Effect of the vital activity products of beaver (<Emphasis Type="Italic">Castor fiber</Emphasis> L.) on the formation of zooplankton structure: Changes in the quantitative parameters of two cladocera species of different sizes in a beaver pound (in situ experiment)

The experiments were performed in microcosms situated on a control site and on a site regulated by beavers. Two cladoceran species of different sizes were placed in microcosms in different proportions. It was found that the vital activity products of Castor fiber L. promote an increase in the concentration of total phosphorus (P), a decrease in the N/P ratio in water, and an increase in the chlorophyll a concentration and the abundance and biomass of bacterioplankton. In such conditions, the abundance and biomass of the large species Daphnia (Ctenodaphnia) magna Straus was increased to the greatest extent when compared with background values when it was placed in combination with Ceriodaphnia dubia Richard or without it. Moreover, in the case of the combination of two crustacean species in microcosms, the abundance of C. dubia was lower when compared with the control site, while the abundance of D. magna was higher. The results of a bioassay showed that the productivity of C. dubia decreases in waters where D. magna is most abundant. We concluded that the formation of zooplankton in beaver ponds with specific features determined by the presence of large cladoceran species is provided by changes in the quantitative and qualitative parameters of food resources. This contributes to the massive distribution of large D. magna and its competitive relations with small C. dubia. According to the results of a bioassay, the vital activity products of D. magna can inhibit the fecundity of C. dubia.