Inundation of forested landscapes by human induced rewetting and beaver dams – effects on dissolved carbon and greenhouse gases in runoff

Rewetting of previously drained wetlands in the forested landscape is suggested as an efficient Nature Based Solution to combat multiple environmental challenges e.g. to protect biodiversity, improve water resilience, and reduce greenhouse gas emissions. In parallel with this human induced re-creation of wet areas there are also non-human activities i.e. damming by beavers, that that could have a similar water retaining function. However, the consequence of these different water retaining actions on carbon exported via runoff is often ignored in climate benefit and water quality assessments. Here we explored the effect of rewetting and beaver dams on total organic carbon (TOC), carbon dioxide (CO₂) and methane (CH₄) in runoff by comparing data collected in a coordinated sampling campaign across hemiboreal Sweden. In total, runoff from 68 sites were sampled across five site types including rewetted sites (n=15), beaver dams (n=14) and pristine wetlands (11). To assess the effect of rewetting and beaver dams, runoff samples was also collected in drained wetlands (n=15) and at sites without beaver dams (n=14), both used as reference sites.

It was evident that rewetted sites stood out and displayed higher runoff TOC and CO₂ concentrations (on average 2-fold higher) than observed in all other site types. In contrast, no difference in TOC or CO₂ concentrations was observed among the remaining four site types. Rewetted sites also showed the highest CH₄ concentrations, 5-fold higher than observed in pristine and reference (drained wetlands and sites without beaver dams) sites. However, CH₄ concentrations in rewetted sites were not statistically different than observed in beaver dams.  Collectively, this suggest that rewetting cause elevated runoff concentrations of all major carbon forms compared to both drained and pristine wetlands. In addition, beaver dams were identified as potential hotspots for CH₄ formation despite not showing any elevated runoff concentrations of the other carbon forms. We attribute the effect of rewetting on runoff carbon to an enhanced connectivity to carbon-rich terrestrial sources, but also to enhanced metabolic production under anoxic conditions, as rewetted sites showed lower oxygen in runoff than all other site types. We further speculate that the elevated CH₄ in beaver dams could be linked to another type of organic substrate, as beaver dams are not necessarily located in areas with peat-rich soils. The findings from the current study suggest that rewetting of drained wetlands has implications for both climate and downstream water quality, and that those implications might differ compared to effects from non-human inundation (i.e. beaver dams).