Atmospheric deposition of mercury (Hg) is increasing in some regions of the Western U.S. When transported to anoxic environments, inorganic Hg can be transformed into methylmercury (MeHg), a bioaccumulating neurotoxin. Simultaneously, there is evidence that American beaver (Castor canadensis) populations are increasing, and their modifications of ecosystems (e.g., construction of dams, lodges, food caches, and burrowing in the floodplain) expand anoxic conditions, highlighting a need to understand how beaver activity alter MeHg production, storage, and transport to downstream water bodies.
In this work we investigate the seasonal variation in Hg concentration, speciation, and dissolved versus particulate fractions across two stream reaches (~1.5 km), one beaver-dominated and one unimpounded. We hypothesize that, compared to the unimpounded reach, the beaver-dominated reach will 1) exhibit higher dissolved MeHg transport due to enhanced particulate settling and microbial methylation in anoxic pond sediments and 2) have the greatest MeHg enrichment during midsummer, when prolonged water residence time, warm temperatures, and anoxia stimulate high rates of microbial reduction. To test these hypotheses, we collected surface, ground, and porewater and paired stream discharge throughout the snow-free 2025 season in Coal Creek watershed near Crested Butte, Colorado. We collected samples at the inlet, outlet, and on the floodplain in paired beaver-dominated and impounded reaches and analyzed the water samples for dissolved and particulate total Hg/MeHg along with sulfate, dissolved organic carbon, and physical water quality parameters. Preliminary results suggest that hydrologic connectivity may be a stronger control on Hg cycling than beaver activity, with similar changes across the hydrograph observed in both the control and beaver dominated reach. The outcomes of this work will provide valuable information to land and water managers and restoration practitioners who must consider the expanding presence of beaver activity and its impacts on water quality and the bioavailability of MeHg in the river corridor.