Abstract
Permafrost thawing and erosion results in the enrichment of northern lakes by soil organic matter. These allochthonous inputs favour bacterial decomposition and may cause the draw-down of dissolved oxygen to anoxic conditions that promote methanogenesis. Our objective in the present study was to determine the seasonal variations in dissolved oxygen in a set of permafrost peatland lakes in subarctic Quebec, Canada, and to relate these changes to metabolic rates, ice cover, and mixing. The lakes had high dissolved organic carbon concentrations, and their surface waters in summer had greenhouse gas concentrations that were up to one (CO2) to three (CH4) orders of magnitude above air-equilibrium values, indicating their strongly heterotrophic character. Consistent with these observations, the peatland lakes had elevated rates of bacterial production and oxygen consumption. Continuous measurements of oxygen by in situ sensors and of ice cover by automated field cameras showed that the lakes became fully anoxic shortly after freeze-up. The waters were partially re-oxygenated by mixing events in spring and fall, but in one lake, the bottom waters remained anoxic throughout the year. These observations provide a foundation for subsequent biogeochemical and modelling studies of peatland thaw lakes as an abundant class of Arctic freshwater ecosystems.
Highlights
The dissolved oxygen pool in thermokarst lakes and ponds is primarily controlled by three simultaneous and interacting processes: (1) consumption of oxygen by respiration, (2) production of oxygen by photosynthesis, and (3) gas exchange with the atmosphere (del Giorgio and Williams 2005; Holtgrieve et al 2010)
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 to extend our previous results (Deshpande et al 2015) to focus on thaw lakes that occur in permafrost peatlands
Study sites We investigated five thaw lakes in a peatland valley to the north and south of the Sasapimakwananisikw River (Fig. 1) near the village of K-W, which lies in northern Quebec
Summary
The dissolved oxygen pool in thermokarst lakes and ponds (hereafter referred to as thaw lakes) is primarily controlled by three simultaneous and interacting processes: (1) consumption of oxygen by respiration, (2) production of oxygen by photosynthesis, and (3) gas exchange with the atmosphere (del Giorgio and Williams 2005; Holtgrieve et al 2010). Organic matter is known to be mainly of allochthonous origin, as a result of erosion of the surrounding permafrost soils (Olefeldt and Roulet 2012). This provides substrates for bacterial heterotrophic production and includes high concentrations of coloured dissolved organic matter (CDOM) that strongly attenuate light throughout the water column (Watanabe et al 2011), thereby reducing autotrophic phytoplankton production at depth (Shirokova et al 2009; Forsström et al 2013; Roiha et al 2015). Subarctic permafrost peatland lakes are favorable to both particle-based and free-living aquatic bacterial communities due to their high carbon and nutrient availability (Deshpande et al 2016)
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