Distribution, Size-Morphological Structure and Production of Heterotrophic Bacterioplankton in Gorky Reservoir

The abundance, biomass, size-morphological structure, growth rate and production of bacterioplankton, the intensity of primary phytoplankton production and dark fixation of CO₂, as well as the abundance and biomass of heterotrophic nanoflagellates were determined in a large plain eutrophic reservoir (Gorky Reservoir, Middle Volga). The abundance, biomass, and production of bacterioplankton were relatively high and averaged 7.6 × 10⁶ cells/mL, 117.9 mg C/m³, and 59.2 mg C/(m³ × day), respectively. Heterotrophic nanoflagellates reached a high level of quantitative development – 6.9 × 10³ cells/mL, 47.9 mg C/m³. Their biomass averaged 41.6 ± 18.4% of the bacterioplankton biomass, which indicates that, in addition to bacteria, nanoflagellates used other food sources. Small rods and cocci dominated among the size-morphological bacterioplankton groups and accounted for 36.3 and 33.3% of its total abundance, respectively. Small rods averaged more than a half (56.2%) of the total biomass and were the most stable component of the community. The growth rate and production of bacterioplankton increased in those parts of the reservoir where the medium-sized cocci and coccobacilli accounted for from 18.2 to 29.3% of the total abundance. The highest bacterial activity was recorded in the area affected by the warm waste waters of the Volgorechensk State District Power Plant. Based on the data obtained, the reservoir divided on the upper river section and the lower lake section.