The influence of humic substances on the speciation and bioavailability of dissolved mercury and methylmercury, measured as uptake by Chaoborus larvae and loss by volatilization

Abstract

The influence of dissolved humic substances (HS) on the bioavailability of dissolved inorganic and methyl mercury (Hg) was quantified by measuring the direct uptake of 203Hg in Chaoborus larvae using laboratory microcosms containing artificial freshwater. The animals were exposed individually in triplicate aquaria at 10 different concentrations of HS covering the whole range found in natural freshwaters (0–110 mg C l −1). Mercury-203 concentrations were monitored repeatedly in the same individuals and in their ambient water during up to 10 days. Near-steady state Hg concentrations in Chaoborus were usually reached within 5 days. The bioconcentration factor (BCF, direct uptake only) for the larvae in the absence of HS was 0.55±0.09 (S.E.) ml individual −1 for inorganic Hg and 5.3±0.7 ml individual −1 for methyl Hg, thus showing a 10-fold difference. Normalizing to the organic carbon content of the larvae yields a BCF OC in the absence of HS of 2.8±0.4×10 3 ml (gC) −1 for inorganic Hg and 2.7±0.3×10 4 ml (gC) −1 for methyl Hg. The uptake of both inorganic and methyl Hg decreased markedly with increasing concentration of HS. For inorganic Hg, the decrease in uptake was most pronounced at HS concentrations below 0.2 mg C l −1. For methyl Hg, the relationship between uptake and log([HS]) was sigmoid, showing a reduction by >90% when increasing HS concentrations from 1 to 50 mg C l −1. Similar patterns were observed for losses of Hg from the water phase, mainly through volatilization. These results have implications for both the biouptake and the abiotic cycling of Hg in natural ecosystems and suggest that most dissolved inorganic Hg is bound to dissolved organic matter in most natural freshwaters, whereas dissolved methyl Hg is bound only in humic waters. Assuming that only free aqueous Hg is taken up by the organisms, the rather simple methodology employed here can be used for estimating distribution coefficients ( K OC) for Hg between HS and water. In this study, the K OC values were 2.5±0.7 (S.E.)×10 7 ml (gC) −1 for inorganic Hg and 1.5±0.6×10 5 ml (gC) −1 for methyl Hg. Values of similar magnitude were derived from observed losses of Hg from the water phase, supporting the assumption of an immobilization of both inorganic and methyl Hg by HS. The strong negative influence of dissolved HS on the bioavailability of both inorganic and methyl Hg in freshwater suggests that the high Hg levels often found in fish from humic lakes in the boreal forest zone cannot be explained alone by direct uptake of methyl Hg from the water phase into biota at low trophic levels.