Microbiological Characteristics of Three Stratified Lakes in the Nizhny Novgorod Region

Abstract—Three karst lakes were investigated in the Nizhny Novgorod region: Staropustynskie lakes Svyato and Nekrasov Bay and Lake Svetloyar. The studied lakes belonged to the mesotrophic-eutrophic polyhumous type and were characterized by stable stratification with signs of meromixia. Their water columns were divided into aerobic and anaerobic zones, with the bottom water containing sulfide. Fe(II) compounds were also present in the Staropustynskie lakes. In the Lake Nekrasov Bay, the mixolimnion showed a high rate of oxygenic photosynthesis, up to 1.2 µg С L^–1 day^–1, as well as a maximum of anoxygenic photosynthesis in the chemocline (0.030–0.706 µg С L^–1 day^–1) at a depth of 1.5–2.5 m. The peak of dark CO₂ assimilation (0.146 µg С L^–1 day^–1) occurred at a depth of 1.5 m in the oxycline zone. Anoxygenic phototrophic bacteria (APB) were found in the Lakes Zaliv Nekrasova and Svyato at the boundary of light penetration. Green sulfur bacteria with short cell prosthecate outgrowths, “Ancalochloris sp.,” predominated. Consortia “Chlorochromatium aggregatum” and filamentous chlorobacteria “Chloronema giganteum” were also found, their cell sheaths accumulated ferric iron salts. In the Staropustynskie lakes, various morphotypes of iron bacteria formed clusters in the microaerobic zone. In Lake Svetloyar the chemocline was located at a depth of 16 m, outside the photic zone, and the conditions were unfavorable for APB growth. In the studied lakes, heterotrophic aerobic bacteria played the main role in the aerobic oxidation of sulfur compounds in the chemocline zone.