Oxygen dynamics in permafrost thaw lakes: Anaerobic bioreactors in the Canadian subarctic

Abstract

Permafrost thaw lakes occur in high abundance across the subarctic landscape but little is known about their limnological dynamics. This study was undertaken to evaluate the hourly, seasonal, and depth variations in oxygen concentration in three thaw lakes in northern Quebec, Canada, across contrasting permafrost regimes (isolated, sporadic, and discontinuous). All lakes were well stratified in summer despite their shallow depths (2.7-4.0 m), with hypoxic or anoxic bottom waters. Continuous automated measurements in each of the lakes showed a period of water column oxygenation over several weeks in fall followed by bottom-water anoxia soon after ice-up. Anoxic conditions extended to shallower depths (1 m) over the course of winter, beginning 18-137 d after ice formation, depending on the lake. Full water column anoxia extended over 33-75% of the annual record. There was a brief period of incomplete spring mixing with partial or no reoxygenation of the bottom waters in each lake. Conductivity measurements showed the build-up of solutes in the bottom waters, and the resultant density increase contributed to the resistance to full mixing in spring. These observations indicate the prevalence of stratified conditions throughout most of the year and underscore the importance of the fall mixing period for gas exchange with the atmosphere. Given the long duration of anoxia, subarctic thaw lakes represent an ideal environment for anaerobic processes such as methane production. The intermittent oxygenation also favors intense methanotrophy and aerobic bacterial decomposition processes.Northern landscapes underlain by permafrost (perennially frozen ground) are currently in rapid transition in response to atmospheric warming (AMAP 2012). The unfrozen surface layer (active layer) has deepened in certain regions, and the vast, ancient reserves of organic carbon stored in the permafrost have begun to be mobilized (Schuur et al. 2015). Considerable research has focused on the influence of climate warming on greenhouse gas emissions from permafrost soils (e.g., McGuire et al. 2012;Elberling et al. 2013), but the lakes on permafrost landscapes and their biogeochemical responses to climate change have only recently begun to receive close attention.Thaw lakes (thermokarst lakes and ponds) are one of the most abundant freshwater ecosystem types in the circumpolar North, with a likely total area of 2.5-3. et al. 2006). Small lakes at high northern latitudes dominate the total land-water interface (perimeter) of lakes throughout the world (Verpoorter et al. 2014), and the large total perimeter of eroding permafrost soils that surround northern thaw lakes further indicates their potential biogeochemical significance. Lakes across the circumpolar North are responding in variable ways to climate warming, with drainage, infilling or evaporation of thaw lakes in some areas, but increases in their abundance and size in others (Vincent et al. 2013). In the subarctic region of eastern Canada, the southern

Oxygen depletion in subarctic peatland thaw lakes

“…These results build upon our observations from other sites in this region, and they reinforce earlier evidence of the tendency of these waters to be well stratified in summer despite their shallow depths (Laurion et al 2010;Deshpande et al 2015).…”

“…The overall aim of the present study was to examine the effects of ice cover, stratification, and mixing on seasonal changes in underwater dissolved oxygen concentrations and specifically to extend our previous results (Deshpande et al 2015) to focus on thaw lakes that occur in permafrost peatlands. This is a major class of thermokarst systems that has received relatively little attention to date, but the waterbodies of this type have been shown to have unusually high rates of methane emission to the atmosphere .…”

Landscape-gradient assessment of thermokarst lake hydrology using water isotope tracers

“…Rapid degradation of permafrost since the mid-20 th century along the eastern coast of Hudson Bay has contributed to an increase in the number of shallow thermokarst lakes (Payette et al, 2004). Thermokarst lakes constitute an important landscape feature and recent studies have documented the global implications of these aquatic ecosystems as a potential source of greenhouse gases, especially methane (Laurion et al, 2010;Comte et al, 2015;Crevecoeur et al, 2015;Deshpande et al, 2015;Przytulska et al, 2015). They are rich in dissolved organic carbon (DOC), most of which originates from thawing permafrost.…”

“…As a result, most of the lakes have anoxic bottom waters despite their shallow depth (< 5 m). This chemical gradient of oxygen, with an upper oxic and bottom anoxic layer, represents an ideal environment for anaerobic processes such as methane production (Laurion et al, 2010;Deshpande et al, 2015;Matveev et al, 2016). During water column mixing, methane is released to the atmosphere.…”

Impact of permafrost thaw on the turbidity regime of a subarctic river: the Sheldrake River, Nunavik, Quebec

“…In northern Quebec, near the southern limit of permafrost distribution, thawing of permafrost in large areas of palsas, lithalsas, peat plateaus, and permafrost plateaus has led to the reduction of permafrost extent by roughly 40% over the last 50 years (Payette et al 2004;Marchildon 2007;Vallée and Payette 2007;Fortier and Aubé-Maurice 2008;Jolivel and Allard 2013). Continued warming will lead to further degradation, releasing sediments and making previously frozen organic matter available for bacterial decomposition and recycling into bioavailable carbon and greenhouse gases (Schuur et al 2008;Deshpande et al 2015;Vonk et al 2015).…”