The role of the phytoplankton community in the formation of the chemical composition of waters in Koporskaya Bay of the Gulf of Finland, serving as the cooling pond of the Leningrad nuclear power plant, was assessed. The work was performed with the use of data of systematic year-round monitoring. Tables of stable correlations for individual seasons were obtained. A decisive role of phytoplankton in the dynamics of inorganic forms of phosphorus, nitrogen, organic material, and elements of the carbonate equilibrium system was demonstrated. The problem of the applicability of BOD5 as an indicator of organic pollution is discussed.