The Extent and Regulation of Summer Methane Oxidation in Northern Lakes

Inland waters have a significant influence on atmospheric methane (CH4) levels. However, processes determining the strength of CH4 emissions from these systems are not well defined. Aerobic oxidation is a major sink of CH4 in freshwater environments and thus an important determinant of aquatic CH4 emissions, yet strikingly little is known about its drivers. Here we assessed the extent of water column CH4 oxidation at the whole ecosystem scale using stable carbon isotopic (C-13) mass balance of CH4 in 14 northern lakes spanning wide range of dissolved organic carbon (DOC) concentrations. We show that the extent of oxidation can vary from near zero to near complete, and for concentrations of 1.9-11mg/L, DOC is a key modulator of CH4 oxidation during the summer stratification period. Increasing DOC concentrations enhances oxidation in the upper layers by reducing light inhibition on methanotrophic activity, while reducing oxygen available for oxidation in the deeper layers. The effect of this light inhibition was also observable over the diurnal cycle. We developed simple predictive empirical models (r(2)>0.82) to estimate the extent of oxidation in the different layers of lakes for the summer period. Applying our surface layer model to a larger data set suggests that about 30% of CH4 transported to or generated within the epilimnion of Quebec lakes is oxidized during summer. Our results imply that DOC concentration, through its effect on the light regime of lakes, has the potential to affect strongly the magnitude and patterns of summer CH4 emissions.