framework explicitly establishes the interactions between SAV ecosystems and 

 human economic systems. Direct and indirect effects of alternative management 

 scenarios were assessed in terms of economic values and ecological processes . 

 Finally, this framework, provided a heuristic format for understanding some 

 principles of resource management. 



This scheme is illustrated in figure 7 as a cluster of interconnected models 

 representing the influence of human activities, as modified by physical forces 

 (e.g., rain, sunlight, tides), on SAV ecosystem dynamics which in turn affect 

 resources valued by society. Briefly, meteorological conditions coupled with 

 agricultural practices are shown as inputs to the Watershed Runoff Model (Holton 

 and Yaramanglou 1979) which links the Universal Soil Loss Equation to hydrologic 

 and chemical process models, thereby routing water, nutrients, sediments and 

 herbicides from fields to estuary. A simplified 2-layer "box model" of 

 Estuarine Circulation, based largely on continuity at steady-state (Officer 

 1980), receives agricultural runoff and sewage nutrient wastes and transports 

 water and materials through the estuary providing an ambient water quality field 

 to which SAV are exposed. These materials, along with direct agricultural run- 

 off, provide inputs to the SAV Ecosystem Management Model (SEMM), the details 

 of which will be discussed in the next section. Outputs from SEMM, including 

 fisheries and recreational activities are input functions to the Resource 

 Economics Model which estimates equivalent economic values associated with 

 these features (Kahn 1981, Boynton et al., 1981). 



Depicted on the left side of figure 7, the marginal costs and benefits 

 associated with various economic activities (Land Development, Agricultural Pro- 

 duction, and Sewage Treatment) are calculated (Boynton et al., 1981). Also asso- 

 ciated with these economic processes are direct or indirect effects on waste 

 loading to the estuary. Resource values are combined with costs and benefits 

 of watershed activities to establish viable Resource and Waste Control Options 

 which managers and citizens consider towards developing resource policies. In 

 general, connections between submodels are undirectional , with feedback occurring 

 only indirectly through the management decision process. For example, materials 

 enter the estuary from the watershed, while the estuary, per se, has little direct 

 influence on watershed activities. In this scheme the modeler serves as the 

 interface between connected submodels, and piece-wise simulations can be per- 

 formed with no loss of information since there are no direct feedbacks. In 

 other words, the output information from simulations in one submodel are used 

 by the modeler to define input conditions for the next sub-model in the sequence. 



SAV Ecosystem Management Model 



At the focal point of this resource management framework is the SAV Eco- 

 system Management Model (SEMM). The SEMM was designed to emphasize interactions 

 between SAV ecosystems and human systems (fig. 8), specifically water quality 

 effects on SAV production and abundance, and the habitat and food-chain factors 

 whereby SAV enhances fish production. The structure of SEMM aggregates much of 

 the complexity which had been emphasized in the SAV ecosystem submodels (e.g., 

 Autotroph and Nekton Models in fig. 3 and 5). Our intention here was to 

 preserve sufficient detail in ecological function so that relevant interactions 

 with socio-economic activities could also be included without losing conceptual 

 and computational control. Sensitivity analyses performed for the ecosystem 

 submodels provided some guidance on strategies of aggregation, wherein crucial 



147 



