MODELING ECOSYSTEM CHANGES CAUSED BY THE ZEBRA MUSSEL INVASION IN SAGINAW BAY, LAKE HURON

To understand and implement the ecosystem approach to managing the Great Lakes and other aquatic ecosystems, a quantitative understanding of the response of a system to the aggregate effects of multiple stressors is required. The current state-of-the-art models address individual management issues. Those models do not integrate the effects of multiple ecosystem stressors (e.g., nutrient and chemical loadings and invasive species) on individual management issues. An integrated multi-class phytoplankton-zebra mussel ecosystem model (SAGEM) was developed to understand the interactions between the trophic state and contaminant concentrations of a system that is perturbed by zebra mussels (Dreissena polymorpha). SAGEM is a dynamic mass balance model that represents nutrients, contaminants (such as PCBs), five phytoplankton and one benthic algal functional group, zooplankton and three cohort groups of zebra mussels. SAGEM successfully reproduced observed spatial and temporal trends of water quality parameters in Saginaw Bay immediately prior to the zebra mussel invasion in late 1991. SAGEM was used to test various hypotheses on zebra mussel, phytoplankton and phosphorus dynamics to better understand the potential role of zebra mussels in promoting Microcystis blooms in the bay (Bierman et al., 2003).In this paper, two application examples of SAGEM are presented: a) example 1 evaluates the effect of zebra mussels on total primary productivity of the system, and b) example 2 evaluates the effect of zebra mussels on total PCB fate and transport. Model results of example 1, showed that the primary productivity of the system did not change, but the distribution of primary production shifted from a pelagic-dominated (in the pre-zebra mussel period) to a benthic-pelagic coupled system with the introduction of zebra mussels. These model results are consistent with the reported field studies. Model results of the application example 2, showed that the particulate matter filtration by zebra mussels has caused an increased flux of suspended particulate matter to the sediments, which has manifested itself in an increase in sediment PCB concentrations. These application examples demonstrate the feasibility and utility of a multi-stressor ecosystem model such as SAGEM for aquatic ecosystem management.