Methane Concentration and Fluxes in Volga River Reservoirs

The paper presents the results of methane flux and its concentration measurements in the reservoirs of the Volga cascade: Ivankovskoye, Rybinskoye, Gorkovskoye, Kuybyshevskoye, and Volgogradskoye reservoirs. The article summarizes the materials from the 2017–2023 seasonal observations archive. Measurements of the gas flux were carried out by the floating chamber method, the methane concentration determination in the samples was carried out by the headspace method. The spatial and seasonal variability of both methane content and its emissions depending on the coefficient of water exchange, weather conditions, the nature of bottom sediments, and depth was revealed. High values of methane concentration and methane flux are observed in the presence of stratification, while during vertical mixing, the flux values decrease significantly. The highest methane flux values are characteristic of the heavily populated by macrophytes shallow Shoshinskiy reach of the Ivankovskoye reservoir (up to 334 mgC-CH₄/(m² day)), the flooded left bank floodplain of the Gorkovskoye reservoir (up to 548 mgC-CH₄/(m² day)), where they are associated with weak flow and intra-mold circulation, and also for Chesnava bay of the Rybinskoye reservoir (up to 1086 mgC-CH₄/(m² day)), associated with anthropogenic pollution and low flow rates. In the bays of Kuybyshevskoye and Volgogradskoye reservoirs that receive inflows with increased mineralization, stratification may increase due to density stratification, the formation of zones with oxygen deficit, and increase in methane flux despite the small amount of organic matter in sediments. The example of the Gorkovskoye reservoir shows the effect of the dam on the spatial structure of methane flux and concentration. Comparison with generalized data on specific methane flows from moderate water reservoirs showed that in the Volga cascade, these values are lower in all months of the open water period except August.