Sediment fluxes rather than oxic methanogenesis explain diffusive CH₄ emissions from lakes and reservoirs

Methane emissions from lakes and reservoirs are a major natural source in the global budget of atmospheric CH₄. A large fraction of these emissions are due to diffusive transport of CH₄ from surface waters to the atmosphere. It was suggested recently that CH₄ production in the oxic surface waters is required to compensate for diffusive CH₄ emissions from lakes. In contrast, we demonstrate here that typical diffusive CH₄-fluxes from sediments in shallow water zones, F_sed,S, suffice to explain CH₄ emissions to the atmosphere. Our analysis is based on the combination of an exceptional data set on surface concentrations of CH₄ with a mass balance model of CH₄ that is focused on the surface mixed layer and considers CH₄-fluxes from sediments, lateral transport, gas exchange with the atmosphere, and includes temperature dependencies of sediment fluxes and gas exchange. F_sed,S not only explains observed surface CH₄ concentrations but also concentration differences between shallow and open water zones, and the seasonal variability of emissions and lateral concentration distributions. Hence, our results support the hypothesis that diffusive fluxes from shallow sediments and not oxic methanogenesis are the main source of the CH₄ in the surface waters and the CH₄ emitted from lakes and reservoirs.