Riverine exports of mercury and methylmercury from dammed and undammed rivers of Quebec, Eastern Canada

Abstract

Coastal rivers represent an important flux of mercury (Hg) to marine systems, but large uncertainties remain due to a lack of field data in key regions of the northern hemisphere. Given the increasing number of dams built on rivers globally and their influence on the Hg cycle, damming may affect broad-scale Hg budgets, but empirical studies on the contribution of dammed vs undammed rivers on fluxes at broad spatial extents remain scarce. Here we have compiled measurements of the concentrations, fluxes and yields of Hg, methylmercury (MeHg), dissolved organic carbon (DOC) concentration and quality at the mouth of 9 dammed and 21 undammed coastal rivers across Quebec, Eastern Canada. Total and methyl Hg concentrations were typically lower for dammed vs. undammed rivers (mean Hg of 1.40 vs. 2.79 ng L−1; mean MeHg of 0.10 vs. 0.13 ng L−1). The yields displayed the same pattern (mean Hg of 0.82 vs. 1.50 g km−2 year−1; mean MeHg of 0.05 vs. 0.07 g km−2 year−1) although dammed river fluxes were typically higher (mean Hg of 25.66 vs. 15.02 kg year−1; mean MeHg of 1.25 vs. 0.48 kg year−1) because of higher discharge. DOC fluxes were good predictors of Hg fluxes and could be used as proxies. We extrapolated these fluxes to the entire province of Quebec and provide a first order estimate of annual Hg flux of 454 ± 72 kg from dammed and of 1879 ± 240 kg from undammed rivers, compared to 33 ± 5 kg and 89 ± 11 kg of MeHg, respectively. These findings are in the lower range of current estimates for other northern regions and suggest that dammed rivers export mercury in amounts that are roughly proportional to river discharge.