Effects of Differential Growth and Mortality in the Seasonal Succession of Phytoplankton Populations in Lawrence Lake, Michigan

An investigation was made of factors controlling algal succession in a small, oligotrophic lake during summer stratification. Phytoplankton population densities, growth rates, sedimentation losses, and mortality due to zooplankton grazing were measured weekly. Cyclotella michiganiana was the dominant alga through the end of June, at which time Cyclotella comensis began to increase, becoming the dominant by August. In August, high grazing pressure caused the rapid declines of both C. michiganiana and C. comensis, which were followed by an increase of Sphaerocystis schroeteri. The combined effect of greater growth rates and lower loss rates of C. comensis resulted in its dominance over C. michiganiana. In contrast, the C. comensis—S. schroeteri succession clearly resulted from differential mortality alone. It is likely that losses due to sedimentation and/or grazing are important in many lakes and that interspecific competition may be less important in actually controlling seasonal succession.