A first run-of-river hydropower plant development in a permafrost-rich subarctic Canadian region: short-term fate of mercury and carbon.

A first run-of-river power plant built in Nunavik (QC, Canada) lies in a continuous permafrost zone, a substantial carbon (C) rich mercury (Hg) reservoir. Its impoundment could promote permafrost thaw and remobilize Hg and lead to the enhanced production of highly toxic methylmercury (MeHg). To elucidate how RORs can influence C and Hg dynamics and transformations in a subarctic landscape, soils, water and benthic invertebrates were sampled shortly before and after the flooding. Soil Hg concentrations were higher in the organic active layer than in frozen ground. Three months after river impoundment, MeHg concentrations and proportions in the surface organic layer of flooded soils were seven and four times higher, respectively. A similar increase was observed in surface waters of the newly created bay, where MeHg concentration and proportion were, respectively, ∼10 and four times higher. Biological MeHg concentrations increased in low trophic level organisms associated with this flooded environment, namely primary consumers (∼4×) and omnivores (∼3×). However, these rises were limited to the small (<1 km2) newly created bay, highlighting spatial heterogeneity in the production and trophic transfer of MeHg at the river scale in response to recent impoundment.