EARLY SUMMER ZOOPLANKTON OF LAKE VOZHE AND LACHA (VOLOGDA REGION)

In June 2015, zooplankton (Cladocera, Copepoda, Rotifera) of lakes Vozhe and Lacha, Svid’ River and the headwaters of the Onega River (catchment of the Onega River, White Sea basin) was examined. It was found that crustaceans (mainly the copepods Mesocyclops leuckarti and Eudiaptomus gracilis) dominated the community. The biomass of zooplankton (<1 g / m3) indicated a low food supply for fish (poorly fed water bodies), compared with the beginning of the 1970s, it decreased by 4–10 times. The composition and structure of the community characterized the ecosystem status of the Lake Vozhe as eutrophic, and Lake Lacha as mesotrophic. Changes in the structure of zooplankton over 40 years testified to the eutrophication of the ecosystem of Lake Vozhe and stabilization at the level of mesotrophy of Lake Lacha. In the lakes, the dispersal and relatively high abundance of some thermophilic species (Polyarthra luminosa, Thermocyclops crassus), indicators of eutrophic conditions that were previously absent in the community, were revealed. The productivity (14–34 cal / m2 × day) and the respiration rate of zooplankton (61–122 cal / m2 × day) of the lakes corresponded to the level of northern oligotrophic water bodies. It was shown that at the beginning of summer a significant part of the zooplankton production was consumed by planktonic predators (mainly copepods). Fishes had access to about 15% of the daily production of zooplankton in Lake Vozhe and up to 43% in Lake Lacha. A small proportion of filter feeders (30–60% of the total biomass) was recorded in the modern zooplankton of lakes. By 2015, the filtration capacity of plankton from lakes Vozhe and Lacha decreased by an order of magnitude compared to that in the 1980s. The low self-cleaning ability of lake waters is discussed as the main reason for the growing organic pollution and increased silt accumulation in their ecosystems.