Factors Contributing to High CH4 and CO2 Efflux Rates from Thermokarst Lakes in the Rapidly Warming Hudson Bay Region

Abstract:

Thermokarst lakes and ponds that form on thawing permafrost landscapes have long been recognized as biogeochemical reactors that emit significant amounts of CH₄ and CO₂. However, there is considerable uncertainty in the exact contribution of these water bodies to the global carbon cycle, in large part because of the paucity of observations from different ecosystem types across the circumpolar North, and the incomplete understanding of factors that control the balance between methane production (methanogenesis) and loss (methanotrophy). The aim of our research was to address these gaps by focusing on thermokarst lakes in subarctic Canada (eastern Hudson Bay), primarily at the southern limit of permafrost that is experiencing rapid warming, but where limnological changes have received little attention to date. Thermokarst lakes were sampled at five geographical locations that differed in their degree of permafrost degradation, as well as in carbon content and lability. All sampled lakes were supersaturated with CH₄, with epilimnetic concentrations varying from CO₂ undersaturation in turbid mineral (lithalsa) lakes of the continuous and discontinuous permafrost landscapes, to oversaturation by several orders of magnitude of both CO₂ and CH₄ in the organic-rich (palsa) lakes, especially in the areas of highly degraded permafrost at its southern limit. Concentrations and fluxes of CH₄ and CO₂ in these palsa lakes were at or above the highest values reported for thermokarst waters elsewhere. In addition, methane oxidation experiments showed high rates of methanotrophy that substantially reduced the net emission of CH₄ from both lithalsa and palsa lake types. Our results imply that subarctic thermokarst lakes, especially those at the northward migrating permafrost margin, may be a major source of greenhouse gases as the circumpolar North continues to warm.