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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 (CH4) 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 CH4 production (OMP) have remained ambiguous. Based on the summer CH4 mass balance in the surface mixed layer (SML) of five small temperate lakes (surface area, SA, of 0.008-0.44 km2), 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 CH4 (δ13C-CH4) indicates direct photoautotrophic release as the most likely source of oxic CH4. Furthermore, we show that the oxic CH4 contribution to the SML CH4 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 km2. Given that global lake CH4 emissions are dominated by small lakes (SA of <1 km2), the large contribution of oxic CH4 production (up to 76%) observed in this study suggests that OMP can contribute significantly to global CH4 emissions.