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

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.Plain Language Summary Waterbodies that are formed by the thawing and collapse of ice-rich permafrost ("thermokarst lakes") are known to be major sources of greenhouse emissions in northern landscapes, but the seasonal variation in these emissions is not well understood. In this study, we measured the concentrations of methane and carbon dioxide beneath the ice of five thermokarst lakes in late winter and compared these with summer concentrations and profiles. These "black-water lakes" are located in subarctic peatlands and are darkly colored because of their high concentrations of permafrost-derived, colored organic carbon. The results showed a winter accumulation of gases beneath the ice that would result in 10 to 100 times greater emissions from the surface waters at spring ice breakup than during summer open water conditions. However, continuous in situ measurements of water temperature and oxygen showed that lakes with smaller area to depth ratios may partially retain greenhouse gases that accumulated in their bottom waters throughout winter, thereby limiting the loss of methane in spring that would otherwise occur. More complete mixing occurred during fall cooling and circulation, and release of gases accumulated in the bottom waters during winter and summer may occur at that time. The exact volume of lake water that is mixed during the spring and fall periods is likely related to the wind fetch and depth of the basin. These...

Seasonal patterns in greenhouse gas emissions from thermokarst lakes in Central Yakutia (Eastern Siberia)

“…Depending on the lake type, we also found substantial seasonal variations in the amount of dissolved greenhouse gas in lake water, spanning two orders of magnitude throughout the year. Since most greenhouse gas studies are limited to summer quantification (e.g., Abnizova et al 2012; Bouchard et al 2015), such seasonal assessments are precious (Langer et al 2015; Matveev et al 2019). The wide seasonal ranges observed here underscore the need to consider such heterogeneities when attempting to estimate global greenhouse gas emissions from thermokarst lakes and permafrost landscapes.…”

“…6). Increasing concentrations under the ice cover have been noted in other Arctic, subarctic and boreal regions (Langer et al 2015;Matveev et al 2019). In productive systems, the isolation of the water mass under an ice cover quickly generates anoxic conditions favoring methanogenesis and suppressing methanotrophy.…”

“…In summer, periods of wind-induced mixing would likely generate regular venting of the greenhouse gas produced in summer by these shallow lakes, although the very high CH 4 concentrations observed in fall indicate either some release of CH 4 stored at the bottom or very high production rates during this period. Other studies on thermokarst lakes have documented that the timing and magnitude of greenhouse gas storage flux can vary between lakes even when they are subject to the same local climate conditions (Matveev et al 2019). Table 2.…”

“…However, the CH 4 released through diffusive flux, or through ebullition in deeper stratified lakes, remains in the water column long enough so that a significant fraction can be oxidized to CO 2 by methane-oxidizing bacteria (Vachon et al 2019). In winter, the CH 4 emitted through both processes can be oxidized under the ice cover (Matveev et al 2019). Significant release of CO 2 from thermokarst lakes in northern latitudes has also been documented (e.g., Abnizova et al 2012).…”

Water column gradients beneath the summer ice of a High Arctic freshwater lake as indicators of sensitivity to climate change

“…Both CH 4 and N 2 O concentrations dropped substantially over summer, consistent with active photosynthesis at the bottom of the water column at this time of year, and provision of oxygen for methanotrophy above the mats, along with more complete nitrification to nitrate and less production of N 2 O, which may be ultimately consumed by denitrifiers deep within the mats. In the absence of summer ice, these gradients would be lost, and greenhouse gases that accumulated during winter would be vented to the atmosphere at ice break-up and mixing in spring, without this opportunity for biogeochemical conversion 28 . The more oxygenated benthic environment may be less conducive to methane production, and colder water conditions may dampen all gas-producing processes.…”