273 - Mercury Export From Central Canadian Boreal Headwater Catchments Before, During, and After Forest Harvest

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 Hg, and numerous forestry operations, few multi-watershed studies of Hg export in watersheds undergoing harvest have been conducted in the region. To first elucidate the potential factors governing background Hg export, we examined how landscape and hydrological factors (such as surface geology, forest type, and young water fraction) were related to pre-harvest Hg export measured during the ice-free season of 2019 in 19 headwater streams across 5 forested regions in northwestern Ontario. Catchments within boreal shield landscapes dominated by shallow soils and exposed bedrock exported more methylmercury than catchments within glaciolacustrine plain landscapes with thicker sand deposits. Wet forest cover, defined by the proportion of catchment cover by tree species favoring wet conditions, was shown to be an important indicator of watershed Hg export. Then, to better understand how forest harvesting impacts mercury export, we repeated the same streamflow, Hg, and hydrochemical measurements during the ice-free seasons of 2020 (during harvest) and 2021 (post-harvest). Pre- and post-harvest results were compared and patterns in Hg export varied among regions. While the fraction of particulate mercury generally increased post-harvest, total mercury export was not significantly higher post-harvest except in catchments where substantial surface disturbance related to stream crossings by machinery was observed. Understanding the variation in Hg export in boreal forests under baseline and disturbed conditions is a key step in more accurately assessing how different boreal watersheds may be impacted by forest harvesting and other disturbances.

Authors: Wai Ying Lam, Rob Mackereth, Carl Mitchell