Variability of methane content and fluxes in the rybinsk reservoir based on field observations in different seasons of the year

The formation of methane fluxes in the Rybinsk reservoir and the variability of its content in water were investigated during several field campaigns on the reservoir in different seasons. The Rybinsk reservoir is a very large, relatively shallow, low-flow, mesotrophic-eutrophic, morphologically complex basin-valley type reservoir with perennial flow regulation, created on the Upper Volga in 1941. In total, water and air sampling was carried out at 71 stations to determine methane concentration, which was combined with measurements of water column characteristics . As a result, a network of reference stations was established, where regular measurements are made, as well as additional stations in the water area and river estuaries. For control purposes, water samples were taken at the hydroelectric power plant units and downstream of the Sheksninskaya HPP dam, placed within the Rybinsk city. In general, the Rybinsk reservoir is characterised by relatively low methane concentrations - at most stations the average content of dissolved CH4 in water does not exceed 20 µl/l. The lowest concentrations were observed during the winter period. Methane fluxes from the surface of the Rybinsk reservoir vary from 4 to 718 mgC-CH4 m-2 day-1. Specific fluxes in summer period are larger than those measured in autumn, also in summer the spatial variability of the measured fluxes is more significant. Flux values depend on reservoir stratification, oxygen content in water, organic matter in bottom sediments and other factors. Methane degassing through the dam is significantly less than the fluxes from the surface.