Based on samples collected during the International Field Year for the Great Lakes, the phytoplankton assemblage of Lake Ontario is dominated by taxa indicative of degraded water quality, including many potentially nusiance producing species. Many taxa characteristic of the offshore waters of the upper Great Lakes are either absent from the flora or very rare. Compared to the upper lakes, the flora of Lake Ontario undergoes extreme seasonal succession, with diatoms predominating during the winter and early spring, green algae becoming abundant during the summer, and blue-green algae showing a distanct fall peak. Various species of microflagellates are a relatively important element of the flora during all seasons. Succession during the spring bloom appears to be controlled by the thermal bar, and data suggest control by depletion of essential nutrients following stratification. Striking differences were apparent in samples collected on comparable dates in the spring of two successive years.

Prepared for National Environmental Research Center, Office of Research and Development, U.S. Environmental Protection Agency under grant R-800605, program element 1 BA026, ROAP/Task no. 21AKP/007. Includes bibliographical references (pages 369-373).