Temporal patterns of greenhouse gas emissions from two small thermokarst lakes in Nunavik, Canada

Small thermokarst lakes, formed by the thawing of ice-rich permafrost, are significant sources of greenhouse gases (GHG). Most estimates of emissions rely solely on daily measurements, which may bias annual flux calculations. In this study, we combined GHG flux measurements from two intensive summer campaigns with nearly two years of continuous temperature, oxygen, and conductivity profiling in two small (<200 m²) thermokarst lakes in Nunavik (56°33'28.8"N, 76°28'46.5"W), Canada. One campaign occurred during a colder summer (0.7°C above the seasonal mean) and the other during a warmer one (2.6°C above the seasonal mean), with one lake being humic and sheltered and the other more transparent and wind-exposed. Average diffusive fluxes of CO₂ (22.1 ± 20.5 mmol m⁻² d⁻¹; mean ± standard deviation) and CH₄ (14.3 ± 14.2 mmol CO₂-eq m⁻² d⁻¹) were consistent with values reported for similar thermokarst lakes, while N₂O fluxes were negligible (–0.8 ± 1.3 mmol CO₂-eq m⁻² d⁻¹). Emissions increased 4-fold during the warmer summer, alongside the emergence of a diel trend, where daytime (09:00-17:00) CO₂ fluxes increased by 47%, CH₄ by 95%, and negative N₂O fluxes by 75% relative to nighttime fluxes. Moreover, ebullitive CH₄ fluxes were six times higher than diffusive fluxes in the humic, sheltered lake, reaching 117.0 ± 44.7 mmol CO₂-eq m⁻² d⁻¹. Seasonal flux estimates indicate that emissions peaked in fall and spring, as they were almost four times higher than those in summer. Our findings highlight the importance of including both daytime and nighttime measurements, as well as storage fluxes (emitted in spring and fall), to improve the accuracy of GHG emission estimates from thermokarst lakes.