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. 

 Correll, pers. comra.). 



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, 



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