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 Québec. The diffusive CH₄ flux was highly variable (mean 11.6 ± 26.4 SD mg m^?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 Q₁₀ of CH₄ flux was 3.7, once other covarying factors were accounted for, but this apparent Q₁₀ 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.