Total and methylmercury export in central Canadian boreal forest headwater catchments before and after forest harvest

Abstract:

Anthropogenic mercury (Hg) inputs have led to the accumulation of Hg in terrestrial and aquatic boreal environments. Fish Hg concentrations in boreal lakes are elevated relative to many other freshwater systems. Forest harvesting has been reported to mobilize Hg from soils to waterways and further exacerbate this problem. Despite the Canadian boreal forest's extensive network of waterways, high capacity for storing mercury, and numerous forestry operations, few multi-watershed studies of mercury export in watersheds undergoing active harvesting have been conducted in the region. To better understand how forest harvesting impacts mercury export we measured streamflow, filtered and unfiltered total mercury (THg) and methylmercury (MeHg), dissolved organic carbon (DOC), pH, specific conductance, temperature, and dissolved oxygen in 12 headwater streams across 3 undisturbed forested regions near Dryden, Ontario during the ice-free seasons of 2019 (pre-harvest) and 2020 (during and post-harvest). To elucidate the potential factors governing background mercury export across these catchments we examined how landscape and hydrological factors (such as mean sub-catchment slope, baseflow index, sub-catchment area coverage of soil types, primary material, open wetland, and forest type) were related to pre-harvest hydrochemical measurements. Pre- and post-harvest results were compared and patterns in THg and MeHg concentrations and loads varied among regions. Catchments underlain by fine-grained glaciolacustrine material exported significantly higher concentrations of THg than those underlain by shallow bedrock. Catchments underlain by shallow bedrock exported significantly higher MeHg loads. THg was more strongly positively correlated with DOC concentration than was MeHg, whereas MeHg was more strongly positively correlated with pH. Understanding the variation in THg and MeHg export in the boreal forest under baseline and disturbed conditions is a key step in being able to more accurately assess how different boreal watersheds may be impacted by forest harvesting and other disturbances.