Winter Accumulation of Methane and its Variable Timing of Release from Thermokarst Lakes in Subarctic Peatlands

Abstract

Abstract Previous studies of thermokarst lakes have drawn attention to the potential for accumulation of CH 4 under the ice and its subsequent release in spring; however, such observations have not been available for thermokarst waters in carbon‐rich peatlands. Here we undertook a winter profiling of five black‐water lakes located on eroding permafrost peatlands in subarctic Quebec for comparison with summer profiles and used a 2‐year data set of automated water temperature, conductivity, and oxygen measurements to evaluate how the annual mixing dynamics may affect the venting of greenhouse gases to the atmosphere. All of the sampled lakes contained large amounts of dissolved CH 4 under their winter ice cover. These sub‐ice concentrations were up to 5 orders of magnitude above air equilibrium (i.e., the expected concentration in lake water equilibrated with the atmosphere), resulting in calculated emission rates at ice breakup that would be 1–2 orders of magnitude higher than midsummer averages. The amount of CO 2 dissolved in the water column was reduced in winter, and the estimated ratio of potential diffusive CO 2 to CH 4 emission in spring was half the measured summer ratio, suggesting a seasonal shift in methanogenesis and bacterial activity. All surface lake ice contained bubbles of CH 4 and CO 2 , but this amounted to <5% of the total amount of the dissolved CH 4 and CO 2 in the corresponding lake water column. The continuous logging records suggested that lake morphometry may play a role in controlling the timing and extent of CH 4 and CO 2 release from the water column to the atmosphere.