No Longer a Paradox: The Interaction Between Physical Transport and Biological Processes Explains the Spatial Distribution of Surface Water Methane Within and Across Lakes

Lakes play an important role in the global carbon cycle, emitting significant amounts of the carbonic greenhouse gases, CO2 and methane (CH4). Nearly all lake studies have reported oxygenated surface waters oversaturated with (and thus continuously emitting) CH4, yet no consistent explanation exists to account for why CH4, which is produced in anoxic zones and consumed in the presence of oxygen, remains in oxic waters across the range of lake sizes. Here, we developed a physical model that defines the spatial CH4 distribution in the surface waters of lakes as a function of CH4 transport from the littoral zone including air-water gas exchange, and tested this in a set of 14 lakes that ranged widely in size (0.07-19,000km(2)). Although the model adequately resolved the overall CH4 decline within a lake relative to distance from shore across the range of lake sizes, discrepancies between observations and predictions suggest that other processes modulate surface CH4 distributions. Coupled trends in the stable carbon isotopic signature of CH4 further indicate that the spatial pattern in 30% of the lakes was dominated by a net loss via oxidation, whereas a net input of C-13-depleted CH4 dominated the spatial pattern in 70% of the lakes, suggesting the predominance of pelagic CH4 production in the oxic epilimnia of these lakes. The spatial patterns imposed by the interaction between physical and biological processes may result in a size-dependent underestimation of whole-lake CH4 emissions when based on center samples. Whereas the actual contributions of oxidation and eplimnetic CH4 production are still not well understood, our results demonstrate that the ubiquitous CH4 oversaturation observed in most lakes can be explained through the interaction between horizontal transport of littoral CH4, air-water gas exchange and the balance between epilimnetic CH4 oxidation and production.