method has been modified (Dettmann 2002) to model the fate and concentrations of total 
nitrogen in estuaries. Preliminary work in extending model applicability also has shown that 
total N concentrations predicted by the model appear to correlate well with peak annual 
phytoplankton concentrations and peak macroalgal abundance in estuaries. The overall goal of 
this work has been to explain the response of estuaries to nitrogen loading using as few 
parameters as possible. The model appears to reasonably describe annual net N, the dependence 
of annual denitrification on water residence time, and the annual average concentrations of total 
N in estuaries where it has been tested. The results emphasize the importance of water residence 
time in determining export, denitrification, and concentrations, and give quantitative expressions 
for these dependencies. At present, the model provides annualized results averaged over the 
entire estuary, although recent results indicate that it may also have application to seasonal 
response as well. The final extended model is expected to have direct applications to the 
evaluation of estuary sensitivity to nutrient loading, which will be useful in setting nutrient 
criteria. The model may also serve as part of the foundation of a classification system for 
estuarine sensitivity to nutrient loading. 
Food Web Models 
Models of food webs link the food web components to the overall ecosystem through an explicit 
quantification of exchanges. This makes it possible to evaluate how changes in the model 
components directly effect ecosystem processes. For example, the cycling of carbon and 
nutrients directly result from food web interactions in which many species play a role. Within 
many ecosystems, species that contribute little biomass still may have a laige influence on 
nutrient cycling and energy flow, and thus affect the functioning of other species. Examples of 
this include bacterial grazers, which can stimulate microbial activity through nutrient recycling, 
and algal grazers which stimulate the productivity of submerged macrophytes by providing better 
light conditions through the grazing of periphytic algae. Extinction or changes in abundance of 
such species can have a disproportionately large influence on ecosystem function. Hence, food 
web approaches can be used for analyzing the effects of nutrient stressors on key target species, 
on the biological diversity in communities, and on the functioning of ecosystems. In this way, 
food webs are the wiring on the circuit board of the ecosystem, spanning different levels of 
ecological organization. Food web models allow managers to identify the critical food web 
flows within the estuarine ecosystem, for which small changes in an ecosystem component will 
cascade through the system and result in large changes in eutrophication, extinction of important 
habitats, or changes in the tropic structure of the overall ecosystem (Vezina and Pace 1994). 
Food web models now being developed and evaluated in ORD are used to calculate metrics that 
define the state of an ecosystem (Ulanowicz 1986, Hagy 2002). These metrics are similar to 
diversity and commonness, but are much more sensitive to ecosystem condition than the older 
metrics. These new metrics can be calculated directly from the food web models now being 
developed at ORD. In the following two examples, we describe how these indices can be used 
to develop useful relationships between nutrient loading and trophodynamics: 
1. Indices of ecosystem trophic efficiency can be used to quantify how carbon and nutrients 
supplied to the estuary are passed thought the food web to the higher tropic levels. If more 
nutrients and carbon are moved into higher tropic levels, then the nutrient capacity of the estuary 
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