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WP4 Metals and Metalloids
(441) The effects of DOC and forestry on the photo-production of dissolved gaseous mercury.
O'Driscoll, N.1, Lean, D.1, Loseto, L.1, Carignan, R.2, Siciliano, S.3, 1 University of Ottawa, Ottawa, Ontario, Canada2 Université de Montréal, Montreal, Quebec, Canada3 University of Saskatchewan, Saskatoon, Saskatchewan, Canada
ABSTRACT- The production of dissolved gaseous mercury (DGM) in freshwater lakes is induced by solar radiation, and is also thought to be linked to processes mediated by dissolved organic carbon (DOC). Studies investigating these processes using comparisons between lakes are often confounded by differences in DOC content and structure. In this study, we investigated the link between DOC concentrations and DGM production by using tangential ultrafiltration to manipulate DOC concentrations in water samples taken from a given lake. In this way, a range of samples with different DOC concentrations was produced without substantial changes to DOC structure or dissolved ions. This was repeated for four lakes in northern Quebec; two with highly logged drainage basins and two with minimally logged drainage basins. On two separate days for each lake, water samples (filtered free of microorganisims) with varying DOC concentrations were incubated in clear and dark Teflon bottles on the lake surface. Temperature and DGM concentrations were measured at 3.5-hour intervals over the course of 10.5 hours. Levels of DGM increased with increasing cumulative irradiation for all lakes until approximately 4000 kJ m-2 (400-750 nm, photosynthetically active radiation (PAR)), when DGM concentration reached a plateau (between 20 and 200pg L-1). Reversible first-order reaction kinetics fit the observed data well (r2 ranging from 0.59 to 0.98). The DGM plateaus were independent of DOC concentrations but differed between lakes. In contrast photo-production efficiency (DGMprod), i.e. the amount of DGM produced per unit radiation (fg L-1 (kJ/m2)-1) prior to 4000 kJ m-2 PAR, was linearly (P<0.0005) proportional to DOC concentration. Furthermore, logged lakes had a lower DGMprod per unit DOC (P<0.006) than the non-logged lakes. In these four lakes, the rate of DGM production per unit PAR was dependent on the concentration of DOC, with significant differences between lakes presumably due to different DOC structures and dissolved ions. These results may help explain why elevated levels of mercury in biota that have been observed by other researchers in lakes with logged drainage basins.
Key words: dissolved organic carbon, dissolved gaseous mercury, freshwater lakes, forestry
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