Mercury stocks and methylmercury production remain unaffected by 17 years of experimental permafrost thaw in a Swedish sub-Arctic peatland

Mercury (Hg) has been stored in permafrost peatlands for millennia. As permafrost thaw is predicted to increase with ongoing climate warming, Hg is at risk to be remobilized from those peatlands and could potentially form hotspots for Hg methylation. Monomethylmercury (MeHg) is a known neurotoxin and a health concern to northern communities if Hg is remobilized, transformed to MeHg and bioaccumulated in the food chain. It is uncertain how Hg cycles in thawing permafrost systems and how much of it could potentially be remobilized by thaw processes. In this study, we have investigated Hg dynamics in a permafrost peatland of northern Sweden, where a snow-fence field experiment was set up in 2005 to simulate accelerated permafrost thaw through winter warming. We compared total mercury (THg) and MeHg concentrations in soil plots representing intact versus thawed permafrost conditions, investigated seasonal variations and examined how microbial community composition influences MeHg concentration. We found that permafrost thaw had no impact on overall THg (7.9+/-3.4 vs. 7.4 +/-2.7 mg m-2) or MeHg stocks (0.029 +/-0.012 vs. 0.059+/-0.048 mg m-2) in peat and that thaw-affected plots did not indicate increased MeHg formation. While putative methylators dominated in the fall and the broader microbial communities differed in both abundance and composition between seasons, permafrost thaw has not (yet) altered the Hg pool. Our findings highlight the importance of studying Hg cycling within a known time frame of permafrost thaw. Our observations are in contrast to previous studies and emphasize the complexity of Hg dynamics in thaw-affected permafrost landscapes.