Dissolved gaseous mercury in peat porewater increases as trophic status decreases

Peatlands are long-term sinks of mercury (Hg) and hotspots of neurotoxic methyl-Hg production. Dissolved gaseous mercury (DGM or Hg⁰_aq) is a key component in peatland Hg cycling due to its high volatilization and potential oxidation to Hg^II, the precursor of neurotoxic methyl-Hg (MeHg). Despite the importance of DGM in Hg cycling, DGM dynamics in peatlands remain poorly understood. Here, we present temporal changes in porewater DGM concentrations in six different boreal peatlands with a gradient of trophic levels and chronologies. DGM concentrations are significantly higher in older, more oligotrophic ones (> 2500 years) than in younger, more mesotrophic peatlands (< 1500 years). There is also a more pronounced seasonal variation in older, more oligotrophic peatlands, with a marked DGM peak in summer. The lower DGM concentrations in younger, more mesotrophic peatlands are likely due to stronger oxidation capacity in the presence of chloride and more nutrients, leading to less DGM production in the porewater and more bioavailable Hg^II for methylation. Our study suggests that older, more oligotrophic peatlands with lower MeHg concentrations likely have the potential to evade more Hg⁰ into the atmosphere in the boreal landscape.