Abstract
The aim of the present study was to identify seasonal changes in net phytoplankton communities as indicators of the trophic state in two lakes with differing morphometry and trophic states. These studies were conducted in Lake Maróz from May to November 1995 and 1996 and in Lake Jeziorak Mały in 1996 and 1997. Differences in the abundance, biomass, species diversity, and dominant taxa of particular groups of phytoplankton between these lakes were analyzed. Lake Jeziorak Mały, which has a smaller area and shallower depth, is more susceptible to degradation than is Lake Maróz. The results of studies confirm that there are differences in the net phytoplankton communities and physiochemical water parameters in the studied lakes. The abundance of blue-greens (60760 indiv. dm-3), dominated by Limnothrix redekei (Van Goor) Meffert, noted from May to October was almost nine-times higher in Lake Jeziorak Mały. The oxygen deficiency in the spring season and low mean visibility (0.7 m) also confirmed that this lake is polytrophic, while the lower abundance of blue-green algae (7022 indiv. dm-3), the great abundance of dinoflagellates, and the higher mean visibility (2.12 m) in Lake Maróz indicated that these waters are mesoeutrophic.