EN
Phytoplankton composition was analysed in the Maltański Reservoir and the inlet and outlet of the Cybina River. Samples were collected from three sampling stations in November 2007, February, May and August 2008. Considerable changes in the qualitative and quantitative composition of the phytoplankton in the infl owing and outfl owing water and in the central part of the reservoir were noted. Among 142 taxa chlorophytes were the most numerous group. There were also groups represented by the large values of phytoplankton abundance and biomass such as Cyanobacteria in autumn, chrysophytes in winter, diatoms and cryptophytes in spring and chlorophytes in summer. The diff erences in the qualitative and quantitative composition of the phytoplankton among the analysed sampling stations were noticed. The species composition of the potamoplankton was changing as it passed through the reservoir along the course of the river. With regard to all seasons the lowest number of taxa were noticed in samples from the inlet. The most diversifi ed was phytoplankton composition in the reservoir. The highest similarity between the analysed stations was observed in the phytoplankton composition in the reservoir and outlet of the river from this reservoir.