Magnitude and Drivers of Oxic Methane Production in Small Temperate Lakes.

Methanogenesis is traditionally considered as a strictly anaerobic process. Recent evidence suggests instead that the ubiquitous methane (CH 4 ) oversaturation found in freshwater lakes is sustained, at least partially, by methanogenesis in oxic conditions. Although this paradigm shift is rapidly gaining acceptance, the magnitude and regulation of oxic CH 4 production (OMP) have remained ambiguous. Based on the summer CH 4 mass balance in the surface mixed layer (SML) of five small temperate lakes (surface area, SA, of 0.008-0.44 km 2 ), we show that OMP (range of 0.01 ± 0.01 to 0.52 ± 0.04 μmol L -1 day -1 ) is linked to the concentrations of chlorophyll- a , total phosphorus, and dissolved organic carbon. The stable carbon isotopic mass balance of CH 4 (δ 13 C-CH 4 ) indicates direct photoautotrophic release as the most likely source of oxic CH 4 . Furthermore, we show that the oxic CH 4 contribution to the SML CH 4 saturation and emission is an inverse function of the ratio of the sediment area to the SML volume in lakes as small as 0.06 km 2 . Given that global lake CH 4 emissions are dominated by small lakes (SA of <1 km 2 ), the large contribution of oxic CH 4 production (up to 76%) observed in this study suggests that OMP can contribute significantly to global CH 4 emissions.