The effect of warm summer 2012 on seasonal and annual methane dynamics in adjacent small lakes on the ice-free margin of Greenland

In thermally stratified lakes, the greatest annual CH4 emissions typically occur during thermal overturn events. In July of 2012, Greenland experienced significant warming that resulted in substantial melting of the Greenland Ice Sheet and enhanced runoff events. This unusual climate phenomenon provided an opportunity to examine the effects of short-term natural heating on lake thermal structure and CH4 dynamics and compare these observations with those from the following year when temperatures were within normal conditions. In this study, we present CH4 concentrations within the water column of 5 adjacent small lakes on the ice-free margin of Southwest Greenland under open-water and ice-covered conditions from 2012-2014. Enhanced warming of the epilimnion in 2012 lead to strong thermal stability and the development of an anoxic hypolimnia in each of the lakes. As a result, mean dissolved CH4 concentrations were significantly (p < 0.0001) greater under open water conditions in 2012 than in 2013. In all of the lakes, mean CH4 concentrations under ice-covered conditions were significantly (p < 0.0001) greater than under open-water conditions, suggesting spring overturn may be the period with the largest annual CH4 flux to the atmosphere. As the climate continues to warm, greater heat income and warming of lake surface waters will lead to increased thermal stratification and hypolimnetic anoxia, which will result in increased water column inventories of CH4. Additionally, continual warming will result in shorter ice cover durations, which may reduce the winter inventory of CH4 and lead to a decrease in total CH4 flux during ice-melt. Taken together, these results indicate that in Arctic lakes, a shortening of winter ice cover and increased thermal stratification during open-water conditions will lead to increased CH4 production, higher water column CH4 inventories and greater CH4 emissions at fall overturn.