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The Biological Services Program was established within the U.S. Fish and Wildlife Service
to supply scientific information and methodologies on key environmental issues that
impact fish and wildlife resources and their supporting ecosystems. The mission of the
program is as follows:
To strengthen the Fish and Wildlife Service in its role as a primary source of
information on national fish and wildlfe resources, particularly in respect to
environmental impact assessment.
To gather, analyze, and present information that will aid decisionmakers in
the identification and resolution of problems associated with major changes in
land and water use.
To provide better ecological information and evaluation for Department of
the Interior development programs, such as tho*e relating to energy deve-
lopment.
Information developed by the Biological Services Program is intended for use
in the planning and decisionmaking process to prevent or minimize the impact of
development on fish and wildlife. Research activities and technical assistance services are
based on anaysis of the issues, a determination of the decisionmakers involved and their
information needs, and an evaluation of the state of the art to identify information gaps
and determine priorities. This is a strategy that will ensure that the products produced
and disseminated are timely and useful.
Projects have been initiated in the following areas: coal extraction and conver-
sion; power plants; geothermal, mineral, and oil-shale development; water resource
analysis, including stream alterations and western water allocation; coastal ecosys-
tems and Outer Continental Shelf development; and systems inventory, including
National Wetland Inventory, habitat classification and analysis, and information transfer.
The Biological Services Program consists of the Office of Biological Services in
Washington, D.C., which is responsible for overall planning and management; National
Teams, which provide the Program's central scientific and technical expertise and arrange
for contracting biological services studies with states, universities, consulting firms, and
others; Regional Staff, who provide a link to problems at the operating level; and staff at
certain Fish and Wildlife Service research facilities, who conduct in-house research
studies.
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FWS/OBS-78/69
September 1978
A CONCEPTUAL ECOLOGICAL MODEL
FOR CHESAPEAKE BAY
by
Katherine A. Green
11801 Rockville Pike, No. 802
Rockville, Maryland 20852
Order Number-SFWB 144807
Project Officer-David A. Flemer
Coastal Ecosystems Project
Office of Biological Services
Fish and Wildlife Service
Washington, D.C. 22022
This study was conducted in cooperation with the Chesapeake Bay Pro-
gram, U.S. Environmental Protection Agency.
j* Performed for
CoaStai Ecosystems Project
Office of Biological Services
Fish and Wildlife Service
U.S. Department of the Interior
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DISCLAIMER
The opinions, findings, conclusions, or recommendations expressed in
this report are those of the author, and do not necessarily reflect the
views of the Office of Biological Services, Fish and Wildlife Service,
U.S. Department of the Interior, or the U.S. Environmental Protection
Agency; nor does mention of trade names or commercial products con-
stitute endorsement or recommendation for use by the Federal Govern-
ment.
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EXECUTIVE SUMMARY
A conceptual model for the Chesapeake Bay ecosystem (wetlands, tributaries, and bay-
proper) has been developed as an interrelated series of diagrams showing carbon and nutri-
ent pathways. Information was based on an analysis of local literature and discussions with
scientists who are studying the Bay. The ecological functions that produce the resources of
commercial and recreational fisheries, habitat for migratory birds and other wildlife, waste
disposal, and aesthetic water quality are indicated. Physical (light, turbidity, mixing, trans-
port, sedimentation) and chemical (sediment-water interactions, presence of pollutants)
aspects of the environment modify the rates of biological processes (primary production,
nutrient regeneration, larval survival).
Marshes and other wetlands export carbon as detritus into the Bay system. They also trap
nutrients, and release them gradually. Their natural buffering capacity can be, at times, ex-
ceeded by excessive nutrient loading from sewage or fertilizers.
Natural nutrients and detritus as well as pollutants such as trace metals, refined hydro-
carbons, herbicides, and pesticides enter the Bay system through river flow and overland
runoff.
In the Bay and tributaries, primary producers are phytoplankton, seagrasses, and benthic
algae. Plankton dynamics facilitate nutrient regeneration, as do sediment chemistry and
benthic organisms. Plankton, benthos, and marsh organisms provide food for fin- and shell-
fishes of commercial importance.
A detailed ecosystem model combining the wetlands, plankton, seagrasses, other ben-
thos, and fish trophic dynamics submodels shows the importance of material transfer and
interactions between subsystems. In hierarchical research designs, there is a tendency to
focus on interactions within subsystems. Exchanges between subsystems should also be
studied. Quantitative data and estimates of flows on a Bay-wide, annual basis are needed.
In relating observed changes in the system, such as the decline of submerged aquatic
vegetation or reduction in oyster spatfall, to water quality, the ecosystem context is useful
in indicating possible causal mechanics and pathways. Potential indicators of water quality
and ecosystem health are distribution and abundance of seagrasses, chlorophyll a, dissolved
oxygen, water transparency, blue crab abundance, larval setting, and concentrations of pol-
lutants in the tissues of commercial fin- and shellfishes, plankton, and forage fishes.
To provide information on the relative importance of various biological processes for
water-quality maintenance, and the relative magnitude of different pollutant impacts on the
Bay, quantitative estimates of the flows in the conceptual models should be made.
in
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PREFACE
The Chesapeake Bay Program of the U.S. Environmental Protection Agency (EPA)
has as a principal objective the development of a Chesapeake Bay Water Quality Manage-
ment Plan. A major problem for the U.S. Fish and Wildlife Service (FWS) and other agen-
cies that are responsible for the living resources in the Bay has been the significant decline
in the submerged aquatic vegetation (SAV). It is hypothesized that the reduction in abun-
dance and distribution of SAV is the result of changes in water quality.
Through an interagency agreement, FWS and EPA are cooperating to develop informa-
tion concerning the ecology and value of SAV in the Chesapeake Bay. To serve as a base
of reference, A Conceptual Ecological Model for Chesapeake Bay was devised to indicate
the major components of the ecosystem and to illustrate their interrelationships. Funding
to support this report was provided by Region 3, Chesapeake Bay Program, EPA, through the
coordinating efforts of the Office of Biological Services, FWS.
Any suggestions or questions regarding this publication should be directed to:
Information Transfer Specialist
National Coastal Ecosystems Team
U.S. Fish and Wildlife Service
National Space Technology Laboratories
NSTL Station, Miss. 39529
This report should be cited as follows:
Green, K. A. 1978. A conceptual ecological model for Chesapeake Bay. U.S. Fish and
Wildlife Service, Biological Services Program. FWS/OBS-78/69. 22 pp.
IV
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ACKNOWLEDGMENTS
I wish to thank R. Ulanowicz, D. Heinle, R. Morgan, W. Boynton, M. Kemp, C. Steven-
son, and L. E. Cronin from the Center for Environmental and Estuarine Studies, University
of Maryland; L. Bongers, T. Polgar, A. Lippson, and L. Moran from Martin-Marietta; J. Taft
and L. Brush from the Chesapeake Bay Institute, Johns Hopkins University; D. Flemer from
the Office of Biological Services; D. Correll, Chesapeake Bay Center, Smithsonian Institu-
tion; R. Wetzel, K. Webb, R. Orth, D. Boesch, M. Lynch, R. Harris, D. Haven, J. Merriner,
and D. Markle from the Virginia Institute of Marine Science. All have generously provided
information and ideas for the development of this conceptual model of the Chesapeake Bay.
Insights are attributable to them; errors are my own.
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A CONCEPTUAL ECOLOGICAL MODEL FOR CHESAPEAKE BAY
Katherine A. Green1
INTRODUCTION
PURPOSE
The main objective of this project was the de-
velopment of a conceptual model of the Chesapeake
Bay ecosystem. The model indicates carbon and
nutrient pathways in the Bay.
The Chesapeake Bay and adjacent wetlands
provide habitat for migratory birds and other wild-
life, maintain an aesthetically pleasing environ-
ment, and support recreational and commercial
fisheries. Resources are affected by biological inter-
actions and the physical and chemical processes of
Bay waters, as well as by water quality and the im-
pacts of human activities.
For planning research to support management
decisions on renewable resources, Chesapeake Bay
should be viewed as an estuarine ecosystem. Such a
broad perspective is practical using a conceptual
model to indicate interrelationships among re-
sources and habitats. Within the ecosystem context,
key processes and potential indicator species can
be identified.
A conceptual model, as an explicit statement
of the functioning of Bay ecosystem, will provide a
biologically realistic context for considering the
ramifications of changes in water quality.
The information used to develop the concep-
tual model comes mainly from discussions with
scientists currently doing research on the Bay.
(Questions asked in interviews are listed in appen-
dix A). Some references are given, but the author's
principal role was synthetic, that is, combining
ideas and information from various sojdxes into a
conceptualization of the ecosystem gestalt. Previous
models of the Bay ecosystem have been implicit
mental concepts. This report presents a concrete
i
11801 Rockville Pike, No. 802, Rockville, Md. 20852.
ecosystem concept to facilitate the objective ex-
amination of assumptions.
The project represents only 8 weeks of work
for interviews and writing. The model is general
and simplified for any given area of research. Its
utility lies in its holistic perspective, placing the
relationships among systems components into the
ecosystem context.
This model, while necesarily limited in scope,
is a starting point for an ecosystem perspective on
the Bay, and should serve as a basis for discussion
on systems structure and relationships.
The conceptual model is structured as a set of
box-and-arrow diagrams. Boxes represent system
components; arrows represent flows between com-
ponents or compartments. Components repre-
sented are carbon (C), nitrogen (N), and phos-
phorus (P), but the same basic structure could be
used for energy units.
PHILOSOPHY
An ecosystem is a system open in at least one
property, and in which at least one entity is living
(Dale 1970). Ecosystem behavior is regulated by
feedback loops, time lags, and external physical
factors (King and Paulik 1967). More generally, an
ecosystem consists of organisms including plants,
herbivores, carnivores, and decomposers, with asso-
ciated abiotic resources used by those organisms,
all located within a definable geographic area and
interrelated through a food web. It is an open sys-
tem, with radiant energy entering from, and matter
and energy lost to, the surrounding environment.
Energy is dissipated within an ecosystem, but
nutrients are recycled (Green 1975).
A model should simplify the real system, while
preserving essential features (Levins 1966). Ecolo-
gical theory looks upon ecosystems as hierarchical
systems that can be subdivided for analysis, with
1
-------
complexity derived from successional addition of
organizational states (King and Paulik 1967). But
an ecosystem is more than a collection of subsys-
tems. Present modeling research is focusing on the
linkages among systems components. The coupling
structure has been demonstrated to be important
to overall system behavior (Walsh 1975, Lane and
Levins 1977).
Throughout this report it is assumed that the
Chesapeake Bay, adjacent wetlands, and tributaries
comprise a single ecosystem. Subsystems can be
identified and studied, but a holistic perspective is
necessary to understand the responses of the Bay
system to changes in water quality.
CONCEPTUAL MODEL OF THE BAY SYSTEM
OVERVIEW
The Chesapeake Bay system as defined here
includes the Wetlands, the Bay proper, and its
tributaries. It can be considered a single system
from an ecological point of view. Few species are
found throughout the system; their distributions
vary with salinity, depth, and time of year. But the
web of species interactions does span the whole
system, and includes opportunistic feeding, the
movement of fish from one end of the Bay to the
other, and the large-scale impacts of human
activities.
The Bay system can be viewed as a mechanism
for turning oak leaves into bluefish, or as an enor-
mous nutrient-cycling system, or as a menhaden-
blue crab community, or as a nursery ground for
Bay and Atlantic fisheries. Each of these perspec-
tives is appropriate for some purpose, and all share
the concept of the entire Bay as a single system.
Exchanges of material and energy between the
Bay system and its air, land, and water environ-
ments are indicated in figure 1. Sunlight is the ma-
jor energy input, but winds and tides also add
energy to the system. Water enters the system from
groundwater, rainfall, land runoff, and tides, but
the biggest input is river flow. Water is lost through
tides, evaporation, and flow into the Atlantic
Ocean. Natural nutrients and detritus eojer the
system from river flow and land runoff. Pollutants
are introduced from rivers, runoff, pleasure boats
and ship traffic, and sewage and industrial effluents.
Chemical nutrients can be lost to the deep sedi-
ments or exported to Atlantic waters. Organic car-
bon exchanges occur through migration of birds
and other wildlife, movements of adult and larval
fishes between the Bay and the Atlantic, and remo-
val by commercial and recreational fishing. Possibly
the biggest single carbon loss from the living com-
ponents of the system is the CO2 loss through res-
piration. The gasses CO2, O2, and N2 are exchanged
with the atmosphere.
Losses of carbon and nutrients through respira-
tion, to the sediments, and by export to the Atlan-
tic will not be indicated on more detailed ecosys-
tems diagrams, to keep them as simple as possible.
However, such losses should be taken into consi-
deration in any carbon or nutrient budgets based
on the conceptual diagrams.
Driving the Bay system are inputs of light, nut-
rients, and carbon (a measure of organic matter
derived from photosynthesis). Carbon sources vary
throughout the system. Detritus of external origin
is the main source in the upper reaches of the estu-
ary and tributaries. Marsh plants and "seagrasses"
(used here loosely as a term for submerged aquatic
vegetation) fix carbon in some shallow areas; most
of it enters the system as detritus. In the deeper
parts of the estuary, carbon is fixed in situ by phy-
toplankton, as well as being transported from
shallower areas. A large part of the carbon fixed in
the system goes through detrital pathways and sup-
ports an abundant shallow benthic community that
turns over rapidly. Nutrients from drainage are ab-
sorbed in the marshes and the shallows, and are re-
cycled there and in deeper waters by the activities
of microplankton and microbenthos.
Zooplankton in the Bay are eaten by cteno-
phores, Atlantic menhaden (Brevoortia patronus),
and other fishes, and have abundant algal food
sources on which to graze. Plankton support men-
haden and other forage fishes, which in turn sup-
port commercial and recreational fisheries as well
as unexploited fish groups. Most of the fishes are
transient, spending only part of their life cycle or
part of the year in the Bay system. Atlantic conti-
nental-shelf fisheries are partially supported by the
Bay. The benthic communities support a large pop-
ulation of blue crabs (Callinectes sapidus), which
are effective predators as well as scavengers. Oysters
and clams, also commercially important, derive
most of their nutrition from water-column sources.
This broad overview of the biology of the Bay
system (fig. 2) provides a framework for more
detailed discussions of its ecological dynamics.
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I E
Et*S
£ cc
=> o
a a-
UJ o
x u
O- .
SUN ^ ,,^ »
(ENERGY) p
WIND fc
(ENERGY) W
MIGRATORY BIRDS »
& MAMMALS
(CARBON, NUTRIENTS) ^
GROUNDWATER ^
(FRESHWATER) *
u
ill
it
CC!±.OQ. u> ^
Figure 2. Major processes in a general conceptual model of the Chesapeake Bay system.
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RESOURCES OF THE BAY
The Chesapeake Bay provides a variety of re-
sources. The most obvious are commercial and
recreational fisheries for clams, oysters, blue crabs,
menhaden, striped bass (Morone saxatilis) and
other species. Other resources are habitat for wild-
life and migratory birds, waste treatment, and
water of quality that is suitable for recreation.
Each of these resources is dependent on the
ecological functions of the Bay system. One objec-
tive of these conceptual model diagrams is to indi-
cate the supporting biology for the various re-
sources. Many ecological processes involve loops or
cycles within the system, which are vulnerable to
disturbance at any point.
WETLANDS
The wetlands considered part of the Chesa-
peake Bay system are tidal mudflats and marshes
that experience tidal flushing (fig. 3). Most wet-
lands are above mean low water, but some emer-
gent wetlands extend to shallow (2 m) depths.
Marshes can be divided into several types on
the basis of plant communities. Through their func-
tion as sediment traps and nutrient absorbers, mar-
shes play a role in maintaining water quality of the
Bay. Fresh- and brackish-water mixed vegetative
marsh communities, salt marsh cordgrass communi-
ties, and arrow-arum/pickerel weed communities
are the most valuable marsh types in terms of pro-
duction, habitat, and erosion buffering (Silberhorn
etal. 1974).
Some marshes provide a spawning area for fish-
es, and marsh invertebrates serve as fish food (Wass
and Wright 1969).
Marshes also provide food and habitat for mi-
gratory waterfowl, resident birds, and other wild-
life (Wass and Wright 1969). Migratory geese,
whistling swans, and ducks use adjacent farmlands,
as well as the marsh, as food sources during part of
the year (L. E. Cronin, pers. comm.). Herons,
egrets, and ibises nesting in the marshes are impor-
tant consumers of fish and crustaceans. Gulls,
during the winter, and terns, during the summer, eat
fin- and shellfish in the marshes (R. Andrew, pers.
comm.)
By feeding in farmlands and defecating in the
marsh, birds may import carbon and nutrients. The
magnitude of such imports is not known. Birds also
export some material when they leave the marshes
to migrate. Results from studies of the role of birds
in other ecosystems suggest that the quantity of
carbon and nutrients cycled by migratory birds
during feeding and elimination within the system is
much greater than that of material imported to or
exported from the system.
Mammals which use the marsh habitat include
nutria (Myocastor coypus), muskrat (Ondatra
zibethica), mink (Mustela vison), and raccoon
(Procyon lotor) (Wass and Wright 1969).
Marshes with sufficient tidal flushing export
some of their annual carbon production to the Bay
(R. Wetzel, pers. comm. based on salt marsh model
research). Most export occurs in the winter, by ice
scouring and tidal flushing when standing dead
material is greatest (Heinle et al. 1976). Poorly
flooded marshes may not exchange any carbon
with the estuary on the Patuxent River, although
dissolved nitrogen and phosphorus are exported to
the estuary (Heinle and Flemer 1976). After Teal
(1962), it has been widely assumed that marshes
enhance the productivity of estuaries by exporting
much of the carbon produced. While it is probable
that there is a net export of carbon from marshes
along the Chesapeake, there is still disagreement
among scientists as to the role of marshes as con-
tributors to Bay production.
Another unresolved aspect of the relationship
between the estuary and adjacent marshes is nu-
trient exchange. Marshes trap nutrients (both
natural and pollutants) from tributaries and upland
drainage (Silberhorn et al. 1974). Nutrients can be
temporarily stored in the marshes, and released
gradually to the Bay. The buffering capacity of
marshes is, however, limited and can be lost if large
nutrient inputs from sewage effluent saturate the
marsh (R. Wetzel, pers. comm.).
Organic and inorganic nutrients also enter
marshes from the Bay. Nutrient exchange between
marshes and Bay waters involves changes in the
chemical forms of N and P, superimposed on a
probable net export of N and P to the Bay
(Axelrad 1974). The role of marshes in nutrient ex-
change is debated among scientists and requires
further research.
The primary producers in coastal wetlands are
marsh plants with attached periphyton and benthic
algae (fig. 3). Species vary with the type of marsh
or mudflat. Most marsh plants enter the marsh
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LIGHT
MARSH PLANTS
& EPIPHYTES
MIGRATORY
& RESIDENT
BIROS
OTHER
WILDLIFE
CRUSTACEANS
SMALL FISHES
BENTHOS
(SMALL CONSUMERS)
BENTHIC
ALGAE
BAY PROPER
BAY FISHES
| BAY PROPER
BAY PROPER
Figure 3. Conceptual model of wetlands.
food web through the detrital pathway, and de-
composers are consumed by crabs and other ben-
thic crustaceans, bivalves, mollusks, and small
fishes. In any particular marsh, the small (in physi-
cal size) marsh consumer box will be dominated by
one particular species (R. Wetzel, pers. comm.),
e.g., fiddler crabs, insects, or small fishes. However,
the dominant consumer member of the compart-
ment will vary with the type of marsh and perhaps
with the time of year. Small consumers of marsh
detritus and benthic algae play a role in any marsh
food web. In turn, these small marsh consumers
support fishes which enter from the Bay during
high water, birds, and other wildlife.
Material enters the Bay system with runoff
over lands which are not classified as wetlands.
This flow appears in figure 2 as drainage to the Bay
(organic and inorganic N, P, and C). Runoff intro-
duces a variety of substances into the Bay. Pollu-
tants include herbicides and pesticides from agri-
cultural land, and toxic substances impregnating
wooden bulkheads, as well as natural (as opposed
to man-induced) nutrients and detrital material.
River flow brings natural nutrients, trace
metals, and detritus, as well as chlorine and nutrient
loads from partially treated sewage, effluents from
power plants, and chemicals from industrial activi-
ties, into the Bay system. Trace metals and petrol-
eum hydrocarbons are introduced from shipping
traffic, Baltimore Harbor, and storm-sewer runoff.
I found no estimates of the relative inputs of
natural and pollutant materials from rivers, land
runoff, pleasure and commercial boats, and Balti-
more Harbor. Some work along this line has been
done at Chesapeake Bay Institute at Johns Hopkins
University. Point sources such as sewage treatment
plants are monitored (A. J. Lippson, pers. comm.,
Potomac River survey). Nonpoint sources such as
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land runoff are much more difficult to measure or
estimate.
Pollutants have reached all parts of the Chesa-
peake Bay system (L. E. Cronin, pers. comm.).
BAY AND TRIBUTARIES
Those parts of the Bay system that are perma-
nently under water exhibit a marked salinity
gradient from fresh water in the more landward
locations of the estuary to approximately 30 o/oo
salinity at the mouth of the Bay. At any given loca-
tion in this partially mixed estuary, salinity is high-
est in the summer and fall when river runoff is low,
and lowest in the winter and spring when rainfall
and runoff are high. Pritchard (1968) provides a
general description of water movement in the Bay.
Storms can produce abrupt changes in salinity
distribution.
The salinity regime affects the distribution of
species from plankton to benthos to fishes. An eco-
logical structure appropriate to any salinity region
within the Bay is illustrated in figure 2. However,
species composition inside the boxes varies with
salinity and season; for example, net zooplankton
species are dominated by copepods: Eurytemora in
fresh and brackish water, and Acartia in more
saline water. Net phytoplankton dominants vary
from blue-green algae in fresh water to diatoms in
more saline waters. The distribution of fish species
(eggs, larvae, juveniles, and adults) depends upon
the salinity regime as well as the time of year.
Species of benthic infauna and epifauna vary with
salinity; oysters and clams are found in the middle
salinity regions of the Bay (Lippson 1973).
A second gradient in the Bay system is that of
depth. Community structure changes from emer-
gent wetlands to shallows to deeper waters.
The upper Bay derives most of its carbon
from allochthonous sources; particulate organic
carbon is transported into the system by the Sus-
quehanna River. Only about 10 percent of new car-
bon is derived from primary production in situ; in
contrast, most new carbon in the middle Bay is
fixed by phytoplankton and relatively little is im-
ported from upstream (Biggs and Flemer 1972).
Total annual carbon inputs to the whole Bay from
river transport, marshes, seagrasses, and phyto-
plankton production are estimated roughly in
appendix B. Phytoplankton carbon production
appears to be the most important, followed by that
of marshes and seagrasses, and then by river trans-
port. Inputs from land runoff have not been esti-
mated.
Primary producers in emergent wetlands are
marsh plants, epiphytes, and benthic algae. Most of
this carbon enters the food web by the detrital
pathway. Shallow regions receive carbon from
three sources: transport of detrital material from
marshes and rivers; production by seagrasses; and
production by phytoplankton. In waters too deep
for seagrass growth, phytoplankton production in
situ and transportation from upstream are the
carbon sources.
Much of the biological activity in the Bay oc-
curs in the shoal or shallow waters that are most di-
rectly influenced by runoff from the land. Sedi-
ment-trap areas may remove sediments, nutrients,
and toxic materials from the water column in shal-
low waters, preventing much of that material from
reaching deeper waters.
PLANKTON
Figure 2 was originally planned to distinguish
shallow and deeper water communities in the Bay.
However, for plankton and some nekton, the dis-
tinction is not clear. Plankton species have not
been found to vary from shallow to deeper water
(Heinle, pers. comm.), although they do vary with
salinity (Lippson 1973). The following discussion
of the plankton community applies to all Bay
waters (fig. 4).
In Chesapeake Bay, phytoplankton standing
stock is apparently limited by availability of P in
the spring and inorganic N in the summer (Taft and
Taylor 1976). In the winter, biomass is limited by
light or temperature (Taft, pers. comm.). Sediment-
water interactions in oxygenated and anoxic waters,
as well as regeneration by organisms, determine
abundance and chemical form of available P and N
in the system.
Primary production rates may be determined
by nutrient regeneration rates. While total nutrients
place an upper limit on standing stock during the
summer, turnover rates may be as rapid as every 2
days (Heinle, pers. comm.). It is possible that sum-
mer primary production is limited only by the
physiological capabilities of plant cells (Taft, pers.
comm.). Nannoplankton (plant cells less than 10
microns in diameter) contribute at least two-thirds
of total primary production on an annual basis
(Van Valkenburg and Flemer 1974).
6
-------
LIGHT
FISHES
CTENOPHORES
& JELLYFISH
NET ZOOPLANKTON
COPEPODS
NETPHYTOPLANKTON
DIATOMS
OYSTERS
& CLAMS
MICROZOOPLANKTON
ROTIFERS
TINTINNIDS
NANNO PHYTOPLANKTON
OINOFLAGELLATES
BENTHIC
FILTER
FEEDERS
DISSOLVED
INORGANIC
N, P, & C
BACTERIA
FUNGI
PROTOZOA
' DISSOLVED
ORGANIC
*\ N, P, & C
\
BENTHIC
DECOMPOSERS
MICROFAUNA
OYSTERS
& CLAMS
Figure 4. Conceptual model of plankton and nutrient interactions.
Zooplankton also can be considered in two size
classes, the larger or net zooplankton such as cope-
pods, and smaller or microzooplankton such as tin-
tinnids and rotifers. In the upper Patuxent River,
zooplankton must consume detritus because plant
production in situ is not sufficient to support them
(Heinle and Flemer 1975);in the middle Bay, larger
zooplankton eat only about 10 percent of daily net
primary production (Heinle, pers. comm.). What
happens to the rest of the production? Figure 4
shows one possible microzooplankton-dissolved in-
organic N, P, C-nannoplankton loop whose mecha-
nism for consumption and rapid regeneration of nu-
trients in the euphotic zone would facilitate rapid
turnover of the plankton community, but its exis-
tence has not been demonstrated (Heinle, pers
comm.).
Zooplankton tend to maximize living material in
the diet, and can eat bacteria (Heinle, pers. comm.).
Bacteria and protozoa can take up inorganic (as
well as organic) N and P, and may be competing
with phytoplankton for nutrients (Webb, pers.
comm.). Bacteria and fungi can break down cellu-
lose and chitin, the main components of detritus
(Webb, pers. comm.).
Plankton must be viewed in the context of or-
ganic and inorganic nutrient dynamics. Nutrients are
affected not only by plankton, but also by excretion
by larger organisms, absorption and regeneration
through chemical processes in sediments, regenera-
tion by biological processes in the euphotic zone
and benthos, and by physical transport. There are
still many questions about nutrient dynamics in the
Bay system, and figure 4 should be regarded as ex-
pressing present hypotheses. Plankton are usually
viewed as the starting point of a food web, but
they are one step in a nutrient-cycling loop that in-
volves the whole Bay system.
One other aspect of plankton and nutrient dy-
namics involves tidal exchange of water between
the Atlantic Ocean and the Chesapeake Bay at the
Bay mouth. Studies of the tidal exchange are under
-------
way at Virginia Institute of Marine Science. The
relative magnitudes of inputs and losses are not yet
known, but some nutrient losses are probable.
The zooplankton compartments of the concep-
tual Bay model also contain icthyoplankton and
larvae of benthic organisms. Any animal feeding on
zooplankton can also consume larval fishes and
benthos, thus playing a role in regulation of those
populations. All larval fishes are zooplankton
feeders, and may be a very significant factor in the
trophic dynamics of the Bay system.
BENTHOS
Benthic organisms are important in the flow
dynamics of C, N, and O in the Bay (D. Boesch,
pers. comm.).
Eelgrass (Zostera) communities cover much of
the shallow bottom from mean low water to about
2 m depth in the upper mesohaline and polyhaline
areas of the Bay (Orth 1975). Other seagrass spe-
cies, generally more abundant on the eastern shore
where there are wide shallow areas, include
Potamogeton and Vallisneria in fresh and brackish
water, and Ruppia in middle and higher salinities
(Lippson 1973).
Seagrasses (fig. 5) provide structure and habitat
for epiphytic plants and a diverse epifauna of
amphipods, isopods, barnacles, tunicates, poly-
chaetes, and gastropods (Marsh 1973, 1976).
Macrofauna consume about 55 percent of the net
production of eelgrass, phytoplankton, and benthic
algae of a Zostera community, with the rest avail-
able to bacteria, microfauna, and meiofauna
(Thayer et al. 1975). There is little grazing pressure
on the leaf blades (Zieman 1975, Marsh 1970),
which enter the food chain as detritus. Benthic in-
fauna densities are higher in Zostera communities
than any other benthic habitat in Chesapeake Bay
since the grass stabilizes the sediments (Orth 1973).
Seagrass communities also provide protection for
larval and juvenile fishes and blue crabs in soft and
peeler stages, as well as food for fishes, crabs,
shrimps, and water birds. Fishes associated with
eelgrass [e.g., anchovies (Anchoa spp.) in summer,
spot (Leiostomus zanthurus) and sihersides
(Antherinidae) in winter] feed on detritus, plank-
tonic copepods, and epifaunal crustaceans, deriving
about half of their nutrition from the seagrass com-
munity (Adams 1976a, b, c).
The cownose ray (Rhinoptera bonasus) tends
to uproot seagrasses as it grubs for benthic or-
ganisms (Orth 1975).
Oyster reefs constitute another type of benthic
community (Larsen 1974). Oysters feed on dino-
flagellates and detritus, and possibly bacteria and
lipids; clams feed on coarser particles (D. Haven,
pers. comm.). Oysters, clams, and other filter
feeders remove sediments and detritus from the
water column much faster than possible through
sinking alone. Fecal pellets make the material avail-
able to benthic grazers. Biodeposits on the sedi-
ment surface are enriched by bacteria, and turned
over by sediment mixers, such as shrimp and
worms (D. Haven, pers. comm.).
Benthic organisms also are present in sandy and
muddy bottoms without seagrasses or oyster-reef
structure (see fig. 6). Standing stocks are lower,
but turnover may be very rapid. Exclosure studies
indicate that blue crabs and some demersal fishes
are voracious predators, and may control benthic
standing stocks on unprotected bottoms (Boesch et
al. 1976, Virnstein 1976). Benthic populations
may also be partly controlled by predation on
planktonic reproductive stages during the summer.
A bimodal spring and fall setting pattern is com-
mon, with setting reduced during the summer
when predation by ctenophores and fishes on zoo-
plankton is highest (D. Boesch, pers. comm.).
The distinction between shallow and deep parts
of the bay is more obvious for benthos. Benthic
communities are characterized by surface and sus-
pension feeders in shallower waters, and deposit
feeders in deeper waters. Predation on infauna may
be the most important controlling factor for shal-
low populations, and competitive interference is a
controlling factor in deeper waters. Species also tend
to be associated with sediment type; shallow
sediments are usually sand, and deeper sediments
are usually mud. In the deep channels experiencing
anoxic conditions due to stratification of the water
column, benthos are depauperate (D. Boesch, pers.
comm.). Presumably, low oxygen conditions also
exclude fish and other mobile organisms from
some of the deeper waters of the Bay during the
summer (Haefner 1971).
Biological and chemical processes in the sedi-
ments are thought to be important in regulating
the abundance and chemical form of N and P in
the water column. More research is necessary to
elucidate the mechanisms as well as the magnitude
of these processes (K. Webb, pers. comm.).
-------
LIGHT
FISHES
FISHES
WATER COLUMN
DETRITUS
BENTHIC
EPIFAUNA
&INFAUNA
LIGHT
FISHES
COWNOSE RAYS
CRABS
SHRIMPS
Figure 5. Conceptual model of seagrass communities.
LIGHT
INORGANIC
N, P, ft C
i
DISSOLVED
ORGANIC
N, P, & C
COLUMN COLUMN
SEAGR ASSES
BENTHIC ALGAE
BACTERIA
OYSTERS
& CLAMS
UINUFLAGELLATES
/
BENTHIC
DETRITUS
i 1
IVNOSE MAN
YS
i
r
MEIO-&
MICROFAUNA
^-^^
^^**
II
N
i
COLUMN
DETRITUS
& BACTERIA
t \ >
CRUSTACEANS
POLYCHAETES
TUBE WORMS
SHRIMPS
I 1
(ORGANIC FISHES
. P, &C
1
FISHES
, i
.UE CRABS
i I
FISHES MAN
i
DISSOLVED
ORGANIC
N, P, & C
INORGANIC
N, P. & C
Figure 6. Conceptual model of other benthic communities.
-------
NEKTON
Ctenophores and jellyfish are a nuisance to
swimmers in the Bay, and can be extremely abun-
dant in certain areas in the summer time. Their
abundance is controlled by salinity, temperature,
and possible unidentified factors, as well as the
breaking up of their tissues by high wave activity in
the fall and winter. They are a major predator of
zooplankton, but arc apparently a dead end in
terms of trophic dynamics; they are not known to
provide a major food source for any other group.
The fish community of the Bay can be consid-
ered to be dominated by the Atlantic menhaden
which as an adult consumes phytoplankton, and
during the larvae stages, consumes zooplankton. In
fresh waters, the menhaden are replaced by other
clupeid fishes. In the diagram of major Bay proces-
ses (fig. 1), fishes are considered in two groups, At-
lantic menhaden and all others. This indicates the
importance of menhaden in grazing on the phyto-
plankton and in providing food resources for other
fishes. Menhaden also support a large commercial
fishery and are the main resource removed from the
Bay in terms of fishery yield. Removal of menhaden
is also removal of fish food; menhaden feed other
populations of commercial and recreational fishes in
the Bay.
The dominance of fishes by menhaden is pre-
sented as a simplifying perspective on the fish com-
munity. Some Bay scientists disagree with this view.
There are approximately 200 fish species in
Chesapeake Bay. Figure 7 shows a generalized troph-
ic dynamics model of this diverse fauna (Hildebrand
and Schroeder 1928, Bigelow and Schroeder 1953,
Reintjes 1969,Markleand Grant 1970). Adult men-
haden and anchovies, and all fish larvae are primarily
plankton feeders. At various times of the year, how-
ever, the larvae of most of the fish species in the Bay
will be included among the plankton. Some fishes
such as bluefish (Pomatomus saltatrix), weakfishes
(Cynoscion spp.), and striped bass are piscivorous.
Bluefish are particularly voracious predators, feed-
ing on the juveniles and adults of all other fishes
found in the Bay. There is a middle group of omni-
vores, such as eels and Atlantic croaker (Micropo-
gon undulatus), whose diets may include the plank-
tonic crustaceans (copepods, amphipods, and my-
sids); benthic crustaceans such as crabs or shrimps;
bivalves; small forage fishes such as killifishes (Cy-
prinodontidae); mummichog (Fundulus heterocli-
tus); silversides, anchovies, and menhaden; and
benthic fauna from marshes. They may also feed
on the epifauna and epiphytes of seagrass com-
munities.
A diagram of food-web dynamics for Bay fishes
is very difficult to construct. To attempt accuracy
only produces a diagram which is too complex to
be useful, with arrows from every box to every
other box, or with dozens of boxes. In devel-
oping figure 7, fishes were grouped by major food
habits. A different set of simplifying assumptions
would produce a different diagram of fish trophic
dynamics. Common names in the compartments in-
dicate adults or late juveniles. Larval fishes are all
plankton eaters. Production of larvae contributes
to Bay zooplankton, which includes icthyoplank-
ton. Larvae in the Bay system are indicated by
broken arrows; spawning may occur in the Bay or
outside the Bay mouth, with larvae then coming
into the Bay.
As a broad overview, Chesapeake Bay supports
resident and migratory fishes, with planktivorous,
omnivorous, and piscivorous feeding habits. Men-
haden, striped bass, and anchovies can be found in
the Bay year round even though they may not nec-
essarily spend their entire life cycle there. Men-
haden are most abundant in the spring and sum-
mer. Other small forage fish such as killifishes,
mummichog, silversides, hogchoker (Trinectes ma-
culatus] and gobies (Gobiidae) are found in the
shallow waters, among seagrasses, or feeding out of
the marshes throughout the year. In the spring (ap-
proximately March), adult fishes migrate into the
Bay from the Atlantic. The anadromous alosines,
or shad and river herring, migrate to fresh water to
spawn, and feed as they return to sea. White perch
(Morone americana) and striped bass, which are
resident in the Bay, migrate into fresher water to
spawn. This wave is followed by a migration of the
sciaenids, or croaker, drum, weakfish, and spot,
entering the Bay to feed after spawning in near-
shore shelf waters of the Atlantic. Then their larvae
also enter the Bay. Bluefish spawn in the ocean and
enter the Bay only for feeding. Bluefish are a major
fish predator as well as an important recreational
fish. They can be found up to the fresh water limits
in the Bay system, although they are more abun-
dant in waters of higher salinity. In the fall the mi-
gration process is reversed as many of the fishes
10
-------
NET
PHYTOPLANKTON
NET
ZOOPLANKTON
i
MENHADEN
LARVAL FISHES
ANCHOVIES
SHAD
S ALEWIFE
MARSH
CONSUMERS
KILLIFISHES
SILVERSIDES
MUMMICHOG
GOBIES
HOGCHOKER
t
WHITE PERCH
EELS
WINTER FLOUNDER
CATFISHES
S- ATLANTIC CROAKER
S BLUEFISH
STRIPED BASS
& WEAKFISHES
S SPOT
S DRUM
BENTHIC
CRUSTACEANS
POLYCHAETES
BIVALVES
BLUE
CRABS
SEAGRASS
EPIFAUNA
S MARCH TO NOVEMBER IN SYSTEM
LARVAL FORMS
Figure 7. Trophic dynamics of fish.
leave the Bay for the ocean (Cronin and Mansueti
1971).
Dominant fish fauna of the Bay, in terms of
biomass present and food consumed or material
flow, are the Atlantic menhaden, the sciaenids,
and the bluefish (Merriner, pers. comm.).
Fishes feed on plankton and other fishes in the
water column, on bivalves, mollusks, crustaceans,
polychaetes and other benthic organisms, on the
epifauna and epiflora of seagrass communities, and
on consumers in the marsh. Thus all portions of
the Bay are important for the feeding ecology of
fishes. While spatial variations are not indicated in
figures 2 and 7, they may be very important in
terms of overall fish population dynamics because
of the need for suitable spawning areas. Different
areas of the Bay are crucial for different fishes, but
every part of the Bay serves as a spawning aiea for
some species.
The bottle-nosed dolphin (Tursiops truncatus),
which consumes fishes, is found in the Bay. Other
dolphins, porpoises, and the smaller toothed whales
are rarely recorded (Wass 1972).
Migratory waterbirds feed in open water. Diving
ducks such as scaup (Aythya spp.) or canvasback
(Aythya valisineria] feed primarily on mollusks;
scoters (Melanitta spp.), oldsquaw (Clangula hy-
cmalis), goldeneyes (Bucephala spp.), and ruddy
ducks (Oxyura jamaicensis] eat crustaceans. Loons
(Gavia spp.), mergansers, (Mergus spp.) and nesting
ospreys (Pandion haliaetus) feed on fish. Fin- and
shellfishes are consumed by gulls (Laridae) in the
winter and terns in the summer (R. Andrew, pers.
comm.).
DETAILED BAY MODEL
To integrate the wetlands, plankton and nu-
trients, seagrass community, other benthos, and fish
trophic dynamics submodels (fig. 3 through 7) into
one model more detailed than the system overview
in figure 2, the connectivity matrix of figure 8 was
constructed. This matrix model contains the same
information as a combined box and arrow diagram,
but in a different format. With a little practice, the
matrix format is easier to read than the diagrams.
Compartments in the connectivity matrix
11
-------
WETLANDS
WATER COLUMN
o
a;
z
GO
Z
S
z
o
z
S
O
tL
TO
1 PLANTS & EPIPHYTES
2 BENTHIC ALGAE
3 SMALL CONSUMERS
4 BIRDS
5 OTHER WILDLIFE
E DETRITUS
7 DECOMPOSERS
8 ORGANIC N, P, 8 C
9 INORGANIC N, P, & C
10 NET PHYTOPLANKTON
11 NANNOPLANKTON
2 NET ZOOPLANKTON
3 MICROZOOPLANKTON
4 CTENOPHORES & JELLYFISH
5 WATERBIRDS
6 MENHADEN, LARVAL FISH ETC
7 KILLIFISH, ETC
18 CROAKER. ETC
19 BLUEFISH. ETC
20 DETRITUS
21 BACTERIA. PROTOZOA
22 ORGANIC N, P, & C
23 INORGANIC N. P, & C
24 DISSOLVED OXYGEN
25 SEA GRASSES
26 EPIPHYTES
27 BENTHIC ALGAE
28 EPIFAUNA
29 INFAUNA
30 OYSTERS & CLAMS
31 BLUE CRABS
32 CRUSTACEANS. ETC
33 DETRITUS
34 MEIO- & MICROFAUNA
35 POLLUTANTS
36 RIVER DRAINAGE
37 LAND RUNOFF
38 ATMOSPHERE
39 ATLANTIC OCEAN
40 DEEP SEDIMENTS
41 MAN
WETLANDS
1 PLANTS 8 EPIPHYTES
2 BENTHIC ALGAE
3 SMALL CONSUMERS
a
a:
5 OTHER WILDLIFE
r
3
7 DECOMPOSERS
o
9 INORGANIC N, P, 8 C
WATER COLUMN
z
0
t-
z
4
O
Z
Z
o
z
a
z
z
2 NET ZOOPLANKTON
z
£
X
00
CC
o
1
z
5 WATERBIRDS
6 MENHADEN, LARVAL FISH, ETC
u
8 CROAKER, ETC
UJ
GO
0 DETRITUS
1 BACTERIA, PROTOZOA
2 ORGANIC N, P. & C
u
00
Z
cc
o
z
4 DISSOLVED OXYGEN
BENTHOS
X
a.
a.
o
X
Z
28 EPIFAUNA
29 INFAUNA
30 OYSTERS 8 CLAMS
i
cc
o
GO
32 CRUSTACEANS. ETC
33 DETRITUS
34 MEIO- & MICROFAUNA
35 POLLUTANTS
ENVIRONMENT
36 RIVER DRAINAGE
u.
O
Z
=3
or
o
8 ATMOSPHERE
z
u
O
u
Z
0 DEEP SEDIMENTS
1 MAN
Figure 8. Connectivity matrix for a detailed conceptual model of the Chesapeake Bay ecosystem.
model are plants and epiphytes, benthic algae, small
consumers, birds, other wildlife, detritus, decom-
posers, and organic and inorganic N, P, and C for
wetlands; net phytoplankton, nannoplankton, net
zooplankton, microzooplankton, ctenophores and
jellyfish, water birds, menhaden and fish larvae,
killifishes, etc., croaker, etc., bluefish, etc., detritus,
bacteria and protozoa, organic and inorganic N, P,
and C, and dissolved oxygen for the water column;
seagrasses, epiphytes, benthic algae, epifauna, ajid
infauna for seagrass communities; and oysters and
clams, blue crabs, crustaceans, etc., detritus, and
meio- and micro fauna for benthos; pollutants are
also part of the system. The environmental elements
are river drainage, land runoff, atmosphere, Atlantic
Ocean, deep sediments, and man.
Each compartment appears as a heading for
one column in the matrix. Cells of the matrix in-
dicate material flow between compartments. For
example, the element in row 12 (net zooplankton)
and column 10 (net phytoplankton) indicates flow
from net phytoplankton to net zooplankton, or
grazing on phytoplankton by zooplankton. Pro-
cesses covered by elements of the matrix include
grazing, predation, respiration and mineralization
of nutrients (from animals to inorganic N, P,
12
-------
and C), heterotrophy (from organic N, P, and C to
plants), migration (between fishes and Atlantic
Ocean), waterborne inputs of detritus, nutrients,
and pollutants, gas exchange between water and
the atmosphere, chemical exchanges between sedi-
ments and the water column, etc. Empty cells rep-
resent exchanges that do not or are not known to
occur.
The matrix format shows interactions within
and between the submodels for wetlands, water
column, and benthos. In a hierarchical study ap-
proach to the Bay, with research focused on sub-
systems, connections between the system under
study and other parts of the Bay ecosystem should
also be addressed.
For any compartment, elements in its row in-
dicate the inputs to it, and elements in its column
list its losses. The diagram does not show the rela-
tive magnitudes of such inputs and losses. Informa-
tion on magnitudes will be useful in assigning
priorities for research and environmental con-
cerns.
MODEL INTERPRETATION
WATER QUALITY AND ECOLOGICAL
CONCERNS
The Chesapeake Bay estuary is a very dynamic
environment in which organisms must continually
cope with changing conditions. Many researchers
are concerned that human activities such as increas-
ing sediment loads from land development; nutrient
loads from sewage disposal; herbicides and pesti-
cides from agriculture; other toxic chemicals from
industrial effluents; refined petroleum hydrocar-
bons from pleasure boating, commercial shipping,
or runoff from storm sewers (oil changes, etc.) may
be altering the Bay environment too rapidly. If the
limits of organisms to adapt or adjust to environ-
mental change are exceeded, then the ecological
structure of the system could change, with the loss
of desirable species and introduction or increase of
undesirable ones. The Bay system is self-cleaning
to some extent, but if its capacity to recover is ex-
ceeded, water quality will continue to deteriorate.
The shallow waters of the Bay system are most
likely to be affected by pollutants from river flow,
land runoff, or sewage and industrial effluents. The
sediment-trap function of some shallow waters also
serves to keep pollutants in the shallower areas.
But shallows are regions of the greatest biological
activity and concentration of biomass, and so are
most affected by pollutants. Pollutants are here de-
fined as materials introduced into the system, as a
result of human activities, that are excessive or
harmful to the system.
Toxic substances that have entered the Bay
system can be found in the water column, the
sediments, and the biota. Material-flow pathways
in the Bay model indicate potential routes for bio-
accumulation of toxic materials and concentration
up the food chain. Physical transport processes af-
fect the distribution of toxic materials and their
availability to biota. Chemical processes in the
sediments and water column also influence the
availability of such materials.
An example of a system response to changing
environment is the recent decline of seagrass com-
munities. The present decrease in abundance may
be within the range of normal variation for the sys-
tem; one of the problems in evaluating observable
changes is the tremendous natural (i.e. undisturbed)
variability of the Chesapeake Bay system. It is dif-
ficult to attribute the changes in the seagrasses to
man-related causes when large population fluctua-
tions have been observed historically. Within the
model framework, however, are pathways that may
contribute to the decline. First, some researchers
argue that nutrient loading (from sewage inputs and
land runoff of fertilizers) has increased the phyto-
plankton standing crop, which has in turn increased
turbidity and reduced the light available for seagrass
growth. A second hypothesis is that herbicides
from land runoff may be responsible for killing sea-
grasses. Current research indicates that there are
toxic concentrations of herbicides in the Bay (D.
Corrcll, pers. comm.).
Another observed change is the great reduction
in oyster spatfall since Hurricane Agnes. The cause
is unknown. Model processes and pathways that af-
fect plankton or benthic community conditions are
possibilities.
Aspects of water quality affecting the ecology
of the Bay include water transparency, dissolved
oxygen concentration, chemical forms and concen-
trations of N and P, presence and concentrations
of trace metals and toxic chemicals, rates of bio-
logical activities such as plant growth or nutrient
regeneration, and abundance of desirable or unde-
sirable species. Each aspect is affected by biological,
13
-------
chemical, and physical processes within the Bay
system. In interpreting the model for the ecosys-
tem, remember that where directions of flow are in-
dicated by arrows, the rates along those pathways
depend on the environmental parameters of tem-
perature, light, nutrient or pollutant concentra-
tions, mixing, water and sediment chemistry, trans-
port, and salinity and oxygen distribution, as well
as on the abundance of the donor and receiver
compartments for each flow.
The fishery, estuarine habitats, waste treatment,
and recreational resources of the Bay system are
supported by its underlying ecology as indicated in
the conceptual models for the system.
Fishes appear as larvae and as adults in the con-
ceptual model. Production of fishes sufficient for a
commercial or recreational fishery requires suitable
habitat for spawning, survival of some larvae
through juvenile stages to adult and recruitment
size, and availability offish food. For species spawn-
ing inside the Bay, suitable unpolluted habitats are
required. The model indicates fish food require-
ments as well. Nutrient concentrations affect phy-
toplankton and other plant growth, which in turn
provides food for the zooplankton and epifaunal
and infaunal benthic communities supporting for-
age fishes, which are then fed upon by fish of com-
mercial importance. Many components of the food
web are necessary to sustain the fishery. Larval
fishes in the plankton are especially vulnerable to
pollution or other changes in water quality. Even
though the biomass represented by larvae is low,
they are essential to the continuation of the fishery.
Fish habitats are more difficult to pull from the
model, since each species uses a different part of the
Bay, and spatial relationships are not indicated.
The Bay system functions naturally in waste
treatment, as indicated by the nutrient uptake and
regeneration cycles, the role of decomposers in the
water column and benthos, and sediment chem-
istry. Loops need emphasis, since cycles for nutrient
import, regeneration, and export involve the whole
food web, as well as chemical and physical processes.
Toxic substances that interfere with organisms,
particularly plants and decomposers, interrupt the
nutrient cycle and hence the self-cleaning action of
the system. The conceptual models indicate :he nu-
trient cycles and the presence of pollutants as po-
tential rate modifiers. Nutrient inputs from river
drainage and land runoff are also indicated in the
model. Sewage disposal, if input rates are too great,
may overload the system with nutrient concentra-
tions higher than biological turnover rates can
handle.
The maintenance of habitat for fishes, birds, and
other wildlife is indicated indirectly by the concep-
tual models. Food supply and the ecological mech-
anisms for its continuance are indicated. Species di-
versity, and the abundance of desirable species for
different habitats are indicated only indirectly. With
sufficient pollution stress, the structure of the food
web might change, so that the one presented in the
models no longer applies.
To be suitable for recreational purposes, water
should be clear, have a pleasant smell, be free of
weeds or stinging jellyfish; in short, be aesthetically
pleasing. It is the ecology of the whole system that
produces these qualities; the entire system needs to
be healthy to maintain them. Plants, nutrients, and
decomposers affect water chemistry. Biological as
well as physical processes control the abundance of
weeds or jellyfish. Spawning habitat, acceptable
conditions for larval development, food availability,
and predation pressure (including commercial fish-
ing) influence recreational fisheries.
It is the healthy function of the whole Bay eco-
system, not just single parts of it, that allows the
Bay to provide abundant resources.
INDICATORS
The conceptual model provides a perspective
on the role of the following potential indicators of
water quality in the Chesapeake Bay ecosystem.
Seagrasses. Seagrasses, present in some of the
shallow areas of the Bay, occupy both sediment
and the water column; they can respond to condi-
tions in both. Abundance and distribution of sea-
grass communities may reflect herbicide concentra-
tions, water transparency, and other factors.
Chlorophyll a. Chlorophyll a concentration is
proportional to phytoplankton standing stock, but
not turnover rate. It can be measured throughout
the year in the upper few meters of the whole sys-
tem for comparison of conditions over space and
time. It is related to nutrient abundance and
possibly with herbicides or other toxic pollutants.
Dissolved Oxygen. Dissolved oxygen distributions
are affected by biological processes (increased by
plant production, decreased by animal respiration
and decomposition) and physical ones (exchange
with the atmosphere, mixing and vertical stratifica-
14
-------
tion of the water column). It can be measured at
all depths in the system, and provides another
parameter for comparison in space and time.
Transparency. Water transparency is easily mea-
sured by Sccchi disc. It indicates both sediment
loading and biological contributions to turbidity.
Blue crabs. Because blue crabs require different
habitats during various parts of their life cycle, from
the water at the Bay mouth to the tributaries, they
can integrate information for the whole Bay sys-
tem. Abundance may be affected by climate and
fishing pressure, as well as water quality, and there
may be wide natural variations in abundance. Con-
sideration of additional system information will be
necessary to interpret changes in abundance.
Larval forms. Larval forms are potentially good
indicators of pollution levels because larvae are
much more sensitive to pollutants than adults.
Forms that eventually settle on the bottom, such
as oyster spat, are the easiest to measure.
Pollutant concentrations in tissues. Concentra-
tions of pollutants in the tissues of commercial
fishes and shellfishes can provide indications of bio-
accumulation in the benthic and pelagic environ-
ments. Concentrations in forage fishes and plank-
ton, while a little more difficult to analyze, would
provide earlier indications of dangerous accumula-
tions that could eventually be passed to commercial
species.
RESEARCH NEEDS
One problem in assessing the impact of man's
activities on the Bay as a system is lack of adequate
information on how the Bay system operates, on
both short-term and long-term time scales. Short-
term information requirements involve such mat-
ters as feeding habits, spawning habits, relation-
ships of species to toxic substances, migration
patterns, fishing patterns, and so forth. The long-
term natural variation in population sizes is more
difficult to handle, but the information is impor-
tant. Since the Bay is a very dynamic system, it is
difficult to distinguish between the biological
responses to human activities and the undisturbed
or "normal" changes which are long-term cyclic or
successional phenomena due to the nature of the
Bay as an estuary. Spatial scales are also important.
Most studies are quite localized, and extrapolation
of their results over the whole Bay, a very large
system, presents serious problems in interpretation
and impact assessment.
Thus one important aspect of research in the
Bay is large-scale, long-term work. Parameters
which will provide effective monitoring of water
quality and ecosystem conditions need to be identi-
fied. Some possible indicators have been discussed.
There is also a need for ecosystem-level studies
in the Bay. While a great deal of information is
available on Bay ecology, it is still difficult to
answer definitively such questions as (1) the rela-
tive impact of marsh detritus, upland detritus, sea-
grass production, and phytoplankton production in
driving the system; (2) the relative importance of
physical processes such as climate, temperature, ice
regime, or sediment loading; (3) the importance of
biological processes such as reproduction and pre-
dation; or of human impact such as the effects of
toxic substances on commercially important spe-
cies in the Bay, or on populations which are impor-
tant in their food chains; or (4) the relative impor-
tance of water column and benthic processes in re-
generation of nutrients, which are crucial to the
overall productivity of the system. These questions
are pertinent to management decisions as to habi-
tat that must be protected, processes to be moni-
tored, and the important aspects of water quality
and their long-term economic effects. Answering
these questions may be an ambitious undertaking,
but coordination among investigators would con-
tribute toward providing answers.
In the conceptual model for the Chesapeake
Bay ecosystem, (figs. 1 through 8), about 40 key
system components are identified by the compart-
ments or boxes of the diagrams. Their interactions
(arrows) and interrelationships are indicated. Phy-
sical and chemical processes that affect the biology
of the system are identified. The relationships of
several system processes to water quality have been
discussed. This conceptualization of the system can
be debated, compartments redefined, and new in-
teractions included. As understanding of Bay ecol-
ogy increases, the diagrams will be modified. The
conceptualization reflects current hypotheses
about Bay ecology.
Even at this simple level of resolution, the rela-
tive magnitudes (or importance) of the various
flows on a Bay-wide, annual basis are not known.
Compartment sizes or biomass measured in carbon
units can be estimated easily for only a few of the
compartments. One approach to research about
15
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water quality of the Bay is to take this or a similar
general conceptualization of the system and esti-
mate magnitudes (at least relative magnitudes) of
the flows. It will not be easy. An example of the
kind of information and calculations required is
given in appendix B, a rough attempt to quantify
carbon fixed in the Bay system by marsh plants,
seagrasses, and phytoplankton, and to compare
that quantity to carbon imported by rivers. Rela-
tive flow information can be useful for manage-
ment. The largest inputs to primary production are
most important to protect. The largest point or
nonpoint sources of pollutants are the most impor-
tant to regulate. The nutrient regeneration path-
ways having the highest turnover rate should be
measured accurately so that the waste-treatment
capacity of the system can be calculated. The eco-
system context provides a perspective on the rela-
tive importance of various management problems,
if quantitative information can be obtained.
In a hierarchical approach to the Bay ecosys-
tem, such as currently taken, subsystems are
studied. Practicality dictates some subdivision of
the ecosystem for study purposes, and a communi-
ty approach (seagrasses, plankton, fishes, etc.) is
workable. It is important to include information
about exchanges between the particular communi-
ty under study and the rest of the system (seagrass
export of detrital carbon, consumption of zoo-
plankton by fishes, proportions of phytoplankton
production consumed by various grazers) as well as
interactions within the subsystem itself. Quantita-
tive information, with seasonal and spatial varia-
tions or year to year variations, is most useful.
Whenever quantitative estimates of any of the
model compartments or flows are made for any part
of the Bay system, an attempt should also be made
to extrapolate the estimates to the whole Bay for a
year. If such an extrapolation cannot be made, the
information necessary to complete it should be
identified. If an extrapolation is made, the underly-
ing assumptions on spatial and temporal variations
should be defined.
The attempt to quantify standing stocks and
annual total flows for the conceptualized Bay sys-
tem may eventually lead to development of a simu-
lation of Bay ecology. In the meantime, it provides
a meana for examining a variety of assumptions
about dynamics of subsystems in the context of
the whole ecosystem.
A list of specific research questions of concern
to scientists interviewed for this project is presented
in appendix C.
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Adams, S. M. 1976c. The ecology of eelgrass, Zostera ma-
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Axelrad, D. M. 1974. Nutrient flux through the salt marsh
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Green, K. A. 1975. A simulation of the pelagic ecosystem
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Haefner, P. A., Jr. 1971. Avoidance of anoxic conditions by
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Heinle, D. R., andD. A. Flemer. 1975. Carbon requirements
of a population of the estuarine copepod Eurytemora
affinis. Mar. Biol. 31:235-247.
Heinle, D. R., and D. A. Flemer. 1976. Flows of materials
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1976. Contributions of tidaly wetlands to estuarine
food chains. University of Maryland Water Resources
Research Center TR 29. 34 pp.
Hildebrand, S. F., and W. C. Schroeder. 1928. Fishes of the
Chesapeake Bay. Bull. U.S. Bur. Fish. 43. 388 pp.
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251-267.
Lane, P., and R. Levins. 1977. The dynamics of aquatic
systems. 2. The effects of nutrient enrichment on
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22:454-471.
Larsen, P. F. 1974. Quantitative studies of the macrofauna
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Levins, R. 1966. The strategy of model building in popula-
tion biology. Am. Scient. 54:421-431.
Lippson, A. I. 1973. The Chesapeake Bay in Maryland: An
atlas of natural resources. Johns Hopkins University
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Mann, K. H. 1973. Seaweeds: their productivity and
strategy for growth. Science 182:975-981.
Markle, D. F., and G. C. Grant. 1970. The summer food
habits of young-of-the-year striped bass in three
Virginia rivers. Chesapeake Sci. 11:50-54.
Marsh, G. A. 1970. A seasonal study of Zostera epibiota in
the York River, Virginia. Ph.D. Dissertation. William
and Mary University. 156 pp.
Marsh, G. A. 1973. The Zostera epifaunal community in
the York River, Virginia. Chesapeake Sci. 14:87-97.
Marsh, G. A. 1976. Ecology of the gastropod epifauna of
eelgrass in a Virginia estuary. Chesapeake Sci. 17:182-
187.
Orth, R. J. 1973. Benthic infauna of eelgrass, Zostera
marina, beds. Chesapeake Sci. 14:258-269.
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by the cownose ray, Rhinoptera bonasus, in the Chesa-
peake Bay. Chesapeake Sci. 16:205-208.
Pritchard, D. W. 1968. Chemistry and physical ocean-
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Governor's Conference on Chesapeake Bay. State of
Maryland, Annapolis.
Reintjes.J. W. 1969. Synopsis of biological data on the
Atlantic menhaden, Brevoortia tyrannus. FAO Species
Synopsis 42. USDI Circular 230.
Silberhorn, G. M., G. M. Dawes, and T. A. Barnard, Jr.
1974. Coastal wetlands of Virginia. Interim report 3.
Virginia Institute of Marine Science special report in
applied marine science and ocean engineering 46.
Taft.J. L., and W.R.Taylor. 1976. Phosphorus distribu-
tion in the Chesapeake Bay. Chesapeake Sci. 17:67-73.
Teal.J. J. 1962. Energy flow in the salt marsh ecosystem of
Georgia. Ecology 43:614-624.
Thayer, G. W., S.M.Adams, and M. W. LaCroix. 1975.
Structural and functional aspects of a recently estab-
lished Zostera marina community. Pages 518-540
in L. E. Cronin, ed. Estuarine research. Vol. I. Aca-
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Van Valkenburg, S. D., and D. A. Flemer. 1974. The distri-
bution and productivity of nannoplankton in a tem-
perate estuarine area. Estuarine Coastal Mar. Sci.
2:311-322.
Virnstein, R. W. 1976. The effects of predation by epiben-
thic crabs and fishes on benthic infauna in Chesapeake
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87pp.
Walsh, J. J. 1975. Utility of systems models: a considera-
tion of some possible feedback loops of the Peruvian
upwelling ecosystem. Pages 617-632 in L. E. Cronin,
ed. Estuarine research. Vol. I. Academic Press, New
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Wass, M. L. and T.D.Wright. 1969. Coastal wetlands of
Virginia. Interim report. Virginia Institute of Marine
Science special report in applied marine science and
ocean engineering 10.
Wass, M. L. 1972. A checklist of the biota of lower Chesa-
peake Bay. Virginia Institute of Marine Science special
scientific report 65. 290 pp.
Zieman,J. C. 1975. Quantitative and dynamic aspects of
the ecology of turtle grass, Thalassia testudinum. Pages
541-562 in L. E. Cronin, ed. Estuarine research. Vol. I.
Academic Press, New York.
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APPENDIX A
QUESTIONS ASKED OF BAY SCIENTISTS
1. What is the relative importance of macrophytes and phytoplankton in total primary production?
2. What proportion of primary production goes through the herbivore food chain, and what pro-
portion through detritus?
3. What is the role of detrital feeding in the ecosystem?
4. Why is the estuary productive?
5. What are critical pollution problems, and impacts?
6. What are the main stresses on living resources (harvest, habitat, etc.)?
7. What research is needed on the Bay system?
8. What is the role of jellyfish and ctenophores in the system?
9. How do seasonal variations in salinity, temperature, or other physical parameters affect the biology of
the system?
10. How do shallow areas compare with the deeper open bay for primary and secondary productivity, bio-
logical activity, resource availability, pollution stress?
11. What are the most important inputs and outputs for each compartment? What are turnover rates?
12. What are the relative importance of physical and biological controls on population sizes and energy and
carbon flow rates?
13. What controls biological population sizes?
14. Identify critical habitat types. What are the requirements for their maintenance?
15. What are inputs to and outputs from the system by migratory populations?
16. How does Bay ecology vary spatially? (e.g., shallows vs. deeper waters, upper vs. lower Bay.)
17. What are food resources, competitors, predators, and parasites for commercially harvested species?
18. What are the greatest threats to commercial anSi sport fisheries, habitats, and aesthetic values of the
Bay?
19. What are the greatest threats to water quality?
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20. What are potential indicators of water quality, ecosystem health?
21. What has been the impact of power plants on tributaries?
22. What is the impact of pesticides and herbicides on the Bay?
23. What is the impact of commercial shipping, other sources of petroleum hydrocarbons?
24. Arc relevant studies on particular questions available from other estuaries?
25. What processes are most important to nutrient regeneration?
26. Are C, N, and P flows all in the same direction?
27. What is the significance of migratory and resident birds on biological populations, nutrient and carbon
flow?
28. What historical changes can be perceived in the Bay? Last 10 years? Last 300 years?
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APPENDIX B
PRELIMINARY PRIMARY PRODUCTIVITY CALCULATIONS
Carbon input calculations here are intended as an example of information required to quantify the
conceptual Bay model. Results are not considered to be realistic.
Annual marsh carbon production
300,OOOA(4000m2/A) (337.5g/m2/yr)=4X10ng/yr=400,000 ton/yr
Marsh area and productivity from discussion with R. Wetzel.
Seagrass annual production
21,OOOA(4,OOOm2/A) (l,750g/m2/yr)=1.47X1011g/yr=147,000 ton/yr
Productivity estimate from Mann (1973) for Nova Scotia. Acreage estimate, roughly 7,000 acres for
the western shore at present (R. Orth, reluctant estimate), and assuming 14,000 acres for eastern
shore.
Phytoplankton annual production
Estimated in situ carbon production for the Maryland portion of the Bay, 282,000 ton/yr (Biggs
and Flemer 1972). Assuming the same rate and similar area for the Virginia portion, total phyto-
plankton carbon production is roughly 600,000 ton/yr.
River input of carbon
POC (participate organic carbon) input from the Susquehanna River, which is 85 percent of river
flow into the Bay, is 84,000 ton/yr (Biggs and Flemer 1972). Assuming the same input rate for other
rivers, total river carbon input is 100,000 ton/yr.
Summary
Carbon sources for the Bay system, and annual totals over the whole Bay:
Phytoplankton 600,000 ton/yr.
Marshes 400,000 ton/yr.
Seagrasses 147,000 ton/yr.
River input 100,000 ton/yr.
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APPENDIX C
RESEARCH PROBLEMS IDENTIFIED BY BAY SCIENTISTS
1. Relate commercial harvest of menhaden to that of other species.
2. What is the sustainable yield for Bay fisheries, and where should it be cropped?
3. More information is needed about fish food species, Atlantic menhaden, anchovies, penaeid
shrimp, mysids.
4. What eats hogchokers?
5. What are the food web pathways for phytoplankton production if zooplankton only consume
10% of daily net production?
6. What is the Bay nutrient budget, including benthic regeneration?
7. How are changes in nutrient inputs and primary productivity transferred up the food chain?
8. What are food chain consequence of species changes in phytoplankton?
9. What will happen to the ecosystem if nutrient loading is (is not) cleaned up?
10. What is the N vs. P limitation for primary productionseasonal and spatial variations?
11. What are the effects of land-use policies on the Bay?
12. What are the nutrient uptake kinetics in the estuary, or how can productivity be kept down?
13. What is the abundance and distribution of benthic organisms and plankton?
14. Better fisheries monitoring data is needed, both of commercial and recreational catch.
15. What are the synergistic effects of pollutants?
16. Large-scale regional studies are needed rather than very localized studies in the Bay.
17. What is the impact of agricultural pesticides and herbicides?
18. Generally more quantitative work is needed.
19. What are the impacts of PCBs, toxins?
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20, How do benthos serve as indicators of Bay health?
21. What is natural variability in Bay populations vs. impacts of human activities?
22. More studies are needed about the Bay edges, where biological activity and control are.
23. What is the role of mudflats in Bay ecology?
24. Mass balance studies of marsh/estuary exchanges arc needed.
25. Develop a conceptual model to focus on systems level information.
26. What is the value of benthos as support for other resources, and as a pollution reservoir?
27. Emphasize control of populations and processes.
28. What are the relative values of different bottom types?
29. Information is needed on utilization of shallows, where it isn't deep enough for trawl samples.
30. What eats bacteria?
31. Studies of anaerobic processes and nutrient regeneration are needed.
32. What are the chemical and biological interactions between water column and benthos and
sediments?
33. Recent and widely distributed chemical data is needed.
34. More coordination among institutes and research programs is needed.
35. Research the basic biology of eelgrassphysical, chemical environment, recruitment, cul-
turing.
36. What is the importance of seagrasses to the Bay systemprotection and food for shrimp,
crabs, fish, etc.
37. Bioassay the effects of pollutants and herbicides.
38. What has caused the decline in setting of oysters and other larvae?
39. What are the environmental cues for fish migration?
40. Quantitative information for food webs is needed.
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