| Summary: | Deposition, catchment runoff concentrations and fluxes, lake water concentrations of total mercury (TotHg) and methyl mercury (MeHg), and potential Hg methylation in different compartments of boreal ecosystem and TotHg concentrations of fish in boreal and tropical reservoirs were studied. The results provide new knowledge of behaviour and cycling of Hg for Hg pollution protection policy. Anthropogenic and natural Hg emissions have led to increased Hg deposition and further accumulation of Hg in soil. A decline of 50% in atmospheric TotHg deposition from the late 1980s to 2000 was observed in southern Finland. During the period of 1995–2000 TotHg and MeHg deposition remained unchanged in southern Finland. The vast storage of Hg in forest soil had a determining role as a source of TotHg and MeHg for forest drainage lakes. Only small variations in TotHg concentrations and output fluxes in runoff waters were detected among the catchments, but clearly the highest MeHg concentrations and output fluxes came from the pure peatland and the lowest from forested upland catchments. This indicates more effective MeHg production in peatlands than in uplands. Silvicultural treatment of a small spruce forest catchment increased significantly the runoff concentrations and export of TotHg and MeHg. The results indicated that clear cutting and soil treatment may significantly increase the mobility of TotHg and MeHg accumulated in forest soil and silvicultural treatment is thus an important factor for the total input of Hg and MeHg to boreal lakes. Flooding of forest soils (humus and peat) released TotHg and MeHg to water column and enhanced Hg methylation. Moreover, Hg methylation process was favoured by anoxic conditions. Flooding of soils on a large scale, i.e. when constructing man-made lakes (reservoirs) caused elevated fish Hg levels through enhanced Hg methylation. Hg accumulation as elevated TotHg concentrations in fish (especially predatory fish) was observed both in boreal (Ostrobothnia and Lapland, Finland) and tropical ...
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