Inter-annual changes in zooplankton in the Volga delta kultuk zone in summer (low-water period)

   The paper presents the results of our studies of inter-annual changes in the structure of summer zooplankton in the kultuk zone of the Volga delta depending on hydrological and meteorological conditions. The study shows the main trends in zooplankton differences in a series of low-water years. It was found that in low-water years in the kultuk zone with low runoff, short floods and weak watering of floodplain territories, zooplankton was characterized by low species richness, reduced average individual mass of crustaceans and the number of Rotifera, but a high proportion of the number of juvenile Copepoda. In the year when the water level decreased before the flood during the period of temperature increase, and then a rapid rise with the inflow of cold waters and the removal of fish larvae, zooplankton was characterized by a minimum species richness and quantitative values of Rotifera, but increased biomass of Cladocera and the Shannon index. In every low-water year, with a stable and relatively high water level before the flood, fish spawning in the kultuk zone, biomass of zooplankton reduced due to Cladocera, and the number of species in the sample. In every mid-water year, the flood had the largest runoff and prolonged, increased flooding of the floodplain, zooplankton was characterized by the maximum total and specific species richness, abundance of Rotifera, total biomass on account of Cladocera, and the individual mass of crustaceans, the Shannon index, but the minimum proportion of juvenile Copepoda.