Limnological properties of permafrost thaw ponds in northeastern Canada

Arctic warming has recently accelerated, triggering the formation of thaw ponds and the mobilization of a carbon pool that has accumulated over thousands of years. A survey of 46 thaw ponds in the Canadian arctic and subarctic regions showed that these ecosystems have high concentrations of dissolved organic matter (DOM) and nutrients and are relatively productive. This activity was reflected in the optical properties of DOM that indicated a dominance of allochthonous sources but a significant contribution of low molecular weight compounds. Several subarctic ponds were stratified in summer, resulting in a hypoxic hypolimnion. Most ponds were supersaturated in CO2 and CH4, with higher gas concentrations in bottom waters. However, arctic thaw ponds colonized by benthic microbial mats showed lower CO2 concentrations, likely caused by active photosynthesis. CO2 was correlated with both the quantity and the optical properties of DOM, suggesting the significant role of dissolved compounds from melting organic soils and catchment vegetation on the balance between heterotrophy and autotrophy. The large variability observed in limnological properties of this series of ponds precludes generalisations about their role in greenhouse gas production. However, the fact that all thaw ponds were supersaturated in CH4 underscores the importance of estimating their global significance.