Diatom assemblages in Lake Imandra sediments (Kola Peninsula) were investigated in the area affected by heated discharges from the Kola NPP. Diatoms responses to the environmental changes were analyzed; the main stages in the lake ecosystem development were revealed; the water characteristics (pH, saprobity) were reconstructed by diatoms models. It is shown that the influence of thermal pollution has been the most conspicuous at the present stage of the lake development. The consequences of climatic changes since the Little Ice Age were revealed.