Role of organic matter and microbial communities in mercury retention and methylation in sediments near run-of-river hydroelectric dams

• Published in: The Science of the total environment Vol. 774; p. 145686
• Main Authors: Millera Ferriz, L., Ponton, D.E., Storck, V., Leclerc, M., Bilodeau, F., Walsh, D.A., Amyot, M.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 20.06.2021
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2021.145686

Run-of-river power plants (RoRs) are expected to triple in number over the next decades in Canada. These structures are not anticipated to considerably promote the mobilization and transport of mercury (Hg) and its subsequent microbial transformation to methylmercury (MeHg), a neurotoxin able to biomagnify in food webs up to humans. To test whether construction of RoRs had an effect on Hg transport and transformation, we studied Hg and MeHg concentrations, organic matter contents and methylating microbial community abundance and composition in the sediments of a section of the St. Maurice River (Quebec, Canada). This river section has been affected by the construction of two RoR dams and its watershed has been disturbed by a forest fire, logging, and the construction of wetlands. Higher total Hg (THg) and MeHg concentrations were observed in the surface sediments of the flooded sites upstream of the RoRs. These peaks in THg and MeHg were correlated with organic matter proportions in the sediments (r2 = 0.87 and 0.82, respectively). In contrast, the proportion of MeHg, a proxy for methylation potential, was best explained by the carbon to nitrogen ratio suggesting the importance of terrigenous organic matter as labile substrate for Hg methylation in this system. Metagenomic analysis of Hg-methylating communities based on the hgcA functional gene marker indicated an abundance of methanogens, sulfate reducers and fermenters, suggesting that these metabolic guilds may be primary Hg methylators in these surface sediments. We propose that RoR pondages act as traps for sediments, organic matter and Hg, and that this retention can be amplified by other disturbances of the watershed such as forest fire and logging. RoR flooded sites can be conducive to Hg methylation in sediments and may act as gateways for bioaccumulation and biomagnification of MeHg along food webs, particularly in disturbed watersheds. [Display omitted] •The St. Maurice watershed was impacted by logging, wildfire and hydropower dams•Sediment Hg levels were higher in the pondages created by run-of-river dams.•Sediment Hg and methylHg levels were strongly correlated to organic matter.•Hg methylation potential was related to the C/N ratio in sediments.•Hg-methylating community included sulfate reducers, methanogens and fermenters.