Characteristics of the Distribution and State of Phytoplankton at Various Depths in Kislo-Sladkoe Lake (White Sea)

— Phytoplankton of the stratified Kislo-Sladkoe Lake (White Sea coast) was studied using chlorophyll fluorescence methods in 2018. A layer of red water was observed with a chlorophyll a concentration of 1.7 µg/L at a depth of 3.3 m in the chemocline (at the aerobic and anaerobic zone interface). Cryptomonad algae ( Rhodomonas sp.) were dominant in the layer; high functional activity of the photosynthetic apparatus was recorded in the algae based on the PI ABS fluorescence parameter (high values of the quantum yield of the primary photochemical reaction in photosystem 2 (PS2) ( F V / F M )); the proportion of active reaction centers (RCs) (RC/ABS); and the quantum yield of electron transport in PS2 ( ). The high functional activity of the photosynthetic apparatus of cryptomonad algae in the chemocline points toward their stability in the presence of hydrogen sulfide. By comparison, the phytoplankton in the surface water layer showed lower photosynthetic activity. Experiments with photoinhibition showed that the chemocline phytoplankton community is characterized by a greater reparative capacity after photooxidative stress despite the increased photosensitivity. It is proposed to employ chlorophyll fluorescence methods to study a state of phytoplankton in stratified meromictic water bodies.