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Lake Sevan is a meso-eutrophic water body, which was severely impacted by anthropogenic level decrease, pollution and eutrophication during the last century. Starting in the 1970s, these processes resulted in the formation of an oxygen-depleted hypolimnion during summer–autumn stratification of the lake. In this work, we demonstrate for the first time that eutrophication of the lake leads not only to the full depletion of oxygen and nitrate in the hypolimnion but as well to the presence of sulfate-reducing microorganisms and toxic hydrogen sulfide. Concentrations of hydrogen sulfide in the hypolimnion of Major and Minor Sevan in October were as high as 9 and 39 μM, respectively. In October 2019, 66 % of lake's bottom was covered by sulfidic waters, while the fraction of sulfidic water volume reached 19 %. Values of δ34S for hypolimnetic sulfide are lower by only 7–12 ‰ compared to epilimnetic sulfate, while δ33S values of sulfide are similar to the δ33S values of sulfate. These isotopic fingerprints are not consistent with microbial sulfate reduction as the sole source of hydrogen sulfide in the hypolimnion. We attribute the formation of a sulfidic deep-water layer to a combination of microbial sulfate reduction in the water column and diffusion of hydrogen sulfide from the sediments. [ABSTRACT FROM AUTHOR]

Lake Sevan is the largest freshwater lake in the Caucasus region and one of Eurasia's largest alpine lakes. The lake has been experiencing growing cyanobacteria blooms; however, the relevance of external and internal phosphorus (P) loading to its eutrophication is still not known. We carried out a sediment investigation of Lake Sevan to estimate the potentially mobile P, which could be a source of internal total phosphorus (TP)-loading; we also estimated external TP-loading and retention. The study was carried out at four sampling points of different depths to determine the spatial variability of P in 2018. The sediments had elevated TP contents at the sediment surface; potentially mobile P ranged from 20 to 60% in the top 2 cm. The upper 4 cm sediment had an elevated content of TN (8 to 16.1 mg N g-1 dw) and TP (1.2 to 1.7 mg P g-1 dw). Spatial variability of most of the measured parameters was more prominent in the upper 3 cm. External TP loading was estimated to be 110 tons annually, and the TP retention was 85%. The estimated TP stored in the top cm of the sediment is 1,500 tons. The potential for P release is high; the short-term exchange between oxic and anoxic overlying water could release 0.01 to 0.02 mg P L-1 from the top cm of the sediment, and long-term diagenesis and burial could release about 0.12 mg P cm-2. Internal P-loading in Lake Sevan may play an essential role in eutrophication, especially given the long flushing time of Lake Sevan. Internal P-loading in Lake Sevan may play an important role in eutrophication, especially given the long flushing time of Lake Sevan. [ABSTRACT FROM AUTHOR]