V = V 



N +k*N, 

 r a b -I 



L K +(N +k*N u ) J ' (5) 



s a b 



where N a and N^ are aqueous and benthic nitrogen concentrations of dissolved 

 inorganic nitrogen (mostly NHt) , and K is the half-saturation constant for uptake 

 of N fl and (K g /k ) is the half-saturation for N^. Again, these coefficients were 

 calculated from our own experimental data, primarily for the SAV species, 

 Potamogeton perfoliatus (Kemp et al., 1981). Similar expressions were used to 

 describe light, nutrient and temperature interactions in primary production of 

 other autotrophic groups. 



The basic behavior of this model is illustrated in the calibration output 

 (fig. 4). The close correspondence between model and field data is also 

 apparent here. For clarity the variances associated with these data are not 

 given. However, the model trace is generally well within the 95 percent confi- 

 dence interval for field observations. Subsequently, the veracity of this 

 model was compared to a second independent data set, and again good agreement 

 was obtained between model and measurements (Kemp et al., 1983b). The effects 

 of nutrient additions to this model system were also very similar to those 

 observed in large experimental ponds, and the model was used to extrapolate 

 results from these systems to actual estuarine conditions (Kemp et al., 1983 

 a,b). It is interesting to note the slight asynchrony of peak summer abundance 

 for these 4 components, indicating some temporal separation of niches to minimize 

 competition toward system homeostasis (Lewis 1980). 



The Nekton Subsystem Model 



The hypothesis to be investigated with the Nekton model was that changes 

 in SAV abundance would influence total fish abundance and would shift the balance 

 among various trophic and habitat fish groups. This model is important in 

 the overall simulation framework because nekton provide a crucial feedback control 

 for the other ecosystem submodels (fig. 2) and because its output provides a 

 principal linkage to management concerns. 



The general organization of the Nekton submodel is described in figure 5, 

 where categories of fish (including total biomass, and adult or juvenile 

 numerical abundance for each) compete for various food items, an important one 

 of which, benthic infauna, is explicitly included in this model. Other food 

 sources are external to the model, and most are variables in other subsystems. 

 The nekton system here is defined by 9 state variables in 4 categories. There 

 are 3 variables within 2 fish groups, and 2 within the third, "Resident Fish". 

 There is some direct predator-prey interaction among the 3 fish groups; however, 

 competition for limited foods also represents an indirect mechanism of inter- 

 action. Model fish groups are connected to external fish populations, with 

 immigration and emigration controlled by temperature cues and density dependent 

 factors. 



The 3 categories of fish are functional classifications defined on the basis 

 of similar habitat, trophic relations and life histories. The ecological units 

 were developed as a compromise allowing aggregation but retaining some of the 

 mechanistic relationships which characterize populations in nature (namely where 

 they live, what they eat, and how and when they reproduce). This condensation 



141 



