Long-Term Dynamics of the Heterotrophic Bacterioplankton Development in the Mouth of Usinskii Bay of the Kuibyshev Reservoir

Data of our long-term observations of the development of bacterioplankton in the mouth section of Usinskii Bay (Kuibyshev Reservoir) during the summer–autumn periods are presented. The total abundance of heterotrophic bacterioplankton varied within (1.61–7.44) × 10⁶ cells/mL, and the biomass was 39.01–249.53 μg C/L, which corresponds to water quality classes III–V. Between 2010 and 2017, the total abundance of heterotrophic bacterioplankton was nonmonotonically decreasing, and since 2021 a significant increase has been recorded. Among heterotrophic bacteria, single free-floating cells predominated, mainly cocci and coccobacilli, and the proportion of rod-shaped cells increased during a cyanobacterial bloom. Picocyanobacteria and picodetritus particles were almost always present in plankton, but their average contribution to the total organic carbon of picoseston was insignificant.