Methylmercury in surface runoff from beaver- and human-induced inundations in the forest landscape

Rewetting of previously drained wetlands is a strategy to mitigate the effects of climate change via increased carbon sequestration as well as improve water holding capacity and biodiversity in the forest landscape. The damming of watercourses by beavers creates similar water retention features. In both cases, the inundation of forest soils may affect the mobilization of mercury (Hg), and subsequent production of toxic and bioavailable methylmercury (MeHg), due to the generation of anoxic conditions where microbial methylators can proliferate. Elevated MeHg concentrations in water may accumulate in the food web and increase the already high Hg levels in inland fishes in Sweden. Here, we studied the impacts of rewetting and beaver ponds on total Hg (THg) and MeHg concentrations in runoff water collected throughout Sweden.

A total of 72 sites were sampled, comprising rewetted sites (n=15), beaver dams (n=15), and pristine wetlands (n=12). Each of the inundated sites was paired with a nearby reference, i.e. rewetted sites were compared to drained wetlands (n=15) and beaver ponds to watercourses without beaver dams (n=15).

Concentrations of MeHg were higher in runoff from both rewetted and beaver-impacted sites compared to natural wetlands, but only beaver ponds exhibited significantly higher MeHg concentrations relative to their reference sites. Meanwhile, THg concentrations were significantly higher in rewetted sites compared to drained sites. A critical difference between rewetted sites and inundations caused by beavers is that the latter will be found on a wide variety of land types, whereas the former typically represent previously drained wetlands, often peatlands. These results indicate that rewetting drained wetlands increases the mobilization of THg but does not promote the formation of MeHg to the same extent as inundation of other terrestrial soils, including upland soils, by beavers.