In July 2010, abnormally high water temperature (25-29 degrees C), as well as increased biomass and phytoplankton production caused intensive development of heterotrophic bacteria and heterotrophic nanoflagellates. It was found that the abundance, biomass, and production of heterotrophic bacterioplankton, as well as the abundance and biomass of heterotrophic nanoflagellates, and the number of planktonic viruses, which were calculated on average for the reservoir under study, turned out to be higher in the years with lower water temperature (20-23 degrees C). The virus-induced mortality of bacterioplankton in the Oka River and the Cheboksary Reservoir averaged 25.4 +/- 3.4 and 22.4 +/- 2.7% of the daily bacterioplankton production.