Abstract
The objective of the study was to identify seasonal changes in phytoplankton taxonomic structure and development rates to characterize phytoplankton communities as determinants of the trophic and ecological conditions of Lake Kortowskie in northeast Poland which has been undergoing restoration with selective hypolimnetic withdrawal. The lake was designated as being of high trophic state and bad ecological status based on intense phytoplankton growth, the strong dominance and persistent occurrence of blue-green algae, and seasonal modifications in phytoplankton taxonomic structure and assemblage growth rates. The ecological status of the lake assessed in 2011 corresponds to that determined in 1987-1990 and 1999 and indicates that eutrophication in Lake Kortowskie is progressing.