Large-scale patterns in summer diffusive CH4 fluxes across boreal lakes, and contribution to diffusive C emissions

Lakes are a major component of boreal landscapes, and whereas lake CO2 emissions are recognized as a major component of regional C budgets, there is still much uncertainty associated to lake CH4 fluxes. Here, we present a large-scale study of the magnitude and regulation of boreal lake summer diffusive CH4 fluxes, and their contribution to total lake carbon (C) emissions, based on in situ measurements of concentration and fluxes of CH4 and CO2 in 224 lakes across a wide range of lake type and environmental gradients in Quebec. The diffusive CH4 flux was highly variable (mean 11.6 +/- 26.4 SD mgm(-2)d(-1)), and it was positively correlated with temperature and lake nutrient status, and negatively correlated with lake area and colored dissolved organic matter (CDOM). The relationship between CH4 and CO2 concentrations fluxes was weak, suggesting major differences in their respective sources and/or regulation. For example, increasing water temperature leads to higher CH4 flux but does not significantly affect CO2 flux, whereas increasing CDOM concentration leads to higher CO2 flux but lower CH4 flux. CH4 contributed to 8 +/- 23% to the total lake C emissions (CH4+CO2), but 18 +/- 25% to the total flux in terms of atmospheric warming potential, expressed as CO2-equivalents. The incorporation of ebullition and plant-mediated CH4 fluxes would further increase the importance of lake CH4. The average Q(10) of CH4 flux was 3.7, once other covarying factors were accounted for, but this apparent Q(10) varied with lake morphometry and was higher for shallow lakes. We conclude that global climate change and the resulting shifts in temperature will strongly influence lake CH4 fluxes across the boreal biome, but these climate effects may be altered by regional patterns in lake morphometry, nutrient status, and browning.