The changes in abundance and biomass of phytoplankton as well as in the concentration of chlorophyll ɑ in the pelagial and in the main tributary of the reservoir are presented. Physical and chemical indicators of water quality were correlated with parameters characterizing phytoplankton. Much more significant correlations occurred in the pelagial of the reservoir than in the river flowing into it. Chlorophyll ɑ had the most stable set of correlations in respect of number and type of agents, significant at α < 0.01 level. To this (down to a depth of 3 m) belonged water temperature, turbidity, pH, CO2, oxygen saturation, BOD5, oxidability, biomass of algae and dry mass of seston.