Diurnal variations in greenhouse gas emissions from two contrasting thermokarst lakes in Nunavik, Canada

Small thermokarst lakes, formed by the thawing of ice-rich permafrost, can emit significant amounts of methane (CH₄) and carbon dioxide (CO₂) to the atmosphere. The physical processes behind diurnal variations in greenhouse gas (GHG) emissions from thermokarst lakes remain poorly understood due to a lack of observational data in subarctic regions. This study focuses on the dynamics of GHG emissions from two small lakes (< 200 m²) located in the Tasiapik Valley, near the village of Umiujaq, Nunavik, Canada (56°33'28.8"N 76°28'46.5"W). One lake is characterized by a more humic and sheltered environment, while the other is characterized by greater transparency and exposure to wind. Continuous measurements of temperature, conductivity, and oxygen in the water column, as well as meteorological conditions (wind, pressure, heat exchanges) have been conducted since October 2021. CO₂ fluxes measured using a floating chamber and dissolved gases (CO₂, CH₄, N₂O) at the surface were measured on a daily cycle for 2-week periods in July 2022 and August 2023, with bubble traps quantifying ebullition rates. Diffusive CO₂ fluxes are in line with estimates for other thermokarst lakes, ranging from –2 to 17 mmol m^–2 d^–1 in July 2022 (during a particularly cold period) and from 8 to 66 mmol m^–2 d^–1 in August 2023, a period of stronger stratification. Turbulence, characterized by the gas transfer coefficient k₆₀₀, was higher in 2023 (0.4 to 10.4 cm h^–1) than in 2022 (1.1 to 4.7 cm h^–1). CH₄ emission through ebullition was more than 6 times higher than through diffusion in the more humic and sheltered lake (13 ± 5.5 mol m^–2 d^–1) and almost 7 times higher than in the more transparent and exposed lake (2 ± 1.5 mmol m^–2 d^–1), where ebullition was of the same order of magnitude than diffusion. Diurnal cycles were characterized by the nocturnal mixing of surface waters with deeper waters enriched in CO₂, leading to a peak in CO₂ fluxes in the morning, which gradually decreased over the course of the day with the establishment of thermal stratification due to solar radiation, and the potential uptake by primary production. Overall, these results highlight the complex interactions between environmental factors influencing GHG emissions in thermokarst lakes, and the major differences that can exist between adjacent lakes.