Inputs, storage, and transport of total and methyl mercury in two temperate forest wetlands

The fluxes and pools of total mercury (THg) and methyl mercury (MeHg) were evaluated for two northern temperate forest wetlands: an abandoned beaver meadow and a riparian wetland. Mass balance calculations reveal that the wetlands are net sources of THg and MeHg, although, the magnitude of the source is a function of wetland connectivity to stream water. The volatilization rate of elemental mercury (Hg-o) in the beaver meadow wetland is of similar magnitude compared to total atmospheric Hg deposition and constitutes an important component of the overall annual Hg budget. In contrast, annual Hg-o fluxes exhibited net transfer from the atmosphere to soil at the riparian wetland possibly because volatilization rates were retarded by slower diffusion of Hg-o through soil to the air-water interface in water-saturated soil. The storage of THg and MeHg in wetland soil is a large pool which appears coupled with carbon and sulfur accumulation. In the current scenario of decreasing Hg emissions and atmospheric deposition, the large active soil pool in wetlands is a potential short-term and long-term source of Hg and MeHg to downstream aquatic ecosystems.