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Climate change is expected to alter the hydrology and vascular plant communities in peatland ecosystems. These changes may have as yet unexplored impacts on peat mercury (Hg) concentrations and net methylmercury (MeHg) production. In this study, peat was collected from PEATcosm, an outdoor, controlled mesocosm experiment where peatland water table regimes and vascular plant functional groups were manipulated over several years to simulate potential climate change effects. Potential Hg(II) methylation and MeHg demethylation rate constants were assessed using enriched stable isotope incubations at the end of the study in 2015, and ambient peal total Hg (THg) and MeHg concentration depth profiles were tracked annually from 2011 to 2014. Peat THg and MeHg concentrations and the proportion of THg methylated (%MeHg) increased significantly within the zone of water table fluctuation when water tables were lowered, but potential Hg(II) methylation rate constants were similar regardless of water table treatment. When sedges dominate over ericaceous shrubs, MeHg concentrations and %MeHg became significantly elevated within the sedge rooting zone. Increased desorption of Hg(II) and MeHg from the solid phase peat into pore water occurred with a lowered water table and predominant sedge cover, likely due to greater aerobic peat decomposition. Deeper, more variable water tables and a transition to sedge-dominated communities coincided with increased MeHg accumulation within the zone of water table fluctuation. Sustained high water tables promoted the net downward migration of Hg(II) and MeHg. The simultaneous decrease in Hg(II) and MeHg concentrations in the near-surface peat and accumulation deeper in the peat profile, combined with the trends in Hg(II) and MeHg partitioning to mobile pore waters, suggest that changes to peatland hydrology and vascular plant functional groups redistribute peat Hg(II) and MeHg via vertical hydrochemical transport mechanisms. (C) 2019 Elsevier B.V. All rights reserved.