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Zebra mussels (Dreissena polymorpha) are an invasive species that have been implicated in the reduction of algae stocks in the near‐shore environment of western Lake Erie. To determine their basin‐wide effects, we applied a two‐dimensional hydrodynamic and water‐quality model for 1994. The model accurately reproduced lake‐wide hydrodynamics and water quality. When modeled as true benthic organisms (resting on the bottom), the dreissenids grazed 53% of the western basin May through September net algal growth. This grazing resulted in a ~0.1‐mg L^‐1 reduction in the pelagic algae concentration relative to the case without dreissenids. In comparison, dreissenids grazed 77% western basin net algal growth when the lake was modeled as a fully mixed water column. We found that the biomass grazed was governed by a balance between the timescales of vertical wind‐induced mixing and benthic …