Conceptual Processes for Linking Eutrophication and Network Models

BACKGROUND: In recent years new ideas for nutrient management to control eutrophication in estuarine environments have been under consideration. One popular approach being considered in the Chesapeake Bay Program is called the “top down” approach based on the premise that restoring algal predators, such as oysters and menhaden, will limit excess phytoplankton production and possibly eliminate costly nutrient control programs. The approach is being considered to replace or use in conjunction with the “bottom up” approach of reducing nutrient loads. Guidance for nutrient control programs is frequently obtained from eutrophication models such as CE-QUAL-ICM (Cerco and Cole 1994). Eutrophication models provide temporal representations of carbon, nutrient, and oxygen cycling on a discrete spatial grid. These models usually represent the rate of primary production and/or phytoplankton biomass but extend no further to higher trophic levels. More complex eutrophication models that incorporate higher trophic level organisms (i.e., zooplankton and oysters) have limits and can run into numerical difficulties from multiple interacting partial differential equations needed to describe the food web. No models are presently available to provide guidance for “top down” management.