Seasonal patterns of mercury dynamics in thermokarst lakes from sporadic permafrost.

Mercury (Hg) is a natural occurring element but is often emitted from anthropogenic sources and reaches the Arctic via long-range atmospheric transport. Organic matter (OM)-rich thermokarst lakes are characteristic features of the permafrost landscape in this region, where monomethylmercury (MMHg) production can be enhanced, as this process is mainly carried out by prokaryotes. To better understand the complex Hg biogeochemical cycle, two distinct thermokarst lakes (SAS 1A and SAS 2A) in sporadic permafrost in the Sasapimakwananistikw (SAS) River Valley, Canadian Subarctic, were sampled during winter and summer of 2022. Water column analysis showed no seasonal variation in total Hg (THg) and MMHg concentrations in SAS 2A but significantly higher THg and MMHg in winter in SAS 1A. Biogeochemical parameters affecting the activity of known methylating communities drive both inter-lake and seasonal variations in the water column. Strong correlations between MMHg and dissolved organic carbon (DOC) were found, with SAS 1A showing almost seven times more MMHg variability with DOC than SAS 2A. This difference is potentially linked to variations in OM composition between the sites. The lakes showed high THg seasonality in sediments, with higher concentrations in winter. In contrast, no seasonal variation was observed in MMHg concentrations with SAS 1A exhibiting higher values. Different divalent mercury (Hg(II)) bioavailability might explain these differences. By conducting incubation experiments with isotope-enriched Hg in the sediment, the important role of sulfate-reducing bacteria in the methylation process was revealed. This study highlights the complexity of thermokarst lakes which are increasing in Northern landscapes and might be hotspots for MMHg formation.