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.