Using dissolved organic matter fluorescence to predict total mercury and methylmercury in forested headwater streams, Sleepers River, Vermont USA
Aqueous transport of mercury (Hg) across the landscape is closely linked to dissolved organic matter (DOM). Both quantity and quality of DOM affect Hg mobility, as well as the formation and transport of toxic methylmercury (MeHg), but only a limited number of field studies have investigated Hg and MeHg with respect to specific DOM components. We investigated these interactions at the 41-ha forested W-9 catchment at Sleepers River, Vermont, which has a long history of mercury and other biogeochemical research. We examined spatial and temporal patterns of filtered Hg fractions and dissolved organic carbon (DOC) concentration, DOM quality, and major solutes at 12 stream sites within W-9 and the downstream W-3 gage (837 ha) over five sampling campaigns including a large (79 mm) fall storm, spring snowmelt, and three seasonally contrasting base flow periods. Filtered total Hg (THg), MeHg, and DOC concentrations increased in order base flow < snowmelt < fall storm, except that MeHg remained at baseflow levels during snowmelt. Ranges of median concentrations across sites for the five campaigns were THg, <0.2–4.1 ng L −1 ; MeHg, <0.03–0.45 ng L −1 ; and DOC, 0.8–14.0 mg L −1 . Humic-like DOM fluorescence components, as determined by parallel factor analysis (PARAFAC), dominated the fluorescence across sites and sampling campaigns. THg correlated strongly ( r > 0.94) with these humic components, but even more strongly with bulk DOC and absorbance at 254 nm (UV 254 ; r > 0.96), and less strongly with protein-like DOM (0.7 < r < 0.9). MeHg correlated in the same order but less strongly with humic- (0.8 < r < 0.9) and protein-like (0.6 < r < 0.8) DOM. MeHg increased in summer, potentially in response to enhanced microbial production in warmer periods. MeHg formation may have been linked to protein-like DOM, but its transport was linked to humic-like DOM.