title: Elevated concentrations of methyl mercury in streams after forest clear-cut: a consequence of mobilization from soil or new methylation?
abstract: Concentrations of inorganic, mercuric mercury (Hg(II)), methyl mercury (MeHg) and ancillary chemistry measured in first-order streams draining 0-4 (N = 20) and 4-10 (N = 27) year-old clear-cuts of former Norway Spruce Picea abies (Karst.) forest stands were compared with concentrations in streams draining >70 year-old Norway Spruce reference stands (N = 10). Concentrations of MeHg, and ratios of MeHg TOC(-1) and Hg(II) TOC(-1), were significantly (p < 0.01) elevated in 0-4 year-old clear-cuts, as compared to references. The only ancillary variable showing a significant elevation for 0-4 year-old clear-cuts was Mn (p < 0.02). The 4-10 year-old clear-cuts showed intermediate concentrations with nonsignificant differences as compared to references. pH, nitrate, sulfate, Ca, Fe, TOC, TON, and the aromaticity of TOC (SUVA(254 nm)) showed nonsignificant differences between clear-cut age classes and references. Assuming that MeHg and Hg(II) are mobilized from soil to stream to a similar relative extent as a consequence of clear-cutting, a calculation showed that (1)/(6) of the elevated MeHg concentration was due to enhanced mobilization from soil and (5)/(6) was due to new methylation of Hg(II) 0-4 years after clear-cut. New methylation after clear-cut is suggested to be stimulated by an increased availability of electron donors for methylating bacteria, as a consequence of degradation of logging residue ("slash") and soil organic matter. A subdivision of sites situated above and below the highest postglacial coastline (HC) revealed a significant elevation of MeHg, MeHg TOC(-1) and Hg(II) TOC(-1) (p < 0.05) beyond their references in 0-4 year-old clear-cuts above (but not below) the HC. This suggests that postglacial deposits of FeS(s) and FeS(2)(s) were not an important factor for elevation of MeHg after clear-cut.
authors.affiliation: Department of Forest Ecology, Swedish University of Agricultural Sciences, SE-90183 Umeå, Sweden. ulf.skyllberg@sek.slu.se
authors.forename: Ulf
authors.initials: U
authors.lastname: Skyllberg
authors.forename: Mattias Björkman
authors.initials: MB
authors.lastname: Westin
authors.forename: Markus
authors.initials: M
authors.lastname: Meili
authors.forename: Erik
authors.initials: E
authors.lastname: Björn
mesh_terms: D016468:Forestry / Q000379:methods*; D008670:Metals / Q000737:chemistry; D008767:Methylmercury Compounds / Q000737:chemistry*; D045483:Rivers / Q000737:chemistry*; D012987:Soil / Q000032:analysis*; D014874:Water Pollutants, Chemical / Q000737:chemistry*
chemical_list: D008670:Metals; D008767:Methylmercury Compounds; D012987:Soil; D014874:Water Pollutants, Chemical