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 Jie 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- 



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