Impacts of forest harvesting on mercury concentration, methylation and demethylation in soils and sediment in Canadian boreal forests 

Abstract

<p>Methylmercury (MeHg) bioaccumulates through terrestrial and aquatic food webs and can irreversibly damage the central nervous system, particularly in frequent fish consumers. Forest harvesting can result in increased MeHg exports from watersheds. To date, little empirical research has been conducted to examine mercury (Hg) methylation and MeHg demethylation processes happening within soils and sediment of forest-harvested watersheds; and mechanistic details remain relatively scarce. A field investigation was therefore carried out in 5 forest-harvested and 2 unharvested boreal watersheds in northwestern Ontario Canada to better understand MeHg production and degradation in this ecosystem. Total Hg (THg) and MeHg concentrations as well as first-order potential rate constants for Hg methylation and MeHg demethylation potentials (K<sub>meth</sub> and K<sub>demeth</sub>) in soils and stream sediment were determined. Specifically, these values were compared between years before (2019) and after (2020) forest harvesting activities, as well as between harvested and unharvested watersheds in the same years. We found some increases in THg and MeHg concentrations in upland soils in harvested forests, but the overall concentrations were still relatively low. Concentrations of THg and MeHg in wetland/riparian soils remained relatively consistent and some even declined in the first year after harvesting. We found considerable increases in THg and MeHg concentrations, as well as K<sub>meth</sub> in stream sediment in two of the harvested watersheds, both which had significantly narrower than normal vegetated buffer zones. These results suggest that short-term (< 1 year) mercury-related impacts of harvesting activities are mostly constrained within the harvested upland zones but may stretch into downgradient stream sediment when machinery damage is very close to streams. </p>