Abstract
The study of planktonic animals of the Caspian Sea is topical and, during the last centuries, has brought and continues to bring new results. This is an inevitable process attributed to the introduction of non-indigenous predatory representatives of zooplankton by ballast water of ships. During the study period, the zooplankton of the Middle Caspian Sea was represented by 13 taxa and consisted mainly of non-indigenous species typical of the Palearctic region. Native fauna was represented by three species—cladoceran Evadne anonyx, Podonevadne camptonyx, and copepod Halicyclops sarsi during the study period. The quantitative variables of zooplankton did not reach a high level in May 2020 and 2021 compared to the data of previous years. Cladocerans Podonevadne camptonyx, copepods Acartia tonsa, and larvae of Cirripedia dominated in 2020. By 2021, the dominant species of last year had been replaced by the cladoceran Evadne nordmanni. One-way ANOVA analysis detected significant differences in the quantitative variables of cladocerans and copepods between different years of zooplankton study. The decreasing abundance, biomass, and alteration of the dominant zooplankton species in the Middle Caspian Sea during the study period may be associated with the feeding type of predator species and a slight elevation of water salinity.