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Mercury (Hg) levels are alarmingly high in fish from lakes across Fennoscandia and northern North America. The few published studies on the ways in which silviculture practices influence this problem indicate that forest operations increase Hg in downstream aquatic ecosystems. From these studies, we estimate that between one-tenth and one-quarter of the Hg in the fish of high-latitude, managed forest landscapes can be attributed to harvesting. Forestry, however, did not create the elevated Hg levels in the soils, and waterborne Hg/MeHg concentrations downstream from harvested areas are similar to those from wetlands. Given the current understanding of the way in which silviculture impacts Hg cycling, most of the recommendations for good forest practice in Sweden appear to be appropriate for high-latitude regions, e.g., leaving riparian buffer zones, as well as reducing disturbance at stream crossings and in moist areas. The recommendation to restore wetlands and reduce drainage, however, will likely increase Hg/MeHg loadings to aquatic ecosystems.

Recovery from anthropogenic acidification in streams and lakes is well documented across the northern hemisphere. In this study, we use 1996-2009 data from the four Swedish Integrated Monitoring catchments to evaluate how the declining sulfur deposition has affected sulfate, pH, acid neutralizing capacity, ionic strength, aluminum, and dissolved organic carbon in soil water, groundwater and runoff. Differences in recovery rates between catchments, between recharge and discharge areas and between soil water and groundwater are assessed. At the IM sites, atmospheric deposition is the main human impact. The chemical trends were weakly correlated to the sulfur deposition decline. Other factors, such as marine influence and catchment features, seem to be as important. Except for pH and DOC, soil water and groundwater showed similar trends. Discharge areas acted as buffers, dampening the trends in streamwater. Further monitoring and modeling of these hydraulically active sites should be encouraged.

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This is an author produced version of a paper published in Science of The Total Environment. This paper has been peer-reviewed and is proofcorrected, but does not include the journal pagination.Citation for the published paper: Löfgren, S., Gustafsson, J. P., Bringmark, L. (2010) Recently, a hypothesis was presented on how various spatial and temporal factors affect 4 the DOC dynamics. It was concluded that declining sulphur deposition and thereby 5 increased DOC solubility, is the most important driver for the long-term DOC 6 concentration trends in surface waters. If this recovery hypothesis is correct, the DOC 7 levels should increase both in the soil solution as well as in the surrounding surface 8 waters as soil pH rises and the ionic strength decline due to the reduced input of SO 4 2-9 ions. In this project a geochemical model was set up to calculate the net humic charge and 10 DOC solubility trends in soils during the period 1996-2007 at two integrated monitoring 11 sites in southern Sweden, showing clear signs of acidification recovery. The Stockholm 12Humic Model was used to investigate whether the observed DOC solubility is related to 13 the humic charge and to examine how pH and ionic strength influence it. Soil water data 14 from recharge and discharge areas, covering both podzols and riparian soils, were used. 15The model exercise showed that the increased net charge following the pH increase was 16 in many cases counteracted by a decreased ionic strength, which acted to decrease the net 17 charge and hence the DOC solubility. Thus, the recovery from acidification does not 18 necessarily have to generate increasing DOC trends in soil solution. Depending on 19 changes in pH, ionic strength and soil Al pools, the trends might be positive, negative or 20 indifferent. Due to the high hydraulic connectivity with the streams, the explanations to 21 the DOC-trends in surface waters should be searched for in discharge areas and 22 peatlands. 23 24 25

Surface water concentrations of dissolved organic carbon ([DOC]) are changing throughout the northern hemisphere due to changes in climate, land use and acid deposition. However, the relative importance of these drivers is unclear. Here, we use the Integrated Catchments model for Carbon (INCA-C) to simulate longterm (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)

The mobility of mercury (Hg) deposited on soils controls the concentration and toxicity of Hg within soils and in nearby streams and lakes, but has rarely been quantified under field conditions. We studied the in situ partitioning of Hg in the organic top layer (mor) of podsols at two boreal forest sites differing in Hg deposition and climatic regime (S. and N. Sweden, with pollution declining to the north). Soil solution leaching from the mor layer was repeatedly sampled using zero-tension lysimeters over 2 years, partly in parallel with tension lysimeters. Concentrations of Hg and dissolved organic carbon (DOC) were higher while pH was lower at the southern site (means ± SD: Hg=44±15 ng L −1 , DOC=63.0±31.3 mg L −1 , pH=4.05±0.53) than at the northern site (Hg=22±There was a positive correlation over time between dissolved Hg and DOC at both sites, even though the DOC concentration peaked during autumn at both sites, while the Hg concentration remained more constant. This correlation is consistent with the expected strong association of Hg with organic matter and supports the use of Hg/C ratios in assessments of Hg mobility. In the solid phase of the overlying O f layer, both Hg concentrations and Hg/C ratios were higher at the southern site (means ± SD: 0.34±0.06 μg g −1 dw and 0.76±0.14 μg g −1 C, respectively) than at the northern site (0.31±0.05 μg g −1 dw and 0.70±0.12 μg g −1 C, respectively). However, concentrations in the solid phase differed less than might be expected from the difference in current atmospheric input, suggesting that the fraction of natural Hg is still substantial. At both sites, Hg/C ratios in the upper half of the mor layer were only about two thirds of those in the lower half, suggesting that the recent decrease in anthropogenic Hg deposition onto the soil is offset by a natural downward enrichment of Hg due to soil decomposition or other processes. Most interestingly, comparison with soil leachate showed that the average Hg/C ratios in the dissolved phase of the mor layers at both sites did not differ from the average Hg/C ratios in the overlying solid organic matter. These results indicate a simple mobilisation with negligible fractionation, despite differences in Hg deposition patterns, soil chemistry and climatic regimes. Such a straight-forward linkage between Hg and organic matter greatly facilitates the Water Air Soil Pollut (parameterisation of watershed models for assessing the biogeochemical fate, toxic effect and critical level of atmospheric Hg input to forest soils.

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