Dynamics of Lake Michigan Plankton: A Model Evaluation of Nutrient Loading, Competition, and Predation

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Abstract
Lake Michigan's offshore ecosystem has been altered dramatically during the past decade. Summer zooplankton dominance has changed from calanoid copepods to Daphnia and the substantial contribution of filamentous blue-green algae to summer phytoplankton has been replaced by phytoflagellates. These changes occurred concurrently with reduced P load, P concentration, and abundance of the dominant zooplanktivore, the alewife (Alosa pseudoharengus). In this analysis we pose alternative hypotheses of nutrient loading and species interactions as determinants of zooplankton and phytoplankton species composition in the summer epilimnion. We evaluate these hypotheses with a food web model that was calibrated to measurements of the 1980s Lake Michigan plankton composition and algal production, sedimentation, and growth rates and literature estimates of zooplankton secondary production and nutrient excretion. The model simulates the influence of gradients of both P load and alewife abundance on predation–competition interactions. We conclude that summer plankton composition in Lake Michigan is controlled largely by predation. The model further predicts a return to a plankton community similar to that of the 1970s under a scenario of increasing invertebrate predation by a new zooplankton species for Lake Michigan, Bythotrephes cederstroemi.