Patterns of mercury methylation in urban artificial wetlands

English Abstract:

Surface-flow artificial wetlands (SFAWs) can effectively manage erosion, flooding, and pollutant loadings, but these vegetated aquatic habitats can also be sources of methylmercury (MeHg), a bioaccumulative neurotoxin. Mercury methylation varies at fine spatial scales and can reach very high levels in young wetlands for reasons which are poorly understood, thus impairing our ability to predict and manage MeHg accumulation. Stormwater management ponds and created habitat wetlands are two important subcategories of SFAWs which are similar in size, design, construction, and hydrology, but differ in subsequent management. We investigated the extent and fine scale distribution of total mercury and methylmercury concentrations at three to five points, defined by plant microhabitat, in three stormwater and five habitat wetlands in the Greater Toronto area. Plant biomass, area cover, and standard water quality parameters were also collected. Sites ranged in age from one to fourteen years, in size from 0.43-1.9 hectares, and in maximum sampled depth from 0-61cm. Both the total burden of MeHg in sediments, and the percent of mercury present as methylmercury (%-MeHg) were significantly lower in stormwater wetlands compared to habitat wetlands (MeHg: 0.507 ng/g dw ± 0.373 vs. 1.55 ng/g dw ± 0.859, p-value of 0.0001; %-MeHg: 1.507 % ± 1.185 vs 3.995% ± 2.227, p-value of <0.0001). Mercury methylation potential rates (K_meth) were also lower in stormwater wetlands (0.742% ± 0.643 vs. 1.252% ± 1.098), suggesting that these environments support lower abundances or activities of mercury methylating microorganisms, although these differences were not significant. Ongoing statistical analysis will explore possible relationships between parameters reflecting MeHg production and accumulation, and wetland age, plant communities and soil characteristics, which may help explain observed differences between MeHg burdens of stormwater and habitat wetlands and inform practical decisions by wetland managers.