Planktonic Community of a Large Eutrophic Reservoir during a Period of Anomalously High Water Temperature

The rise in temperature to anomalously high values for the Middle Volga (27–29°С) in the Cheboksary Reservoir in the summer of 2010 significantly increased (2–14 times) the biomass of some plankton components (phytoplankton, bacterioplankton, protozoans, and zooplankton) and the community as a whole. The total plankton biomass reached a record value of 3 g C/m3 for deep parts in the Upper and Middle Volga. The cyanobacterial bloom of water was observed in most of the reservoir. Differences in the structure of the plankton community and the bacterioplankton-to-phytoplankton production ratio were found between riverine and lacustrine parts of the reservoir. Autotrophic (in the riverine part) and heterotrophic (in the lower lacustrine part) stages of the plankton community development were observed simultaneously in different parts of the reservoir. The functioning of the plankton community in a water temperature that exceeded usual summer heating by almost 9°C led to the deterioration of water quality in the reservoir.