The temporal (seasonal and long-term) and spatial (in cascade of reservoirs) variations in efficiency of
solar radiation utilization (ESRU) under phytoplankton photosynthesis in the Volga and Angara river reservoirs are
under consideration. ESRU values change in a range of 0,02–1 % that is comparable with the ones in water bodies
of other regions. ESRU depends on input of solar radiation (R2 = 0,41–0,61), water temperature (R2 = 0,34–0,49),
and chlorophyll content (R2 = 0,52–0,68) that shows its fluctuations according to reservoirs trophic state.