Modeling the effects of nutrient concentrations on ecosystem stability:

The effects of nutrients on algal productivity are not transferred directly to production at higher trophic levels and can not be modelled as if they were. Rates of processes and time scales at different trophic levels require a modular approach to a comprehensive model of the Great Lakes. Predator growth is based ultimately on prey availability and life histories of predators. Factors that control availability of preferred prey (e.g. top-down vs. bottom up) are unresolved. Prey availability is ultimately controlled by C-flow from the base of the food web, but many factors besides nutrient loading influence C-flow to forage fish. Here we focus on factors at the base of the food web that influence Cand P-flow and the efficiency of energy transfer from phytoplankton to forage fish. Specifically, we consider a detailed model of the base of the food web that includes phytoplankton production, grazing interactions and processes in the microbial food web (MFW): bacterial production and bacterivory. The model is constructed from authentic field data and fit into an exact steady state model in a unique way. Phosphorus concentration is the major forcing in this model. The model is examined for ecosystem stability to loading perturbations. Examination of factors to which Cand P-flow is most sensitive indicates much more attention needs to be given to the significance of MFW, especially in offshore communities.