Olga Marzena Żygadłowska, Jessica Venetz, Wytze K. Lenstra, Niels A. G. M. van Helmond, Robin Klomp, Thomas Röckmann, Annelies J. Veraart, Mike S. M. Jetten, Caroline P. Slomp

The production of methane in coastal sediments and its release to the water column is intensified by anthropogenic eutrophication and bottom water hypoxia, and it is yet uncertain whether methane emissions to the atmosphere will be enhanced. Here, we assess seasonal variations in methane dynamics in a eutrophic, seasonally euxinic coastal marine basin (Scharendijke, Lake Grevelingen). In-situ benthic chamber incubations reveal high rates of methane release to the water column (74 – 163 mmol m-2 d-1) between March and October 2021. Comparison of in-situ benthic and calculated diffusive fluxes indicates that methane was primarily released in the form of bubbles. In spring and fall, when the water column was oxic, most of the dissolved methane was removed aerobically in the bottom water. In early summer, in contrast, methane accumulated below the oxycline. Enrichments in δ13C–CH4 and δD-CH4 and the abundant presence of methane oxidizing bacteria point towards removal of methane around the oxycline, possibly linked to iron oxide reduction. Methane emissions to the atmosphere were substantial in all seasons with the highest, in-situ measured diffusive chamber fluxes (1.2 mmol m-2 d-1) observed upon the onset of temperature-induced mixing at the end of summer. Methane release events in the floating chamber and model calculations point towards a high year-round flux of methane to the atmosphere in the form of bubbles (55 - 120 mmol m-2 d-1), which bypass the microbial methane filter. Because of bubble formation methane emissions from eutrophic coastal systems are likely higher than previously thought.