Impact of Forest Harvesting Operations on the Export of Total Mercury and Methylmercury to Aquatic Ecosystems

Forest harvesting operations have been reported to increase the levels of both total mercury (THg) and methylmercury (MeHg) in runoff water and downstream biota. In a series of full-scale experiments based on high-frequency monitoring of stream waters draining forested catchments in Sweden, we evaluated how runoff water chemistry was affected by forest harvesting as well as following site preparation for replantation and stump harvest. Stump harvesting is a forest management strategy that aims at increasing the supply of biofuels by recovering also the underground biomass when harvesting a stand. However, little is known about the local environmental impacts of stump harvest, within the catchment as well as downstream, as a result of changes in soil and surface water chemistry. Among other effects, concentrations or fluxes of THg and MeHg in soil and runoff waters may increase after such soil disturbances. The effects of forest harvesting and site preparation on runoff water chemistry were studied in three boreal catchments in northern Sweden using a paired-catchment approach (one of them an unharvested control). In the first two years after clearcutting, but before site preparation, there was no significant concentration response for either THg or MeHg relative to the control catchment, possibly due to very low soil disturbance. This raises the question of whether a major soil disturbance, such as that intentionally created by site preparation, is needed to generate the forestry effect on THg and MeHg reported in the literature, rather than just removal of trees alone. During the seven months following site preparation, MeHg and THg concentrations in the two site-prepared areas seemed to increase somewhat compared to the reference catchment. While not significant, the MeHg increase after site-preparation was 5-36% relative to the pre-harvest period and 4-8% relative to the harvest-only period. The updated assessment to be presented at the conference will include results from one full year after site preparation. The effects of stump harvesting were assessed in the south of Sweden using a paired-catchment approach, comparing a stump-harvested site with two reference sites. One was clearcut and subjected only to traditional site preparation, and one was left with intact forest. During the first year after stump harvest the stump harvested catchment showed no significant changes in the stream THg and MeHg concentrations, compared to the site prepared reference and the forested reference site. In all sites, TOC was an important factor controlling the concentrations of THg, but not of MeHg. Hydrology, being the main controlling factor for TOC concentrations, was also important and highflow episodes resulted in very high loads of THg and TOC. In conclusion, none of the treatments, harvesting, site-preparation and stump-harvesting, caused any increase in THg and MeHg concentrations as large as reported from some earlier studies. It is important to learn why the response to forestry activities is so variable with respect to THg and MeHg concentrations.