Effects of rewetting and ditch cleaning on total- and methylmercury concentrations in surface water

The management options for historical drainage to improve forest productivity are highly politicized in many high latitude regions, often related to potential climate and biodiversity values. However, important consequences of this legacy and current management to mercury (Hg) mobilization and the microbial formation of methylmercury (MeHg) should also be considered. As the historical ditches age, ditch cleaning could be an option to maintain forest growth rates. However, reducing the drainage capacity by rewetting peatlands are also a viable option, to increase water holding capacity, carbon sequestration and biodiversity. While ditch cleaning may cause erosion influencing Hg mobilization, flooding of peat soil during rewetting may increase the microbial formation of MeHg. Here, we studied how both recent rewetting and ditch cleaning influence MeHg and total Hg (THg) in stream runoff across Sweden.

We used two field infrastructures, (a) a spatial study where 25 rewetted wetlands and 25 cleaned ditches distributed from north to south of Sweden were sampled during two (rewetting) or three (ditch cleaning) campaigns and (b) a temporal study where two rewetted wetlands and two cleaned ditches located in Trollberget Experimental Area in the north of Sweden that were sampled at a monthly basis both before (>1 year) and after (>2 years) rewetting or ditch cleaning. All sites at both (a) and (b) had nearby references, i.e. non-restored and non-cleaned ditches.

In the spatial study (a) THg concentrations, but not MeHg concentrations, were elevated in rewetted wetlands compared to references. Concentrations of MeHg were lower in cleaned ditches than in non-cleaned, but only in those located in forests, and not in clear-cuts. THg concentrations did not differ between cleaned and non-cleaned ditches. In the temporal study in Trollberget (b), rewetting and ditch cleaning had more pronounced effects. Both THg and MeHg increased after rewetting in one of the two wetlands, and decreased after ditch cleaning.

We suggest that the observed changes on MeHg and THg concentrations were largely driven by changes in groundwater levels, where rewetting resulted in higher levels and ditch cleaning in lower levels, which resulting in altered redox conditions stimulating or inhibiting MeHg formation. Also, altered groundwater levels affected dominating hydrological flowpaths, which influenced MeHg and THg mobilization from soils to water.

Although MeHg and THg concentrations may increase initially after rewetting, these sites may also benefit from enhancing other important ecosystem services, such as promoting carbon sequestration and providing a diversity of plants, animals, and microorganisms. This research will be an important contribution when forming guidelines concerning where and how wetlands can be restored to maximize benefits and reduce potential negative impacts.