Craig J Allan | Search USGS

USGS2025

The effects of forest harvesting on total and methylmercury concentrations in surface waters depend on harvest practices and physical site characteristics

Forest harvesting can lead to mercury (Hg) mobilization from soils to aquatic habitats and promote the transformation of inorganic Hg to highly neurotoxic and bioaccumulative methyl-Hg (MeHg). Multiple past studies reveal broad variation of stream water MeHg and total Hg (THg) concentrations responses to forest harvesting, which has confounded messaging to forest and resource managers. To advance beyond divergent and sometimes contradictory findings, we synthesize information for 23 previously studied catchments in North America and Fennoscandia, and compiled a uniform set of soil, landscape, and harvesting properties to identify forest management, riparian, and hillslope factors that influence responses of stream water MeHg and THg concentrations. From this synthesis, we found catchments with high soil moisture and organic soil layers > 100 cm to be at highest risk for disturbance-induced increases in MeHg formation after harvest, but not necessarily affecting concentrations of MeHg in stream waters. Instead, the combination of MeHg formation in soils along with factors that affect mobilization with runoff to streams most influenced how forest harvest affects MeHg concentrations in stream waters.

Environmental Science and Technology↗

USGS2025

Connecting tributary mercury loads to nearshore and offshore sediments in Lake Superior

Lake Superior has a vast and largely undeveloped watershed in comparison to the other Great Lakes, which makes it challenging to study mercury (Hg) sources and cycling. To examine Hg inputs to Lake Superior, we conducted an expansive binational assessment in 40 watersheds from a diverse range of landcover types. We further paired tributary Hg data to sediment source portfolios in the nearshore and offshore zones of Lake Superior through partnership with the Great Lakes Sediment Surveillance Program. We observed that total Hg loads were highest in the spring driven by the combination of elevated Hg concentrations and increased water discharge from snowmelt. In addition, total Hg concentrations in tributaries from remote, heavily forested regions, such as Pukaskwa National Park and the Minnesota Northshore, were higher than the Southshore and Thunder Bay regions. Methylmercury concentrations and loads were more spatially dependent, often corresponding to regions with more wetlands (e.g., Michigan Upper Peninsula). We estimated that the total Hg tributary load to Lake Superior in 2021 was 126 kg per year. To further examine the fate of watershed Hg sources, we examined sediments from 28 sites in Lake Superior using Hg stable isotopes. At open water sites, precipitation was the primary Hg source to sediments, but within nearshore sites Hg originated predominantly from watershed runoff. This work further defines the sources and fate of Hg within Lake Superior and highlights how Hg delivery is intrinsically tied to varying hydrologic regimes.

Michigan, Minnesota, Ontario, Wisconsin↗