Seasonal Dynamics of Dissolved Methane in Lakes of the Mackenzie Delta and the Role of Carbon Substrate Quality

Dissolved CH4 among lake waters of the Mackenzie River Delta was tracked in 2014 to assess how river‐to‐lake connection times, plus carbon substrate quantity and quality, affects the patterns and dynamics of CH4. An under‐ice survey of 29 lakes, three open‐water surveys of 43 lakes, and weekly surveys of 6 lakes revealed that CH4 among lake‐waters ranged from very high concentrations at the end of winter, with highest concentrations linked to shortest annual river connection times, to considerably lower concentrations as open water progressed, with a limited concentration range among lakes by late summer. Lakes most strongly affected by thermokarst varied irregularly from this pattern and did not have the highest CH4 concentrations. CH4 among lake waters was generally related to measures of carbon substrate quantity, where relations with macrophyte biomass and dissolved organic carbon in lake water were statistically stronger than % organic matter within the lake sediments. CH4 was also directly related to the molecular weight (a[250]:a[365]) of dissolved organic matter at the end of winter, but was inversely related to this measure during open water. Carbon quality per se, after accounting for differences in carbon quantity (macrophyte biomass, dissolved organic carbon concentrations, or organic content of lake sediments), appears to play a significant role in controlling CH4 concentrations among the lake waters, particularly during winter ice cover. Carbon quality in lake sediments and of dissolved organic matter in winter lake waters appears to be as important as thermokarst augmentation of carbon quantity for enhancing methanogenesis in this lake‐rich Arctic system.