Composition, Structure, and Peculiarities of the Spatial Distribution of Zooplankton in the Sheksna Reservoir (Upper Volga, Russia)

The pelagic zooplankton of the Sheksna Reservoir (Upper Volga basin) was studied in July 2017. It contains 60 species, among which there are 5 species new for the reservoir ( Сyclops kikuchii copepod, Cephalodella gibba, Trichocerca rattus, Euchlanis pyriformis, and Notholca labis rotifers). Limnosida frontosa and Daphnia galeata cladocerans and Eudiaptomus gracilis and Mesocyclops leuckarti copepods (common for the reservoir) dominate in most of the water area. The mass development of the rare copepod Arctodiaptomus laticeps is detected for the first time. The average biomass of the reservoir zooplankton reaches 1.22 ± 0.15 g/m 3 ; the maximum (2.5–3.4 g/m 3 ) is registered in the western part of Lake Beloye (twice as low as observed in the 1970s and 1990s). It is established that the individual fecundity of mass species of crustaceans is noticeably lower when compared with the other water reservoir of the Volga cascade.