Phytoplankton community composition integrates numerous environmental drivers, some managed and others currently unmanaged. Phytoplankton community composition is of particular interest to resource managers because it influences food quality for fisheries and because some phytoplankton species can produce fish, wildlife, and human toxins. We compared historical (1971) to recent (2015) phytoplankton community data for Grand Traverse Bay (Lake Michigan) to assess continued recovery from the direct disposal of fruit pulp into the bay. We further compared abiotic conditions and phytoplankton community composition among Grand Traverse Bay, Saginaw Bay, and western Lake Erie; three Great Lakes Bays with contrasting nutrient loads. Finally, among year community variability in western Lake Erie was assessed relative to the influence of nutrient and weather variability and the potential for nutrient management to shift community composition.
Results/Conclusions
Historical species composition data from shortly after reductions in fruit pulp disposal showed a sharp contrast to recent community composition indicative of oligotrophication. For example, in 1971, a Microcystis aeruginosa synonym was noted as exceeding 5% of phytoplankton cell counts in September, but this harmful algal bloom species was not detected in 2015. Both phytoplankton biovolume and community composition varied with average nutrient concentrations among bays. However, preliminary analysis indicated that interannual variability within Lake Erie was almost as great as that between Lake Erie and Saginaw bay as the Lake Erie community varied in response to both nutrient loading and the timing of that loading relative to spring warming. This suggests that, while phytoplankton management through nutrient loading reduction is feasible, interaction with other drivers could mask or delay expected responses.