Cumulative Effects of Beaver Impoundments and Forest Harvest on Surface Water Mercury Concentrations in Boreal Watersheds

Studies in boreal regions have shown that natural wetlands and artificial reservoirs contribute to high levels of the bioaccumulative neurotoxin methylmercury (MeHg) in downstream waters, stream sediments, and aquatic organisms. Higher methylation rates are thought to result from increased anaerobic microbial activity in saturated, organic-rich soil and sediment.

Over the past century, beaver (Castor canadensis) populations have increased steadily in response to decreased predation, resulting in an increase in dams and flooded forest areas. The creation of beaver impoundments promotes the accumulation and subsequent decay of organic matter, leading to enhanced anaerobic microbial activity, which is a precursor for mercury (Hg) methylation. Although these impoundments are ubiquitous natural reservoirs in the Canadian boreal region, impacts of beaver impoundments resulting from dams in various stages of preservation are not well quantified, particularly in areas experiencing additional disturbances thought to impact stream mercury, such as forest harvest.

This study aimed to determine the combined impacts of (a) active and abandoned beaver dams in (b) recently (< 5 years) and previously (>10 years) harvested watersheds on stream mercury. Water samples were taken upstream and downstream of 5 beaver impoundments and in 2 reference stream reaches in harvested and unharvested watersheds near Dryden, in northwestern Ontario, Canada, from June to October 2021. Though total Hg concentrations generally increased downstream of impoundments in watersheds that experienced forest harvest, neither harvest status nor beaver dam preservation stage had a significant impact on MeHg concentrations or methylation efficiency. Indeed, downstream MeHg concentrations were lower than or equivalent to upstream concentrations for all but one site. This suggests that catchment characteristics may have a greater influence on the Hg impact of a beaver impoundment than catchment harvest status or dam preservation stage. Understanding the Hg impacts of dams in conjunction with forest harvest is important for predicting the ultimate effectiveness of management decisions aimed at mitigating mercury risks.