Phytoplankton of Lake Simcoe During the Ice-Free Periods of 1980–82 and Potential Response to Reduced Phosphorus Loadings

Phytoplankton biomass and species composition during 1980–82 demonstrated a clear gradient in trophic status through Lake Simcoe ranging from most enriched in southern Cook Bay to least enriched in the open waters of the main lake. Inner Kempenfelt Bay and Cook Bay phytoplankton showed influence of the nutrient inputs in these regions. A clustering of phytoplankton similarity coefficients demonstrated that the lake contained identifiable regions with high percentage similarities linking sites a) located in proximity and/or b) in areas of the lake defined by similar conditions of nutrient loading and water depth. Two-dimensional ordinations of the ten dominant phytoplankton taxa at each of the nine sampling sites on the lake showed that the best definition of these regions was during the springtime when material inputs from point sources were greatest. After diversion of the two major municipal sewage sources out of the Lake Simcoe basin in late 1984, calculations show the potential “improvements” in trophic status of Cook Bay will be a 0.3 mm 3/L decline in average May-October phytoplankton biomass and an increase in Secchi disc visibility of about 80 cm.