Seasonal and daily patterns of methane content and emission in the estuary of the Chernaya River (Crimea)

Estuaries are important sources of methane (CH₄) to the atmosphere. Estimates of the emission of this greenhouse gas are necessary to determine its contribution to the global atmospheric budget. The paper discusses the results of measuring the temporal dynamics of CH₄ concentration and its emission at the atmosphere- water boundary in the Chernaya River estuary, influenced by the seiche waves. The hydrochemical studies together with hydrophysical measurements were carried out in 2021 winter, spring and summer seasons using the RCM 9 LW (Aanderaa) multi-parameter sound. High rates and a wide range of daily-measured CH₄ concentration changes in water of the Chernaya River estuary was shown which correspond to the dynamics of recorded wave processes. The calculated periods of hydrological parameters oscillation were 45, 19–21, 14–17 and 9 min. The trend of CH₄ concentration variations was consistent with salinity changes: higher CH₄ values correspond to fresh waters, and lower values to seawaters. The highest correlation was between the values of СН₄ concentration and water temperature (R = 0,61), and the content of dissolved oxygen (R = –0,61). The calculated CH₄ flux from water surface to the atmosphere for different seasons was in the range from 71 to 1680 μmol/m²· day. Balance estimates showed that the turnover time of dissolved methane in the estuary of the Chernaya River, i. e. the ratio of methane content in 1 m³ to the sum of its fluxes from the bottom and into the atmosphere, did not exceed one day for the selected conditions. It was shown that high rates and a wide range of changes in CH₄ concentration in areas subject to wave processes, as well as the patterns of such changes, are of high importance in the context of assessing methane fluxes into the atmosphere.

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