Newly inundated forest landscapes - hotspots for mercury in water and benthic fauna

Sammanfattning: Mercury (Hg) is a widespread contaminant in high-latitude aquatic ecosystems, where it can undergo microbial transfor-mation into toxic methylmercury (MeHg). Due to its chemical properties, MeHg tends to accumulate in the aquatic food chain and will eventually end up in predatory species at the top of the terrestrial food web. This study aimed to assess the effects of new inundations in the forest landscape induced by beaver and human activity on the concentrations of Hg in water and benthic fauna. It was hypothesized that inundated systems would have elevated concentrations of THg and MeHg in water downstream of the flooded area, as well as concentrations of THg in benthic fauna. This study covered 72 sites across central Sweden including natural and restored wetlands, beaver ponds, and respective wetlands. Water was analysed for Hg, MeHg, and standard water chemistry and benthic fauna was analysed for THg. The results showed a significant increase in concentrations of THg and TOC at the outlet of restored wetlands, compared to both natural wetlands and previously drained wetlands (reference sites), indicating release of TOC and associated THg following restoration. MeHg concentrations and %MeHg of THg were significantly higher downstream of beaver impoundments, compared to their reference sites indicating new formation of MeHg. The results showed no significant effect of inunda-tion on THg concentrations in the benthic fauna. The results suggest that new inundations caused by construction of beaver dams are hotspots for Hg methylation whereas, rewetting of previously drained wetlands promotes mobilisation of THg and organic matter (OM).