SPECIFIC FEATURES OF LACUSTRINE PLANKTIC COMMUNITUES IN DIFFERING ECOLOGICAL PERIODS (THE "ICE-COVER" AND THE "OPEN-WATER" PERIODS)
The splitting of the annual ecological cycles in the lakes into two periods, the “open-ice” (“vegetational”) and “ice-cover” (“ lifeless ”) periods does not conform to all diversities of the states of environment and plankton. The lakes studied originally and analyzed on the basis of published data situated at the upper and middle Volga River basins, Transbaikalia, and Chelyabinsk oblast differ in terms of one of the key factors -intensity of impact of the solar radiation. This radiation is higher in the studied Transbaikalian lakes owing to their more southern (≈ 8-9º) geographic location. During the ice-cover period the same factor prevails in Transbaikalia in the low-snow periods. The winter maxima of phytoplankton and zooplankton are the characteristics of the studied Transbaikalian lakes. In the upper and middle Volga basins the vegetation of phytoplankton, bacterioplankton and zooplankton starts under ice, enhancing during March-April following the melting of the over-ice snow and, hence, increased insulation. In additions, there is an ice-cover vegetation of heterotrophyc cyanobacteria (predominantly, of the species of g. Oscillatoria ), along with development of the methane-oxidizing saprophytic bacteria and the zooplankton dwelling in the oxycline layer. These facts seems true for all studied megathermic lakes The period of ice laying, as it was specified in our studies, plays a special role in the annual cycle of plankton development in the upper Volga basin lakes. The development of the “winter zooplankton” reversely depends on the duration of the later period.