Redefining the role of wetlands as methyl mercury sources

Current literature identifies boreal wetlands as net sources for the potent neurotoxin methyl mercury (MeHg). Combined with national environmental aims of restoration of previously drained wetlands, there is a possible conflict between the ecological benefits of wetlands and their role as MeHg sources. This thesis presents a four-year study of seven Swedish boreal wetlands of different nutrient status subjected to restoration measures. Wetlands were characterized according to climate/geography, vegetation type and ancillary chemistry. Mercury (Hg) biogeochemistry was assessed by determination of proxies for long- and short-term net MeHg production rates in soils, i.e., %MeHg (of total Hg) and potential methylation and demethylation rate constants (km and kd). MeHg exports from each wetland catchment were calculated. In addition, each wetland was assessed as a net MeHg source or sink, based on mass balance budgets. Results follow similar patterns for %MeHg, km/kd and budgets among the wetlands. The nutrient status of the wetlands affect the net production of MeHg, with wetlands of intermediate nutrient status, i.e., poor-fen types of wetlands, having the highest %MeHg, km/kd and the largest net output of MeHg. MeHg budget results showed that six out of seven wetlands were net MeHg sources. The MeHg output varied more among wetlands, than before and after restoration measures of an individual wetland. This suggests that the nutrient status of a wetland is more important to the MeHg production than the performed restoration measures. A nutrient-rich Alnus glutinosa swamp was a net sink for MeHg during the entire study period. A spatial analysis along a gradient into the Alnus swamp showed an increased degradation of MeHg in the swamp soil. Snapshot budgets from nine additional swamps suggest that net degradation of MeHg is a general phenomenon for Alnus swamps. Results from this thesis have implications for forest managers and landscape planners. Previously drained wetlands can be restored based on informed decisions, avoiding restoration of poor-fen types of wetlands. In addition, Alnus swamps should be maintained and restored if possible, helping to mitigate the production of MeHg in boreal landscapes