| Summary: | The work aimed to study the structural variables of zooplankton to assess the water quality of the Caspian Sea. Studies of zooplankton were conducted in the spring and summer of 2008 and 2010. Abundance, biomass, an average individual mass of a specimen, Shannon Bi, Shannon Ab, Δ-Shannon indices, and Clarke’s W-statistic were calculated for zooplankton. Quantitative variables of zooplankton were the highest in the Northeastern and Northern Caspian, decreasing towards the Middle Caspian. In the Northeastern and Northern Caspian from spring to summer, the number of zooplankton, and the values of Shannon Bi and Shannon Ab indices decreased; the values of Δ-Shannon and Clarke’s W-statistic increased. In the Middle Caspian, the biomass of the community increased; the values of Δ-Shannon and Clarke’s W-statistic decreased. From spring to summer, the value of an average individual mass of a specimen decreased over the entire surveyed area. The jellyfish <i>Blackfordia virginica</i> and <i>Moerisia pallasi</i> significantly influenced the size structure of the holoplankton. Seasonal dynamics of structural variables of zooplankton as well as changes in water transparency showed that water quality improved from spring to summer in the shallow northern and northeastern areas of the sea, and decreased in the deep-water Middle Caspian.
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