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-<i>a</i>, 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.