Unusual vertical distribution of zooplankton and fish in the pelagic zone of Lake Sevan during summer stratification

Previous studies in Lake Sevan during summer stratification have shown that the highest zooplankton abundance and biomass was found at the lower boundary of the epilimnion. At the same time, whitefish (Coregonus lavaretus), being main planktivorous species in the lake, preferred the hypolimnion, but was leaving this layer if dissolved oxygen concentrations decreased to 1–5 mg/L. The study was conducted in Bolshoy Sevan (the southern part of Lake Sevan) on July 29–30, 2019. The vertical profiles of water temperature and dissolved oxygen concentration, as well as distribution of major taxonomic groups of aquatic organisms were assessed four times a day: in the dark (night time), in the light (day time), and in twilight (morning and evening). Fish population density was assessed using a “Simrad EK80” echosounder. Vertical distribution of zooplankton and fish observed within the study period did not correspond to that described earlier. Maximal zooplankton biomass during the study was recorded in the hypolimnion, and minimal biomass, in the epilimnion. Most of fish (89–100%) located in the metalimnion. At night, about 10% of the fish population descended to the hypolimnion, despite the low dissolved oxygen concentrations found there (3.0–3.1 mg/L). At dawn, about 7% of the fish ascended to the epilimnion, reaching a 7-m depth, where the water temperature was 20.8 °C. The water temperature range preferred by most fish individuals varied during the observation period and did not correspond to previously reported ranges.