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

Abstract

Small thermokarst lakes, formed by the thawing of ice-rich permafrost, are significant sources of greenhouse gases (GHGs). 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 2 years of continuous temperature, oxygen and conductivity profiling in two small (< 200 m2) thermokarst lakes in Nunavik (56°33028.800 N, 76°28046.500 W), Canada. One campaign occurred during a colder period (8.8 °C average temperature) and the other during a warmer one (14.6 °C average temperature), 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 fourfold during the warmer summer, alongside the emergence of a diel trend, where daytime (09:00–17:00 EST) 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 could peak in autumn and spring, as the lakes accumulated large concentrations of GHG at the bottom. Our findings highlight the importance of including both daytime and nighttime measurements, as well as storage fluxes (emitted in spring and autumn), to improve the accuracy of GHG emission estimates from thermokarst lakes.