complexity derived from succcssional 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 eater 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 C0 2 loss through res- 

 piration. ThegassesC0 2 , 2 , and N 2 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. 



