Effects of destratification aeration on physicochemical parameters and cyanobacterial blooms in a Himalayan lake

Past studies on Himalayan lakes provide limited understanding of the effectiveness of destratification aeration in improving different water quality characteristics of eutrophicated lakes. Scientific literature lacks a comprehensive understanding of factors influencing artificial aeration's impact on thermal regimes and water quality in subtropical Himalayan lakes. Addressing the knowledge gap, here, we investigate the effects of bubble plume-type artificial aeration on physicochemical parameters and cyanobacterial blooms in Nainital lake, a eutrophic freshwater lake in the Himalayas. Results show a substantial reduction but not complete elimination of thermal stratification and anoxic conditions that prevailed before aeration. Observed temperature profiles reveal weak thermal stratification during summer and an early (October) lake overturn causing a well-mixed system till late spring. Adequate dissolved oxygen (DO) levels are found for most of the year except for summer hypoxia (< 2 mg/L) limited within 5-7 m of the lake bottom. Significant concentrations (0.5-2.2 µg/L) of phycocyanin only emerge between spring and early summer. Study results highlight a prominent influence of seasonal variability in air temperature on lake temperature; local wind patterns, and rainfall (through nutrient-laden inflows) on DO levels; and solar radiation, mixing intensity, and nutrient levels on cyanobacterial blooms in the aerated system. These factors can be critical in defining the effectiveness of artificial mixing in a subtropical lake, especially in the Himalayas. Thus, the influencing factors should be adequately considered in the design and planning of destratification aeration systems for other eutrophicated lakes in the region.