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