Features of Structural Changes in the Plankton Community of an Alpine Lake with Increasing Fish Density in Summer and Autumn

— It is found that a large species of the order Cladocera, Daphnia ( Ctenodaphnia ) magna Straus, has disappeared from the zooplankton composition in the alpine Lake Sevan, Armenia, with an increase in the density of whitefish, Coregonus lavaretus L. This led to an increase in the quantitative characteristics of heterotrophic nanoflagellates, resulting in a decrease in the number and biomass of bacterioplankton. At the same time, a number of seasonal features of plankton transformation were observed. In particular, the number and biomass of planktonic invertebrates, including representatives of Cladocera, D. longispina O.F. Müller, and Diaphanosoma lacustris Kořínek increased in July. The main reason for this was the change in the distribution of whitefish caused by an increase in their abundance, depletion of food supply due to the disappearance of D. magna , and a high water temperature. This contributed to the formation of the maximum fish density at greater depths than usual, where the trophic and temperature conditions were optimally combined. The reduction of a significant effect of fish on zooplankton can be associated with an increase in the trophic status of the lake under which grazing is compensated by a higher fecundity of invertebrates. An increase in the trophic status of the lake is indicated by an increase in the phytoplankton biomass, which is determined by the increasing amount of phosphorus regenerated by Cladocera against the background of a high water temperature. This not only compensated for grazing of algae and cyanobacteria, but also contributed to an increase in their biomass. In October, the pressure of fish on invertebrates in the water column increased due to an increase in their density and the patterns of their vertical distribution due to oxygen deficiency in the near-bottom layers. Under these conditions, the number of species and the proportion of Rotifera in the total abundance and biomass of zooplankton increased, while the total abundance and biomass of zooplankton decreased due to the filter-feeding crustaceans D. lacustris and D. longispina . However, the decrease in their abundance did not lead to changes in the biomass and structure of phytoplankton, which is associated with the decrease in the input of phosphorus excreted by Cladocera. Thus, the response of invertebrates and the groups of planktonic organisms they control to an increased density of the fish population depends on the vertical distribution of planktophages determined by the water temperature, oxygen concentration, and the quantitative characteristics of food objects.