This summarizes studies conducted during 1980 to assess the effects of reductions in phosphorus loading to Saginaw Bay on phytoplankton in the bay and the adjacent waters of Lake Huron. Quantitative estimates of phytoplankton abundance were developed from an array of stations sampled during the ice-free season. Distribution and abundance of major species and multivariate statistical representations of associations were compared to similar data collected during 1974, prior to phosphorus loading reductions. Results show a substantial reduction in the abundance and range of distribution of eutrophication tolerant and potentially nuisance-producing phytoplankton populations in Saginaw Bay and reduced export of such populations to the main Lake Huron system.