Spatial-Dynamic Modeling of Algal Biomass in Lake Erie: Relative Impacts of Dreissenid Mussels and Nutrient Loads

Over the past several decades, reductions in phytoplankton stocks and increased water clarity in Lake Erie have resulted from phosphorus load abatement and the introduction of zebra (Dreissena polymorpha) and quagga mussels (D. bugensis). The relative impacts of these developments and their implications for lake management have remained difficult to delineate. To address this issue, we numerically model the complex biophysical interactions occurring in Lake Erie using a two-dimensional hydrodynamic and water quality model that is extended to include dreissenid mussel and zooplankton algorithms. The model reasonably simulates longitudinal trends in water quality as well as the dynamics of central basin hypoxia. Phosphorus is the limiting nutrient through the euphotic zone and its control decreases the algal growth rate and biomass ( ～ 55–60%). Filter feeding by dreissenid mussels also decreases algal biomass ( ～ 25–30%), simultaneously stimulating increased net algae growth through enhanced algal consumption and subsequent phosphorus recycling. Effective recycling implies that algae stocks are ultimately regulated by external phosphorus loads. Returning phosphorus loads to pre-abatement 1960s levels, in the presence of dreissenid mussels, results in a western basin algae concentration of ～ 0.7?mg?dry?weight?L?1 with a potential for nuisance algae growth.