United States Office of Research and
Environmental Protection Development
Agency Washington DC 20460
EPA/620/R-94/029
December 1994
SEPA
Indicator Development:
Seagrass Monitoring and
Research in the
Gulf of Mexico
Environmental Monitoring and
Assessment Program
National Biological Survey
-------
EPA/620/R-94/029
December 1994
Indicator Development:
Seagrass Monitoring and
Research in the Gulf of Mexico
Report of a Workshop Held
at
Mote Marine Laboratory
in
Sarasota, FL
January 28-29, 1992
Edited by
Hilary A. Neckles
National Biological Survey
Southern Science Center
Lafayette, LA
Project Officer
J. Kevin Summers
U.S. Environmental Protection Agency
EMAP-Estuaries
Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Gulf Breeze, FL 32561
Printed on Recycled Paper
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DISCLAIMER
This document has been reviewed in accordance with U.S. Environmental Protection Agency
policy and approved for publication. Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
v)
V)
Seagrass Monitoring and Research -1992 Page in
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PREFACE
This document is a final report of a workshop co-sponsored by U.S. EPA, EMAP-Estuaries, The National
Biological Survey and NOAA in 1992.
The appropriate citation for this report is:
Neckles, H.A. (ed.) 1994. Indicator Development: Seagrass Monitoring and Research in the Gulf of
Mexico. U.S. Environmental Protection Agency, Office of Research and Development, Environmental
Research Laboratory, Gulf Breeze, FL. EPA/620/R-94/029.
Seagrass Monitoring and Research -1992 Page iv
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ACKNOWLEDGMENTS
This workshop was sponsored by the U.S. Environmental Protection Agency, the U.S. Fish and Wildlife
Service, and the National Oceanic and Atmospheric Administration. We thank the scientists and resource
managers who participated in the workshop and whose ideas and discussions form the basis of this report.
We are especially grateful to Mike Durako, Ken Haddad, and Chris Onuf for leading working groups on
research, mapping, and monitoring; to Ernie Estevez, Mark Fonseca, Ken Moore, and Judy Stout for
leading working groups on conservation objectives; to Julie Morris for assisting with workshop
organization and for facilitating all discussions; and to Bill Dennison for chairing the plenary sessions and
finding the common threads among working group reports. Kumar Mahadevan and the staff of Mote
Marine Laboratory graciously hosted the workshop and provided on-site support critical to workshop
success, and Martha Griffis, Lois Haseltine, and Cam Wiik assisted in preparation of this report.
Workshop Steering Committee:
Hilary A. Neckles, National Biological Survey
W. Judson Kenworthy, National Marine Fisheries Service
William L. Kruczynski, U.S. Environmental Protection Agency
J. Kevin Summers, U.S. Environmental Protection Agency
Seagrass Monitoring and Research -1992 Page v
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WORKSHOP PARTICIPANTS
Susan S. Bell, Ph.D., University of South Florida, Tampa, FL
Douglas A. Bulthuis, Ph.D., Padilla Bay National Estuarine Research Reserve, Ml. Vemon, WA
Otto S. Bundy, Horticulture Systems, Inc., Parrish, FL
JoAnn M. Burkholder, Ph.D., North Carolina State University, Raleigh, NC
Paul R. Carlson, Jr., Ph.D., Florida Department of Natural Resources, St. Petersburg, FL
James K. Culter, Mote Marine Laboratory, Sarasota, FL
Clinton J. Dawes, Ph.D., University of South Florida, Tampa, FL
Robert Day, Indian River Lagoon National Estuary Program, Melbourne, FL
William C. Dennison, Ph.D., University of Queensland, St. Lucia, Qld., Australia
L. Kellie Dixon, Mote Marine Laboratory, Sarasota, FL
Kenneth W. Dunton, Ph.D., University of Texas at Austin, Marine Science Institute
Port Arkansas, TX
Michael J. Durako, Ph.D., Florida Department of Natural Resources, St. Petersburg, FL
Lionel N. Eleuterius, Ph.D., Gulf Coast Research Laboratory, Ocean Springs, MS
Ernest D. Estevez, Ph.D., Mote Marine Laboratory, Sarasota, FL
Randolph L. Ferguson, Ph.D., National Marine Fisheries Service Laboratory, Beaufort, NC
David A. Flemer, Ph.D., U.S. Environmental Protection Agency-Environmental Research Laboratory
Sabine Island, Gulf Breeze, FL
Ruth Folit, New College Environmental Studies, Sarasota, FL
Mark S. Fonseca, National Marine Fisheries Service Laboratory, Beaufort, NC
Charles L. Gallegos, Ph.D., Smithsonian Environmental Research Center, Edgewater, MD
Holly S. Greening, Tampa Bay National Estuary Program, St. Petersburg, FL
Ken D. Haddad, Florida Department of Natural Resources, St. Petersburg, FL
Margaret O. Hall, Ph.D., Florida Department of Natural Resources, St. Petersburg, FL
Lawrence R. Handley, National Biological Survey, Southern Science Center, Lafayette, LA
M. Dennis Hanisak, Ph.D., Harbor Branch Oceanographic Institute, Ft. Pierce, FL
Kenneth L. Heck, Jr., Ph.D., Dauphin Island Sea Laboratory, Dauphin Island, AL
Jeff G. Holmquist, Ph.D., University of Puerto Rico, Lajas, PR
Roger Johansson, City of Tampa, Tampa, FL
James B. Johnston, Ph.D., National Biological Survey, Southern Science Center,
Lafayette, LA
W. Judson Kenworthy, Ph.D., National Marine Fisheries Service Laboratory, Beaufort, NC
William L. Kruczynski, Ph.D., U.S. Environmental Protection Agency-Environmental Research
Laboratory, Sabine Island, Gulf Breeze, FL
Brian E. Lapointe, Ph.D., Harbor Branch Oceanographic Institute, Big Pine Key, FL
Lynn W. Lefebvre, Ph.D., U.S. Fish and Wildlife Service, Gainesville, FL
Jay Leverone, Mote Marine Laboratory, Sarasota, FL
Helene Marsh, Ph.D., James Cook University, Townsville, Qld., Australia
Mike J. Marshall, Ph.D., Mote Marine Laboratory, Sarasota, FL
Peggy H. Mathews, Department of Environmental Regulation, Tallahassee, FL
John M. Macauley, U.S. Environmental Protection Agency-Environmental Research Laboratory,
Sabine Island, Gulf Breeze, FL
Benjamin F. McPherson, Ph.D., U.S. Geological Survey, Tampa, FL
Seagrass Monitoring and Research -1992 Page vi
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Kenneth A. Moore, College of "William and Mary, Virginia Institute of Marine Science,
Gloucester Point, VA
Julie Morris, New College Environmental Studies, Sarasota, FL
Hilary A. Neckles, Ph.D., National Biological Survey, Southern Science Center,
Lafayette, LA
Walter Nelson, Ph.D., Florida Institute of Technology, Melbourne, FL
John C. Ogden, Ph.D., Florida Institute of Oceanography, St. Petersburg, FL
Christopher P. Onuf, Ph.D., National Biological Survey, Corpus Christi, TX
Robert J. Orth, Ph.D., College of William and Mary, Virginia Institute of Marine Science,
Gloucester Point, VA
Ronald C. Phillips, Ph.D., Battelle, Pacific Northwest Laboratories, Richland, WA
Warren M. Pulich, Jr., Ph.D., Texas Department of Parks and Wildlife, Austin, TX
Thomas F. Ries, Southwest Florida Water Management District, Tampa, FL
Frederick T. Short, Ph.D., University of New Hampshire, Jackson Estuarine Laboratory, Durham, NH
Kenneth N. Smith, Florida Department of Natural Resources, Tallahassee, FL
Judy P. Stout, Ph.D., Dauphin Island Sea Laboratory, Dauphin Island, AL
Michael J. Sullivan, Ph.D., Mississippi State University, Mississippi State, MS
J. Kevin Summers, Ph.D., U.S. Environmental Protection Agency-Environmental Research Laboratory,
Sabine Island, Gulf Breeze, FL
John Thompson, Continental Shelf Associates, Jupiter, FL
David A. Tomasko, Ph.D., Sarasota Bay National Estuary Program, Sarasota, FL
Dean A. Ullock, U.S. Environmental Protection Agency, Coastal Programs Section, Atlanta, GA
Robert W. Virnstein, Ph.D., St. Johns River Water Management District, Palatka, FL
Richard L. Wetzel, Ph.D., College of William and Mary, Virginia Institute of Marine Science,
Gloucester Point, VA
Susan L. Williams, Ph.D., San Diego State University, San Diego, CA
Joseph C. Zieman, Ph.D., University of Virginia, Charlottesville, VA
Richard C. Zimmerman, Ph.D., University of California, Los Angeles, CA
Seagrass Monitoring and Research -1992 Page vii
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TABLE OF CONTENTS
DISCLAIMER iii
PREFACE iv
ACKNOWLEDGMENTS v
WORKSHOP PARTICIPANTS vi
INTRODUCTION 9
RECOMMENDATIONS 11
BACKGROUND FOR RECOMMENDATIONS: WORKSHOP PRESENTATIONS AND
DELIBERATIONS 15
CONSERVATION AND RESTORATION OF THE SEAGRASSES OF THE GULF OF MEXICO
THROUGH A BETTER UNDERSTANDING OF THEIR MINIMUM LIGHT REQUIREMENTS
AND FACTORS CONTROLLING WATER TRANSPARENCY 17
W. Judson Kenworthy
SUBMERGED AQUATIC VEGETATION MAPPING 33
Lawrence R. Hundley
ECOLOGICAL INDICATORS 43
Hilary A. Neckles
SUBMERGED AQUATIC VEGETATION RESEARCH NEEDS 51
William L. Kruczynski
SEAGRASS CONSERVATION IN THE GULF OF MEXICO: AN ACTION AGENDA 59
Hilary A. Neckles
APPENDIX A 63
Seagrass Monitoring and Research -1992 Page viii
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INTRODUCTION
Seagrass habitats in the Gulf of Mexico have
declined precipitously during the past 50 years.
Most habitat losses can be attributed to effects of
coastal population growth and accompanying
municipal, industrial, and agricultural
development. Although proximate causes of
local declines can sometimes be identified, the
majority of habitat loss has resulted from
widespread deterioration of water quality.
Restoration and preservation of these important
habitats depend foremost on improving scientific
understanding of the complex causal
relationships between anthropogenic stress and
seagrass ecosystem persistence, and on
developing scientifically based management
programs for seagrass conservation.
On January 28-29,1992, approximately 60
researchers, State and Federal regulators, and
environmental managers met at Mote Marine
Laboratory in Sarasota, Florida, to discuss
strategies for monitoring the environmental
status of seagrass habitats and to determine the
research needed to increase our knowledge of
seagrass responses to anthropogenic stress. The
workshop was sponsored by the U.S.
Environmental Protection Agency (EPA), the
U.S. Fish and Wildlife Service, and the National
Oceanic and Atmospheric Administration. The
goals of the workshop were to provide technical
guidance on monitoring requirements to the
EPA's Environmental Monitoring and
Assessment Program-Estuaries and to assist in
the development of a coastal wetlands research
program at the EPA's Environmental Research
Laboratory-Gulf Breeze. In addition, the
workshop provided a forum for coordinating
research and monitoring activities among
government agencies, universities, and private
organizations with interest and mandates in the
protection of submerged aquatic vegetation
(SAV) resources in the Gulf of Mexico.
EMAP-Estuaries is designed to characterize the
ecological condition of the nation's estuarine and
coastal resources over broad geographic regions
and long time periods. The program is intended
to provide quantitative information on the extent
and potential causes of adverse environmental
changes. In an effort to provide one indicator of
nearshore environmental quality, EMAP-
Estuaries is mapping the location and extent of
SAV in the coastal region of the Gulf of Mexico.
All maps are scheduled to be completed in 1995.
Baseline information on the distribution and
abundance of SAV will then be used to develop a
monitoring program to assess the status and
trends of these habitats. This assessment will be
based on measurement of defined parameters
that serve as indicators of SAV habitat quality.
The workshop developed recommendations for
SAV mapping, classification, and monitoring in
the Gulf of Mexico, and identified a set of
ecological indicators for accurate assessment of
SAV habitat condition.
The EPA Wetlands Research Program included
funding in 1992 for the initiation of coastal
wetlands research. The EPA Science Advisory
Board recommended that initial research be
conducted on the effects of cumulative impacts
within watersheds on coastal SAV communities.
The workshop identified and prioritized research
needs to develop a pilot project and a future EPA
Coastal Wetlands Research Initiative on a
national scale. As a result of this workshop a
study of seagrass responses to long-term light
limitation was initiated at three field sites in the
Gulf of Mexico.
Following introductory presentations on EMAP,
the Wetlands Research Program, and the state of
current knowledge of seagrass environmental
requirements, workshop participants divided into
three working groups to address the workshop
Seagrass Monitoring and Research -1992
Page 9
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objectives: seagrass mapping, ecological
indicators, and research needs. At the end of the
workshop, participants reorganized into four
working groups, each charged with developing a
list of the highest priority actions for
preservation and restoration of seagrass systems.
The working groups reconvened periodically in
plenary sessions to report conclusions, solicit
input from other workshop participants, and to
integrate and synthesize recommendations. This
report summarizes results of the workshop,
emphasizing the recommendations of
participants in an attempt to guide development
of a comprehensive seagrass conservation
program in the Gulf of Mexico.
Seagrass Monitoring and Research -1992 Page 10
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RECOMMENDATIONS
Knowledge of seagrass systems and our ability
to preserve and restore these important habitats
will be advanced most effectively through the
integration of mapping, monitoring, and research
across a range of spatial and temporal scales
(Fig. 1). Specific recommendations within each
of these components of a comprehensive
seagrass conservation program are listed below.
MAPPING
All maps should be produced at a scale of
1:24,000 to conform to the standard of U.S.
Geological Survey topographic quadrangles.
Maps should be developed from aerial
photographs combined with extensive concurrent
field ground-truthing. A minimum list of ground
data to verify photointerpretation includes
submerged aquatic vegetation (SAV) species
present, confirmation of the signature
identification, nonvegetated features, and
location. Other data that can be collected during
ground truthing yet are not critical to map
verification may either assist in
photointerpretation or make the map more
useful. These data include SAV density, water
depth, presence or abundance of epiphytes and
macroalgae, evidence of prop scars, sediment
type, turbidity, and salinity.
Global Positioning System technology should be
used whenever possible during routine collection
of SAV field data to provide true locations for
correlation with historical, present, and future
maps.
SAV beds should be stratified for Environmental
Monitoring and Assessment Program (EMAP)
sampling based on geomorphic type: hypersaline
lagoons, estuaries, open coastal, and deltaic
formations. To ensure equal representation
within geomorphic strata a second tier of
sampling stratification should be introduced,
based on bed size, water depth, and surficial
sediment type.
Mapping of the Louisianan Province should be
repeated every four years to assist other SAV
monitoring, to ensure the repeatability of sample
locations, and to establish long-term trends in
SAV distribution and abundance.
ECOLOGICAL INDICATORS
Various parameters reflecting SAV responses to
environmental stressors can be measured to
quantify the ecological condition of the habitat.
Response indicators fall into three classes,
according to their readiness for incorporation
into a long-term monitoring program:
• Parameters that are ready for implementation
- macrophyte depth limit, shoot density,
aboveground and belowground biomass,
species composition of SAV and macroalgae;
• Parameters for which field evaluations are
necessary to define temporal and spatial
variability and to further characterize
relationships to multiple environmental
stressors - algal biomass, leaf width, plant
constituents, stress proteins, grazer densities;
• Parameters dependent on newly available
technologies that with, significant additional
development, might be important future
ecological response indicators - leaf area
index measured with an automatic meter and
genetic diversity using DNA fingerprinting.
Seagrass Monitoring and Research -1992
Page 11
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V) (/)
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0) o
c
o
o
Minimum Light
Requirement
Figure 1. Integration of mapping, monitoring, and research Into a comprehensive program for
seagrass conservation in the Golf of Mexico. Each component must consider scales in space, time,
and ecological complexity.
Seagrass Monitoring and Research -1992
Page 12
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None of the proposed response indicators have
been tested at the regional and decadal scales
used by EMAP. Seagrass beds are dynamic,
complex systems, and many of the parameters
used to characterize habitat condition, therefore,
exhibit considerable temporal and spatial
variability. We recommend strongly that the
EMAP network be supplemented with increased
sample density at selected sites. The ability to
detect change from widely spaced samples taken
annually must be validated before meaningful
statistical confidence can be placed in the use of
the proposed indicators to assess regional long-
term trends in seagrass ecosystem health.
The most important parameters to measure as
indicators of the extent of pollutant exposure or
habitat degradation present in Gulf of Mexico
seagrass systems are water column light
attenuation, turbidity, chlorophyll
concentrations, dissolved nutrient
concentrations, and diel fluctuation in dissolved
oxygen concentrations. All of these exposure
indicators exhibit extreme temporal variability,
so that single, annual samples would yield no
useful information. To provide the needed data,
exposure indicators must be evaluated either
from frequent sampling or from continuous
monitoring at permanent stations.
Although the proposed indicators exhibit general
relationships with habitat quality, threshold
values separating desirable conditions from
undesirable ones cannot be identified for any of
the variables. Research is needed to better define
and validate criteria for interpreting specific
values of candidate response and exposure
indicators in terms of ecosystem health.
RESEARCH NEEDS
Research is needed to determine the species-
specific minimum light requirements for long-
term persistence and restoration of subtropical
seagrasses.
Assessment of the responses of seagrass
communities to environmental stresses (e.g.,
light quantity and quality, nutrients, sediment
loading, salinity, temperature) is needed to better
project the effects of environmental management
strategies. This area of research should examine
potential changes in seagrass species,
productivity, genetic diversity, and reproductive
success in response to these parameters. The
roles of macroalgae and epiphytes in these
changes and the potential complicating effects of
plant-animal interactions should be evaluated.
Available maps should be used as a research tool
rather than simply as an assessment method.
Information on seagrass distribution and
abundance should be used in correlative and
other analyses to generate specific hypotheses on
interrelationships among seagrass condition,
depth, and other key forcing variables.
Very little is known about the environmental
requirements of deepwater Halophila spp.
communities. This seagrass community requires
significant general research to understand its role
and importance in marine ecosystems.
CONSERVATION OBJECTIVES
No permitted losses of existing seagrass
communities should be tolerated. This is
particularly important in the case of Thalassia
beds, for which few examples of successful
replacement have been documented.
Restoration of seagrasses to historical levels in
the Gulf of Mexico will require widespread water
quality improvements. This requires foremost
that anthropogenic nutrient and sediment loading
be reduced.
Legislative initiatives to protect and restore Gulf
of Mexico seagrass communities depend
ultimately on strong public support. Public
education programs should be developed to
increase awareness of, and appreciation for, the
Seagrass Monitoring and Research -1992
Page 13
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ecological and economic values of seagrass
habitats.
A seagrass working group including research'
scientists, Federal, State, and local resource
managers, and representatives of user groups
should be formed to coordinate seagrass
conservation efforts in the Gulf of Mexico.
Seagrass Monitoring and Research -1992 Page 14
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BACKGROUND FOR RECOMMENDATIONS:
Workshop Presentations and Deliberations
^ Seaerass Monitoring and Research -1992 Pane 15
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CONSERVATION AND RESTORATION OF THE
SEAGRASSES OF THE GULF OF MEXICO THROUGH
A BETTER UNDERSTANDING OF THEIR MINIMUM
LIGHT REQUIREMENTS AND FACTORS
CONTROLLING WATER TRANSPARENCY
by
W. Judson Kenworthy
Beaufort Laboratory
National Oceanic and Atmospheric Administration
National Marine Fisheries Service
Southeast Fisheries Science Center
Beaufort, NC 28516
Seaerass Monitoring and Research • 1992 Paee 17
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INTRODUCTION
Spanning nearly 5 degrees of latitude and 15
degrees of longitude, the Gulf of Mexico is the
ninth largest body of water in the world. The
shallow coastal waters of the gulf consist of an
assortment of physicochemical environments
including extensive barrier island lagoons, 33
major river systems, and 207 estuaries. These
range from the clear subtropical carbonate
sediment-based systems of the Florida Keys to
the temperate hypersaline Laguna Madre in
Texas. The physical and chemical diversity
provided by these environments supports
extensive and highly productive plant
communities that are valuable habitats for
resident and migratory species of fish and
wildlife. The Gulf of Mexico has the largest and
most valuable shrimp fishery in the United States
as well as numerous other important commercial
and recreational fisheries, many of which depend
on the shallow vegetated ecosystems fringing the
gulf.
The Gulf of Mexico is experiencing the second
fastest rate of growth of the five coastal regions
of the United States. Most growth and
development are occurring within a few miles of
the shoreline or along the watersheds draining
into the gulf. Two-thirds of the land area of the
contiguous United States eventually drains into
the Gulf of Mexico, delivering organic matter,
inorganic nutrients, and fresh water. Unless
growth and water quality are properly managed,
the consequences are a predictable
environmental degradation and a serious threat
to the health and well-being of the coastal living
marine resources.
Seagrasses are an important component of the
coastal plant communities in the Gulf of Mexico
(Durako et al. 1987, Zieman and Zieman 1989).
Four genera, including five of the six tropical
western hemisphere species, grow in the gulf:
Thalassia testudinum, Syringodium filiforme,
Halodule wrightii, Halophila decipiens, and
Halophila engelmanni (Fig. 2). Almost always
found growing completely submerged,
seagrasses stabilize unconsolidated sediments
and recycle nutrients while providing food,
shelter, and substrate for hundreds of species of
flora and fauna (Durako et al. 1987, Zieman and
Zieman 1989). Despite the low diversity of
species, seagrasses occupy a wide variety of
habitats including, but not restricted to, sand
shoals, shallow muddy and sheltered lagoons,
high-energy tidal channels, and relatively deep
open-water continental shelves (Continental
Shelf Associates Inc. and Martel Laboratories
Inc. 1985, Iverson and Bittaker 1986, Durako et
al. 1987, Zieman et al. 1989, Zieman and Zieman
1989). Their ability to grow in these very
different environments results from their
phenotypic plasticity and the wide diversity of
morphology and life history strategies provided
by a remarkably few species. Size alone
illustrates the heterogeneity furnished by the
limited species pool. Fully mature seagrass
communities in the Gulf of Mexico span two
orders of magnitude in canopy height and
belowground structure and three orders of
magnitude in weight, from the small low-relief
meadows of Halophila decipiens and Halophila
engelmanni up to the robust and dense beds of T.
testudinum (Zieman and Wetzel 1980). In
between these extremes are two conspicuous
plants, Halodule wrightii and S. filiforme, which
are intermediate in size and reproduce
vegetatively at a moderately high rate (Eleuterius
1987,Fonsecaetal. 1987).
Our understanding of the role seagrasses have in
supporting the living marine resources
of the Gulf of Mexico, and our ability to predict
what the effects of altered water quality will have
on these functions, depend on a comprehensive
understanding of the mechanisms controlling
their distribution and abundance. Light,
temperature, substrate, nutrients, and water
motion constitute the major environmental
Seaerass Monitoring and Research -1992
Paee 18
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Halodule \vrightii
Thalassia tatudinum
Halophila decipiens
Halophila engebnanni
Syringodium flliforme
Figure 2. IHostntlon of the flve species of snb-tropkaJ and tropical seagrasses found growing In
the Gulf of Mexico (from Fonseca, 1993). Horizontal ban = 1 on scale.
Seaerass Monitoring and Research -1992
Paee 19
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factors controlling seagrass growth. Of these
five parameters, light is the most important. The
quality and quantity of available
photosynthetically active radiation (PAR; 400-
700 nm) drive the photosynthetic processes to
fix carbon and produce oxygen, two metabolic
processes critical for the survival and growth of
seagrasses. Ultimately, the amount of light
reaching seagrasses depends on water
transparency, which is a function of the water
quality parameters that influence light
attenuation in the water column and on the
surfaces of the leaves (Neckles 1991, Morris and
Tomasko 1993).
In general, it is difficult to assign overall
dominance or abundance to any one of the five
species in the Gulf of Mexico. Areal distribution
is patchy and, depending on the location, relative
abundance of species will shift within a few
meters. In spite of this variability, consistent
patterns of depth distribution provide evidence
for the interrelationships between seagrasses and
water quality, particularly water transparency
(Iverson and Bittaker 1986, Dennison 1987,
Durako et al. 1987, Zieman and Zieman 1989,
Duarte 1991, Kenworthy and Haunert 1991,
Morris and Tomasko 1993). The observed
patterns suggest that the five species can be
collapsed into three groups with different
minimum light requirements. In descending
order from highest to lowest light requirements
they are 1) T. testudinum, 2) Halodule wrightii/S.
flliforme, and 3) Halophila decipiens/Halophila
englemanni.
SEAGRASS MINIMUM LIGHT
REQUIREMENTS
Two by-products of photosynthesis,
carbohydrates and oxygen, form the basis of two
working hypotheses seeking to explain the
mechanisms controlling the distribution of
seagrasses (Dennison and Alberte 1986, Marsh et
al. 1986, Dennison 1987, Smith et al. 1988,
Zimmerman et al. 1989, Zimmerman et al. 1991,
Morris and Tomasko 1993). Carbon fixed in
photosynthesis is used to build nonstructural
carbohydrates, which support maintenance
respiration, and structural carbohydrates, which
support new growth (carbon balance;
Zimmerman et al. 1989). Oxygen produced in
photosynthesis is critical to the metabolic needs
of the roots and rhizomes of seagrasses, which
often grow in chronically anoxic sediments and
are exposed to the potentially toxic effects of
reduced sulfur compounds (Penhale and Wetzel
1983, Smith et al. 1984, Smith et al. 1988).
Carbon balance and oxygen production are not
necessarily competing hypotheses, yet they may
operate to different degrees in affecting seagrass
distribution, depending on the available species
pool and the prevailing submarine light regime.
Recent studies have indicated that the minimum
light requirements of seagrasses growing in the
Gulf of Mexico are much higher than originally
suggested by physiological studies of leaf
photosynthesis alone (Vincente and Rivera 1982,
Onuf 1991, Fourqurean 1991, Fourqurean and
Zieman 1991, Kenworthy and Haunert 1991,
Kenworthy 1992, Morris and Tomasko 1993).
The traditional definition of the light
compensation point (Ic), which historically has
been 1-5% of the surface incident light, may be
appropriate for phytoplankton, macroalgae, and
charophytes, but it underestimates the
requirements of many seagrasses, including
those residing in the Gulf of Mexico (Dennison
1987,1991, Duarte 1991, Kenworthy and
Haunert, 1991).
Whole plant minimum light requirements, rather
than the requirement of leaves alone, define an
ecological light compensation point (sensu
Goldsborough and Kemp 1988) estimated to be
approximately 15-20% of the average annual
surface incident light for Halodule wrightii and
S. flliforme (Onuf 1991, Kenworthy et al. 1991,
Morris and Tomasko 1993). Because of the
extensive belowground storage mass of T.
testudinum (Dawes 1987, Fourqurean and
Seaerass Monitoring and Research -1992
Paee20
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Zieman 1991), this species may be capable of
withstanding periods of reduced light (Hall et al.
1991). During periods of low light, carbohydrate
reserves may be diverted from the rhizomes to
support whole plant carbon balance (Tomasko
and Dawes 1989). The ability to utilize reserves
may depend on the previous light history of the
plant. During periods of light stress, carbon
reserves may become depleted more rapidly in
species like Halodule wrightii and S. filiforme
that have less belowground storage capability
than does T. testudinum. Thalassia testudinum
may be better adapted to avoid short-term
deficiencies in carbon balance; however,
observations of depth distribution suggest that
its long-term light requirements are as high or
perhaps even higher than those of Halodule
wrightii and 5. filiforme (Vincente and Rivera
1982, Phillips and Lewis 1983, Iverson and
Bittaker 1986, Kenworthy and Haunert 1991).
Reported patterns of depth distribution almost
always indicate that Halodule wrightii and 5.
filiforme grow deeper than T. testudinum,
reinforcing the hypothesis that factors other than
carbon balance alone are important in
determining light requirements and depth
distribution (Zimmerman et al. 1991).
The larger reservoir of belowground tissues in T.
testudinum meadows may be vulnerable to the
phytotoxic effects of reduced sulfur compounds
at the lower light levels in relatively deeper
water, or during temporary and seasonal periods
of poor water transparency. The primary
mechanism controlling T. testudinum light
requirements may be phytotoxicity rather than
carbon balance, whereas Halodule wrightii is
better adapted to minimize both problems.
Halodule wrightii can avoid phytotoxicity and
maximize carbon balance by having greater
oxygen production at low light levels (high
alpha), a higher maximum photosynthetic rate
(high Praax; Williams and McRoy 1982,
Fourqurean 1991, Kenworthy 1992), a shallower
rooting depth, and lower root-rhizome to shoot
ratios (Fourqurean and Zieman 1991, Fourqurean
1991, Kenworthy 1992). Therefore, Halodule
wrightii can sustain growth at lower light levels
for longer periods of time than can T. testudinum
and will survive in deeper water as well as water
with lower overall transparency (a lower
minimum light requirement).
This comparison, and the emerging general
understanding of the minimum light
requirements of seagrasses, suggest that we may
be able to predict what species should be
growing under different conditions of water
transparency as well as the maximum depth and
overall areal coverage we should expect in a
particular water body. This predictive capability
would be a powerful tool for resource managers
to use in water management programs designed
for the protection and restoration of seagrasses
(Kenworthy and Haunert 1991, Zimmerman et al.
1991, Morris and Tomasko 1993). The adequacy
of this prediction will depend on the assumptions
that an average annual attenuation coefficient or
some other relevant variable is a reliable
predictor of seagrass condition and that the
factors responsible for light attenuation can be
isolated for management attention. Currently,
these are two areas of active research interest and
should draw the attention of scientists and
managers during development of the U.S.
Environmental Protection Agency's Coastal
Submerged Aquatic Vegetation Initiative (see,
for example, Morris and Tomasko 1993).
Seaerass Monitoring and Research -1992
Paee 21
-------
bJ
HOBE SOUND MONTHLY Kd VALUES
0.00
-0.25
-0.50:
-0.75 -
-1.00
-1.25
-1.50 -
I I
JAN FEE MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
MONTH
Figure 3- Seasonal cyde of diffuse PAR light attenuation (K* PAR) In a shallow tidal lagoon, Kobe Sound, hi the Southern Indian
UNDERSTANDING AND
PREDICTING SEAGRASS
DISTRIBUTION
Because of the wide diversity of watersheds and
coastal geomorphology in the Gulf of Mexico,
there are a variety of nearshore ecosystems and
water qualities. The result is a range of water
transparencies that are detectable along spatial
and temporal gradients and directly controlled by
three commonly monitored water quality
variables: 1) total suspended solids (TSS,
frequently measured as turbidity by
nephelometric turbidity units), 2) chlorophyll
(CHL), and 3) dissolved organic matter (DOM,
usually measured as color; Kirk 1988, Gallegos
et al. 1990, Gallegos et al. 1991, Moore 1991,
Kenworthy and Haunert 1991, Morris and
Tomasko 1993). Also important are the indirect
Seaerass Monitoring and Research • 1992
Pane 22
-------
controls on water transparency operating through
CHL and DOM. These are dissolved inorganic
nutrients, mainly inorganic nitrogen and
phosphorous, which stimulate phytoplankton
and macroalgal blooms as well as the growth of
epiphytes on the leaves of seagrasses. Both the
direct and indirect controls of light attenuation
have very strong seasonal signatures (Moore
1991, Kenworthy et al. 1991, Neckles 1991,
Kenworthy 1992, Morris and Tomasko 1993)
which suggest that mechanisms controlling
short-term light limitation may be masked by
estimating the minimum light requirements of
seagrasses through a simple correlation between
an average annual light attenuation coefficient
and the maximum depth of growth (Dennison
1987, Duarte 1991, Kenworthy and Haunert
1991, Zimmerman et al. 1991).
The problem with inferring light limitation by a
correlation between maximum depth of growth
and the average annual attenuation coefficient
was illustrated by an intensive study of the
submarine light regime and seagrass distribution
in the southern Indian River (Kenworthy et al.
1991, Kenworthy 1992). Weekly sampling of
the light attenuation coefficients in two coastal
lagoons showed significant effects related to
time and distance from an inlet. During four
years of sampling, a repeatable cycle of summer
maximum and winter minimum transparencies
was detected (Fig. 3). Based on the average
annual light attenuation coefficient, Halodule
wrightii and 5. filiforme grew to a maximum
depth corresponding to light levels of 15 and
37% of the incident light. Even though light
levels exceeded this average value in deeper
water during the summer months (May-August),
these two species could not establish permanent
populations there. Yet, healthy populations of a
smaller species, Halophila decipiens, grew in the
deeper water between May and October
(Kenworthy 1992). If observations on seagrass
distribution were obtained only during the clear
summer period the depth transects would have
suggested that Halophila decipiens was the
species adapted to the lowest light levels, a
paradigm frequently assigned to this species for
several reasons. The small, low-density, low-
relief canopy minimizes self-shading and allows
more light to enter, while a shallow rooting
depth avoids the potentially phytotoxic effects in
the deeper, more reduced sediments.
Additionally, Halophila decipiens has a
simplified anatomy including thin cell walls and
densely packed chloroplasts, which maximize
the amount of light reaching the chlorophyll
molecules (Josselyn et al. 1986). The individual
leaves grow rapidly and turnover is fast enough
to minimize the establishment of epiphytes that
could further attenuate light on the surfaces of
the leaves (Josselyn et al. 1986, Kenworthy et al.
1989). Based on these physiological and
morphological attributes, Halophila decipiens
should have a lower minimum light requirement
than the larger species (Josselyn et al. 1986), yet
they grow only in summer during maximum
photoperiod and intensity.
There is another plausible explanation for the
observed seagrass depth distribution that is
based on the life history of these species rather
than just their physiological and anatomical
characteristics. Although sexual reproduction
occurs with all the species growing in the Gulf of
Mexico (McMillan 1985, Moffler and Durako
1987), Halophila decipiens is by far the most
fecund. In the southern Indian River
(Kenworthy 1992), and even in the tropical
environment of the Salt River Canyon in St.
Croix (Josselyn et al. 1986, Williams 1988,
Kenworthy et al. 1989), ephemeral populations
of Halophila decipiens are reestablished
annually by seed. During the winter periods of
low light and low temperatures in the Indian
River, populations of Halophila decipiens
disappear except in the immediate vicinity of
inlets, where relatively warmer and clearer water
prevails throughout the winter. In other
subtropical and tropical locations fall and winter
storms contribute to the erosion and burial of
existing Halophila decipiens beds, leading to a
seasonal decline in abundance and cover; this
occurs even in tropical deepwater beds of the
Seaerass Monitoring and Research -1992
Paee 23
-------
Virgin Islands (Williams 1988). In the Big Bend
region of the eastern Gulf of Mexico there are
vast areas of deep water (depth > 10 m) on the
continental shelf that are vegetated by Halophila
decipiens and Halophila engelmanni
(Continental Shelf Associates Inc. and Martel
Laboratories Inc. 1985, Contintal Shelf
Associates Inc. 1989). Although there are no
detailed seasonal studies of these deepwater
beds, an evaluation of Hurricane Elena's impact
in 1985 revealed that Halophila meadows were
completely destroyed, yet they recovered during
the following growing season (Continental Shelf
Associates Inc. 1987). This indicates that these
populations are based on an annual life history
strategy.
The storm-impacted beds in the Gulf of Mexico
and St. Croix, and the seasonally ephemeral
Halophila decipiens beds of the southern Indian
River, are reestablished by seed. In the Indian
River seedlings emerge in early spring (March-
May) and continue to germinate throughout the
summer, forming patchily distributed meadows
in deeper water but never in the canopy of the
larger species that grow in relatively shallower
water. Seed germination, seedling growth, and
bed development coincide with the highest levels
of PAR observed during the year (Kenworthy
1992). Because Halodule wrightii, S.filiforme,
and T. testudinum reproduce mainly by
vegetative branching (Tomlinson 1974), they
have limited dispersal potential. These larger
species cannot utilize the available light in
deeper water because the time window is too
short for vegetative propagation and dispersal to
take advantage of the resource. Volunteer
fragments, consisting of a few short shoots,
rhizomes, and roots of these three larger species,
recruit to the deeper areas in summer but they do
not survive the reduced light periods of winter
(Kenworthy 1992).
These observations indicate that the depth
zonation patterns and the inferred minimum light
requirements of seagrasses in the Gulf of Mexico
are more complex than can be described by
physiology, anatomy, or temporally static depth
transects alone. Seagrass light requirements
depend in part on the life history patterns of the
individual species, reinforcing the argument that
an average annual attenuation coefficient may
not adequately predict the distribution of some
seagrasses (Zimmerman et al. 1991). The
survival, growth, and year-to-year persistence of
Halophila decipiens in the Indian River
communities may depend largely on the water
quality in summer, when actively growing
populations are forming seed stocks that will be
the basis for the next year's population. If
growth and fruiting slow or cease during cooler
months (October-April), the light attenuation
values obtained in winter will have no bearing at
all on predicting the success of populations in
subsequent years.
This same argument probably applies to the three
larger species as well, but for different reasons.
During the active growing periods of spring,
summer, and early fall, good water transparency
may ensure an adequate production of
belowground storage carbohydrates that can be
mobilized to short shoots during periods of algal
overgrowth or low light in winter, or for
regrowth the following spring (Dawes 1987,
Tomasko and Dawes 1989). Equally or perhaps
more important, for the larger species that
produce considerable belowground biomass, is
the immediate production of oxygen and carbon
skeletons. These end products of photosynthesis
detoxify reduced sulfur compounds and nitrogen
(nitrate), whereas the production of alternate end
products (carbon compounds) minimizes the
phytotoxic effects of ethanol during nighttime
and during daytime periods of low light
(Pregnall et al. 1984, Smith et al. 1988).
Because the three larger species are perennial,
growing and metabolizing all year, winter light
attenuation will have a greater effect on them
than it would on a species like Halophila
decipiens, which overwinters in a seed bank. An
average annual light attenuation coefficient may
be a better predictor of depth distribution for the
larger species in the more southerly latitudes of
Seaerass Monitoring and Research -1992
Paee 24
-------
the Gulf of Mexico; however, we should
continue to examine the concept of a critical time
period in order to develop a more sensitive
predictor for each of the species, regardless of
size (Moore 1991). For example, in more
northerly regions of the gulf the annual growth
period of Halodule wrightii may be shortened by
low winter temperatures; this argues for a
smaller time window in which light attenuation
should impact seagrass growth.
Even within the genus Halophila the two species
appear to have different requirements for growth
(Dawes et al. 1986, Dawes et al. 1989).
Halophila decipiens will grow right up to the
edge of a meadow but is rarely found growing
within the canopy of the larger species.
Halophila engelmanni grows in the understory
of the larger species or in mixed beds with
Halophila decipiens (McMillan 1985,
Continental Shelf Associates Inc. and Martel
Laboratories Inc. 1985, Onuf 1991, Kenworthy,
personal observations in the Banana River,
Florida). Both species are often the deepest
dwelling but Halophila engelmanni behaves
more like a perennial than an annual plant.
Based on the above discussion, water quality,
particularly water transparency, is expected to
have a major influence on determining the
species composition and abundance of
seagrasses in the Gulf of Mexico. The five
seagrass species, with their diverse anatomy,
varying structural complexity, and widely
ranging habitat requirements, provide different
functions and values for the flora and fauna of
the gulf. Presumably, seagrasses can act as a
mediary in transmitting the detrimental effects of
degraded water quality to secondary production
and the health and well-being of fish and wildlife
(Kenworthy and Haunert 1991).
SEAGRASS CONSERVATION AND
RESTORATION
Our efforts to protect and maintain the diversity
and productivity of seagrass communities in the
Gulf of Mexico will depend on our ability to
sustain good water quality, hi order to do this
we must develop comprehensive water
management plans that include functional and
reliable optical water quality models that enable
resource managers to identify the parameters
having the greatest influence on transparency
(Kirk 1988, Gallegos et al. 1990, Gallegos et al.
1991, Kenworthy 1992, Morris and Tomasko
1993). Within a comprehensive plan, regional
and local resource agencies would establish
desirable goals for seagrass species and coverage
based on existing and/or historical seagrass
distribution and abundance data. These goals
would be matched with the species pool, current
water quality conditions, and the bathymetry of
the watershed, lagoon, or estuary in order to
evaluate the goals with respect to the cost of
achieving such goals. An essential feature to this
plan is a scientifically based water quality
monitoring program that identifies a functional
variable (e.g., the attenuation coefficient) for
predicting seagrass species and their distribution.
In addition, the monitoring program must be
capable of identifying the water quality factors
that control the functional variable (e.g., DOM,
TSS, CHL, and dissolved inorganic nitrogen).
When properly calibrated, optical water quality
models can be used to quantitatively compare the
relative contributions of the individual factors.
For example, a dependent variable such as the
light attenuation coefficient or the percent of
surface irradiance can be evaluated as a function
of one or several independent variables on a
contour plot to estimate their relative
contributions to PAR attenuation (McPherson
and Miller 1987, Vant 1990, Gallegos et al.
1991, Dennsion et al. 1993, Gallegos and
Kenworthy 1993, Gallegos In Press). This type
of comprehensive analysis provides a means for
determining the target parameter for
Seaerass Monitoring and Research -1992
Pane 25
-------
management efforts neededto improve water
transparency, fhis approach avoids the
inadequacies of traditional water quality criteria
and standards where single numerical values or
vague narratives are assigned as targets for
which water quality parameter values cannot be
exceeded. Given the diversity of environments
and seagrass habitat requirements known to exist
in the Gulf of Mexico, a more flexible approach
is needed. Any effort to impose the same
standard for water transparency in the Florida
Keys, the barrier island lagoons of Mississippi,
and the Laguna Madre will probably fail because
the species pools and factors controlling water
transparency in these coastal ecosystems are
likely to be very different.
Future efforts to conserve and restore the
valuable seagrass resources of the Gulf of
Mexico depend on a scientifically based
understanding of the light requirements of the
individual species and the environmental and
anthropogenic factors affecting the submarine
light regime. Research efforts should continue
to focus on developing scientifically based water
quality monitoring programs that not only
measure water quality but also analyze and
interpret the parameters so that factors
influencing water transparency can be evaluated
for the protection of seagrasses.
Seaerass MonitoHne and Research -1992 Page 26
-------
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Seaerass Monitoring and Research • 1992 Page 31
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SUBMERGED AQUATIC VEGETATION MAPPING
WORKING GROUP REPORT
by
Lawrence R. Handley
National Biological Survey
Southern Science Center
700 Cajundome Blvd.
Lafayette, LA 70506
Seagrass Monitoring and Research • 1992 Page 33
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The primary goal of this working group was to
review, discuss, and recommend criteria for
mapping the location and extent of submerged
aquatic vegetation (SAV) in the coastal region of
the Gulf of Mexico as an indicator of nearshore
environmental quality. The area under
consideration for this discussion is the
Louisianan Province of Environmental
Monitoring and Assessment Program -Estuaries,
which includes the coastline of the Gulf of
Mexico from Brownsville, Texas, to Anclote
Key, Florida. This includes the coastlines of the
states of Texas, Louisiana, Mississippi, and
Alabama, and the northwestern coast of Florida.
The area of concern includes bays, sounds, and
estuaries from their offshore limit to the inland
line of astronomical tidal influence.
EXISTING MAPPING PROJECTS
What maps, data, or information are available
and planned by Federal and State agencies
within the Gulf of Mexico area? What is the
areal coverage of the projects? What kind of
cooperation exists among agencies for each
project? EMAP-Estuaries has begun a mapping
project of SAV within the Louisianan Province.
This project will acquire 1:24,000 scale natural
color aerial photography of the Gulf Coast over
four years. The photography will be interpreted,
mapped as overlays to U.S. Geological Survey
(USGS) 1:24,000 scale quadrangles, and
digitized to provide SAV acreages. The National
Biological Survey's (NBS) Southern Science
Center is the project leader and is responsible for
completing the photointerpretation and mapping.
Other project participants include the National
Oceanic and Atmospheric Administration
(NOAA) for seagrass mapping protocols, and the
National Park Service (NPS) and the states of
Florida, Alabama, Mississippi, Louisiana, and
Texas for review and ground-truthing.
The NBS is responsible for most of the seagrass
mapping in the Louisianan Province completed
to date. They prepared seagrass atlases for the
Gulf of Mexico from aerial photography
acquired in 1983, and their wetland and upland
maps developed for each of the coastal states
include SAV; Inconsistent identification of SAV
limits the utility of these historical wetland and
upland maps. Seagrass trend maps at a scale of
1:100,000 have been developed for the Laguna
Madre of Texas using 1988 field mapping and
master's theses for two dates. Seagrass maps
have been developed for trends analysis for the
Chandeleur Islands of Louisiana for nine time
periods, for Perdido Bay, Florida-Alabama, for
four time periods, and for the NPS Gulf Islands
National Seashore, Mississippi-Alabama-Florida,
for three time periods. Seagrass mapping for
four time periods for St. Andrew's Bay, Florida,
is in progress.
Although NOAA has developed a major seagrass
mapping program for the North Carolina coast,
they have not mapped any areas in the Gulf of
Mexico. They have been the principal instigator
and coordinator in the development of a seagrass
mapping protocol. The Minerals Management
Service funded the development of seagrass
atlases for the Florida Big Bend area in 1985 and
the Florida Bay area in 1985-1987. These atlases
are being done at a scale of 1:100,000.
The NPS has funded the U.S. Fish and Wildlife
Service (USFWS) to map the Gulf Islands
National Seashore for three time periods and is
currently developing a contract for field
inventory of seagrasses present. If funding is
available they will fund the USFWS to develop
seagrass maps for the Gulf Islands for the early
1940's.
The states around the Gulf of Mexico coast have
varied in program development related to
seagrass inventory and mapping. Texas and
Florida have active mapping programs, and
Alabama mapped the seagrasses of Mobile and
Perdido bays in the late 1970's, but Louisiana
and Mississippi have never inventoried or
mapped their SAV (although the Louisiana
Department of Natural Resources Coastal
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Management Division was very active in the
review and ground-truthing of the aquatic beds
in the 1988 USFWS wetlands habitat maps for
coastal Louisiana).
The University of Texas Bureau of Economic
Geology has mapped the entire coast of Texas
for SAV for the late 1950's and for 1979-1980.
The Texas Department of Parks and Wildlife has
inventoried and mapped Galveston Bay, San
Antonio Bay, and Nueces Bay. They also have
an ongoing project for Corpus Christi Bay.
The Florida Department of Natural Resources
has completed trends analysis for St. Andrews
Bay, and has ongoing projects to redo the 1983
USFWS seagrass photographic atlases and to
establish trends for Tampa Bay.
BASE MAPS
Do we have adequate base maps? Base maps are
a common map depiction of the coastline in a
common coordinate system at some level of map
accuracy standards, such as USGS 1:24,000
topographic quadrangles or 1:40,000 National
Ocean Survey (NOS) nautical charts. It will be
difficult to complete historical mapping or to
match mapping projects from one state or region
to another without the application of a common
base map series.
The 1:24,000 USGS topographic quadrangles are
the most available and widely used base maps
and should be considered the minimum base
map, in terms of format and scale, to be used for
seagrass mapping.
There are advantages and disadvantages to
working with the USGS 1:24,000 maps:
ADVANTAGES:
• USGS is the national standard.
• Matches other programs (e.g., National
Wetlands Inventory).
• Applicable to county- and parish-level
planning.
• Used for screening permits and regulatory
monitoring.
• Widely available.
• Allow visualization in the field.
• USGS has an active updating program.
DISADVANTAGES:
• Too coarse for small impacts (e.g., prop scars,
boat docks).
• Minimum of 1/4 acre mapping.
• For some quadrangles only orthophotos have
been produced.
• Digital data for quadrangles lacking for many
areas.
• Lack of submerged information (e.g.,
bathymetry).
• Many maps are outdated for changing
coastlines.
There are alternatives to the USGS topographic
quadrangles as base maps. The NOS shoreline
manuscript maps are at scales of 1:20,000 or
1:10,000. However, they are limited in their
availability, they are not available in a digital
form, and other coastal features (e.g., marsh,
cultural features, and roads) are lacking. Other
projects are developing their own base maps
because the scope of the projects demands maps
at scales of 1:20,000, 1:12,000, or 1:6,000.
Other mapping efforts may use 1:100,000 scale
USGS base maps that cover large offshore areas
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and/or areas where detailed data are not available
nor deemed necessary.
during photo interpretation and even slight
amounts of turbidity destroy the signature.
GROUND DATA TO COLLECT IN
THE MAPPING EFFORT
What site-specific data should be collected in the
field as indicators for photointerpretation?
Ground-truthing for SAV mapping should be
done by the photointerpreter. The ground-
truthing focuses on the identification of the SAV
photo signature. Point-specific locations of
"questionable interpretation" are primary field
check sites, and other areas of "confidently
interpreted" SAV are covered by a grid system,
transects, or random point field check sites.
There is a difference between ground data to be
acquired as aids to photointerpretation and that
used as verification of the mapping completed.
EMAP requires that if a monitoring location is
placed in an area where seagrasses are mapped
on the 1:24,000 quadrangles, seagrasses must be
present. Therefore, mapped seagrass beds must
be verified as present or absent through field
review. The key elements in tying ground data
to the photointerpretation are species present,
signature identification confirmation,
nonvegetated features, and location. Other field
data that may be collected as part of the mapping
effort but are not critical as aids to the
photointerpreter in SAV delineation include
vegetation density, water depth, presence or
abundance of epiphytes, evidence of prop scars,
sediment type, light attenuation, salinity, and
presence or abundance of macroalgae.
Species Present: In most instances, species
cannot be distinguished in a photo (although
there are a few exceptions). Although aerial
photographic signatures for Halophila sp. have
been identified through fieldwork in water
depths greater than 15m, aerial photography is
not practical for mapping in deep water because
the water often obscures detection of grass beds
Signature Identification: The photointerpreter
(PI) gains confidence with repeated signature
identification. That identification includes
photointerpretation and the collection of data in
the field. Although study has shown
interpretation can be done without PI field
participation, it is generally recommended that
the PI be involved in the field effort. The PI has
to know how the signatures correspond to SAV
in the water. In addition, a PI lacking field
experience may not know all the pertinent
information to ask the field worker, thus
affecting the accuracy of the final product.
Nonvegetated Features: When at a site, the field
person should be aware of objects that could be
confused with SAV on the aerial photographs
(e.g., eroding peat banks, geologic formations,
etc.).
Location: The correlation of a field collection
site, transect, or plot with a location on the aerial
photographs is essential to photointerpretation
and verification of signatures. This is even more
important when looking at vegetation density
and species composition.
Density: Estimating density requires extensive
field sampling and is potentially a major
resource expenditure that can limit the number of
sampling stations. There are two types of
density that can be estimated: 1) the ground
cover approach estimates the density of SAV
within a patch or bed, i.e, the percent of surface
covered by blades and stems, and 2) the patch
density approach estimates the density of patches
of SAV across the area, i.e., the number, size,
and distribution of patches compared to the
amount of bare ground across the surface. The
ground cover approach is certainly the most
desired, but it requires considerable control in
terms of scale, emulsion, water clarity, water
depth, and field work. Accurate estimation of
ground cover through photointerpretation is not
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always achievable because water clarity and
depth can lighten submerged vegetation, making
it appear less dense. Also the relation between
photographic signature and density can vary
between species and even within a species
depending on blade length, width, pigmentation,
degree of epiphytization and aspect (lying over
versus standing upright). To achieve an estimate
of ground cover with a stated degree of accuracy
requires considerable fieldwork. As a result, it is
time-consuming and expensive. Estimating
patch density is more economical and feasible
through photointerpretation, thus providing
accurate descriptions of SAV distribution.
Water Depth: Although not essential in the
photointerpretation process, data on water depths
(soundings, pole measurements, bathymetric
maps, etc.) can aid the PI by identifying the areas
within the optimal range for SAV growth, or by
identifying dark signatures in the water as
resulting from deep water rather than from SAV.
Water depth at the time the aerial photography is
acquired can affect the signature of the SAV
present. The vegetation will appear darker if the
water level is low and the blades are lying over
than if the water level is high and the blades are
more upright.
Epiphytes: The amount and type of epiphytes
present are important field data to be collected to
determine the health and condition of SAV, but
they are very seasonal in occurrence and it is not
possible to photointerpret them. In addition,
sampling epiphytes is extremely time consuming
and would therefore require a large committment
of resources.
Scats: Prop scars are easily identified on low-
level (e.g., 1:6,000 scale) aerial photography.
They may be mapped and their revegetation
followed for subsequent time periods. They help
in establishing locations on the aerial
photography.
Sediment type: Surficial sediments are easily
collected at field sites. Data derived from
samples may include redox readings, dry weight
organic content, statistics on sediment grain size,
or the percent sand, silt, and clay. Although data
on sediment types are important in the
comparison of sediment type and organic content
to submerged vegetation density and species
composition, the sediment type data have little
application in the photointerpretation process.
Light Attenuation: Turbidity plumes in the
water column caused by suspended sediment or
algae can obscure the signature of SAV. Data on
light attenuation can be gathered by use of light
meters lowered within the water column. Such
data are important in understanding the growth,
structure, and composition of SAV. Turbidity is
easily identified in aerial photographs, but
attenuation data are not essential in the
identification of SAV signatures.
Salinity: Although salinity can cause
considerable variation in SAV species
composition, density, and growth, salinity data
are not necessary in the identification of SAV
signatures.
Macroalgae: Macroalgae take two forms: drift
algae moving with bottom currents, and attached
macroalgae, generally found in shallow low-
energy water. Although macroalgae are
generally found in shallow water they may
appear darker than other SAV and often have
circular patterns within the signature. More
often the signature of macroalgae is
indistinguishable from that of other forms of
SAV, and field determinations are necessary.
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NEW TECHNOLOGY IN
SEAGRASS MAPPING
The most important questions surrounding new
technology associated with image acquisition
are:
• What remotely sensed imagery exists that we
could successfully use?
• Is the remotely sensed imagery effective at
identifying SAV?
• Should programs be looking at these new
advances in the long term?
The working group expressed the opinion that
eventually SAV mapping should get away from
the use of aerial photos because of the increasing
costs and complexity of flight mission planning
and coordination. The general consensus of the
working group was that, at the present time,
remotely sensed imagery, i.e., satellite and
airborne scanner data, cannot provide accurate
and consistent SAV identification. However, the
group believes that future technological
advances will bring scanner data on SAV within
the realm of consideration. Therefore we should
stay current with all new technology even though
we may not be able to use it at this time.
Use of Global Positioning System (GPS) is
considered to be the most important
technological advancement in the mapping of
SAV. Points to consider in using GPS include:
• Aircraft navigation technology is advancing
rapidly and positioning using airborne GPS
technology allows precise location of the plane
at the moment the photo was taken.
• GPS provides real-time display of location
and coverage of the photography collected to
ensure acquisition of the areal coverage
specified.
• GPS technology is advancing very rapidly.
• GPS can reduce the amount of time needed to
physically locate the beds appearing in aerial
photographs during post-flight ground surveys.
• To allow for statistical accuracy, more pre-
flight field time is required for putting out
targets that can be seen on the photo (unless
permanent visible features are already present).
• GPS provides horizontal control of aerial
photography and maps.
• GPS solves digitization problems with
rectification and geopositioning because it
incorporates GPS digital data into the process.
• GPS-controlled photography costs four-to-
five times as much as standard photography.
• Certain photos require a target present in
order to be triangulated with other photos, which
increases cost and time for coordination.
The working group strongly recommended that
anyone collecting point data in SAV fieldwork
use GPS technology and perform differential
correction on the GPS data acquired.
The use of the analytical stereoplotter is another
technological advance that can potentially be of
importance in SAV mapping because it will
reduce the time required for mapping and the
cost of mapping in the long term. The analytical
stereoplotter incorporates the use of GPS field
data to rectify the aerial photography and allows
for photointerpretation and digitization in a one-
step process.
CLASSIFICATION
The primary question formulated by the working
group was: What has been traditionally
classified when seagrasses are mapped?
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Most historic mapping projects have simply
delineated the presence versus absence of
seagrasses, because of the scale of the mapping
effort, the limited funding often available, a
demanding schedule, lack of fieldwork, and lack
of PI expertise in recognizing seagrass
signatures. Also, the simple presence versus
absence of seagrasses allows easier replication of
effort to determine trends of seagrass change.
Macroalgal presence is also identified as a
separate category in some projects.
The working group agreed that it is better to
have fewer classifications. The simpler the
classification, the fewer "gray" areas for
interpretation. The interpretation of seagrass
density has been attempted in several projects in
the past, are each with inconsistent results.
Generally, the intent in seagrass mapping is to
describe the morphology of the bed as a whole,
not the density of seagrasses within the bed. In a
USFWS study, preliminary data indicated the
accuracy of interpretation of seagrass densities
over 70% and under 30% was nearly 70%,
whereas interpretation of the moderate or
medium density range (30-70%) was
approximately 50%. The classification of
seagrasses by species, or the separation of
freshwater species from seagrasses, has also been
attempted, but it cannot be consistently
interpreted from aerial photography and requires
extensive fieldwork and ground-truthing.
Interpretation of species composition and density
is affected by the lack of homogeneity (although
turtlegrass may be interpreted with some
consistency as a darker signature, other seagrass
species are not as easily discerned and the
species composition of mixed beds is impossible
to determine from aerial photography) and
changing water depths (as water depth increases
the water signature gets darker).
Morphologic classification can be accomplished
from aerial photography. Past projects have
identified "continuous" beds (large areas of
seagrasses) and "patchy" beds (small scattered
units of seagrasses). "Patchy" seagrass beds can
be further classified by a range of densities of
patches within an area. A density classification
system is presented as part of the implementation
plan for NOAA's Coastwatch-Change Analysis
Project (C-cap; Dobson et al. 1994).
The Louisianan Province SAV mapping
classification for EMAP includes a gradient of
SAV patch densities (from continuous coverage
through four density classes), and the presence
of macroalgae beds.
STRATIFICATION OF SAMPLING
Once the baseline mapping for SAV present in
the Louisianan Province is completed , EMAP
intends to develop a monitoring program to
assess status and trends of these habitats.
Because SAV beds vary widely in size, shape,
and ecological characteristics, some form of a
priori stratification is necessary to ensure
adequate and representative sampling.
Sampling may be stratified by several criteria:
• Geographic location (distance from shore,
estuaries, river mouths, islands, behind
barriers, open Gulf).
• Salinity (may not be repeatable from year to
year as salinity can change radically).
• Substrate type.
• Anthropogenic stresses (dredging, boating
access, contaminants).
• System size.
• Areal extent of SAV beds.
• Water depth.
• Relationship to physical stress (fetch).
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Special management/jurisdiction.
A tiered approach is suggested for the
stratification of sampling.
FIRST TIER
The first tier of sampling strata is based on
geographic location and system size. This will
provide adequate distribution of sampling
throughout all ecological systems available
within the Louisianan Province.
Lagoons - sounds or bays, protected by barrier
islands without large freshwater inflow.
Estuaries - includes large and small systems (for
sampling purposes include all systems on an
equal basis rather than weighting systems by
area! extent).
Big Bend Area, Florida - unique area (open
coastal).
River deltas - freshwater to brackish SAV.
SECOND TIER
The second tier of sampling strata is based on
seagrass bed morphology and ensures
representation regardless of size, coverage, water
depth, or sediment type. This tier is based,
primarily, on the ability to delineate seagrasses
and map them. The polygon delineates the
boundary of SAV beds or patches on a map.
Potential stratification variables include:
Polygon class - continuous or patchy, density of
patches over area.
Polygon size - ranges of polygon sizes should be
formulated.
Polygon shape - probably not feasible because of
wide variation of shapes within each geographic
location to be sampled.
Depth - ranges of depths should be sampled.
Sediment type - sand, silt, organic mucks.
MONITORING
The mapping working group suggested that the
EMAP monitoring of SAV should:
Tie in with existing mapping and monitoring
programs of NOAA, the NBS, state agencies, the
U.S. Environmental Protection Agency Gulf of
Mexico Program-habitat degradation committee,
and regional and county mapping programs.
Repeat the mapping for the Louisianan Province
every four years to assist monitoring, ensure
repeatability of sampling locations, and establish
trends related to the ecological health of the
province's SAV.
Rely on the protocols developed from the 1990
NOAA-sponsored seagrass workshop and the C-
CAP program (Dobson et al. 1994).
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REFERENCES
Dobson, J.E., E.A. Bright, R.L. Ferguson, D.M. Field, L.L. Wood, K.D. Haddad, H. Iredale III, J.R. Jensen,
V.V. Klemas, R.J. Orth, and J.P. Thomas. 1994. NOAA coastwatch change analysis project-guidance
for regional implementation. U.S. Department of Commerce. NOAA/Coastal Ocean
Program/Coastwatch: Change Analysis Project, NOAA/NMFS, Beaufort Laboratory, Beaufort, NC.
121 pp+ 19 figures.
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ECOLOGICAL INDICATORS
WORKING GROUP REPORT
by
Hilary A. Neckles
National Biological Survey
Southern Science Center
700 Cajundome Blvd.
Lafayette, LA 70506
Seagrass Monitoring and Research -1992 Page 43
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The charge to this working group was to identify
a suite of indicators of the ecological condition
of submerged aquatic vegetation (SAV) beds
appropriate for long-term monitoring. Ideally,
indicators would function on a regional scale
over a period of decades. Indicators must be
applicable across a range of SAV habitat types,
related to ecological condition in a way that can
be quantified and interpreted, quantifiable in a
standardized manner with a high degree of
repeatability, and appropriate within the
constraints (financial, logistical) imposed by the
spatial and temporal scale of a regional, long-
term monitoring program: i.e., the long-term and
regional variability of an indicator must not be
masked by short-term or local variability. The
Environmental Monitoring and Assessment
Program (EMAP) attempts to limit broad scale
data collection for indicators of environmental
quality to a single index period per year, when
responses to anthropogenic and climatic stresses
are anticipated to be most severe (see Summers
etal. 1991). The overriding concern expressed
by this working group during discussions of
candidate indicators was the potential to yield
meaningful information if sampled only once
during the year at sites separated by kilometers.
Working group members were asked to consider
parameters that could be measured to quantify
integrated responses of SAV to individual or
multiple stressors ("response indicators"), and
parameters that could be measured to quantify
pollutant exposure or habitat degradation
("exposure indicators"). Participants were asked
also to recommend the optimal timing and
methods for measurement and to suggest
whether threshold values separating desirable
from undesirable habitat conditions exist for
candidate indicators. It became clear during
discussions that, in many areas, further research
is necessary to improve our ability to
characterize SAV habitat condition over broad
spatial and temporal scales.
RESPONSE INDICATORS
Many plant processes can be expected to respond
to changes in environmental conditions.
Consequently, a wide range of plant
characteristics should reflect environmental
change. The working group generated an
exhaustive list of candidate response indicators
and then combined them into general categories
for discussion. The categories in Table 1 were
selected from the more extensive list by
consensus as the best indicators of habitat
condition.
None of the proposed indicators has been tested
at the regional and decadal scales inherent in
EMAP sampling. Seagrass beds are dynamic,
complex systems, and many of the parameters
used to characterize habitat condition exhibit
considerable temporal and spatial variability.
The working group agreed overwhelmingly that
to accurately assess seagrass ecosystem
condition the EMAP sampling network should
include frequent sampling at selected permanent
stations. The proposed indicators would yield
the most information on seagrass habitat status
and trends if sampled along permanent transects
established perpendicular to the depth gradient.
ABUNDANCE
Measures of plant abundance are among the most
important indicators of habitat condition.
Working group members identified various
morphometric and population parameters that
tend to respond to environmental change and
then prioritized these measures for inclusion in a
long-term monitoring program. All candidate
indicators are estimable from quadrat-based
sampling. Prioritization was based on ease of
measurement, predictability of response to
environmental stress, and degree of temporal and
spatial variability (Table 1). Abundance
measures should be added to a monitoring
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program in order of priority as resources permit.
RESPONSE INDICATORS
Abundance
Shoot density by species
SAV biomass
Algae biomass
Leaf width
Leaf area index
Plant constituents
Soluble carbohydrate concentration
Ratio of C:N:P
Species composition
Seagrasses
Macroalgae
Filamentous algae
Depth limit of bed
Genetic diversity
Stress proteins
Animals
Productivity
EXPOSURE INDICATORS
Light
Nutrients
Total nitrogen, total phosphorus
Ammonium, nitrate, soluble reactive phosphate
Dissolved oxygen
Physical conditions
Physical energy regime
Sediment characteristics
Table 1. Ecological indicator) proposed for Inclusion In the EMAP
sampling network.
Shoot density: Density decreases predictably
with declining availability of light and sediment
nutrients and is less subject to variability caused
by grazing than are other measures of
abundance. Shoot density has historically been
one of the most frequently measured parameters
of seagrass populations. Thus, a long-term data
base for seagrass density under varying
environmental conditions exists in the literature
from which thresholds of responses could
probably be generated. Because seagrass species
differ in their susceptibility to environmental
stress and competitive interactions, a record of
densities of individual species would yield more
information than would a record of total
macrophyte density. For example, among
tropical seagrasses, Thalassia is the most
sensitive to certain environmental stressors and
is the strongest competitor for nutrients. An
increase in density of other seagrasses could thus
signal a decline in environmental condition if
coupled with a decrease in density of Thalassia.
SAV Biomass: Although susceptible to the
confounding effect of grazer leaf removal,
biomass integrates leaf length and width and
therefore may be more responsive to
environmental stress than leaf morphometry.
The allocation of resources between
aboveground and belowground biomass can also
yield insight into environmental stressors.
Algal Biomass: Algal growth is frequently
correlated with nutrient enrichment, such that
high biomass of either epiphytes or unattached
macroalgae may signal declining water quality.
Algal biomass is a result of interactions among
many abiotic and biotic controls, however, and
ordinarily exhibits extreme temporal and spatial
variability. Therefore, environmental condition
cannot be interpreted definitively from algal
biomass alone. To improve the utility of algal
biomass as an indicator of SAV condition,
research is particularly needed to elucidate the
complex interrelationships among light
availability, nutrient concentrations, grazing
intensity, and algal response.
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Leaf Morphometry: Leaf width can be used to
diagnose environmental changes within
Thalassia populations; in general, declining leaf
widths suggest environmental stress.
Information does not exist, however, to interpret
differences in leaf width among populations.
Therefore, leaf width should be considered as a
local response indicator for Thalassia when
monitored at permanent stations only. Since leaf
width varies with shoot age as well as with
environmental conditions, trends in other
response and exposure indicators should be
considered to help interpret any temporal
changes in leaf width. Before leaf width is
considered as a geographic indicator, further
research is needed to quantify the spatial
variability of this parameter and its relationship
to environmental gradients.
Leaf Area Index: LAI integrates leaf size and
density and thus may be more responsive to
stressors than leaf width alone. The effort
required to determine LAI manually, however,
reduces the utility of this measure for large-scale
manual sampling. A meter that measures LAI by
light obstruction is currently used in terrestrial
systems and is adaptable for underwater
applications. Research is needed to determine
whether such a meter can be calibrated reliably
in aquatic systems. Instrument-automated LAI
measures may thus be available in the future.
PLANT CONSTITUENTS
Concentrations of soluble carbohydrates and
ratios of C:N:P in plant tissue generally reflect
environmental conditions. For example,
carbohydrate concentrations in Thalassia have
been shown to decline with light limitation.
Because concentrations of chemical constituents
also show considerable seasonal variation,
samples for comparative purposes must be
restricted to similar times of year, plant growth
phases, and tissue types. Although a long-term
change in soluble carbohydrate concentrations or
C:N:P ratios at a site would indicate a change in
environmental conditions, insufficient data exist
to assign critical levels for any seagrass species.
Research is needed to determine thresholds of
constituent concentrations indicative of
environmental stress.
Constituent concentrations in SAV tissue will be
the most useful for evaluating environmental
conditions. There may be some benefit in
sampling C:N:P ratios of macroalgae also, as an
index of recent water-column nutrient
availability. Despite the ephemeral nature of
macroalgal growth, repeated sampling over
broad geographic areas might be useful to detect
patterns of nitrogen and phosphorus loading.
SPECIES COMPOSITION
The physical and chemical requirements of SAV
species differ, making SAV species composition
a good indicator of environmental conditions.
The species composition of existing macro- and
filamentous algal communities can also yield
information on habitat quality. The presence of
Enteromorpha, for example, may indicate
nutrient-enriched waters. Little is known about
the species response of epiphytic microalgae,
primarily diatoms, to specific conditions.
DEPTH LIMIT
Declines in cover of submerged macrophytes
associated with degrading water quality usually
occur first at the deepest edge of the beds. The
depth limit of a grass bed is thus a reliable
indicator of environmental quality; shoreward
migration of the edge of the bed over time
indicates a decrease in the availability of light at
depth. Scuba diving is most often used to locate
the outer limit of a grass bed. Most of the
seagrass communities in the Gulf of Mexico
exist in water shallow enough to use scuba for
bed delineation. Alternatively, an underwater
video camera can be mounted on a sled and
pulled behind a boat. Use of remote sensing
technology is the only practical technique for
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locating the edge of deeper grass beds such as
those in Florida's Big Bend region. Side scan
sonar may offer a second remote sensing
technique for determining the presence of
vegetation in deeper waters. The potential to
map distributions of seagrasses on the U.S. west
coast has been investigated using this technique.
Further research is necessary to determine the
applicability of side scan sonar to the Gulf of
Mexico. The mixed species composition of
seagrass communities may limit the utility of
this technique in Gulf waters, as it is unlikely
that side scan images would allow species
determinations.
GENETIC DIVERSITY
The magnitude of genetic variability within plant
populations is a function of environmental,
demographic, and genetic events. Genetic
diversity is necessary for long-term persistence
of populations and adaptation to changing
environmental conditions. A decline in genetic
diversity may signal reduced resistance to
environmental stresses and disease. Gel
electrophoresis surveys of specific loci have
been performed for seagrass beds. Although
biomolecular techniques for extraction and
fingerprinting of seagrass DNA are currently in
research and development stages, rapid advances
in forensic technology and applications suggest
that routine genetic processing of biological
material will soon be commercially available.
The incorporation of genetic diversity into a
seagrass monitoring program is dependent on the
availability of technology to process large
numbers of samples for genetic composition.
However, starch gel electrophoresis of isozymes
is well-established for Zostera marina and at
least 1000 samples a week can be processed
easily.
STRESS PROTEINS
Stress proteins are a group of compounds that are
highly conserved evolutionarily and that form in
response to sublethal stresses. The use of stress
proteins as condition indicators stems primarily
from crop research, where high levels have been
correlated with such stresses as anoxia and
repeated metal toxicity. The applicability of
stress proteins for monitoring seagrass condition
is unknown. Research is needed to determine the
environmental factors and duration of exposure
eliciting stress protein expression in seagrasses,
as well as the thresholds of response indicating
degraded habitat conditions.
ANIMALS
Animals exert strong direct and indirect
influences on many of the macrophyte
parameters proposed as ecological indicators.
For example, urchin grazing can directly reduce
leaf height and biomass. Alternatively, by
controlling accrual of epiphyte biomass,
mesograzers can indirectly regulate macrophyte
biomass, growth, and long-term survival. The
importance of higher order interactions in the
control of macrophyte dynamics argues for the
inclusion of meso- and macrograzers in any
monitoring program; without information on
animal population densities it will be difficult to
ascribe changes in macrophyte and epiphyte
characteristics unequivocally to habitat
conditions. Grazers exhibit such extreme
temporal and spatial variability that
incorporation into a monitoring program using
widely spaced, infrequent samples would yield
little information. However, monitoring grazers
concurrent with epiphyte and macrophyte
parameters regularly (e.g., monthly) at
permanent stations representative of larger
geographic areas would contribute substantially
to the understanding of local and regional habitat
trends.
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PRODUCTIVITY
Leaf productivity responds rapidly to changes in
environmental conditions, and it is
straightforward, albeit labor-intensive, to
measure using leaf-marking techniques. Because
of seasonal variability in productivity, annual
sampling is insufficient to detect regional or
long-term trends. If sampled at the appropriate
time scale, however, this parameter may be one
of the most diagnostic early indicators of
environmental change. Monthly productivity
measurements at representative permanent
stations would provide an excellent assessment
of local conditions.
EXPOSURE INDICATORS
Most of the parameters that stress seagrass
populations exhibit extreme temporal variability,
so that single, annual samples would yield no
information on the extent of pollutant exposure
or habitat degradation present. Working group
members agreed that the only way to quantify
habitat quality in terms of many of the most
important stress variables is by frequent
sampling or continuous monitoring at permanent
stations. The number of permanent stations
established would be dictated by funding.
Station location should be stratified by degree of
anthropogenic impacts. Sites close to urban
areas are the most susceptible to change, and
sites away from urban areas can provide baseline
data for comparison.
Although it is possible to list exposure variables
that are correlated with seagrass health and
therefore should form part of a monitoring
program (Table 1), scientific understanding of
the causal relationships between multiple
environmental stressors and macrophyte
response is limited. The need for further
research to validate the proposed variables as
exposure indicators cannot be overemphasized:
the evolution of seagrass management requires
elucidation of the complex interrelationships
among light availability, nutrient concentrations,
epiphyte biomass and composition, macro- and
mesograzer activity, and macrophyte response.
LIGHT
The most important indicator of seagrass habitat
quality is the availability of photosynthetically
active radiation (PAR) at depth. PAR should be
monitored continuously at permanent stations.
The sampling array for each station consists of a
data logger connected to two spherical sensors
offset vertically and separated by 0.25 - 0.5 m,
depending on water clarity (see also Morris and
Tomasko 1993). The sensors will have to be
cleaned regularly. The frequency of
maintenance visits required will be site specific;
the maximum interval between cleanings will
probably be two weeks or less. Light is already
monitored intensively at several sites in the Gulf
of Mexico as part of ongoing research efforts.
EMAP should attempt to collaborate with these
existing programs.
Technology is also available for continuous
monitoring of chlorophyll concentration and
turbidity. These measurements should be
coupled with light monitoring as funds permit.
NUTRIENTS
Nutrient enrichment enhances growth of
phytoplankton and epiphytic algae, and therefore
can indirectly limit the amount of light reaching
leaf surfaces. Dissolved nutrient concentrations
are subject to considerable temporal variability;
data are most meaningful if derived from
frequent samples. Ideally, water quality should
be measured at the same permanent stations used
for continuous light monitoring. The need to
visit sites regularly for light sensor maintenance
provides at least biweekly opportunities to take
water samples. Samples should be analyzed for
Seagrass Monitoring and Research • 1992
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total nitrogen, total phosphorus, nitrate,
ammonium, and soluble reactive phosphate.
To provide a spatial assessment of nutrient
concentrations and the potential sources of
nutrient enrichment, frequent water quality
sampling at a small number of sites should be
coupled with annual or semiannual sampling at
all of the sites forming the EMAP network. All
nutrient sampling should be restricted to a 6-8
week window. The precise timing of nutrient
sampling should be determined from existing
records to minimize confounding effects of
temporal variability. Ideally, periods of
maximum and minimum runoff should both be
included for each site in order to identify
potential extremes of nutrient concentration.
Samples should be analyzed for chlorophyll in
addition to those nutrients identified for frequent
sampling.
DISSOLVED OXYGEN
The diel fluctuation in dissolved oxygen
concentration is an index of ecosystem health.
Hypoxia limits secondary producers directly, and
effects may also cascade to seagrasses by
limiting grazers and consequently enhancing
epiphyte growth. Dissolved oxygen should be
measured continuously at each sampling site
long enough to characterize the magnitude of
diel variation and the duration of hypoxic
conditions. Pilot tests of up to a week of
continuous measurement should be undertaken
at a limited number of sites to determine an
appropriate monitoring interval for use in
regional sampling. Continuous PAR monitoring
at the same sites as oxygen measurement could
assist in interpreting temporal and spatial
patterns of oxygen concentration.
may be assisted by classifying sampling sites
according to energy regime and sediment
characteristics. Valuable data for such
postsampling stratification include wave energy
density, physical exposure index, effective fetch,
tidal current velocity, sediment depth, sediment
grain size distribution, and sediment carbonate
and organic contents.
PHYSICAL CONDITIONS
Most of the seagrass parameters considered as
response indicators are affected by physical
conditions. Interpretation of response variables
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REFERENCES
Morris, L. J. and D. A. Tomasko (eds.). 1993. Proceedings and conclusions of workshops on: submerged
aquatic vegetation and photosynthetically active radiation. Special Publication SJ93-SP13. Palatka,
FL: St. Johns River Water Management District. 244 pp. + Appendices.
Summers, J. K., J. M. Macauley, and P. T. Heitmuller. 1991. Implementation plan for monitoring the
estuarine waters of the Louisianian Province - 1991 demonstration. EPA/600/5-91/228. U.S.
Environmental Protection Agency, Office of Research and Development, Environmental Research
Laboratory, Gulf Breeze, FL. 160 pp.
Seagrass Monitoring and Research • 1992 Page 50
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SUBMERGED AQUATIC VEGETATION
RESEARCH NEEDS
WORKING GROUP REPORT
by
William L. Kruczynski
and
David A. Flemer
U.S. Environmental Protection Agency
Environmental Research Laboratory
Sabine Island
Gulf Breeze, FL 32561
Seagrass Monitoring and Research -1992 Page 51
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The purpose of this portion of the workshop was
to identify and prioritize research requirements
for submerged aquatic vegetation (SAV)
ecosystems and give some direction to the U.S.
Environmental Protection Agency concerning
which research issue could be addressed with
1992 fiscal year funds. This working group also
discussed problems associated with designing,
implementing, and interpreting an assessment
program (Environmental Monitoring and
Assessment Program; EMAP) for SAV
communities.
ESTABLISHMENT OF
ECOLOGICAL LIMITS
There was general agreement among the
members of the working group that the quality
and quantity of light are the principal ecological
factors that control the presence and growth of
SAV and that light requirements for subtropical
SAV species have not been adequately
determined. Also, the minimal ecological
requirements for establishment and growth of
SAV species are species-specific and may vary
geographically within the range of a species.
Data on northern species (e.g., Zostera in
Chesapeake Bay) are not directly transferable to
predictive models for southern, subtropical
systems (e.g., Thalassia in Florida Bay). Many
more species of SAV exist in warmer waters,
which compounds the problem of establishing
ecological limits for SAV communities. Further,
species found in coastal waters stained by
organic acids probably have different ecological
requirements than do different species or the
same species growing in spring-fed waters.
Light requirements cannot be considered alone,
because the availability and quality of light are
controlled by other environmental factors.
Absorption of incident light can occur as a result
of water column attenuation and macroalgae-
epiphyte attenuation. The amount of light-
absorbing phytoplankton and epiphytic growth
may be proportional to nutrient concentrations.
Thus, although there is agreement that light is
the principal controlling mechanism, it is
necessary to quantify the relationship between
light, nutrients, phytoplankton standing crop and
species composition, suspended sediments,
color, macroalgae and epiphyte standing crop
and species composition, and grazers for each
SAV community in different geographic areas.
Research to establish the minimal ecological
requirements must be multifaceted and should
proceed in two directions to determine:
• The causes and mechanisms of light
reduction; and,
• How plants and their community respond to
changes in the quality and quantity of light and
other ecological stressors.
Information is needed on the effects of stressors
on plant morphology and carbon balance. Also,
the association between nutrients and light
availability must be quantified.
Research must be performed in the field and in
the laboratory (microcosms and mesocosms)
through manipulation of environmental
variables. Results must be modeled and their
predictability tested. Research is required on
development of culture methods for subtropical
species of seagrasses before mesocosms can be
used to establish and test limits to growth.
RESTORATION
Restoration and creation of Thalassia and other
SAV communities was discussed at length and it
was concluded that there are no documented
examples of successful replacement of a
Thalassia community. Once Thalassia
disappears from an area, it will take a long time
for that area to recover. The reasons for poor
recovery, whether the area is planted or not, are
Seagrass Monitoring and Research -1992
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many and complex. Resuspension of sediments
in unvegetated areas and changes in sediment
chemistry are primary factors that inhibit
colonization by Thalassia. The working group
concluded that all existing Thalassia meadows
must be preserved and that no losses of "climax"
SAV species caused by development should be
tolerated.
Halodule is a pioneer species of seagrass that
may recolonize a site within several growing
seasons. Once a bed of Halodule is established,
sediments become stabilized and the area may be
invaded by Thalassia. Species, population, and
community responses during decline may not be
the same as those observed during recovery of an
SAV community. Research is required to define
optimum conditions for revegetation by SAV
species and determine plant, population, and
community parameters indicative of declining
and recovering systems.
SPECIFIC RESEARCH TOPICS
The working group identified specific research
projects for consideration for future funding, two
of which were discussed in some detail. First, it
was suggested that a detailed mapping and
monitoring program could be used to identify
research priorities. If SAV communities are
mapped on a regular basis, areas of decline may
be detectable before vegetation completely
disappears. Research could then be initiated to
assess ecological conditions and identify
indicators of stress at various levels of ecological
organization. It was noted that there appears to
be a strong empirical correlation between
presence of fringing emergent wetland
communities and presence of SAV communities.
Regional mapping efforts are required to
substantiate this observation and, if documented,
research must be performed to establish the
mechanisms controlling this phenomenon.
The second research project discussed concerned
establishment of the absolute maximum depth
for each SAV species throughout its geographic
range. Physical and chemical measurements
taken over the depth distribution could be used
to establish minimum ecological requirements
for each species.
General concern was expressed over
extrapolation of measurements and observations
determined on one scale to other scales. It was
agreed that scaling experiments must be
performed before generating predictive models
based upon site-specific observations or
mesocosm manipulations.
The following is a list of the highest priority
SAV research given by each member of the
working group. Although specific research
topics were later consolidated into broader areas,
there is a benefit in reproducing the complete list
here to identify the range of specific topics that
were identified. Also, although many topics
listed appear nearly duplicative, there is a benefit
in listing the slightly different emphasis that
different scientists gave to areas of similar
concern.
Priority Research Topics
1. Quantify minimum and optimum
physical and chemical requirements for
all SAV species.
2. Quantify the link between nutrient input
and light regime in different near-coastal
systems.
3. Establish the "lethal dose" that results in
a declining SAV community.
4. Identify the suite of environmental
variables that best predicts the
abundance and survival of SAV species.
Seagrass Monitoring and Research • 1992
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5. Identify and quantify combinations and
interactions of environmental parameters
that control SAV distribution and
abundance.
6. Identify the interaction of sublethal and
lethal effects on SAV communities that
are associated with water and sediment
quality.
7. Investigate mechanisms of recovery of
seagrass ecosystems including
comparison of the relative importance of
sexual and asexual propagation and
community succession.
8. Determine whether remotely sensed sea
turtle distribution can be used as an
approximation of distribution of
seagrasses.
9. Assess the usefulness of carbon balance
of plants in detecting stress caused by
subtle changes in water or sediment
chemistry that may otherwise be
undetec table.
10. Monitor genetic differences within a
plant species, because they may result in
regional differences in tolerance of
physical and chemical parameters.
11. Investigate the intensity of plant
reponses to alterations of light quantity
for SAV species.
12. Quantify response of entire seagrass
community, including fisheries
productivity, to nutrient loading.
13. Quantify the effects of epiphytes,
epiphyte grazers, and macroalgae on
seagrass survival and growth.
14. Determine the impact of stressors on the
balance of vegetative multiplication and
flowering. Is increased flowering an
indicator of stress?
15. Map distribution of SAV species over
the entire region and overlay with
regional maps of depth, currents,
nutrient loadings, sediment plumes, and
other stressors. Use mapping exercise as
a hypothesis-generating tool and
determine multivariate response surface
for each species.
16. Analyze all existing information and
make best estimate on indicators of
stress and thresholds.
17. Investigate the potential impact of
changes in sea levels to seagrass
distribution.
18. Determine the framework for
extrapolation of measurements made on
one scale to other scales.
19. Investigate the biology and ecology of
Halophila spp. Species of Halophila
have not received much research
attention and may be important to
sediment stability, food chain
productivity, and ecosystem dynamics.
Research topics were grouped into five main
areas and summary statements were made to
consolidate individual areas of concern. Major
areas of required research were summarized in a
model that identifies important stressors to SAV
ecosystems (Fig. 4). Research efforts are
required to identify and quantify responses at
various levels of ecological organization to
environmental stresses, including determination
of thresholds.
1. Physiological responses of plants to
ecological factors.
a. Determine the physiological
responses of SAV species to
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various levels of stresses. Does
plant sensitivity change
seasonally?
b. Identify a suite of plant-level
responses to evaluate sublethal
stresses so that environmental
controls can be implemented
before thresholds of population
decline and change in
community structure are
reached.
c. Investigate the interaction
between light intensity and light
quality.
d. Determine the response surface
of SAV to temperature, salinity,
and light.
2. Responses at population level.
a. Quantify the species-specific
water quality and light
requirements and their
interaction on long-term
maintenance and establishment
of SAV species.
b. Determine the mechanisms of
recruitment.
3. Responses at community level.
a. Determine the interaction of
epiphytes, epiphyte grazing,
and nutrient loading on growth
and survival of SAV species.
b. Investigate ecological variables
and functions of communities
dominated by Halophila spp.
• Does Halophila stabilize
sediments?
• How rapidly does it turn
c.
d.
over?
• Does Halophila enhance
biodiversity and abundance?
• Is it a good indicator of
ecological conditions?
Investigate the effects of
macroalgae in light attenuation.
Determine the relationship
between nutrient levels and
community structure.
4. Mapping exercises.
Develop regional maps of SAV
distribution and physical and
chemical parameters to generate
hypotheses and predications
concerning the effects of
stressors.
5. Overriding factors.
b.
Research is required to
distinguish between natural and
anthropogenic changes in
seagrass distribution and
community structure. Natural
cycles and impacts of episodic
events must be considered.
Analysis of long cores may be
useful in detecting changes in
community structure and
correlating with historical
events. Does succession or do
episodic events control species
dominance? What is the
temporal scale of response?
The management policy should
be no net loss of climax SAV
species because conditions
required for their recruitment are
difficult or impossible to
replicate.
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Geographic Coverage/Distribution
Mapping Exercise
Watershed
Impacts
Minimum and Optimum
Light Requirements
Color(X)
Macroalgae
Phytoplankton
Epiphytes
Total Suspend?
Solids
Species-
Specific
Changes
e.g., Halophila sp.
Factors:
Dissolved oxygen
Carbon dioxide
Temperature
Salinity
Nutrients
Toxins
Figure 4. Summary of areas of research emphasis for SAV communities. All areas must consider scale, time,
and space components.
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c. It is necessary to determine the
effects of watershed
management and nutrient and
salinity effects on light regimes
in estuaries and near coastal
waters.
d. Scaling considerations are
necessary to allow confidence in
predictive models.
e. The effects of meadow
fragmentation on ecosystem
function must be determined.
What is the minimum patch
size? Do many small patches
function as well as a continuous
meadow?
f. New and emerging technologies,
such as DNA fingerprinting,
should be applied to seagrass
communities.
g. Genetic diversity of SAV
species must be maintained in
transplant efforts.
ENVIRONMENTAL ASSESSMENT
Environmental assessment programs (e.g.,
EMAP) must be carefully designed so that they
have the sensitivity required to detect changes
(deterioration or improvement) in environmental
conditions. Careful consideration must be given
to the selection of ecological indicators to assess
the status of the "health" of SAV ecosystems.
The nature of environmental problems and their
indicators may change with regions and species.
Thus, preparing a plan to assess the status of
seagrasses in a large geographical area is not a
simple matter.
Once a problem has been identified, the next step
is to determine the causes of the problem. An
environmental assessment program must be
sensitive to the fact that it is extremely difficult
to distinguish changes that result from
anthropogenic causes, natural successional
processes, or long- or short-term variations in
climatic conditions. In many cases, there is not
enough information on the response of SAV
species to various stressors to determine the
causality of SAV decline. For example, although
disease (e.g., caused by Labyrinthula sp.) is
known to play an important role in the demise of
eelgrass (Zostera) on the North Atlantic coast
and Europe, it is not clear if disease organisms
play a similar role for subtropical seagrass
species. Further, the association between other
environmental stresses and incidence of disease
must be determined and quantified.
A monitoring and assessment program must also
be sensitive to the fact that SAV species may
respond to stressors slowly and that
environmental conditions observed when the
decline is observed may not represent the same
conditions that initiated the decline. Because of
this time lag, natural SAV communities may not
be good indicators of current environmental
perturbations. However, they are excellent
integrative indicators of long-term ecological
conditions.
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SEAGRASS CONSERVATION IN THE
GULF OF MEXICO:
AN ACTION AGENDA
SUMMARY OF WORKING GROUP REPORTS
by
Hilary A. Neckles
National Biological Survey
Southern Science Center
700 Cajundome Blvd.
Lafayette, LA 70506
Seagrass Monitoring and Research -1992 Page 59
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Following a day and a half spent summarizing
knowledge of mapping, monitoring, and research
on seagrass habitats, workshop participants
reorganized to translate this information into
specific actions necessary to reduce habitat
degradation. Four working groups met to
address the question, "What can our agencies and
institutions do together to begin to reverse the
trend of seagrass loss in the Gulf of Mexico?"
Each group was asked to develop, by consensus,
a list of the four highest priority actions for
seagrass conservation. Proposed actions were to
adhere to the following four criteria: 1) actions
must lead to significant habitat improvements;
2) it must be possible to verify or measure
whether actions have been accomplished; 3)
responsible parties must be willing and able to
undertake the proposed actions; and 4) any
necessary financial resources must be available.
Working groups were given 1.5 hours to produce
their lists. The short time frame served to focus
working group attention on the most urgent
conservation needs.
As a springboard for discussion, each working
group developed a fairly exhaustive list of
potential conservation actions. Individual
suggestions fell into the categories of water
quality improvement, public education, habitat
restoration, regulation, enforcement, research,
coordination, monitoring, and seagrass
sanctuaries. Various approaches were used
within working groups to reach a consensus on
the top priorities, including combining like
statements into inclusive conservation objectives
and ranking proposed actions by democratic
vote. The final lists from each working group
are presented in Appendix 1.
As evidenced by the high degree of overlap
among the lists generated independently by each
work group, seagrass experts generally agree on
the immediate courses of action necessary to
reverse habitat losses in the Gulf of Mexico.
Workshop participants reconvened in a plenary
session to consolidate the four groups of
conservation objectives into a single action
agenda. The following actions were concluded
to represent the four highest priority objectives
for conservation of Gulf of Mexico seagrass
systems.
ESTABLISH A POLICY OF NO
SEAGRASS LOSS
The National Wetlands Policy Forum
recommended that United States adopt a policy
of "no net loss" of wetlands, to be achieved
through compensatory mitigation for all
permitted habitat conversions. This policy
implies a 1:1 replacement for all permitted losses
so that the net wetland acreage remains constant.
It is exceedingly difficult, however, to
successfully establish seagrass beds, so that
compensatory mitigation has not yet been
effective for this habitat. The best way to ensure
no net loss of seagrass systems is thus to avoid
impacts altogether. Written policy must allow
no loss of existing seagrass communities
through any permitting programs. This is
particularly important in the case of Thalassia
beds, which are the most difficult to establish
through planting. Thalassia population growth
and coverage rates are very slow, so that it takes
many years for transplants to coalesce. The
potential for physical disturbance, bioturbation,
and depletion of fauna in the interim further
reduces the likelihood of establishing a
functional Thalassia meadow through
transplanting. Therefore, permitted conversions
of Thalassia beds invariably result in a net loss
of seagrass. To date, no examples of successful
replacement of Thalassia habitat have been
documented. The only way to avoid reductions
in total Thalassia acreage through the permit
process is to stop all permitted losses.
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IMPROVE WATER QUALITY
The primary cause of declines in seagrass habitat
is deterioration of water quality. Restoration of
seagrasses to historical levels in the Gulf of
Mexico will require widespread water quality
improvements, which in turn will require
reduction of anthropogenic nutrient and
sediment loading. Public and legislative support
for necessary changes in watershed management
could be gained through the development of
demonstration projects linking specific
reductions in nutrient discharge or sediment
inputs with seagrass recovery. Research is
needed to define the minimum water quality
requirements of subtropical seagrass species and
the sources of water quality degradation, thereby
providing targets for management efforts.
Minimum water quality requirements can be
derived from empirical relationships between
water quality gradients and seagrass distribution,
as has been done in the Chesapeake Bay
(Dennison et al. 1993), and the factors
contributing to water column light attenuation
can be determined from models relating optical
properties of the water to specific water quality
parameters (Gallegos et al. 1991). Experimental
research should be promoted to elucidate the
causal relationships between environmental
variables and seagrasses at various temporal and
spatial scales.
DEVELOP PUBLIC EDUCATION
PROGRAMS
Legislative initiatives to protect and restore Gulf
of Mexico seagrass communities depend
ultimately on strong public support. Education
programs must be developed to increase public
awareness of and appreciation for the ecological
and economic values of seagrass habitats. Public
appreciation for natural resources is enhanced by
involvement. Programs should therefore be
designed not only to disseminate information,
but also to encourage public participation in
seagrass conservation. For example, regional
and local programs should be developed to
include citizens in monitoring and restoration
activities. The words of a vocal seagrass
constituency can translate into legislative
support for necessary conservation measures.
FORM A SEAGRASS WORKING
GROUP TO DEVELOP POLICY
AND IMPLEMENT DECISIONS
Effective seagrass conservation requires the
cooperative efforts of Federal, State, and local
agencies, research institutions, and various user
groups. A coordinated approach to Gulf of
Mexico seagrass habitat conservation should be
formalized through establishment of a working
group representing all interests. A lead
coordinating agency must be selected to
facilitate interaction among representatives and
to act as a clearinghouse for information. The
SAV Working Group of the Chesapeake Bay
Program serves as a model for coordinated
efforts of scientists, resource managers,
politicians, and the public; collaboration among
the interest groups resulted in the development
of baywide and regional submerged aquatic
vegetation water quality requirements and
distribution restoration targets (Batiuk et al.
1992).
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REFERENCES
Batiuk, R.A., R.J. Orth, K.A. Moore, W.C. Dennison, J.C. Stevenson, L.W. Staver, V. Carter, N.B. Rybicki,
R.E. Hickman, S. Kollar, S. Bieber, P. Heasly. 1992. Chesapeake Bay submerged aquatic vegetation
habitat requirements and restoration targets: A technical synthesis. U.S. Environmental Protection
Agency, Chesapeake Bay Program, CBP/TRS 83/92.
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 vegetation. Bioscience 43:86-94.
Gallegos, C.L., D.L. Correll, and J. Pierce. 1991. Modeling spectral light available to submerged aquatic
vegetation. Pages 114-126 in W.J. Kenworthy and D.E. Haunert (eds.), The light requirements of
seagrasses: proceedings of a workshop to examine the capability of water quality criteria, standards and
monitoring programs to protect seagrasses. NOAA Technical Memorandum NMFS-SEFC-287.
Seagrass Monitoring and Research -1992 Page 62
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APPENDIX A
Highest priority actions for seagrass conservation in the Gulf of Mexico, as determined by individual
working groups.
Group 1
• Develop, fund, and implement cost-effective sewage and storm water treatment systems.
• Establish a written seagrass policy and an implementation plan including research, agency, and
public interests.
• Develop a baseline of information on seagrass distribution and abundance for the Gulf of Mexico.
• Develop and coordinate a system of citizen advisory, public education, and monitoring groups
across the Gulf of Mexico.
Group 2
• Demonstrate the linkage between improvements in point source discharges and seagrass community
response at specific sites.
• Reduce point source and non-point source nutrient and sediment loading to attain defensible,
historical values of light attenuation for individual estuaries.
• Develop legislative and public support for seagrass systems through education.
• Require that local comprehensive plans include potential impacts to seagrass ecosystems.
Group 3
• Actively support the preservation and restoration of seagrass habitats.
• Establish a seagrass management working group with scientific, management, regulatory, and user
group representatives to develop policy and a strategic management plan for the Gulf of Mexico
grassbeds.
• Build a seagrass constituency by increasing public and user group appreciation of the importance of
seagrasses.
• Improve the water quality of seagrass habitats.
Seagrass Monitoring and Research -1992 Page 63
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(Appendix A, continued)
Group 4
• Change no net loss to no loss of seagrasses because mitigation and enforcement are not effective.
• Promote experimental research and mapping at various scales to determine causes of habitat loss.
• Revise and enforce water quality criteria to protect submerged aquatic vegetation.
• Encourage enforcement of existing laws, policies, and rules through public education.
Seagrass Monitoring and Research -1992 Page 64^
•tflJ.S. GOVERNMENT PRINTING OFFICE: 1995 - 650-006/00240
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