Size distribution of methylmercury associated with particulate and dissolved organic matter in freshwaters.

Water samples were collected from 20 wetland, river and lake sites across Eastern Ontario and Western Quebec to investigate the distribution of methylmercury (MeHg) associated with various size fractions of dissolved organic matter (DOM). Tangential Flow UltraFiltration (TUF) was used to fractionate DOM by nominal molecular size (<0.2 microm, <300 kDa, <30 kDa, <5 kDa and <1 kDa). DOM fluorescence (DOM FL) and absorbance (DOC Abs) were used to quantify DOM photoreactivity and aromaticity in each sample. Significant differences in the size-associated distribution of MeHg, Dissolved Organic Carbon (DOC), DOM FL, and DOM Abs were observed between wetlands, rivers, and lakes. The low molecular weight (LMW) fraction (<5 kDa) in wetlands contained the majority of MeHg (70.0+/-13.8%), DOC (56.1+/-9.4%), and DOM FL (77.4+/-7.5%). DOM FL was also high in the LMW fraction for rivers (60.6+/-25%) and lakes (75.2+/-16.9%). Mean MeHg concentrations in the LMW fraction of lakes (41+/-26 pg L(-1)) and rivers (32+/-19 pg L(-1)) were substantial but much lower than wetlands. Rivers had the highest percentage of methylmercury (38.0+/-23.5%) in the particulate (>0.2 microm) fraction. This research highlights the importance of low molecular weight dissolved organic matter in methylmercury fate. For example, a large proportion of MeHg was found in the LMW weight fractions (mean=47.3+/-25.4%) of the wetlands, rivers, and lakes in this study.