Impacts of Land Use and Hydrology on Methylmercury Concentrations in Water in Prairie Wetlands

Prairie wetlands situated across the Great Plains of North America possess many characteristics of ecosystems that offer ideal conditions for high rates of methylmercury (MeHg) production, including unique chemical and physical characteristics and high hydrologic variation. We explored two possible factors that may influence MeHg concentrations in water from ephemeral wetlands in central Saskatchewan: the roles that fluctuating water levels and the type of land use adjacent to wetland ponds may have in controlling MeHg concentrations. We sampled 28 wetland ponds for MeHg and THg concentrations and ancillary chemical characteristics. Wetland ponds were located in areas that fell into four land use categories and two hydrology patterns. Land use groupings included lands converted to native grasslands prior to 1995, land converted to native grasslands in 2004, land cultivated using non-organic methods, and land cultivated using certified organic methods. Based on the water level data during the preceding decade (2000-2009), we were able to categorize two types of patterns in respect to water levels: “Mainly Wet”- ponds which either did not dry up at all over the course of each summer or stayed wet until October of each year and “Mainly Dry” - ponds which dried up in the earlier parts of each summer. MeHg in whole water ranged from 0.110 ng L-1 to 2.617 ng L-1. Forward stepwise multiple linear regression on data from all ponds determined sulphate and dissolved organic carbon (DOC) concentrations, as well as specific UV absorption (SUVA; a measure of DOC quality) to be significant variables predicting MeHg concentrations in whole water. We did observe differences in MeHg and %MeHg concentrations in Type III wetland ponds when data were grouped into land use categories, although the differences were generally not significant. Average whole water MeHg concentrations and %MeHg were lowest in water from ponds that have recently been converted to grasslands compared to those in water from cultivated or grassland ponds. A stronger relationship was observed between water MeHg concentrations and the water level patterns. When we examined MeHg concentrations and %MeHg in these two groups of ponds, we found that both average MeHg and %MeHg concentrations were significantly higher in the Mainly Wet ponds compared to the Mainly Dry ponds. Sulphate concentrations were also significantly higher, and SUVA values significantly lower, in wet ponds compared to dry ponds. Examining environmental factors regulating MeHg production in prairie wetlands is vital to addressing fundamental gaps in understanding of environmental controls on the production of MeHg in general, and specifically, in prairie wetlands, and will provide information necessary to protect wildlife populations inhabiting these vulnerable habitats and make inferences on the impacts of climate change to these vulnerable systems.