Mercury dynamics in the pore water of peat columns during experimental freezing and thawing

Biogeochemical processes in northern peatland ecosystems are influenced by seasonal temperature fluctuations that are changing with the climate. Methylmercury (MeHg), commonly produced in peatlands, affects downstream waters; therefore, it is important to understand how temperature transitions affect mercury (Hg) dynamics. We investigated how the freeze-thaw cycle influences belowground peat pore water total Hg (THg), MeHg, and dissolved organic carbon (DOC). Four large, intact peat columns were removed from an ombrotrophic peat bog and experimentally frozen and thawed. Pore water was sampled across seven depths in the peat columns during the freeze-thaw cycle and analyzed for THg, MeHg, and DOC concentrations. Freezing results showed increased concentrations of THg below the ice layers and limited change in MeHg concentrations. During thawing, THg concentrations significantly increased, whereas MeHg concentrations decreased. Limited bromide movement and depth decreases in THg and DOC concentrations were associated with increased bulk density and degree of humification in the peat. The experiment demonstrates the effects of the freeze-thaw cycle on Hg concentrations in northern peatlands. Changes to freeze-thaw cycles with climate change may exacerbate Hg cycling and transport processes in peatland environments.