Frigörelse av kvicksilver och metylkvicksilver till bäckvatten under olika perioder efter skogsavverkning

Sammanfattning: High levels of Hg in fish in Swedish lakes have been a problem along time in Sweden. The major concern is production and accumulation of the most toxic form of mercury; methyl mercury (MeHg). Consequently, in about half of our 83 000 lakes fish contain concentrations above the recommended limit for human consumption of 0,5 mg mercury per kg fresh weight (most of it in the form of MeHg). Consumption of fish containing too high levels of MeHg may cause problems for both humans and animals. Three major sources of MeHg in surface waters are: 1) runoff from terrestrial areas, 2) deposition from the atmosphere, and 3) internal production of MeHg in lakes and streams. The aim of this study was to investigate the effect of clear-cutting on the concentration of Hg and MeHg into draining streams. Increased Hg export is due to an increased mobilization of complexes associated with inorganic sulphides and/or dissolved organic matter (DOM), whereas increase in MeHg concentrations may in addition be due to an enhanced production in soils of clear-cut areas. The latter process is called methylation and mediated by sulphate and iron reducing bacteria (SRB and FeRB, respectively). The activites of SRB and FeRB may be limited by the presence of sulphate and iron (III), respectively. In terrestrial systems above the highest coast line (HCL) sulphate is supposedly less available than iron (III). This is because soils high in FeS (s) (that is oxidized to sulphate) are found only below the HCL. Therefore selection of clear-cuts above and below the HCL was one major focus in this study. It could be hypothesized that if MeHg net production is mainly related to the activity of SRB, clear-cuts below the HCL may be more of a problem (causing a high net production of MeHg due to a presumably higher availability of sulphur in soils) than clear-cuts above the HCL. In the study 24 clear-cuts and 4 reference sites above and 25 clear-cuts and 6 reference sites below HCL were selected. All clear-cuts were at least covered by 70% Picea abies prior to harvest and the references were all mature (> 70 years) Picea abies stands. Streams originating within the clear-cut and draining most of its area were sampled during 2-week period in September and October 2007. Single-factor ANOVA and Two-tailed Dunnett´s test were used to statistically evaluate the results. As expected, concentrations of sulphate were significantly higher below as compared to above the HCL (p=0.004), when all sites (including references) were compared. No significant differences were observed for concentrations of DOC, Fe and Hg above and below HCL. Interestingly enough, concentrations of MeHg (absolute or normalized to Hg or DOC) were significantly higher above the HCL (p<0.03). This result suggests that MeHg was either mobilized and/or produced at higher extent above the HCL. When all sites were compared (independent on their position in relation to the HCL), the concentrations of MeHg (absolute and normalized to DOC) and Hg (normalized to DOC) was significantly (p<0.01) higher in streams draining 0-4 years-old clear-cuts than in reference stands. Concentrations of Hg-tot were significantly higher at p<0.05. The only ancillary parameter which showed a statistic significance was the C/N-ratio, which was lower (p<0.01) in the 0-4 year-old clear-cuts. The lower C/N ratio may indicate a contribution from biomass (microorganisms) and low-molecular-weight organic molecules in the streams of the young clear-cuts. If so, the high MeHg concentrations may be due to a higher availability of energy-rich electron-donators to SRB and FeRB at the youngest clear-cuts. When only sites above the HCL were considered, concentrations of MeHg were significantly higher in streams draining 0-4 years old clear-cuts (p<0.01 for absolute concentrations and p<0.05 when normalized to Hg or DOC), as compared to the references. For the older clear-cuts (4-10 years after harvest) no statistical differences were observed in relation to the references. Below the HCL no significant differences at p<0.05 were observed. It cannot with certainty be resolved whether an increased mobilization or a net-production of MeHg (or both processes) caused the enhanced concentrations of MeHg at 0-4 years-old clear-cuts, as compared to the references. Concentrations of inorganic Hg and Fe were significantly correlated to DOC, which is expected given the very strong complexation of Hg and Fe(III) to organic matter under oxidized conditions. The Hg/DOC ratio may be taken as a measure of Hg mobilized in the soil together with inorganic sulphides. Similarly, the Fe/DOC ratio may be taken as an estimate of Fe(II) (an indirect measure of the activity of FeRB). MeHg was only weakly correlated to DOC, indicating that a net production of MeHg was contributing to the concentration of MeHg in streams of young clear-cuts. This assumption was further corroborated by significant relationships between MeHg and the Fe/DOC ratio in streams. Correlations observed between MeHg and Fe/DOC (significantly, positive) and sulphate (weakly negative) indicate that FeRB may play a larger role than previously recognized. If that is correct, the focus on low-lying terrain susceptible to sulphate reduction as problematic soils after clear-cut should be modified to include more well-drained soils that after a clear-cut get intermediately reduced (so called sub-oxic conditions). Further research is needed to establish the relative importance of iron(III) and sulphate reducing conditions for MeHg production and export as a consequence of forestry activities. Not until these issues are resolved, recommendations on how to modify forestry practices to minimize the negative effects on Hg biogeochemistry and MeHg accumulation in fish cannot be established.