During the anomalously hot summer of 2010, the water temperature in the Gorky reservoir reached 27–33°C. Pronounced cyanobacterial blooms occurred in the limnetic part of the reservoir. The average values for bacterioplankton abundance (11.58 ± 1.25 × 106 cell/mL), biomass (886 ± 96 mg/m3), and production [169 ± 32 mg C/(m3 day)] were twice as high as in the year with temperatures comparable to long-term average values. These parameters were higher in the limnetic part than in the river one. The abundance (4.86 ± 0.75 × 103 cell/mL) and biomass (138 ± 9 mg/m3) of heterotrophic nanoflagellates were 2.3 and 1.7 times higher, respectively, than in years with regular temperature regimes. The average number of plank-tonic viral particles (N
v) in 2010 was 48.89 ± 9.54 × 106 particles/mL, while virus-induced bacterial mortality (VMB) accounted for 26.9 ± 4.6% of the bacterial production. The N
v and VMB values in the limnetic part of the reservoir were, respectively, 1.5 and 1.8 times higher than in the river one.