COS 207-6 - Effects of beaver impoundments on surface water mercury concentrations in boreal watersheds along a gradient of forest harvest disturbance

Background/Question/Methods

Studies in boreal regions concerning the bioaccumulative neurotoxin methylmercury (MeHg) in experimental reservoirs and natural wetlands have shown that these waterbodies contribute to high MeHg levels in underlying sediments, inundated vegetation, and aquatic organisms. Beaver impoundments are natural reservoirs that are ubiquitous in the Canadian boreal region, yet the impacts of beaver impoundments on stream MeHg are not well quantified, particularly in areas experiencing additional disturbances like forest harvest. Forest management in Ontario is largely based on emulating impacts of natural disturbances, and beaver impoundments may be the most impactful natural disturbance affecting MeHg in aquatic systems against which to compare. As such, quantifying MeHg impacts of beaver impoundments in landscapes along a gradient of harvest disturbance could assist in defining acceptable export levels of MeHg post-harvest. To examine landscape factors that may play a role in determining the impact of beaver impoundments on stream mercury, water samples were taken upstream and downstream of 10 in-channel beaver impoundments along a gradient of % watershed harvested near Thunder Bay, Ontario, from July to October, 2021. To quantify the impact of beaver impoundments on stream MeHg, the ratio of downstream to upstream MeHg concentrations (D:U MeHg) was calculated for each site.

Results/Conclusions

Mercury concentrations at impoundment outflows were up to twenty times higher than at inflows, though the median ratio of outflow to inflow concentrations was much lower (1.6). Type of tree cover, % harvested area, and % watershed draining directly to the impoundment were significant predictors of D:U MeHg. Higher values of % harvest and % coniferous cover, landscape factors associated with higher MeHg concentrations in boreal streams, led to lower median D:U MeHg. These results suggest that MeHg impacts of beaver impoundments can be very substantial but are highly variable among streams, and that impoundment impacts on stream MeHg are lessened in landscapes already conducive to higher MeHg concentrations.