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

Oxic lake surface waters are frequently oversaturated with methane (CH). The contribution to the global CH 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 CH emissions to the atmosphere is due to CH produced within the oxic surface mixed layer (SML) during the stratified period, while the often observed CH maximum at the thermocline represents only a physically driven accumulation. Negligible surface CH oxidation suggests that the produced 110 ± 60 nmol CH L d efficiently escapes to the atmosphere. Stable carbon isotope ratios indicate that CH in the SML is distinct from sedimentary CH production, suggesting alternative pathways and precursors. Our approach reveals CH 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.