmful algal blooms. For example, some
harmful blooms may respond more to nutrient ratios than absolute concentrations, or
may be regulated by top-down controls (e.g., grazer dynamics) more than by nutrient
availability. This section represents a valuable compilation of information, focusing
on several Chesapeake Bay species that have been observed to correlate with chloro-
phyll a concentrations. As illustrated below using the four previously cited species,
the likelihood of bloom conditions being produced by some harmful or nuisance
algal species tends to be associated with elevated chlorophyll a levels. Future moni-
toring and research is expected to provide more insight into the practicality and
methodology for managing blooms of these and other species.
Microcystis aeruginosa. A substantial body of literature deals with the negative
effects of toxic cyanobacteria on the feeding, growth, behavior and survival of
micro- and mesozooplankton. Numerous studies have documented the avoidance of
ingestion of toxic and nontoxic strains of Microcystis aeruginosa by specific taxa of
zooplankton (Clarke 1978; Lampert 1981; Gilbert and Bogdan 1984; Fulton and
Paerl 1987, 1988; DeMott and Moxter 1991) while others indicate physiological and
behavioral problems associated with its ingestion (Lampert 1981, 1982; Nizan et al.
1986; Fulton and Paerl 1987; DeMott et al. 1991; Henning et al. 1991).
From laboratory studies, 10,000 cells milliliter1 was determined to be the threshold
above which zooplankton communities can be adversely altered by the poor food
quality, large particle size of the colonies, increased density of particles in the water
column or directly by the toxin (Lampert 1981; Fulton and Paerl 1987; Smith and
Gilbert 1995). (See Appendix G for more detailed descriptions of the determination
of the effects threshold.)
Upon matching the chlorophyll a concentrations to samples containing M. aerugi-
nosa, normalized frequency distribution plots were constructed for M. aeruginosa
bloom frequency and the frequency of both bloom and non-bloom abundances
versus chlorophyll a concentrations (figures V-2 and V-3, respectively). Chlorophyll
a concentrations <15 «g liter1 characterize M. aeruginosa concentrations less
<10,000 cells milliliter1 (Figure V-2). Increasing concentrations of chlorophyll a
above 15 fig liter1 leads to increasing frequencies of bloom samples > 10,000 cells
milliliter1 (Figure V-3).
Colonies of M. aeruginosa vary in their cell counts but colony counts provide an
additional measure of bloom conditions (Figure V-4). The ratio of cells per colony is
approximately 17:1, providing an estimate of 588 colonies containing 10,000 cells
as a translation to threshold levels for zooplankton community impacts.
chapter v • Chlorophyll a Criteria
-------
118
" o
Chlorophyll a Concentration (ug liter1
Figure V-2. Normalized frequency of Microcystis aeruginosa abundances above-threshold
(i.e., >10,000 cells milliliter-1) versus summer tidal fresh chlorophyll a concentration. The
number of above-threshold Microcystis aeruginosa abundances in each chlorophyll a inter-
val is divided by the total number of phytoplankton records in that interval. For summer
tidal fresh, there were 16 above-threshold occurrences in a total of 266 samples.
Source: Chesapeake Bay Phytoplankton Monitoring Program Database
h tt p ://www. chesapeakebay. net/data
70
Microcystis aeruginosa 0-10,000 cells millilter1
60 ~ Microcystis aeruginosa >10,000 cells millilter1
50
& 40 -
30
20
10
JL
0-5 5-10 10-15 15-20 20-25 25-30 30-35
Chlorophyll a Concentration (jug liter1)
35-40
>40
Figure V-3. Normalized frequency of above- and below-threshold Microcystis aeruginosa
abundances versus summer tidal fresh chlorophyll a concentration. The number of above-
and below-threshold Microcystis aeruginosa abundances in each chlorophyll interval is divid-
ed by the total number of phytoplankton records in that interval. For summer tidal fresh,
there were 62 total occurrences of Microcystis aeruginosa in a total of 266 samples. The
increasing trend in total occurrences of Microcystis aeruginosa identify it as an indicator
species of eutrophication.
Source: Chesapeake Bay Phytoplankton Monitoring Program Database
h tt p ://www. chesapeakebay. net/data
chapter v • Chlorophyll a Criteria
-------
119
c
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ll
c =
11
q5 m
.to O
1
o
50000
45000
40000
35000
30000
25000
20000
15000
10000
1000 1500 2000 2500
Microcystis aeruginosa Colony counts
3000
Figure V-4. Relationship of Microcystis aeruginosa colony counts versus cell counts.
Cell counts = 1 6.97 x colony counts; r2=0.66; n=20.
Source: Maryland Department of Natural Resources unpublished data.
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4000
3000
2000
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Lack of Blooms
Bloom region
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20
40
60
80
100
120
A
Chlorophyll a Concentration (|jg liter ')
Figure V-5. Microcystis aeruginosa colony counts versus a gradient of chlorophyll a concentrations illustrat-
ing the boundary between bloom and non-bloom conditions.
Source: Maryland Department of Natural Resources unpublished data.
chapter v • Chlorophyll a Criteria
-------
120
M. aeruginosa counts were made from water samples collected by the Maryland
Department of Natural Resources through a separate water quality monitoring
program from the tidal-fresh and oligohaline waters of Maryland's Chesapeake Bay
Between 1985 and 2000, M. aeruginosa colony counts showed low concentrations
(<588 colonies milliliter1) and low variance between 0-33 fig liter1 chlorophyll a
(Figure V-5). Beyond 33 fig liter1 chlorophyll a, the variance of colony counts
increases significantly and counts exceeding the 588 colonies milliliter1 threshold
increase to 42 percent beyond 33 fig liter1 chlorophyll a, providing a threshold and
probability for potentially harmful blooms of this cyanobacteria with respect to
chlorophyll a measures. The chlorophyll a range of 15-33 fig liter"1 provides a
threshold region between levels that protect against M. aeruginosa blooms versus
conditions with a high likelihood for blooms.
One of the primary locations for M. aeruginosa blooms in the Chesapeake Bay
estuary is the tidal-fresh Potomac River. Extensive blooms of M. aeruginosa were
documented over the period of 1965-1983, before the initiation of the coordinated
Chesapeake Bay monitoring program. During the period of 1965-1974, summer
chlorophyll a concentrations in the vicinity of Indian Head (near monitoring station
TF2.3) were typically above 50 fig liter1 and often exceeded 100 fig liter1 in the
surface layer (Pheiffer 1975). During the same period, cyanobacteria blooms were
extensive in this portion of the river in summer, although there are very few data
reflecting cell densities. Total cyanobacteria densities ranged from 20,000-120,000
cells milliliter1 in the summer of 1971 near Possum Point (Simmons et al. 1974).
In 1983, a massive bloom of M. aeruginosa was documented in this portion of the
Potomac River (mile 12 - mile 46) (Thomann et al. 1985). Chlorophyll a concentra-
tions averaged over 200 fig liter"1 for the Indian Head area in August 1983. Again,
little species composition data is documented for this bloom.
With the initiation of the Chesapeake Bay phytoplankton monitoring program in
August 1984, a steady flow of phytoplankton species composition and chlorophyll a
data was recorded for a station in the tidal-fresh Potomac River near Indian Head
(TF2.3). Figure V-6 summarizes these data for M. aeruginosa during the summer
months of 1985-2002. The data show that the threshold is rarely exceeded during
this period after 1988, but one can assume that during the severe blooms of the 1970s
and early 1980s, this threshold may have been surpassed on a regular basis. The fact
remains that this taxon is an impairment to zooplankton assemblages above a
specific threshold and this threshold density has been surpassed on a number of
occasions in the tidal-fresh Potomac River during the past several decades.
Strengths and Limitations. The strength of this line of evidence for establishing a
chlorophyll a threshold for the tidal-fresh and oligohaline regions of the estuary lies
in the evidence provided in the many laboratory and field studies that indicate
adverse affects on zooplankton populations caused by cyanobacteria in general and,
more specifically, by M. aeruginosa. M. aeruginosa has been found in many of the
tidal-fresh locations sampled as part of the Chesapeake Bay water quality
chapter v • Chlorophyll a Criteria
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121
co
a;
CO
c
CD
Q
CD
CO
O
c
co
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fc .9
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.CO E,
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40
= ^ 30
CD i_
O 0
20
10
II - III
I
Jtu
Liu
JL
198519861987198819891990199119921993199419951996199719981999200020012002
Year
JUNE
~
JULY
AUGUST
~
SEPT
Figure V-6. Mean summer Microcystis aeruginosa cell densities from 1985-2002 from
the surface mixed layer of the Potomac River tidal fresh phytoplankton monitoring
station TF2.3.
Source: Chesapeake Bay Phytoplankton Monitoring Program Database
h tt p ://www. chesapeakebay. net/data
monitoring program, implying that this 'indicator' species is ubiquitous to this
particular tidal-fresh habitat during the summer under certain hydrodynamic con-
ditions and with a given set of nutrient requirements.
Numerous field studies have documented changes in zooplankton community struc-
ture associated with blooms of cyanobacteria in general (Infante and Riehl 1984;
Orcutt and Pace 1984; Threlkeld 1986; Burns et al. 1989; Gilbert 1990; Fulton and
Jones 1991). These studies most frequently cite the inability of many zooplankton
taxa in using cyanobacteria as a nutritive food source. Therefore, it can reasonably
be stated that high chlorophyll a concentrations in tidal-fresh and oligohaline regions
of the Chesapeake Bay estuary in summer often are associated with high densities of
cyanobacteria, which can adversely alter the zooplankton community structure in
these areas.
Colony counts have a lower variance than, and a positive relationship to, M. aerug-
inosa cell counts, providing a robust indicator to describe bloom conditions. Both
data sets in these analyses independently define a relatively narrow range of condi-
tions that separate the bloom from non-bloom regions of the chlorophyll a gradient
based on threshhold level effects on living resources of 10,000 cells milliliter1.
chapter v • Chlorophyll a Criteria
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122
The threshold value for the cell density that affects zooplankton populations was
derived from two laboratory studies citing impairment thresholds at very different
cell densities (see Appendix G). A third study has been identified that documented
negative effects on zooplankton at M. aeruginosa cell densities of 50,000 cells milli-
liter1, which is an intermediate value compared to the two previously cited studies
(Smith and Gilbert 1995).
Some of the detrimental effect of M. aeruginosa on zooplankton assemblages is
related to the toxin content of a particular strain of this cyanobacterium (one reason
that the threshold density of the two laboratory studies is so different). The toxin
content of the strains of M. aeruginosa found in the Chesapeake Bay has not been
determined, which forced the extrapolation of the threshold for this document to be
chosen as a midpoint between the thresholds of the two laboratory studies.
Colony counts are not interchangeable with cell counts, since the variance increases
as the counts increase. The risks have been stated based on a threshold for
zooplankton effects using an abundance of cells, while the risks to toxin production
or toxic effects are less well understood in relation to cell or colony concentrations.
Research Needs. Two obvious research studies would strengthen this line of
evidence. The first would be to assess the toxin content in the populations of M.
aeruginosa found in various tidal-fresh regions of the Chesapeake Bay. The second
would be to use some strains of the cyanobacterium in specific laboratory experi-
ments that studied effects on zooplankton feeding, reproduction and survival at
specific cell densities and associated chlorophyll a concentrations.
The estimate of colony counts as a threshold can be refined using the conversion
with cell counts through results of the Chesapeake Bay phytoplankton monitoring
program. Additional work is needed to correlate the concentration data with levels
associated with detrimental levels of microcystin toxins in the ecosystem. Spatial
and temporal resolution of M. aeruginosa levels in relation to cell and colony
concentration would provide valuable information for any reassessment of the
density driven thresholds being proposed.
Prorocentrum minimum. P. minimum effects may be a function of bloom
density or toxicity. In Japan in 1942, P. minimum was attributed as the cause of a
shellfish poisoning in Japan in which 114 people died (Nagazima 1965, 1968). P.
minimum isolated from a 1998 bloom in the Choptank River and subsequently grown
in the laboratory was found to be toxic to scallops (G. H. Wickfors, personal commu-
nication). Blooms of P. minimum in the source intake waters to Virginia and
Maryland oyster hatcheries were suspected to have caused oyster larvae mortality at
the two hatcheries in 1998 (Mark Luckenbach and Don Merritt, personal communi-
cation). There has been no documented case of shellfish toxicity or mortality as a
result of the 1998 P. minimum bloom in the Chesapeake Bay, but clearly the poten-
tial exists for toxic repercussions to shellfish and other organisms as a result of this
bloom.
chapter v • Chlorophyll a Criteria
-------
123
The P. minimum density of 3,000 cells milliliter1 was chosen as a threshold for the
chlorophyll a criteria analysis based on laboratory analyses (Wickfors and
Smolowitz 1995; Luckenbach et al. 1993; see Appendix G). When the threshold is
applied to Chesapeake Bay phytoplankton monitoring program data, the normalized
frequency distribution of chlorophyll a concentrations associated with bloom densi-
ties (>3,000 cells milliliter1) illustrates that concentrations > 5 fig liter1 can generate
densities that may impair the survival of various life stages of oysters (Figure V-7).
The likelihood of bloom level events tends to increase with increasing chlorophyll a
concentrations (Figure V-8).
When the threshold is applied to Chesapeake Bay phytoplankton monitoring
program data, the normalized frequency distribution of chlorophyll a concentrations
associated with the P. minimum bloom densities (greater than 3,000 milliliter1) indi-
cates a large increase at chlorophyll a concentrations of 25 to 30 fig liter1 (Figure
V-9). More than 19 percent of samples containing P. minimum in mesohaline waters
in spring are characterized by densities that exceed the threshold whereby oyster life
stages are impaired and fall within the chlorophyll a range of 25 to 30 fig liter1. In
addition, more than 70 percent of the above-threshold data for/! minimum occur at
chlorophyll a concentrations greater than 25 fig liter1 (Figure V-10). These normal-
ized frequency distributions thus indicate that chlorophyll a concentrations of greater
than 25 fig liter1 in spring in mesohaline waters often are associated with densities
of P. minimum that may impair the survival of various life stages of oysters.
In an analysis of a separate Maryland Department of Natural Resources database
from 1985-2000, a probability analysis illustrated that no blooms of P. minimum
occurred at or below chlorophyll a concentrations of 4 fig liter1 (Figure V-l 1). This
analysis of an independent data set complements the previously described Chesa-
peake Bay Phytoplankton Monitoring Program database analysis confirming the low
target chlorophyl a concentration needed to eliminate conditions for blooms of P.
minimum in the mesohaline Chesapeake Bay. Figure V-ll shows that as the chloro-
phyll a concentration increases, the probability of detecting a P minimum bloom
level above the 3,000 cells milliliter1 threshold in a sample increases in a non-linear
fashion. The possibility increases rapidly at first above 4 fig liter1 and then slows as
the maximum potential detection of 11 percent of samples is reached at high chloro-
phyll a concentrations. Maximum bloom probability was 11 percent in the spring,
or 1 in every 9 samples when conditions are optimal. Protecting against the condi-
tions for 50 percent of maximum bloom potential occurred at approximately 25-30
fig liter1 (Figure V-ll).
Currently, the impairment thresholds are usually reached in spring in mesohaline
waters, but P. minimum commonly occurs in both spring and summer in oligohaline,
mesohaline and polyhaline habitats.
Strengths and Limitations. P. minimum blooms occur in many mesohaline portions
of the estuary. The appearance of the major bloom events in these areas occur on
regular seasonal basis. Therefore this would be a useful indicator species to monitor.
chapter v • Chlorophyll a Criteria
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124
Chlorophyll a Concentration gug liter
Figure V-7. Normalized frequency of Microcystis aeruginosa abundances above-
threshold (i.e., >3,000 cells milliliter1) versus spring mesohaline Chesapeake Bay and
tidal tributary chlorophyll a concentration. The number of above-threshold Prorocentrum
minimum abundances in each chlorophyll a interval is divided by the total number of
phytoplankton records in that interval. For spring mesohaline stations, there were
35 above-threshold occurrences out of a total of 648 sampling records.
Source: Chesapeake Bay Phytoplankton Monitoring Program Database
http://www.chesapeakebay.net/data
100
£ 70 --
60
a) 50
40 --
O 30
20
10
Mesohaline
Oligohaline
Combined CFD
Chlorophyll a Concentration (jjg liter"
Figure V-8. All occurrences of Prorocentrum minimum abundances above threshold
versus combined spring and summer, mesohaline and oligohaline Chesapeake Bay and
tidal tributary chlorophyll a concentration. The number of above threshold Prorocentrum
minimum densities in each chlorophyll a interval is divided by the total number of above-
threshold densities (n=44).
Source: Chesapeake Bay Phytoplankton Monitoring Program Database
http://www.chesapeakebay.net/data
chapter v • Chlorophyll a Criteria
-------
125
Prorocentrum minimum Versus Chlorophyll a
TO
3
C
o
c
CD
O
c
o
o
o
100
75
50
^ 25
jr
Q.
O
0
1000 1200 1300 1400 1700 1900 2500 3300 4200 5200 790013000
Prorocentrum minimum cell densities
(cell milliliter1)
Figure V-9. Prorocentrum minimum cell densities and associated chlorophyll a
concentrations in the Chesapeake Bay, 1985-2000, Cell density threshold associated
with impacts on the oyster community is indicated by the vertical black line at
3,000 cells milliliter1.
Source: Chesapeake Bay Phytoplankton Monitoring Program Database
http://www,chesapea kebay.net/data
O
D)
3
c
o
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-------
126
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2 8
^ 0
E 3
o
o
DQ
12
10
8
6
4
2
0
~ Bloom Prob
-------
127
Bay. Defining density relationships to light field requirements is likely to be a fertile
area of analysis with this species, since its distribution coincides with spring growth
of underwater bay grass beds in the mesohaline portion of the Chesapeake Bay
Additional studies also are needed to determine if adverse effects of P. minimum
occur in mixed algal diets. Finally research is needed to determine effective manage-
ment strategies for P. minimum. This will require a better understanding of the
physical, biological and chemical controls on blooms of this taxon.
Cochlodinium heterolobatum. This species forms intense blooms in warm
months at the mouth of the York River and in the lower Chesapeake Bay
(Mackiernan 1968; Zubkoff and Warriner 1975; Zubkoff et al. 1979; Marshall 1995).
The bloom appears to begin at the mouth of the York River and is dispersed into the
lower Chesapeake Bay from this point of origin and has been documented to affect
-215 km2 in this part of the estuary (Marshall 1995). In this bloom area, cell densi-
ties were generally >1,000 cells milliliter1. Laboratory studies indicated a threshold
concentration of ~ 500 cells milliliter1 resulted in mortality of oyster larvae (Ho and
Zubkoff 1979). Further analysis of these data published by Zubkoff et al. (1979)
yielded a chlorophyll a concentration of approximately 50 «g liter1 at the threshold
concentration of 500 cells milliliter1.
Karlodinium micrum. K. micnim, synonymous with Gyrodinium galatheanum
Braarud and Gymnodinium micnim, and historically reported as Gyrodinium estiiar-
iale in Maryland, is a common and widespread estuarine dinoflagellate in the
Chesapeake Bay. Recent work by Deeds et al. (2002) has demonstrated that Mary-
land isolates of the dinoflagellate produced toxins with hemolytic, cytotoxic and
ichthyotoxic properties. Initial studies indicate K. micnim may produce sufficient
toxin to result in fish mortality in the field at cell densities of 10,000 to 30,000 cells
milliliter1 and above (Deeds et al. 2002; Goshorn et al. 2003).
K. micnim is present year-round in the water column of the Chesapeake Bay. Peak
monthly average abundances occur between April and September, favoring mesoha-
line salinities and elevated concentrations showing a preferred temperature of
21.5-27.5°C (Goshorn et al. 2003). Between 1985 and 2002, there were 1,312
samples from approximately 7,000 collected from Maryland's Chesapeake Bay that
contained K. micnim. Mean density of the cell counts when present was 589 cells
milliliter1, with nine samples (0.7 percent) exceeding the potential lethal threshold
of 10,000 cells milliliter1 (Goshorn et al. 2003).
A historical review of a fish kill database maintained by the Maryland Department
of the Environment showed eight events where kills were associated with the pres-
ence of potential acutely lethal concentrations of K. micnim (Goshorn et al. 2003).
Cell concentrations in these near-shore creek environments not sampled in routine
monitoring provided a range in concentrations from 10,270 to 322,968 cells
milliliter1. Deeds et al (2002) however, also report on fish kills in aquaculture ponds
on the lower Eastern Shore of Maryland that implicate K. micnim in fish kill events
chapter v • Chlorophyll a Criteria
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128
with densities > 10,000 cells milliliter1. Kempton et al. (2002) related^. micrum to
a South Carolina fish kill in a brackish water retention pond with evidence of toxi-
city and concentrations of 64,000-68,000 cells milliliter1. Nielsen (1993) showed
that juvenile cod exposed to 100,000 cells milliliter1 of K. micrum resulted in death
within 2 days.
A subset of the K. micrum (n=684) database had chlorophyll (/-associated data. K.
micrum was more likely to exceed 2,000 cells milliliter"1 when chlorophyll a concen-
trations exceeded 10 fig liter1 in open-water habitat (Figure V-12). One count
exceeded the 10,000 cells milliliter1 boundary and the associated chlorophyll a was
75 fig liter1. Kempton et al. (2002) found chlorophyll a concentrations of 117 fig
liter1 in association with acutely lethal concentrations (64,000-68,000 cells
milliliter1) of K. micrum at the South Carolina fish kill site. Variance in K. micrum
cell counts increases with increasing chlorophyll a measures suggesting the risk of
acutely toxic levels coincidentally increasing with the rise in chlorophyll a out to
75 fig liter"1. However, the present Maryland data set does not presently demonstrate
a clear threshold level for chlorophyll a with acutely toxic boundary conditions of
K. micrum densities.
Strengths and Limitations. K. micrum represents an abundant, relatively easy to
identify potential harmful algal bloom species in the Chesapeake Bay. Maryland
isolates from fish kill events have generated toxicity at many levels from cytotoxi-
city to hepatotoxicity and ichythyotoxicity. Lab results demonstrated acutely lethal
levels of K. micrum. The aquatic life impairment associated with fish kills is clear.
Sublethal effects are essentially unknown. Concentration alone does not imply toxi-
city but co-occurring conditions that induce disintegration of the cells may be needed
12000
D
O
O
S
2 _
.o :=
£ E
£ CO
¦a ®
,C o
¦5 "
0
1
10000 -
® 8000 -
6000 -
4000 -
2000 -
150
200
Chlorophyll a (|jg liter)
Figure V-1 2. Karlodinium micrum cell counts versus chlorophyll a concentrations in the
Maryland portions of the Chesapeake Bay. A total of 684 samples are illustrated.
Source: Maryland Department of Natural Resources unpublished data.
chapter v • Chlorophyll a Criteria
-------
129
in order for the toxins to be released (Deeds et al. 2002). The habitats where fish kills
have been most commonly associated with potentially lethal densities of
K. micrum are shallow-water and near-shore habitats, and small tributary systems
(Goshorn et al. 2003), aquaculture facilities (Deeds et al. 2002) and brackish reten-
tion ponds (Kempton et al. 2002). To date, these habitats are not typical of areas
routinely sampled by water quality monitoring programs of the Chesapeake Bay.
Thus far, while the risk of acutely lethal concentrations increases with increasing
chlorophyll a, only two instances are noted with chlorophyll a data at 75 fig liter1
(this chapter), 117 fig liter1 (Kempton et al. 2002) and > 10,000 cells milliliter1.
Although the probability of elevated densities is higher when chlorophyll a exceeds
10 fig liter1, above this concentration there is no strong correlation between cell
density and chlorophyll a concentration.
Research Needs. More detailed knowledge of the relationship between densities
above the acute threshold boundary and chlorophyll a levels is needed from near-
shore monitoring and fish kill responses to refine the critical range of chlorophyll a
levels that we should avoid in managing for reducing levels of harmful algal blooms
in the Chesapeake Bay. The sublethal effects of K. micrum on the environment is in
an obvious area for further study. Understanding toxin concentration relationships of
K. micrum under field conditions that result in cell disintegration enhancing the like-
lihood of toxin interaction with living resources also needs additional research. And
it is necessary to better understand the physical, chemical and biological processes
that control K. micrum blooms in order to develop even more effective management
strategies.
CHLOROPHYLLS CONCENTRATIONS CHARACTERISTIC
OF TROPHIC-BASED CONDITIONS
Table V-8 categorizes, by trophic status, chlorophyll a concentrations that charac-
terize desired (oligotrophic and mesotrophic) and stressed (eutrophic) ecological
conditions in Chesapeake Bay open-water tidal habitats. These concentrations were
drawn from scientific literature values related to trophic status, historically observed
concentrations in the Chesapeake Bay and those characteristic of reference phyto-
plankton communities versus potentially harmful algal blooms.
Chlorophyll a concentrations characteristic of oligohaline conditions published by
Ryding and Rast (1989), Wetzel (2001), Smith et al. (1999), Molvaer et al. (1997)
and Novotny and Olem (1994) are listed first in Table V-8 in each salinity-regime
specific row under the heading 'oligohaline conditions.' Seasonal mean chlorophyll
a concentrations derived from Olson's analysis (2002) of the 1950s Chesapeake Bay
mainstem chlorophyll a conditions using the same historical data set as Harding and
Perry (1997) are listed next in each 'oligohaline conditions' salinity-regime specific
row.
Mesotrophic conditions expressed as ranges of chlorophyll a concentrations charac-
terized in the scientific literature by several authors (Ryding and Rast 1989; Wetzel
chapter v • Chlorophyll a Criteria
-------
130
Table V-8. Summary of chlorophyll a concentrations reflecting trophic-based water quality,
phytoplankton community and ecological conditions.
Salinity
Regime
Chlorophyll a Concentrations (jug liter1
Oligotrophic Conditions
Mesotrophic
Conditions
Eutrophic
Conditions
Average or
General
Range
Peak Range
Average or
General
Range
Peak Range
Average or
General
Range
Peak Range
Spring (March - May)
Tidal-Fresh
0.8 - 3.4a
0.3 - 3b
<3.5°
<4®
1.1f
2.6 - 7.6a
3.0 - 7.4a
2 - 1 5b
3.5 - 9C
4-10®
4.3g
15'
8.9 - 29a
13.5g
6.7 - 31a
10-500b
9-25°
>10®
6.7h
16.9 - 107a
42.9h
<33'
Oligohaline
2.3f
9.6g
15'
24.3g
5.0h
29.8h
<33'
Mesohaline
3.7f
5.6g
5j
24.6g
11.1h
44.9h
<25-30j
Polyhaline
<1°
<2d
3.9f
1-3°
2 - 7d
2.9g
5j
6.7g
3-5°
>7d
9.1h
18.0h
<25-30j
Summer (July - September)
Tidal-Fresh
0.8 - 3.4a
0.3 - 3b
<4®
1.1f
2.6 - 7.6a
3.0 - 7.4a
2 - 1 5b
3.5 - 9C
4-10®
8.6g
8.9 - 29a
15.9g
1 5'
6.7 - 31a
10-500b
>10®
25.3h
16.9 - 1 07a
62.1h
33'
Oligohaline
2.0f
6.0g
25.2g
1 5'
17.1h
60.5h
33'
Mesohaline
4.4f
7.1 g
5j
14g
12.2h
52.5h
<25-30j
Polyhaline
<1°
<2d
1 - 3C
2 - 7d
4.4g
5j
8.7g
3-5°
>7dc
6.1h
25.8h
<25-30j
aRyding and Rast, 1989; bWetzel, 2001; cSmith, 1998; dMolvaer et al., 1997; eNovotny and Olem, 1994; f01son 2002;
gTable V-7 this chapter; hAppendix F, Figure F-3 this volume; Microcystis aeruginosa section this chapter;
JProrocentrum minimum section this chapter.
chapter v • Chlorophyll a Criteria
-------
131
2001; Smith et al. 1999 and Novotny and Olem 1994) are listed first in each salinity-
regime specific row first in Table V-8 under the 'mesotrophic conditions' column
heading. The trophic status data shows a narrow range of chlorophyll a concentra-
tions that characterize mesotrophic aquatic ecosystems (Table V-8). For freshwater
areas, seasonal average chlorophyll a concentrations in mesotrophic systems should
fall in the range of 2 to 15 fig liter1 with a mean around 7 fig liter1. In high-salinity
marine ecosystems, mesotrophic status is characterized by seasonal average chloro-
phyll a concentrations from 1 to 7 fig liter1 with a mean around 3 fig liter1
The paired general and peak values that follow are the median and 95th percentile
concentrations of chlorophyll a in waters supportive of the phytoplankton reference
community. These chlorophyll a concentrations reflect conditions in which water
clarity is sufficient for healthy algae and bay grasses growth and the concentrations
of one or both of the critical nutrients are low enough to limit excess algal growth
(e.g., 'best,' 'better' and sometimes the 'mixed better light' categories). The range of
chlorophyll a concentrations that follow in the mesotrophic conditions' peak range
column are those characteristic of algal communities not containing cell densities of
Microcystis aeruginosa and Prorocentrum minimum exceeding thresholds above
which adversely impact zooplankton and oyster communities, respectively.
The spring and summer chlorophyll a concentrations characterizing each of these
salinity-based phytoplankton reference communities provide the most direct water
quality measures of a more balanced phytoplankton assemblage (see Table V-7).
Chlorophyll a concentrations characteristic of the phytoplankton reference commu-
nities, which straddle the boundary between mesotrophic and eutrophic (Table V-8)
conditions, are higher than those observed in the 1950s (see Table V-5) which reflect
oligotrophic conditions.
Ryding and Rast (1989); Wetzel (2001); Smith et al. (1999); Molvaer et al. (1997)
and Novotny and Olem (1994) have all published ranges of chlorophyll a concentra-
tions characterizing eutrophic conditions listed first in Table V-8 under the 'eutrophic
conditions' in each salinity regime specific row. The paired general and peak range
values listed next in each row are the median and 95th percentile concentrations,
respectively, of chlorophyll a in waters categorized as 'poor' during the process for
characterizing the reference phytoplankton communities (Appendix F, Figure F-l).
These chlorophyll a concentrations reflect water quality conditions in which both
critical nutrients (nitrogen and phosphorus) exceed the empirically determined
growth-limiting thresholds for algae, and water clarity is not sufficient for healthy
algae or underwater bay grasses growth. The range of chlorophyll a concentrations
that follow in the eutrophic conditions' peak range column are those characteristic of
harmful algal blooms exceeding cell density thresholds derived from literature-based
values for M. aeruginosa and P. minimum.
Trends in chlorophyll a concentrations observed over the past fifty years indicate
that water quality in many tidal habitats of the Chesapeake Bay has changed from
oligotrophic-mesotrophic to eutrophic and even highly eutrophic. Chlorophyll a
chapter v • Chlorophyll a Criteria
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132
concentrations in the highly saline waters at the mouth of the Chesapeake Bay were
characteristic of oligotrophic marine conditions in the 1950s (<2 fig liter1). They
now reflect mesotrophic conditions, with a mean concentration of 5.6 fig liter1 and
maxima exceeding 18 fig liter"1. Chlorophyll a concentrations in the middle and
upper Chesapeake Bay mainstem were indicative of mesotrophic conditions during
the 1950s, with mean concentrations well below 7 fig liter1. They now reflect
eutrophic conditions, with mean chlorophyll a concentrations above 7 fig liter1 in
mid-Bay waters and above 10 fig liter1 in the tidal-fresh, upper Chesapeake Bay
waters. Peak concentrations often exceed 30 fig liter1.
Eutrophic conditions also characterize all the major Bay tidal tributaries. Smaller,
urbanized watershed tidal tributaries with poor flushing, such as the Back River,
experience highly eutrophic conditions. Excessive nutrient and sediment loadings
are the cause of the shift towards eutrophic conditions in the Chesapeake Bay's tidal
waters. The results are more deep-water habitats prone to anoxia, further losses of
underwater bay grasses and more extensive harmful algal blooms.
Decisions on what chlorophyll a value should be applied to protect a designated use
against a specific impairment should be made at local or regional water-body scales.
More specific implementation procedures and guidelines are provided in Chapter VI.
CHLOROPHYLLS CONCENTRATIONS PROTECTIVE
AGAINST WATER QUALITY IMPAIRMENTS
Contributions to Reduced Light Levels
Phytoplankton attenuate or reduce the amount of light reaching the leaves of bay
grasses by absorbing or scattering the light (see Chapter IV). Additional reductions
in light occur at the leaf surface, as the remaining light must pass through algal
epiphytes and suspended solids settled there (see Appendix J). Chesapeake Bay
scientists have developed a diagnostic tool to calculate the relative contributions of
chlorophyll a versus total suspended solids to reducing light penetration through the
water column (Batiuk et al. 2000; Gallegos 2001).
Water-Column Diagnostic Tool. Water-column attenuation of light measured by
the light attenuation coefficient Kd can be divided into contributions from four
sources: water, dissolved organic matter (color), chlorophyll a and total suspended
solids. The basic relationships can be expressed in a series of simple equations,
which were combined to produce the equation for the percent water-column diag-
nostic tool (Gallegos 2001). The resulting equation calculates linear combinations of
chlorophyll a and total suspended solids concentrations that just meet the percent
light-through-water (PLW) criteria for a particular water-column depth at any site or
season in the Chesapeake Bay and its tidal tributaries. This diagnostic tool can also
be used to consider management options for improving water quality conditions
when the water clarity criteria are not currently met (see Chapter VII).
chapter v • Chlorophyll a Criteria
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133
Derived Chlorophyll a Concentrations. A finite yet significant number of
possible chlorophyll a concentrations exist that support attainment of the percent
light-through-water criteria, depending on the ambient total suspended solids
concentration and water-column application depth. For the purpose of deriving
chlorophyll a criteria applicable across a wide array of tidal habitats, total suspended
solids concentrations were assumed to range from 5 to 20 mg liter-1 (Table V-9). The
Table V-9. Chlorophyll a concentrations (jjg liter1) that reflect attainment of
the Chesapeake Bay water clarity criteria given a range of total
suspended solids concentrations and shallow-water application
depths. Areas in gray indicate exceedance of the water clarity criteria.
Total
Tidal-Fresh and Oligohaline
Mesohaline and Polyhaline
Suspended
Solids
Water-Column Depth (meters)
(mg liter"1)
0.5 m
1 m
2 m
0.5 m
1 m
2 m
5
199
71
9
122
34
10
171
43
95
8
15
144
16
68
20
116
42
water-column application depths were set at 0.5, 1 and 2 meters to reflect the range
of shallow-water designated use boundary depths (U.S. EPA 2003).
Chlorophyll a concentrations of 16 fig liter'(tidal-fresh and oligohaline) and 8 fig
liter1 (mesohaline and polyhaline) were identified as protective against negative
water clarity effects. Values were selected as they corresponded with total suspended
solids concentrations in the range of 10-15 mg liter1, which were previously identi-
fied as habitat requirements for underwater bay grasses (Batiuk et al. 1992;
Dennison et al. 1993; Stevenson et al. 1993) and the 1-meter shallow-water applica-
tion depth (mid-depth between 0.5 and 2 meters; U.S. EPA 2003).
Strengths and Limitations. The assignment of water clarity criteria application
depths and the selection of appropriate total suspended solids ambient concentration
assumptions should be made on a Chesapeake Bay Program segment by segment
basis. These values will vary on temporal and spatial scales. In some regions, chloro-
phyll a!algal biomass is a negligible component of the total light attenuation,
compared with non-algal solids. In such regions, chlorophyll a reductions would not
be expected to significantly improve water clarity.
Contribution to Low Dissolved Oxygen Conditions
Algae that are not consumed by zooplankton, oysters and fish becomes fuel, through
its breakdown by the microbial community, for reducing dissolved oxygen levels.
Seasonal chlorophyll a concentrations (e.g., algal biomass) that lead to desired
chapter v • Chlorophyll a Criteria
-------
134
dissolved oxygen conditions can be estimated using the Chesapeake Bay water
quality model.
The Chesapeake Bay watershed model and the 13,000-cell version of the Chesa-
peake Bay water quality model can be used to determine the seasonal average
chlorophyll a concentrations associated with estimated nutrient and sediment reduc-
tions needed to attain the Chesapeake Bay dissolved oxygen criteria.
The model-simulated chlorophyll a levels were extracted from the nutrient and sedi-
ment loading reduction allocation scenario which attained the Chesapeake Bay
dissolved oxygen criteria across all designated uses and tidal waters. The simulated
chlorophyll a concentrations were compiled for spring (March-May) and summer
(July-September) by salinity regime—tidal-fresh, oligohaline, mesohaline and poly-
haline. The seasonal mean chlorophyll a concentration for each season and salinity
regime combination was then calculated (Table V-10). See Chapter VI for details on
how the Chesapeake Bay water quality model and Chesapeake Bay water quality
monitoring results have been integrated for assessing criteria attainment under
various management scenarios in support of setting loading allocations.
Strengths and Limitations. Like the water clarity criteria, the chlorophyll a
concentrations that are needed to attain the dissolved oxygen criteria are expected to
vary over temporal and spatial scales. Table V-10 shows the general relationship
between chlorophyll a concentrations and attainment of the dissolved oxygen
criteria. Depending on their location in the Chesapeake Bay system and hydrologic
and hydrodynamic factors, individual segments or tributaries may exceed these
concentrations without experiencing dissolved oxygen-related impairments.
Table V-10. Model-simulated seasonal mean and salinity regime-specific chlorophyll a
concentrations (jjg liter1) estimated to characterize conditions supporting
attainment of the Chesapeake Bay dissolved oxygen criteria.
Season
Tidal-Fresh
Oligohaline
Mesohaline
Polyhaline
Spring
4
5
6
5
Summer
12
7
5
4
METHODOLOGIES FOR DERIVING WATERBODY-SPECIFIC
CHLOROPHYLLS CRITERIA
Water Clarity Impairment-Based Methodology
Regional and segment-specific chlorophyll a criteria can be derived to protect
against water clarity impairments by applying the water-column diagnostic tool
described previously. When applied to local and regional tidal waters, more site-
specific assumptions about existing or anticipated ambient total suspended solids
chapter v • Chlorophyll a Criteria
-------
135
concentrations and the shallow-water bay grasses designated use boundary depths
can be factored into the derivation of the chlorophyll a criteria.
Dissolved Oxygen Impairment-Based Methodology
Region-specific chlorophyll a concentrations can be derived by applying the Chesa-
peake Bay water quality model and analyzing the segment-specific results.
Confidence in the derived chlorophyll a criteria can be increased by focusing on
those Chesapeake Bay Program segments that are the principal contributors to low
dissolved oxygen conditions due to an excess production of unconsumed algae.
Nuisance Bloom-Based Methodology
Regional and segment-specific chlorophyll a targets can be derived using studies—
either user perception surveys or algal condition assessments—to identify
chlorophyll a concentrations that protect against nuisance blooms.
User Perception Surveys. User perception surveys can be conducted to rate a
user's satisfaction with a water body's color, clarity and overall appearance. Surveys
have been successfully applied in lake settings by several states, including Vermont
and Minnesota. User perception surveys require careful design and their form
depends on the type of water body and its uses. All such studies should include
certain elements:
1. Surveys should be conducted in conjunction with water quality and phyto-
plankton monitoring to allow correlation of user perceptions with ambient
conditions.
2. Commercial and recreational users should be targeted for the survey.
3. Questions should be worded to avoid bias.
4. Questions should focus on present, specific conditions rather than on general
perceptions of the Chesapeake Bay's water quality.
5. Surveys should be conducted under a variety of water quality and sky condi-
tions, and under a range of chlorophyll a and clarity conditions.
6. Surveys should be conducted in conjunction with objective, scientific assess-
ments of algal conditions in the water body, as described below.
Vermont and Minnesota used lake user surveys to identify specific total phosphorus,
chlorophyll a or Secchi disk values at which algal nuisances and impairment of
recreation were perceived by the public (Heiskary and Walker 1988; Smeltzer and
Heiskary 1990; North American Lake Management Society 1992). Using the results
of a survey on physical appearance and recreation potential, Smeltzer and Heiskary
(1990) defined the statistical relationships between eutrophication-related water
chapter v • Chlorophyll a Criteria
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136
quality variables (Secchi and chlorophyll a) and user perceptions of lake quality in
Minnesota and Vermont.
In Minnesota, surveyors calibrated user response by determining Secchi depth and
chlorophyll a levels that correspond to perceived nuisance conditions or impairment
of water uses. A nonparametric procedure was used to cross-tabulate the water
quality measurements against the user categories. Results showed a distinct contrast
between observations of 'definite algae' and 'high algae' for chlorophyll a measure-
ments. Also, 'impaired swimming' and 'no swimming' ratings generally had
chlorophyll a levels exceeding 20-40 fig liter1 (Heiskary and Walker 1988). In
Minnesota some distinct ecoregional patterns in user perception emerged, whereby
expectations were much greater in the deeper lakes of the northern forested region
(similar to Vermont) as compared to the shallow prairie lakes of southern Minnesota.
The final steps necessary for setting chlorophyll a criteria include specifying the
nuisance criterion (e.g., extreme chlorophyll a >30 fig liter1) or recreation potential
and the acceptable risk level (i.e., probability that nuisance condition will be encoun-
tered 1 percent). Although Minnesota has not yet adopted total phosphorus or
chlorophyll a criteria into water quality standards, the state used the methodology as
a basis for setting lake management goals for total phosphorus. The various chloro-
phyll a and Secchi depth thresholds can be related to total phosphorus based on
empirical relationships (e.g., total phosphorus and frequency of various levels of
chlorophyll a) as noted in Heiskary and Walker (1988).
Algal Condition Assessments. Algal condition assessments involve qualitative
descriptions and ordinal ratings of algal conditions by monitoring personnel. These
constitute the 'scientific' version of the user perception survey. Qualitative informa-
tion to be recorded includes the presence or absence of floating algae, its color, odor,
etc. As with user perception surveys, algal condition assessment should be
performed in conjunction with water quality and phytoplankton monitoring. It is
highly recommended that states develop and apply standard indices for use with
algal condition assessments. For example, Table V-ll provides an example devel-
oped for coastal waters in Oregon.
Algal condition assessments should be conducted by trained scientists or techni-
cians. The more highly trained the personnel, the more detailed information can be
collected on the size, texture and density of blooms. States that decide to pursue this
approach should consider adding algal assessments to their existing Chesapeake Bay
and tidal tributary monitoring programs. Ideally, user perception surveys and algal
condition assessments would be conducted in tandem. However, algal condition
assessments will have some utility for setting chlorophyll a targets independent of
user perception surveys. It is expected that surveys and assessments would result in
different chlorophyll a targets for different salinity regimes. For example, bright-
green algae that form surface scums (e.g., M. aeruginosa) in some tidal freshwater
segments might be more perceptible at lower chlorophyll a concentrations than
brownish, more dispersed blooms.
chapter v • Chlorophyll a Criteria
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137
Table V-11. Example of an algal condition index.
Algal Index
Value
Category
Description
0
Clear
Conditions vary from no algae to small populations
visible to the naked eye.
1
Present
Some algae visible to the naked eye but present at
low to medium levels.
2
Visible
Algae sufficiently concentrated that filaments
or balls of algae are visible to the naked eye.
May be scattered streaks of algae on water surface.
3
Scattered
Surface
Blooms
Surface mats of algae scattered. May be more
abundant in localized areas if winds are calm. Some
odor problems.
4
Extensive
Surface
Blooms
Large portions of the water surface covered by mats
of algae. Windy conditions may temporarily
eliminate mats, but they will quickly redevelop as
winds become calm. Odor problems in localized
areas.
Source: Coastnet, 1996, Sampling Procedures: A Manual for Estuary Monitoring, prepared for the
Coastnet Water quality Monitoring Project administered by the Oregon State University Extension Sea
Grant Program, http://secchi.hmsc.orst.edu/coastnet/manual/index.html.
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chapter v • Chlorophyll a Criteria
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chapte VI
Recommended
Implementation Procedures
This chapter presents implementation procedures as regional guidance to the Chesa-
peake Bay watershed states and other agencies, institutions, groups or individuals
applying the criteria to determine the degree of attainment. In accordance with
Section 117(b)(2)(B)(iii) of the Clean Water Act, these procedures accompany the
regional criteria to promote their consistent, baywide application in common tidal-
water designated uses across jurisdictional boundaries.
The Chesapeake Bay criteria, as presented in the previous three chapters, will protect
designated uses if they are applied strictly following current EPA national guide-
lines. The regional implementation procedures described in this chapter are tailored
to the Chesapeake Bay and its tidal tributaries, the refined tidal-water designated
uses and the current and anticipated enhancements to the baywide coordinated moni-
toring program. Adoption and application of the Chesapeake Bay-specific
implementation procedures across jurisdictions will give the states and other partners
a greater degree of confidence in assessing the attainment of criteria and protection
of designated uses. The extensive shared tidal waters should be assessed consistently
across the four jurisdictions using these recommended procedures that account for
natural conditions and processes, highlight the magnitude and extent of remaining
impairments and provide up-front diagnostics of possible reasons for criteria nonat-
tainment. The EPA strongly encourages states to adopt these implementation
procedures into their water quality standards.
The chapter includes:
• A brief review of the criteria, defining the spatial and temporal boundaries
within which criteria attainment will be measured;
• A method for quantifying and visualizing the degree of criteria attainment or
exceedance that incorporates the amount of area or volume of a region that
meets or exceeds a criterion and how often a criterion is met or exceeded;
• A description of successful criteria attainment recognizing that 100 percent
attainment is not necessary to protect designated and existing uses;
chapter vi • Recommended Implementation Procedures
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144
• A practical description of how monitoring information may be used to assess
attainment, including statistical estimation methods for addressing assessment
of the short-interval criteria, such as the 7-day mean, 1-day mean and instanta-
neous minimum dissolved oxygen criteria; and
• A description of how mathematical model-simulated information may be used
to assess the effect on future criteria attainment under various nutrient/sediment
reduction scenarios, which support decisions on load reductions and caps on
loadings to maximize the beneficial effect on attainment.
DEFINING CRITERIA ATTAINMENT
DISSOLVED OXYGEN CRITERIA
The Chesapeake Bay dissolved oxygen criteria were derived to protect species and
communities in the five tidal-water designated uses during specific seasons (Table
VI-1). See Chapter III for detailed information on the designated use-specific criteria
and appropriate periods for applying them. Refer to Appendix A and the Technical
Support Document for the Identification of Chesapeake Bay Designated Uses and
Attainability (U.S. EPA 2003) for details on the five designated uses and their bound-
aries. The Chesapeake Bay dissolved oxygen criteria should not be applied to a
designated use or during a period of the year for which they were not specifically
derived (see Chapter III).
The EPA expects the states to adopt the fall set of dissolved oxygen criteria that will
protect the refined tidal-water designated uses, presented in Table VI-1. Given recog-
nized limitations in direct monitoring at the temporal scales required for assessing
attainment of the instantaneous minimum, 1-day mean and 7-day mean criteria (see
section titled "Monitoring to Support the Assessment of Criteria Attainment" for
more details), states can waive attainment assessments for these criteria until moni-
toring at the required temporal scales is implemented or apply statistical methods to
estimate probable attainment. Where sufficient data at these temporal scales exist for
specific regions or local habitats, states should assess attainment of the fall set of
applicable dissolved oxygen criteria.
WATER CLARITY CRITERIA
The Chesapeake Bay water clarity criteria were derived based on the minimum
percent light-through-water (PLW) requirements of underwater bay grasses (Table
VI-2). These criteria apply only to shallow-water designated use habitats. The water
clarity criteria are not intended to apply in areas where underwater bay grasses are
precluded from growing by non-water clarity-related factors such as excessive wave
action or at depths where natural and other physical habitat factors will prevent
sufficient light penetration required by the plants. See Chapter IV for a discussion of
the salinity regime-specific criteria and time periods for application. Refer to
chapter vi • Recommended Implementation Procedures
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145
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-------
146
Table VI-2. Summary of Chesapeake Bay water clarity criteria for application to shallow-water
bay grass designated use habitats.
Salinity
Regime
Water Clarity
Criteria as
Percent Light-
through-Water
Water Clarity Criteria as Secchi Depth
Temporal
Application
Water Clarity Criteria Application Depths
0.25
0.5
0.75
1.0
1.25
1.5
1.75
2.0
Secchi Depth (meters) for above Criteria Application Depth
Tidal-fresh
13%
0.2
0.4
0.5
0.7
0.9
1.1
1.2
1.4
April 1 - October 31
Oligohaline
13 %
0.2
0.4
0.5
0.7
0.9
1.1
1.2
1.4
April 1 - October 31
Mesohaline
22 %
0.2
0.5
0.7
1.0
1.2
1.4
1.7
1.9
April 1 - October 31
Polyhaline
22%
0.2
0.5
0.7
1.0
1.2
1.4
1.7
1.9
March 1 - May 31,
September 1 - November 30
'Based on application of Equation IV-1, PLW = 100exp(-KdZ), the appropriate PLW criterion value and the selected application
depth are inserted and the equation is solved for Kj. The generated Kj value is then converted to Secchi depth (in meters) using
the conversion factor Kj = 1.45/Secchi depth.
Appendix A and U.S. EPA (2003) for broad and detailed descriptions, respectively,
of the shallow-water designated use and its boundaries.
The Chesapeake Bay water clarity criteria should not be applied to a designated use
or in a period during the year for which they were not derived. The March 1 through
May 31 and September 1 through November 30 temporal application for the polyha-
line water clarity criteria was originally established for protection of eelgrass
(Zostera marina) beds (Batiuk et al. 1992). Widgeon grass (Riippia maritima) co-
occurs with eelgrass in polyhaline habitats. In shallow-water habitats where both
species currently or historically co-occur1, states and other users should assess water
clarity criteria attainment using a March 1 through November 30 or April 1 through
October 31 temporal application period.
When the water clarity criteria were derived, there was an insufficient scientific basis
for deriving a set of water clarity or related (e.g., total suspended solids) criteria for
protection of open-water designated use habitats.
The EPA expects the states to adopt the salinity regime-specific water clarity criteria
to protect their shallow-water designated uses, presented in Table VI-2. States are
expected to measure the achievement of the shallow-water designated use at the
Chesapeake Bay Program segment scale by achieving an established acreage of
underwater bay grasses, attainment of the applicable water clarity criteria at an
'Maps of the potential and recent distributions of both species were published by Batiuk et al. (1992);
see page 125 for eelgrass and page 128 for widgeon grass. Further information on underwater bay grass
aerial survey findings on the distribution of these two species can also be found at the Virginia Institute
of Marine Science's website at http://www.vims.edu/bio/sav.
chapter vi • Recommended Implementation Procedures
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established application depth or attainment of the applicable water clarity criteria
throughout an established potential shallow-water habitat acreage. The available
supporting technical information on segment-specific underwater bay grass
acreages, application depths and potential shallow-water habitat acreages are
described in the "Monitoring to Support the Assessment of Criteria Attainment,"
section of this chapter and published in detail in the Technical Support Document for
the Identification of Chesapeake Bay Designated Uses and Attainability (U.S. EPA
2003).
CHLOROPHYLLS CRITERIA
Because of the regional and site-specific nature of algal-related water quality impair-
ments, only narrative chlorophyll a criteria have been published here. The
chlorophyll a concentrations tabulated in Chapter V are not numerical EPA criteria.
Along with the documented methodologies, they are provided as a synthesis of the
best available technical information supporting the states' development and adoption
of site-specific numerical chlorophyll a criteria or the derivation of numerical trans-
lators for their narrative chlorophyll a criteria.
The narrative Chesapeake Bay chlorophyll a criteria were derived to address the fall
array of possible impairments, all of which may not manifest themselves within a
particular water body at a given time (Table VI-3). The site-specific nature of impair-
ments caused by the overabundance of algal biomass supports the states' adoption of
the EPA-recommended narrative criteria, with application of site-specific numeric
criteria only for localized waters addressing local algal-related impairments.
The EPA expects states to adopt narrative chlorophyll a criteria into their water
quality standards for all Chesapeake Bay and tidal tributary waters. The EPA
strongly encourages states to develop and adopt site-specific numerical chlorophyll
a criteria for tidal waters where algal-related impairments persist after the Chesa-
peake Bay dissolved oxygen and water clarity criteria have been attained.
The formulation and ultimately the assessment of numerical chlorophyll a criteria
should be based upon seasonal dynamics and concentrations of chlorophyll a in the
Chesapeake Bay and its tributaries. Spring and summer were chosen for these
purposes. Any site-specific numerical impairment-based chlorophyll a criteria
should be applied as salinity regime-based spring (March through May) and summer
(July through September) seasonal mean concentrations.
Table VI-3. Recommended Chesapeake Bay chlorophyll a narrative criteria.
Concentrations of chlorophyll a in free-floating microscopic aquatic plants (algae) shall
not exceed levels that result in ecologically undesirable consequences-such as reduced
water clarity, low dissolved oxygen, food supply imbalances, proliferation of species
deemed potentially harmful to aquatic life or humans or aesthetically objectionable
conditions-or otherwise render tidal waters unsuitable for designated uses.
chapter vi • Recommended Implementation Procedures
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ADDRESSING MAGNITUDE, DURATION,
FREQUENCY, SPACE AND TIME
To define and measure criteria attainment, a number of factors are taken into
account. According to a recent National Research Council (2001) review, estab-
lishing the "magnitude, duration and frequency" of a condition is crucial for
successful development and application of state water quality standards. Equally
important is the spatial extent of a condition, and the spatial and temporal dimen-
sions of attainment assessment must be defined.
Magnitude refers to how much of the pollutant—or a given quantifiable measure of
condition—can be allowed while still achieving the designated uses. Magnitude is
assessed through a direct comparison of ambient concentrations with the appropriate
Chesapeake Bay criterion value. The magnitude of nonattainment of a criterion value
also provides information useful to making management decisions on taking correc-
tive actions.
Attainment of all three Chesapeake Bay criteria should be assessed by Chesapeake
Bay segment (Figure VI-1; Table VI-4), separately for each designated use habitat.
Therefore, each designated use habitat in an individual Chesapeake Bay Program
segment is considered a spatial assessment unit. This is consistent with the scale of
data aggregation and reporting for Chesapeake Bay tidal-water quality monitoring
and the physical scale of the designated use areas.
Criteria attainment should be presented in terms of spatial extent, i.e., the percentage
of the volume (dissolved oxygen) or surface area (water clarity, chlorophyll a) of the
particular designated use habitat in each Chesapeake Bay Program segment that
meets or exceeds the applicable criteria. Measuring spatial extent will be enabled
through the use of spatial interpolation methods, which are described later in this
chapter. Such 'interpolators' work by dividing a water body into a three-dimensional
grid, with cell size depending on data density and the application's resolution
requirements, among other factors.
Duration is defined as the period over which exposure to the constituent of concern
is to be averaged within the assessment period (see below) to prevent detrimental
effects. Duration can also be thought of as the allowable time of exposure before
effects occur. For example, the open-water dissolved oxygen criteria includes a crite-
rion with a magnitude of 5 mg liter1 evaluated as a 30-day mean; another criterion
has a magnitude of 4 mg liter1 evaluated as a 7-day mean.
The dissolved oxygen, water clarity and chlorophyll a criteria are season-specific,
and attainment should be measured only over the applicable season. For example,
attainment of the dissolved oxygen criteria for the migratory fish spawning and
nursery designated use should be assessed and reported for the period of February 1
through May 31; attainment of the open-water fish and shellfish designated use
chapter vi • Recommended Implementation Procedures
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criteria, as applied to both open- and shallow-water bay grass designated uses,
should be assessed and reported seasonally, in winter (December, January and
February), spring (March, April and May), summer (June, July, August and
September) and fall (October and November). Tables VI-1 and VI-2 define 'seasons'
and applicable criteria for dissolved oxygen and water clarity, respectively Numer-
ical chlorophyll a criteria should be applied to the spring and summer seasons
defined previously
The assessment period refers to the most recent three consecutive years for which
relevant monitoring data are available. In circumstances where three consecutive
years of data are not available, a minimum of three years within the most recent five
years should be used.
A three-year period is consistent with the water quality status assessment period
used for over a decade by the Chesapeake Bay Program partners (e.g., Alden and
Perry 1997). A three-year period includes some natural year-to-year variability
largely due to climatic events, and it also addresses residual effects of one year's
conditions on succeeding years. Two years is not enough time to assess central
tendency, and four or more years delay response to problems that may be detected.
Longer periods are more appropriate for detecting trends than for characterizing
current water quality conditions.
A comparison of criteria attainment across one-, three- and five-year assessment
periods confirmed the selection of three years as the appropriate temporal averaging
period. Attainment levels were highly variable using single-year periods. The five-
year period smoothed much of the variability and resulted in little difference
between one assessment period and the next.
The allowable frequency at which the criterion can be violated without a loss of the
designated use also must be considered. Frequency is directly addressed through
comparison of the generated cumulative frequency distribution with the applicable
criterion reference curve. All values falling below the reference curve are considered
biologically acceptable exceedances of the applicable Bay criteria. Through its deri-
vation, the reference curve directly incorporates a biologically acceptable frequency
of exceedances of the applicable Chesapeake Bay criteria.
By combining these factors to measure attainment, the spatial extent of violation or
attainment of the criterion can be determined for each designated use within each
Chesapeake Bay Program segment at temporal increments defined by the criterion.
As the next section describes, the frequency of these occurrences is tallied for each
season over the assessment period.
chapter vi • Recommended Implementation Procedures
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NORTF
CB1TF
BSHOH—
GUNOH—
MIDQH,
ELKOH
C&DOH
BOHOH
SASOH
ANATF
POTO
JMSTF
APPTF
BACOH
PATM
MAGMH
SEVMH
SOUMH
RHDM
WSTMH
CHSTF
CHSOH
CHSMH
CB3M
EASmH
CHOTF
PAXTF
WBRTF
CHOMH1
CHOOH
CHOMH2
POTTF
PISTF
NANTF
NANOH
NANMH
PAXuH
MATTF
LCHMH
HNGMH
FSBMH
PAXMH
WICMH
MANMH
- BIGMH
POTMH
CB5MH
RPPTF
RPPuH
RPPMH
MPNTF
CRRMH
PMKTF
MPNOH
PIAMH
TANMH
CB6PH
PMKOH
CHKOH
YRKM
CB7PH
YRKP
JMSOH
JMSMH
JMSPH
POCTF
POCOH
POCMH
MOBPH
CB8PH
ELIPH -1
WBEMH —1
—LAFMH
¦- EBEMH
SBEMH
Figure VI. The geographical location of the 78 Chesapeake Bay Program segments.
Source: Chesapeake Bay Program 1999.
chapter vi • Recommended Implementation Procedures
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Table VI-4. Chesapeake Bay Program segmentation scheme segments.
Northern Chesapeake Bay CB1TF
Upper Chesapeake Bay CB20H
Upper Central Chesapeake Bay .... CB3MH
Middle Central Chesapeake Bay ... CB4MH
Lower Central Chesapeake Bay .... CB5MH
Western Lower Chesapeake Bay .... CB6PH
Eastern Lower Chesapeake Bay .... CB7PH
Mouth of Chesapeake Bay CB8PH
Bush River BSHOH
Gunpowder River GUNOH
Middle River MIDOH
Back River BACOH
Patapsco River PATMH
Magothy River MAGMH
Severn River SEVMH
South River SOUMH
Rhode River RHDMH
West River WSTMH
Upper Patuxent River PAXTF
Western Branch Patuxent River .... WBRTF
Middle Patuxent River PAXOH
Lower Patuxent River PAXMH
Upper Potomac River POTTF
Anacostia River ANATF
Piscataway Creek PISTF
Mattawoman Creek MATTF
Middle Potomac POTOH
Lower Potomac POTMH
Upper Rappahannock River RPPTF
Middle Rappahannock River RPPOH
Lower Rappahannock River RPPMF1
Corrotoman River CRRMH
Piankatank River PIAMH
Upper Mattaponi River MPNTF
Lower Mattaponi River MPNOH
Upper Pamunkey River PMKTF
Lower Pamunkey River PMKOH
Middle York River YRKMH
Lower York River YRKPH
Mobjack Bay MOBPH
Upper James River JMSTF
Appomattox River APPTF
Middle James River JMSOH
Chickahominy River CHKOH
Lower James River JMSMH
Mouth of the James River JMSPH
Western Branch Elizabeth River . . . WBEMH
Southern Branch Elizabeth River . . . SBEMH
Eastern Branch Elizabeth River .... EBEMH
Lafayette River LAFMH
Mouth to mid-Elizabeth River ELIPH
Lynnhaven River LYNPH
Northeast River NORTF
C&D Canal C&DOH
Bohemia River BOHOH
Elk River ELKOH
Sassafras River SASOH
Upper Chester River CHSTF
Middle Chester River CHSOH
Lower Chester River CHSMH
Eastern Bay EASMH
Upper Choptank River CHOTF
Middle Choptank River CHOOH
Lower Choptank River CHOMH1
Mouth of the Choptank River .... CHOMH2
Little Choptank River LCHMH
Honga River HNGMH
Fishing Bay FSBMH
Upper Nanticoke River NANTF
Middle Nanticoke River NANOH
Lower Nanticoke River NANMH
Wicomico River WICMH
Manokin River MANMH
Big Annemessex River BIGMH
Upper Pocomoke River POCTF
Middle Pocomoke River POCOH
Lower Pocomoke River POCMH
Tangier Sound TANMH
Source: Chesapeake Bay Program 1999.
chapter vi • Recommended Implementation Procedures
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DEVELOPING THE CUMULATIVE
FREQUENCY DISTRIBUTION
The use of cumulative frequency distributions (CFDs) is recommended for assessing
spatial and temporal water quality criteria exceedance in the Chesapeake Bay CFDs
offer a number of advantages over other techniques that are applied for this purpose.
First, the use of CFDs is well established in both statistics and hydro logic science.
CFDs have been used for much of the past century to describe variations in hydro-
logic assessments (Haan 1977). For example, the U.S. Geological Survey has
traditionally used CFDs to describe patterns in historical streamflow data for the
purpose of evaluating the potential for floods or droughts (Helsel and Hirsch 1992).
Second, the application of the CFD for evaluating water quality criteria attainment
in the Chesapeake Bay allows for the evaluation of both spatial and temporal varia-
tions in criteria exceedance. Methods currently used for the assessment of criteria
attainment are based only on temporal variations because measurements are usually
evaluated only at individual monitoring station locations. One of the limitations of
this approach is that it is often difficult to determine whether an individual sampling
location is representative, and there is always potential for bias. In a water body the
size of the Chesapeake Bay, accounting for spatial variation can be very important
and in that respect, the CFD approach represents a significant improvement over
methods used in the past.
A CFD is developed first by quantifying the spatial extent of criteria exceedance for
every monitoring event during the assessment period. Compiling estimates of spatial
exceedance through time accounts for both spatial and temporal variation in criteria
exceedance. Assessments are performed within spatial units defined by the intersec-
tion of Chesapeake Bay Program segments (see Figure VI-1) and the refined
tidal-water designated uses (see U.S. EPA 2003 for specific boundaries), and
temporal units of three-year periods. Thus, individual CFDs will be developed for
each spatial assessment unit over three-year assessment periods. Details on the steps
involved in developing CFDs are described below.
STEP 1. INTERPOLATION OF WATER QUALITY MONITORING DATA
The Chesapeake Bay Program partners collect monitoring data over a range of spatial
scales and frequencies. Much of the water quality monitoring data collected in the
Chesapeake Bay and its tidal tributaries is drawn from a limited number of fixed
stations that are visited on a monthly (or more frequent) basis. Other types of data are
collected at different spatial frequencies. For example, some chlorophyll a data are
collected in a spatially continuous in-situ manner along the cruise tracks of moni-
toring vessels. All of the different types of data are useful for assessing criteria
attainment; however, they must be connected to a single spatial framework in order to
provide a common basis for interpretation. Assessment of criteria attainment requires
that conclusions be drawn for all locations within a spatial unit and not just the loca-
chapter vi • Recommended Implementation Procedures
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tions where data may have been collected. Thus, the data must be extrapolated in
order to evaluate criteria attainment for the larger spatial unit that the data represent.
For the Chesapeake Bay and its tidal tributaries, using a grid-based spatial interpola-
tion software provides a common spatial framework and spatial extrapolation. Spatial
interpolation provides estimates of water-quality measures for all locations within a
spatial assessment unit. This is accomplished at any single location by linear interpo-
lation of the data of all its nearest neighbors. This approach provides an estimate of
the water quality measure at all locations within the spatial unit being considered.
An example of the use of spatial interpolation is illustrated in Figure VI-2, which
displays the monitoring segment boundaries and fixed-station locations in the area
Figure VI-2 Chesapeake Bay Program segment boundaries, fixed monitoring station
locations and summer chlorophyll a concentration (jug liter1) distribution in the Tangier
Sound area of the Eastern Shore of Maryland and Virginia. Summer chlorophyll a
concentration distribution is defined by spatial interpolation.
chapter vi • Recommended Implementation Procedures
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around Tangier Sound and the adjacent portion of the Eastern Shore of Maryland and
Virginia. Using spatial interpolation, chlorophyll a concentrations were estimated
for all locations in the Tangier Sound area. Based on those estimates, the spatial
distribution of chlorophyll a is illustrated by shading the area according to the esti-
mated concentration (darker shading represents higher chlorophyll a
concentrations). The results illustrate the spatial gradients that tend to occur
throughout an area of this size. Those gradients need to be accounted for in order to
accurately assess the extent of criteria exceedance.
The Chesapeake Bay Program spatial-interpolation software (or 'CBP interpolator')
computes water quality concentrations throughout the Chesapeake Bay and its tidal
tributaries from measurements collected at point locations or along cruise tracks
(Bahner 2001). It estimates water quality concentrations at all locations in a two-
dimensional area or in a three-dimensional volume. The CBP interpolator is
cell-based. Fixed cell locations are computed by interpolating the nearest number (n)
of neighboring water quality measurements, where n is normally 4, but is adjustable.
Typically an interpolation is performed for the entire Chesapeake Bay for a single
monitoring event (e.g., a monthly cruise). In this way all monitoring stations are used
to develop a baywide picture of the spatial variation of the parameter being consid-
ered. Segment and designated use boundaries can then be superimposed over the
baywide interpolation to assess the spatial variation of the parameter in any one
segment's designated use(s).
Cell size in the Chesapeake Bay was chosen to be 1 kilometer (east-west) by 1 kilo-
meter (north-south) by 1 vertical meter, with columns of cells extending from the
surface to the bottom of the water column, thus representing the three-dimensional
volume as a group of equal-sized cells. The tidal tributaries are represented by
various cell sizes, depending on the geometry of the tributary, since the narrow
upstream portions of the tidal rivers require smaller cells to represent the river's
dimensions accurately. This configuration results in a total of 51,839 cells for the
mainstem Chesapeake Bay and a total of 238,669 cells for the Chesapeake Bay and
its tidal tributaries.
The CBP interpolator is tailored for use in the Chesapeake Bay in that the code is
optimized to compute concentration values that closely reflect the physics of strati-
fication. The Chesapeake Bay is very shallow despite its width and length; hence
water quality varies much more vertically than horizontally. The CBP interpolator
uses a vertical filter to select the vertical range of data for each calculation. For
instance, to compute a model cell value at 5-meters deep, monitoring data at 5 meters
are preferred. If fewer than n (4) monitoring data values are found at the preferred
depth, the depth window is widened to search up to d (normally ± 2 m) meters above
and below the preferred depth, with the window being widened in 0.5-meter incre-
ments until n monitoring values have been found for the computation. The user is
able to select the smallest n value that is acceptable. If fewer than n values are
located, a missing value (normally a -9) is calculated for that cell.
chapter vi • Recommended Implementation Procedures
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A second search radius filter is used to limit the horizontal distance of monitoring data
from the cell being computed. Data points outside the radius selected by the user
(normally 25,000 meters) are excluded from calculation. This filter is included so that
only data near a specific location are used for interpolation. In the current version of
the CBP interpolator, segment and region filters have been added (Bahner 2001).
The Chesapeake Bay Program segments are geographic limits for interpolation. For
instance, the mainstem Chesapeake Bay is composed of eight segments (see Figure
VI-1 and Table VI-4). The tidal tributaries are composed of 70 additional segments,
using the Chesapeake Bay Program 1998 segmentation scheme (CBP 1999). Each
segment represents a geographic area that has somewhat homogeneous environ-
mental conditions. Segmentation enables users to report findings on a
segment-by-segment basis, which can reveal localized changes compared to the
entire Chesapeake Bay ecosystem.
As stated above, the CBP interpolator uses monitoring data to fill in the three-dimen-
sional space of the Chesapeake Bay. The CBP interpolator assumes a linear
distribution of the data between points. Given the dynamic nature of estuaries, this
is obviously a conservative assumption. However, the spatial limitations of the data
make the simplest approach the most prudent. The strength of the CBP interpolator's
output is directly related to the quality and spatial resolution of the input data. As
sample size increases, interpolation error decreases. For more detailed documenta-
tion on the Chesapeake Bay Program interpolator and access to a downloadable
version, refer to the Chesapeake Bay Program web site at http://www.
chesapeakebay.net/tools.htm.
STEP 2. COMPARISON OF INTERPOLATED WATER QUALITY
MONITORING DATA TO THE APPROPRIATE CRITERION VALUE
To quantify the spatial extent of criteria exceedance, the interpolated water quality
monitoring data must be compared to the appropriate criteria value. In all cases, the
water quality criteria are defined within specific spatial limits and with varying spatial
values. In order to define the spatial extent of criteria exceedance, the appropriate
criteria values must be aligned with the water quality measures throughout the spatial
assessment unit. Accordingly, the spatial definition of each criterion is superimposed
on the interpolator grid structure to assign a criteria value to each cell. Criteria assess-
ments can then be made on a cell-by-cell basis using the water quality estimate from
the interpolator and the criteria value defined for each cell. Figure VI-3 illustrates a
schematic of the process for spatially defined criteria assessment. Chlorophyll a esti-
mates generated from the interpolator (such as that for Tangier Sound, Figure VI-2)
are combined with the grid-based definition of criteria values. The integration of those
two layers allows the comparison of 'measured' chlorophyll a to the applicable
criteria value in each cell to determine if that cell exceeds the criterion for the time
period for which data were collected (Figure VI-3).
chapter vi • Recommended Implementation Procedures
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156
SEGMENT
• Station
Figure VI-3 Chlorophyll a concentration values estimated for each interpolator cell
are compared to the appropriate criterion value on a cell-by-cell basis to determine the
spatial extent of exceedance.
STEP 3. IDENTIFICATION OF INTERPOLATOR CELLS
THAT EXCEED THE CRITERION VALUE
When the appropriate criterion value has been assigned to each interpolator cell,
comparisons can be made on a cell-by-cell basis to determine if the estimated water
quality values met or exceeded the criteria at the time of the monitoring event. Eval-
uation of criteria exceedance is performed for each cell in a spatial unit (Figure
VI-4a), enabling the entire spatial unit to be characterized. The percentage of cells
that exceed the criteria represents the spatial extent of exceedance in that spatial unit
and for that sampling event. The same process is repeated for every sampling event
(Figure VI-4b) and the compilation of the estimates of the extent of spatial
exceedance provides an indication of the frequency of exceedance.
STEP 4. CALCULATATION OF THE CUMULATIVE PROBABILITY
OF EACH SPATIAL EXTENT OF EXCEEDANCE
The spatial extent of exceedance (represented by the colored cells in Figure VI-4) is
calculated as the percentage of area or volume exceeding the criteria. This is accom-
plished by simply dividing the area or volume of all the cells exceeding the criteria
by the total area or volume of the spatial assessment unit and multiplying by 100.
chapter vi • Recommended Implementation Procedures
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157
Mar
1999
Apr
1999
May
1999
Jun
1999
Mar
2000
Apr
2000
May
2000 >
r
C
Meets criteria
Exceeds criteria
Figure VI-4. For a given sampling event, cells that exceed the criterion are determined by comparing the interpo-
lator estimated water quality value in each cell (e.g., chlorophyll a) to the appropriate criterion value (a) as in
Figure VI-3. The same process is repeated for each sampling event through the assessment period (b).
The development of CFD is based on the estimates of spatial exceedance percent-
ages for all monitoring events conducted during the assessment period (Figure VI-5).
CFDs are based on the concept of 'cumulative frequency,' where each observed
value is assigned a probability that represents the potential for observing a lower
value. To calculate cumulative frequency data are sorted in ascending order and then
ranked. This approach is typically used for evaluating streamflow data (Helsel and
Hirsch 1992). It is similar to that used in assessing water quality criteria except that
the values are ranked in descending order (Figure VI-5), because the interest lies in
the potential for observing a spatial exceedance rate greater, not less, than the one
observed.
Once the data are sorted and ranked, the cumulative probability is calculated using a
'plotting position' formula (Helsel and Hirsch 1992). The Weibull formula,
rank/(«+l), developed by Weibull (1939) was chosen as the simplest and most
commonly used; there is a strong precedent for the use of this formula in the hydro-
logic literature (Helsel and Hirsch 1992).
Figure VI-6 summarizes the results of the calculations for the development of the
CFD. Cumulative probability represents the frequency of occurrence of each value
of spatial exceedance or a greater value. For example, more than 50 percent spatial
exceedance was observed 46 percent of the time. At the lower end of the plot, the
point (100, 0) is included because more than 100 percent of the area or volume will
be in exceedance 0 percent of the time. At the upper end of the plot, the point (0,
100) was included because 0 percent of the area or volume will be in exceedance
more that 100 percent of the time.
chapter vi • Recommended Implementation Procedures
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158
a.
b.
Month
Percent Area/
Volume
Month
Percent Area/
Volume
Rank
March 1998
72
June 1998
75
1
April 1998
55
March 1998
72
2
May 1998
65
May 1999
67
3
June 1998
75
May 1998
65
4
March 1999
49
April 1998
55
5
April 1999
34
June 2000
50
6
May 1999
67
March 1999
49
7
June 1999
25
April 2000
39
8
March 2000
20
May 2000
35
9
April 2000
39
April 1999
34
10
May 2000
35
June 1999
25
11
June 2000
50
March 2000
20
12
Figure VI-5. To develop a CFD for an area/volume, estimates of spatial extent of criteria exceedance for all of the
sampling events conducted over a three-year assessment period (See Figure Vl-4b) are compiled (a). To prepare for
developing the CFD the estimates of spatial extend of exceedance are sorted in descending order (b) and ranked.
a.
b.
Month
Percent Area/
Volume Rank
Month
Percent Area/
Volume Rank
Cumulative Probability
[Rank/(n + 1)]
June 1998
75
1
100
0.00%
March 1998
72
2
June 1998
75
1
7.69%
May 1999
67
3
March 1998
72
2
15.38%
May 1998
65
4
May 1999
67
3
23.08%
April 1998
55
5
May 1998
65
4
30.77%
June 2000
50
6
April 1998
55
5
38.46%
March 1999
49
7
June 2000
50
6
46.15%
April 2000
39
8
March 1999
49
7
53.85%
May 2000
35
9
April 2000
39
8
61.54%
April 1999
34
10
May 2000
35
9
69.23%
June 1999
25
11
April 1999
34
10
76.92%
March 2000
20
12
June 1999
25
11
84.62%
March 2000
20
12
92.31%
0
100.00%
Figure VI-6. To develop a CFD, estimates of spatial extent of criteria exceedance for all of the sampling events
conducted over a three-year assessment period (see Figure VI-4) are compiled, sorted in descending order and
ranked (a). Cumulative probability is calculated using the formula 'rank/(n + 1)' (b).
chapter vi • Recommended Implementation Procedures
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159
STEP 5. PLOT OF SPATIAL EXCEEDANCE VS.
THE CUMULATIVE FREQUENCY
The CFD is a graphical illustration that summarizes criteria exceedance by plotting
the temporal and spatial exceedance values listed in Figure VI-6. Temporal
frequency of exceedance is plotted on the vertical axis and spatial extent of
exceedance on the horizontal axis (Figure VI-7). The resulting figure can be used to
draw conclusions about the extent and pattern of criteria exceedance. Each point on
the curve represents the cumulative amount of space and time in which the criteria
were exceeded. The potential for observing a spatial extent of exceedance greater
than the one observed is indicated by the temporal frequency. The curve in Figure
VI-7 shows two examples of the interpretations of individual points. In addition to
the interpretation of individual point, the area beneath the curve represents a spatial
and temporal composite index of criteria exceedance. This area is recommended as
the basis for defining criteria attainment for all Chesapeake Bay segments and desig-
nated uses.
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0
"o
CD
Q_
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E
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100.0
80.0
60.0
40.0
20.0
0.0
39% or more of the area/volume
exceeds the criteria
in 62% of the sampling events
during the
three-year assessment period
65% or more of the area
exceeds the criteria
in 31 % of the sampling events
during the
three-year assessment period
0.0 20.0 40.0 60.0 80.0
Percentage of Area/Volume Exceeding the Criteria
100.0
Figure VI-7. The horizontal axis is the spatial extent of criteria exceedance based on monitoring data
extrapolated using spatial interpolation. The vertical axis is the cumulative frequency of criteria exceedance
for the monitoring events conducted during the assessment period.
chapter vi • Recommended Implementation Procedures
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160
The shape of the curve also indicates the spatial and temporal pattern of criteria
exceedance. Figure VI-8 illustrates three potentially observable CFD plots. Curve (a)
indicates a situation in which the water quality criteria are chronically exceeded in a
relatively small amount of a given segment. Managers could use this information to
target segments for further monitoring and assessment and to identify chronic prob-
lems and tailor management plans to address them. Curve (b) illustrates a situation
where criteria are exceeded on a broad spatial scale, but relatively infrequently. Such
broad-scale acute problems should be evaluated individually. If the frequency and
duration of broad-scale criteria exceedances were low enough, ecological impacts
could be limited. On the other hand, some short-term exceedances can have signifi-
cant ecological effects. Curves (a) and (b) reflect a similar degree of overall criteria
exceedance; however, the exceedance of curve (a) is primarily temporal, and the
exceedance of curve (b) is primarily spatial. Curve (c) reflects broad-scale criteria
exceedance in both space and time. The shape of the curves should be used for diag-
nostic purposes only. Decisions regarding fall attainment should be based on the
overall amount of criteria exceedance indicated by the area under the curve.
As discussed above, it is possible that some spatial and temporal criteria exceedances
could be observed, without necessarily having significant effects on ecological
health or on the designated use of a portion of the Chesapeake Bay. Such
exceedances are referred to as 'allowable exceedances.' Such exceedances have been
00.0
00.0
Percentage of Area/Volume Exceeding the Criteria
Figure VI-8. Use of cumulative frequency distribution to characterize patterns of water
quality criteria exceedance. Curve (a) indicates that criteria are chronically exceeded in a
relatively small portion of the spatial unit. Curve (b) indicates that criteria are exceeded
over a large portion of the spatial unit on a relatively infrequent basis. Curve (c) indicates
that criteria are exceeded over large portions of space and time.
chapter vi • Recommended Implementation Procedures
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161
provided for in EPA national guidance for assessing criteria attainment (U.S. EPA
1997). Ten percent of the samples collected at a point are allowed to reflect nonat-
tainment of water quality criteria without indicating nonattainment of designated
uses. These criteria exceedances are considered 'allowable exceedances' that had
limited impact on the designated use. The 10-percent rule is not directly applicable
in the context of the CFD methodology for defining criteria attainment because it
was designed for samples collected at one location and, therefore, is only reflective
of time.
A more appropriate approach for defining 'allowable exceedances' in the CFD
context is to develop a reference curve (described below) that identifies the amount of
spatial and temporal criteria exceedance that can occur without causing significant
ecological degradation. Such curves can be based on biological indicators of ecolog-
ical health that are separate from the criteria measures themselves. Biological
indicators can be used to identify areas of the Chesapeake Bay and its tidal tributaries
that have healthy ecological conditions and supportive water quality conditions. CFDs
can be developed for those areas as well. Since healthy ecological conditions exist in
the selected areas, CFDs developed for the area would reflect an extent and pattern of
criteria exceedance that did not have significant ecological impact. Thus, the refer-
ence curve approach takes the development of criteria levels beyond those developed
in a laboratory setting and provides actual environmental context. Small incidents of
spatial and temporal criteria exceedance that do not have ecological impacts are iden-
tified and allowed in the assessment of criteria attainment. A description of the
application of the reference curve is provided in this section, with more details on
reference curves in the section titled "Defining the Reference Curve."
Figure VI-9 illustrates the use of the reference curve and the interpretation of criteria
attainment using the CFD. The light blue line illustrates a possible reference curve,
below which a certain amount of spatial or temporal exceedance is allowed. An
actual reference curve could be asymmetrical, indicating that the system could with-
stand either short-term excursions in time or chronic exceedances in small portions
of space, but not both.
Development of the reference curve is intended to identify such specifics to more
accurately reflect what the ecological system needs to thrive. It also is intended to be
developed as a benchmark that is not changed on a regular basis, recognizing the
potential for updates as new information is gathered. By contrast, the attainment
curve is developed over every assessment period during which monitoring data are
collected.
The attainment curve is the assessment of the condition in the segment during the
assessment period and is compared to the reference curve. The area above the refer-
ence curve and below the attainment curve reflects criteria attainment and is referred
to as "non-allowable exceedances." It is recommended that separate attainment
curves be developed for each criteria component, for subsequent application in every
spatial assessment unit (Chesapeake Bay Program segment/designated use) and for
at least one full assessment period of three years.
chapter vi • Recommended Implementation Procedures
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Attainment Curve
Area of 'Non-Allowable'
Criteria Exceedance
© x
Q. LU
Area of 'Allowable'
Criteria Exceedance
Reference Curve
0 10 20 30 40 50 60 70 80 90 100
Percentage of Area/Volume Exceeding the Criteria
Figure VI-9. Light area reflects amount of 'allowable' criteria exceedance defined as
the area under the reference curve (light line). Dark area reflects the amount of 'non-
allowable' criteria exceedance defined as the area between the attainment
curve (black line) and the reference curve.
In cases where the amount of 'non-allowable exceedances' is large (e.g., Figure VI-
8, line c; Figure VI-9), decisions regarding the attainment of designated uses will be
unequivocal. However, situations could arise where small amounts of non-allowable
exceedance could render the decisions less clear. Figure VI-10 illustrates a situation
in which a decision on nonattainment might be clear (a) and one in which the deci-
sion might be less clear (b). In the latter case, questions could arise about the
certainty of the analysis and whether the data were adequate to unequivocally decide
that the portion of the Chesapeake Bay was not attaining its designated use. In some
cases, many data points could have contributed to the development of the CFD,
whereas in other cases there may have been only a few. It is possible to define the
decision rule that any non-allowable exceedance would indicate nonattainment of
the established designated use. However, a decision rule based on a statistical test
could help to address some of the uncertainty involved by accounting for differences
in the number of observations on which the analysis is based.
Work is currently under way to devise a statistical test for the application of CFDs
to assess water quality criteria attainment in the Chesapeake Bay. The test currently
chapter vi • Recommended Implementation Procedures
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(D X
o_ LU
o 0
Attainment Curve
Area of 'Non-Allowable'
Criteria
Exceedance
Attainment Curve
Q_ LU
Area of 'Non-Allowable'
Criteria
Exceedance
0 10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70 80 90 100
Percentage of Area/Volume Exceeding the Criteria
Percentage of Area/Volume Exceeding the Criteria
Figure VI-10. Light area reflects amount of 'allowable' criteria exceedance defined as the area under the
reference curve (light line). Dark area reflects the amount of 'non-allowable' criteria exceedance defined as
the area between the attainment curve (black line) and the reference curve.
being evaluated and refined is the
Kolmogorov-Smirnov (KS) test, which
was originally developed to test for signif-
icant differences between cumulative
density functions (Haan 1977). The KS
test is nonparametric and is based on the
maximum difference between curves
(Figure VI-11). The maximum difference
is somewhat different than the area
between the curves, which is the preferred
indicator for assessing attainment.
However, it can be shown that the
maximum difference and the area
between the curves are closely correlated
and, therefore, evaluation of one will
reflect an evaluation of the other.
The KS test is well-documented and
accepted in the statistical literature. Some
refinements that may be necessary are
currently being evaluated. Overall,
however, the KS test has a strong potential
for evaluating water quality criteria attain-
ment in the Chesapeake Bay.
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0 10 20 30 40 50 60 70 80 90100
Percentage of Area/Volume Exceeding the Criteria
Figure VI-11. Illustration of the basis of the Kolmogorov-
Smirnov statistical test for identifying statistically significant
differences between cumulative density functions. In this case,
the test is applied to identify statistically significant differences
between the reference and attainment curves.
chapter vi • Recommended Implementation Procedures
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DIAGNOSING THE MAGNITUDE OF
CRITERIA EXCEEDANCE
The CFD is a useful tool for evaluating water quality criteria attainment, but it is
based on pass/fail principles and provides no information on the magnitude of
criteria exceedance, which would interest managers, because it indicates how much
effort is needed to correct any impairment. To fill this need and provide supporting
information for the CFD, it is recommended that interpolator plots be generated for
each monitoring event conducted during an assessment period. Viewed either
individually or as a movie, interpolator plots will shed light oil the magnitude of
exceedance during the assessment period.
Two types of interpolator plots are useful for this purpose. The first is the basic inter-
polator plot of the criteria parameter (i.e., concentration for dissolved oxygen and
chlorophyll a, and percent light-through-water for water clarity; Figure VI-12). Such
Chlorophyll a
Relative to Possible Criteria
Figure VI-12. Example plot of chlorophyll a concentration (/jg liter1) estimates
generated through spatial interpolation for purposes of evaluating the magnitude
of criteria exceedance.
chapter vi • Recommended Implementation Procedures
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plots show problem areas and indicate their distance from criteria attainment.
However, they are limited in evaluating the overall picture of magnitude of criteria
exceedance for the entire Chesapeake Bay. Criteria values vary spatially and thus the
magnitude of exceedance will depend on both actual interpolator values and the
criteria values themselves. To address this need, a second set of interpolator plots
illustrating the magnitude of exceedance as a percentage of the criteria values them-
selves should be generated (Figure VI-13). Any estimated values below the criteria
level will be less than one and bounded at zero, whereas estimated values above the
criteria level will be in percentage of criteria level.
Other information is available to evaluate the significance of the criteria attainment
assessment results and to place them in context. This includes the size of the desig-
nated use (as surface area or volume) and the percentage of the total habitat that is
represented by the designated use. This particular data is especially useful for
dissolved oxygen criteria attainment assessment. The information is used to under-
stand the relative percentage of the total habitat that is accounted for by the
Figure VI-13. Example plot of chlorophyll a concentration (pg liter1) estimates
generated through spatial interpolation, expressed as a percentage of a possible spring
season criteria value, for purposes of evaluating the magnitude of criteria exceedance.
Interpolator-
Chlorophyll ;
(iug liter1)
¦ Attains
101-120
121-120
200-300
300-400
H 400-500
r-p i
Interpolator-Estimated
Chlorophyll a Concentration
500-600
600-1100
chapter vi • Recommended Implementation Procedures
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open-water, deep-water or deep-channel designated use habitats in the entire water
column. For example, if the deep-water use was found in nonattainment at a rate of
50 percent but only accounted for 10 percent of the total habitat of the water column,
the management actions taken in response would differ from those taken if the deep-
water use accounted for 75 percent of the total habitat. This may prove to be a useful,
additional source of data when difficult decisions must be made.
DEFINING THE REFERENCE CURVE
The recommended criteria attainment assessment approach is designed to protect the
living resources as defined by the designated uses. The criteria levels themselves
were largely based on scientific studies performed in laboratory settings or under
controlled field conditions. The criteria establish the level of a given habitat condi-
tion that living resources need for survival. They do not account for many other
environmental factors that could affect survival.
Reference curves were developed to provide a scientific-based, direct measure of the
'allowable' criteria exceedances. These exceedances are defined to be those that last
a short enough time or cover a small enough area to have no adverse affects on the
designated use. It is assumed that the designated uses can be attained even with some
limited level of criteria exceedances and thus, the reference curves define those
criteria exceedances deemed to be allowable—chronic in time but over small areas,
or infrequent occurrences over large areas. Exceedances that occur over large areas
of space and time would be expected to have significant detrimental effects on
biological communities, which would imply nonattainment of designated uses.
STRENGTHS AND LIMITATIONS
Although the Chesapeake Bay and its tidal tributaries are listed as impaired water
bodies, there are some places that have met or usually meet the Chesapeake Bay
criteria and support healthy aquatic living resource communities. Reference curves
derived from monitoring these areas reveal patterns of criteria attainment or
exceedances that support the healthy community. That is, they show whether areas
that support a relatively healthy target community: 1) never exceed the applicable
criteria, 2) exceed the criteria frequently, but over a small area or volume, 3) exceed
the criteria infrequently over a large area or volume or 4) exhibit some other pattern.
The EPA recognizes that there are currently a limited number of reference sites, given
the Chesapeake Bay's nutrient-enriched status. In addition, there are limited data avail-
able—both for criteria parameters as well as measures of the biological health of target
communities—with adequate spatial and temporal coverage from which to develop a
fall array of biological-based reference curves. However, where sufficient data exist,
the reference curves appear to be stable. The reference curve for the deep-water desig-
nated use dissolved oxygen criteria is the most solidly grounded in data.
chapter vi • Recommended Implementation Procedures
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This biological reference curve (see below for details) is based on dissolved oxygen
concentration distributions at sites associated with bottom sediment-dwelling
benthic communities scoring 3 or higher on the Chesapeake Bay benthic index of
biotic integrity (benthic-IBI). If several of the reference segments were randomly
removed, the regenerated reference curves do not change much, suggesting that
within designated uses, the attainment curves for reference segments appear to be
very similar. Although less firmly grounded, the reference curves for other desig-
nated uses and other criteria also seem to be relatively stable.
APPROACHES TO DEFINING REFERENCE CURVES
At least three options exist for defining a reference curve (Figure VI-14). Fixed
percentages could be selected based on a policy decision or other basis similar to the
10 percent level of acceptable exceedances allowed in 305(b) EPA national guidance
(Figure VI-14a; U.S. EPA 1997). Alternatively, laboratory or empirical field data
from areas known to be unimpaired by the stressor can be used to derive a biologi-
cally-based reference curve (Figure VI-14b). Even this second approach, however,
requires technical or policy decisions regarding the acceptable level of biological
effect. Finally, a reference curve could be established to reflect uncertainty based on
the assumption of a normal distribution, and using observed or estimated error vari-
ance for both time and space (Figure VI-14c).
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(b) Biologically-Based
(c) Accounting for
Sampling/Measurement Error
-r
10
-r~
20
60 70 80 90 100
Percentage of Area/Volume Exceeding the Criteria
Figure VI-14. Three possible options for setting reference curves for application
to the cumulative frequency distribution approach for defining criteria attainment:
(a) fixed percentages based on policy decisions; (b) biological effects-based empirical
field or laboratory data and; (c) observed or estimated uncertainty data.
chapter vi • Recommended Implementation Procedures
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The reference curves described below for the dissolved oxygen and water clarity
criteria are based on empirical, biologically-based field data where possible. Where
no corroborating field data exist, a normal distribution curve representing approxi-
mately 10 percent exceedance is used (see Figure VI-18). Appendix H contains
supporting analyses and detailed descriptions of the methodologies used for defining
these reference curves, as well as the list of reference locations.
REFERENCE CURVES FOR DISSOLVED OXYGEN CRITERIA
Reference curves for dissolved oxygen are intended to represent the spatial and
temporal distribution of dissolved oxygen concentrations in areas supporting healthy
species and communities the criteria were established to protect. The deep-water
designated use, for example, contained the necessary water quality and biological
source data collected over similar temporal and spatial scales. When such data were
not available at the scales necessary to establish quantitative relationships between
the criteria parameter and measured living resource community health, surrogate
measures of biological and habitat conditions were explored. Ideally, each set of
designated use-based dissolved oxygen criteria should have a separate, individually
derived reference curve. However, satisfactory synoptic water quality and biological
indices data or surrogate measures of habitat condition were found only for the open-
water fish and shellfish and deep-water designated uses and were tested only against
the 30-day mean criteria for those uses.
Migratory Fish Spawning and Nursery Dissolved Oxygen
Criteria Reference Curve
Current Chesapeake Bay water quality monitoring in migratory fish spawning and
nursery habitats is limited to midchannel stations. There also are insufficient
spawning success fisheries-independent data available to identify biologically-based
reference sites for these criteria. In addition, the criteria duration components for this
designated use are an instantaneous minimum and 7-day mean, and methodologies
to translate less frequently monitored dissolved oxygen measurements into these
time steps have not been finalized.
An attainment curve for exploratory purposes was created for the February-May
spawning period, using a 30-day criterion of 6 mg liter1 and reference sites identified
using nitrogen, phosphorus, chlorophyll a and total suspended solids as parameters
(Figure VI-15). Attainment was very close to 100 percent. Until more data are
collected to assess the attainment of the 7-day mean and instantaneous minimum
criteria in the migratory fish spawning and nursery designated use, however, the open-
water dissolved oxygen criteria reference curve should be applied (Figure VI-16).
Open-Water Dissolved Oxygen Criteria Reference Curve
In the absence of a Chesapeake Bay open-water fish community index of biotic
integrity, reference Chesapeake Bay Program segments with 'good' water quality
chapter vi • Recommended Implementation Procedures
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CD CD
o j
CD O
100
Percentage of AreaA/olume Exceeding the Criteria
Figure VI-1 5. Initial attempt at developing a dissolved oxygen criteria reference curve for
migratory, spawning and nursery habitat designated use areas using the 6 mg liter1 7-day
mean criterion assessed as a 30-day mean.
Percentage of AreaA/olume Exceeding the Criteria
Figure VI-1 6. Dissolved oxygen criteria reference curve for defining criteria attainment in
open-water designated use habitats.
were identified based on assessments of surface and above-pycnocline concentra-
tions of four parameters: total nitrogen, total phosphorus, chlorophyll a and total
suspended solids (see Appendix F for details). Cumulative frequency distribution
reference curves for migratory spawning and nursery designated use habitats from
February through May (Figure VI-15) and for open-water designated use habitats in
summer (Figure VI-16) were derived using dissolved oxygen concentration data
from these segments.
The Chesapeake Bay Program's Tidal Monitoring and Analysis Workgroup devel-
oped a procedure to assess relative status for cases in which an absolute point of
reference for a water quality parameter is not available (Alden and Perry 1997). That
procedure uses the logistic distribution of a parameter in a 'benchmark' data set as a
chapter vi • Recommended Implementation Procedures
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170
standard against which individual data points are assessed. The individual data are
thus scored between 1 and 100. The assessments are conducted separately in salinity
classification and in depth layers corresponding to the designated uses. The median
score of the individual data scores is then calculated. The benchmark distribution is
divided roughly into thirds, which are defined as 'good', 'fair' and 'poor' (these
terms relate only to each other, not necessarily to actual water quality requirements
of living resources). Status of the parameter is assigned depending on where the
median score falls among these divisions.
Using this procedure, open-water concentrations of the four parameters were
assessed for each Chesapeake Bay Program segment, yielding for each parameter an
assessment of 'good,' 'fair' or 'poor' for each segment, year and season (spring and
summer). To qualify as a reference location, at least three out of four parameters had
to be 'good' and only one parameter could be 'fair'. Once the times and locations
were selected, the corresponding monthly average dissolved oxygen concentration
data were evaluated against the migratory fish spawning and nursery dissolved
oxygen criterion value of 6 mg liter1 (evaluated as a 30-day mean, not as a 7-day
mean) and the open-water dissolved oxygen 30-day mean criterion of 5 mg liter1 for
spring and summer, respectively. The percent volume failing the criterion was calcu-
lated for each month of the season/year. The resulting cumulative frequency
distribution curves are shown in figures VI-15 and VI-16, respectively. Figure VI-16
currently serves as the recommended reference curve for both the migratory fish
spawning and nursery and open-water fish and shellfish designated uses for purposes
of assessing dissolved oxygen criteria attainment.
Deep-Water Dissolved Oxygen Criteria Reference Curve
Reference areas were identified using a measure of benthic community health, the
Chesapeake Bay Benthic Index of Biological Integrity (benthic-IBI; Weisberg et al.
1997). Sessile benthic communities are good indicators of water quality conditions of
overlying waters. Although relatively tolerant of lower oxygen concentrations, a
dissolved oxygen concentration of 2 mg liter1 is considered the lower threshold below
which benthic infaunal communities become severely stressed (see Chapter III). A
healthy benthic community, therefore, could indicate that dissolved oxygen conditions
meeting deep-water dissolved oxygen criteria were met. Benthic infaunal community
samples are collected as part of a long-term Chesapeake Bay Benthic Monitoring
Program. Samples are collected at fixed and random locations in the summer season,
usually in August/September. If the benthic-IBI of that sample is 'good', in this case 3
or greater on a scale of 1 to 5, then it is likely that dissolved oxygen conditions have
been adequate for the previous one to two months of the summer.
The benthic-IBI data from 1985 through 1994 were assessed and a list of deep-water
reference locations identified by year and segment was compiled. Then, the summer
(June through September) dissolved oxygen data that were collected as part of the
Chesapeake Bay Water Quality Monitoring Program at the times and places on the
list were evaluated relative to the deep-water criteria. Figure VI-17 shows the
chapter vi • Recommended Implementation Procedures
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Percentage of Area/Volume Exceeding the Criteria
Figure VI-17. Dissolved oxygen criteria reference curve for defining criteria attainment in
deep-water designated use habitats.
resulting cumulative frequency distribution curve, which serves as the recommended
reference curve for the deep-water seasonal fish and shellfish designated use for
assessing dissolved oxygen criteria attainment (see Appendix H for documentation
of the validation curves used to confirm the reference curve).
Deep-Channel Dissolved Oxygen Criteria Reference Curve
The deep-channel seasonal refuge designated use contains dissolved oxygen concen-
trations that are inadequate to support most Chesapeake Bay species, and the
criterion is set to protect the survival of benthic organisms. Unfortunately, a biolog-
ically-based reference curve could not be developed for the deep-channel use at this
time. This area is assumed to be severely degraded and is not now sampled as part
of the Chesapeake Bay Program long-term benthic monitoring program. No other
appropriate biological data were available with which to identify reference sites.
While a biologically-based reference curve is recommended for the future, a default
reference curve such as the normal distribution curve representing approximately 10
percent exceedance is appropriate in this case to account for anticipated natural
criteria exceedances (Figure VI-18). States and other users must recognize that the
deep-channel dissolved oxygen criterion is stated as an instantaneous minimum, thus
any exceedance is assumed to have direct consequences to the survival of the
bottom-dwelling community.
REFERENCE CURVES FOR WATER CLARITY CRITERIA
Reference areas for development of the water clarity criteria reference curve were
identified as Chesapeake Bay Program segments or parts of segments where under-
water bay grasses were abundant historically and thriving or increasing in coverage
chapter vi • Recommended Implementation Procedures
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CFD Curve
Area of Allowable
Criteria
Exceedance
Percentage of Area/Volume Exceeding the Criteria
Figure VI-18. Cumulative frequency distribution curve in the shape of a hyperbolic curve
that represents approximately 10 percent allowable exceedances equally distributed
between time and space.
in recent years. Separate reference curves were developed for low salinity—tidal-
fresh and oligohaline-and higher salinity-mesohaline and polyhaline-zones. The
supporting analyses for deriving the water clarity criteria reference curves are
provided in Appendix H.
Once the reference Chesapeake Bay Program segments were identified, the water
clarity data (as measured by Secchi depth) for those segments were extracted from
the Chesapeake Bay water quality monitoring program data base. Percent light-
through-water (PLW) is the operational parameter used for assessing attainment of
the water clarity criteria. PLW = 100exp(-KdZ), where Z is the target restoration
depth and Kd, the coefficient of extinction, is estimated as Kd= 1.45/Secchi depth
(see Chapter III for details). Kd values calculated from the Secchi depth data were
averaged by month for each station. The monthly data were then spatially interpo-
lated baywide for each month in the underwater bay grass growing season from 1985
through 1994 to match the Chesapeake Bay water quality model hydrologic simu-
lation period. PLW was calculated for each interpolation cell using the interpolated
Kd value and the defined segment-specific restoration depth. The PLW values were
compared to the criterion value appropriate to the Chesapeake Bay Program
segment's salinity zone, and the percent of the shallow-water area (< 2 meters)
failing the criterion in each segment was calculated for each month. The monthly
chapter vi • Recommended Implementation Procedures
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attainment percentages for each reference Chesapeake Bay Program segment were
pooled in their respective low and higher salinity groups and plotted as cumulative
frequency distribution curves (figures VI-19 and VI-20). Appendix H contains the
reference curves generated using the more recent 1995-2000 data. All these water
clarity criteria reference curves were derived using data spanning decadal scales,
capturing the full range of wet, dry and average hydrologic conditions.
The derived water clarity criteria reference curves reflect findings published in the
scientific literature for Chesapeake Bay species that indicate that underwater plants
can survive reduced light conditions for periods of days to weeks. Field and labora-
tory experiments indicated that lower salinity species were more tolerant of longer
periods of reduced light conditions (Rybicki et al. 2002) compared with species
inhabiting higher salinity waters (Goldsborough and Kemp 1988). These salinity
regime differences also are reflected in the different shapes of the derived reference
curves. The lower salinity reference curve allows for more exceedances over time
and space than are allowed for by the higher salinity reference curve (figures VI-19
and VI-20, respectively).
It should be noted that the water clarity criteria were derived, in part, on the basis of
underwater bay grass growing season medians (Batiuk et al. 1992, 2000), but
attainment is measured on a monthly basis over the growing season (see "Devel-
oping the Cumulative Frequency Distribution," p. 152, for details). Appendix H also
shows water clarity reference curves based strictly on growing season median
assessments.
rn m
CO (D
Percentage of Area/Volume Exceeding the Criteria
Figure VI-19. Water clarity criteria reference curve for defining criteria attainment in
tidal-fresh/oligohaline shallow-water bay grass designated use habitats.
chapter vi • Recommended Implementation Procedures
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C (D I I I I I I I I I I I
So o 10 20 BO 40 50 60 70 80 90 100
o ^
^ o Percentage of Area/Volume Exceeding the Criteria
Figure VI-20. Water clarity criteria reference curve for defining criteria attainment in
mesohaline/polyhaline shallow-water bay grass designated use habitats.
REFERENCE CURVES FOR CHLOROPHYLL A CRITERIA
As states derive numerical regional and local specific chlorophyll a criteria, they
should either derive biologically-based reference curves that reflect the 'allowable'
exceedances of local impairments or apply the normal distribution curve representing
approximately 10 percent 'allowable' exceedance in time and space (see Figure VI-18).
The cumulative frequency distributions derived from the subset of Chesapeake Bay
water quality monitoring program chlorophyll a data associated with the 'Better' and
'Best,' and sometimes 'Mixed_Better Light' water quality categories closely
matched the normal distribution curve in both spring and summer (figures VI-21 and
VI-22). These categories formed the basis for characterizing the Chesapeake Bay
phytoplankton reference community (see Chapter V and Appendix F for details). The
cumulative frequency distributions were derived from applying the 95th percentiles
of chlorophyll a values occurring in these categories (see Table V-6). In figures
VI-21 and V-22, respectively, the cumulative frequency distributions of spring
(March-May) and summer (July-September) chlorophyll a concentration exceeding
the 95th percentile phytoplankton reference community values (a) are overlaid with
the normal distribution curve (b). The normal distribution curve matches well with
both seasonal biological-based cumulative frequency distributions, providing further
justification for applying the normal distribution curve as a chlorophyll a criteria
reference curve in the absence of a directly derived biological reference curve.
REFERENCE CURVE IMPLEMENTATION
As the states adopt the Chesapeake Bay criteria and concomitant procedures into
their water quality standards, they may decide to: 1) allow for no criteria exceedance,
2) select the normal distribution curve representing approximately 10 percent
allowable criteria exceedance or 3) apply a biological reference curve. The first two
chapter vi • Recommended Implementation Procedures
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.5 100
CT>
O Tidal-Fresh
~ Oligohaline
A Mesohaline
• Polyhaline
60
q_ LLl
Cf) 0
40
CO as
20 --
100
Percentage of Area/Volume Exceeding the Criteria
Figure VI-22. Cumulative frequency distribution of summer (July-September) chlorophyll
a concentration exceeding the 95th percentile phytoplankton reference community values
(a) compared with the normal distribution curve (b).
chapter vi • Recommended Implementation Procedures
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176
options are likely to be more restrictive than the biological reference curve approach.
If states choose to apply the biological reference curve, then there should be a strong
incentive to collect relevant data to strengthen the scientific basis of those reference
curves in the future.
MONITORING TO SUPPORT THE ASSESSMENT
OF CRITERIA ATTAINMENT
To support the development of cumulative frequency distributions for criteria attain-
ment assessment purposes, additional monitoring will be required. The current
fixed-station Chesapeake Bay Water Quality Monitoring Program will support many
aspects of the effort to assess criteria attainment. However, some aspects will require
new monitoring in areas of Chesapeake Bay tidal waters from which data have not yet
been collected. Other aspects will require new types of monitoring based on new tech-
nologies that will better address the technical requirements of the criteria as they are
currently defined. The Chesapeake Bay Program has developed a tidal monitoring
network design that identifies the needs and proposes options for addressing those
needs. Many of those options can feasibly be implemented, but additional monitoring
will be expensive, and it is expected that available funds will limit what can be done.
The following describes options for conducting monitoring to support the assess-
ment of criteria attainment. Given that funding may be limited, the monitoring
options are divided into three categories based on funding level. The first category,
'recommended', assumes that funding will be available to conduct monitoring to
fully support the assessment of criteria attainment. The 'recommended' level of
monitoring is based on technological needs to provide a set of data that can be
defended legally and scientifically in making decisions regarding the attainment of
designated uses. The second category, 'adequate', assumes that funding will be
somewhat limited, but will be sufficient to collect enough data to support the devel-
opment of cumulative frequency distributions for most criteria components in most
Chesapeake Bay Program segments and tidal-water designated uses. The third cate-
gory, 'marginal', assumes that monitoring will be significantly limited by available
funding and that it will not be possible to assess all criteria components in all
segments of the Chesapeake Bay and its tidal tributaries.
Efforts are underway to develop the tools necessary to generate verifiable and quan-
titative estimates of error and the levels of monitoring required for given levels of
accuracy acceptable to management agencies. The three general categories defined
above were developed to give the reader some perspective on the range of options
available and the adequacy of the options.
SHALLOW-WATER MONITORING
Resource managers rely upon habitat and water quality monitoring data to charac-
terize problem areas in a watershed, such as areas of low dissolved oxygen, and to
chapter vi • Recommended Implementation Procedures
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detect changes related to management strategies to reduce nutrients and sediments
on a tributary to baywide level. Traditional monitoring programs have collected peri-
odic data at a small number of fixed sampling locations, often in the deeper
midchannel areas. These measurements provide a good baseline for watershed
assessment and long-term trends, but may miss small-scale gradients in tidal water
quality and neglect critical shallow-water habitats.
In the past, intensive water quality monitoring of these shallow-water habitats has
been time-intensive and cost-prohibitive. The advent of a new suite of technologies
known as the DATAFLOW water quality monitoring system, however, has brought
intensive monitoring of shallow-water habitats into reach (http://mddnr.
chesapeakebay.net/sim/index.html). DATAFLOW is a system of shipboard water
quality probes that measure spatial position, water depth, water temperature, salinity,
dissolved oxygen, turbidity (a measure of clarity of the water) and chlorophyll a
from a flow-through stream of water collected near the water body's surface. The
system allows data to be collected rapidly (approximately every four seconds) and
while the boat is traveling at speeds up to 25 knots. Because the DATAFLOW system
is compact, it can be housed on a small boat, enabling sampling in shallow water and
the ability to map an entire small tributary in less than a day. Typical DATAFLOW
research cruise sampling paths traverse shallow and channel areas to obtain a full
characterization of a tributary's water quality.
The discussion below focuses on migratory spawning and nursery, open-water, deep-
water and deep-channel designated uses. The DATAFLOW system is the only viable
option for monitoring water quality conditions in the shallow-water designated use.
The high temporal and spatial variability expected in shallow-water areas implies
that intensive data collection would be required for any assessment to have credi-
bility. A probability-based approach was considered as a less expensive approach for
shallow-water monitoring, but the cost savings were not sufficient to justify the
reduced amount of information that this approach would provide. The only option
for reduced costs in shallow-water monitoring is to limit the amount that is
conducted during any one year.
DISSOLVED OXYGEN CRITERIA ASSESSMENT
'Recommended' Level of Monitoring
Monitoring for dissolved oxygen criteria attainment should address all four frequen-
cies of dissolved oxygen criteria: 30-day mean, 7-day mean, 1-day mean and
instantaneous minimum. The current fixed-station monitoring program is designed
to provide a long-term record of dissolved oxygen concentrations that reflect
seasonal and interannual variation. For that reason, even though instantaneous meas-
urements are collected, the current monitoring is best suited for assessing the 30-day
mean dissolved oxygen criteria component and poorly suited for assessing the 7-day,
1-day mean and instantaneous minimum criteria components. To address the need
for data that will address the 7-day, 1-day mean and instantaneous minimum criteria
chapter vi • Recommended Implementation Procedures
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components, continuous monitors mounted to buoys or piers will be required. At
least one continuous monitor should be located at each assessment location. The
continuous record will then be combined with fixed-station data, used to calibrate
the spectral-analysis model (described below), and all criteria components could be
quantified using that model. Individual criteria component estimates would be
assessed at all fixed locations and interpolated for incorporation in a cumulative
frequency distribution.
'Adequate' Level of Monitoring
Assuming that funding will not be available for the 'recommended' monitoring
approach, an alternative would be to place a limited number of continuous monitors
at representative locations in the Chesapeake Bay and tidal tributaries. The number
of continuous monitors would be relatively small, but the number would be estab-
lished to characterize different types of settings in the Chesapeake Bay. Those
representative temporal records would then be combined with fixed-station data in
similar settings, and spectral models would be developed for each fixed-station loca-
tion. Dissolved oxygen criteria components would be assessed based on the spectral
models, interpolated and used to develop the cumulative frequency distributions.
This approach would entail much greater uncertainty in the assessments. The
absolute variation would be characterized well by regular monthly measurements at
the fixed-stations. However, the higher frequency assessments would be based on
data collected at only a few locations, which would then be extrapolated over large
distances.
'Marginal' Level of Monitoring
Assuming that funding will not be available for even the 'adequate' level of moni-
toring, assessments would need to rely on the fixed-station data only. As stated
above, this type of monitoring was designed for long-term assessments and would
only be truly appropriate for the 30-day mean criteria component. If the 'marginal'
level of monitoring was selected, it is likely that higher frequency criteria compo-
nents would not be assessed in most designated use areas.
Assessing Dissolved Oxygen Criteria Attainment
Addressing Duration Issues. The dissolved oxygen criteria have several
different durations: 30-day mean, 7-day mean, 1-day mean (deep-water only) and
instantaneous minimum. A state's ability to assess these criteria and to have certainty
in the results depends on the time scale of available data and on the capacity of
models to estimate conditions at those time scales. At present, long-term, fixed-
station, midchannel water quality monitoring in the Chesapeake Bay and its tidal
tributaries provides dissolved oxygen measurements twice monthly at most or
approximately every 15 days between April and August. Proposed enhancements to
the tidal water quality monitoring program include shallow-water monitoring, as
chapter vi • Recommended Implementation Procedures
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well as high-resolution spatial and temporal monitoring in selected locations.
However, these new components are only in the planning and early implementation
stages at this point, and because of financial constraints or limitations to current
technology, direct monitoring at the scales of the criteria may not be possible in the
foreseeable future. Therefore, the direct assessment of attainment for some
geographic regions and for some short-term criteria elements (e.g., instantaneous
minimum, 1-day mean and 7-day mean) must be waived for the time being or based
on statistical methods that estimate probable attainment. Several approaches to
addressing the duration issue are described below.
Thirty-Day Mean Attainment Procedure. This duration appears to be within the
temporal scale of the current Chesapeake Bay water quality monitoring programs.
The simplest assessment approach is to use the one value or average of two values
collected within a month as the best estimate of the true 30-day mean. At present,
this is the approach recommended for assessing attainment of criteria with this dura-
tion. However, it is debatable how well one or two samples per month represent what
is intended as protective by the 30-day mean.
These procedures assume the existence of a baywide tidal-water monitoring program
with a fixed-station sampling design and sampling frequency at least once per month
during the seasons defined by the criteria. The procedures assume that horizontal and
vertical measurements of dissolved oxygen will be sufficiently dense that the inter-
polator can create an accurate three-dimensional representation of dissolved oxygen
in the defined designated uses. It also assumes that data are sufficient to define the
boundaries of the designated uses where boundaries are variable, depending on
pycnocline depth.
To simplify computations, if there is more than one observation per month, then the
monthly average is calculated prior to input to the volumetric interpolator. Prior to
averaging for the month, each station's dissolved oxygen profile is interpolated verti-
cally to obtain a value at each half-meter interval from surface to bottom. The
monthly average concentrations at each fixed station at each half-meter are then
interpolated horizontally by the Chesapeake Bay interpolator to yield a basinwide
grid of concentrations for each month. A comparable reference grid or a table of grid
coordinates and depths can be used to relate the monthly cell concentrations to be
evaluated with the correct designated use and corresponding criteria concentrations.
The cell is scored as meeting or not meeting the criterion level and cell volume is
accumulated in the pool of passing or failing totals for each designated use in each
Chesapeake Bay Program segment. From this, the spatial extent of nonattainment,
i.e., the percentage of the total volume exceeding the criterion in each designated use
in each Chesapeake Bay Program segment is tallied for each month in the assess-
ment period (most recent three years).
Dissolved oxygen criteria attainment is reported seasonally (see Table VI-1). To
assess, for example, attainment of the summer season 30-day mean criterion for the
deep-water seasonal fish and shellfish designated use, the percent exceedance data
chapter vi • Recommended Implementation Procedures
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for the months of June through September for a three-year period for all Chesapeake
Bay Program segments with deep-water designated use habitats would be extracted
and evaluated individually using the cumulative frequency distribution approach.
The cumulative frequency distribution attainment curve would be calculated (and
plotted, if desired) and compared to the appropriate reference curve for the desig-
nated use and season using the statistical test described earlier. If the two curves are
significantly different, then the segment/designated use is considered out of attain-
ment, and failing by the amount defined by the area between the two curves.
Seven-Day Mean Attainment Procedure. The 7-day time frame is much shorter
than the temporal scale of the current baywide water quality monitoring programs,
and statistical forecasting models are necessary to assess criteria of this duration.
The proposed approach, referred to as the spectral analysis approach in this chapter
and discussed in more detail below, uses long-term, low-frequency data from the
monitoring program and shorter-term, high-frequency data from in situ semi-contin-
uous monitors to synthesize a data set that incorporates both long- and short-term
patterns of variability. The synthetic data set is created at user-specified time inter-
vals, e.g., weekly, daily and hourly. The minimum interval will depend on the
interval length of the continuous data. The synthetic data set is then analyzed at the
appropriate temporal scale, which in this case is seven days. At present there are
insufficient high-frequency data and insufficient validation of the approach to
recommend its implementation. For now, attainment of 7-day mean criteria should
not be assessed unless data are available for a specific location/segment at a temporal
scale consistent with the 7-day duration.
One-day Mean Attainment Procedure. The 1-day attainment procedure is the
same as the 7-day mean procedure described above. For now, attainment of the
1-day mean criteria should not be assessed unless data are available for a specific
location/segment at a temporal scale consistent with the 1-day duration.
Instantaneous Minimum Attainment Procedure. Again, the instantaneous
minimum time frame is much shorter than is currently sampled. The spectral
analysis approach presented above is one way to estimate attainment of these
dissolved oxygen criteria. Another approach, referred to as the logistic regression
approach in this chapter and described in more detail below, applies by restating the
criterion in slightly different temporal terms. An instantaneous minimum implies
that the criterion is not met if dissolved oxygen concentrations are below the crite-
rion value at any time. The logistic regression approach estimates the relative
frequency or percent of time that a region falls below a specified concentration based
on the empirical relationship between seasonal or monthly mean values and the
percent of dissolved oxygen concentrations above or below the specified level as
observed in the historical data record (of the Chesapeake Bay water quality moni-
toring program). This method has been applied experimentally with reasonable
results (Jordan et al. 1992) and can approximate criteria exceedance/attainment
frequency. However, at this time the method has not been adequately validated to
recommend implementation for formally assessing criteria attainment. Attainment of
chapter vi • Recommended Implementation Procedures
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instantaneous minimum criteria should not be assessed unless data are available for
a specific location/segment at a temporal scale consistent with the instantaneous
minimum duration.
Spectral Analysis Approach. The foundation for this method was developed by
Neerchal et al. (1992) in the context of implementing the Chesapeake Bay dissolved
oxygen restoration goal (Jordan et al. 1992) and has been modified for criteria appli-
cation. The method uses spectral analysis to extract the cyclical components of the
long- and short-term time-series records and combines them to create a synthesized
time-series data set with data synthesized at user-specified time steps. At present, the
synthetic data are hourly with cyclic components limited to two cycles per day. The
synthetic data have the annual and seasonal cyclic and trend characteristics of the
long-term record as well as the tidal, diurnal and any other periodic characteristics
of the short-term, high-frequency record. The long-term record comes from fixed-
station monitoring data collected at regular once or twice monthly intervals in the
seasons of interest. The short-term data come from in-situ semicontinuous oxygen
monitors deployed on buoys or other fixed structures at designated locations around
the Chesapeake Bay and its tidal tributaries. These semicontinuous oxygen monitors
are put in place for various lengths of time at many different locations and depths.
Sites are chosen in order to best characterize the dissolved oxygen conditions in each
designated use. The sampling interval of the semicontinuous monitors are commonly
5, 10 or 20 minutes. To be most useful, the interval should be no longer than one
hour. More details are provided in Appendix I.
Application of the Spectral Analysis Approach. The spectral analysis application
shown in Figure VI-23 uses long-term data from station CB4.2C, a monitoring
station in the midregion of the Chesapeake Bay, and a two-month series of contin-
uous dissolved oxygen measurements at a buoy deployment near that station at
approximately 9 meters below the surface. Figure VI-23 shows the observed monthly
dissolved oxygen concentrations (asterisks) at station CB4.2C (8- to 10-meter depth)
and the long-term forecast (line) from the spectral equation.
CD ~
01JAN1985
01 JAN 1990
01 JAN 1995
01JAN2000
Date
Figure VI-23. Observed monthly dissolved oxygen concentrations (*) at Chesapeake Bay
Monitoring Program station CB4.2C (at the 8 to 10 meter depth) from January 1985 to
January 2000 and the long-term 'forecast' (—) from application of the spectral equation.
chapter vi • Recommended Implementation Procedures
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The synthetic data record is obtained by combining the long- and short-term equa-
tions. A sample two-month period, August through September 1987 (indicated by
the two, close-together vertical reference lines in Figure VI-23), is illustrated in
Figure VI-24. This synthetic record can then be analyzed relative to the applicable
criteria elements. In the example shown, the 9-meter depth at station CB4.2C is near
or below the pycnocline and is, therefore, subject to criteria for the deep-water desig-
nated use. Summer dissolved oxygen criteria for the deep-water designated use is a
3 mg liter1 30-day mean, 2.3 mg liter1 1-day mean and 1.7 mg liter1 instantaneous
minimum. For demonstration purposes, let a 7-day mean of 2.5 mg liter1 also apply.
5 16
12
q *-|—i 1 i i i i i 1 n—i i i i 1 | i i i i i i i i i—]—i—i—i—i—i i i i—' | i i i i i 1——i—|—i i i i 1 i i i
01AUG1987 11AUG1987 21AUG1987 31AUG1987 10SEP1987 20SEP1987 30SEP1987
Date
Dot=observed monthly dissolved oxygen; dashed line=long-term forecast; solid line=combined forecast
Figure VI-24. Expanded view from Figure VI-23 of the two-month period August-
September 1987 synthetic data record obtained by combining the long- and short-term
spectral equations.
Based on monitoring data alone (two observations each month), the August and
September mean monthly values are 3.4 mg liter1 and 4.2 mg liter1, respectively.
Basing assessment on the synthetic data record, attainment can be measured either
in sequential or rolling time windows, as described below. In some cases the results
vary depending on which option is used (Table VI-5). For the 30-day duration, the
sequential option results in two 30-day periods within the 61 days, between August
1 and September 30, 1987; the rolling time window option yields 31 periods. If there
was a 7-day criterion for deep-water designated use, there would be 8 sequential
versus 55 rolling-window periods in those 61 days. For the 1-day minimum duration,
the question of sequential and rolling-window is moot.
Verifying the Spectral Analysis Approach. The number and distribution of high
frequency semicontinuous dissolved oxygen data sets is small compared to the
variety of habitats, times of year and layers of the water column that need to be char-
acterized. There are gaps in critical seasons, geographic coverage and designated
chapter vi • Recommended Implementation Procedures
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Table VI-5. Sample attainment results when assessing with varying time windows
Dissolved Oxygen Criterion
Time Windows
Meeting Criterion
Percent of
Observations
at or above Criterion
30-day Mean (3 mg liter-1):
Sequential
Rolling window
2 of 2
31 of31
100%
100%
7-day Mean (2.5 mg liter-1):
Sequential
Rolling window
7 of 8
46 of 55
87.5%
83.6%
Instantaneous Minimum (1.7 mg liter -1)
Pool of hourly measurements
1,250 of 1,484
84.2%
uses. Nevertheless, the number of such data sets on hand is substantial and growing,
relative to the number and location of fixed monitoring stations.
Developing and verifying the method will be an ongoing process. Short-term fore-
casts based on synthetic data are created and compared to actual semicontinuous
records not used in the original forecasting process. There are some, but not many,
instances in which semicontinuous data are available at the same site in different
years. Also, in some instances, multiple semicontinuous records are available for the
same region. In these cases, one record is used in the spectral analysis and equation
development and the other is used to verify the results. With data recorders deployed
for the specific purpose of validating and refining the forecasting models, better veri-
fication will be available in the future.
Even with these issues resolved, there are still questions concerning how synthetic
time-series data sets should be adapted to enable an assessment of spatial extent and
frequency of attainment in a manner consistent with criteria assessed by other analyt-
ical methods.
Logistic Regression Approach. This method modifies and significantly updates
a method developed originally to measure attainment of the 1992 Chesapeake Bay
dissolved oxygen restoration goal (Jordan et al. 1992). The early work demonstrated
predictable relationships, on a segment-by-segment basis, between seasonal mean
dissolved oxygen concentrations and the percent of observations above a target
concentration. The relationships proved to be strong and applicable in areas where
dissolved oxygen concentrations ranged above and below the goal target concentra-
tions. Given the tidal water quality monitoring data record that spans more than 17
years with the measurements from multiple depths (the vertical dissolved oxygen
profile is collected at 1- to 2-meter intervals), the regression models are now month-
and depth-specific in many segments. Based on the monthly mean dissolved oxygen
concentration measured at a specified depth, the models predict the percent of time
chapter vi • Recommended Implementation Procedures
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that the dissolved oxygen concentration at that depth in a segment is at or above a
user-specified concentration, e.g., an instantaneous minimum of 1.7 mg liter"1 (see
Appendix I for more details).
Application of the Logistic Regression Approach. The method can be applied
using the three-dimensional baywide interpolations of monthly average dissolved
oxygen, as described for the determination of 30-day duration criteria. The monthly
average concentrations at each fixed station at each half-meter are interpolated hori-
zontally by the Chesapeake Bay interpolator to yield a basinwide grid of
concentrations for each month. A comparable reference grid or a table of grid coor-
dinates and depths relate the monthly cell concentrations to be evaluated with the
correct designated use and corresponding criteria concentration (e.g, instantaneous
minimum of 1.7 mg liter1). In the data processing step, a segment- and criterion
level-specific prediction model uses the cell's monthly average concentration, depth
and month as factors in predicting the percent of the time that particular cell is at or
above the criterion. The cell is scored as passing or failing the criterion level
depending on the model results. The cell volume is accumulated in the pool of
passing or failing totals for each designated use in each segment. Like the method
for assessing the 30-day mean, the spatial extent of nonattainment, i.e., the
percentage of the total volume exceeding the criterion in each designated use in each
segment, is tallied for each month in the assessment period (most recent three years).
The cumulative frequency distribution attainment and reference curves can then be
derived, and the same statistical test for determining attainment as described for the
direct assessment method can be applied.
Strengths and Current Limitations. The logistic models are based on conditions
represented by the fixed stations in the current monitoring program, which in most
tributaries are sited in the main channel. Until more data are collected, the similarity
of shallow areas to the midchannel in the same segment is not known. This approach
would assume, in the absence of other data, that the main channel data are represen-
tative of similar depths in the shallows. If salinity or other physical data from the
shallows indicate that all or part of the shallow water column is more similar to a
different depth in the midchannel (as is sometimes the case for various reasons), then
the more representative depth would be used to estimate percent attainment. For
example, the pycnocline typically is deeper in the portion of the Chesapeake Bay
than on the flanks, and the depth of the pycnocline on one flank is typically deeper
than the other. Thus a 4-meter-deep, above-pycnocline water parcel on one flank
may be most similar to the 4-meter-above-pycnocline depth in the midchannel
profile, while the 4-meter-deep, subpycnocline parcel on the opposite flank is more
similar to the 5-meter depth in the midchannel profile.
To date, dissolved oxygen concentrations have shown little significant trend in most
areas of the Chesapeake Bay and its tidal tributaries and, therefore, history-based
estimation models are reasonable. However, where significant trends are detected, it
would be important to review the models and their basis in light of new, emerging
empirical relationships at those locations. This approach provides an estimate of the
chapter vi • Recommended Implementation Procedures
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amount of time a water parcel is above or below a particular concentration, but does
not address the length of individual exposure, rate of re-exposure, or a specific event-
duration such as daily or 7-day mean.
WATER CLARITY CRITERIA ASSESSMENT
'Recommended' Level of Monitoring
Because the DATAFLOW technology is the only viable approach for assessing water
quality conditions in shallow-water designated use areas, there is only a 'recom-
mended' level of monitoring for assessing the water clarity criteria. Significant
spatial and temporal variability are expected in the shallow-water designated use
area. The DATAFLOW is best suited to address the high level of variability and
provide data for credible assessments of criteria attainment. The only option for
reduced costs in shallow-water monitoring is to limit either the total number of tidal
systems assessed and/or the frequency of monitoring events for each system that are
conducted during a single year.
Assessing Attainment of the Shallow-Water Bay Grass
Designated Use
Restoring underwater bay grasses to areas supporting "the propagation and growth
of balanced, indigenous populations of ecologically, recreationally and commer-
cially important fish and shellfish inhabiting vegetated shallow-water habitats" is
ultimately the best measure of attainment of the shallow-water bay grass designated
use. To determine the return of water clarity conditions necessary to support restora-
tion of underwater grasses and, therefore, attainment of the shallow-water designated
use, states may: 1) evaluate the number of acres of underwater bay grasses present
in each respective Chesapeake Bay Program segment, comparing that acreage with
the segment's bay grass restoration goal acreage; and/or 2) determine the attainment
of the water clarity criteria within the area designated for shallow-water bay grass
use. The shallow-water bay grass use designated use area may be defined by either:
1) applying the appropriate water clarity criteria application depth (i.e., 0.5, 1 or 2
meters) along the entire length of the segment's shoreline (with exception of those
shoreline areas determined to be underwater bay grass no-grow zones; see U.S. EPA
2003 for details); or 2) determining the necessary total acreage of shallow-water
habitat within which the water clarity criteria must be met using a salinity regime
specific ratio of underwater bay grass acres to be restored within a segment to acres
of shallow-water habitat that must meet the water clarity criteria within the same
segment (regardless of specifically where and at what exact depth those shallow
water habitat acreages reside within the segment). These approaches to assessing
attainment of the shallow-water bay grass designated use are described below in
more detail.
Assessing Underwater Bay Grasses Restoration. In response to a commit-
ment in the Chesapeake 2000 agreement, the Chesapeake Bay watershed partners
chapter vi • Recommended Implementation Procedures
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adopted a baywide underwater bay grasses restoration goal of 185,000 acres. This
baywide restoration goal was established "to reflect historic abundance, measured as
acreage and density from the 1930s to present" (Chesapeake 2000, Chesapeake
Executive Council 2000).
The single best year of underwater bay grasses growth observed in each Chesapeake
Bay Program segment from the entire available record of aerial photographs (1938-
2000) was determined by state and federal agency resource managers and
Chesapeake Bay scientists as the best available data on underwater bay grasses
occurrence over the long-term. The underwater bay grasses goal acreage was set
using the single best year acreage out to a Chesapeake Bay Program segment-
specific application depth determined as summarized in Table VI-6 and described in
detail in the Technical Support Document for the Identification of Chesapeake Bay
Designated Uses and Attainability (U.S. EPA 2003). Based on the implementation
Table VI-6. Methodology for establishing the 185,000 Chesapeake Bay baywide
underwater grasses restoration goal.
The baywide underwater bay grasses goal acreage was set using the single best year
acreage out to an application depth determined as follows:
1. Bathymetry data and aerial photographs were used to divide the single best year
underwater bay grasses acreage in each Chesapeake Bay Program segment into three
depth zones: 0-0.5 meters, 0.5-1.0 meters and 1-2 meters.
2. The aerial photographs were then used to determine the maximum depth to which the
underwater bay grass beds grew in each segment with either a minimum abundance or
minimum persistence. The underwater bay grass goal for a Chesapeake Bay Program
segment is the portion of the single best year acreage that falls within this determined
depth range. The decision rules for this were as follows:
In all segments, the 0-0.5 meter depth interval was designated for shallow-water
bay grass use. In addition, the shallow-water bay grass use was designated for
greater depths within a segment if either:
A) The single best year of underwater bay grasses distribution covered at least
20 percent of the potential habitat in a deeper zone; or,
B ) The single best year of underwater bay grasses distribution covered at least
10 percent of the potential habitat in the segment-depth interval, and at least
three of the four five-year periods of the more recent record (1978-2000)
show at least 10 percent SAV coverage of potential habitat in the segment-
depth interval.
3. The single best year underwater bay grasses distribution acreage of all Chesapeake
Bay Program segments were clipped at the deeper depth of the segment-depth
interval, determined above. The resulting underwater bay grass acreages for each
segment were added, resulting in the total baywide underwater bay grass restoration
goal of 185,000 acres.
Source: U.S. Environmental Protection Agency 2003
chapter vi • Recommended Implementation Procedures
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187
of this methodology, each Chesapeake Bay Program segment (see Figure VI-1 and
Table VI-4) has an underwater bay grass restoration goal acreage, with the exception
of those segments documented as underwater bay grass no-grow zones along their
entire shoreline, with the total acreage summed up from all segments equaling
185,000 acres.
In adopting and implementing their water quality standards for protecting the
shallow-water bay grass designated use, states may: 1) adopt the segment-specific
underwater bay grass restoration goal acreages that make up the baywide 185,000
restoration goal; or 2) adopt a lower initial Chesapeake Bay Program segment-
specific underwater bay grass acreage, below the established goal acreage for a
segment, and use their upcoming triennial reviews of state water quality standards to
continually evaluate and appropriately increase the segment-specific acreages
towards the ultimate underwater bay grass restoration goal acreage. If states choose
to adopt a lower initial segment-specific acreage, at a minimum they must adopt an
underwater bay grass acreage for that Chesapeake Bay Program segment equal to or
greater than the existing use underwater bay grasses acreage defined as either the
single best year of composite acreage of underwater bay grasses mapped through the
baywide underwater bay grasses aerial survey since 1975. The Chesapeake Bay
Program segment-specific acreages that, added together, make up the baywide
185,000 restoration goal are documented in the Technical Support Document for the
Identification of Chesapeake Bay Designated Uses and Attainability along with the
segment-specific existing use underwater bay grasses acreages (U.S. EPA 2003).
Achieving the Chesapeake Bay Program segment-specific underwater bay grass
restoration acreages should be measured as the single best year of acreage as
observed through the most recent three years of data from the Chesapeake Bay
underwater bay grasses aerial survey. All mapped acreages of underwater bay
grasses in a segment should be counted towards achievement of each segment-
specific restoration goal regardless of the depth. Chesapeake Bay segment level
acreages of underwater bay grasses are published annually and can be accessed
through the Chesapeake Bay Program's web site at http://www.chesapeakebay.
net/data, or directly through the Virginia Institute of Marine Science's "Bay Grass in
Chesapeake Bay and Delmarva Peninsula Coastal Bays" web site at http://www.
vims.edu/bio/sav/index.html.
Assessing Water Clarity Criteria Attainment at an Established Applica-
tion Depth. The recommended method for assessing water clarity criteria
attainment is, first, to interpolate monthly values of Kd to obtain a Kd value for each
interpolator cell, then to calculate PLW for each cell using the interpolated value of
Kd and the Chesapeake Bay Program segment-specific shallow-water bay grass
designated use boundary depth (see U.S. EPA 2003 for a fall listing of the recom-
mended shallow-water bay grass designated use boundary depths). Note that for
statistical reasons, the interpolations are performed using a log transformation of the
light values (log[Kdj). The resulting interpolated cell values are converted back to
their untransfonned status for the PLW calculation.
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As described previously in this chapter, the interpolator cells can be associated with
the proper Chesapeake Bay Program segment and salinity zone so that each cell's
PLW value can be compared to the proper salinity regime-based water clarity crite-
rion value. The cell area is then accumulated in the 'fail' or 'pass' tally for each
Chesapeake Bay Program segment for each month. The cumulative frequency distri-
bution curve resulting from the monthly percent attainment measures over the
respective underwater bay grass growing season (see Table VI-2) and three-year
attainment period is then compared statistically to the reference curve for the appro-
priate salinity zone to determine the degree of attainment or nonattainment. If the
curves are differ significantly then the segment/designated use is considered out of
attainment and fails by the amount defined by the area between the two curves.
Assessing Water Clarity Criteria Attainment throughout a Defined
Shallow-Water Habitat Acreage. Restoring underwater bay grasses within a
segment requires that the particular shallow-water habitat meets the Chesapeake Bay
water clarity criteria across acreages much greater than those actually covered by
underwater bay grasses. The ratio of underwater bay grass acreage to the required
shallow-water habitat acreage achieving the necessary level of water clarity to
support return of those underwater bay grasses varies, based upon the different
species of bay grasses inhabiting the Chesapeake Bay's four salinity regimes. The
average baywide ratio of underwater bay grass acreage to suitable shallow-water
habitat acreage is approximately one acre of underwater bay grasses for every three
acres of shallow-water habitat achieving the Chesapeake Bay water clarity criteria
(U.S. EPA 2003).
The salinity regime and, therefore, bay grass community-specific underwater bay
grass acreage to shallow-water habitat acreage ratios have been derived through an
evaluation of extensive underwater bay grass distribution data within tidal-fresh,
oligohaline, mesohaline and polyhaline salinity regimes (reflecting different levels
of coverage by different underwater bay grass communities). The Technical Support
Document for the Identification of Chesapeake Bay Designated Uses and Attain-
ability documents the methodology followed and the resulting underwater bay
grasses acreage to shallow-water habitat acreage ratios derived for each of the four
salinity regimes (U.S. EPA 2003).
The same procedures as described above in "Assessing Water Clarity Criteria Attain-
ment at an Established Application Depth" are followed for determining attainment
of the water clarity criteria across the total required shallow-water habitat acreage for
a specific Chesapeake Bay Program segment. The only difference is that a segment-
specific water clarity criteria application depth is not applied. Instead, the depth of
attainment of the water clarity criteria is determined for each interpolator cell. The
area in each interpolator cell from the intertidal zone out to the water-column depth
at which the water clarity criteria was attained is combined along with other similar
areas determined for the other interpolator cells comprising the shallow-water areas
in a specific segment.
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Factoring in the Influence of Tidal Range
on Water Clarity Attainment
Chesapeake Bay Submerged Aquatic Vegetation Water Quality and Habitat-Based
Requirements and Restoration Targets: A Second Technical Synthesis specifies that
half the diurnal tidal range for that Chesapeake Bay Program segment should be
added to the restoration depth Z before calculating PLW or PLL (Batiuk et al. 2000,
page 102). These half tidal-range values, taken from tidal-range tables and averaged
by Chesapeake Bay Program segment, were listed on page 202 of that document in
Table D-4. However, for the purposes of testing attainment of the water clarity
criteria, the EPA recommends using the water clarity criteria application depths
without adding half the diurnal tidal range to it (see U.S. EPA 2003). This recom-
mendation is based on the biologically-based water clarity criteria reference curves.
The methodology followed in the derivation of those reference curves did not
include adding the half tidal range to the restoration depth, Z (see Appendix H). The
EPA believes it is important to maintain consistency throughout the entire set of
procedures for determining water clarity criteria attainment.
Using Midchannel Data to Estimate Shallow-water Conditions
The majority of baywide, regional and local tidal Bay water quality monitoring
programs in the past have collected data only from fixed midchannel stations. Incor-
porating a rotational shallow-water monitoring into the tidal monitoring network is
leading to the generation of shallow-water data for evaluating attainment for the
water clarity criteria. However, given the rotational nature of this shallow-water
monitoring network component, fixed midchannel stations are still going to be used
in criteria assessment. It is relevant, in assessing water clarity criteria attainment, to
note the extent to which water quality monitoring data collected from midchannel
stations in the Chesapeake Bay and its tidal tributaries represent conditions at
shallow-water sites where underwater bay grasses potentially occur and the water
clarity criteria apply.
Evaluation of Midchannel and Nearshore Data Comparability. Several
studies have addressed the shallow-water versus midchannel sampling issue in the
Chesapeake Bay (Stevenson et al. 1991; Batiuk et al. 1992; Ruffin 1995; Bergstrom,
unpublished data; Parham 1996; Karrh 1999; Hunley, unpublished data). While most
studies indicate that midchannel data can be used to describe shallow-water condi-
tions, several suggest the opposite. There is no doubt that demonstrable differences
in water quality can occur between shallow-water and midchannel stations over
varying temporal and spatial scales, especially when bay grasses are present (Ward
et al. 1984; Moore 1996). Other possible causes of variability between shallow-water
and midchannel environments include localized resuspension of sediments, algal
patchiness, point source effluents or sediment chemistry variability (Goldsborough
and Kemp 1988; Moore 1996).
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Using Shallow-water Water Quality Data where Available. Because of
these sources of variability, the use of midchannel data to evaluate the water-clarity
criteria should be avoided whenever shallow-water data are available. Managers of
tidal-water quality monitoring programs should consider the need for enhanced eval-
uation of the shallow-water environment in future monitoring efforts and requests for
funding.
Guidance for Using Midchannel Data when Shallow-water Information
Is Absent. When nearshore shallow-water monitoring data are not available, Karrh
(1999) and Batiuk et al. (2000) provide guidance on the use of midchannel informa-
tion. The findings published by Karrh (1999) and reported by Batiuk et al. (2000)
were based on a comprehensive analysis of shallow-water and midchannel data in
the Chesapeake Bay, which have been collected since 1983 to determine whether
such data can be used to characterize shallow-water environments. Data for the
Karrh (1999) study, obtained from state monitoring efforts, academic researchers
and citizen monitors, were incorporated from the entire Chesapeake Bay and its tidal
tributaries, including the upper Chesapeake Bay region; the Middle, Magothy,
Rhode, Chester, Choptank, Patuxent, Potomac, Rappahannock, Poquoson, York and
James rivers; and Mobjack Bay.
These reports indicated that underwater bay grass habitat quality conditions (relative
to attainment or nonattainment of the 1992 bay grass habitat requirements published
by Batiuk et al. in 1992 and Dennison et al. in 1993) were comparable between
nearshore and adjacent midchannel stations 90 percent of the time, when station
pairs were separated by less than two kilometers.
Midchannel and nearshore areas usually show similar attainment/nonattainment of
the individual water quality parameters—Kd or Secchi depth, dissolved inorganic
nitrogen, dissolved inorganic phosphorus, chlorophyll a and total suspended
solids—published in 1992 as the original set of Chesapeake Bay underwater bay
grass habitat requirements (Batiuk et al. 1992; 2000). These same water quality
parameters are used in calculating percent light-at-the-leaf (PLL) and applying the
supporting diagnostics tools (see Chapter VII).
The Karrh (1999) study results also indicated that individual water quality parameter
concentrations at many of the comparison sites differed significantly between shallow-
water and midchannel areas, from a statistical standpoint. These differences suggest
that although the attainment/nonattainment status may have been comparable, the
magnitude of attainment/nonattainment and the diagnosis of the water quality factors
involved between the shallow-water and midchannel areas could be affected.
It should be noted that the comparisons made between shallow-water and
midchannel areas may also have been affected by temporal factors, given that the
pairs were not sampled on the same day. Water quality managers should also be
aware that these reports were developed to support the application of nonregulatory
bay grass habitat requirements and restoration goals, not regulatory aquatic life
chapter vi • Recommended Implementation Procedures
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water quality criteria. Therefore, the report's recommendations for the allowable use
of midchannel data should be used with appropriate caution only in the absence of
shallow-water quality monitoring data.
Estimating Areas Characterized by Midchannel Stations. It is possible to
determine a distance from a specific midchannel station for which it is appropriate
to use the midchannel distance to characterize the shallow-water environment.
Results revealed that the underwater bay grass habitat quality conditions are indis-
tinguishable between shallow-water and adjacent midchannel stations 90 percent of
the time, when station pairs were separated by less than two kilometers. This radius
differs on a site-by-site basis (see Batiuk et al. 2000, Chapter IX, Table IX-3 and
figures IX-4a through IX-4o). Decisions to use midchannel data to characterize
shallow-water conditions should be made on a site-by-site, tributary-by-tributary
basis. Karrh (1999) provides detailed illustrations of estimated distances from
midchannel monitoring stations to the farthest point where the shallow-
water/midchannel data are comparable.
River Input and Flow Considerations
States responsible for measuring water clarity/shallow-water bay grass designated
use attainment near the fall-lines of where major free flowing rivers enter tidal
waters should recognize the strong influences of intra- and interannual flows on
conditions in the shallow-water habitats. The quality of the waters entering the tidal-
fresh reaches of these rivers is greatly influenced by flow levels. The decadal scale
record of underwater bay grasses distributions and concurrent water quality moni-
toring data provides the states and other users with a wealth of information from
which to gather information on the relative influence of flow conditions on observed
attainment. In the case of water clarity attainment and restoration of underwater
grasses, the EPA recommends recognition within states' water quality standards of
the influence of river flow conditions on water clarity and underwater bay grasses
(through chlorophyll a and suspended solids contributions to reduced light penetra-
tion) particularly for the tidal reaches just below the major river fall lines.
Management actions directed toward attaining the water clarity criteria and shallow-
water bay grass designated use attainment in these tidal reaches should also reflect
the long-term flow conditions and influences on local shallow-water habitat quality.
CHLOROPHYLLS CRITERIA ASSESSMENT
'Recommended' Level of Monitoring
Monitoring for chlorophyll a criteria assessment requires a significant amount of
spatially and temporally intensive data. Algal blooms tend to occur sporadically and
in patches throughout the Chesapeake Bay. The severe nature of blooms, associated
dissolved oxygen extremes and associated releases of toxins are what cause ecolog-
ical impacts.
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To capture data that reflect those blooms, spatially and temporally intensive data are
required. In the shallow-water designated use areas, the DATAFLOW system can
adequately characterize the spatial variability in chlorophyll a.
A 'recommended' monitoring program for the open-water and migratory spawning
and nursery designated use areas would include a combination of fixed-station,
continuous track and remotely sensed data collection. Fixed-station data is usually
considered the most reliable type of data collection because it includes ambient
sample analysis in the laboratory. For that reason, it serves as the baseline for all
other types of chlorophyll a measurement. Continuous-track ('flow-through') moni-
toring should be developed for all vessels used to conduct the fixed-station
monitoring program. Like the DATAFLOW system, the continuous-track monitoring
would provide intensively collected data that would significantly improve our
assessment of the spatial variation in chlorophyll a. One of the limitations of contin-
uous-track monitoring is that it does not cover the entire Chesapeake Bay. Thus, the
third type of recommended monitoring is remote sensing, which can provide esti-
mates of chlorophyll a for most locations in the Bay. It is not clear at this point that
remote sensing is ready for the criteria assessment application, but it does offer great
potential. All three types of monitoring (fixed-station, continuous track, remote
sensing) are recommended because each provides complementary types of informa-
tion that are useful for evaluating different parts of the Chesapeake Bay.
'Adequate' Level of Monitoring
Assuming that funding will not be available for the recommended monitoring
approach, an alternative would be to collect only fixed-station data enhanced with
continuous track monitoring. This provides spatially intensive data wherever cruises
occur, including most tidal tributaries. Furthermore, it represents a relatively small
cost, particularly when considered in proportion to the amount of information that
could be generated. The improvement of this approach over current monitoring is
that spatially intensive data collection would be collected on a regular basis in most
large tidal tributaries. The limitation would be that data would only be collected
along cruise tracks and not intensively throughout the Chesapeake Bay (i.e., as might
be possible with remote sensing). For that reason, the uncertainty associated with the
'adequate' monitoring plan would be higher than the 'recommended' plan.
'Marginal' Level of Monitoring
If funding is not available for even the adequate level of monitoring, assessments
would need to rely on fixed-station data only. This type of monitoring is limited in
its ability to assess the spatial and temporal variability of chlorophyll a found in most
of the Chesapeake Bay. The uncertainty associated with the assessment of chloro-
phyll a criteria attainment using only the fixed-station monitoring program would be
expected to be quite high.
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Assessing Chlorophyll a Criteria Attainment
Phytoplankton are actively growing and consuming nutrients throughout the surface
mixed layer of the water column. The pycnocline region below the this layer, as well
as other depth strata below the pycnocline, rarely contain sufficient light for active
photosynthesis. Therefore, there is little or no autotrophic growth below the surface
mixed layer, although phytoplankton accumulate within and below the pycnocline
due to the physical processes of sinking and estuarine circulation. Given that the
chlorophyll a concentrations throughout the water column will be expressed at the
surface at some point during the natural cycling of phytoplankton and for the
sampling reasons described above, the chlorophyll a criteria are applied to surface
waters only.
Chlorophyll a samples used in determining attainment of numerical chlorophyll a
criteria should be collected at 0.5 to 1 meter below the surface. The majority of
historical and current chlorophyll a data are collected from a discrete surface depth.
The potential for assessing broad areas of the estuary via high-speed vessels and
flow-through technologies or remote sensing can only be tapped if the criteria apply
only to surface chlorophyll a distributions. In general, chlorophyll a concentrations
are highest in the surface layer of the water column.
The formulation and ultimately the assessment of numerical chlorophyll a criteria
should be based upon seasonal dynamics and concentrations of chlorophyll a in the
Chesapeake Bay and its tidal tributaries. Spring and summer were chosen for these
purposes because chlorophyll a concentrations attain annual peaks during these
months in the estuary's various salinity regimes. Any site-specific numerical
impairment-based chlorophyll a criteria should be applied as salinity regime-based
spring (March through May) and summer (July through September) seasonal mean
concentrations.
In spring, river inputs with high dissolved inorganic nitrogen dominate, dissolved
inorganic nitrogen is abundant, phytoplankton are primarily limited by the avail-
ability of phosphorus, and bottom waters are oxygenated. By contrast, under summer
conditions, recycling of nitrogen and phosphorus is the dominant supply, both
dissolved inorganic nitrogen and dissolved inorganic phosphorus are low, phyto-
plankton are primarily limited by the availability of nitrogen and deep bottom waters
are anoxic. The ecological implications of chlorophyll a concentrations in spring and
summer are vital to physical and chemical processes such as hypoxia and anoxia,
nutrient recycling and light attenuation, and biological processes such as the avail-
ability of sufficient and appropriate food for filter and suspension-feeders.
After years of monitoring the Chesapeake Bay and its tidal tributaries, characterizing
phytoplankton dynamics and analyzing these data, Bay scientists have found that
June is indeed a 'transition' month from spring to summer. During certain years,
June behaves more like spring in the types and quantity of phytoplankton that are
present, while in other years, the flora reflect the summer patterns of composition
and densities. This means that in attempts to measure 'spring' and 'summer'
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phytoplankton populations, June is either springlike, summerlike or uniquely
different from either season.
At present, the recommended method for assessing numerical chlorophyll a criteria
attainment is to interpolate monthly chlorophyll a concentrations for each surface
interpolator cell from the available fixed stations. The interpolator cells can be asso-
ciated with the proper segment and salinity zone, so that each cell's chlorophyll a
concentration can be compared to the proper chlorophyll a criterion value. The cell
area is then accumulated in the fail or pass tally for each Chesapeake Bay Program
segment for each month. The cumulative frequency distribution curve resulting from
the monthly percent attainment measures over the spring or summer seasons and the
three-year attainment period is then compared statistically to the reference curve to
determine the degree of attainment/nonattainment. If the curves are significantly
different, then the segment/designated use is considered out of attainment, and
failing by the amount defined by the area between the two curves.
River Input and Flow Considerations
States responsible for measuring chlorophyll a criteria attainment near the fall lines
where major free-flowing rivers enter tidal waters should recognize the strong influ-
ences of intra- and interannual flows on conditions in the adjacent tidal-fresh
habitats. In addition to their upstream contributions of chlorophyll a, the flow levels
of waters directly entering the tidal-fresh reaches of these rivers greatly influence the
resulting tidal habitat chlorophyll a concentrations. The decadal scale record of
water quality monitoring data provides the states and other users with a wealth of
information from which to understand the relative influence of flow conditions on
observed chlorophyll a criteria attainment. The EPA recommends recognition within
states' water quality standards of the influence of river flow conditions on chloro-
phyll a concentrations, particularly in the tidal reaches just below the major fall
lines. Management actions directed toward chlorophyll a criteria attainment in these
tidal reaches should also reflect the long-term flow conditions and influences on
local water quality.
EVALUATION OF CHESAPEAKE BAY
WATER QUALITY MODEL OUTPUT
The Chesapeake Bay Program has developed what have become standard estuarine
modeling tools, including a watershed model (Donigian et al. 1994; Linker et al.
1996, 2000), airshed model (Shin and Carmichael 1992; Appleton 1995, 1996),
estuarine hydrodynamic model (Wang and Johnson 2000), estuarine water quality
model (Cerco 1993, 1995a, 1995b; Thomann et al. 1994; Cerco and Meyers 2000;
Cerco 2000; Cerco and Moore 2001; Cerco et al. 2002) and estuarine sediment
diagenesis model (Di Toro 2001). Together these linked simulations provide a
system to estimate dissolved oxygen, water clarity and chlorophyll a in 35 major
segments of the Chesapeake Bay and its tidal tributaries. The same criteria
chapter vi • Recommended Implementation Procedures
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attainment assessment process applied to observed data is applied to integrated
modeling/monitoring 'scenario' data to determine likely criteria attainment under
management loading scenarios.
The watershed and airshed models are loading models. As such, they provide an esti-
mate of management actions through air controls, agricultural best management
practices, or point source controls which will reduce nutrient or sediment loads to
the Chesapeake Bay tidal waters. The advantage of using loading models is that the
full simulation through different hydrologies of wet, dry and average periods can be
simulated on existing or hypothetical land use patterns. All of the Chesapeake Bay
Program models used in this system simulate the 10-year period from 1985 to 1994
(Linker and Shenk 2000).
CHESAPEAKE BAY WATERSHED MODEL
The Chesapeake Bay Watershed Model is designed to simulate nutrient and sediment
loads delivered to the Chesapeake Bay under different management scenarios
(Donigian et al. 1994; Linker et al.1996; Linker 1996). The simulation is an overall
mass balance of nitrogen and phosphorus in the basin, so the ultimate fate of the
input nutrients is incorporation into crop or forest plant material, incorporation into
soil, or loss through river runoff.
The Chesapeake Bay Watershed Model has been in continuous operation in the
Chesapeake Bay Program since 1982 and has had many upgrades and refinements.
The current version of the Watershed Model, Phase 4.3, is a comprehensive package
for the simulation of watershed hydrology, nutrient and sediment export from
pervious and impervious land uses and the transport of these loads in rivers and
reservoirs. The model is based on a modular set of computer codes called Hydro-
logic Simulation Program—Fortran (HSPF). A slightly modified version of HSPF
release 11.1 (Bicknell et al. 1996) is applied in the watershed simulation. Version 11
is a widely-used public-domain model supported by the U.S. EPA, U.S. Geological
Survey and U.S. Army Corps of Engineers (Shenk et al. 1998).
The Watershed Model allows for the integrated simulation of land and soil contam-
inant runoff processes with in-stream hydraulic and sediment-chemical interactions.
The model takes into account watershed land uses and the application of fertilizers
and animal manure; loads from point sources, atmospheric deposition and onsite
wastewater management systems; and best management practice reduction factors
and delivery factors. Land uses, including cropland, pasture, urban areas and forests,
are simulated on an hourly time-step.
Fourteen calendar years (1984-1997) of varying hydrology are simulated by the
Watershed Model, although only 10 of those years (1985-1994) are used in this
study because of the more limited simulation period of the Chesapeake Bay water
quality model. Scenarios are run on a 1-hour time step and results are often aggre-
gated into 10-year-average annual loads for reporting and comparisons among
scenarios. Watershed Model results, in the form of daily flows and nutrient and sedi-
ment loads, are used as input to the Chesapeake Bay water quality model.
chapter vi • Recommended Implementation Procedures
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CHESAPEAKE BAY WATER QUALITY MODEL
The complex movement of water within the Chesapeake Bay, particularly the density
driven vertical estuarine stratification, is simulated with a Chesapeake Bay hydrody-
namic model of more than 13,000 cells (Wang and Johnson 2000).
Three-dimensional equations of the intertidal physical system, including equations
of continuity, momentum, salt balance and heat balance, are employed to provide the
correct simulation of the movement, or the barriers to movement, of the water quality
constituents of dissolved oxygen, water clarity and chlorophyll a. Correct formula-
tion of vertical mixing, including the simulation of vertical eddy diffusion
coefficients in the hydrodynamic model is particularly important for the dissolved
oxygen criteria as the principal barrier to vertical movement of dissolved oxygen
from surface waters to the deep water is the pycnocline simulated by the hydrody-
namic model.
The water quality model is linked to the hydrodynamic model and uses complex
nonlinear equations describing 26 state variables relevant to the simulation of
dissolved oxygen, water clarity and chlorophyll a (Cerco 1993, 1995a, 1995b, 2000;
Thomann et al. 1994; Cerco and Meyers 2000). Dissolved oxygen is simulated as the
mass balance calculation of reaeration at the surface, respiration of algae, benthos
and underwater bay grasses; photosynthesis of algae, benthic algae and underwater
bay grasses; and the diagenesis, or decay of organics, by microbial processes in the
water column and sediment. This mass balance calculation is made for each model
cell and for associated sediment cells at each hourly time step, providing an estimate
of dissolved oxygen from nutrient loads from the watershed and airshed to the waters
of the 35 major segments of the Chesapeake Bay and its tidal tributaries. Chlorophyll
a is estimated based on Monod calculations of algal growth given resource
constraints of light, nitrogen, phosphorous or silica. Water clarity is estimated from
the daily input loads of sediment from the watershed and shoreline acted on by
regionally-calibrated settling rates, as well as estimated advection due to hydrody-
namics. Coupled with the water quality model are simulations of settling to the
sediment of organic material and its subsequent decay and flux of inorganic nutri-
ents from the sediment (Di Toro 2001) as well as a coupled simulation of underwater
bay grasses in shallow waters (Cerco and Moore 2001).
INTEGRATION OF MONITORING AND MODELING
FOR CRITERIA ASSESSMENT
The load allocation process requires that specific water quality conditions be met
over critical time periods within designated use areas. These areas are given either a
'pass' or 'fail' status. While the Chesapeake Bay water quality model can estimate
changes in water quality due to changes in input loads with reasonable accuracy, an
exact match of the simulated and observed data is impossible. The following method
was developed to make the best use of the strengths of the Chesapeake Bay water
chapter vi • Recommended Implementation Procedures
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quality model and the Chesapeake Bay Water Quality Monitoring Program in
assessing criteria attainment.
The observed data is used to assess criteria attainment during a 'base' period corre-
sponding to the years of calibration for the Chesapeake Bay water quality model,
1985-1994. The Chesapeake Bay water quality model is used in scenario mode to
determine the effect of changes in nutrient and sediment loads on water quality
concentrations. A modified 1985-1994 observed data set is generated for each
scenario using both the model and the observations. The same criteria attainment
assessment process applied to the observed data is then applied to this scenario data
to determine likely criteria attainment under modified loading scenarios.
To generate the modified data set for a particular scenario (e.g., 2010 Clean Air Act),
the EPA compared the output of the scenario to the output of the calibration on a
point-by-point and month-by-month basis. For each point in space and time where
an observation exists during the 1985-1994 period, a mathematical relationship
between the model scenario and the model calibration was established by regressing
the 30 or so daily values for the month when the observation occurred in the water
quality model cell that contains the observation. The regression generates a unique
equation for each point and month that transforms a calibration value to a scenario
value. This relationship is then applied to the monitored observation as an estimate
of what would have been observed had the Chesapeake Bay watershed been under
the scenario management rather than the management that existed during
1985-1994. This procedure is repeated for each monitored observation of dissolved
oxygen, water clarity and chlorophyll a to generate an 'observed' data set for the
scenario. For a full discussion of this procedure, see A Comparison of Chesapeake
Bay Estuary Model Calibration With 1985-1994 Obsen'ed Data and Method of
Application to Water Quality Criteria (Linker et al. 2002).
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R. Reynolds. 1994. Watershed Model Application to Calculate Bay Nutrient Loadings: Final
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Goldsborough, W. J. and W. M. Kemp. 1988. Light responses of a submersed aquatic macro-
phyte: Implications for survival in turbid waters. Ecology 69:1775-1786.
Haan, C.T. 1977. Statistical Methods in Hydrology. Iowa State University Press. Ames, Iowa.
378 pp.
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Helsel, D. R. and R. M. Hirsch. 1992. Statistical Methods in Water Resources. Elsevier
Science Publishing Company, Inc. New York. 522 pp.
Jordan, S. J., C. Stenger, M. Olson, R. A. Batiuk and K. Mountford. 1992. Chesapeake Bay
Dissolved Oxygen Goal for Restoration of Living Resource Habitats: A Synthesis of Living
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toring Information. CBP/TRS 88/93. Chesapeake Bay Program, Annapolis, Maryland.
Karrh, L. 1999. Comparison of Nearshore and Midchannel Water Quality Conditions.
Chesapeake Bay Program, Annapolis, Maryland. 200 pp.
Linker, L.C., 1996. Models of the Chesapeake Bay. Sea Technology 37(9):49-55.
Linker, L.C., G. W. Shenk, P. Wang, C. F. Cerco, A. J. Butt, P. J. Tango and R. W. Savidge.
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conditions in a lower Chesapeake Bay tributary. Ph.D. dissertation. University of Maryland,
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chapte \/ii
Diagnostic Procedures for
Natural Processes and
Criteria Nonattainment
ADDRESSING NATURAL EXCEEDANCE
OF THE CHESAPEAKE BAY CRITERIA
Through the refinement of tidal-water designated uses to better reflect natural habi-
tats defined by season and physical features (e.g., bathymetry, stratification and
hydrodynamic process) and the development of criteria that specifically support
these uses, a fall consideration of natural conditions has been directly interwoven
into the two major components of state water quality standards. Within the recom-
mended implementation procedures for defining criteria attainment, occasional
exceedance of criteria, often natural in origin, has been directly accounted for in
deriving and applying biologically based reference curves (see Chapter VI). Finally,
possible errors in sampling and natural spatial and temporal variability have been
accounted for, in part, through applying a statistical test for the significance of the
observed nonattainment. Outside of extreme climatic events, application of the
complete set of integrated Chesapeake Bay criteria, designated uses and attainment
determination procedures will clearly identify nonattainment of desired water
quality conditions due to anthropogenic impacts.
This combination of refined uses, habitat-tailored criteria and comprehensive imple-
mentation procedures factors in many circumstances, described below, in which
natural conditions affect criteria attainment. In some situations extreme weather
events or conditions may result in criteria exceedances beyond those accounted for
in the combined criteria-uses-implementation procedures. In such situations, addi-
tional steps should be taken to quantify, where possible, exceedances that are due to
natural events or conditions versus anthropogenic, pollutant-based stresses. This
section describes known natural events or conditions that will influence attainment
of the Chesapeake Bay dissolved oxygen, water clarity and chlorophyll a criteria.
Tools that can be used to diagnose and quantify factors contributing to nonattainment
also are described.
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NATURAL EXCURSIONS OF LOW
DISSOLVED OXYGEN CONDITIONS
Physical (e.g., temperature, stratification or wind- and tide-driven mixing), chemical
(e.g., salinity) and biological (e.g., respiration and photosynthesis) processes can
independently and interactively affect the concentration of dissolved oxygen faster
than new equilibrium can be reached with the atmosphere. As a result, for relatively
short periods of time, or under sustained conditions of reduced physical mixing (i.e.,
stratification of the water column), dissolved oxygen concentrations can be driven
well below saturation. Dissolved oxygen concentrations can decrease to near zero
(anoxia), especially in deep or stratified bodies of water, or to 20 mg liter1 (supersat-
uration) during dense algal blooms in surface waters.
The refined tidal-water designated uses were defined largely on the basis of natural
conditions that divide the Bay and its tidal tributaries into different habitat zones. By
devising Bay dissolved oxygen criteria to protect each designated use habitat, natural
conditions that directly influence dissolved oxygen conditions have been largely
accounted for through this process. In addition, by definition, the biologically-based
reference curves derived for the respective designated uses directly incorporate
allowable criteria exceedances due to natural causes in those habitats. The applica-
tion of the statistical test of significant differences between the curves also addresses
sampling error. Nevertheless, extreme occurrences in the natural processes may
occur and the EPA strongly recommends that managers consider the natural factors
listed below when evaluating criteria attainment.
Temperature and Salinity Effects
The amount of oxygen dissolved in the water changes as a function of temperature,
salinity, atmospheric pressure and biological and chemical processes. The equilib-
rium (or saturated) concentration of dissolved oxygen in natural waters ranges from
about 6 to 14 mg liter1. Seawater at equilibrium at a given temperature contains
substantially less dissolved oxygen than freshwater. The higher the temperature and
salinity, the lower the equilibrium dissolved oxygen concentration. The saturation
concentration for dissolved oxygen decreases with increasing salinity (about -0.05
mg literVpsu1) and increasing temperature (about -0.2 mg literV°C).
An analysis of the degree of saturation given existing temperature and salinity condi-
tions within a designated use habitat can indicate whether these natural conditions
will or are preventing criteria attainment. A spreadsheet analysis tool for conducting
such analyses is described below and available on the Chesapeake Bay Program's
web site at http://www.chesapeakebay.net/tools.
High or Low River Flow Events
Because of its morphology and estuarine circulation, the Chesapeake Bay and some
of its tidal tributaries have a natural tendency to produce reduced dissolved oxygen
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conditions, particularly in deeper waters. The Chesapeake Bay's highly productive
shallow waters, coupled with its tendency to retain, recycle and regenerate nutrients
delivered from the atmosphere and surrounding watershed, create a nutrient-rich
environment. The mainstem Chesapeake Bay and the major tidal rivers flowing off
of shallower, broad shoal waters, along with the significant influx of freshwater
flows, produce a stratified water column that prevents the water at the bottom from
mixing with more highly oxygenated surface waters. The combination of nutrient
retention and recycling and water-column stratification leads to severe reductions in
dissolved oxygen concentrations, usually from June to September.
The timing and extent of hypoxic and anoxic water conditions vary from year to year
because of regional weather patterns, the timing and magnitude of freshwater river
flows, the flow of nutrients and sediments into tidal waters and the corresponding
springtime phytoplankton bloom. The actual freshwater flow is the natural condition
that should be considered in determining attainment. It is important to remember that
under the low-flow conditions between 1950 and 1965, there was far less hypoxia in
the mainstream Chesapeake Bay than there has been in the comparable low-flow
years of the late-1980s to the present. Likewise, historical high-flow years produced
less hypoxia and anoxia than current high-flow years (Hagy 2002). The impact from
extremely high or low river flows can be evaluated by accounting for variations in
the stratification of the water column. Basing the determination of the boundaries
between the open-water, deep-water and deep-channel designated uses on sampling
event calculations of the upper and lower pycnocline depths is the most straightfor-
ward means of addressing the effects of river flow on dissolved oxygen criteria
attainment.
The data required to calculate sampling event-based pycnocline boundary depths can
be found on the Chesapeake Bay Program's web site at http://www.chesapeakebay.
net/data. Analysts are urged to use the Chesapeake Bay Water Quality Monitoring
Program's protocol for calculating the upper and lower boundaries of the pycnocline
(found at http://www.chesapeakebay.net/tools.htm), as this protocol was used to set
the designated use boundaries. (Also see Appendix J in U.S. EPA 2003.) Extensive
data on river flow can be found on the U.S. Geological Survey's Chesapeake Bay
web site at http://chesapeake.usgs.gov.
Upwelling of Hypoxic Water
Nearshore, shallow waters in the Chesapeake Bay periodically experience episodes
of low- to no-dissolved-oxygen conditions that result in part from intrusions of
bottom water forced onto the shallows by sustained winds. Such seiching events are
natural, but a large percentage of the low dissolved oxygen that intrudes into these
shallow habitats is not due to natural causes. Therefore, attaining the deep-water and
deep-channel dissolved oxygen criteria will greatly reduce or even prevent the influx
of oxygen-depleted bottom waters into the shallows.
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These pycnocline seiche events often take place over time scales that are missed by
the monitoring program's sampling frequency. When they have occurred during a
sampling cruise, the seiching events result in a clear tilting of the pycnocline. Such
events often are triggered by sustained winds in a single direction over a period of
several days. To verify that observed tilting of the pycnocline and the resulting
excursion of less than 5 mg liter1 waters into shallow- and open-water designated use
habitats were due to natural seiching events, it is recommended that offshore salinity
with depth profiles and the wind direction and speed data be analyzed.
Extensive salinity with depth profile data are available on the Chesapeake Bay
Program's web site at http://www.chesapeakebay.net/data. For the Chesapeake Bay's
tidal waters, the best sources of information on continuous wind direction and speed
are the Patuxent Naval Air Station, Baltimore-Washington International Airport and
Norfolk International Airport1. Data from these wind monitoring stations can be
accessed through the NOAA National Climatic Data Center at http://ww.ncdc.
noaa.gov.
Natural Diel Fluctuations
Diel cycles of low dissolved oxygen conditions often occur in nonstratified shallow
waters where nightly water-column respiration temporarily depletes dissolved
oxygen levels. The lowest dissolved oxygen readings, generally observed in the early
morning hours from 0.5 to 2 hours after sunrise, are frequently missed by typical
daytime shipboard water quality monitoring, where sampling usually starts in the
morning and continues into the late afternoon. These diel fluctuations are the result
of natural processes such as daily temperature cycles and photoperiod cycles, but
anthropogenic stresses further exaggerate the fluctuations.
The Chesapeake Bay dissolved oxygen criteria were derived to protect aquatic
animals in the defined designated uses during the applicable time frames, regardless
of time of day. It should be noted that daytime measurements of dissolved oxygen
may not fully reflect actual attainment of the criteria over the 24-hour cycle.
To achieve the most protective degree of criteria attainment, the oxygen dynamics of
a particular water body should be characterized using oxygen meters that monitor
semicontinuously. If diel fluctuations in oxygen conditions are found to exist, two
further steps should be taken. The level of oxygen saturation should be analyzed to
confirm that the criteria meet the given natural temperature and salinity conditions.
Users also should build in a determination of diurnal minimum concentrations
through translation or correction of fixed stations using semicontinuous buoy data.
1 A time-series of hour/wind direction and velocity for 1985-1994 for each of these three stations was
developed for use in the Chesapeake Bay water quality model. Wind data was adjusted to account for
over-water conditions by multiplying the east-west component by a factor of 1.0, 1.43 and 1.25 for
BWI, Patuxent and Norfolk, respectively. Likewise, the north-south component was multiplied by
factors of 1.50, 2.05 and 1.25, respectively.
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The Maryland Department of Natural Resources (MD DNR) is developing a method
to temporally standardize dissolved oxygen measurements to a diurnal minimum.
Averaged spring and summer data from MD DNR's continuous monitors indicate
that dissolved oxygen minima are reached at approximately 6:30 a.m., while
dissolved oxygen maxima are achieved at 3:30 p.m. These diurnal fluctuations in
dissolved oxygen produce increasing values during water quality mapping cruises,
where thousands of point samples are collected throughout a tributary over the
course of several hours. In order to produce realistic interpolated surfaces of the
spatially intensive monitoring data, the 'time of day' artifact must be removed from
the dissolved oxygen data. MD DNR has chosen to standardize data to the dissolved
oxygen minimum time of 6:30 a.m. to represent the worst conditions that living
resources might face in the tributary, even though this methodology could just as
easily be applied to other times of the day.
The first step in temporal standardization is to obtain a 15-minute interval average of
continuous monitoring data during a two-week period that encompasses a water
quality mapping cruise. The two-week average is somewhat arbitrary, but helps to
filter out small-scale noise in the dissolved oxygen signal. In MD DNR's case, the
two-week period will be reevaluated in the coming months with additional, concur-
rent continuous and spatial data collected in 2002. A third-order polynomial is fit to
the two-week dissolved oxygen average from 5:30 a.m. (one hour before dissolved
oxygen minimum) to one hour after the completion of the water quality mapping
cruise of interest. The third-order polynomial model is used to back-calculate each
water quality mapping sample to its theoretical 6:30 a.m. value. The standardized
data is then put into geostatistical interpolation models to produce a dissolved
oxygen minimum map.
Methods to incorporate multiple monitors into the standardization process should be
developed. Also, the effect of chlorophyll a concentrations on dissolved oxygen
concentrations should be studied and possibly included in the correction.
Release of Organic Materials from Tidal Wetlands
Tidal wetlands are a valuable component of estuarine systems. They have been
shown as net sinks for sediments (Neubauer et al. 2001) and in most cases also serve
to remove nutrients from overlying water (Anderson et al. 1997). High rates of
organic production, accompanied by high rates of respiration (Neubauer et al. 2000),
can significantly reduce dissolved oxygen and enhance dissolved inorganic carbon
levels both in sediment pore water and overlying water in wetland systems. Another
process that can deplete dissolved oxygen in wetland sediments is nitrification,
which converts ammonium to nitrite and nitrate (Tobias et al. 2001).
Studies of South Carolina estuaries demonstrate that small tidal salt marsh creeks
have significantly lower dissolved oxygen levels than large tidal creeks (Van Dolah
et al., in press). Cai et al. (1999, 2000) determined that a significant export of high
dissolved inorganic carbon from marshes was responsible for the low dissolved
oxygen concentrations observed in five estuaries in South Carolina and Georgia. In
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a series of studies of the York River estuary, Raymond et al. (2000) showed that the
system is supersaturated with respect to carbon dioxide pressure (pC02); conserva-
tive mixing diagrams demonstrated a mid-estuary source of dissolved organic
carbon, which caused respiration to exceed production in the system. Further studies
by Neubauer and Anderson (2003) showed that the export of dissolved inorganic
carbon from tidal freshwater and saltwater marshes could account for approximately
47 percent of the excess dissolved inorganic carbon observed by Raymond et al.
(2000) in the York River estuary.
These effects need to be considered in cases where there is a large wetland-to-water
ratio or high residence times of water in extensive nearby wetlands. The Mattaponi
and Pamunkey rivers, two large tidal tributaries to the York River in Virginia, are the
two best examples of such systems in the Chesapeake Bay region. Computer
simulation modeling may be used to help quantify the impact on dissolved oxygen
criteria attainment.
NATURAL REDUCTIONS IN WATER CLARITY LEVELS
The shallow-water bay grasses designated use excludes those habitats where natural
physical factors (e.g., wave action) will prevent underwater bay grasses from ever
growing. Other natural conditions found in potential and current underwater bay
grass habitats (e.g., resuspension) are addressed using a comparison of ambient data
with a biologically-based reference curve. This reference curve defines the water
clarity criteria exceedances through time and space that can occur without impairing
the underwater bay grass community.
High Flow Events
High river flows resulting from major storms will carry elevated loads of suspended
solids from the upper watersheds and lead to reduced water clarity levels in the
midchannel and shallow-water habitats. According to recent U.S. Geological Survey
studies, most of the sediment that has been delivered to free-flowing stream corri-
dors occurred during land clearance in the 1800s. Much of the sediment mobilized
from stream banks and adjacent flood plains and delivered to the tidal rivers and
mainstem Chesapeake Bay may be these 'legacy' sediments. The U.S. Geological
Survey is conducting research to determine the amount of sediment that is caused by
recent erosion from land sources versus the sediment that is eroded from within the
stream corridors themselves. The latest findings and extensive data on river flows
can be found on the U.S. Geological Survey's Chesapeake Bay web site at
http://chesapeake.usgs.gov.
The influence of high flow events is largely accounted for through the derivation
(and application) of the biologically-based water clarity criteria reference curves.
These reference curves were developed based on almost two decades' worth of
underwater bay grass distributions and water quality data. The mid-1980s to early
2000s data record contains the full array of long-term drought to extreme storm
events (e.g., hurricanes) to sustained, very wet hydrological conditions.
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Wind-Driven Events
Sustained high winds can cause shallow-water sediments to become resuspended
and thus lead to reduced water clarity levels. The U.S. Geological Survey is identi-
fying areas where poor water-clarity conditions are likely to exist due to wind-driven
events. The latest research findings for management application can be found on the
U.S. Geological Survey's Chesapeake Bay web site at http://chesapeake.usgs.gov.
The biologically-based reference curves should account for allowable criteria
exceedances due to such short-term wind-driven events.
Estuarine Turbidity Maximum Zones
The area in the Bay's larger tidal tributaries and the upper Bay mainstem where the
warmer, lighter freshwater flows first mix with saltier, denser water flowing
upstream (originally from the coastal Atlantic Ocean) is called the zone of maximum
turbidity, or estuarine turbidity maximum zone (Lin and Kuo 2001; Sanford et al.
2001). The intersection of these two water masses causes nutrients and sediment to
be naturally mixed and continually resuspended. The general locations of these
zones are illustrated in Figure VII-1, which was mapped using long-term salinity and
total suspended solids records over the past 20 years. The actual location varies from
year to year, depending on the timing and volume of freshwater flows.
The natural effect of the estuarine turbidity maximum zone on water clarity in shallow
habitats has been directly factored into the selection of the Chesapeake Bay water
clarity criteria application depths (see U.S. EPA 2003 for more details). The historical
(1930s to early 1970s) and more recent (1978-2001) record of bay grasses distribu-
tions included the effects of the estuarine turbidity maximum zones located in the
tidal tributaries and the mainstem Chesapeake Bay. The shallow-water bay grass
designated use depth boundaries for Chesapeake Bay Program segments, within
which the estuarine turbidity maximum zones are located, generally have lower water
clarity application depths, reflecting the fact that total suspended solids concentra-
tions would be naturally elevated leading to less water clarity (U.S. EPA 2003).
Natural Water Color
Several tidal tributaries throughout the Chesapeake Bay drain extensive tidal,
wetland-dominated watersheds. The organic materials from those areas tend to color
or stain the water naturally, which reduces water clarity. A background level of water
color was factored into the scientific basis for the Chesapeake Bay water-clarity
criteria and the supporting diagnostic tools (see Batiuk et al. 2000 and Gallegos 2001
for details). However, in tidal-fresh habitats along the lower Eastern Shore where
water color plays a significant role in reducing water clarity, the habitats were
considered underwater bay grass no-growth zones. Since no shallow-water bay grass
designated use applies in these habitats, the water clarity criteria do not apply (see
U.S. EPA 2003 for details).
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T urbidity
< Median (50%)
> Median (50%)
> 90%, salinity < 7 ppt
Figure VI1-1. The estuarine turbidity maximum zone is generally found at the interface
of fresh and salt water. It is illustrated here as the region within each river basin where
mean concentration of total suspended solids is at or above the 90th percentile of con-
centrations measured within that basin in the last decade, i.e., between 1991-2000.
The regions of lesser turbidity are divided into two categories: those with mean con-
centrations less than the median (50th percentile) or greater than the median, but less
than the 90th percentile. 'Hot spots' of relatively high turbidity in downstream meso-
and polyhaline areas are not shown. 'Major' basins are the mainstem Bay (including
Mobjack Bay) and the Chester, Choptank, Nanticoke, Pocomoke, Patuxent, Potomac,
Rappahannock, York and James rivers. In some of these river basins, the turbidity
maximum is too far upriver to be clearly displayed on this map.
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NATURAL ELEVATED CHLOROPHYLLS CONCENTRATIONS
Many of the factors influencing chlorophyll a concentrations are related to physical
processes affecting the residence time of a water mass in a tidal river, creek or
embayment, and light penetration due to channel morphology or physical mixing. In
regions or specific tidal-water habitats where these listed physical processes lead to
chlorophyll ^-related impairments, states should derive local scale numerical chloro-
phyll a criteria directly addressing these natural conditions.
High Residence Time and Reduced Flushing Rates
In many small tidal rivers, the reduced flushing of more confined open-water habitats
often leads to elevated chlorophyll a concentrations, given that phytoplankton popula-
tions are exposed to nutrient-enriched conditions for longer periods. Nutrient loadings
that would not otherwise lead to increased chlorophyll a concentrations in well-flushed
tidal open-water habitats generate bloom conditions in these smaller systems.
There has been relatively little analysis of the appropriateness and attainability of
specific chlorophyll a values in poorly flushed tidal systems. For example, most of the
analyses performed in support of generating chlorophyll a target concentrations have
focused on well-flushed open-water systems (see Chapter V). Natural elevations of
chlorophyll a should be considered when setting designated use boundaries and when
setting specific numeric targets and criteria for addressing regional and local algal-
related impairments.
Through the development and application of biologically-based reference curves, the
numerical chlorophyll a criteria attainment methodology can factor in the spatial
extent of criteria attainment or nonattainment. This allows for limited spatial extent
with elevated chlorophyll a concentrations and larger spatial areas with lower, yet
nonattaining, chlorophyll a concentrations. If a Chesapeake Bay Program segment
contains a very high portion of tidal habitats with high residence times, more
detailed analyses of the relative contribution of naturally reduced flushing rates
versus excessive anthropogenic nutrient loadings should be undertaken.
Channel Morphology
Tidal rivers and creeks with shallow and wide channels (versus narrower and deep
channels) will tend to have higher chlorophyll a concentrations, given the greater
volume of the photic zone relative to the total channel volume. In addition, the
shallow and wide channels tend to be less well-flushed, allowing greater accumula-
tion of phytoplankton and chlorophyll a.
Natural Algal Blooms Independent of Nutrient Conditions
Although anthropogenic nutrient loading is a principal factor in the overall primary
productivity of the Chesapeake Bay system, its relationship to blooms of specific
taxa is not well understood. Such blooms have been observed to occur in the absence
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of elevated nutrient conditions as a result of a complex set of physical, chemical and
biological stimuli. Species composition data from the Chesapeake Bay Phyto-
plankton Monitoring Program should be consulted to determine if the observed algal
bloom conditions are due principally to species that fall within this category. These
phytoplankton monitoring data can be accessed through the Chesapeake Bay
Program website at http://www.chesapeakebay.net/data.
DIAGNOSING CAUSES OF CRITERIA NONATTAINMENT
DISSOLVED OXYGEN CRITERIA
Percent Saturation
An analysis of the degree of saturation given existing temperature and salinity condi-
tions within a designated use habitat can be performed by applying the following
equation. For temperature in degrees Celsius and salinity in mg liter1:
dissolved oxygen saturation = 14.6244 - 0.367134(Temp°C) + 0.0044972
(Temp°C)2 - 0.0966(salinity) + 0.00205 (salinity) (Temp°C) + 0.0002739
(salinity)2.
A spreadsheet version of this diagnostic analysis tool is available on the Chesapeake
Bay Program's web site at http://www.chesapeakebay.net/tools.htm.
Chesapeake Bay Water Quality Model
As explained in Chapter VI, the Chesapeake Bay water quality model is linked to the
Chesapeake Bay hydrodynamic model and uses complex nonlinear equations
describing 26 state variables relevant to the simulation of dissolved oxygen, chloro-
phyll a and water clarity. Dissolved oxygen is simulated as the mass balance
calculation of reaeration at the surface; respiration of algae, benthos and underwater
bay grasses; photosynthesis of algae, benthic algae and underwater bay grasses; and
the diagenesis, or decay of organics, by microbial processes in the water column and
bottom sediments. This mass balance calculation is made for each model cell and for
associated bottom sediment cells at each hourly time step. Estimates of dissolved
oxygen from nutrient loads from the watershed and airshed are simulated in the tidal
waters of the 35 major segments of the Chesapeake Bay and its tidal tributaries. This
state-of-the-science modeling tool is available to management agencies and others to
help diagnose the reasons behind nonattainment of the Chesapeake Bay dissolved
oxygen criteria.
For the dissolved oxygen criteria, the daily output of dissolved oxygen concentration
for 10 years (1985-1994) for the 13,000 cells provides a detailed estimate of the
transport and transformation of nutrients and organic matter that ultimately consume
oxygen in the waters of the Chesapeake Bay and its tidal tributaries. Influential
aspects, such as the limiting nutrient, seasonal changes in dissolved oxygen, changes
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in the nutrient flux of bottom sediments that change with bottom-water oxygen
levels, and other temporal and spatial aspects of dissolved oxygen concentrations
and dynamics, can be diagnosed by evaluating water quality model output to gain
insights into the reasons behind nonattainment of the dissolved oxygen criteria.
WATER CLARITY CRITERIA
In Chesapeake Bay Submerged Aquatic Vegetation Water Quality and Habitat-Based
Requirements and Restoration Targets: A Second Technical Synthesis, a set of diag-
nostic tools were developed not only to better interpret the relative degree of
achievement of the Bay water clarity criteria, but also to understand the relative
contributions of different water quality parameters to overall light attenuation
(Batiuk et al. 2000). Two management-oriented diagnostic tools have been devel-
oped. The water-column diagnostic tool quantifies the relative contributions to total
light attenuation in the water column that is attributable to light absorption and scat-
tering by total suspended solids and chlorophyll a. The leaf surface attenuation
diagnostic tool further quantifies the light attenuation at the leaf surface attributable
to epiphytes and total suspended solids settled out on the leaf surface. Both diag-
nostic tools are available as spreadsheet-based application tools and can be accessed
through the Chesapeake Bay Program's web site at http://www.chesapeakebay.net
/tools.htm.
Water-Column Light Attenuation Diagnostic Tool
Water-column attenuation of light measured by the light attenuation coefficient, Kd,
can be divided into contributions from four sources: water, dissolved organic matter,
chlorophyll a and total suspended solids. The basic relationships can be expressed in
a series of simple equations, which were combined to produce the equation for the
water-column diagnostic tool (Gallegos 2001). The resulting equation calculates
linear combinations of chlorophyll a and total suspended solids concentrations that
just meet the percent light-through-water (PLW) criteria value for a particular depth
at any site or season in the Chesapeake Bay and its tidal tributaries. This diagnostic
tool can also be used to consider various management options for improving water
quality conditions when the water clarity criteria are not currently met.
Generation of Management Options. The water-column diagnostic tool
spreadsheet program calculates median water quality concentrations and evaluates
them in relation to PLW criteria for growth to 0.5-, 1- and 2-meter restoration depths.
Provisions are included for specifying a value for PLW criteria appropriate for
mesohaline and polyhaline regions (22 percent) or for tidal-fresh and oligohaline
areas (13 percent). When the observed median chlorophyll a and total suspended
solids concentrations do not meet the PLW criteria, up to four target chlorophyll a
and total suspended solids concentrations that do meet the PLW criteria are calcu-
lated based on four different management options (Figure VII-2). Under some
chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment
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A. Projection to Origin
B. Normal Projection
Median
conditions
Median
conditions
Habitat
requirement
Target
Chlorophyll a
Chlorophyll a
C. TSS Reduction Only
D. Chlorophyll a Reduction Only
Median
conditions
Median
conditions
CO
I
E
3
C/D
cn
CO
I
£
S
co
w
Chlorophyll a Chlorophyll a
Figure VII-2. Illustration of management options for determining target concentrations
of chlorophyll a and total suspended solids. It illustrates the use of the diagnostic tool to
calculate target growing-season median concentrations of total suspended solids (TSS)
and chlorophyll a for restoration of underwater bay grasses to a given depth. Target
concentrations are calculated as the intersection of the percent light-through-water
criteria line, with a line describing the reduction of median chlorophyll a and TSS concen-
trations calculated by one of four strategies: (A) projection to the origin (i.e., chlorophyll
a=0, TSS=0); (B) normal projection, i.e., perpendicular to the percent light-through-water
requirement; (C) reduction in total suspended solids only; and (D) reduction in chlorophyll
a only. A strategy is not available (N/A) whenever the projection would result in a 'nega-
tive concentration.' In (D), reduction in chlorophyll a also reduces TSS due to the dry
weight of chlorophyll a, and therefore moves the median parallel to the line (long dashes)
for ChIVS, which describes the minimum contribution of chlorophyll a to TSS.
chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment
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conditions, some of the management options are not available because a 'negative'
chlorophyll a or total suspended solids concentration would be calculated.
Option 1 is based on projections from existing median conditions to the origin
(Figure VII-2a). This option calculates target chlorophyll a and total suspended
solids concentrations as the intersection of the PLW criteria line with the line
connecting the existing median concentration and the origin, i.e., chlorophyll a = 0,
TSS = 0. Option 1 always results in positive concentrations of both chlorophyll a and
total suspended solids.
Option 2 is based on normal projections (Figure VII-2b). It calculates target chloro-
phyll a and total suspended solids concentrations as the projection from existing
median conditions perpendicular to the PLW criteria. Geometrically, Option 2
requires the least overall reductions in chlorophyll a and total suspended solids
concentrations. In practice, target chlorophyll a and total suspended solids concen-
trations for the normal projection, when permissible (i.e., no negative concentrations
are calculated), are frequently very similar to those calculated in Option 1 using
projection to the origin.
Option 3 is based on a total suspended solids reduction only (Figure VII-2c). This
option calculates target chlorophyll a and total suspended solids concentrations,
assuming the target can be met only by reducing the concentration of total suspended
solids. Option 3 is not available whenever the median chlorophyll a exceeds the total
suspended solids = 0 intercept. When a system is nutrient-saturated and light-limited,
a reduction of total suspended solids alone poses the risk of relieving light limitation
and promoting farther phytoplankton growth. Such a tendency is indicated on the
diagnostic tool plot whenever data points tend to align parallel to the PLW criteria
lines (Figure VII-2c).
Option 4 is based on a chlorophyll a reduction only. This option calculates target
chlorophyll a and total suspended solids concentrations, assuming that the target can
be met only by reducing the concentration of chlorophyll a (Figure VII-2d). Due to
the suspended solids removed by reduction of phytoplankton and associated carbon,
i.e., ChlV, the target total suspended solids concentration reported for Option 4 is
actually lower than the existing median. Option 4 is not available whenever the
median total suspended solids concentration exceeds the chlorophyll a = 0 intercept
of the PLW criteria line.
The precision of the calculations implies a degree of control over water quality
conditions that clearly is not always attainable. Nevertheless, reporting of four
potential targets provides managers with an overall view of the magnitude of the
necessary reductions and some of the available tradeoffs. Furthermore, the spread-
sheet reports the frequency with which the PLW criteria for each restoration depth
are not achieved by the individual measurements.
Evaluating Management Options. Option 1 will likely be the most useful for
generating target concentrations because it always results in the calculation of posi-
tive concentrations. Also, most efforts to control loadings involve a reduction of total
chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment
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214
runoff, which reduces both suspended solids and nutrients. Under certain conditions
managers may choose to apply Option 3, when data plots indicate that attenuation is
dominated by flood-borne or resuspended sediments (Figure VII-3a). Similarly
Option 4 may be useful when diagnostic plots indicate that light attenuation is domi-
nated by algal blooms (Figure VII-3b). For details on how best to evaluate the four
possible management options, refer to Chesapeake Bay Submerged Aquatic Vegeta-
tion Water Quality and Habitat-Based Requirements and Restoration Targets: A
Second Technical Synthesis (Batiuk et al. 2000, pp. 47-49).
Leaf Surface Light Attenuation Diagnostic Tool
Building from the diagnosis and quantification of water-column contributions to
attenuation of light, a second diagnostic tool focuses on how changes in water
quality variables alter the light available to underwater plant leaves and considers
effects of light attenuation resulting from substances both in the overlying water
column (phytoplankton, suspended particles and dissolved organics) and attached to
underwater bay grass leaves (epiphytic algae, organic detritus and inorganic parti-
cles). A simple model was developed to calculate photosynthetically available
radiation (PAR) at the leaf surface for plants growing at a given restoration depth (Z)
under specific water quality conditions. The computed value for PAR at the plant
leaves is compared to the applicable Bay water clarity criteria.
The overall objective is to apply this model using water quality monitoring data to
estimate growing season mean light levels at bay grass leaves for a particular site or
geographic region. The calculated light levels at bay grass leaves are then compared
to the applicable light-at-the-leaf water clarity requirement to assess whether water
quality conditions are suitable to support the survival and growth of underwater bay
grasses. The relative contributions of water-column versus epiphytic substances in
attenuating incident light to underwater bay grass leaves also are computed.
The scientific basis of this model is described in detail in Batiuk et al. (2000) and
Kemp et al. (in review).
Generating Diagnostics. To compute median PAR at the bay grass leaf surface,
the diagnostic spreadsheet model requires underwater bay grass growing season
medians for four water quality variables: 1) dissolved inorganic nitrogen (nitrate +
nitrite + ammonia), or DIN; 2) dissolved inorganic phosphorus (primarily phosphate),
or DIP; 3) total suspended solids (TSS); and 4) diffuse downwelling PAR attenuation
coefficient (Kd). Values for Kd are either obtained from direct measurements of
decrease in PAR with water depth using a cosine-corrected sensor, or calculated from
observations on the depth at which a Secchi disk disappears (see Chapter III in Batiuk
et al. 2000 for the details on the recommended Secchi depth/Kd conversion of Kd =
1.45/Secchi depth). The restoration depth is defined by the Chesapeake Bay Program
segment-specific shallow-water designated use outer depth boundary (U.S. EPA
2003). Figure VII-4 and Table VII-1 lays out the steps for running the spreadsheet
model, the data required, and the scientific basis for the calculation.
chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment
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A Upper Bay, CB2.2
90
80
70
60
CD
co 40
f— 30
20
T3
i r
,r 03
0 10 20 30 40 50 60 70 80 90
Chlorophyll a (mg m "3)
B. Baltimore Harbor, MWT5.1
5
0 30 60 90 120 150 180
Chlorophyll a (mg m 3)
Measured
*
Median
•• 0.5 m H. R.
— 1 m H. R.
• - 2 m H. R.
- ChIVS
C. Lower mesohaline,
15
12
9
6
3
0
0
30
60
Chlorophyll a (mg m ~3)
Figure VII-3. Application of the water column light attenuation diagnostic tool to two mainstem Chesapeake
Bay stations and one tidal tributary station, which demonstrates three primary modes of variation in the data:
(A) variation in diffuse attenuation coefficients is dominated by (flow-related) changes in concentrations of
total suspended solids (TSS) (upper Chesapeake Bay station, CB2.2); (B) variations in attenuation coefficients is
dominated by changes in chlorophyll a concentration (Baltimore Harbor, MWT 5.1); and (C) maximum
chlorophyll a concentration varies inversely with TSS, indicating light-limited phytoplankton (lower middle
Chesapeake Bay, CB5.2). Plots show individual measurements (points) and growing season median (asterisk) in
relation to the percent light-through-water (PLW) criteria for restoration to depths of 0.5m (short dashes), 1m
(solid line) and 2m (dotted line); and PLW calculated by equations IV-1 and J-1 (see Chapter IV and Appendix J)
Note the change in scale. Approximate minimum contribution of chlorophyll a to TSS (ChIVS) is calculated by
Equation IV-11 (long dashes) in Batiuk et al. 2000. The data is from the Chesapeake Bay Water Quality
Monitoring Program, April through October, 1986-1996.
Sources: Batiuk et al. 2.000; Gallegos 2001.
chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment
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Percent Light-through-Water (PLW)
Inputs
Percent Light-at the-Leaf (PLL)
Kd measured directly
Secchi depth
Calculation
PLW=100exp(-KdZ)
100% Ambient Light of Water Surface
ywatef
Color
V
V**'; v'.;'.."i'/Total Suspended
\ / Solids
V; /
¦f
/Epiphyte ***>¦^
j Attenuation
Algae
-ELW
.PJJ-
Inputs
Kd
Total suspended solids
Dissolved inorganic nitrogen
Dissolved inorganic phosphorus
Calculation
Evaluation
Calculated PLW vs. PLW criteria
PLL=100[exp(-KdZ)][exp(-K,B,)]
*Ke = Epiphyte attenuation
*Be = Epiphyte biomass
Evaluation
PLL vs. PLL Diagnostic Requirement
Figure VII-4. Illustration of percent light-at-the-leaf (PLL) and percent light-through-water (PLW) calculation
comparisons for underwater bay grasses in the Chesapeake Bay.
Source: Batiuk et al. 2000
Evaluating Diagnostic Outputs. To examine the components of light attenua-
tion, as determined by the spreadsheet percent light-at-the-leaf (PLL) calculator,
several fields in addition to PLL are shown. This permits insight into the contribu-
tion to light total attenuation from the water column, leaf surface epiphytes and leaf
surface total suspended solids (TSS). The additional fields are:
• PLW—percent-light-through-water. Comparing PLL to PLW gives an indi-
cation of the contribution of leaf surface light attenuation to the total
attenuation.
• PLLnoTSS—PLL calculated without TSS light attenuation. Indicates the
relative importance of epiphytes and TSS.
• %EpiAtten. Refers to the percentage of the light attenuation on the leaf
surface that is due to the growth of epiphytes.
• %LeafTSSAtten. Refers to the percentage of the light attenuation on the leaf
surface that is due to deposited TSS.
• Requirement. Indicates whether the calculated PLL meets or fails the PLL
diagnostic minimum light requirement. Assessment takes into account the
salinity regime of the station.
chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment
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Table VI1-1. Summary of the approach to estimate photosynthetically available radiation
at the leaf surface of underwater bay grasses using water quality data routinely monitored
in the Chesapeake Bay.
Step in Model Calculation Input Source of Model
Functional Relation Data Relationship Units
1) Decide limiting nutrient DIN, DIP Fisher et al. 1992 //M
DIN/DIP > 16 , use DIP
DIN/DIP < 16 , use DIN
2) Derive general equation
to calculate epiphyte biomass
Be = (Be)m [1 + 208 (DINKN(0D))]"1
• (Be)m = maximum Be value
• KN(0D) = characteristic coeff.
3) Calculate PAR effect on KN(0D) K,|, Z Numerical model K,|, m_|
and (Be)m (Madden and Kemp 1996) Z, m
(Be)m = 2.2 - [0.251 (OD123)]
• OD = Optical Depth = (K^HZ)
KN(0D) = 2.32 (1 - 0.031 OD142)
DIN, DIP Numerical model Be, gCgC"1
(Madden and Kemp 1996) DIN, //M
Kniod)- none
4) Calculate epiphyte dry weight TSS
Bde = 0.107 TSS + 0.832 Be Be
5) Calculate epiphyte biomass- Be,
specific PAR attenuation coeff. Bde
Ke = 0.07 + 0.32 (Be/Bde)-°88
Regression from
experimental data
(e.g., Staver 1984)
Regression from
experimental and
field data
TSS, mg 1_!
Be, mg chl gdw-1
Bde, gdw gdw"1
Be, jug chl cm 2
Bde, mg dw cm 2
Kg, cm2 jug chl1
6) Calculate PAR at SAV leaves (Ize)
Ize/I0 = [expf-KdZ)] [exp(-KeBe)]
DIN, DIP, Combining steps 1-5
K,|, TSS, Z (from above)
DIN, //M
DIP, //M
TSS, mg l1
Kd, m1
7) Compare SAV leaf PAR with
Light-at-the-Leaf Requirement
fze^o
See Chapter VII
in Batiuk et al. 2000
Note that units used for specific variables change at different steps in calculation, but are consistent with
conventions of data and model sources.
Source: Batiuk et al. 2000.
chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment
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218
Chesapeake Bay Water Quality Model
Outputs from the Chesapeake Bay water quality model include quantification of the
various components of light attenuation from sediment, algae or color. Further evalua-
tion of the relative contributions of these various components of light attenuation can
provide insights into the reasons behind nonattainment of the water clarity criteria.
CHLOROPHYLLS CRITERIA
Chesapeake Bay Water Quality Model
The Chesapeake Bay community also has access to water-quality models that repre-
sent excellent tools for diagnosing the causes for nonattainment of the chlorophyll a
criteria. Time and space aspects of the criteria and the understanding of the funda-
mental behavior and significant influences on chlorophyll a in the Chesapeake Bay
designated use habitats is based primarily on resource limitation of algae. Resource
limitation on the growth of algae include nitrogen and phosphorus limitation, light
limitation and, for diatoms, limitation of silica. Interactions of the chlorophyll a and
water clarity criteria include algal self-shading and light attenuation due to sediment
or the color imparted to natural waters due to dissolved organic material. Through
the Chesapeake Bay water quality model, the total fate and transformation of algae
based on the Monod structure of temperature corrected algal growth operating on a
hourly time step can be evaluated. Diagnostics of chlorophyll a criteria nonattain-
ment that can be examined through model outputs include nitrogen and phosphorus
limitation, light limitation and, for diatoms, limitation of silica. See the Water Clarity
section above for diagnostics related to factors limiting light.
LITERATURE CITED
Anderson, I. C., C. R. Tobias, B. B. Neikirk and R. L. Wetzel. 1997. Development of a
process-based mass balance model for a Virginia Spartina altemiflora salt marsh: Implica-
tions for net DIN flux. Marine Ecology Progress Series 159:13-27.
Batiuk, R. A., P. Bergstrom, M. Kemp, E. Koch, L. Murray, J. C. Stevenson, R. Bartleson, V.
Carter, N. B. Rybicki, J. M. Landwehr, C. Gallegos, L. Karrh, M. Naylor, D. Wilcox, K. A.
Moore, S. Ailstock and M. Teichberg. 2000. Chesapeake Bay Submerged Aquatic Vegetation
Water Quality and Habitat-based Requirements and Restoration Targets: A Second Technical
Synthesis. CBP/TRS 245/00 EPA 903-R-00-014. U. S. EPA Chesapeake Bay Program,
Annapolis, Maryland.
Cai, W. J., W. J. Weibe, Y. Wang and J. E. Sheldon. 2000. Intertidal marsh as a source of
dissolved inorganic carbon and a sink of nitrate in the Satilla River-estuarine complex in the
southeastern U. S. Limnology and Oceanography 45:1743-1752.
Cai, W. J., L. R. Pomeroy, M. A. Moran and Y. Wang. 1999. Oxygen and carbon dioxide mass
balance for the estuarine-intertidal marsh complex of five rivers in the southeastern U. S.
Limnology and Oceanography 44:639-649.
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219
Fisher, T. R., E. R. Peele, J. W. Ammerman and L. W. Harding, Jr. 1992. Nutrient limitation
of phytoplankton in Chesapeake Bay. Marine Ecology Progress Series 82: 51-63.
Gallegos, C. L. 2001. Calculating optical water quality targets to restore and protect
submersed aquatic vegetation: Overcoming problems in partitioning the diffuse attenuation
coefficient for photosynthetically active radiation. Estuaries 24:381-397.
Hagy, J. D. 2002. Eutrophication, hypoxia and trophic transfer efficiency in Chesapeake Bay.
Ph.D. dissertation, University of Maryland, College Park, Maryland.
Kemp, W. M., R. Batiuk, R. Bartleson, P. Bergstrom, V. Carter, C. L. Gallegos, W. Hunley, L.
Karrh, E. W. Koch, J. M. Landwehr, K. A. Moore, L. Murray, M. Naylor, N. B. Rybicki, C.
Stevenson and D. J. Wilcox. In review. Habitat requirements for submerged aquatic vegetation
in Chesapeake Bay: Water quality, light regime and physical-chemical factors. Estuaries.
Lin, J. and A. Y. Kuo. 2001. Secondary turbidity maximum in a partially mixed microtidal
estuary. Estuaries 24:707-720.
Madden, C. J. and W. M. Kemp. 1996. Ecosystem model of an estuarine submersed plant
community: Calibration and simulation of eutrophication responses. Estuaries 19(2B): 457-
474.
Neubauer, S. C. and Anderson, I. C. 2003. Transport of dissolved inorganic carbon from a
tidal freshwater marsh to the York and Pamunkey river estuary. Limnology and Oceanog-
raphy 48:299-307.
Neubauer, S. C., I. C. Anderson, J. A. Constantine and S. A. Kuehl. 2001. Sediment deposi-
tion and accretion in a mid-Atlantic (U.S.A.) tidal freshwater marsh. Estuarine Coastal and
Shelf Science.
Neubauer, S. C., W. D. Miller and I. C. Anderson, I. C. 2000. Carbon cycling in a tidal fresh-
water marsh ecosystem: A gas flux study. Marine Ecology Progress Series 199:13-30.
Raymond, P.A., J. E. Bauer and J. J. Cole. 2000. Atmospheric C02 evasion, dissolved inor-
ganic carbon production and net heterotrophy in the York River estuary. Limnology and
Oceanography 45:1701-1717.
Sanford, L. P., S. E. Suttles and J. P. Halka. 2001. Reconsidering the physics of the Chesa-
peake Bay estuarine turbidity maximum. Estuaries 24:655-669.
Staver, K. 1984. Responses of epiphytic algae to nitrogen and phosphorus enrichment and
effects on productivity of the host plant, Potamogeton perfoliatus L., in estuarine waters.
Master's thesis, University of Maryland, College Park, Maryland.
Tobias, C. R, I. C. Anderson, E. A. Canuel and S. A. Macko. 2001. Nitrogen cycling through
a fringing marsh-aquifer ecotone. Marine Ecology Progress Series 210:25-39.
U.S. Environmental Protection Agency. 2003. Technical Support Document for the Identifi-
cation of Chesapeake Bay Designated Use and Attainability. EPA 903-R-03004. Chesapeake
Bay Program Office, Annapolis, Maryland.
Van Dolah, R. F., D. E. Chestnut, J. D. Jones, P. C. Jutte, G. Reikirk, M. Levinson and W.
McDermott. In press. The Importance of Considering Spatial Attributes in Evaluating Estu-
arine Habitat Conditions: The South Carolina Experience.
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append ix^
Refined Designated Uses for the
Chesapeake Bay and Tidal Tributaries
BACKGROUND
Federal water quality standards regulations establish that states must specify appro-
priate water uses to be achieved and protected. Current designated uses applied to
the waters of the Chesapeake Bay and its tidal tributaries do not fully reflect natural
conditions and are too broad in their definition of 'use' to support the adoption of
more habitat-specific aquatic life criteria. Furthermore, they change across jurisdic-
tional borders in the same body of water.
Under the federal water quality standards regulation, states may adopt subcategories
of uses, seasonal uses and may remove uses under certain conditions (including
natural, physical and socio-economic conditions). If a state wishes to remove or
establish a subcategory of a designated use that requires a less stringent water quality
criteria, the state must conduct a use-attainability study. States must also demon-
strate that all water uses present on or after November 28, 1975, will always be
protected.
The Chesapeake 2000 agreement and the subsequent six state, District of Columbia
and EPA memoranda of understanding challenged the Bay watershed jurisdictions
to, "by 2010, correct the nutrient- and sediment-related problems in the Chesapeake
Bay and its tidal tributaries sufficiently to remove the Bay and the tidal portions of
its tributaries from the list of impaired waters under the Clean Water Act" (Chesa-
peake Executive Council 2000; Chesapeake Bay Watershed Partners 2001).
These agreements included commitments to "define the water quality conditions
necessary to protect aquatic living resources" by 2001 and to have the jurisdictions
with tidal waters "use their best efforts to adopt new or revised water quality stan-
dards consistent with the defined water quality conditions" by 2003. Against this
backdrop of a renewed commitment to restore Bay water quality (in part through the
adoption of a consistent set of Chesapeake Bay water quality criteria as state stan-
dards), it was recommended that the underlying tidal-water designated uses must be
refined to better reflect desired Bay water quality conditions.
appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries
-------
A-2
In considering the refinement of the tidal-water designated uses, the six Bay water-
shed states and the District of Columbia should take into account five principal
considerations:
• Habitats used in common by sets of species and during particular life stages
should be delineated as separate designated uses;
• Natural variations in water quality should be accounted for by the designated uses;
• Seasonal uses of different habitats should be factored into the designated uses;
• The Chesapeake Bay criteria for dissolved oxygen, water clarity and chloro-
phyll a should be tailored to support each designated use; and
• The refined designated uses applied to the Chesapeake Bay and its tidal tribu-
taries will support the federal Clean Water Act and state goals for uses existing
in these water since 1975 and for potential uses not currently met.
The five proposed designated uses were derived to reflect the habitats of an array of
recreationally, commercially and ecologically important species. The supporting prey
communities were given fall consideration along with the 'target species' in defining
the designated uses. The Chesapeake Bay criteria were based on effects data from a
wide array of species and biological communities to capture the range of sensitivities
of the thousands of aquatic species inhabiting the Chesapeake Bay and tidal tributary
estuarine habitats. As the U.S. Environmental Protection Agency (2003a) documents
extensively, the only species formally listed as threatened or endangered that would
be affected by the Chesapeake Bay criteria was the shortnose sturgeon. Low dissolved
oxygen effects data for shortnose sturgeon were part of the larger scientific data base
used to derive the Chesapeake Bay dissolved oxygen criteria.
This appendix broadly describes the five designated uses and the general boundaries
between the migratory fish spawning and nursery; shallow-water bay grass; open-
water fish and shellfish; deep-water seasonal fish and shellfish; and deep-channel
seasonal refage designated use habitats (Table A-l). Figure 1 in the Executive
Summary illustrates the conceptual framework of the refined tidal-water designated
uses. More detailed descriptions of and documentation on the five designated uses
are published in the Technical Support Document for the Identification of Chesa-
peake Bay Designated Uses and Attainability (U.S. EPA 2003b).
CHESAPEAKE BAY TIDAL-WATER DESIGNATED USES
The following descriptions of designated uses provide the context for deriving
dissolved oxygen, water clarity and chlorophyll a water quality criteria for the
Chesapeake Bay provided in this Regional Criteria Guidance. Correct application of
water quality criteria depends on the accurate delineation of these designated uses.
For example, each of these designated uses have distinct dissolved oxygen criteria
derived to match the respective level of protection required.
appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries
-------
A-3
Table A-1. General descriptions of the five proposed Chesapeake Bay tidal-water designated uses.
Migratory Fish Spawning and Nursery Designated Use: Aims to protect migratory finfish
during the late winter/spring spawning and nursery season in tidal freshwater to low-salinity
habitats. This habitat zone is primarily found in the upper reaches of many Bay tidal rivers and
creeks and the upper mainstem Chesapeake Bay and will benefit several species including
striped bass, perch, shad, herring and sturgeon.
Shallow-Water Designated Use: Designed to protect underwater bay grasses and the many
fish and crab species that depend on the shallow-water habitat provided by grass beds.
Open-Water Fish and Shellfish Designated Use: Designed to protect water quality in the
surface water habitats within tidal creeks, rivers, embayments and the mainstem Chesapeake
Bay year-round. This use aims to protect diverse populations of sportfish, including striped
bass, bluefish, mackerel and seatrout, bait fish such as menhaden and silversides, as well as the
listed shortnose sturgeon.
Deep-Water Seasonal Fish and Shellfish Designated Use: Aims to protect living resources
inhabiting the deeper transitional water column and bottom habitats between the well-mixed
surface waters and the very deep channels during the summer months. This use protects many
bottom-feeding fish, crabs and oysters, as well as other important species, including the bay
anchovy.
Deep-Channel Seasonal Refuge Designated Use: Designed to protect bottom sediment-
dwelling worms and small clams that act as food for bottom-feeding fish and crabs in the very
deep channel in summer. The deep-channel designated use recognizes that low dissolved
oxygen conditions prevail in the deepest portions of this habitat zone and will naturally have
very low to no oxygen during the summer.
The watershed states with tidally influenced Chesapeake Bay waters—Maryland,
Virginia, Delaware and the District of Columbia—have the ultimate responsibility
for defining and adopting the designated uses into their state water quality standards.
These uses will be adopted as subcategories of current state tidal-water designated
uses, which are designed to protect aquatic life. The formal process for refining
designated uses will meet the requirements of the Clean Water Act. The adopted
designated uses will protect existing aquatic and human uses of the tidal waters that
have been present since 1975, as well as potential uses. The specific use definitions
and the spatial application of the final designated uses will undergo public review
and the four jurisdictions' respective regulatory adoption processes prior to final
approval by EPA.
MIGRATORY FISH SPAWNING AND NURSERY DESIGNATED USE
Waters with this designated use support the survival, growth and propagation of
balanced indigenous populations of ecologically, recreationally and commercially
appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries
-------
A-4
important anadromous, semi-anadromous and tidal-fresh resident fish species inhab-
iting spawning and nursery grounds from February 1 through May 31.
Chesapeake Bay tidal waters support spawning areas and juvenile nurseries for a
host of anadromous and semi-anadromous fish, important not only to Chesapeake
Bay fishery populations, but also to those of the entire East Coast, such as striped
bass. The eggs, larvae and early juveniles of anadromous and semi-anadromous
species often have more sensitive habitat quality requirements than other species and
life stages (Funderburk et al. 1991; Jordan et al. 1992). Thus, the combined migra-
tory spawning and nursery habitats were delineated as a refined tidal-water
designated use for the Chesapeake Bay and its tidal tributaries.
Designated Use Boundary Delineation
The boundaries of the migratory fish spawning and nursery designated use are
broadly delineated from the upriver extent of tidally influenced waters to the down-
river and lower Bay spawning and nursery habitats that have been determined
through a composite of all targeted anadromous and semi-anadromous fish species'
spawning and nursery habitats. Free-flowing streams and rivers, where several of the
target species (such as shad and river herring) migrate for spawning, are protected
through other existing state water quality standards.
From February 1 through May 31, the migratory fish spawning and nursery desig-
nated use coincides with and, therefore, encompasses portions of the shallow-water
bay grass and open- water fish and shellfish designated use habitats. Therefore, the
horizontal and vertical delineations for the migratory fish spawning and nursery
designated use are the same as those of the open-water fish and shellfish designated
uses. For those areas designated for migratory spawning and nursery uses, the desig-
nated use extends horizontally from the intertidal zone (mean low water) across the
body of water to the adjacent intertidal zone, and down through the water column to
the bottom sediment-water interface.
SHALLOW-WATER BAY GRASS DESIGNATED USE
Waters with this designated use support the survival, growth and propagation of
rooted, underwater bay grasses necessary for the propagation and growth of
balanced, indigenous populations of ecologically, recreationally and commercially
important fish and shellfish inhabiting vegetated shallow-water habitats.
Designated Use Rationale
The shallow-water bay grass designated use protects a wide variety of species, such
as largemouth bass and pickerel, which inhabit vegetated tidal-fresh and low-salinity
habitats; juvenile speckled sea trout in vegetated higher salinity areas; and blue crabs
that inhabit vegetated shallow-water habitats covering the full range of salinities
encountered in the Chesapeake Bay and its tidal tributaries. Underwater bay grasses,
appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries
-------
A-5
the critical community that the designated use protects, provide the shelter and food
that make shallow-water habitats so unique and integral to the productivity of the
Chesapeake Bay ecosystem. Many Chesapeake Bay species depend on vegetated
shallow-water habitats at some point during their life cycle (Funderburk et al. 1991).
Given the unique nature of this habitat and its critical importance to the Chesapeake
Bay ecosystem, shallow waters were delineated as a refined tidal-water designated
use for the Chesapeake Bay and its tidal tributaries.
The shallow-water bay grass designated use is intended specifically to delineate the
habitats where the water clarity criteria would apply. The open-water fish and shellfish
designated use and the accompanying dissolved oxygen criteria will fully protect the
biological communities inhabiting shallow-water habitats. The open-water fish and
shellfish designated use extends into the intertidal zone and protects shallow-water
organisms that do not depend on bay grasses. The seasonal shallow-water bay grass
designated use, similar to the migratory fish spawning and nursery use, actually coin-
cides with the year-round open-water designated use and provides specific protection
for underwater bay grasses through the application of water clarity criteria.
Designated Use Boundary Delineation
The shallow-water bay grass designated use covers tidally influenced waters from
the intertidal zone to a Chesapeake Bay Program segment-specific depth contour
from 0.5 to 2 meters. The shallow-water designated use applies during the bay grass
growing season: April 1 through October 31 for tidal-fresh, oligohaline and mesoha-
line segments, and March 1 through May 31 and September 1 through November 30
for polyhaline segments.
OPEN-WATER FISH AND SHELLFISH DESIGNATED USE
Waters with this designated use support the survival, growth and propagation of
balanced, indigenous populations of ecologically, recreationally and commercially
important fish and shellfish species inhabiting open-water habitats.
Designated Use Rationale
The natural temperature and salinity stratification of open waters influence dissolved
oxygen concentrations, and thus the distribution of Chesapeake Bay species. Waters
located above the pycnocline with higher oxygen levels support a different commu-
nity of species than deeper waters from late spring to early fall. Several well-known
species that inhabit these open waters are menhaden, striped bass and bluefish. Their
habitat requirements and prey needs differ from those of species and communities
inhabiting deeper water habitats during the summer months.
Designated Use Boundary Delineation
From June 1 through September 30, the open-water designated use includes tidally
influenced waters extending horizontally from the shoreline measured at mean low
appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries
-------
A-6
water, to the adjacent shoreline, and extending through the water column to the
bottom. If the presence of a pycnocline prevents oxygen replenishment, the open-
water fish and shellfish designated use extends only as far as the upper boundary of
the pycnocline. If a pycnocline exists but other physical circulation patterns (such as
the inflow of oxygen-rich oceanic bottom waters) provide oxygen replenishment to
the deep waters, the open-water fish and shellfish designated use extends to the
bottom water-sediment interface.
From October 1 through May 31, the boundaries of the open-water designated use
include all tidally influenced waters extending horizontally from the shoreline,
measured at mean low water, to the adjacent shoreline, and down into the water
column to the bottom water-sediment interface.
DEEP-WATER SEASONAL FISH AND SHELLFISH DESIGNATED USE
Waters with this designated use protect the survival, growth and propagation of
balanced, indigenous populations of important fish and shellfish species inhabiting
deep-water habitats.
Designated Use Boundary Delineation
This designated use refers to tidally influenced waters located between the measured
depths of the upper and lower boundaries of the pycnocline, where a measured pycn-
ocline is present and presents a barrier to oxygen replenishment from June 1 through
September 30. In some areas, the deep-water designated use extends from the upper
boundary of the pycnocline down to the sediment/water interface at the bottom,
where a lower boundary of the pycnocline is not calculated due to the depth of the
water column.
DEEP-CHANNEL SEASONAL REFUGE DESIGNATED USE
Waters within this designated use must protect the survival of balanced, indigenous
populations of ecologically important benthic infaunal and epifaunal worms and
clams, which provide food for bottom-feeding fish and crabs.
Designated Use Boundary Delineation
Deep-channel seasonal refuge designated use waters are defined as tidally influenced
waters at depths greater than the measured lower boundary of the pycnocline in
isolated deep channels. The deep-channel designated use is defined laterally by
bathymetry of the trough and vertically by the lower boundary of the pycnocline
above, and below, at the sediment-water interface on the bottom.
appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries
-------
A-7
LITERATURE CITED
Chesapeake Bay Watershed Partners. 2001. Memorandum of Understanding among the State
of Delaware, the District of Columbia, the State of Maryland, the State of New York, the
Commonwealth of Virginia, the State of West Virginia and the United States Environmental
Protection Agency Regarding Cooperative Efforts for the Protection of the Chesapeake Bay
and its Rivers. Chesapeake Bay Program, Annapolis, Maryland
Chesapeake Executive Council. 2000. Chesapeake 2000 agreement. Chesapeake Bay
Program, Annapolis, Maryland.
Funderburk, S. L., J. A. Mihursky, S. J. Jordan and D. Riley (eds.). 1991. Habitat Require-
ments for Chesapeake Bay Living Resources: Second Edition. Chesapeake Research
Consortium, Solomons, Maryland.
Jordan, S. J., C. Stenger, M. Olson, R. Batiuk and K. Mountford. 1992. Chesapeake Bay
dissolved oxygen goal for restoration of living resource habitats: a synthesis of living
resource requirements with guidelines for their use in evaluating model results and moni-
toring information. CBP/TRS 88/93. Chesapeake Bay Program, Annapolis, Maryland.
U.S. Environmental Protection Agency. 2003a. biological Evaluation for the Issuance of
Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for
the Chesapeake Bay and its Tributaries. Chesapeake Bay Program Office, Annapolis, Mary-
land.
U.S. Environmental Protection Agency. 2003b. Technical Support Document for the Identifi-
cation of Chesapeake Bay Designated Uses and Attainability. Chesapeake Bay Program
Office, Annapolis, Maryland.
appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries
-------
B-1
a ppend ix
Sensitivity to Low Dissolved Oxygen
Concentrations for Northern and
Southern Atlantic Coast Populations
of Selected Test Species
This appendix provides the following lines of evidence to support the conclusion that
the data used in the calculation of the Virginian Province saltwater dissolved oxygen
criteria (U.S. EPA 2000) are appropriate for the Chesapeake Bay dissolved oxygen
criteria development.
For the juvenile criterion (Final Acute Value), most test temperatures ranged from
19°C to 30°C. Three species were tested at temperatures less than 19°C: Homarus
americanus (15°C), Carcinus maeniis (10°C) and Rithropanopeus harrisii (10°C).
Fourteen genera were tested at 19°C to 21.5°C and eight genera were tested at 23°C
to 30°C. Figure B-1 shows the cumulative rank plot for genus mean acute values
(GMAV) using data from Appendix B of the Virginian Province saltwater dissolved
oxygen criteria document (U.S. EPA 2000). The data were segregated into '20°C'
and '26°C'groups, representing the 14 and 8 genera groups mentioned above,
respectively. Plots of the two sets of data overlap, showing that both groups give a
similar estimate of the range for the juvenile community's sensitivity to hypoxia. The
criteria minimum concentrations (CMC) calculated for the two sets of data are like-
wise very similar, 2.36 mg/L for the '20°C' group, and 2.26 mg/L for the '26°C'
group.
The same type of analysis was conducted using the 24-hour larval LC50 data (lethal
concentration at which 50 percent mortality of the test organisms was observed)
from Appendix D of the Virginian Province document (Figure B-2; U.S. EPA 2000).
The temperature ranges were also similar, 18°C to 22°C for the '20°C' group, and
23°C to 30°C for the '26°C' group. There were 14 genera in the former and 9 gen-
era in the latter. The conclusion is the same for larvae as for juveniles, a similar dis-
tribution of community sensitivity to hypoxia for both sets of temperatures.
In addition to the data from the Virginian Province document, the EPA has conduct-
ed tests comparing the sensitivity to hypoxia for northern and southern populations
of two invertebrates (the mud crab, Dyspanopeus sayi and the grass shrimp,
Palaemonetes vulgarus, larvae for both species) and one fish (the inland silverside,
Menidia heiyllina, juveniles and larvae; Thursby, personal communication). All of
the northern populations were from Rhode Island. The southern populations of
appendix B • Sensitivity to Low Dissolved Oxygen Concentrations
-------
B-2
invertebrates were from Georgia, and the fish were from Florida. The exposure
response data are shown in figures B-3 through B-6. The northern and southern pop-
ulations of each species responded similarly to low dissolved oxygen conditions,
even though they were conducted at different temperatures.
L
0
O)
CD
c
0
O)
X
O
T3
0
_>
O
w
w
b
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
•O
. 8
• O
• •
• O
• •
o
• GMAV20C'
O GMAV26 C'
0% 20% 40% 60% 80% 100%
Percent Rank
Figure B-1. Plot of juvenile genus mean acute values (GMAVs ) against percent rank.
Data are from Appendix B in the Virginian Province saltwater dissolved oxygen criteria
document (U.S. EPA 2000).
Vs 3.00
L_
0)
ra 2.50
E
> 2.00
<
0 1.50
c
0
O)
5? 1.00
O
o
•
o
•
•o
•
o
•
o <
°# "
o •
0 0.50
>
o
• GMAV20C'
yJ
O GMAV26 C'
w
« 0.00
Q 0% 20% 40%
60% 80%
100%
Percent Rank
Figure B-2. Plot of larval genus mean acute values (GMAVs ) against percent rank. Data
are from Appendix D in the Virginian Province saltwater dissolved oxygen criteria docu-
ment (U.S. EPA 2000).
appendix B • Sensitivity to Low Dissolved Oxygen Concentrations
-------
B-3
110
o__
— A"/Yi
100
90
80
70
60
50
/////
40
30
A RI-20 C
O RI-25 C
• GA-28C
20
10
0
0
1
2
3
4
6
8
5
7
Dissolved Oxygen (mg liter_1)
Figure B-3. Ninety-six hour dose-response for larvae of the marsh grass shrimp
Palaemonetes vulgaris exposed to various levels of low dissolved oxygen. Open symbols are
for tests conducted with populations from Rhode Island (Rl) (three at 20°C and four at
25°C). The closed circles are data for a population from Georgia (GA) conducted at 28°C. All
of the Rl data are from tests included in the Virginian Province saltwater dissolved oxygen
criteria document and are listed in Poucher and Coiro (1997). Tests were initiated with larval
less than 24 hours old.
Source: U.S. EPA 2000.
A RI-20 C
O RI-25 C
~ RI-30C
• GA-28C
1 2 3 4 5 6 7 8
Dissolved Oxygen (mg liter-1)
110
100
90
80
70
60
50
40
30
20
10
0
Figure B-4. Ninety-six hour dose-response for larvae of the Say mud crab Dyspanopeus
sayi exposed to various levels of low dissolved oxygen. Open symbols are for tests con-
ducted with populations from Rhode Island (Rl) (one at 20°C, seven at 25°C, and one at
30°C). The closed circles are data for a population from Georgia (GA) conducted at 28°C.
All of the Rhode Island data are from tests included in the Virginian Province saltwater
dissolved oxygen criteria document and are listed in Poucher and Coiro (1997). Tests were
initiated with larval animals ranging from stage 1 to stage 3 in development.
Source: U.S. EPA 2000.
appendix B • Sensitivity to Low Dissolved Oxygen Concentrations
-------
B-4
110
100
90
o
c=
o
o
80
70
o
c
(D
O
60
a)
Q.
40
as
>
'>
30
13
CD
20
O RI-25 C
• GA-28C
0
1
2
3
4
6
8
5
7
Dissolved Oxygen (mg liter1)
Figure B-5. Ninety-six hour dose-response for larvae of the inland silverside Menidia
beryllina exposed to various levels of low dissolved oxygen at two temperatures. Open cir-
cles are for a test conducted with a population from Rhode Island (Rl) (25°C) and closed
circles are for two tests with a population from Georgia (GA) conducted at 28°C. The
Rhode Island data are from a test listed in Poucher and Coiro (1997). Tests were initiated
with 7-day-old larvae.
Source: U.S. EPA 2000.
110
-o
100
90
o
i—
c
o
o
80
70
o
60
4—1
c
0
O
50
0
Q_
40
CC
>
>
i—
30
=3
(/)
20
O RI-25 C
• GA-28C
0
1
2
3
4
6
8
5
7
Dissolved Oxygen (mg liter1)
Figure B-6. Seventy-two hour dose-response for juveniles of the inland silverside
Menidia beryllina exposed to various levels of low dissolved oxygen at two temperatures.
Open circles are for a test conducted with a population from Rhode Island (Rl) (25°C) and
closed circles are for a test with a population from Georgia (GA) conducted at 28°C.
Source: U.S. EPA 2000.
appendix B • Sensitivity to Low Dissolved Oxygen Concentrations
-------
B-5
LITERATURE CITED
Poucher, S. and L. Coiro. 1997. Test Reports: Effects of Low Dissolved Oxygen on Saltwater
Animals. Memorandum to D. C. Miller. U. S. EPA, Atlantic Ecology Division, Narragansett,
Rhode Island.
U.S. EPA 2000. Ambient Aquatic Life Water Quality Criteria for Dissolved Oxygen (Salt-
water): Cape Cod to Cape Hatteras. EPA-822-R-00-012. Office of Water, Office of Science
and Technology, Washington, D.C. and Office of Research and Development, National
Health and Environmental Effects Research Laboratory, Atlantic Ecology Division,
Narragansett, Rhode Island.
appendix B • Sensitivity to Low Dissolved Oxygen Concentrations
-------
C-1
Summary of Literature on the Tolerance
of Chesapeake Bay Macrobenthic Species
to Low Dissolved Oxygen Conditions
Species
Life
Stage
Dissolved
Oxygen
(mg liter')
Temp
(°C)
Observed Response
Reference
Mollusca
A bra alba
Adult
0
10
LD50 in 200 hrs
Dries and Theede 1974
Cardium
ednle
Adult
0
10
50% mortality in 7 days
Thamdrup 1935
referenced in O'Connor
(unpublished manuscript)
Adult
0.15
10
50% mortality in 102 hrs
(4.3 days) without sulfide,
96 hrs (4 days) with sulfide
(50 mg liter"1 Na2S.9H20
Theede et al. 1969; Theede
1973
Carium
lamarki
Adult
0
10
LD50 in - 220 hrs (9.2 days)
Dries and Theede 1974
Littorina
littoria
Adult
0.15
10
LD50 in 365 hrs (15.2 days)
without sulfide, 180 hrs
(7.5 days) with sulfide;
50 mg liter"1
Theede et al. 1969; Theede
1973
Littorina
saxatilus
Adult
0.15
10
LD50 in 365 hrs (15.2 days)
without sulfide, 72 hrs
(3 days) with sulfide;
50 mg liter"1
Theede et al. 1969; Theede
1973
Macoma
balthica
Adult
0
10
4% mortality in 7 days
Thamdrup 1935; referenced
in O'Conner (unpublished
manuscript)
Adult
0
10
LD50 in 500 hrs (20.8 days)
Dries and Theede 1974
Mercenaria
mercenana
Larvae
0.9-2.4
25
Reduced growth
Morrison 1971
0.2
25
100% mortality in 14 days
Morrison 1971
NR
0.9
25
0% mortality in 14 days
Morrison 1971
appendix C • Summary of Literature on the Tolerance of Chesapeake Bay Macrobenthic Species
-------
C-2
Species
Life
Stage
Dissolved
Oxygen
(mg liter"1)
Temp
(°C)
Observed Response
Reference
Juvenile/
Adult
(31-38
mm)
5.7
19-24
Maximum burrowing rate
Savage 1976
NR
0.9-1.8
17-24
Reduced burrowing rate
Savage 1976
NR
0.9
19
No mortality in 21 days
and 30 days (two trials)
Savage 1976
Mulina
lateralis
Juvenile
(5 mm)
0
10
LT50 in 10.5 days without
sulfide, 4.3 days with
sulfide; 644 mg liter"1
Na2S.9H20
Shumway and Scott 1983;
referenced in O'Conner
(unpublished manuscript)
NR
0
20
LT50 in 7.5 days
Shumway and Scott 1983;
referenced in O'Conner
(unpublished manuscript)
Mulina
lateralis
NR
0
30
LT50 in 2 days
Shumway and Scott 1983;
referenced in O'Conner
(unpublished manuscript)
Adult
(10 mm)
0
10
LT50 in 10 days without
sulfide, 3.8 days with
sulfide; 644 mg liter"1
Na2S.9H20
Shumway and Scott 1983;
referenced in O'Conner
(unpublished manuscript)
NR
0
20
LT50 in 2.5 days
Shumway and Scott 1983;
referenced in O'Conner
(unpublished manuscript)
NR
0
30
LT50 in 1.8 days
Shumway and Scott 1983;
referenced in O'Conner
(unpublished manuscript)
Mva armaria
NR
0
'very
low'
Survived for 'weeks'
Collip 1921; referenced in
O'Conner (unpublished
manuscript)
NR
0
14
Survived 8 days
Collip 1921; referenced in
O'Conner (unpublished
manuscript)
NR
0
31
Survived 1 day
Collip 1921; referenced in
O'Conner (unpublished
manuscript)
appendix C • Summary of Literature on the Tolerance of Chesapeake Bay Macrobenthic Species
-------
C-3
Species
Life
Stage
Dissolved
Oxygen
(mg liter"1)
Temp
(°C)
Observed Response
Reference
My a arenaria
Adult
0.2
10
LC50 in 21 days without
sulfide, 17 days with
sulfide.
Theede et al. 1969; Theede
1973; referenced in
O'Conner (unpublished
manuscript)
Mvtilus
ednlis
Adult
0.2
10
LC50 in 35 days without
sulfide, 25 days with
sulfide
Theede et al. 1969; Theede
1973; referenced in
O'Conner (unpublished
manuscript)
Adult
0
10
20% mortality in 7 days
Thamdrup 1935; referenced
in O'Conner (unpublished
manuscript)
Spisula
solidissima
Adult
(49-64
mm)
5.3-6.0
11-22
Maximum burrowing rate
Savage 1976
NR
0.8-1.6
11-22
Reduced burrowing rate,
mortality
Savage 1976
NR
1.6
21.7
1 of 9 dead in 5 days
Savage 1976
NR
0.9
21.0
3 of 9 dead in 5 days
Savage 1976
Juvenile/
Adult
(31-
28mm)
5.7
19-24
Maximum burrowing rate
Savage 1976
NR
0.9-1.8
17-24
Reduced burrowing rate
Savage 1976
Spisula
solidissima
NR
0.9
19
No mortality in 21 days
and 30 days (two trials)
Savage 1976
Midinia
lateralis
Juvenile
(5 mm)
0
10
LT50 in 10.5 days without
sulfide, 4.3 with sulfide;
644 mg liter"1 Na2S.9H20
Shumway and Scott 1983;
referenced in O'Conner
(unpublished manuscript)
NR
0
20
LT50 in 7.5 days
Shumway and Scott 1983;
referenced in O'Conner
(unpublished manuscript)
NR
0
30
LT50 in 2 days
Shumway and Scott 1983;
referenced in O'Conner
(unpublished manuscript)
appendix C • Summary of Literature on the Tolerance of Chesapeake Bay Macrobenthic Species
-------
C-4
Species
Life
Stage
Dissolved
Oxygen
(mg liter"1)
Temp
(°C)
Observed Response
Reference
Adult
(10 mm)
0
10
LT50 in 10 days without
sulfide, 3.8 days with
sulfide; 644 mg liter"1
Na2S.9H20
Shumway and Scott 1983;
referenced in O'Conner
(unpublished manuscript)
NR
0
20
LT50 in 2.5 days
Shumway and Scott 1983;
referenced in O'Conner
(unpublished manuscript)
NR
0
30
LT50 in 1.8 days
Shumway and Scott 1983;
referenced in O'Conner
(unpublished manuscript)
Adult
(100
mm)
1.0
10
LC50 in 15 days; initial
mortality in 8 days; total
mortality in 30 days
Thurberg and Goodlett 1979
Mulinia
NR
3.0
10
No mortality in 2 months
Thurberg and Goodlett 1979
lateralis
Juvenile/
Adult
(3.7-5
cm)
1.0
10
LC50 in 7 days
Thurberg and Goodlett 1979
Juvenile/
Adult
(3.8-4.6
cm)
2.0
10
LC50 in 21 days
Thurberg and Goodlett 1979
Polychaeta
Capitella
capitata
Adult
0
12
Mortality in 8 days
Jacubowa and Malm 1931;
referenced in O'Conner
(unpublished manuscript)
Capitomastus
minimus
Adult
0
12
Mortality in 8 days
Jacubowa and Malm 1931;
referenced in O'Conner
(unpublished manuscript)
Etoeone picta
Adult
0
12
Mortality in 6 days
Jacubowa and Malm 1931;
referenced in O'Conner
(unpublished manuscript)
Glvcera
convoluta
Adult
0
12
Mortality in 10 days
Jacubowa and Malm 1931;
referenced in O'Conner
(unpublished manuscript)
appendix C • Summary of Literature on the Tolerance of Chesapeake Bay Macrobenthic Species
-------
C-5
Species
Life
Stage
Dissolved
Oxygen
(mg liter')
Temp
(°C)
Observed Response
Reference
Hcirmothae
incerta
Adult
0
12
Mortality in 5 days
Jacubowa and Malm 1931;
referenced in O'Conner
(unpublished manuscript)
Nephtys
ciliata
Adult
0
10
LD50 in 140 hr (5.8 days)
Dries and Theede 1974
Nerevis
diversicolor
Adult
0.2
10
LC50 in 5 days without
sulfide, 4 days with
sulfide; referenced in
O'Conner (unpublished
manuscript)
Theede et al. 1969; Theede
1973
Adult
0
10
LD50 in 120 hrs (5 days)
Dries and Theede 1974
Adult
0
6-8
72 hrs with no mortality;
ATP conc. 59% of initial
value (after 72 hrs)
Schottler 1979
Nereis
pelagica
Adult
0
6-8
40% mortality after 36 hrs;
ATP conc. 51% of initial
value (after 72 hrs)
Schottler 1979
Nereis virens
Adult
0
6-8
72 hrs with no mortality;
ATP conc. 57% of initial
value (after 72 hrs)
Schottler 1979
Pectinaria
neapolitana
Adult
0
12
Mortality in 8 days
Jacubowa and Malm 1931;
referenced in O'Conner
(unpublished manuscript)
Terebellides
stroemi
Adult
0
10
LD50 in 72 hrs (3 days)
Dries and Theede 1974
Source: Holland et al. 1989.
NR = not reported.
LC50 = lethal concentration at which 50 percent mortality of the test organisms was observed.
LD50= lethal dose (same as LC50).
LT50 = lethal threshold (same as LC50).
appendix C • Summary of Literature on the Tolerance of Chesapeake Bay Macrobenthic Species
-------
C-6
LITERATURE CITED
Dries, R. R. and H. Theed. 1974. Saverstoffmangelresistenz Mariner Bodenvertebraten aus
der Westlichen Ostsee. Marine Biology 25:327-333.
Holland, A. F., A. T. Shaughnessy, L. C. Scott, V. A. Dickens, J. Gerritsen and J. A. Ranas-
inghe. 1989. Long-Term Benthic Monitoring and Assessment Program for the Maryland
Portion of Chesapeake Bay: Interpretative Report. CBRM-LTB/EST-2. Maryland Depart-
ment of Natural Resources, Annapolis, Maryland.
Morrison, G. 1971. Dissolved oxygen requirements for embryonic and larval development of
the hardshell clam Mercenaria mercenaria. Journal of Fisheries Research Board Canada
28:379-381.
Savage, N. B. 1976. Burrowing activity in Mercenaria mercenaria (L.) and Spisula solidis-
sima (Dillwyn) as a function of temperature and dissolved oxygen. Marine Behavior and
Physiology 3:221-234.
Schottler, U. 1979. On the anaerobic metabolism of the three species of Nereis (Annelida).
Marine Ecology Progress Series 1:249-254.
Theede, H. 1973. Comparative studies on the influence of oxygen deficiency and hydrogen
sulphide on marine bottom invertebrates. Netherlands Journal of Sea Research 7:244-252.
Theede, H., A. Ponat, K. Hiroki and C. Schlieper. 1969. Studies on the resistance of marine
bottom invertebrates to oxygen-deficiency and hydrogen sulphide. Marine Biology 2:325-
337.
Thurberg, E. R and R. B. Goodlett. 1979. Impact on clams and scallops. Part II. Low
dissolved oxygen concentrations and surf clams, a laboratory study. In: Oxygen Depletion
and Associated Benthic Mortalities in New York Bight, R. B. Swanson and C. J. Sindermann,
(eds.). 1976. NOAA Professional Paper 11, U.S. Government Printing Office, Washington,
D. C. Pp. 227-280.
appendix C • Summary of Literature on the Tolerance of Chesapeake Bay Macrobenthic Species
-------
D-1
a ppend ix
Narrative, Numerical and Method-based
Chlorophyll a Criteria Adopted as Water Quality
Standards by States Across the U.S.
State
Water Body Type or Designated Use
Numeric or Narrative Chlorophyll a Criteria
Alabama
Narrative criteria for specific lakes and
reservoirs
Walter F. George
West Point
Weiss
Numeric chlorophyll a criteria
-16 |ug liter"1
-27 |ag liter"1
-20 |ag liter"1
Chlorophyll a levels set
for samples collected
between April-October.
Samples collected
monthly at deepest
points.
Alaska
(1) Fresh water
(A) water supply
(i) drinking, culinary and food processing;
(ii) agriculture, including irrigation and
stock watering;
(iii) aquaculture;
(iv) industrial;
(B) water recreation
(i) contact recreation;
(ii) secondary recreation;
(C) growth and propagation of fish, shellfish,
other aquatic life and wildlife; and
(2) Marine water
(A) water supply
(i) aquaculture;
(ii) seafood processing;
(iii) industrial;
(B) water recreation
(i) contact recreation;
(ii) secondary recreation;
(C) growth and propagation of fish, shellfish,
other aquatic life and wildlife; and
(D) harvesting for consumption of raw
mollusks or other raw aquatic life.
Narrative criteria
Aesthetic Qualities
CRITERIA
All waters free from substances attributable to
wastewater or other discharges that:
(5) Produce undesirable or nuisance aquatic life.
Arizona
Designated uses of a surface water may
include full body contact, partial body contact,
domestic water source, fish consumption,
aquatic and wildlife (warm-water fishery),
aquatic and wildlife (ephemeral), aquatic and
wildlife (effluent dependent water) agricultural
irrigation, and agricultural livestock watering.
The designated uses for specific waters are
listed in Appendix B of the article.
R 18-11-108 Narrative Water Quality Standards
A. A surface water shall be free from pollutants in
amounts or combinations that: 6) cause the growth of
algae or aquatic plants that inhibit or prohibit the
habitation, growth or propagation of other aquatic life or
that impair recreational uses.
D
appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S.
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D-2
State
Water Body Type or Designated Use
Numeric or Narrative Chlorophyll a Criteria
California
Water Quality
Control Board
Region 5
Central Valley Sacramento and San Joaquin
River Basins
Wineries with stills produce substantial quantities of
stillage waste which is high in concentrations of BOD
and nitrogen. The stillage is normally discharged directly
to land without any prior treatment. There is a potential
for the waste to affect water quality and to create
nuisance conditions. A study has been conducted to
develop recommendations for minimizing water quality
effects and nuisance conditions resulting from land
application of stillage waste. There is a need to
implement guidelines for land disposal of stillage waste
that can be used by the industry as a general indication of
minimum disposal practices when accompanied with
suitable soil, weather, groundwater and other conditions
affecting the discharge.
California
Water Quality
Control Board
San Francisco Bay/
Sacramento-San Joaquin
Delta Estuary
Water Quality Compliance and Baseline Monitoring
Monthly monitoring for chlorophyll a at several stations.
Water Quality Objective: To prevent nuisance.
California
Water Quality
Control Board
Region 2
San Francisco Bay region
One criterion to protect the aesthetic value of water used
for recreation from excessive algal growth is based on
chlorophyll a.
Biostimulatory substances can cause high chlorophyll a
level rise.
Colorado
Numeric water quality criteria by designated
use and for indicated rivers and streams.
Bear Creek Reservoir .. .Traditionally, the average
concentration of chlorophyll a has been selected by the
commission as the indicator of lake condition. For Bear
Creek Reservoir, however, peak algal biomass
(chlorophyll a) was selected as the most important of
these indicators upon which to assess trophic response
because algal blooms are most often associated with
impaired uses. To achieve the goal of change in trophic
status, a 16 percent reduction in the frequency of
nuisance algal blooms during the growing season would
need to be achieved, as well as a reduction in frequency
and magnitude of the peak chlorophyll a concentrations.
Connecticut
Inland Surface waters
-Class AA
Existing or proposed drinking water supply;
fish and wildlife habitat; recreational use;
agricultural, industrial supply and other
purposes (recreational uses may be restricted).
-Class A
Potential drinking water supply; fish and
wildlife habitat; recreational use; agricultural,
industrial supply and other legitimate uses,
including navigation.
-Class B
Recreational use; fish and wildlife habitat;
agricultural and industrial supply and other
legitimate uses, including navigation.
-Class C and Class D (goal to be class A or B)
Lake: Oligotrophic
May be class A, AA or class
B water.
0-2 |ag liter"1 mid-
summer
Lake: Mesotrophic
May be Class AA, Class A,
or Class B.
2-15 |ag liter"1 mid-
summer
Lake: Eutrophic
May be Class AA, Class A,
or Class B water.
15-30 |ag liter"1 mid-
summer
Lake: Highly Eutrophic
May be Class AA, Class A,
or Class B water.
30+ |ag liter"1 mid-
summer
appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S.
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D-3
State
Water Body Type or Designated Use
Numeric or Narrative Chlorophyll a Criteria
Georgia
Chlorophyll a criteria for only Lakes and
Major Lake tributaries
West Point Lake: shall not exceed 27 |ig liter"1 (April-
October)
Walter F. George Lake: shall not exceed 18 |ig liter"1
(April-October)
Lake Jackson: shall not exceed 20 |ig liter"1 (April -
October)
Lake Alatoona
Upstream from the Dam - 10 |ig liter"1
Allatoona Creek upstream from 1-75 10 |ig liter"1
Mid-Lake downstream from Kellogg Creek - 10|.ig liter"1
Little River upstream from Highway 205 - 15 |ig liter"1
Etowah River upstream from Sweetwater Creek- 12 |ig
liter"1
Lake Sidney Lanier
Upstream from the Flowery Branch confluence - 5 |ig
liter"1
At Browns Bridge Road (State Road 369) - 5 |ig liter"1
At Boiling Bridge (State Road 53) on Chestatee River -
10 |ig liter"1
At Lanier Bridge (State Road 53) on Chattahoochee
River - 10 |ig liter"1
Hawaii
Criteria for all Estuaries except Pearl Harbor
Criteria for Pearl Harbor Estuary
Open coastal waters (note that criteria for open
coastal waters differ, based on fresh water
discharge.)
Oceanic waters
Chlorophyll a (|ig liter"1) - geometric mean not to
exceed the given value of 2.00 |ig liter"1. Not to exceed
the given value more than 10 percent of the time 5.00 |ig
liter"1. Not to exceed the given value more than 2 percent
of the times of 10.00 |ig liter"1.
Chlorophyll a (|ig liter"1) - geometric mean not to exceed
the given value of 3.50 |ig liter"1. Not to exceed the given
value more than 10 percent of the time - 10.00 |ig liter"1.
Not to exceed the given value more than 2
percent of the time - 20.00 |ig liter"1.
Chlorophyll a (|ig liter"1) - geometric mean not to
exceed the given value of 0.30 |ig liter"1*, 0.15 |ig liter"
Not to exceed the given value more than 10 percent of the
time - 0.90 |ig liter"1*, 0.50 |ig liter"1**
Not to exceed the given value more than 2 percent of the
time - 1.75 |ig liter"1*, 1.00 |ig liter"1**
*"Wet" criteria apply when the coastal waters receive
more than three million gallons per day of fresh water
discharge per shoreline mile.
** "Dry" criteria apply when the open coastal waters
receive less than three million gallons per day of fresh
water discharge per shoreline mile.
Chlorophyll a
0.06 |ig liter"1 - geometric mean not to exceed the given
value
0.12 |ig liter"1 - not to exceed the given value more than
10 percent of the time
0.20 |ig liter"1 - not to exceed the given value more than 2
percent of the time
appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S.
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D-4
State
Water Body Type or Designated Use
Numeric or Narrative Chlorophyll a Criteria
Idaho
All waters
Excess Nutrients. Surface waters of the state shall be free
from excess nutrients that can cause
visible slime growths or other nuisance aquatic growths
impairing designated beneficial uses.
Iowa
By designated uses
General use segments.
General Water Quality criteria
b. Such waters shall be free from floating debris, oil,
grease, scum and other floating materials attributable to
wastewater discharges or agricultural practices in
amounts sufficient to create a nuisance.
Designated use segments:
Primary contact recreation (Class "A").
Cold water aquatic life (Class "B (CW)").
High quality water (Class "HQ").
High quality resource water (Class "HQR").
Significant resource warm water (Class
"B(WWJ").
Limited resource warm water (Class"B(LR)").
Lakes and wetlands (Class "B(LW)").
Drinking water supply (Class "C").
c. Such waters shall be free from materials attributable to
wastewater discharges or agricultural practices producing
objectionable color, odor or other aesthetically
objectionable conditions.
Kansas
Aquatic Life support use
Nutrients. The introduction of plant nutrients into
streams, lakes or wetlands from artificial sources shall be
controlled to prevent the accelerated succession or
replacement of aquatic biota or the production of
undesirable quantities or kinds of aquatic life.
Recreation use
The introduction of plant nutrients into surface waters
designated for primary or secondary contact recreational
use shall be controlled to prevent the development of
objectionable concentrations of algae or algal by-products
or nuisance growths of submersed, floating or emergent
aquatic vegetation.
Louisiana
Narrative criteria for all waters
Nutrients. The naturally occurring range of nitrogen-
phosphorous ratios shall be maintained. This range shall
not apply to designated intermittent streams. To establish
the appropriate range of ratios and compensate for natural
seasonal fluctuations, the administrative authority will
use site-specific studies to establish limits for nutrients.
Nutrient concentrations that produce aquatic growth to
the extent that it creates a public nuisance or interferes
with designated water uses shall not be added to any
surface waters.
appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S.
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D-5
State
Water Body Type or Designated Use
Numeric or Narrative Chlorophyll a Criteria
Maine
All Lakes
1. Class GPA waters. Class GPA shall be the
sole classification of great ponds and
natural ponds and lakes less than 10 acres in
size.
B. Class GPA waters shall be described by
their trophic state based on measures of the
chlorophyll a content, Secchi disk
transparency, total phosphorus content and
other appropriate criteria. Class GPA waters
shall have a stable or decreasing trophic state,
subject only to natural fluctuations and shall be
free of culturally induced algal blooms which
impair their use and enjoyment.
Trophic state - Maine
Trophic State Index (TSI)
Trophic state is the ability of
a body of water to produce
algae and other aquatic
plants. The trophic state of a
body of water is a function of
its nutrient content and may
be estimated using the Maine
Trophic State Index (TSI) as
follows....
In addition, a scale of 0 to
100 is established in order to
measure the trophic state or
degree of enrichment of lakes
due to nutrient input.
TSI = 70 log (mean
chlorophyll a + 0.7)
Massachusetts
All waterbody - Narrative criteria
Control of Eutrophication. .. .there shall be no new or
increased point source discharge of nutrients primarily
phosphorus and nitrogen that would encourage cultural
eutrophication or the growth of weeds or algae in these
Lakes or ponds. Any existing point source discharge
containing nutrients in concentration which encourage
eutrophication or growth of weeds or algae in these lakes
or ponds shall be provided with all reasonable control for
non-point source.
Michigan
All waters narrative criteria
R 323.1060 Plant nutrient
Nutrients shall be limited to the extent necessary to
prevent stimulation of growths of aquatic rooted,
attached, suspended, and floating plants, fungi or bacteria
which are or may become injurious to the designated uses
of the waters of the state.
Minnesota
Narrative criteria for all waters
Nuisance conditions prohibited.
-Excessive growths of aquatic plants, or other offensive
or harmful effects.
Missouri
General criteria for all waters
Waters shall be free from substances or conditions in
sufficient amounts to cause unsightly color or turbidity,
offensive odor or prevent full maintenance of beneficial
uses.
Montana
Eight classifications by designated use
(h) No increases of carcinogenic, bioconcentrating, toxic
or harmful parameters, pesticides and organic and
inorganic materials, including heavy metals, above
naturally occurring concentrations, are allowed.
Nebraska
Agricultural use
This use applies to all surface waters of the state. To be
aesthetically acceptable, waters shall be free from human-
induced pollution which causes: 1) noxious odors; 2)
floating, suspended, colloidal, or settleable materials that
produce objectionable films, colors, turbidity, or deposits;
and 3) the occurrence of undesirable or nuisance aquatic
life (e.g., algal blooms). Surface waters shall also be free
of junk, refuse, and discarded dead animals.
appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S.
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D-6
State
Water Body Type or Designated Use
Numeric or Narrative Chlorophyll a Criteria
Nevada
Lake Mead - 445A. 194 Requirements to
maintain existing higher quality for area of
Lake Mead; standards for beneficial uses for
area not covered by NAC 445A. 196. (NRS
445A.425,
445A.520)
Designated use
Recreation involving contact with water,
propagation of aquatic life, including, without
limitation, a warm water fishery, recreation not
involving contact with water and municipal or
domestic supply, or both.
Chlorophyll a -|ag liter"1 requirement to maintain existing
Higher Quality
b. The requirements for chlorophyll a are:
(1) Not more than one monthly mean in a calendar year at
Station 3 may exceed 45 |ag liter"1.
(2) The mean for chlorophyll a in summer (July 1 -
September 30) must not exceed 40 |ag liter"1 at Station 3,
and the mean for 4 consecutive summer years must not
exceed 30 |ag liter"1. The sample must be collected from
the center of the channel and must be representative of
the top 5 meters of the channel. "Station 3" means the
center of the channel at which the depth is from 16 to 18
meters.
(3) The mean for chlorophyll a in the growing season
(April 1-September 30) must not exceed 16 |ag liter"1 at
LM4 and 9 |ag liter"1 at LMS. LM4 is located just outside
of the Las Vegas Bay launch ramp and marina, next to
buoy RW " 1." LM5 is located next to buoy RW "A" with
the southshore landmark of Cresent Island.
(4) The mean for chlorophyll a in the growing season
(April I - September 30) must not exceed 5 |ag liter"1 in
the open water of Boulder Basin, Virgin Basin, Gregg
Basin and Pierce Basin. The single value must not exceed
10 jag liter"1 for more than 5 percent of the samples.
(5) Not less than 2 samples must be collected between the
months of March and October. During months when
only one sample is available, that value must be used in
place of the monthly mean.
New
Hampshire
Narrative criteria related to all waters
e) There shall be no new or increased discharge(s)
containing phosphorus or nitrogen to tributaries of lakes
or ponds that would contribute to cultural eutrophication
or growth of weeds or algae in such lakes and ponds.
New Jersey
Narrative criteria for all waters
2. Except as due to natural conditions, nutrients shall not
be allowed in concentrations that cause objectionable
algal densities, nuisance aquatic vegetation, or otherwise
render the waters unsuitable for the designated uses.
3. Activities resulting in the non-point discharge of
nutrients shall implement the best management practices
determined by the Department to be necessary to protect
the existing or designated uses.
New Mexico
Narrative criteria for all waters
E. Plant Nutrients: Plant nutrients from other than natural
causes shall not be present in concentrations which will
produce undesirable aquatic life or result in a dominance
of nuisance species in surface waters of the State.
New York
Narrative criteria for all State waters
Waters shall contain no phosphorus and nitrogen in
amounts that will result in growths of algae, weeds and
slimes that will impair the waters for their best usage.
appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S.
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D-7
State
Water Body Type or Designated Use
Numeric or Narrative Chlorophyll a Criteria
North Carolina
Freshwater - Class C waters and tidal salt
water
For lakes and reservoirs and other waters
subject to growths of macroscopic and
microscopic vegetation not designated as trout
waters
Lakes, reservoirs and other waters subject to
growths of macroscopic or microscopic
vegetation designated as trout waters ( not
applicable to lakes and reservoirs less that 10
acres in surface area)
Not to exceed 40 |ag liter 1
Not to exceed 15 |ag liter"1
North Dakota
1) Municipal and domestic water.
2) Recreation Fishing and Wildlife
3) Agricultural uses
4) Industrial water
1) Free from substances attributable to municipal,
industrial, or other discharges or agricultural practices
that will cause the formation of putrescent or otherwise
objectionable sludge deposits.
(2) Free from floating debris, oil, scum, and other floating
materials attributable to municipal, industrial, or other
discharges or agricultural practices in sufficient amounts
to be unsightly or deleterious.
(3) Free from materials attributable to municipal,
industrial, or other discharges or agricultural practices
producing color, odor, or other conditions to such a
degree as to create a nuisance or render any undesirable
taste to fish flesh or, in any way, make fish inedible.
Ohio
Narrative criteria for all waters
3745-1-04 Criteria applicable to all waters.
(E) Free from nutrients entering the waters as a result of
human activity in concentrations that create nuisance
growths of aquatic weeds and algae.
Oklahoma
Narrative criteria for all waters
To determine excess nutrient by using Carlson's Trophic
State Index. Using chlorophyll a, a value of 62 or greater,
is otherwise listed as "NLW" in Appendix A of chapter.
Water are to be designated as "Nutrient-limited
watershed" which means a watershed of a waterbody
with a designated beneficial use which is adversely
affected by excess nutrients as determined by Carlson's
Trophic State Index.
A) Narrative criterion applicable to all waters of the state.
Nutrients from point source discharges or other sources
shall not cause excessive growth of periphyton,
phytoplankton, or aquatic macrophyte communities
which impairs any existing or designated beneficial use.
appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S.
-------
D-8
State
Water Body Type or Designated Use
Numeric or Narrative Chlorophyll a Criteria
Oregon
Water use designation by basin.
340-041-0150
Nuisance Phytoplankton Growth
The following values and implementation program shall
be applied to lakes, reservoirs, estuaries and streams,
except for ponds and reservoirs less than ten acres in
surface area, marshes and saline lakes:
(1) The following average chlorophyll a values shall be
used to identify water bodies where phytoplankton may
impair the recognized beneficial uses:
(a) Natural lakes which thermally stratify: 0.01 mg liter"1;
(b) Natural lakes which do not thermally stratify,
reservoirs, rivers and estuaries: 0.015 mg liter"1;
(c) Average chlorophyll a values shall be based on the
following methodology (or other methods approved by
the Department): A minimum of three samples collected
over any three consecutive months at a minimum of one
representative location (e.g., above the deepest point of a
lake or reservoir or at a point mid-flow of a river) from
samples integrated from the surface to a depth equal to
twice the Secchi depth or the bottom (the lesser of the
two depths); analytical and quality assurance methods
shall be in accordance with the most recent edition of
Standard Methods for the examination of Water and
Wastewater.
Rhode Island
Narrative criteria related for all waters
Freshwater
10 b. None in such
concentration that would
impair any usages
specifically assigned to said
Class or cause undesirable or
nuisance aquatic species
associated with cultural
eutrophication, nor cause
exceedance of the criterion of
10(a) above in a downstream
lake, pond, or reservoir.
Seawater
Where waters have low
tidal flushing rates,
applicable treatment to
prevent or minimize
accelerated or cultural
eutrophication may be
required for regulated
nonpoint source
activities.
South Dakota
(1) Domestic water supply waters;
(2) Coldwater permanent fish life propagation
waters;
(3) Coldwater marginal fish life propagation
waters;
(4) Warmwater permanent fish life
propagation waters;
(5) Warmwater semipermanent fish life
propagation waters;
(6) Warmwater marginal fish life propagation
waters;
(7) Immersion recreation waters;
(8) Limited contact recreation waters;
(9) Fish and wildlife propagation, recreation,
and stock watering waters;
(10) Irrigation waters; and
(11) Commerce and industry waters.
74:51:01:09. Nuisance aquatic life.
Materials which produce nuisance aquatic life may not be
discharged or mused to be discharged into surface waters
of the state in concentrations that impair a beneficial use
or create a human health problem.
appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S.
-------
D-9
State
Water Body Type or Designated Use
Numeric or Narrative Chlorophyll a Criteria
Texas
Narrative criteria for all waters
§307.4. General Criteria.
(e) Nutrient parameters. Nutrients from permitted
discharges or other controllable sources shall not cause
excessive growth of aquatic vegetation which impairs an
existing, attainable, or designated use. Site-specific
nutrient criteria, nutrient permit limitations, and/or
separate rules to control nutrients in individual
watersheds will be established where appropriate after
notice and opportunity for public participation and proper
hearing.
Utah
High Quality Waters - Category 1, 2, 3
6.1 Class 1 - Protected for use as a raw water
source for domestic water systems.
a. Class 1A- Reserved.
b. Class IB - Reserved.
c. Class 1C - Protected for domestic purposes
with prior treatment by treatment processes as
required by the Utah Division of Drinking
Water
6.2 Class 2 - Protected for recreational use and
aesthetics.
a. Class 2A - Protected for primary contact
recreation such as swimming.
b. Class 2B - Protected for secondary contact
recreation such as boating, wading, or similar
uses.
6.3 Class 3 - Protected for use by aquatic
wildlife.
a. Class 3 A - Protected for cold water species
of game fish and other cold water aquatic life,
including the necessary aquatic organisms in
their food chain.
b. Class 3B - Protected for warm water
species of game fish and other warm water
aquatic life, including the necessary aquatic
organisms in their food chain.
c. Class 3C - Protected for non-game fish and
other aquatic life, including the necessary
aquatic organisms in their food chain.
d. Class 3D - Protected for waterfowl, shore
birds and other water-oriented wildlife not
included in Classes 3A, 3B, or 3C, including
the necessary aquatic organisms in their food
chain.
e. Class 3E - Severely habitat-limited waters.
Narrative standards will be applied to protect
these waters for aquatic wildlife.
6.4 Class 4 - Protected for agricultural uses
including irrigation of crops and stock
watering.
6.5 Class 5 - The Great Salt Lake. Protected
for primary and secondary contact recreation,
aquatic wildlife, and mineral extraction.
7.2 Narrative Standards
It shall be unlawful, and a violation of these regulations,
for any person to discharge or place any waste or other
substance in such a way as will be or may become
offensive such as unnatural deposits, floating debris, oil,
scum or other nuisances such as color, odor or taste; or
cause conditions which produce undesirable aquatic life
or which produce objectionable tastes in edible aquatic
organisms; or result in concentrations or combinations of
substances which produce undesirable physiological
responses in desirable resident fish, or other desirable
aquatic life, or undesirable human health effects, as
determined by bioassay or other tests performed in
accordance with standard procedures.
appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S.
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D-10
State
Water Body Type or Designated Use
Numeric or Narrative Chlorophyll a Criteria
Virginia
Narrative criteria/plan of action for all state
waters.
The Board recognizes that nutrients are contributing to
undesirable growths of aquatic plant life in surface waters
of the Commonwealth. This standard establishes a
designation of "nutrient enriched waters". Designations
of surface waters of the Commonwealth as "nutrient
enriched waters" are determined by the Board based upon
an evaluation of the historical water quality data for one
or more of the following indicators of nutrient
enrichment: chlorophyll a concentrations, dissolved
oxygen fluctuations, and concentrations of total
phosphorus.
appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S.
-------
D-11
State
Water Body Type or Designated Use
Numeric or Narrative Chlorophyll a Criteria
Washington - Class AA (extraordinary)
Water quality of this class shall markedly and
uniformly exceed the requirements for all or
substantially all uses.
Characteristic uses:
i) water supply (domestic, agricultural,
industrial)
ii) stock watering
iii) fish and shellfish *
iv) wildlife habitats
v) Recreation
vi) Commerce and navigation
vii) Aesthetic Values **
- Class A (excellent)
Same as class AA except for fecal coliform
levels are lower in the AA category in
freshwater.
- Class B (good)
Water quality for this class shall meet or
exceed the requirements for most uses.
Characteristic uses:
i) water supply (industrial and agricultural)
(all other uses stay the same as above classes;
different numeric criteria for DO and fecal
coliform.)
- Class C (fair)
Water quality of this class shall meet or exceed
the requirements of selected essential uses,
i) water supply (industrial) (different criteria
for DO and fecal coliform in this class).
Lake class
-Establishing Lake Nutrient criteria
Narrative or Numeric Chlorophyll a Standards
Lakes in the Willamette, East Cascade Foothills, or Blue
Mountain ecoregions do not have recommended values and
need to have lake-specific studies in order to receive criteria
as described in (c)(i) of this subsection.
(b) The following actions are recommended if ambient
monitoring of a lake shows the epilimnetic total phosphorus
concentration, as shown in Table 1 of this section, is below
the action value for an ecoregion:
(i) Determine trophic status from existing or newly gathered
data. The recommended minimum sampling to determine
trophic status is calculated as the mean of four or more
samples collected from the epilimnion between June through
September in one or more consecutive years. Sampling must
be spread throughout the season.
(ii) Propose criteria at or below the upper limit of the trophic
state; or (iii) Conduct lake-specific study to determine and
propose to adopt appropriate criteria as described in (c) of
this subsection, (c) The following actions are recommended
if ambient monitoring of a lake shows total phosphorus to
exceed the action value for an ecoregion shown in Table 1 of
this section or where recommended ecoregional action
values do not exist:
(i) Conduct a lake-specific study to evaluate the
characteristic uses of the lake. A lake-specific study
may vary depending on the source or threat of
impairment. Phytoplankton blooms, toxic
phytoplankton, or excessive aquatic plants, are
examples of various sources of impairment. The
following are examples of quantitative measures that a
study may describe: Total phosphorus, total nitrogen,
chlorophyll-a, dissolved oxygen in the hypolimnion if
thermally stratified, pH, hardness, or other measures of
existing conditions and potential changes in any one of
these parameters.
(ii) Determine appropriate total phosphorus concentrations
or other nutrient criteria to protect characteristic lake
uses. If the existing total phosphorus concentration is
protective of characteristic lake uses, then set criteria at
existing total phosphorus concentration. If the existing
total phosphorus concentration is not protective of the
existing characteristic lake uses, then set criteria at a
protective concentration. Proposals to adopt appropriate
total phosphorus criteria to protect characteristic uses
must be developed by considering technical information
and stakeholder input as part of a public involvement
process equivalent to the Administrative Procedure Act
(chapter 34.05 RCW).
(iii) Determine if the proposed total phosphorus criteria
necessary to protect characteristic uses is achievable. If
the recommended criterion is not achievable and if the
characteristic use the criterion is intended to protect is
not an existing use, then a higher criterion may be
proposed in conformance with 40 CFR part 131.10.
(d) The department will consider proposed lake-specific
nutrient criteria during any water quality standards rule
making that follows development of a proposal. Adoption by
rule formally establishes the criteria for that lake.
appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S.
-------
State
Water Body Type or Designated Use
Numeric or Narrative Chlorophyll a Criteria
Wyoming
Surface water classes and uses.
1) Class 1 - Those surface waters in which
no further water quality degradation by
point source discharge other than from
dams will be allowed.
2) Class 2 - Those surface waters, other than
those classified as Class 1, which are
determined to support game fish.
3) Class 3 -Those surface waters, other than
those classified as Class 1, which are
determined to be presently supporting
non-game fish only.
4) Class 4 - Those surface waters, other than
those classified as Class 1, which are
determined to not have the hydrologic or
natural water quality potential to support
fish and include all intermittent and
ephemeral streams. Class 4 waters shall
receive protection for agricultural uses and
wildlife watering.
(i) USES
(a) Agriculture;
(b) Protection and propagation of fish
and wildlife;
(c) Industry;
(d) Human consumption;
(e) Recreation;
(f) Scenic value.
Section 28. Undesirable Aquatic Life.
All Wyoming surface waters shall be free from
substances and conditions or combinations thereof which
are attributable to municipal, industrial or other
dischargers or agricultural practices, in concentrations
which produce undesirable aquatic life.
Source: http://www.epa. gov/ost/standards/wqslibrary/
appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S.
-------
E-1
1950s-1990s Chesapeake Bay and Tidal
Tributary Chlorophyll a Concentrations
by Chesapeake Bay Program Segment
HISTORICAL DATA SETS
The earliest water quality data in the Chesapeake Bay Program data base date from
the early 1950s. Thus, the historical era referred to here extends from the early 1950s
to 1984, when the coordinated baywide Chesapeake Bay Monitoring Program
began. Most of the early studies focused on the physical and chemical characteriza-
tion of tidal waters. Sometimes measurements of phosphorus species, usually
orthophosphate, and chlorophyll a were taken. The impetus for more nutrient meas-
urements came during the 1960s (possibly exacerbated by the severe drought in that
decade) and 1970s with the increasing awareness of the Chesapeake Bay's eutroph-
ication and other signs of degradation. Nitrate measurements were collected more
frequently, and measurements of a larger suite of phosphorus and nitrogen species
began to be collected. Estimates of total phosphorus and total nitrogen are infrequent
in the historical data, however.
Data from the Johns Hopkins University Chesapeake Bay Institute and the U.S.
Environmental Protection Agency's Annapolis Field Office constitute the largest
contributions to the historical database. Maryland and Virginia state monitoring
programs provided data from various state waters. In Virginia, other major contribu-
tors to the historical database were the Virginia Institute of Marine Science and the
Virginia State Water Control Board slack water surveys. The database also includes
many smaller data sets including, among others, data from University of Maryland
researchers and from environmental impact studies of electric power generation in
Maryland.
Historical to present chlorophyll a concentration data are presented by Chesapeake
Bay Program segment within decades (1950s-1990s) in Table E-1. Table E-2
presents the same chlorophyll a data by Chesapeake Bay Program segment across
the same decades.
appendix E • 1950s—1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-2
BENCHMARK CHLOROPHYLL A DATA ASSESSMENT
The historical and current monitoring data sets (through 1999) were pooled, and the
surface (sampling depth <1.5 meters) values of the parameters were retained. Each
data point was associated with a segment (from the original Chesapeake Bay
Program segmentation scheme) and a salinity regime. Salinity regimes were defined
as: tidal-fresh 0-0.5 ppt; oligohaline >0.5-5.0 ppt; mesohaline > 5-18 ppt and poly-
haline >18 ppt.
If a salinity measurement was associated with the value, then that measurement
determined the regime. Otherwise, the regime was assumed from the median salinity
of the segment in which the measurement was taken. Values were further identified
according to decade (1950s through 1990s) and season. The seasons that were
included were: annual (January through December), spring (March, April and May)
and summer (June, July, August and September).
The individual data values were assessed using the Chesapeake Bay Program
method for calculating relative status (Alden and Perry 1997). The method uses the
logistic distribution of values in a reference data set to assess values in a test data set.
The procedure yields a score between 0 and 100 for each test value. The reference
data, in this case, were Chesapeake Bay Program Water Quality Monitoring data
from 1985 through 1990, which includes the largest number of stations and greatest
seasonal coverage of the monitoring program's history to date. It thus provided the
best available spatial and temporal coverage of the historical record. The time period
also represented a relatively wide variety of flow and other climatic conditions,
although none was particularly extreme.
The reference and test data sets were similarly partitioned by depth, segment, salinity
zone and season. For each reference grouping, the logistic distribution of values was
obtained and cutoff points representing the upper, middle and lower thirds of the
distribution were determined. For nitrogen, phosphorus, chlorophyll a and
suspended solids, high values are undesirable, therefore, the cutoff points repre-
sented 'poor', 'fair' and 'good' quality conditions, respectively, in this context. The
status procedure scored each test value between 0 and 100, based on the distribution
of the complementary reference distribution. Then, for each parameter/segment/
salinity zone/decade/season, the median score was calculated for each calendar
month, from which the median score for the season was obtained. The season
median scores were categorized as 'good', 'fair' or 'poor' by using the reference
cutoff points and adjusted slightly for the number of observations in the test data.
Each segment/zone/decade/season was then evaluated as representing 'healthy'
nutrient and sediment levels. To qualify, none of the critical parameters—total
nitrogen, total phosphorus, chlorophyll a or total suspended solids—could have a
'poor' assessment; only one parameter could have a 'fair' assessment and one or
more parameters had to be 'good'. Benchmark levels for each parameter were then
derived from this set of reference locations by extracting the values only from the
appendix E • 1950s-1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-3
reference locations in which the parameter of interest was assessed as 'good'. These
values were then pooled by salinity regime and decade and, ultimately by salinity
regime alone.
LITERATURE CITED
Alden, R. W. Ill and E. S. Perry 1997. Presenting Measurements of Status: Report to the
Chesapeake Bay Program Monitoring Subcommittee's Data Analysis Workgroup.
Chesapeake Bay Program, Annapolis, Maryland.
appendix E • 1950s—1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-4
Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by Chesapeake Bay Program segment within decade: 1950s-1990s.
Chesapeake Bay
Decade Program Segment
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
1950 Northern Chesapeake Bay
-
-
-
-
1.4
1
Upper Chesapeake Bay
1.1
1
-
-
2.2
7
Upper Central Chesapeake Bay
-
-
1.7
1
3.2
10
Middle Central Chesapeake Bay
3.1
3
2.1
1
4.0
13
Lower Chesapeake Bay
14.1
3
5.6
1
7.0
16
Western Lower Chesapeake Bay
-
-
-
-
0.7
8
Eastern Lower Chesapeake Bay
7.9
3
-
-
4.2
19
Mouth of the Chesapeake Bay
-
-
-
-
1.6
8
Outside of Ches. Bay Mouth
-
-
2.0
1
2.2
2
Northeast River - - - - - -
Elk/Bohemia Rivers - - - - - -
Sassafras River - - - - - -
Chester River - - - - - -
Eastern Bay
-
-
0.5
1
1.5
3
Choptank River
2.4
2
3.4
3
2.8
7
Lower Choptank River
6.9
1
1.7
3
2.6
5
Nanticoke River - - - - - -
Wicomico River - - - - - -
Manokin River - - - - - -
Big Annemessex River - - - - - -
Tangier Sound
-
-
11.8
1
4.3
8
Pocomoke River - - - - - -
Bush River - - - - - -
Gunpowder River - - - - - -
Middle River - - - - - -
appendix E • 1950s-1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-5
Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by Chesapeake Bay Program segment within decade: 1950s-1990s (continued).
Chesapeake Bay
Decade Program Segment
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
1950 Back River
-
-
-
-
-
-
Patapsco River
-
-
-
-
7.5
1
Magothy River
-
-
-
-
-
-
Severn River
-
-
-
-
-
-
South/Rhode/West Rivers
-
-
-
-
-
-
Upper Patuxent River
2.6
1
1.7
2
1.7
4
Middle Patuxent River
-
-
2.1
2
2.9
3
Lower Patuxent River
5.3
3
3.3
4
2.6
14
Upper Potomac River
-
-
-
-
-
-
Middle Potomac River
-
-
26.7
1
26.7
1
Lower Potomac River
10.8
2
5.0
12
6.1
23
Upper Rappahannock River
-
-
-
-
-
-
Middle Rappahannock River
-
-
-
-
3.7
2
Lower Rappahannock River
8.2
1
-
-
4.3
6
Upper York River
-
-
-
-
-
-
Middle York River
-
-
-
-
2.0
1
Lower York River
4.5
1
-
-
1.8
3
Mobjack Bay
-
-
-
-
0.6
2
Upper James River
-
-
-
-
-
-
Middle James River
-
-
-
-
-
-
Lower James River
-
-
3.3
19
2.3
28
1960 Northern Chesapeake Bay
6.1
8
18.2
11
12.4
31
Upper Chesapeake Bay
7.0
10
25.9
15
15.9
42
Upper Central Chesapeake Bay
6.9
29
18.2
59
11.5
122
Middle Central Chesapeake Bay
3.9
18
11.1
25
7.4
69
Lower Chesapeake Bay
2.4
7
10.9
12
9.7
28
appendix E • 1950s—1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-6
Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by Chesapeake Bay Program segment within decade: 1950s-1990s (continued).
Decade
Chesapeake Bay
Program Segment
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
1960
Western Lower Chesapeake Bay
-
-
-
-
-
-
Eastern Lower Chesapeake Bay
5.5
1
1.2
2
2.0
5
Mouth of the Chesapeake Bay
-
-
-
-
-
-
Outside the Ches. Bay Mouth
1.1
2
0.8
4
1.0
8
Northeast River
-
-
-
-
-
-
Elk/Bohemia Rivers
-
-
-
-
-
-
Sassafras River
-
-
18.1
3
20.8
5
Chester River
5.2
11
8.7
14
5.6
36
Eastern Bay
5.3
27
9.2
39
6.5
94
Choptank River
-
-
-
-
-
-
Lower Choptank River
-
-
-
-
-
-
Nanticoke River
-
-
-
-
-
-
Wicomico River
-
-
-
-
-
-
Manokin River
-
-
-
-
-
-
Big Annemessex River
-
-
-
-
-
-
Tangier Sound
-
-
-
-
-
-
Pocomoke River
-
-
-
-
-
-
Bush River
-
-
-
-
-
-
Gunpowder River
-
-
-
-
-
-
Middle River
-
-
-
-
-
-
Back River
-
-
7.7
1
30.9
3
Patapsco River
18.7
17
47.1
41
41.9
64
Magothy River
8.6
13
12.5
21
11.5
56
Severn River
7.1
12
15.9
22
10.8
60
South/Rhode/West Rivers
6.3
17
15.4
38
11.1
73
appendix E • 1950s-1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-7
Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by Chesapeake Bay Program segment within decade: 1950s-1990s (continued).
Chesapeake Bay
Decade Program Segment
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
1960 Upper Patuxent River
20.1
18
32.0
43
22.5
65
Middle Patuxent River
15.0
2
24.8
4
21.5
6
Lower Patuxent River
19.9
2
20.5
4
20.3
6
Upper Potomac River
24.3
50
59.1
81
38.7
176
Middle Potomac River
8.1
26
29.3
35
23.6
83
Lower Potomac River
8.5
24
18.7
33
13.7
76
Upper Rappahannock River
-
-
-
-
-
-
Middle Rappahannock River
-
-
-
-
-
-
Lower Rappahannock River
-
-
-
-
-
-
Upper York River
-
-
-
-
-
-
Middle York River
-
-
-
-
-
-
Lower York River
-
-
-
-
-
-
Mobjack Bay
-
-
-
-
-
-
Upper James River
-
-
-
-
-
-
Middle James River
-
-
-
-
-
-
Lower James River
12.8
2
-
-
12.8
2
1970 Northern Chesapeake Bay
11.7
28
19.3
66
12.1
116
Upper Chesapeake Bay
9.6
26
15.4
66
10.6
125
Upper Central Chesapeake Bay
14.2
156
20.7
266
14.8
589
Middle Central Chesapeake Bay
11.5
99
10.5
142
9.7
325
Lower Chesapeake Bay
11.5
29
7.7
35
8.1
94
Western Lower Chesapeake Bay
-
-
11.0
1
11.0
1
Eastern Lower Chesapeake Bay
14.7
13
4.8
17
7.3
45
Mouth of the Chesapeake Bay
14.8
4
7.7
14
8.7
29
Outside the Ches. Bay Mouth
5.1
7
3.5
8
4.2
31
Northeast River
40.0
11
54.9
35
49.0
53
appendix E • 1950s—1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-8
Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by Chesapeake Bay Program segment within decade: 1950s-1990s (continued).
Chesapeake Bay
Decade Program Segment
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
1970 Elk/Bohemia Rivers
27.3
62
28.8
136
25.9
248
Sassafras River
42.2
26
43.1
61
46.8
106
Chester River
18.2
42
25.6
84
22.7
159
Eastern Bay
6.5
84
21.7
89
14.0
226
Choptank River
18.4
99
17.1
121
18.8
276
Lower Choptank River
11.1
37
21.5
60
17.2
103
Nanticoke River
32.5
37
22.9
80
26.7
168
Wicomico River
36.7
31
41.9
42
31.4
101
Manokin River
15.5
3
7.2
5
12.2
8
Big Annemessex River
-
-
18.2
6
18.2
6
Tangier River
20.3
37
16.6
57
27.6
113
Pocomoke River
23.1
43
19
63
19.9
146
Bush River
7.3
4
13.2
12
10.1
25
Gunpowder River
7.6
24
7.3
39
9.7
94
Middle River
14.7
8
28.2
8
17.7
19
Back River
55.7
115
61.5
167
58.3
392
Patapsco River
14.1
36
40.9
77
23.4
162
Magothy River
33.8
40
37.8
50
32.7
129
Severn River
22.2
12
32.1
43
24.8
75
South/Rhode/West Rivers
25.2
31
29.7
84
29.4
157
Upper Patuxent River
10.9
37
15.8
68
14.3
147
Middle Patuxent River
31.3
2
18.1
8
16.8
14
Lower Patuxent River
10.9
4
15.7
5
11.5
12
Upper Potomac River
17.9
142
31.0
286
18.0
559
Middle Potomac River
20.0
78
19.3
142
16.6
288
appendix E • 1950s-1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-9
Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by Chesapeake Bay Program segment within decade: 1950s-1990s (continued).
Chesapeake Bay
Decade Program Segment
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
1970 Lower Potomac River
8.0
40
8.9
65
11.2
140
Upper Rappahannock River
2.1
66
9.4
142
5.7
313
Middle Rappahannock River
6.4
13
6.6
29
5.6
65
Lower Rappahannock River
6.8
14
8.0
35
7.5
76
Upper York River
3.9
18
9.8
107
7.2
170
Middle York River
5.0
24
9.8
109
7.2
167
Lower York River
7.8
8
5.7
21
5.8
35
Mobjack Bay
8.3
16
7.4
42
6.5
69
Upper James River
5.5
55
8.9
187
5.2
345
Middle James River
7.7
19
4.6
75
4.6
137
Lower James River
7.6
9
3.8
43
3.6
73
1980 Northern Chesapeake Bay
7.6
20
10.9
28
7.8
68
Upper Central Chesapeake Bay
8.4
38
10.1
55
7.3
135
Upper Central Chesapeake Bay
11.5
87
14.7
152
10.7
362
Middle Central Chesapeake Bay
10.4
155
10.7
225
9.4
590
Lower Chesapeake Bay
10.3
111
9.0
158
8.6
454
Western Lower Chesapeake Bay
7.2
60
8.7
80
7.6
236
Eastern Lower Chesapeake Bay
6.2
140
5.8
187
6.5
543
Mouth of the Chesapeake Bay
5.8
45
4.9
62
5.5
181
Outside the Ches. Bay Mouth
6.0
1
2.5
2
4.0
5
Northeast River
23.7
11
54.3
17
31.9
44
Elk/Bohemia Rivers
18.1
34
9.9
52
10.1
141
Sassafras River
34.3
12
70.2
15
47.9
45
Chester River
8.1
46
16.0
83
10.5
205
Eastern Bay
4.3
14
10.2
23
6.6
58
Choptank River
7.0
34
17.4
57
11.2
138
appendix E • 1950s—1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-10
Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by Chesapeake Bay Program segment within decade: 1950s-1990s (continued).
Chesapeake Bay
Decade Program Segment
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
1980 Lower Choptank River
6.4
26
9.3
44
7.0
107
Nanticoke River
11.4
23
18.0
32
13.1
90
Wicomico River
6.6
11
19.6
16
11.3
44
Manokin River
8.2
12
13.8
16
9.0
43
Big Annemessex River
5.0
12
10.0
16
6.5
43
Tangier Sound
9.5
65
10.7
86
8.2
237
Pocomoke River
4.2
12
11.2
15
8.9
45
Bush River
17.6
13
42.9
22
25.3
53
Gunpowder River
22.3
11
20.5
24
17.5
53
Middle River
14.8
11
24.2
19
19.8
48
Back River
105.5
13
101.8
38
83.7
87
Patapsco River
17.5
22
50.3
44
29.3
95
Magothy River
10.0
13
22.1
19
15.0
51
Severn River
13.0
10
22.8
18
16.8
47
South/Rhode/West Rivers
14.9
42
23.8
58
16.5
157
Upper Patuxent River
4.7
94
18.4
160
9.2
414
Middle Patuxent River
15.5
13
14.2
26
17.1
65
Lower Patuxent River
14.7
52
11.4
95
11.4
245
Upper Potomac River
4.5
95
15.9
121
7.9
336
Middle Potomac River
7.4
62
7.4
79
5.8
224
Lower Potomac River
18.2
31
10.3
43
10.7
120
Upper Rappahannock River
4.1
30
15.2
53
8.4
124
Middle Rappahannock River
22.1
24
10.8
39
12.5
103
Lower Rappahannock River
10.9
78
8.9
120
8.3
324
Upper York River
3.1
24
5.1
40
3.8
102
appendix E • 1950s-1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-11
Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by Chesapeake Bay Program segment within decade: 1950s-1990s (continued).
Chesapeake Bay
Decade Program Segment
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
1980 Middle York River
5.4
36
11.0
60
7.0
152
Lower York River
13.5
36
8.3
59
9.7
151
Mobjack Bay
6.3
60
8.4
80
6.9
236
Upper James River
10.2
65
20.7
114
11.2
283
Middle James River
13.8
24
17.3
40
13.7
100
Lower James River
13.8
88
6.2
140
9.6
349
1990 Northern Chesapeake Bay
6.6
27
8.6
40
5.8
102
Upper Chesapeake Bay
5.0
58
6.3
79
4.2
220
Upper Central Chesapeake Bay
7.6
147
14.3
200
9.0
487
Middle Central Chesapeake Bay
7.5
300
9.9
400
8.1
929
Lower Chesapeake Bay
9.5
210
8.5
279
8.0
819
Western Lower Chesapeake Bay
7.1
118
7.5
159
6.7
475
Eastern Lower Chesapeake Bay
6.6
264
6.8
359
6.5
1059
Mouth of the Chesapeake Bay
6.3
88
5.6
120
5.8
354
Outside the Ches. Bay Mouth
-
-
-
-
-
-
Northeast River
23.0
27
53.5
38
31.4
105
Elk/Bohemia Rivers
6.9
88
6.5
113
5.9
326
Sassafras River
39.6
29
66.9
35
46.1
113
Chester River
10.6
89
20.3
117
13.2
350
Eastern Bay
8.3
30
12.8
39
9.2
117
Choptank River
13.3
60
19.9
78
12.8
234
Lower Choptank River
7.4
60
8.4
78
7.4
229
Nanticoke River
10.4
60
26.9
74
15.5
226
Wicomico River
8.1
29
14.3
36
10.6
112
Manokin River
11.8
30
11.2
36
9.8
111
Big Annemessex River
7.5
30
9.6
35
7.4
112
appendix E • 1950s—1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-12
Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by Chesapeake Bay Program segment within decade: 1950s-1990s (continued).
Chesapeake Bay
Decade Program Segment
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
1990 Tangier Sound
10.8
147
10.6
189
9.3
566
Pocomoke River
2.1
30
7.5
39
4.6
113
Bush River
26.4
28
50.9
37
31.0
106
Gunpowder River
21.5
29
18.6
38
17.0
106
Middle River
20.1
29
12.8
38
13.5
107
Back River
104.2
29
82.4
38
75.7
107
Patapsco River
15.5
29
36.1
39
22.3
113
Magothy River
12.2
29
18.3
37
13.6
110
Severn River
13.2
30
19.4
35
14.4
109
South/Rhode/West Rivers
12.4
89
18.4
110
13.0
315
Upper Patuxent River
5.9
234
15.9
307
8.7
863
Middle Patuxent River
17.8
30
15.6
39
15.6
118
Lower Patuxent River
10.7
120
13.0
156
10.4
472
Upper Potomac River
6.0
174
20.3
233
9.8
655
Middle Potomac River
5.0
93
8.4
121
5.6
350
Lower Potomac River
10.8
60
9.4
80
8.7
228
Upper Rappahannock River
3.6
149
14.1
209
7.3
563
Middle Rappahannock River
9.0
66
11.0
85
8.5
250
Lower Rappahannock River
8.2
187
7.9
250
7.1
727
Upper York River
1.5
64
4.4
79
2.5
240
Middle York River
3.5
92
13.3
118
7.4
349
Lower York River
10.3
97
7.6
125
7.6
371
Mobjack Bay
7.3
125
8.5
167
7.3
502
Upper James River
6.3
210
16.3
284
8.9
813
Middle James River
13.3
64
14.1
85
11.1
245
Lower James River
10.7
331
7.9
447
7.7
1295
appendix E • 1950s-1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-13
Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by segment across decades: 1950s-1990s.
Segment
Decade
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
CB1
1950
-
-
-
-
1.4
1
CB1
1960
6.1
8
18.2
11
12.4
31
CB1
1970
11.7
28
19.3
66
12.1
116
CB1
1980
7.6
20
10.9
28
7.8
68
CB1
1990
6.6
27
8.6
40
5.8
102
CB2
1950
1.1
1
-
-
2.2
7
CB2
1960
7.0
10
25.9
15
15.9
42
CB2
1970
9.6
26
15.4
66
10.6
125
CB2
1980
8.4
38
10.1
55
7.3
135
CB2
1990
5.0
58
6.3
79
4.2
220
CB3
1950
-
-
1.7
1
3.2
10
CB3
1960
6.9
29
18.2
59
11.5
122
CB3
1970
14.2
156
20.7
266
14.8
589
CB3
1980
11.5
87
14.7
152
10.7
362
CB3
1990
7.6
147
14.3
200
9.0
487
CB4
1950
3.1
3
2.1
1
4.0
13
CB4
1960
3.9
18
11.1
25
7.4
69
CB4
1970
11.5
99
10.5
142
9.7
325
CB4
1980
10.4
155
10.7
225
9.4
590
CB4
1990
7.5
300
9.9
400
8.1
929
CB5
1950
14.1
3
5.6
1
7.0
16
CB5
1960
2.4
7
10.9
12
9.7
28
CB5
1970
11.5
29
7.7
35
8.1
94
CB5
1980
10.3
111
9.0
158
8.6
454
CB5
1990
9.5
210
8.5
279
8.0
819
CB6
1950
-
-
-
-
0.7
8
appendix E • 1950s—1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-14
Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by segment across decades: 1950s-1990s (continued).
Segment
Decade
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
CB6
1960
-
-
-
-
-
-
CB6
1970
-
-
11.0
1
11.0
1
CB6
1980
7.2
60
8.7
80
7.6
236
CB6
1990
7.1
118
7.5
159
6.7
475
CB7
1950
7.9
3
-
-
4.2
19
CB7
1960
5.5
1
1.2
2
2.0
5
CB7
1970
14.7
13
4.8
17
7.3
45
CB7
1980
6.2
140
5.8
187
6.5
543
CB7
1990
6.6
264
6.8
359
6.5
1059
CB8
1950
-
-
-
-
1.6
8
CB8
1960
-
-
-
-
-
CB8
1970
14.8
4
7.7
14
8.7
29
CB8
1980
5.8
45
4.9
62
5.5
181
CB8
1990
6.3
88
5.6
120
5.8
354
MOUTH
1950
-
-
2.0
1
2.2
2
MOUTH
1960
1.1
2
0.8
4
1.0
8
MOUTH
1970
5.1
7
3.5
8
4.2
31
MOUTH
1980
6.0
1
2.5
2
4.0
5
MOUTH
1990
-
-
-
-
-
-
ET1
1950
-
-
-
-
-
-
ET1
1960
-
-
-
-
-
-
ET1
1970
40.0
11
54.9
35
49.0
53
ET1
1980
23.7
11
54.3
17
31.9
44
ET1
1990
23.0
27
53.5
38
31.4
105
ET2
1950
-
-
-
-
-
-
ET2
1960
-
-
-
-
-
-
appendix E • 1950s-1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-15
Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by segment across decades: 1950s-1990s (continued).
Segment
Decade
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
ET2
1970
27.3
62
28.8
136
25.9
248
ET2
1980
18.1
34
9.9
52
10.1
141
ET2
1990
6.9
88
6.5
113
5.9
326
ET3
1950
-
-
-
-
-
-
ET3
1960
-
-
18.1
3
20.8
5
ET3
1970
42.2
26
43.1
61
46.8
106
ET3
1980
34.3
12
70.2
15
47.9
45
ET3
1990
39.6
29
66.9
35
46.1
113
ET4
1950
-
-
-
-
-
-
ET4
1960
5.2
11
8.7
14
5.6
36
ET4
1970
18.2
42
25.6
84
22.7
159
ET4
1980
8.1
46
16.0
83
10.5
205
ET4
1990
10.6
89
20.3
117
13.2
350
EE1
1950
-
-
0.5
1
1.5
3
EE1
1960
5.3
27
9.2
39
6.5
94
EE1
1970
6.5
84
21.7
89
14.0
226
EE1
1980
4.3
14
10.2
23
6.6
58
EE1
1990
8.3
30
12.8
39
9.2
117
ET5
1950
2.4
2
3.4
3
2.8
7
ET5
1960
-
-
-
-
-
-
ET5
1970
18.4
99
17.1
121
18.8
276
ET5
1980
7.0
34
17.4
57
11.2
138
ET5
1990
13.3
60
19.9
78
12.8
234
EE2
1950
6.9
1
1.7
3
2.6
5
EE2
1960
-
-
-
-
-
-
EE2
1970
11.1
37
21.5
60
17.2
103
appendix E • 1950s—1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-16
Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by segment across decades: 1950s-1990s (continued).
Segment
Decade
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
EE2
1980
6.4
26
9.3
44
7.0
107
EE2
1990
7.4
60
8.4
78
7.4
229
EE3
1950
-
-
11.8
1
4.3
8
EE3
1960
-
-
-
-
-
-
EE3
1970
20.3
37
16.6
57
27.6
113
EE3
1980
9.5
65
10.7
86
8.2
237
EE3
1990
10.8
147
10.6
189
9.3
566
ET6
1950
-
-
-
-
-
-
ET6
1960
-
-
-
-
-
-
ET6
1970
32.5
37
22.9
80
26.7
168
ET6
1980
11.4
23
18.0
32
13.1
90
ET6
1990
10.4
60
26.9
74
15.5
226
ET7
1950
-
-
-
-
-
-
ET7
1960
-
-
-
-
-
-
ET7
1970
36.7
31
41.9
42
31.4
101
ET7
1980
6.6
11
19.6
16
11.3
44
ET7
1990
8.1
29
14.3
36
10.6
112
ET8
1950
-
-
-
-
-
-
ET8
1960
-
-
-
-
-
-
ET8
1970
15.5
3
7.2
5
12.2
8
ET8
1980
8.2
12
13.8
16
9.0
43
ET8
1990
11.8
30
11.2
36
9.8
111
ET9
1950
-
-
-
-
-
-
ET9
1960
-
-
-
-
-
-
ET9
1970
-
-
18.2
6
18.2
6
ET9
1980
5.0
12
10.0
16
6.5
43
appendix E • 1950s-1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-17
Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by segment across decades: 1950s-1990s (continued).
Segment
Decade
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
ET9
1990
7.5
30
9.6
35
7.4
112
ET10
1950
-
-
-
-
-
-
ET10
1960
-
-
-
-
-
-
ET10
1970
23.1
43
19.0
63
19.9
146
ET10
1980
4.2
12
11.2
15
8.9
45
ET10
1990
2.1
30
7.5
39
4.6
113
WT1
1950
-
-
-
-
-
-
WT1
1960
-
-
-
-
-
-
WT1
1970
7.3
4
13.2
12
10.1
25
WT1
1980
17.6
13
42.9
22
25.3
53
WT1
1990
26.4
28
50.9
37
31.0
106
WT2
1950
-
-
-
-
-
-
WT2
1960
-
-
-
-
-
-
WT2
1970
7.6
24
7.3
39
9.7
94
WT2
1980
22.3
11
20.5
24
17.5
53
WT2
1990
21.5
29
18.6
38
17.0
106
WT3
1950
-
-
-
-
-
-
WT3
1960
-
-
-
-
-
-
WT3
1970
14.7
8
28.2
8
17.7
19
WT3
1980
14.8
11
24.2
19
19.8
48
WT3
1990
20.1
29
12.8
38
13.5
107
WT4
1950
-
-
-
-
-
-
WT4
1960
-
-
7.7
1
30.9
3
WT4
1970
55.7
115
61.5
167
58.3
392
WT4
1980
105.5
13
101.8
38
83.7
87
WT4
1990
104.2
29
82.4
38
75.7
107
appendix E • 1950s—1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-18
Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by segment across decades: 1950s-1990s (continued).
Segment
Decade
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
WT5
1950
-
-
-
-
7.5
1
WT5
1960
18.7
17
47.1
41
41.9
64
WT5
1970
14.1
36
40.9
77
23.4
162
WT5
1980
17.5
22
50.3
44
29.3
95
WT5
1990
15.5
29
36.1
39
22.3
113
WT6
1950
-
-
-
-
-
-
WT6
1960
8.6
13
12.5
21
11.5
56
WT6
1970
33.8
40
37.8
50
32.7
129
WT6
1980
10.0
13
22.1
19
15.0
51
WT6
1990
12.2
29
18.3
37
13.6
110
WT7
1950
-
-
-
-
-
-
WT7
1960
7.1
12
15.9
22
10.8
60
WT7
1970
22.2
12
32.1
43
24.8
75
WT7
1980
13.0
10
22.8
18
16.8
47
WT7
1990
13.2
30
19.4
35
14.4
109
WT8
1950
-
-
-
-
-
-
WT8
1960
6.3
17
15.4
38
11.1
73
WT8
1970
25.2
31
29.7
84
29.4
157
WT8
1980
14.9
42
23.8
58
16.5
157
WT8
1990
12.4
89
18.4
110
13.0
315
TF1
1950
2.6
1
1.7
2
1.7
4
TF1
1960
20.1
18
32.0
43
22.5
65
TF1
1970
10.9
37
15.8
68
14.3
147
TF1
1980
4.7
94
18.4
160
9.2
414
TF1
1990
5.9
234
15.9
307
8.7
863
RET1
1950
-
-
2.1
2
2.9
3
appendix E • 1950s-1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-19
Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by segment across decades: 1950s-1990s (continued).
Segment
Decade
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
RET1
1960
15.0
2
24.8
4
21.5
6
RET1
1970
31.3
2
18.1
8
16.8
14
RET1
1980
15.5
13
14.2
26
17.1
65
RET1
1990
17.8
30
15.6
39
15.6
118
LEI
1950
5.3
3
3.3
4
2.6
14
LEI
1960
19.9
2
20.5
4
20.3
6
LEI
1970
10.9
4
15.7
5
11.5
12
LEI
1980
14.7
52
11.4
95
11.4
245
LEI
1990
10.7
120
13.0
156
10.4
472
TF2
1950
-
-
-
-
-
-
TF2
1960
24.3
50
59.1
81
38.7
176
TF2
1970
17.9
142
31.0
286
18.0
559
TF2
1980
4.5
95
15.9
121
7.9
336
TF2
1990
6.0
174
20.3
233
9.8
655
RET2
1950
-
-
26.7
1
26.7
1
RET2
1960
8.1
26
29.3
35
23.6
83
RET2
1970
20.0
78
19.3
142
16.6
288
RET2
1980
7.4
62
7.4
79
5.8
224
RET2
1990
5.0
93
8.4
121
5.6
350
LE2
1950
10.8
2
5.0
12
6.1
23
LE2
1960
8.5
24
18.7
33
13.7
76
LE2
1970
8.0
40
8.9
65
11.2
140
LE2
1980
18.2
31
10.3
43
10.7
120
LE2
1990
10.8
60
9.4
80
8.7
228
TF3
1950
-
-
-
-
-
-
TF3
1960
-
-
-
-
-
-
appendix E • 1950s—1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-20
Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by segment across decades: 1950s-1990s (continued).
Segment
Decade
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
TF3
1970
2.1
66
9.4
142
5.7
313
TF3
1980
4.1
30
15.2
53
8.4
124
TF3
1990
3.6
149
14.1
209
7.3
563
RET3
1950
-
-
-
-
3.7
2
RET3
1960
-
-
-
-
-
-
RET3
1970
6.4
13
6.6
29
5.6
65
RET3
1980
22.1
24
10.8
39
12.5
103
RET3
1990
9.0
66
11.0
85
8.5
250
LE3
1950
8.2
1
-
-
4.3
6
LE3
1960
-
-
-
-
-
-
LE3
1970
6.8
14
8.0
35
7.5
76
LE3
1980
10.9
78
8.9
120
8.3
324
LE3
1990
8.2
187
7.9
250
7.1
727
TF4
1950
-
-
-
-
-
-
TF4
1960
-
-
-
-
-
-
TF4
1970
3.9
18
9.8
107
7.2
170
TF4
1980
3.1
24
5.1
40
3.8
102
TF4
1990
1.5
64
4.4
79
2.5
240
RET4
1950
-
-
-
-
2.0
1
RET4
1960
-
-
-
-
-
-
RET4
1970
5.0
24
9.8
109
7.2
167
RET4
1980
5.4
36
11.0
60
7.0
152
RET4
1990
3.5
92
13.3
118
7.4
349
LE4
1950
4.5
1
-
-
1.8
3
LE4
1960
-
-
-
-
-
-
LE4
1970
7.8
8
5.7
21
5.8
35
appendix E • 1950s-1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
E-21
Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (jjg liter1)
by segment across decades: 1950s-1990s (continued).
Segment
Decade
Spring
Mean
(N)
Summer
Mean
(N)
Annual
Mean
(N)
LE4
1980
13.5
36
8.3
59
9.7
151
LE4
1990
10.3
97
7.6
125
7.6
371
WE4
1950
-
-
-
-
0.6
2
WE4
1960
-
-
-
-
-
-
WE4
1970
8.3
16
7.4
42
6.5
69
WE4
1980
6.3
60
8.4
80
6.9
236
WE4
1990
7.3
125
8.5
167
7.3
502
TF5
1950
-
-
-
-
-
-
TF5
1960
-
-
-
-
-
-
TF5
1970
5.5
55
8.9
187
5.2
345
TF5
1980
10.2
65
20.7
114
11.2
283
TF5
1990
6.3
210
16.3
284
8.9
813
RET5
1950
-
-
-
-
-
-
RET5
1960
-
-
-
-
-
-
RET5
1970
7.7
19
4.6
75
4.6
137
RET5
1980
13.8
24
17.3
40
13.7
100
RET5
1990
13.3
64
14.1
85
11.1
245
LE5
1950
-
-
3.3
19
2.3
28
LE5
1960
12.8
2
-
-
12.8
2
LE5
1970
7.6
9
3.8
43
3.6
73
LE5
1980
13.8
88
6.2
140
9.6
349
LE5
1990
10.7
331
7.9
447
7.7
1295
appendix E • 1950s—1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
-------
F-1
appen
dix
Phytoplankton Reference
Community Data Analyses
This appendix describes various analyses performed with the 1984-2001 Chesa-
peake Bay Program water quality and plankton monitoring data that supported
determination of the phytoplankton reference community chlorophyll a concentra-
tions reported in Chapter V.
REFERENCE PHYTOPLANKTON COMMUNITIES AND
WATER QUALITY CONDITION CLASSIFICATIONS
Biological populations found in pristine or minimally impaired habitats provide
essential information about how restoration efforts might improve ecosystem struc-
ture and function. Called 'reference communities,' these populations serve as
benchmarks for measuring ecosystem impairment. Ecosystem impairment is
assessed with a suite of physical, chemical and biological performance indicators
which are measurable attributes of the ecosystem linked directly to restoration objec-
tives. The properties of the performance indicators in biological reference
communities furnish the evaluation (scoring) criteria needed to quantify ecosystem
impairment at other sites (National Research Council 1992). Chlorophyll a has long
been used as a surrogate measure of phytoplankton biomass and as a performance
indicator of nutrient enrichment across a wide spectrum of aquatic systems (see
Chapter V). Chlorophyll a as an indicator is directly linked to a restoration objective
of the Chesapeake Bay Program, namely the reduction of excess, uneaten phyto-
plankton that accumulates in the water column and contributes to reduced water
clarity and summer oxygen depletion in bottom waters, ultimately stressing the food
webs the phytoplankton support.
Chlorophyll a concentrations for season- and salinity-specific phytoplankton refer-
ence communities for Chesapeake Bay tidal waters are described in this appendix
and elsewhere (Buchanan et al., in review). The reference communities are based on
phytoplankton populations currently found in waters least impaired by poor water
clarity and nutrients in excess of phytoplankton growth requirements. Water quality
appendix F • Phytoplankton Reference Community Data Analyses
-------
F-2
condition classifications were detennined with three parameters crucial to phyto-
plankton growth: light penetration (measured as Secchi depth), dissolved inorganic
nitrogen (DIN) and ortho-phosphate (P04).
ANALYSIS APPROACH
Chesapeake Bay water quality and phytoplankton data collected at Chesapeake Bay
Program biomonitoring stations between 1984 and 2001 were first analyzed to iden-
tify samples that were least impaired by poor water clarity and excess nutrients.
Seasonal and salinity-specific phytoplankton 'reference' communities for the Chesa-
peake Bay were then derived from the populations in those samples. The reference
communities are used in this analysis to quantify chlorophyll a concentrations in the
least-impaired water quality conditions currently found in the Chesapeake Bay and
its tidal tributaries.
The Chesapeake Bay Monitoring Program has coordinated the year-round collection
of plankton and water quality data at more than 26 stations for all salinity zones in
the Chesapeake Bay mainstem and its major tidal tributaries since August 1984. Data
for some parameters were collected over shorter periods or only by one state. The
primary data and data documentation are available at http://www.chesapeakebay.net/
data. Phytoplankton parameters that are measured (primary data) or derived from
measured data include chlorophyll a, pheophytin, species abundances, biomasses of
individual species in the nano (2-20 micron) and micro (20-200 micron) size frac-
tions, phytoplankton biomass in pico (<2 micron) size fractions, average cell size of
the nano-micro phytoplankton and the ratio of phytoplankton biomass (as carbon) to
chlorophyll a. Productivity cannot be used for baywide analyses because Maryland
and Virginia methodologies are different. In this study, water quality and phyto-
plankton data from the mixed upper layer of the water column (usually identified as
'above-pycnocline,' or AP) were analyzed, with the exception of a few tidal-fresh
stations where samples were from the whole water column (WC). Data from each
sampling event at an individual station were sorted into two seasons and four salinity
zones for examination: spring (March, April and May) and summer (July, August
and September); and tidal-fresh (0.0 to 0.5 ppt), oligohaline (>0.5 to 5.0 ppt), meso-
haline (>5.0 tol8.0 ppt) and polyhaline (>18.0 ppt). This minimizes the influence of
season and salinity regime on the analysis.
Phytoplankton and water quality data within each season-salinity group were binned
(further grouped) into six categories using Secchi depth, DIN and P04 thresholds
shown in tables F-l and F-2. The thresholds classify the Secchi depth, DIN, and P04
values of each data record as 'worst,' 'poor,' 'better,' or 'best'. The DIN and P04
thresholds separating 'better' and 'poor' values in tables F-l and F-2 have been
experimentally shown to be resource limitation thresholds for natural Chesapeake
Bay phytoplankton populations (Fisher et al. 1988, 1999; Thomas Fisher personal
communication). The Secchi depth thresholds separating 'better' and 'poor' values
were empirically determined from the monitoring data using the Relative Status,
or benchmark, method (Olson 2002). The 'better' water clarity levels are those
appendix F • Phytoplankton Reference Community Data Analyses
-------
F-3
Table F-1. Spring (March through May) classification criteria for determining 'worst', 'poor', 'bet-
ter' and 'best' water quality parameter conditions. Key: Secchi-Secchi depth (meters);
DIN-average dissolved organic nitrogen in surface mixed layer (mg liter1); P04—aver-
age orthophosphate (SRP) in surface mixed layer (mg liter1); TF—tidal fresh salinities
(0 to 0.5 ppt); OH—oligohaline salinities (>0.5 to 5 ppt); MH—mesohaline salinties
(>5 to 18 ppt); PH—polyhaline (>18 ppt). The 25th percentile, median and 75th per-
centile of the parameter's values at stations identified as 'good' by the Relative Status
Method are given for comparison purposes. See Buchanan et al. (in review) for details.
Parameter
Selected Spring Classification Criteria
Relative Status Method
Worst
Poor
Better
Best
25th%/median/75th%
Secchi
TF
<0.7
=<0.9
>0.9
>1.1
0.7 | 0.9 | 1.10
Secchi
OH
<0.5
=<0.7
>0.7
>1.1
0.5 | 0.7 | 1.10
Secchi
MH
<1.35
=<1.8
>1.8
>2.25
1.35 | 1.80 | 2.25
Secchi
PH
<1.6
=<2.15
>2.15
>2.55
1.6 | 2.15 \ 2.55
Worst
Poor
Better
Best
75th%/median/25th%
DIN
TF
>.585
>0.070
=<0.070
<0.030
.585 | .434 | .290
DIN
OH
>.885
>0.070
=<0.070
<0.030
.885 | .680 | .464
DIN
MH
>.265
>0.070
=<0.070
<0.030
.265 | .150 | .070
DIN
PH
>.070
>0.070
=<0.070
<0.030
.063 | .020 | .011
Worst
Poor
Better
Best
75th%/median/25th%
P04 (SRP)
TF
>0.020
>0.003
=<0.003
=<0.003
.020 | . 136 | .010
P04 (SRP)
OH
>0.010
>0.003
=<0.003
=<0.003
.010|.005 | .004
P04 (SRP)
MH
>0.003
>0.002
=<0.002
=<0.002
.003 | .002 | .0006
P04 (SRP)
PH
>0.005
>0.003
=<0.003
=<0.003
.005 | .004 | .0007
associated with the least impaired stations currently monitored in the Chesapeake
Bay. They also approximate the light levels required for growth of underwater bay
grasses (Batiuk et al. 2000). For the purpose of establishing phytoplankton reference
communities, a water quality parameter classification of 'worst or 'poor' is consid-
ered impaired while a water quality parameter classification of 'better' or 'best' is
considered unimpaired.
When all three parameters were classified as 'worst,' the data record was placed in
the 'worst' water quality category. When all three parameters classified as 'poor' or
'worst' (includes all 'worst'), the data record was placed in the 'poor' water quality
category. 'Poor' and 'worst' water quality conditions are characterized by low levels
of light, and concentrations of DIN and P04 that exceed phytoplankton nutrient
requirements. 'Worst' is an extreme subset of 'poor.' Similarly, when all three param-
eters classified as 'best,' the data record was placed in the 'best' water quality
category. When all three classified as 'best' or 'better' (includes all 'best'), the data
record was placed in the 'better' water quality category, 'better' and 'best' water
quality conditions had high levels of light and limiting (low) concentrations of DIN
and P04. 'Best' is an extreme subset of 'better'. Data records were placed in a
appendix F • Phytoplankton Reference Community Data Analyses
-------
F-4
Table F-2. Summer (July through September) classification criteria for determining 'worst', 'poor,'
'better,' and 'best' water quality parameter conditions. Key: Secchi-Secchi depth
(meters); DIN-average dissolved organic nitrogen in surface mixed layer (mg liter1);
P04—average orthophosphate (SRP) in surface mixed layer (mg liter1); TF—tidal fresh
salinities (0 to 0.5 ppt); OH—oligohaline salinities (>0.5 to 5 ppt); MH—mesohaline
salinties (>5 to 18 ppt); PH—polyhaline (>18 ppt). The 25th percentile, median and
75th percentile of the parameter's values at stations identified as 'good' by the
Relative Status Method are given for comparison purposes. See Buchanan et al.
(in review) for details.
Parameter
Selected Summer Classification Criteria
Relative Status Method
Worst
Poor
Better
Best
25th %/median/75th %
Secchi
TF
<0.6
=<0.8
>0.8
>1.0
0.6 | 0.8 | 1.0
Secchi
OH
<0.55
=<0.6
>0.6
>0.7
0.55 | 0.6 | 0.7
Secchi
MH
<1.2
=<1.45
>1.45
>1.7
1.2 | 1.45 | 1.7
Secchi
PH
<1.55
=<1.85
>1.85
>2.35
1.55 | 1.85 | 2.35
Worst
Poor
Better
Best
75th %/median/25th %
DIN
TF
>.390
>0.070
=<0.070
<0.030
.390 | .240 | .125
DIN
OH
>.090
>0.070
=<0.070
<0.030
.090 | .050 | .028
DIN
MH
>.074
>0.070
=<0.070
<0.030
.074 | .035 | .014
DIN
PH
>.070
>0.070
=<0.070
<0.030
.028 | .011 | .008
Worst
Poor
Better
Best
75th %/median/25th %
P04 (SRP)
TF
>0.025
>0.003
=<0.003
=<0.003
.025 | .020 | .010
P04 (SRP)
OH
>0.010
>0.003
=<0.003
=<0.003
.010 | .009 | .004
P04 (SRP)
MH
>0.008
>0.002
=<0.002
=<0.002
.008 | .005|.0035
P04 (SRP)
PH
>0.010
>0.003
=<0.003
=<0.003
.010 | .008 | .005
'mixed poor light' category if Secchi depth classified as 'poor' or 'worst' and one or
both of the nutrient parameters classified as 'better' or 'best'. Data records were
placed in a 'mixed better light' category if Secchi depth classified as 'better' or 'best'
and one or both of the nutrient parameters classified as 'poor' or 'worst'.
SUMMARY OF CHLOROPHYLL A RESULTS
The 'better' water quality conditions (includes 'best') occurred in 1.6 percent
(spring) and 5.8 percent (summer) of the mesohaline biomonitoring records, and
21.1 percent (spring) and 10.4 percent (summer) of the polyhaline biomonitoring
records collected between 1984 and 2001. Therefore, reference communities could
be characterized directly from the phytoplankton associated with these least-
impaired water quality data. Because values of most phytoplankton parameters in the
mesohaline and polyhaline 'mixed better light' categories, including chlorophyll a,
closely resembled those in 'better' categories, 'mixed better light' data were used to
augment the small number of spring mesohaline 'better' data records. Median
chlorophyll a concentrations were 5.6 (spring) and 7.1 (summer) fig liter1 in the
appendix F • Phytoplankton Reference Community Data Analyses
-------
F-5
mesohaline reference communities, and 2.9 (spring) and 4.4 (summer) fig liter"1 in
the polyhaline reference communities. Reference community chlorophyll a values
are within the 2-7 fig liter1 range identified by Molvaer et al. (1997) for mesotrophic
marine waters, but are slightly higher than the 1-3 fig liter1 chlorophyll a range iden-
tified as mesotrophic by Smith et al. (1990). They can be considered high
mesotrophic. The reference community medians are 50 percent (spring) and 58
percent (summer) of the Poor category median concentrations in mesohaline waters
and 32 percent (spring) and 72 percent (summer) of the 'poor' category median
concentrations in polyhaline waters. These differences are significant (Wilcoxon
test, p<0.01). Chlorophyll a concentrations in the 'poor' categories classify as
eutrophic in mesohaline waters and borderline eutrophic in polyhaline waters.
Tidal-fresh and oligohaline reference community chlorophyll a concentrations are
based primarily on phytoplankton in the 'mixed better light' water quality category,
which is the least impaired category commonly found in low salinity waters of the
Chesapeake Bay. 'Better' water quality conditions occurred in less than 1 percent of
all samples. The combined 'mixed better light' and 'better' categories occurred in 4.7
percent (spring) and 21.5 percent (summer) of the tidal fresh biomonitoring records
and in 18.7 percent (spring) and 29.9 percent (summer) of the oligohaline biomoni-
toring records collected between 1984 and 2001. Median chlorophyll a
concentrations were 4.3 (spring) and 8.6 (summer) fig liter1 in the tidal fresh refer-
ence communities, and 9.6 (spring) and 6.0 (summer) fig liter"1 in the oligohaline
reference communities. Reference community chlorophyll a values are within the
ranges identified by Wetzel (2001) and Novotny and Olem (1994) for mesotrophic
fresh waters, but sometimes exceed the ranges identified by Smith et al. (1998) and
Ryding and Rast (1989). These values can be considered high mesotrophic. Median
chlorophyll a concentrations of the reference community are 64 percent (spring) and
34 percent (summer) of those in tidal fresh 'poor' category waters, and 52 percent
(spring) and 35 percent (summer) of those in oligohaline 'poor' category waters.
These differences are significant (Wilcoxon test, p<0.01). Chlorophyll a concentra-
tions in the tidal-fresh and oligohaline 'poor' categories classify as eutrophic to
highly eutrophic.
Reference communities were also distinguishable from 'poor' category phyto-
plankton populations by their smaller chlorophyll a ranges (Figure F-l). Typically,
ranges of chlorophyll a concentrations in the reference communities were V5 to Zi
the span of those in 'poor' water quality conditions. The large ranges of chlorophyll
a concentrations found in the 'worst,' 'poor,' and 'mixed poor light' water quality
categories of all salinity zones demonstrate the occurrence of frequent algal blooms
in these categories. Marshall et. al. (in draft) show that the species compositions of
phytoplankton associated with the lowest quartile (minimum—25th percentile) of
chlorophyll a values in 'worst', 'poor' and 'mixed poor light' water quality condi-
tions are generally mixed, while species compositions in the highest quartile of
chlorophyll values (75th percentile-maximum) are dominated by 'bloom-forming'
species. Mesohaline and polyhaline bloom-forming species include the diatoms
appendix F • Phytoplankton Reference Community Data Analyses
-------
F-6
SPRING
n = 147 334 as 21
SUMMER
n-S3 171 61 S3
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Water Quality Category
Improving •-
Water Quality Category
Improving —
Figure F-1: Chlorophyll a concentrations (/jg liter1) for six water quality condi-
tions in eight season-salinity groups (see text for details). Symbols: median (•),
average (°), and 5th, 25th, 75th and 95th percentiles (—), Median and 95th
percentile values are shown. A blank indicates <10 data points were available
in the water quality category.
appendix F • Phytoplankton Reference Community Data Analyses
-------
F-7
Chaetoceros spp., Cvclotella spp. and (at times) the small, unidentified centric
diatom, and the dinoflagellates Gymnodinium spp., Katodinium rotundatum and
Prorocentrum minimum. Tidal-fresh and oligohaline bloom-forming species include
colonial bluegreens such as Microcystis aeruginosa, filamentous bluegreen genera
such as Oscillatoria and Raphidiopsis, diatoms such as Coscinodiscus spp., Lepto-
cylindrus minimus, small unidentified centrics and Melosira varians, greens such as
Coelastrum spp., and the dinoflagellate Gymnodinium spp. Coincident water quality
data suggests the high chlorophyll a groups in 'worst,' 'poor,' and 'mixed poor light'
conditions may represent blooms at their peak, while the low chlorophyll a groups
may represent populations unable to use the available nutrients and blooming due to
low light levels. Specifically, DIN concentrations in the high chlorophyll a groups
are sometimes as little as half of those in the low chlorophyll a groups, indicating
increased nitrogen utilization in the high chlorophyll a groups.
The ranges of chlorophyll a concentrations (5th percentile-95th percentile) observed
in the phytoplankton reference communities indicate the peak concentrations that
should be expected in populations currently inhabiting unimpaired Chesapeake
waters. Chlorophyll a concentrations above these peak values constitute excess
phytoplankton production fueled by high nutrient concentrations and are potentially
harmful to the Chesapeake ecosystem. Peak chlorophyll a concentrations of the
reference communities, expressed as fig liter1, are 13.5 (tidal-fresh), 24.3 (oligoha-
line), 24.6 (mesohaline) and 6.7 (polyhaline) in spring, and 15.9 (tidal-fresh), 25.2
(oligohaline), 14.0 (mesohaline) and 8.7 (polyhaline) in summer.
LITERATURE CITED
Batiuk, R. A., P. Bergstrom, M. Kemp, E. Koch, L. Murray, J. C. Stevenson, R. Bartleson, V.
Carter, N. B. Rybicki, J. M. Landwehr, C. Gallegos, L. Karrh, M. Naylor, D. Wilcox, K. A.
Moore, S. Ailstock and M. Teichberg. 2000. Chesapeake Bay Submerged Aquatic Vegetation
Water Quality and Habitat-Based Requirements and Restoration Targets: A Second Technical
Synthesis. CBP/TRS 245/00 EPA 903-R-00-014. U.S. EPA Chesapeake Bay Program,
Annapolis, Maryland.
Buchanan, C., R. V. Lacouture, H. G. Marshall, M. M. Olson and J. Johnson. In review.
Phytoplankton reference communities for Chesapeake Bay.
Fisher, T. R, L. W. Harding, D. W. Stanley and L. G. Ward. 1988. Phytoplankton, nutri-
entsand turbidity in the Chesapeake, Delaware and Hudson estuaries. Estuarine, Coastal and
Shelf Science 27:61-93.
Fisher, T. R., A. B. Gustafson, K. Sellner, R. Lacouture, L. W. Haas, R. L. Wetzel, R.
Magnien, D. Everitt, B. Michaels and R. Karrh. 1999. Spatial and temporal variation of
resource limitation in Chesapeake Bay. Marine Biology 133:763-778.
Marshall, H. G., R. V. Lacouture, C. Buchanan, and J. Johnson. In preparation. Phyto-
plankton assemblages associated with water quality conditions during spring and summer in
salinity regions of Chesapeake Bay derived from a long-term monitoring program.
appendix F • Phytoplankton Reference Community Data Analyses
-------
F-8
Molvaer. J., J. Knutzen, J. Magnusson, B. Rygg, J. Skei and J. Sorensen. 1997. Environ-
mental quality classification in fjords and coastal areas. Statens Fonirensningst.ilsyn TA-1467,
Norway. 36 pp.
National Research Council. 1992. Restoration of Aquatic Systems. National Academy Press.
Washington, D.C. 552 pp.
Novotny V. and Olem H. 1994. Water Qualit\>: Prevention, Identification and Management of
Diffuse Pollution. Van Nostrand Reinhold. New York, New York.
Olson, M. 2002. Benchmarks for Nitrogen, Phosphorus, Chlorophyll and Suspended Solids
in Chesapeake Bay. Chesapeake Bay Program Technical Report Series, Chesapeake Bay
Program, Annapolis, Maryland.
Ryding, S. O. and W. Rast. 1989. The Control of Eutrophication of Lakes and Resen'oirs.
Man and the Biosphere Series, Volume 1, Parthenon Publication Group, Park Ridge, New
Jersey.
Smith, V. H. 1998. Cultural eutrophication of inland, estuarine and coastal waters. In:
Successes, Limitation and Frontiers in Ecosystem Science. Pace, M. L. and P. M. Groffman
(eds.) Pp. 7-49 Springer-Verlag, New York, New York.
Wetzel, R. G. 2001. Limnology-Lake and River Ecosystems, 3rd Edition. Academic Press,
New York, New York.
appendix F • Phytoplankton Reference Community Data Analyses
-------
G-1
Data Supporting Determination of
Adverse Affect Thresholds for
Potentially Harmful Algal Bloom Species
MICROCYSTIS AERUGINOSA EFFECTS TRESHOLD
A substantial body of literature deals with the negative effects of toxic cyanobacteria
on the feeding, growth, behavior and survival of micro- and mesozooplankton.
Numerous studies have documented the avoidance of ingestion of toxic and nontoxic
strains of Microcystis aeruginosa by specific taxa of zooplankton (Clarke 1978;
Lampert 1981; Gilbert and Bogdan 1984; Fulton and Paerl 1987, 1988; DeMott and
Moxter 1991) while others indicate physiological and behavioral problems associ-
ated with the ingestion of Microcystis aeruginosa (Lampert 1981, 1982; Nizan et al.
1986; Fulton and Paerl 1987; DeMott et al. 1991; Henning et al. 1991).
Fulton and Paerl's study (1987) indicated that a unicellular strain of Microcystis
aeruginosa (concentrations of 100,000 cells milliliter1) was toxic to and failed to
support populations of Keratella mixta, Diaphanosoma bracyurum, Daphnia
ambigua and Bosmina longirostris (a rotifer and three cladocerans, respectively).
Other studies have shown additional evidence of inhibitory effects of Microcystis
aeruginosa. For instance, Penaloza et al. (1990), showed that water-soluble frac-
tions of Microcystis aeruginosa were toxic to several rotifers, a copepod and a
cladoceran. De Mott et al. (1991) showed that a calanoid copepod was more sensi-
tive to purified microcystin than the cladoceran that he used in his experiments.
Nutritionally, many zooplankton have been shown to grow poorly on Microcystis
aeruginosa because it lacks certain fatty acids (Ahlgren et al. 1990). The results of
these studies indicate a deleterious effect exerted by blooms of Microcystis aerugi-
nosa on zooplankton communities. Two studies were chosen in the context of
deriving thresholds for impairment because they used densities of cells that could be
used to evaluate data from the Chesapeake Bay Monitoring Program and ultimately
translated into chlorophyll a concentrations. Without doing direct experiments on
inhibitory effects of the Chesapeake Bay strains of Microcystis aeruginosa on
zooplankton populations, certain assumptions of comparable toxicity were made for
the purposes of setting thresholds.
appendix G • Data Supporting Determination of Adverse Affect Thresholds
-------
G-2
Numerous laboratory studies also have documented the acute effects of toxins from
the cyanobacterium Microcystis aeruginosa on fish (Erickson et al. 1986; Rabergh
et al. 1991; Keshavanath et al. 1994; Beveridge et al. 1993; Tencalla et al. 1994; Bury
et al. 1995). Several instances of fish kills resulting from cyanobacterial blooms also
have been documented (Erickson et al. 1986; Penaloza et al. 1990; Rabergh et al.
1991). These studies indicate a variety of negative effects, including inhibition of
filtering rate, liver damage, disturbed ionic regulation, behavioral changes and
mortality. However, these studies addressed the potential damage from the stand-
point of toxin concentrations, not actual cell densities of the phytoplankton species
itself. Therefore, it was not possible to deduce a specific quantitative chlorophyll a
threshold whereby fish can be assessed as being negatively affected by blooms of
Microcystis aeruginosa.
Two laboratory studies were chosen to determine the threshold at which a negative
impact on the zooplankton community occurs-an impact in which the zooplankton
community structure is altered by the poor food quality, large particle size of the
colonies, increased density of particles in the water column or directly by the toxin.
Lampert (1981) conducted a laboratory feeding study in which densities as low as
1,400 cells milliliter1 of Microcystis aeruginosa resulted in the feeding inhibition of
zooplankton. Similarly, Fulton and Paerl (1987) conducted grazing experiments in
which the inhibitory threshold of Microcystis aeruginosa ranged from 10,000-
100,000 cells milliliter1, but was most clearly demonstrated at concentrations of
100,000 cells milliliter1. Since there is a difference of two orders of magnitudes
between the two studies, an intermediate concentration of 10,000 cells milliliter1
was chosen for exhibiting an inhibitory effect on zooplankton feeding.
It should be noted that a third study has been identified which documented negative
impacts on zooplankton at Microcystis aeruginosa cell densities of 50,000 cells
milliliter1 which is an intermediate value compared to the two previously cited
studies (Smith and Gilbert 1995).
PROROCENTRUM MINIMUM EFFECTS THRESHOLD
Certain strains of Prorocentrum minimum are toxic. In Japan in 1942, Prorocentrum
minimum was attributed as the cause of a shellfish poisoning in Japan in which 114
people died (Nagazima 1965, 1968). Prorocentrum minimum isolated from a 1998
bloom in the Choptank River and subsequently grown in the laboratory was found
toxic to scallops (Wickfors, personal communication). Blooms of Prorocentrum
minimum in the source intake water to Virginia and Maryland oyster hatcheries were
suspected to have caused oyster larvae mortality at the two hatcheries in 1998 (Luck-
enbach and Merritt, personal communication). There has been no documented case
of shellfish toxicity or mortality as a result of the 1998 Prorocentrum minimum
bloom in the Chesapeake Bay, but clearly the potential exists for toxic repercussions
to shellfish and other organisms as a result of this bloom.
appendix G • Data Supporting Determination of Adverse Affect Thresholds
-------
G-3
Embryonic development of the Eastern oyster (Crassostrea virginica) was not
affected by living cells or extracts of Prorocentrum minimum, however, larvae
showed poor growth and poor development of the digestive system when fed Proro-
centrum minimum (approximately 4,000 cells milliliter1) (Wickfors and Smolowitz
1995). Juvenile oysters adapted to digesting Prorocentrum minimum, but only after
a two-week period. The study concludes that feeding Prorocentrum minimum to
oyster larvae resulted in clear detrimental effects, but it was not apparent whether the
effects were from toxicity or starvation. In addition, it was concluded that some
component of the Prorocentrum minimum cell interfered with cellular digestive
processes in oyster larvae and spat.
The Wickfors and Smolowitz (1995) study also showed detrimental effects of
various diets containing different proportions of Prorocentrum minimum to oyster
larvae and newly set spat. The larvae showed consistently poorer survival and
growth in the different experimental diets and only those fed the diet with no Proro-
centrum minimum or one-third the maximum concentration developed into
pedi-veligers and set. Both life stages showed difficulties in the digestive system
after Prorocentrum minimum became a major component of their diet. The study
concludes that Prorocentrum minimum blooms impaired the survival, growth and
development of oyster larvae. That the study did not reveal whether the cause of
these detrimental effects was toxicity or starvation is important to the derivation of
numeric chlorophyll a criteria or target concentrations. The highest density used in
the study was 3,900 cells milliliter1 and detrimental effects were seen at densities of
~ 2,600 cells milliliter"1 in a mixed diet. The study is, however, useful in establishing
that 1) Prorocentrum minimum is detrimental to oyster life stages and 2) specific
densities of cells cause impairment.
Another laboratory study indicated more intense impairment of Eastern oyster life
stages when they were subjected to bloom concentrations of Prorocentrum minimum
(Luckenbach et al. 1993). Growth rates were minimal at cell densities of 3,000 cells
milliliter1, as an inverse relationship was documented between grazing rate and cell
density. Ultimately, mortality resulted for 43 percent of the juvenile oysters that
were subjected to this same density of Prorocentrum minimum cells.
The 1993 Luckenbach study was designed to test the effects of Prorocentrum
minimum on the growth and survival of the Eastern oyster. The momentum for this
study came from observations over many years made at the Virginia Institute of
Marine Science oyster hatchery over many years on the impact of dinoflagellate
blooms on the oyster populations in the hatchery. These observations are unpub-
lished but still noteworthy. They include the observation that adult oysters do not
spawn in the presence of bloom densities of Prorocentrum minimum and that early
larval development is impaired and high mortalities occur in the presence of high
densities of this dinoflagellate. The study used densities between 8,900-25,000 cells
milliliter1 for the 100 percent bloom density and 2,964-8,250 cells milliliter1 for a
33 percent bloom density. Mortalities of 100 percent for juvenile oysters took place
appendix G • Data Supporting Determination of Adverse Affect Thresholds
-------
G-4
in the 100 percent bloom diet, while 43 percent mortality was observed in the 33
percent bloom diet.
The density of 3,000 cells milliliter1 that was chosen as a threshold for the chloro-
phyll a criteria analysis is based on the results of these two studies, whereby
detrimental effects were documented at cell densities of 2,600 cells milliliter1 in one
study and 2,964-8,250 cells milliliter1 in the other study Neither study was aimed
specifically at determining the threshold of impairment for Prorocentrum minimum.
but impairments took place in both studies at a bloom density of around 3,000 cells
milliliter1. The fact that two different strains of Prorocentrum minimum were used
in the two studies and negative effects occurred at a very similar density, gives
credence to using 3,000 cells milliliter1 as a threshold for impairment.
COCHLODINIUM HETEROLOBATUM
EFFECTS THRESHOLD
This species forms intense blooms in warm months at the mouth of the York River and
in the lower Chesapeake Bay (Mackiernan 1968; Zubkoff and Warriner 1975;
Zubkoff et al. 1979; Marshall 1995). Laboratory studies indicated a threshold con-
centration of ~ 500 cells milliliter"1 whereby calcium uptake was depressed and
mortality of larvae was significantly elevated (Ho and Zubkoff 1979). Above densi-
ties of- 1000 cells milliliter1, calcium uptake was negligible and mortality extremely
high. Mortality was attributed to 'spatial competition' rather than a 'toxic secretion'
(although this chain-forming dinoflagellate produces copious amounts of mucilage;
Lacouture, personal communication). The densities of this organism during bloom
conditions far exceeds these values and the extent of these densities can cover tens of
square miles (Mackiernan 1968; Zubkoff et al. 1979; Marshall 1995).
LITERATURE CITED
Ahlgren, G., L. Lundstedt, M. Brett and C. Forsberg. 1990. Lipid composition and food
quality of some freshwater phytoplankton for cladoceran zooplankters. Journal of Plankton
Research 12:809-818.
Beveridge, M. C. M., D. J. Baird, S. M. Rahmattulah, L. A. Lawton, K. A. Beattie and G. A.
Codd. 1993. Grazing rates on toxic and non-toxic strains of cyanobacteria by Hypo-
hthalmictys molitrix and Oreochromis niloticus. Journal of Fish Biology 43:901-907.
Clarke, N. V. 1978. The food of adult copepods from Lake Kainji, Nigeria. Freshwater
Biology 8:321-326.
DeMott, W. R. and F. Moxter. 1991. Foraging on cyanobacteria by copepods: Responses to
chemical defenses and resource abundance. Ecology 72:1820-1834.
DeMott, W. R, Q. Z. Zhang and W. W. Carmichael. 1991. Effects of toxic cyanobacteria
and purified toxins on the survival and feeding of a copepod and three species of Daplinia.
Limnology and Oceanography 36:1346-1357.
appendix G • Data Supporting Determination of Adverse Affect Thresholds
-------
G-5
Eriksson, J. E., J. A. O. Meriluoto and T. Lindholm. 1986. Can cyanobacterial peptide toxins
accumulate in aquatic food chains? Proceedings from the IV International Symposium of
Microbial Ecology Ljubljana, Jugoslavia. Pp. 655-658.
Fulton III, R. S. and H. W. Paerl. 1987. Toxic and inhibitory effects of the blue-green alga
Microcystis aeruginosa on herbivorous zooplankton. Journal of Plankton Research 9
(5):837-855.
Fulton III, R. S. and H. W. Paerl. 1988. Effects of the blue-green alga Microcystis aerugi-
nosa on zooplankton competitive relations. Oecologia. 76:383-389.
Gilbert, J. J. and K. G. Bogdan. 1984. Rotifer grazing: In situ studies on selectivity and
rates. In: Trophic interactions within aquatic ecosystems. Meyers, D. G. and J. R. Strickler
eds. American Association for the Advancement of Science Selected Symposium. Vol.
85:97-133. Boulder, Colorado.
Henning, M., H. Hertel, H. Wall and J. G. Kohl. 1991. Strain-specific influence of Micro-
cystis aeruginosa on food ingestion and assimilation of some cladocerans and copepods. Int.
Rev. Ges. Hydrobiol. 76:37-45.
Ho, M. S. and P. L. Zubkoff. 1979. The effects of a Cochlodinium heterolobatum bloom on
the survival and calcium uptake by larvae of the American oyster, Crassostrea vitginica. In:
Toxic Dinoflagellate Blooms. Taylor, F. J. R. and H. H. Seliger, eds. Elsevier North Holland,
New York
Keshavanath, P., M. C. M. Beveridge, D. J. Baird, L. A. Lawton, A. Nimmo and G. A. Codd.
1994. The functional grazing response of a phytoplanktivorous fish Oreocliromis niloticus to
mixtures of toxic and non-toxic strains of the cyanobacterium Microcystis aeruginosa.
Journal of Fish Biology 45:123-129.
Lampert, W. 1981. Inhibitory and toxic effects of blue-green algae on Daplinia.
International Review Gesamten Hydrobiologic 66:285-298.
Lampert, W. 1982. Further studies on the inhibitory effect of the toxic blue-green Microcystis
aeruginosa on the filtering rate of zooplankton. Archives of Hydrobiol ogy 95:207-220.
Luckenbach, M. W., K. G. Sellner, S. E. Shumway and K. Greene. 1993. Effects of two
bloom- forming dinoflagellates, Prorocentrum minimum and Gyrodinium uncatenum, on the
growth and survival of the Eastern oyster, Crassostrea virginica (Gmelin 1791). Journal of
Shellfish Research 12 (2):411-415.
Mackiernan, G. B. 1968. Seasonal distribution of dinoflagellates in the lower York River,
Virginia. Masters thesis. School of Marine Science, College of William and Mary, Gloucester
Point, Virginia. 104 pp.
Marshall, H. G. 1995. Succession of dinoflagellate blooms in the Chesapeake Bay, U.S.A.
In: Harmful Marine Algal Blooms. Lassus, P., G. Arzul, E. Erard, P. Gentien and C.
Marcaillou (eds.) Lavoisier, Intercept Ltd., Paris.
Nagazima, M. 1965. Studies on the source of shellfish poison in Lake Hamana I. Relation
of the abundance of a species of dinoflagellate, Prorocentrum sp. to shellfish toxicity.
Bulletin of Japanese Society of Scientific Fisheries 31:198-203.
Nagazima, M. 1968. Studies on the source of shellfish poison in Lake Hamana IV. Identi-
fication and collection of the noxious dinoflagellate. Bulletin of Japanese Society of Scientific
Fisheries 43:13 0-131.
appendix G • Data Supporting Determination of Adverse Affect Thresholds
-------
G-6
Nizan, S., C. Dimentman and M. Shilo. 1986. Acute toxic effects of the cyanobacterium
Microcystis aeruginosa on Daphnia magna. Limnology and Oceanography 31:497-502.
Penaloza, R., M. Rojas, I. Vila and F. Zambrano. 1990. Toxicity of a soluble peptide from
Microcystis sp. to zooplankton and fish. Freshwater Biology 24:233.
Rabergh, C. M. I., G. Bylund and J. E. Eriksson. 1991. Histopathological effects of micro-
cystis- LR, a cyclic peptide toxin from the cyanobacterium (blue-green alga) Microcystis
aeruginosa on common carp (Cyprinius carpio L.) Aquatic Toxicology 20:131-146.
Smith, A. D. and J. J. Gilbert. 1995. Relative susceptibilities of rotifers and cladocerans to
Microcystis aeruginosa. Archives of Hydrobiology 132:309-336.
Tencalla, F. G., D. R. Dietrich and C. Schlatter. 1994. Toxicity of Microcystis aeruginosa
peptide toxin to yearling rainbow trout (Oncorhyncus mykiss). Aquatic Toxicology 30:215-
224.
Wickfors, G. H. and R. M. Smolowitz. 1995. Experimental and histological studies of four
life- history stages of the Eastern oyster, Crassostrea virginica, exposed to a cultured strain
of the dinoflagellate, Prorocentrvm minimum.. Biological Bulletin 188:313-328.
Zubkoff, R L., J. C. Munday, R. G. Rhodes and J. E. Warriner. 1979. Mesoscale features of
summer (1975 to 1977) dinoflagellate blooms in the York River, Virginia (Chesapeake Bay
Estuary). In: Toxic Dinoflagellate Blooms. F. J. R. Taylor and H. H. Seliger (eds.). Elsevier
North Holland, Inc. New York, New York.
Zubkoff, R L. and J. E. Warriner. 1975. Synoptic sightings of red waters of the lower Chesa-
peake Bay and its tributary rivers (May 1973 to September 1974). In: Proceedings of the
First International Conference on Toxic Dinoflagellate Blooms, LoCicero, V. R. (ed. ). Mass-
achusetts Science and Technology Foundation. Wakefield, Massachusetts.
appendix G • Data Supporting Determination of Adverse Affect Thresholds
-------
H-1
Derivation of Cumulative Frequency
Distribution Criteria Attainment
Reference Curves
Building from the descriptions of reference curves in Chapter VI, this appendix
provides more detailed description of the process and options considered in deriving
the open-water and deep-water dissolved oxygen reference curves and the water
clarity criteria reference curves.
DISSOLVED OXYGEN REFERENCE CURVES
The Chesapeake Bay dissolved oxygen criteria have several duration components:
30-day mean, 7-day mean, 1-day mean and instantaneous minimum. At this time,
reference curves have been developed only for the 30-day mean component.
OPEN-WATER CRITERIA REFERENCE CURVES
The open-water designated use includes surface and surface-mixed water above a
pycnocline. It also includes waters deeper in the water column where there is no
vertical density barrier (pycnocline) or where a vertical barrier is present but does not
prevent exchange with oxygenated water horizontally.
The dissolved oxygen criteria are based primarily on target species that are ecologi-
cally and commercially valuable and have high oxygen requirements. If the criteria
are protective of these species, then by default they are protective of other species
with lower oxygen requirements. Ideally, a reference curve for the open-water criteria
would be based on dissolved oxygen data collected in this habitat at times and places
where these sensitive species are known to thrive. Unfortunately, there is a lack of
open-water column estuarine fish/shellfish-based indices ofbiotic integrity, or similar
biological indicator, in addition to the lack of adequate fisheries-independent data
over the necessary geographic area and time period. Therefore, surrogate indicators
of 'healthy' open-water water quality conditions were employed. To validate the
reference areas, the same indicators were used to identify 'unhealthy conditions.'
Criteria attainment curves were derived for both groups for comparison.
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-2
Four approaches to defining 'healthy' locations by Chesapeake Bay Program
segment were examined for the open-water designated use. Approach 1 identified
Chesapeake Bay Program segments with 'good' and 'poor' water quality conditions
using water quality parameters not including dissolved oxygen. Reference and vali-
dation curves were derived using interpolated dissolved oxygen concentration data
from the reference and validation segments. Approach 2 ranked the Chesapeake Bay
Program segments in order based on seasonal median dissolved oxygen concentra-
tion (spring and summer, separately). Criteria attainment curves of the highest and
lowest 14 segments (10 percent and 10 percent, respectively for a total of 20 percent)
were used to derive reference and validation curves using the interpolated dissolved
oxygen monitoring data. In Approach 3, all the polyhaline Chesapeake Bay Program
segments were selected and similarly processed for comparison with the other
approaches, given these segments were the most likely to have the highest dissolved
oxygen values and least impaired biological communities. Approach 4 involved
selecting open-water CBP interpolator cells from locations (segment, year and
season) where healthy and stressed benthic communities were found (see "Deep-
Water Reference Curves," below, for more details). All the data for this analysis
came from the Chesapeake Bay Water Quality Monitoring Program database, with
the years 1985 through 1994 selected to reflect the years of hydrology currently eval-
uated through the Chesapeake Bay water quality model (see Chapter VI).
Approach 1: Reference and Validation Curves
Using Water Quality Status
The Chesapeake Bay Program's Tidal Monitoring and Analysis Workgroup developed
a procedure to assess relative status for situations in which an absolute point of refer-
ence for a water quality parameter is not available (Alden and Perry 1997). That
procedure uses the (logistic) distribution of the parameter in a 'benchmark' data set as
a standard against which individual data points are assessed. The assessments are done
separately within salinity classification and generally within depth layers. The median
score of the individual data points is then calculated for any user-specified time and
space grouping. In the present context, the benchmark distribution is divided roughly
into thirds, which are defined as 'good,' 'fair' and 'poor'. These terms relate only to each
other, not necessarily to actual water quality requirements of living resources.
For this analysis, the combined status assessments for total nitrogen, total phos-
phorus, chlorophyll a and total suspended solids were used to select reference and
validation locations. Using the above procedure, surface concentrations of the four
parameters for each Chesapeake Bay Program segment, year and season (spring and
summer) were assessed to yield an assessment of 'good', 'fair' or 'poor' for each
parameter. Each segment/year/season was further evaluated. To qualify as a refer-
ence location, at least three out of four water quality parameters had to be 'good' and
only one parameter could be 'fair'. To qualify as a validation location, at least three
parameters had to be 'poor,' the other could be 'fair' and none could be 'good.' The
lists of reference and validation locations using this approach are found in Tables
H-1 through H-4.
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-3
Table H-1. Reference locations for spring open-water, dissolved oxygen criteria
reference curve based on water quality parameters (Approach 1).
Segment
Years
BOHOH
1994
CB20H
1985
1986
1988
1989
1990
CB4MH
1985
1989
1992
CB5MH
1985
1986
1989
1991
1992
CB6PH
1989
CB7PH
1989
CB8PH
1989
1991
1992
CHKOH
1985
1986
1987
CRRMH
1985
1986
1988
1989
1992
EASMH
1987
ELKOH
1991
JMSOH
1985
1986
1987
JMSTF
1992
1993
MIDOH
1993
MPNOH
1985
MPNTF
1985
1986
1987
1988
1989
PAXMH
1992
PIAMH
1985
1986
1989
1992
1994
PMKTF
1985
1986
1987
1988
1989
RPPMH
1989
RPPOH
1985
1986
1987
RPPTF
1985
1986
1987
1991
1992
TANMH
1986
Source: Chesapeake Bay Water Quality Monitoring Program database
http:// www.chesapeakebay.net/data
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-4
Table H-2. Validation locations for spring open water, dissolved oxygen criteria
reference curve based on water quality parameters (Approach 1).
Segment
Years
BIGMH
1990
BOHOH
1986
1990
1992
1993
C&DOH
1986 1987
CB3MH
1990
CB6PH
1993
CB7PH
1993
CB8PH
1987 1993
CHOMH2
1986
1989
1990
1994
CHOOH
1985
1987
1988
1993
1994
CHSMH
1985
CHSOH
1985
1986
1988
1990
1991
1992
EBEMH
1989
1991
1993
1994
ELIPH
1987
1988
1989
1990
1991
1992
ELKOH
1986
1987
FSBMH
1986
1987
1990
1991
1993
GUNOH
1988
1991
JMSMH
1988
1989
1991
1992
1993
JMSOH
1990
JMSPH
1985
1986
1987
1988
1990
1991
LAFMH
1989
1990
MAGMH
1985
1986
1989
1990
MANMH
1987
1990
1994
MOBPH
1987
1993
1994
NANMH
1986
1987
1988
1990
1991
1992
NANTF
1986
1988
1990
1992
1993
PAXMH
1986
1990
PAXOH
1986
1988
PAXTF
1986
1989
1990
1991 1994
POCMH
1993
1994
POTMH
1990
1991
RHDMH
1991
RPPMH
1990
1991
SBEMH
1989
1991
1993
1994
SEVMH
1991
1993
SOUMH
1985
1990
1992
TANMH
1987
WBEMH
1989
1990
1991
1992
1993
1994
WICMH
1986
1987
1988
1989
1990
1991
WSTMH
1986
1988
1991
YRKMH
1989
1991
1992
YRKPH
1986 1987 1988 1990 1991 1993 1994
1994
1994
1994
1994
Source: Chesapeake Bay Water Quality Monitoring Program database
http:// www.chesapeakebay.net/data
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-5
Table H-3. Reference locations for summer open-water, dissolved oxygen criteria
reference curve based on water quality parameters (Approach 1).
Segment
Years
BIGMH
1993
CB1TF
1985
1986
1987
1990
1991
1992
1993
CB20H
1985
1986
1987
1988
1990
1991
1992
CB3MH
1992
1993
CB4MH
1985
1986
1987
1988
1990
1991
1992
CB5MH
1985
1986
1987
1988
1990
1991
1992
CB7PH
1986
1987
CB8PH
1986
1987
1988
1990
1991
CHKOH
1985
1992
CRRMH
1987
1988
1991
1992
EASMH
1986
ELKOH
1991
1992
1994
GUNOH
1985
JMSOH
1985
1986
1987
1990
1994
JMSTF
1991
1992
LCHMH
1986
MATTF
1987
MIDOH
1990
1991
1993
1994
MPNOH
1985
1986
MPNTF
1985
1986
1987
1988
1989
1990
1991
PIAMH
1985
1986
1987
1992
1993
PISTF
1986
1987
PMKOH
1985
PMKTF
1985
1986
1987
1988
1989
1990
1991
POTMH
1985
1986
1987
1991
POTOH
1986
1987
1988
1989
1990
POTTF
1987
1989
1990
RPPMH
1985
1986
1987
RPPOH
1985
1986
1987
1988
1991
1992
1994
RPPTF
1992
1994
TANMH
1986
Source: Chesapeake Bay Water Quality Monitoring Program database
http:// www.chesapeakebay.net/data
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-6
Table H-4. Validation for summer open-water, dissolved oxygen criteria reference
curve based on water quality parameters (Approach 1).
CBP
Segment
Years
APPTF
1988
1990
1991
1992
1993
BOHOH
1986
1987
1988
1989
1992
1994
BSHOH
1985
1989
CB6PH
1989
CHOMH2
1989
1990
1991
1994
CHOOH
1985
1986
1987
1990
1991
1994
CHSMH
1989
1990
1993
CHSOH
1985
1986
1987
1988
1989
1990
1991
1992
ELIPH
1985
1986
1987
1988
1989
1990
1991
1992
FSBMH
1988
1989
GUNOH
1993
JMSMH
1989
JMSPH
1987
1989
1991
1992
1993
1994
LAFMH
1989
1990
LCHMH
1989
MAGMH
1986
1987
1988
1989
1990
1991
1994
MANMH
1986
1987
1988
1989
1990
1991
1993
1994
MOBPH
1986
1989
1990
1991
1993
NANMH
1986
1987
1988
1989
1990
1991
1993
1994
NANTF
1989
1990
1992
1993
1994
NORTF
1989
PATMH
1985
1986
1987
1988
1989
1990
1991
1992
PAXMH
1988
1989
1993
PAXOH
1986
1989
1992
1994
PAXTF
1985
1986
1987
1988
1989
1990
1991
1992
POCMH
1989
1994
POTTF
1994
RHDMH
1985
1986
1987
1988
1989
1990
1991
1992
SASOH
1986
1987
1988
1989
1990
1991
1993
SBEMH
1992
1993
1994
SEVMH
1985
1987
1988
1989
1990
1991
1992
1994
SOUMH
1987
1988
1989
1990
1991
1994
WBEMH
1989
1990
1991
1992
WICMH
1986
1987
1988
1989
1990
1991
1993
1994
WSTMH
1985
1987
1988
1989
1990
1991
1994
YRKMH
1987
1989
1990
1991
1993
YRKPH
1986
1988
1989
1990
1991
1992
1993
Source: Chesapeake Bay Water Quality Monitoring Program database
http:// www.chesapeakebay.net/data
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-7
Monthly mean dissolved oxygen concentration data were then interpolated basinwide
for each month, 1985 to 1994. In addition, a basinwide 'master' interpolated 3-dimen-
sional grid file was created in which each cell has a Chesapeake Bay segment
assignment and a static Designated Use assignment (open-water [OW], deep-water
[DW] and deep-channel [DC]) based on proposed tidal water designated use boundaries
(U.S. EPA 2003). Each cell could thus be identified by the appropriate dissolved oxygen
concentrations) associated with its respective designated use.
For each monthly baywide interpolation, the dissolved oxygen concentration in each
open-water designated use cell was compared to the appropriate criteria concentra-
tion for the season, and the percent of cells passing/failing the criteria calculated for
each segment/designated use. Using the respective lists of 'good' and 'bad' locations
(segment_years), the data for the reference and validation segments were extracted
and pooled in separate groups. For example, segment POCMH in spring 1993 and
1994 were identified as validation locations. The percent volume failing the criterion
in POCMH was calculated for each month—February, March, April and May of
1993 and 1994—and pooled with the percent-volume-failing data from other simi-
larly identified locations. Then, the cumulative frequency distribution attainment
curves were derived for each pooled group. Figures H-l and H-2 show the open-
water designated use dissolved oxygen criteria reference and validation curves for
the spring and summer seasons generated applying water quality status approach.
It is clear that both reference (hatched line) and validation (solid line) areas meet the
spring 30- day 5 mg liter1 criterion almost all if not 100 percent of the time. If there
are areas that do not meet this criterion in spring, this method does not detect them.
There also is little apparent distinction between the illustrated reference and valida-
tion curves in summer (Figure H-2).
However, when the summer data are separated by salinity zone (figures H-3 and
H-4), there are distinct differences between the reference and validation curves. In
tidal fresh and oligohaline segments, overall exceedance is low, but reference areas
have more apparent exceedance than validation areas. The reverse is true for meso-
100
90 ¦
80 ¦
70
60
50 ¦
40 ¦
30
20 ¦
10 ¦
0 i ... -
I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 1 1 1 1 I 11 1 1 I 1 i 1 1 I ' 1 1 1 I 1 11 1 I
0 10 20 30 40 50 60 70 80 90 100
Percentage of Area/Volume Exceeding the Criteria
Figure H-1. Spring open-water reference (hatched line) and validation (solid line) curves:
water quality status approach.
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-8
Percentage of Area/Volume Exceeding the Criteria
Figure H-2. Summer open-water reference (hatched) and validation (solid line) curves:
water quality status.
Percentage of Area/Volume Exceeding the Criteria
Figure H-3: Lower salinity summer open water reference (hatched line) and validation
(solid line) curves: water quality status approach.
100
Percentage of Area/Volume Exceeding the Criteria
Figure H-4. Higher salinity summer open water reference (hatched line) and validation
(solid line) curves: water quality status approach.
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-9
haline-polyhaline segments where exceedance is generally greater. An important
point to remember is that, while we usually think of the open-water habitat as the
surface-mixed layer, the open-water criteria are applicable throughout the water
column in areas that do not experience chronic vertical stratification. There are such
areas in the basin that are quite deep but usually do not have a pycnocline. These
areas are more commonly found in mesohaline and polyhaline segments than in the
tidal-fresh and oligohaline waters. This is one likely factor in the difference between
the reference and validation curves. Another factor could be that surface waters in
the validation segments in the tidal-fresh and oligohaline zones are more affected by
the oxygen-generating processes of algal blooms, whereas the mesohaline and poly-
haline validation segments are more affected by oxygen-consumptive processes
occurring in the deep water layers beneath them.
The cumulative frequency distribution curves for reference locations using the water
quality status method show that areas with low nutrients, chlorophyll a and
suspended solids levels also have dissolved oxygen levels that do not greatly exceed
the applicable criterion. On the other hand, the validation curves suggest these
parameters are not good indicators of locations with dissolved oxygen criteria attain-
ment levels. The mesohaline and polyhaline curve shows some nonattainment, but
conditions are far better than those reflected in the validation curve derived from the
ranking exercise described below. This result is essentially as expected since, in most
of the Chesapeake Bay and tidal tributaries, the link between the water quality
parameters and dissolved oxygen has a number of intermediate steps and the
dissolved oxygen response to water quality parameters is often displaced in time or
space or both.
Approach 2: Segments with Highest and Lowest Long-term
Dissolved Oxygen Concentrations
The ranking procedure for selecting reference and validation segments was based on
observed (i.e., not interpolated) data. Dissolved oxygen measurements are available
for each monitoring station at 1- to 2-meter intervals from surface to bottom. The
depth of the pycnocline, if one existed, also is available. For this analysis, all
dissolved oxygen measurements above the pycnocline, or the shallower of all meas-
urements above 7 meters or above the bottom if there was no pycnocline, were
assumed to be in open-water designated use habitats. To control for supersaturating
conditions, dissolved oxygen concentrations that were above saturation levels
(calculated from temperature and salinity measured concurrently) were set down to
the saturation level.
Spring (March through May) and summer (June through September) data were aver-
aged first by date and station; then by month and segment; then the 10th, 50th and 90th
percentiles of the monthly segment averages were calculated for spring and summer
seasons over the 1985-1994 period. The seasonal median, i.e., 50th percentile, was
used to rank the segments (tables H-5 and H-6). Some segments were excluded,
resulting in 67 segments that were ranked. The excluded Chesapeake Bay Program
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-10
Table H-5. Chesapeake Bay Program segments listed in order of spring open-water designated use,
seasonal median dissolved oxygen concentration.
CBP
10th
90th
CBP
10th
90th
Segment
Median
Mean
percentile
percentile
Segment
Median
Mean
percentile
percent
WICMH
8.0
8.4
6.2
11.5
CB7PH
9.5
9.4
7.9
10.9
PMKTF
8.0
8.1
6.3
10.1
PIAMH
9.6
9.6
8.0
11.2
YRKMH
8.0
8.1
6.3
10.3
PAXMH
9.6
9.3
6.9
11.2
POCTF
8.1
7.9
5.4
10.2
MAGMH
9.6
9.7
7.5
11.5
MPNOH
8.1
8.1
6.1
10.3
NANTF
9.6
9.5
7.5
11.4
PMKOH
8.1
8.2
6.2
10.3
CHSMH
9.6
9.8
8.1
11.7
CHOOH
8.6
8.9
7.2
10.9
POTTF
9.7
9.8
7.9
11.8
MPNTF
8.7
8.6
6.9
10.5
RHDMH
9.7
9.6
7.8
11.6
PAXOH
8.7
oo
od
7.0
10.8
WSTMH
9.7
9.7
7.4
11.9
YRKPH
8.7
8.7
7.0
10.3
CB6PH
9.7
9.7
8.0
11.2
ELIMH
CO
bo
oo
od
6.5
11.0
JMSTF
9.7
9.6
8.2
10.8
ELIPH
CO
CO
oo
od
7.1
10.7
CB3MH
9.7
9.4
7.6
11.2
JMSMH
8.9
9.0
7.4
10.7
SOUMH
9.7
9.2
5.8
11.5
FSBMH
8.9
9.3
7.4
11.7
CB20H
9.7
9.8
7.5
11.8
RPPTF
9.0
9.3
7.3
11.3
POTMH
9.7
9.7
7.9
11.7
CHSOH
9.1
9.1
7.3
10.8
BSHOH
9.8
10.0
8.1
12.0
MANMH
9.1
9.1
7.4
10.9
C&DOH
9.8
10.0
8.0
12.0
JMSPH
9.1
9.2
7.5
11.1
BACOH
9.9
9.9
8.0
12.0
NANMH
9.1
9.3
7.4
11.1
EASMH
9.9
9.9
8.0
11.5
MOBPH
9.1
9.2
7.7
10.7
PISTF
9.9
10.1
7.9
11.9
RPPOH
9.2
9.2
7.4
11.0
SEVMH
9.9
9.8
7.7
11.7
POTOH
9.2
9.4
7.8
11.4
LCHMH
9.9
9.8
8.3
11.4
APPTF
9.2
9.4
8.0
11.2
CB5MH
10.0
10.0
8.3
11.5
POCMH
9.2
9.3
7.9
10.9
MATTF
10.0
10.0
8.5
11.7
BIGMH
9.3
9.3
7.5
11.1
ELKOH
10.0
10.1
8.3
12.1
CB8PH
9.3
9.3
8.0
10.9
CB4MH
10.1
9.9
8.1
11.4
PAXTF
9.3
9.2
7.3
10.6
CHOMH1
10.1
9.8
7.8
11.3
TANMH
9.4
9.3
7.4
11.2
SASOH
10.1
10.1
8.4
11.9
PATMH
9.4
9.4
7.7
11.0
MIDOH
10.3
10.3
8.7
12.2
RPPMH
9.4
9.2
7.3
10.9
NORTF
10.4
10.5
9.2
12.3
CHOMH2
9.4
9.4
7.6
11.4
BOHOH
10.4
10.2
8.6
12.4
CRRMH
9.4
9.2
7.1
10.9
GUNOH
10.5
10.4
8.6
12.1
CHKOH
9.4
9.2
7.1
11.1
CB1TF
11.0
10.7
8.6
12.5
JMSOH
9.5
9.4
7.9
11.0
Source: Chesapeake Bay Water Quality Monitoring Program database
http://www.chesapeakebay.net/data
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-11
Table H-6. Chesapeake Bay segments listed in order of summer dissolved oxygen designated use,
seasonal median dissolved oxygen concentration.
CBP 10th 90th CBP 10th 90th
Segment Median Mean percentile percentile Segment Median Mean percentile percentile
SOUMH
4.2
4.5
2.7
7.1
FSBMH
6.6
6.5
5.8
7.3
MAGMH
4.7
4.8
3.3
6.9
ELKOH
6.6
6.5
5.8
7.2
PMKTF
4.9
5.0
4.4
5.8
CHSMH
6.6
6.5
5.8
7.3
MPNOH
4.9
4.9
4.0
5.6
RHDMH
6.6
6.6
5.5
7.9
POCTF
4.9
5.1
3.7
7.1
POTTF
6.6
6.6
6.0
7.2
PMKOH
5.0
4.9
4.1
5.7
JMSOH
6.7
6.7
6.1
7.2
YRKMH
5.2
5.2
4.5
5.8
CB6PH
6.7
6.8
6.2
7.5
PAXMH
5.4
5.4
4.8
6.1
POCMH
6.7
6.7
6.3
7.3
YRKPH
5.5
5.5
4.9
6.2
LCHMH
6.7
6.7
5.9
7.5
MPNTF
5.5
5.5
4.6
6.4
PIAMH
6.7
6.6
5.7
7.4
ELIMH
5.6
5.7
4.4
7.1
CB4MH
6.7
6.6
6.0
7.2
ELIPH
5.6
5.7
4.6
6.8
CB7PH
6.8
6.9
6.4
7.3
WICMH
5.8
5.7
4.6
6.8
CHOMH1
6.8
6.9
6.3
7.6
CRRMH
5.9
5.8
4.5
6.8
SASOH
6.8
6.5
4.1
8.0
SEVMH
5.9
5.9
5.1
7.6
CB5MH
6.9
6.9
6.4
7.5
PAXOH
5.9
5.9
4.9
7.0
BIGMH
6.9
6.9
6.3
7.4
CHOMH2
6.0
6.0
5.3
6.8
EASMH
6.9
6.9
6.2
7.5
PATMH
6.1
6.0
4.8
7.0
CB8PH
6.9
6.9
6.4
7.5
POTMH
6.1
6.1
5.4
6.8
BOHOH
7.0
7.1
5.7
8.3
WSTMH
6.1
6.1
5.0
7.5
CB1TF
7.0
7.0
6.4
7.8
JMSMH
6.2
6.2
5.6
6.7
JMSTF
7.0
7.0
6.4
7.5
RPPMH
6.2
6.2
5.5
6.7
PISTF
7.0
6.7
5.4
7.6
CB3MH
6.2
6.1
5.4
6.8
APPTF
7.2
7.1
5.8
8.0
CHOOH
6.3
6.3
5.4
7.2
RPPTF
7.2
7.2
6.6
8.0
POTOH
6.3
6.3
5.6
.1
PAXTF
7.2
7.1
6.0
8.1
CB20H
6.4
6.3
5.6
6.8
MIDOH
7.3
7.1
5.7
8.0
NANMH
6.4
6.4
5.7
7.4
CHSOH
7.3
7.2
6.1
8.3
CHKOH
6.5
6.4
5.3
7.4
NANTF
7.4
7.1
5.9
8.3
C&DOH
6.5
6.5
5.8
7.2
GUNOH
7.5
7.3
6.1
8.5
TANMH
6.5
6.5
5.9
7.1
BACOH
7.7
7.3
5.5
8.5
MOBPH
6.5
6.4
5.6
7.0
BSHOH
7.8
7.4
5.5
8.7
JMSPH
6.5
6.6
6.0
7.2
NORTF
7.9
7.8
7.2
8.4
MANMH
6.6
6.6
5.9
7.2
MATTF
7.9
7.9
7.4
8.6
RPPOH
6.6
6.5
5.7
7.3
Source: Chesapeake Bay Water Quality Monitoring Program database
http:// www.chesapeakebay.net/data
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-12
segments were: the Western Branch of the Patuxent River (WBRTF) because it is a
small water body dominated by a waste water treatment plant; the mesohaline tribu-
taries of the Elizabeth River (SBEMH, EBEMH, and WBEMH) because the
dissolved oxygen interpolations did not extend to those segments or the data record
was too short; and the Lafayette River (LAFMH) because it contains no water
quality monitoring station. Within season, the highest ranked 14 segments made up
the list of reference locations and the lowest ranked 14 segments constituted the list
of validation locations.
Monthly mean dissolved oxygen concentration data were then interpolated basin-
wide for each spring and summer month, 1985 to 1994. For each interpolation, the
dissolved oxygen concentration in each cell qualifying as open-water was compared
to the appropriate criteria concentration for the month, and the percent of cells
passing/failing the criteria was calculated for each segment or designated use. Using
the respective lists of 'good' and 'bad' locations, the data for the reference and vali-
dation segments were extracted, pooled and plotted (Figure H-5).
The reference curve (hatched line) using this approach looks too good to be true. The
Chesapeake Bay Program segments with the highest dissolved oxygen levels include
a number of segments known to be eutrophic, with high chlorophyll a concentra-
tions. These segments are likely to have elevated daytime dissolved oxygen
concentrations due to the addition of oxygen from photosynthesis, but these are also
frequently associated with nighttime dissolved oxygen sags when photosynthesis
stops and respiration increases. This curve is, therefore, not a valid reference curve.
cu
Percentage of Area/Volume Exceeding the Criteria
Figure H-5. Open water reference and validation curves for summer based on the best
and worst —20 percent of all segments approach.
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-13
Approach 3: Using only Polyhaline Segments
In this exercise, the interpolated data sets from Chesapeake Bay Program segments
western lower Chesapeake Bay (CB6PH), eastern lower Chesapeake Bay (CB7PH),
mouth of the Chesapeake Bay (CB8PH), mouth of the York River (YRKPH),
Mobjack Bay (MOBPH), mouth of the James River (JMSPH) and Elizabeth River
(ELIPH) were processed as described above. The percent attainment for each month
in spring and summer seasons was calculated for the open-water designated use cells
in each polyhaline segment. These data were pooled and a cumulative frequency
distribution curve generated for each season. The cumulative frequency distribution
curve for summer is shown below (Figure H-6). In the ranking exercise above, the
York (YRKPH) and Elizabeth (ELIPH) river segments fell in the lowest ranked group
of 14 segments while the other polyhaline segments were scattered in the middle
range in both spring and summer seasons (see tables H-5 and H-6, respectively).
Figure H-6. Summer open water reference curve: polyhaline segments only approach.
With regard to this reference curve and all of the validation curves, it should be noted
that summer temperature and salinity conditions, particularly in the Elizabeth River
and occasionally elsewhere, can be such that oxygen saturation concentrations are
below the open-water dissolved oxygen criterion concentration, and it is impossible
to meet the criteria due to natural physical conditions. According to proposed imple-
mentation guidance, nonattainment is forgiven under those conditions. In this
analysis, nonattainment for this reason was not taken into account and, depending on
how often such conditions occurr, this and the other curves may be more accurate.
Approach 4: Reference and Validation Curves using
Benthic Community Health
Benthic community health is the reference and validation site identifier for the deep-
water reference curves. In the absence of other biologically-based indicators for
open-water, open-water reference curves based on benthic health were explored for
QJ
(V
Q_
0 10 20 30 40 50
60 70 80 90 100
Percentage of Area/Volume Exceeding the Criteria
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-14
comparison with the other approaches. The logic was that Chesapeake Bay benthic
organisms have a high tolerance for low dissolved oxygen concentrations, thus
healthy benthos in open-water habitat would not necessarily indicate that the 30-day
mean of 5 mg liter1 was met. On the other hand, a stressed benthic community in an
open-water designated use habitat could indicate that dissolved oxygen criteria in the
habitat zone were not met.
Reference and validation locations (tables H-7 and H-8, respectively) were identified
by methods described below (see section titled "Deep Water Criteria Reference
Curves") and the frequency and extent of criterion attainment were processed as
described below and similar to the other approaches for open-water. Figure H-7
shows the curves resulting from pooling all reference and validation segments in
their respective groups. Figures H-8 and H-9 show the results further segregating
segments by salinity zone.
"O |
to <-
M—
o
u
^ 4/1
v
o ^
o y,
100
90
80
70
60
50
40
30
20
10
0
10 20 30 40 50 60
70
80 90 100
Percentage of Area/Volume Exceeding the Criteria
Figure H-8. Lower salinity summer open-water reference (hatched line) and validation
(solid line) curves: benthic community health approach.
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-15
Table H-7. Reference locations based on benthic index >3 for summer
open-water dissolved oxygen criteria reference curve (Approach 4).
CBP
Segment
Years
CB1TF
1985
1987
1990
1991
1992
CB20H
1986
1988
CB3MH
1988
1993
1994
CB6PH
1986
1990
1991
1992
1993
CB7PH
1988
1990
1992
1993
1994
CB8PH
1985
1986
1987
1989
1990
CHOMH1
1987
CHOMH2
1986
1993
1994
CHSMH
1986
1987
CHSOH
1992
ELKOH
1986
1992
JMSMH
1985
1988
1990
1991
1992
JMSOH
1988
JMSPH
1985
1986
1987
1988
1989
JMSTF
1985
1986
1987
1988
1989
NANMH
1986
1988
PAXMH
1987
1988
PAXOH
1986
1987
PAXTF
1987
1994
PMKTF
1991
1992
1993
1994
POTOH
1986
1987
1988
POTTF
1988
RPPMH
1985
1986
1987
1988
1990
RPPOH
1988
1992
SASOH
1992
YRKMH
1985
1986
1987
1988
1990
1994
Source: Chesapeake Bay Water Quality Monitoring Program database
http:// www.chesapeakebay.net/data
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-16
Table H-8. Validation locations based on benthic index <3 for summer
open-water dissolved oxygen criteria reference curve (Approach 4).
CBP
Segment
Years
BIGMH
1994
CB1TF
1986
1989
CB20H
1987
CB5MH
1986
1987
1989
1992
CB6PH
1987
1988
CB8PH
1988
CHOMH1
1986
1988
1994
CHOMH2
1988
CHOOH
1986
1987
1988
1992
CHOTF
1991
1992
CHSMH
1990
EASMH
1994
ELKOH
1987
1989
1990
1994
HNGMH
1994
JMSMH
1986
1987
1989
1993
JMSOH
1985
1986
1987
1989
LCHMH
1985
1994
PATMH
1985
1987
PAXOH
1994
PAXTF
1989
PMKTF
1985
1986
1987
1988
POTTF
1986
RPPMH
1994
RPPOH
1985
1986
1987
1990
SASOH
1991
SBEMH
1989
1990
1991
1992
YRKMH
1993
1994
Source: Chesapeake Bay Water Quality Monitoring Program database
http:// www.chesapeakebay.net/data
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-17
100
-------
H-18
the previous one to two months of the summer. Thus, in this analysis, if a healthy
benthic sample identified a reference location and it was not otherwise disqualified by
a sample indicating stress, then data for the whole season for that segment/designated
use/season_year were included in the reference distribution. Each benthic sample is
identified by latitude/longitude, segment and bottom depth. This analysis identified each
site by year (assuming months June through September), segment and depth. It did not
take into account a station's specific location within the segment.
The Chesapeake Bay benthic-IBI results from 1985 through 1994 were assessed as
either 3 ('healthy'/'good') or <3 ('stressed'/'not good') and then associated with
season_year (in this case summer), segment and designated use, based on season and
sample depth. 'Healthy' locations were accumulated as the reference distribution.
'Stressed' locations were accumulated as the validation distribution. If both healthy
and stressed sites occurred within the same segment, designated use and
season_year, the location was excluded from both reference and validation distribu-
tions. A listing of the reference and validation locations identified in this way is
attached (tables H-9 and H-10, respectively).
For this exercise, like those described earlier, a baywide master grid file was used in
which each cell has a Chesapeake Bay Program segment assignment and fixed desig-
nated use assignment. In a few segments, both open-water and deep-water
designations occur at the same depth. Because of time limitations,the location of
healthy benthic samples was identified only by segment and depth, not by specific
latitude/longitude, i.e., not by specific grid cell. (Note: Using GIS to locate the
comparable grid cell precisely for each sample would improve the analysis greatly,
but complicate the process.) Thus, when a 'healthy' benthic sample was found at a
segment depth where both open-water and deep-water designated uses were defined,
both were included in their respective list of reference or validation locations.
Monthly mean dissolved oxygen concentration data were interpolated basinwide for
each summer month, June through September, from 1985 through 1994. For each
interpolation, each cell's dissolved oxygen concentration was compared to the
appropriate criteria concentration for the month and designated use, as indicated in
the master grid, and the percent of cells passing/failing the criteria calculated for
Table H-9. Reference locations based on benthic index >3 for summer
deep-water dissolved oxygen criteria reference curve.
CBP
Segment Years
CB3MH 1992
CB6PH 1985 1986 1987 1988 1991 1992 1993 1994
CB7PH 1985 1986 1991
CHSMH 1992 1993
PAXMH 1992
Source: Chesapeake Bay Water Quality Monitoring Program database
http:// www.chesapeakebay.net/data
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-19
Table H-10. Validation locations based on benthic index <3 for summer
deep-water dissolved oxygen criteria reference curve.
CBP
Segment Years
CB3MH
1989
1990
1991
1993
CB4MH
1986
1988
1989
1990
1991
1992
1993
CB5MH
1989
1990
1991
1992
1993
1994
CB6PH
1990
CB7PH
1987
CHSMH
1989
EASMH
1986
PATMH
1989
1990
1991
1992
1993
1994
PAXMH
1987
1988
1989
1990
1993
1994
POTMH
1985
1986
1987
1988
1990
1991
1993
RPPMH
1985
1986
1988
1990
1991
1993
1994
YRKPH
1988
1990
1991
1992
1993
1994
Source: Chesapeake Bay Water Quality Monitoring Program database
http://www.chesapeakebay/net/data
each segment/designated use. Using the respective lists of locations/dates, the data
for the reference and validation locations were extracted, pooled and plotted (Figure
H-10). This approach illustrates a substantial difference between the attainment
curves of healthy and stressed sites. The curves would likely be different (i.e., likely
reduce nonattainment in the reference curve and increasing nonattainment in the
validation curve) if the location selection process were made more specific as
described earlier.
Percentage of Area/Volume Exceeding the Criteria
Figure H-10. Summer deep-water reference (hatched line) and validation (solid line)
curves: benthic community healt approach.
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-20
WATER CLARITY CRITERIA REFERENCE CURVES
The water clarity criteria were developed to be protective of underwater bay grasses.
The criteria apply to the months within the underwater bay grasses growing seasons
and are specific to salinity zone. Reference areas in each salinity zone were selected
by a team of resource managers and underwater bay grasses scientists based on an
extensive review of the available distribution and abundance data record (over 20
years). Chesapeake Bay Program segments or partial segments were identified where
underwater bay grasses distributions had increased significantly in recent years and
had been present historically (Table H-ll). Reference curves were developed for
percent light-through-water (PLW), which is obtained by PLW=100exp(-KdZ),
where Z is the applicaton depth and Kd is a light factor, derived here from Secchi
depth (Kd= 1.45/Secchi depth); see Chapter VI for more detail on implementation of
the water clarity criteria. Application depth (Z) was based on photographic or other
evidence of growth at that depth plus one-half the tide depth in the segment. The
empirical evidence, with the one-half tide height added, provided a range of depths
from which an appropriate depth was selected for inclusion in the PLW calculation.
In some segments, full attainment was achieved at the deepest depth of the range. In
those cases, Z was increased at 0.1 meter increments until exceedance was detected.
Table H-11. Chesapeake Bay Program segments or partial segments used to
establish the water clarity criteria reference curves.
CBP
Segment
Restoration
Target Depth
(meters)
Minimum
Retoration
Target Depth
(meters)
Maximum
Restoration
Target Depth
(meters)
Selected
Restoration
Target Depth
(meters)
CB1TF
2.0
0.5
1.3
0.9
GUNOH
2.0
0.25
0.8
0.5
MATTF
2.0
0.25
0.8
0.5
PISTF
2.0
0.5
1.5
0.5
POTTF
2.0
0.5
1.4
0.6
POTOH
2.0
0.5
1.2
0.75
CB6PH
2.0
0.5
1.3
1.3
CB7PH
1.0
0.5
1.3
1.3
CHOMH1
2.0
0.5
1.3
1.25
EASMH
2.0
0.25
0.8
1.1
MOBPH
2.0
0.5
1.5
1.2
TANMH
2.0
0.5
1.3
0.9
YRKPH
2.0
0.25
1.0
1.2
Source: Chesapeake Bay Water Quality Monitoring Program database
http:// www.chesapeakebay.net/data
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
H-21
Like the methods used to determine attainment of the dissolved oxygen criteria,
ambient light data collected as part of the Chesapeake Bay water quality monitoring
program were averaged monthly and interpolated (using the log transformation).
PLW was calculated for each surface cell using the selected Z depth and the interpo-
lated (back transformed) value for Kd. The PLW value for each cell was compared
to the appropriate criterion for the segment's salinity zone and the cell area desig-
nated as failing or passing the criterion. The spatial extent of attainment, i.e., the
percent area failing the criterion, was tallied for each month in the underwater bay
grass growing season for all years 1985 through 1994, and also for more recent years
through 2000. The monthly figures for percent attainment in each segment were
pooled within salinity classification: tidal-fresh oligohaline and mesohaline polyha-
line and the cumulative frequency distribution calculated and plotted. Note that
segment CB7PH was not included in order to balance the relative contributions from
the different salinity zones. The plots were very similar with and without segment
CB7PH. The reference curves from the 1985-94 period (figures H-l 1 and H-12) are
consistent with the curves developed for the other criteria. The reference curves for
1995-2000 are shown for comparison (figures H-13 and H-14).
It should be noted that the PLW minimum light requirement parameter was origi-
nally developed as a seasonal median measure. For assessing the criteria attainment,
light availability is evaluated on a monthly basis, recognizing that available light
could be less than the requirement level (i.e, 13 percent and 22 percent in lower and
higher salinity waters, respectively) about half the time, and that exceedances will be
more frequent than if the criteria were assessed on a seasonal basis. Because both
criteria attainment and reference curves will be assessed in the same way, the addi-
tional exceedance should be accounted for by the reference curve. Figures H-l5 and
H-l6 illustrate the lower and higher salinity water clarity reference curves, respec-
tively, resulting from assessment on a seasonal median basis.
-------
H-22
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100
90
80
70
60
50
40
30
20
10
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T"
~T~
10 20
30
"T1"
40
"T
~T
50
60
70
80
90 100
Percentage of Area/Volume Exceeding the Criteria
Figure H-1 5. Higher salinity water clarity reference curve: seasonal median
01
I
01
!£ 5
01
100
90
80
70
60
50
40
30
20
10
0
v
10 20 30 40 50 60 70 80
Percentage of Area/Volume Exceeding the Criteria
'i 1 1 1 1 I
90 100
Figure H-1 6. Higher salinity water clarity reference curve: seasonal median.
LITERATURE CITED
Alden, R. W. Ill, and E. S. Perry 1997. Presenting Measurements of Status: Report to the
Chesapeake Bay Program Monitoring Subcommittee's Data Analysis Workgroup. Chesa-
peake Bay Program, Annapolis, Maryland.
Weisberg, S. B., J. A. Ranasinghe, D. M. Dauer, L. C. Schaffner, R. J. Diaz, and J. B.
Frithsen. 1997. An estuarine benthic index of biotic integrity (B-IBI) for Chesapeake Bay.
Estuaries 20:149-158.
appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
-------
1-1
appendix |
Analytical Approaches for Assessing
Short-Duration Dissolved Oxygen Criteria
The Chesapeake Bay dissolved oxygen criteria have several different durations: 30-
day mean, 7-day mean, daily mean and instantaneous minimum. Users' ability to
assess these criteria and to have certainty in the results depends on the time scale of
available data and on the ability of models to estimate conditions at those time
scales. At present, long-term, fixed-station, midchannel water quality monitoring in
the Chesapeake Bay and its tidal tributaries provides dissolved oxygen measure-
ments twice monthly at most or approximately every 15 days between April and
August. Proposed enhancements to the tidal water quality monitoring program
include shallow-water monitoring, as well as high-resolution spatial and temporal
monitoring in selected locations. However, these new components are only in the
planning and early implementation stages at this point, and because of financial
constraints or limitations to current technology, direct monitoring at the scales of the
criteria may not be possible in the foreseeable future. Therefore, the assessment of
attainment for some geographic regions and for some short-term criteria elements
must be waived for the time being or must be based on statistical methods that esti-
mate probable attainment. Several approaches to addressing the duration issue are
described below in more detail.
LOGISTIC REGRESSION MODELS USING ROUTINE
FIXED-STATION MONITORING DATA
This method is a modification and significant update of a method developed origi-
nally to measure attainment of the 1992 Chesapeake Bay dissolved oxygen
restoration goal (Jordan et al. 1992). The early work demonstrated predictable rela-
tionships, on a segment by segment basis, between seasonal mean dissolved oxygen
concentrations and the percent of observations above a target concentration in areas
where dissolved oxygen concentrations ranged above and below goal target concen-
trations (figures 1-1 and 1-2).
appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria
-------
1-2
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TO
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60:
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0 i
P
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afi? *
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-1—I—I—I—I—I—I—I—I—I—I—I—I—I—I—
I—I—I—I—I—I—
2 4 6 8
Season mean dissolved oxygen concentration within designated use
Figure 1-1: Percent of Summer dissolved oxygen concentrations above 1.7 mg liter-1 in
segment CB3MH deep-water designated use habitat.
£=
o
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o
c
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0 ^
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Season mean dissolved oxygen concentration within designated use
Figure 1-2. Percent of Summer dissolved oxygen concentrations above 3 mg liter1 in
segment CB3MH open-water designated use habitat.
The relationships were then expressed as regression equations, which could be used
to predict the percentage of observations above or below target for any seasonal
mean. By extension, the 'percentage of observations' applied both to space and time
within a segment. Most of this pilot work was done for mainstem Chesapeake Bay
segments that were relatively densely populated with fixed monitoring stations later-
ally and longitudinally. Because of the spatial density of stations, the range of
potential dissolved oxygen exposure that any particular point might experience over
a tidal cycle was captured in the models. Contemporaneous, semicontinuous
dissolved oxygen measurements made with in situ sensors deployed on buoys were
used to validate the model estimates. The models did not predict the extreme minima
appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria
-------
1-3
recorded in the continuous record, but were able accurately to predict the frequency
of observations below the mean, 5th and 95th percentile concentrations over a
month's time (Jordan et al. 1992).
With benefit of the long (more than 16 yrs) record of the Chesapeake Bay tidal-water
quality monitoring program and the density of measurements (the vertical dissolved
oxygen profile is characterized at 1- to 2-meter intervals), the simple 1992 regres-
sion models have been improved. These enhanced models use logistic regression,
which is better suited to percent distributions (i.e., distributions between 0 and 100).
The models are now month- and depth-specific in many segments. These models can
be adapted and applied to estimate attainment of the instantaneous minimum
dissolved oxygen criterion, if the user considers that the minimum criterion is not
met if the dissolved oxygen concentration is below the criterion value at any time
and anywhere in the segment-designated use.
The first step is to reconstruct the models using the cruise-by-cruise three-dimen-
sional interpolations of dissolved oxygen monitoring data. That is, collect the
percent volume passing/failing the criterion at each depth in a segment month by
month from, for example, 1985 through 1998, and model the relationship of percent
volume failing/passing the criterion as a function of the monthly mean of that
segment/depth as represented by all the cells in the grid. Using the interpolated data
should improve the spatial representation within the segment.
To assess current attainment, for 1999-2001 for example, the user would interpolate
the monthly average dissolved oxygen concentrations across all tidal waters, as
before, for each month of the season to be evaluated in the assessment period, e.g.,
the summer period including June through September. Then the month/segment/
depth-specific model appropriate for the designated use and cell location would be
applied to estimate the percent of time each cell was likely to be below the instanta-
neous minimum, based on the cell's interpolated monthly average. If the model
predicts that the cell is above the minimum dissolved oxygen level less than 99-100
percent of the time, then the cell is not in attainment. Each cell is assessed in this
way and its volume added to the 'failing' or 'passing' category. Ultimately, the
percent of total volume failing or passing the criterion within the segment and desig-
nated use is calculated for each month. The monthly percentages are tallied over all
months in the season in the assessment period and the cumulative frequency distri-
bution is calculated. Except for the use of the logistic regression model, each of the
steps is consistent with assessment methods of the other criteria (see Chapter VI for
details).
The following is a sample attainment model for Chesapeake Bay Program segment
CB3MH for the open-water instantaneous minimum 3.2 mg liter1 criterion. This
model was based on fixed station data, not on interpolated data as proposed above.
If the time frame is September through March, attainment is likely to be
100 percent, regardless of depth. For other months, the estimated percent
attainment is estimated by
appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria
-------
1-4
LGT = 1.0757 x (mean monthly dissolved oxygen concentration)
-0.0724 x (depth in meters)
-1.8576 for April
-2.9219 for May
-2.7982 for June
-2.8341 for July
-2.5443 for August.
Percent attainment = 100 exp(lgt) / (1 + exp(lgt)).
The figures below illustrate attainment curves for segment CB3MH for summer
deep-water designated use, where the instantaneous minimum criterion is 1.7 mg
liter1 (Figure 1-3) and open-water designated use, where the instantaneous minimum
of 3.2 mg liter1 criterion applies (Fig. 1-4).
At this time the method has not been adequately validated in areas other than the
mainstem Chesapeake Bay. It is, therefore, premature to recommend its implemen-
tation for formally assessing criteria attainment. The issue is not so much the method
itself, but how well the midchannel stations represent the flanks and surrounding
waters where station density is low. The models are only as good as the information
on which they are developed. The day-time sampling schedule cannot detect
nocturnal lows in shallow areas or other areas where dissolved oxygen is quickly
regenerated. In the mainstem Chesapeake Bay, however, the dissolved oxygen
'memory' in the deep waters represents, to some extent, the night-time dissolved
oxygen sags and the station density captures exposure diversity. In other areas and
designated uses, hypoxia is more ephemeral temporally and the density of fixed-
monitoring stations is reduced spatially so the models are likely to be weaker.
o 100
ed
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J* To ."3
1
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1
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1
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Oi O ol on
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£ 8 10-
S. o
0 10 20 30 40 50 60 70 80 90 100
Percentage of Area/Volume Exceeding the Criteria
Figure 1-3. Summer instantaneous minimum deep-water dissolved oxygen 1.7 mg liter1
criterion attainment curve for segment CB3MH based on application of the logistic
regression model.
appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria
-------
Ll—I—I—I—I—I—I—I—I—I—J—II—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—p
0 10 20 30 40 50 60 70 80 90 100
Percentage of Area/Volume Exceeding the Criteria
Figure 1-4. Summer instantaneous minimum open-water dissolved oxygen 3.2 mg liter1
criterion attainment curve for segment CB3MH based on application of the logistic
regression model.
SYNTHETIC, CLOSE-INTERVAL DATA SETS
CREATED BY COMBINING SHORT- AND
LONG-TERM PATTERNS OF VARIATION
IDENTIFIED THROUGH SPECTRAL ANALYSIS
It is critical to obtain real time information about the ephemeral and episodic events
of low dissolved oxygen. However, it is not possible for the states or other partners
to collect such information over vast regions of the Chesapeake Bay and over the
length of time that would be required to address the short duration (i.e., the 7-day
mean, 1-day mean, instantaneous minimum) criteria directly. The following method-
ology is proposed to address the short interval criteria by integrating information
from the long-term, low-frequency monitoring program with short-term, high-
frequency monitoring that can be accomplished using in-situ semi-continuous data
recorders.
The method, still in development, is an adaptation of work by Neerchal (1992). The
method combines temporal variability information from the long-term, low-
frequency monitoring data with that from short-term, high-frequency data such as
collected with in situ continuous recording devices. Since these devices are often
deployed using moored buoys, the associated data are referred to as 'buoy data'.
The method uses spectral analysis to extract the cyclical components of the long- and
short-term dissolved oxygen time series records and combines them to create a
synthesized time-series data set with data synthesized at user-specified time steps.
The synthetic data have the annual and seasonal cyclic and trend characteristics of
the long-term record as well as the tidal, diurnal and other periodic characteristics of
the short-term, high-frequency record. At present, the synthetic data are hourly, with
appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria
-------
1-6
cyclic components limited to two cycles per day The synthetic data are then
analyzed like any other data set relative to the specific elements of the criteria.
The spectral equation for the long-term data (such as the Chesapeake Bay water
quality monitoring program data) is
itm
a) Xtl = Xh+ ajj COS^flfiJt) +b£ Sm^Trf^t) (Equation 1-1)
k=l
where ltm n/2, f\ = fourier frequencies, t = time in months, a,b = spectral coeffi-
cients, and x11, = data.
The spectral equation for the short-term data (for example, from an in situ dissolved
oxygen data recorder) is
stm
b) xf = Xst + £ ajf COS(27rf£t) + sin(27rfjft) (Equation 1-2)
k=l
where stm n/2, = fourier frequencies, t = time in hours, a,b = spectral
coefficients, and xstt = data.
The equation for the spectral forecast (for the synthetic data set) is
x1/ = x11 + a}^ cos^^t) + bsin(27z:f|:tt)
m2 (Equation 1-3)
+ S a^cos(27rf]ft) +bk sin(27rfkt)
k=l
where t = time scaled to suit f, m1 < ltm, is chosen to exclude high frequencies that
would be duplicated in the short-term equation (>/i cycle per month), and m2 < stm
is chosen to exclude frequencies too high to be important (>2 cycles per day).
A SAMPLE APPLICATION OF THE METHOD
The example application below uses long-term data from station CB4.2C, a moni-
toring station in the mid-region of the Chesapeake Bay, and a two-month series of
continuous dissolved oxygen measurements at a buoy deployment in the vicinity of
that station at approximately 9 meters below the surface. Figure 1-5 shows the
observed monthly dissolved oxygen concentrations (asterisks) at station CB4.2C (8-
10-meter depth) and the long-term forecast (line) from the spectral equation.
The synthetic data record is obtained by combining the long- and short-term equa-
tions (Figure I- 6). A sample two-month period, August-September, 1987, indicated
by the two vertical parallel reference lines in Figure 1-5, is expanded in Figure 1-6.
appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria
-------
1-7
16
O)
12
4
0
01JAN1985
01JAN1990
01JAN1995
01JAN2000
Date
Figure 1-5. Observed monthly dissolved oxygen concentrations (*) at Chesapeake Bay
Monitoring Program Station CB4.2C (at the 8- to 10-meter depth) from January 1985 to
January 2000 and the long-term 'forecast' (—) from application of the spectral equation.
Dissolved Oxygen
(m liter-'')
O -P*- CD K) 0)
J
%
W#1'
01AUG1987 11AUG1987 21AUG1987 31AUG1987 10SEP1987 20SEP1987 30SEP1987
Date
Dot=observed monthly dissolved oxygen; dashed Iine=long-term forecast; solid line=combined forecast
Figure 1-6. Expanded view from Figure VI-23 of the two-month period August-
September 1987 synthetic data record obtained by combining the long- and short-term
spectral equations.
Some major difficulties beyond the cost and labor involved in deploying sensors,
which are substantial to begin with, must be overcome to implement this method-
ology The spectral forecasts are essentially temporal interpolations that can be
sampled analytically The forecasts do not lend themselves easily to an analysis of
spatial extent of criteria attainment. For other criteria assessments, the direct meas-
ures of dissolved oxygen in the environment—temporal snap shots—are interpolated
using the Chesapeake Bay Program interpolator, and the spatial extent of attainment
is assessed. Then, the frequencies of the many spatial extent measurements are
collected into a cumulative frequency distribution. The spectral analysis method-
ology developed thus far has yet to incorporate the assessment of spatial extent.
Three-dimensional spatial interpolations at the 10-, 20- and 30-minute short-interval
frequencies is a computational impracticality. Furthermore, more information is
appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria
-------
1-8
needed to determine the sphere of representativeness for the short-term, high-
frequency patterns, both vertically and horizontally
Some compromises will doubtless be required. New pilot projects, some of which
are already underway, and additional analysis of current data already at hand will
answer some of these questions and provide some basic underpinnings. Also, due to
the rapidly developing technology in this area, changes and new, unforeseen oppor-
tunities are likely to present themselves. The importance of the short duration criteria
to the protection of many of the Chesapeake Bay's target species and communities
has been demonstrated. As Bay scientists and managers move forward with devel-
oping assessment tools, it would be prudent to seize new opportunities as technology
evolves.
LITERATURE CITED
Jordan, S. J., C. Stenger, M. Olson, R. Batiuk and K. Mountford. 1992. Chesapeake Bay
Dissolved Oxygen Goal for Restoration of Living Resource Habitats: A Synthesis of Living
Resource Requirements with Guidelines for Their Use in Evaluating Model Results and
Monitoring Information. CBP/TRS 88/93. Chesapeake Bay Program, Annapolis, MD.
Neerchal, N. K., G. Papush and R. W. Shafer. 1992. Statistical Method for Measuring DO
Restoration Goals by Combining Monitoring Station and Buoy Data. Chesapeake Bay
Program, Annapolis, Maryland.
appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria
-------
J-1
appendixJ
Development of Chesapeake Bay
Percent Light-at-the-Leaf
Diagnostic Requirements
The amount of ambient surface light required at the leaf surface to support under-
water bay grasses survival, growth and propagation was determined by comparing
the results of the following three lines of evidence: application of the 1992 bay grass
habitat requirements; accounting for epiphytic light attenuation; and comparison of
field conditions and bay grass growth gradients.
CALCULATION USING THE 1992 BAY GRASS
HABITAT REQUIREMENTS
A set of percent light-at-the-leaf (PLL) requirements was derived by applying the
salinity regime-based values for the 1992 Bay grass habitat requirements for Kd,
dissolved inorganic nitrogen, dissolved inorganic phosphorus and total suspended
solids (Table J-1; Batiuk et al. 1992) into the algorithm (Equation J-1) for deter-
mining PLL:
PLL=100[exp(-Kd Z)][exp(-KeBe)] (Equation J-1).
See Table VII-1 in Chapter VII for how Ke and Be are calculated in Equation J-1.
Using this algorithm, a PLL value of 8.3 percent was calculated for tidal-fresh and
oligohaline salinity regimes. The calculated PLL value was 17.3 percent in mesoha-
line regimes and 13.5 percent in polyhaline regimes. The mesohaline and polyhaline
PLL values differed, despite having the same 1992 Kd, total suspended solids and
dissolved inorganic nitrogen habitat requirements, because their dissolved inorganic
phosphorus bay grass habitat requirements for the two regimes differed (Batiuk et al.
1992; Dennison et al. 1993). By applying the 1992 underwater bay grass habitat
requirements, the PLL requirements of 8 percent for tidal- fresh/oligohaline habitats
and 15 percent (the average of 17.3 and 13.5 values) for mesohaline and polyhaline
habitats were derived from this line of evidence.
appendix J • Development of Chesapeake Bay Percent Light-at-the-Leaf Diagnostic Requirements
-------
J-2
Table J-1. The 1992 underwater bay grasses habitat requirements for the Chesapeake Bay
and its tidal tributaries.
Salinity
Regime
Bay Grass
Growing
Season
Light Attenuation
Coefficient (meter1)
Total
Suspended
Solids
(mg liter1)
Chlorophyll a
(Hg liter1)
Dissolved
Inorganic
Phosphorus
(mg liter"1)
Dissolved
Inorganic
Nitrogen
(mg liter"1)
Tidal-fresh
April-October
1.5
<15
<15
<0.02
none
Oligohaline
April-October
1.5
<15
<15
<0.02
none
Mesohaline
April-October
2.0
<15
<15
<0.01
<0.15
Polyhaline
March-May,
Sept.-November
2.0
<15
<15
<0.02
<0.15
Source: Batiuk et al. 1992.
ACCOUNTING FOR EPIPHYTIC LIGHT ATTENUATION
As noted in Chapter IV, the scientific studies used to derive the percent light-
through-water (PLW) criteria did not consider the shading effects of epiphytes,
which grow on underwater plant leaves at all depths and on experimentally shaded
plants in the field. Several studies in various estuarine habitats indicate that light
attenuation by epiphytic communities tends to contribute an additional 15 to 50
percent shading on underwater plants (e.g., Bulthuis and Woelkerling 1983; van Dijk
1993). A detailed study of turtlegrass beds in Florida coastal waters (Dixon 2000)
showed that, while light levels at the maximum depth of seagrass colonization aver-
aged about 22 percent of surface irradiance (PLW), epiphytic attenuation reduced
this to approximately 14 percent of the surface light that is actually available for
plant photosynthesis (PLL). This represents an average of approximately 35 percent
more shading by epiphytes.
Light attenuation by epiphytic material appears to be important throughout the
Chesapeake Bay, contributing 20 to 60 percent more attenuation (beyond the PLW)
in the tidal-fresh and oligohaline regions, where nutrient and total suspended solids
concentrations were highest, and 10 to 50 percent in the less turbid mesohaline and
polyhaline regions (Figure J-1). These calculated contributions of epiphyte shading
are consistent with the values derived for PLW and PLL by applying the 1992 bay
grass habitat requirement values (see Table J-1) in equations IV-1 and J-1, respec-
tively, where PLL represents approximately 30 percent additional light reduction
beyond PLW.
Epiphytic material was assumed to make a 30 percent additional contribution to light
attenuation throughout Chesapeake Bay shallow-water habitats. This figure was
based on literature values for seagrass minimum light requirements, where epiphyte
effects were either avoided with experimental manipulation (e.g., Czerny and
appendix J • Development of Chesapeake Bay Percent Light-at-the-Leaf Diagnostic Requirements
-------
J-3
60
A. Tidal Fresh & Oligohaline
40
30
20
"oo
75
0
10 20 30 40 50 60
O
60
B. Mesohaline
_l
_l
50
Q_
M—
03
-------
J-4
Dunton 1995) or taken into account with direct measurement (e.g., Dixon 2000) and
results from analysis of Chesapeake Bay data.
Accounting for the epiphytic contribution to light attenuation, PLL requirements for
mesohaline/polyhaline and tidal-fresh/oligohaline habitats were calculated to be
15 percent and 9 percent of surface irradiance, respectively. These values, which
represent the minimum PLL needed to support bay grasses growth, include the
additional 30 percent epiphytic light attenuation beyond the respective PLW require-
ments. For mesohaline/polyhaline habitats, factoring the additional 30 percent
epiphytic light attenuation into the 22 percent PLW requirement yields a 15 percent
PLL requirement as 30% = 100(22-15)/221. A 9 percent PLL requirement for
tidal-fresh/oligohaline habitats was derived by factoring the additional 30 percent
epiphytic light attenuation into the 13 percent PLW requirement, as 30% =
100(13-9)/13.
The derived underwater bay grass PLW and PLL requirements for the Chesapeake
Bay's mesohaline and polyhaline habitats (22 percent and 15 percent surface light,
respectively) are remarkably close to the respective values of 22 percent and 14
percent surface light derived through field experimentation for turtlegrass in Florida
(Dixon 2000).
COMPARISON OF FIELD CONDITIONS AND
BAY GRASSES GROWTH GRADIENTS
Medians of nearshore water quality data (from the Choptank and York rivers) and
Chesapeake Bay Monitoring Program midchannel data were assessed for relationships
between the calculated PLL values, bay grasses growth categories and the proposed
mesohaline/polyhaline and tidal-fresh/oligohaline PLL requirements of 15 percent and
9 percent, respectively. The calculated PLL values from observed water quality con-
ditions associated with 'persistent' and 'fluctuating' bay grass beds were either very
close or well above the PLL requirements, or the limited set of deviations could be
readily explained (Batiuk et al. 2000). These diagnostic PLL requirements were farther
validated through a comprehensive analysis of 14 years (1985-1998) of Chesapeake
Bay water quality monitoring data. The validation results were published in Chapter VII
in Batiuk et al. (2000). From these three lines of evidence, PLL requirements of
15 percent ambient surface light for mesohaline/polyhaline habitats and 9 percent
surface light for tidal-fresh/oligohaline habitats were established.
l6.6 percent represents 30 percent attenuation of the 22 percent light-through-water requirement.
Therefore, light-at-the-leaf requirement is 15.4 percent, which is rounded down to 15 percent.
appendix J • Development of Chesapeake Bay Percent Light-at-the-Leaf Diagnostic Requirements
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J-5
LITERATURE CITED
Batiuk, R. A., R. Orth, K. Moore, J. C. Stevenson, W. Dennison, L. Staver, V. Carter, N. B.
Rybicki, R. Hickman, S. Kollar and S. Bieber. 1992. Chesapeake Bay Submerged Aquatic
Vegetation Habitat Requirements and Restoration Targets: A Technical Synthesis. CBP/TRS
83/92. U.S. EPA Chesapeake Bay Program, Annapolis, Maryland.
Bulthuis, D. A. and W. J. Woelkerling. 1983. Biomass accumulation and shading effects of
epiphytes on leaves of the seagrass, Heterozostera Tasmanica in Victoria, Australia. Aquatic
Botany 16:137-148.
Czerny, A. B. and K. H. Dunton. 1995. The effects of in situ light reduction on the growth of
two subtropical seagrasses, Thalassia testudinum and Halodule wrightii. Estuaries 18:418-
427.
Dennison, W. C., R. J. Orth, K. A. Moore, J. C. Stevenson, V. Carter, S. Kollar, P. W.
Bergstrom and R. A. Batiuk. 1993. Assessing water quality with submersed aquatic vegeta-
tion habitat requirements as barometers of Chesapeake Bay health. Bioscience 43:86-94.
Dixon, L. K. 2000. Establishing light requirements for the seagrass Thalassia testudium: An
example from Tampa Bay, Florida. In: Seagrass Monitoring, Ecology, Physiology and
Management, S.A. Bortone ed., CRC Press, Boca Raton, Florida. Pp. 9-32.
Van Dijk, G. M. 1993. Dynamics and attenuation characteristics of periphyton upon artificial
substratum under various light conditions and some additional observations on periphyton
upon Potamogeton pectinatus L. Hydrobiologia 252:143-161.
appendix J • Development of Chesapeake Bay Percent Light-at-the-Leaf Diagnostic Requirements
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