Structure and Spatial Distribution of Planktonic Ciliates from the Middle and Lower Volga Reservoirs

The species composition of free-living planktonic ciliates and their distribution in the riverbed of the Middle and Lower Volga reservoirs during the end of the hydrological spring of 2016 and low water period of 2017 have been studied. The tendency of decreasing species diversity of ciliates downstream the cascade of the Volga reservoirs is shown and knowledge of the distribution of some rare species and the distribution of the main groups of structure-forming species from the orders Tintinnida and Choreotrichida along the Volga cascade has been broadened. The abundance of ciliates in the area from the dam of the Rybinsk hydroelectric power station to the city of Astrakhan varies from 20 to 4785 thousand cells/m(3) and the biomass varies from 0.1 to 254 mg/m(3). The highest species diversity and quantitative and productive characteristics of the ciliate communities in the Gorky and Cheboksary reservoirs, as well as in the unregulated part of the Lower Volga, are due to their traditionally higher trophic level. Factors determining seasonal and interannual differences in the distribution of ciliates in reservoirs of the Volga River are discussed.