Interannual Variations in Zooplankton of the Volga Delta Kultuk Zone in the Summer Low-Water Period

The results of our studies of the interannual variations in the structure of summer zooplankton in the kultuk zone of the Volga Delta depending on hydrological and meteorological conditions are presented. The main trends in zooplankton differences in a series of low-water years are shown. It was found that in low-water years in the kultuk zone with low discharge volume, short-lasting overflow periods, and weak inundation of floodplain territories, the communities were characterized by low species richness and reduced average individual weight of crustaceans and the number of Rotifera, but a high share of the number of juvenile Copepoda in the total zooplankton abundance. In the conditions of a fall in the water level along with an increase in temperature before the overflow and its subsequent and rapid rise with the arrival of cold waters and displacement of fish larvae, zooplankton was characterized by minimal species richness and quantitative values of Rotifera, but increased biomass of Cladocera and the Shannon index. Under a stable and relatively high water level before the overflow and fish spawning in the kultuk zone, the biomass of zooplankton decreased (due to Cladocera), as did the average individual weight of crustaceans and the number of species in a sample. In medium-water years characterized by the highest discharge volume and duration of the overflow, as well as by an increase in inundation of the floodplain and feeding sites, zooplankton was distinguished by the maximum species richness, number of species in a sample, abundance of Rotifera, total biomass and biomass of Cladocera, the individual weight of crustaceans, and the Shannon index, but a minimal share of juvenile Copepoda in the total abundance and biomass.