Effects of Forest Harvesting and Residual Biomass Harvesting on Mercury Mobilization and Mercury Accumulation in a Peatland Catchment

Abstract:

There is little consensus on how different silviculture practices, such as residual biomass harvesting, impact mercury dynamics, nor are the cumulative effects on downgradient ecosystems well understood. The primary objective of this study was to measure mercury mobilization from harvested hillslopes with and without residual biomass removal, and the associated impacts on mercury cycling and bioaccumulation in a down-gradient peatland. Using a paired, before-after control-impact design, three adjacent hillslopes (“Unharvested Control”, “Biomass Removed”, and “Biomass Left”) were monitored from 2010 to 2011 (pre-harvest) and 2012 to 2013 (post-harvest) from snowmelt to freeze-up at the USDA Forest Service Marcell Experimental Forest, Minnesota. Hillslope runoff and runoff concentrations of total mercury (THg), methylmercury (MeHg), dissolved organic carbon (DOC), nitrate (NO₃^-) and sulfate (SO₄^2-) were measured. The down-gradient peatland was also sampled pre- and post-harvest for peat and macroinvertebrate mercury concentrations. Post-harvest, there was significantly (p< 0.001) greater runoff from both harvested sites, which drove increased loads of all solutes. Both THg and DOC concentrations decreased (p < 0.05) due to harvesting, while NO₃^- concentrations in the runoff increased at the Biomass Left hillslope (p < 0.05). SO₄^2- concentrations did not change following harvesting. Post-harvest there was a significant (p < 0.05) decrease in MeHg and percent MeHg of THg in the down-gradient peatland. It is possible that the increased NO₃^- load entering the peatland limited SO₄^2- reduction and subsequently decreased net MeHg production. There was no discernable change in the bioaccumulation of MeHg in peatland macroinvertebrate communities. This study provides a much-needed mechanistic understanding of the response of mercury dynamics to forest management practices, highlighting that increased solute loads from hillslopes do not necessarily stimulate mercury methylation and biotic impacts to downstream peatlands.