The effects of clear-cutting on stream-water chemistry in northern Sweden remain largely unexplored. Here we report data collected during a reference period and the first two years after logging in two typical partially harvested northern catchments; the objective was to compare water chemistry along the stream with and without a forest buffer. Two typical uncut reference catchments are included for comparison. Runoff was measured at the outlet of each catchment, and water samples were generally taken every second week and analyzed for 20 constituents. Logging resulted in increased runoff and increased concentrations of sodium, potassium, chloride, total nitrogen, total phosphorus, and suspended material from both catchments. Nitrate (NO₃⁻) leaching increased only from the catchment without a forest buffer. It has not yet been possible to evaluate fully the effects of the forest buffer on the NO₃⁻ leaching because the uphill clear-cut area leached minimal amounts of NO₃⁻.

To understand how forest harvest influences the aquatic environment, it is essential to determine the changes in the flow regime. This paper presents changes in the hydrological regime during the first 2 y after harvest in two catchments of the Balsjö Catchment Study in Sweden. The changes were judged relative to a reference catchment, calibrated during an 18-mo pretreatment period starting in September 2004. From August 2006 through March 2008, there was an average of 35% more runoff from the harvested catchments relative to the reference. The flow increased most during the growing seasons and at base flows (<1 mm d⁻¹; 58–99% increase), followed by dormant season and intermediate flows (30–43%). No significant changes were observed during the highest flows (over 5 mm d⁻¹), except for the spring flood a few weeks after harvest, which was delayed and attenuated. Large relative changes in low flow may influence the ecosystem by altering the aquatic habitat.

Forest harvest is hypothesized to increase the mercury (Hg) load in aquatic ecosystems. The Balsjö paired catchment study examined the outputs of methylmercury (MeHg) and total mercury (Hg_tot) from two boreal catchments during the 2 y following forest harvest but prior to site preparation. This enabled us to separate the effect of the two operations that followed best management practices. Hg_tot concentrations increased by approximately 15%, and fluxes by 20–30%. The MeHg concentrations and fluxes either declined or increased by up to 60%, depending on whether annual MeHg peaks during summer low flows were considered to have been influenced by forest harvest. The lack of a severalfold increase in Hg outputs after forest harvest, as reported from other sites, may be the result of minimal soil disturbance during the winter forest harvest operations. If so, there may be a greater Hg response after soil scarification to prepare for planting.

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.

To determine if forestry affects stream water dissolved organic carbon (DOC) concentrations, we conducted high frequency water sampling at a clear-cut catchment experiment in northern Sweden 1 year after harvesting. The overall finding was that harvesting significantly increased stream water DOC in these boreal forest catchments, at least during the growing season. The results indicate a DOC concentration increase of up to 50% during early summer on the two harvested catchments relative to the two control catchments. The analysis supports the hypothesis that a raised groundwater level following harvesting caused the increased DOC concentration during both hydrological episodes and low flow conditions. Harvesting resulted in a 70% increase in DOC export due to the combined effect of runoff and DOC concentration during the June–October study period. Given the extent of forestry activity in the boreal landscape, these results demonstrate that tree harvesting will affect the water quality of the region.

The objective of this study was to add to a traditional forest planning model by incorporating some consideration of the effects of forestry on dissolved organic carbon (DOC) concentrations in streams over time. In a case study, for a watershed in northern Sweden, we present the best possible solution to a forest planning problem that maximizes the net present value (NPV) while the DOC concentration levels in the watershed are maintained below a defined threshold value. Results from the case study show that the decrease in NPV, when taking DOC into account, was considerable. However, the decrease in possible harvest volume was restricted in the case study area because the model moved harvesting activity from the first 20-year period to later periods to avoid high initial DOC concentrations. The model presented could be a useful tool for predicting the effect of forestry on DOC concentrations over time.

The European Union (EU) member states have agreed on the Water Framework Directive. The vision is that all European waters will achieve “good” ecological status. Each member state has agreed to meet these commitments. In Sweden, the Environmental Protection Agency regulates the methods used for classification of the ecological status of inland and coastal waters. The aim of this study was to evaluate how these criteria (using diatoms, benthic fauna, fish, water chemistry, hydromorphology) principally affect the classification of two typical forest streams. Forestland constitutes approximately 70% of the Swedish land area and forestry is the dominating human impact on many waters. Particular attention was paid to evaluate how the classification may vary with catchment size and between years. The results indicate that there is an obvious risk that many Swedish forest streams will not achieve “good” ecological status. The classification outcome may vary between years and regarding fish status; it also seems to depend on stream size.