Variability of Methane Content and Fluxes in the Rybinsk Reservoir Based on Field Observations in Different Seasons of the Year

This study investigates the formation of methane flows in the Rybinsk Reservoir and the variability of methane concentration in the water based on several field campaigns conducted in different seasons. The Rybinsk Reservoir is a very large, relatively shallow, low-flow, mesotrophic–eutrophic, morphologically complex basin–valley reservoir with multiyear flow regulation created on the Upper Volga in 1941. A total of 71 stations have been sampled for water and air to determine methane concentrations, combined with profiling the water column. As a result, a network of reference stations is established for regular measurements, along with additional stations in the reservoir and river mouths. Control samples are taken from the hydroelectric units of the hydropower plant, as well as downstream of the Sheksna HPP dam within the city of Rybinsk. Overall, the Rybinsk Reservoir is characterized by relatively low methane concentrations—at most stations, the average dissolved CH₄ content does not exceed 20 µL/L. The lowest concentrations are observed in the winter period. Methane fluxes from the surface of the Rybinsk Reservoir range from 4 to 718 mgC–CH₄ m^–2 day^–1. Specific fluxes (SFs) measured in the summer period are higher than those measured in the fall, and in summer, the spatial variability of the measured fluxes is more significant. Flux values depend on the stratification of the reservoir, the oxygen content in the water, organic matter in the bottom sediments, and other factors. Methane degassing through the dam is significantly less than the surface fluxes.