Abstract
The aim of the study was to determine the impact water retention time and increased water temperature had on the composition and abundance of zooplankton. Crustacea and Rotifera were examined from lakes Ślesińskie and Licheńskie, which are part of the water cooling system for the Konin and Pątnów power plants. Lake Ślesińskie is included in the cooling system periodically (from May to September) when the long cooling cycle is in operation, and its waters are heated moderately (< 25°C) with a water retention time of about two weeks. The zooplankton biomass in this basin in summer ranges from 1.5 to 4.0 mg dm-3 and was lower than that noted in May (< 25 mg dm-3) before the lake was included in the water cooling system. In Lake Licheńskie, which is heated continuously (< 30°C) and has a water retention time of about 5 days, the zooplankton abundance (< 100 indiv. dm-3) and biomass (< 0.5 mg dm-3) noted was low. In addition to temperature, one of the factors that affects zooplankton development in both lakes is the short water retention time.