The analysis of alternative formulations in a simple model of a coastal ecosystem

Ecosystems are complex and often require complex models if their detailed behaviour is to be replicated. However, such complex models are difficult to analyse due to their nonlinearities and the large number of parameters that most such models have. One approach that allows greater understanding of basic process is the development of simplified models. We have developed a series of simple models describing alternative formulations of a coastal ecosystem, as a tool to aid development and analysis of more sophisticated models. Sediment biogeochemistry plays a critical role in many coastal ecosystems, and much of the nitrogen input load is lost through denitrification, provided eutrophication has not set in. We have dealt with the sediment and water column response separately in simple models by exploiting the different time scales of sediment and water column response. In simple water column models, we have considered a variety of common formulations of phytoplankton–zooplankton interactions, and their implications for the steady-state response of phytoplankton and nutrients to increased nutrient load. For most formulations, we have derived explicit formulae linking model parameters to predicted mean, steady-state concentration and biomass. The simple model results provide considerable insight into the response of the bay to changes in nutrient load. In particular, the sediment model identifies a maximum denitrification capacity for the bay. Once loads exceed this capacity, denitrification declines, and nutrients are instead lost through export. This decline in denitrification results in a switch from mesotrophic to eutrophic conditions. The water column model analysis confirms the importance of the zooplankton mortality formulation in N–P–Z models in determining the dependence of steady-state phytoplankton biomass on nutrient load, and the stability of steady-state solutions.