Horizontal and Vertical Distribution of Phytoplankton in the Alpine Lake Sevan (Armenia) during the Summer Cyanoprokaryota Bloom

A study of the horizontal and vertical distribution of phytoplankton in Lake Sevan was conducted in mid-July 2018 during the maximum Cyanoprokaryota blooms. The summer biomass of algae in the pelagic zone of the lake corresponded to values of mesatrophic waters. The Cyanoprokaryota biomass in Lake Sevan has horizontal and vertical heterogeneity and is related to the depth, morphometry of the lake parts, distribution of currents, transparency, temperature, and biomass of Cladocera. The specific number of phytoplankton species and water transparency decreases in areas where water blooms occur. The following factors contribute to the increase in the average phytoplankton biomass when compared to 2013-2017: (1) increased water temperature, (2) weakening of zooplankton grazing pressure due to the disappearance of Daphnia magna, and (3) increased phosphorus load due to increased contribution of D. longispina and Diaphanosoma lacustris to the zooplankton biomass.