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

Abstract

Run-of-river power plants (RoRs) are expected to triple in number over the next decades in Canada. These struc-tures are not anticipated to considerably promote the mobilization and transport of mercury (Hg) and its subse-quent microbial transformation to methylmercury (MeHg), a neurotoxin able to biomagnify in food webs up tohumans. To test whether construction of RoRs had an effect on Hg transport and transformation, we studied Hgand MeHg concentrations, organic matter contents and methylating microbial community abundance and com-position in the sediments of a section of the St. Maurice River (Quebec, Canada). This river section has been af-fected by the construction of two RoR dams and its watershed has been disturbed by a forestfire, logging, andthe construction of wetlands. Higher total Hg (THg) and MeHg concentrations were observed in the surface sed-iments of theflooded sites upstream of the RoRs. These peaks in THg and MeHg were correlated with organicmatter proportions in the sediments (r2= 0.87 and 0.82, respectively). In contrast, the proportion of MeHg, aproxy for methylation potential, was best explained by the carbon to nitrogen ratio suggesting the importanceof terrigenous organic matter as labile substrate for Hg methylation in this system. Metagenomic analysis ofHg-methylating communities based on thehgcAfunctional gene marker indicated an abundance ofmethanogens, sulfate reducers and fermenters, suggesting that these metabolic guilds may be primary Hg meth-ylators in these surface sediments. We propose that RoR pondages act as traps for sediments, organic matter andHg, and thatthis retention can beamplifiedbyother disturbances of the watershed suchas forestfireand logging.RoRflooded sites can be conducive to Hg methylation in sediments and may act as gateways for bioaccumulationand biomagnification of MeHg along food webs, particularly in disturbed watersheds.