Metal transport in the boreal landscape-the role of wetlands and the affinity for organic matter.

Stream water concentrations of 13 major and trace elements (Al, Ba, Ca, Cr, Cu, La, Mg, Na, Ni, Si, Sr, U, Y) were used to estimate fluxes from 15 boreal catchments. All elements displayed a significant negative correlation to the wetland coverage, but the influence of wetlands was stronger for organophilic metals; 73% of the spatial differences in the normalized element fluxes could be explained based only on the wetland coverage and the affinity for organic matter, which was quantified using thermodynamic modeling. When the analysis was restrained to the smaller streams (<10 km(2)) the explanatory power increased to 88%. The results suggest that wetlands may decrease the fluxes of metals from boreal forests to downstream recipients by up to 40% at otherwise similar runoff. We suggest that the decrease in element fluxes is caused by a combination of low weathering in peat soils and accumulation of organophilic metals in peat. The model could not explain the spatial patterns for some metals with low affinity for organic matter, some redox-sensitive metals, and some metals with exceptionally high atmospheric deposition, but the results still demonstrate that wetlands play an important role for the biogeochemical cycling of many metals in the boreal landscape.