FEATURES OF STRUCTURAL CHANGES IN THE PLANKTON COMMUNITY OF AN ALPINE LAKE WITH INCREASING FISH DENSITY IN SUMMER AND AUTUMN

Daphnia (Ctenodaphnia) magna Straus, a large species of the order Cladocera, was revealed to have disappeared from the zooplankton composition with an increase in the density of whitefish, Coregonus lavaretus L., in Lake Sevan, Armenia. This caused an increase in the quantitative characteristics of heterotrophic nanoflagellates, as a result of which the abundance and biomass of bacterioplankton decreased. At the same time, several seasonal features of plankton transformation were noted. In particular, the abundance and biomass of planktonic invertebrates, including D. longispina O.F. Muller and Diaphanosoma lacustris Korinek, both representatives of Cladocera, increased in July. The main reason for this were changes in the distribution of whitefish caused by an increase in their abundance, a 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 depths greater than usual, where the trophic and temperature conditions were combined optimally. Leveling a noticeable influence of fish on zooplankton could also be associated with an increase in the trophic status of the lake, when grazing was compensated for by a higher fecundity of invertebrates. An increase in the trophic status of the lake is evidenced by a rising phytoplankton biomass, which is determined by an increased amount of phosphorus regenerated by Cladocera against the background of a high water temperature. This not only compensated for the grazing of algae and cyanobacteria, but also contributed to an increase in their biomass. In October, the pressure of fish on invertebrates rose due to an increase in their density and the peculiarities of their vertical distribution due to oxygen deficiency in the bottom water layers. Under these conditions, the number of species and the share 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, a decreased abundance of the latter species failed to lead to changes in the biomass and structure of phytoplankton, which was associated with a decrease in the input of phosphorus excreted by Cladocera. Thus, the response of invertebrates and the groups of plankton organisms they control to an increased density of the fish population depends on the vertical distribution of planktophages, this being determined by water temperature, oxygen concentration, and the quantitative characteristics of food objects.