The dynamic of phytoplankton in Lake Dejguny was investigated in relation to the environmental conditions influencing water quality. The phytoplankton biomass and chlorophyll concentration in water were relatively low (typical of mesotrophic lakes), with a decreasing tendency when the water transparency significantly increased. The phytoplankton was not significantly correlated with the content of nutrients in water. The vast majority of phosphorus was in a form unavailable to phytoplankton, and the TN:TP ratio evidenced the role of nitrogen and phosphorus in colimitation of its development. The general evaluation of trophy based on TSI and TLI indicated meso-eutrophy or even slight eutrophy, because of partial assessments: TSI_TP, TLI_TP, and TLI_TN and domination of filamentous blue-green algae, which revealed a more advanced degree of eutrophication.