An analysis was performed on the effects of radiation exposure on hydrobionts (phytoplankton, zooplankton, zoobenthos) of a freshwater body — Lake Urus Kul, located in the near zone of the accident at the "Mayak" radiochemical chemical plant — over a period of 50 years after the entry of radionuclides into the lake. The radiation doses received by these organisms during their lifetime were estimated. It was shown that in the lake subjected to radiation exposure, phytoplankton, zooplankton, and macrozoobenthos are characterized by significantly lower diversity and biomass, and large-sized forms that received high doses after the accident at the Mayak Chemical Combine were absent. The replacement of large-sized forms of biota species is most pronounced in macrozoobenthos, where species diversity decreased most significantly — from 12 to 7 species, which allows for consideration of macrozoobenthos as a critical group of aquatic organisms. The intensity of doses on hydrobionts decreased rapidly and reached ecologically tolerable limits 5–10 years after the lake's contamination. The restoration of the ecosystem's structure was occurring rather slowly. This suggests that the ecosystem of the affected lake was pushed back to an earlier stage of succession and the restoration of this water body's ecosystem after a radiation exposure is a long-term process.