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.
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机译：含氧湖泊的地表水经常被甲烷（CH4）过饱和。对全球CH4周期的贡献是巨大的，因此导致了越来越多的研究和令人鼓舞的辩论。在这里，我们使用质量平衡表明，在温带中养湖上，向大气中排放的CH4约90％是由于分层期间在含氧表面混合层（SML）中产生的CH4引起的，而经常观察到的CH4跃线的最大值仅表示物理驱动的累积。可以忽略的表面CH4氧化表明所产生的110±60 nmol CH4 L -1 d -1 有效地逸出到大气中。稳定的碳同位素比表明，SML中的CH4与沉积CH4的生产不同，表明存在其他途径和前体。我们的方法揭示了目前被忽视的上层甲烷中的CH4产生，而对甲烷悖论背后的可能机理的研究应另外关注湖泊表层。
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