The paper presents the results of long-term research of phytoplankton of the cooling waterbody of the Lukoml state district power station (Belarus). The dynamics of abundance, biomass, community structure, and composition of the dominant phytoplankton species for the period of 1972 – 2019 were analyzed. The stages of development of the plankton algae community are determined, due to changes in the intensity and direction of anthropogenic factors. The response of phytoplankton to changes in heat load and dependence on the development of the population of Dreissena polymorpha Pallas is shown. The influence of the fish rearing complex on the developmental intensity and phytoplankton structure was revealed. Since 2004, the periods of intensive vegetation of algae alternate with a significant decrease in abundance and biomass, which indicates the unstable state of the ecosystem of the cooling waterbody. The succession of the dominant complex in 2019 was characterized by the loss of the species Aphanizomenon flos-aquae Ralfs ex Bornet & Flahault (family Nostocaceae) and the inclusion of the species Planktothrix agardhii (Gomont) Anagnostidis & Komárek (family Oscillatoriaceae).