Full-scale evaluation of methane production under oxic conditions in a mesotrophic lake

Oxic lake surface waters are frequently oversaturated with methane (CH₄). The contribution to the global CH₄ cycle is significant, thus leading to an increasing number of studies and stimulating debates. Here we show, using a mass balance, on a temperate, mesotrophic lake, that ~90% of CH₄ emissions to the atmosphere is due to CH₄ produced within the oxic surface mixed layer (SML) during the stratified period, while the often observed CH₄ maximum at the thermocline represents only a physically driven accumulation. Negligible surface CH₄ oxidation suggests that the produced 110 ± 60 nmol CH₄ L^-1 d^-1 efficiently escapes to the atmosphere. Stable carbon isotope ratios indicate that CH₄ in the SML is distinct from sedimentary CH₄ production, suggesting alternative pathways and precursors. Our approach reveals CH₄ production in the epilimnion that is currently overlooked, and that research on possible mechanisms behind the methane paradox should additionally focus on the lake surface layer.