Seasonal Patterns of Total and Methyl Mercury Concentrations in Ground and Surface Waters in Natural and Restored Freshwater Wetlands in Northern New York

Among mercury species, methylmercury (MeHg) strongly bioaccumulates and biomagnifies in aquatic food chains, generally controlling the exposure that threatens human and wildlife health. Wetlands are important environments for biogeochemical transformations of Hg, as reducing conditions and wetting/drying cycles promote the production of MeHg. In recent years, nearly half of freshwater wetlands have been lost. In response, there has been an effort to restore freshwater wetlands to improve the ecosystem services they provide. Although wetlands are important landscape features that supply MeHg to downstream aquatic ecosystems, there have been few studies comparing the processing of Hg in restored wetlands with natural wetlands. I measured concentrations of Hg species and ancillary parameters in ground waters and surface waters of four natural and 16 restored wetlands in northern New York for six months, investigating the factors contributing to the differences in concentrations in Hg species among wetlands. I hypothesized that there would be no difference in Hg dynamics between restored and natural wetlands based on concentrations of THg, MeHg and ancillary measurements. Indeed, I found no obvious differences in concentrations of THg and MeHg in surface waters between natural and restored wetlands. Similar seasonal patterns of THg and MeHg concentrations were evident in both natural and restored wetlands, with higher concentrations in late spring and summer, and lower values in early spring and fall. THg concentrations in pond waters were greater than those in ground waters. Ground water stage was generally greater than pond stage, except for the low flow summer period, suggesting the flow of ground waters from the watershed into the surface waters. This pattern coupled with higher concentrations of THg in pond waters than ground waters suggests that Hg in pond waters is partly derived from direct atmospheric deposition or by mobilization from near-wetland shallow sediments, in addition to groundwater inflows. Higher concentrations of THg in pond water than ground water could also be due to loss of water associated with evapotranspiration. The percent MeHg (%MeHg) at the study wetland sites were high in both surface (43.4 ± 25.6%) and ground waters (38.8 ± 27.6%), suggesting that these wetlands are relatively efficient in converting ionic Hg to MeHg regardless if restored or natural. I observed weak or non-existent relations between concentrations of dissolved organic carbon and THg and MeHg. However, large increases in dissolved organic carbon concentrations in pond waters compared to groundwater suggest that dissolved organic matter is important in the supply of Hg to pond waters. Drying and rewetting cycles during summer in both restored and natural wetlands likely promote methylation rates and contribute to relatively high fractions of THg as MeHg. Although the %MeHg values were generally high in study wetlands, in ground waters with high concentrations of SO4 2(> 10 mg S/L) and NO3 (> 0.5 mg N/L) MeHg concentrations and %MeHg were uniformly low, suggesting some chemical limitation on methylation. Seasonal Patterns of Total and Methyl Mercury Concentrations in Ground and Surface Waters in Natural and Restored Freshwater Wetlands in Northern New York