Abstract
In this publication the eutrophication level of Kozlowa Gora dam reservoir base on composition species of algae and biomass of phytoplankton, was defined. The evaluation of phytoplankton biomass was achieved by two methods (direct - for biovolume by comparing the shape of algae to geometrical figures and indirect - base on concentrations of chlorophyll a). The trophic estimation of reservoir water was achieved on the grounds of classifications, which included the boundary values of phytoplankton biomass and concentrations of chlorophyll a. These boundary values are characteristic for individual trophic types of water. The typical species for eutrophic water were observed in the samples of water taking from the Kozlowa Gora dam reservoir, also the highest average percentage share of Cyanophyta in whole biomass of phytoplankton was observed (it may indicate on the advanced eutrophication processes). The research concerning the average biomass of phytoplankton and average concentrations of chlorophyll a allowed classification water of this reservoir to eutrophic and hipertrophic.