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

Ecosystems are complex and often require complex models if their detailed b ehaviour is to be replicated. However, such complex models are difficult to analyse due to their nonlinearities and the large number of parameters tha t 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 ec osystem, as a tool to aid development and analysis of more sophisticated mo dels. Sediment biogeochemistry plays a critical role in many coastal ecosys tems, 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 differ ent time scales of sediment and water column response. In simple water colu mn models, we have considered a variety of common formulations of phytoplan kton-zooplankton interactions, and their implications for the steady-state response of phytoplankton and nutrients to increased nutrient load. For mos t 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 chang es in nutrient load. In particular, the sediment model identifies a maximum denitrification capacity for the bay. Once loads exceed this capacity, den itrification declines, and nutrients are instead lost through export. This decline in denitrification results in a switch from mesotrophic to eutrophi c conditions. The water column model analysis confirms the importance of th e zooplankton mortality formulation in N-P-Z models in determining the depe ndence of steady-state phytoplankton biomass on nutrient load, and the stab ility of steady-state solutions. (C) 1999 Elsevier Science B.V. All rights reserved.