Mercury seasonal biogeochemistry in thermokarst lakes formed by palsa and lithalsa degradation

Palsas and lithalsas are permafrost landforms that are ubiquitous parts of the subarctic landscape, commonly found in areas of discontinuous permafrost. Their degradation can lead to the formation of thermokarst lakes, releasing contaminants previously stored in permafrost, such as mercury (Hg). In this study, two thermokarst lakes (KANG1 and KANG2) were sampled in the winter and summer of 2023 in the Canadian subarctic (Kangiqsualujjuaq, Nunavik). The thermokarst lakes differ in origin, with KANG1 formed through lithalsa degradation and KANG2 through palsa degradation. Water column analyses included total Hg (THg) and monomethylmercury (MMHg). Bottom lake sediments were also analyzed, and incubation assays conducted to evaluate methylation and demethylation potential, as well as the contributions of key microbial methylators. Winter conditions appeared to favor Hg methylation and accumulation in the water column. In KANG1, the fraction of Hg that is present in the form of MMHg (%MMHg) ranged from 56.8 ± 2.9% to 100% (measured as 102.7 ± 1.8%) in winter, with lower summer values averaged 19.8 ± 1.0%, suggesting potential transport to adjacent freshwater systems during spring thaw. Sediment THg concentrations were similar across seasons, but the %MMHg and methylation rates were higher in winter exhibiting the same pattern observed in the water column. Conversely, KANG2 exhibited higher THg, %MMHg and methylation rates, reaching 0.42 ± 0.02 day^-1 during summer. Incubation results highlighted the important role of sulfate-reducing bacteria (SRB) and methanogens in MMHg production in these systems and revealed distinct seasonal patterns in methylation and demethylation rates that may be linked to shifts in microbial guilds responsible for these processes.