The current state of the zooplankton in the Kakhovka reservoir

Abstract

The current state of the quantitative and qualitative development of zooplankton in the Kakhovka Reservoir is examined. The species composition, the representation of taxonomic groups, the number and biomass of zooplankton in the summer period of 2020–2021 were determined. The average number of zooplankton in the summer of 2020 in the studied areas (Bilenke village, Bilenke-Malokaterynivka station, vicinity of Malokaterynivka village) was 40.833 units/m3 with the biomass 0.245 g/m3, in the summer of 2021 — 110,932 units/m3 with the biomass 0.652 g/m3. Rotifers formed the basis of the number and biomass of zooplankton in the summer of 2020 (76% and 86%). Cladocera both in number and biomass (5% and 7%) and copepod (18% and 6%, respectively) had a significantly lower level of development in the reservoir. Dreissena polymorpha larvae were found in a small amount (1%) and biomass (0.1%). In the summer period of 2021, rotifers also formed the basis of the number and biomass of zooplankton (72% and 68%). Cladocera both in number (8%) and biomass (14%), and the copepod (19% and 18%, respectively) reached a lower level of development in the reservoir. As in the previous year, the larva of D. polymorpha was found in a small amount (1%) and biomass (0.1%). In the summer of 2020, the basis of the number and biomass of rotifers was formed by: Asplanchna brightwelli (24% and 40%), Asplanchna priodonta (7% and 24%), Synchaeta pectinata (13% and 8%), Brachionus calyciflorus (6% and 6%), Euchlanis dilatata (6% and 2%, respectively), in the summer of 2021 A. brightwelli (16% and 27%), A. priodonta (5% and 19%), S. pectinata (12% and 8%), E. dilatata (7% and 2%), B. calyciflorus (6% and 6%), Brachionus diversicornis (5% and 2%, respectively). Chydorus sphaericus dominated in number (4%) and biomass (7%) among Cladocera in 2020, and C. sphaericus also dominated in number (7%) and biomass (12%) in 2021. In 2020, copepod in the reservoir are mainly represented by nauplial (17% and 6%) stages, and in 2021 by nauplial (12% and 4%) and copepod (4% and 3%) stages, as well as Diaptomus juv. (1% and 8%), Cyclops sp. (2% and 3%, respectively).