Leclerc et al. 2023 — MeHg bioaccumulation via periphyton in river food webs impounded by run-of-river dams
This study examined how run-of-river (ROR) hydroelectric dam impoundment affects methylmercury (MMHg) production and bioaccumulation in the benthic food web of the St. Maurice River, Québec, Canada. The key finding is that flooded pondage areas created by ROR dams provide favorable environments for periphyton proliferation, and these periphytic biofilms can produce and accumulate MMHg. The proportion of total mercury present as MMHg reached maximum values approximately 2.9 times higher in flooded sites compared to unflooded sites. Periphyton MMHg concentrations significantly predicted MMHg concentrations in some benthic macroinvertebrates, demonstrating a local transfer pathway. Trophic magnification slopes did not differ significantly between flooded and unflooded sectors, but intercepts were higher in flooded sectors, indicating elevated MMHg at the base of the food web rather than altered biomagnification efficiency.
Key numbers
- MMHg ratio (MMHg/THg): up to 2.9 times higher in flooded sites compared to unflooded sites
- Periphyton MMHg concentrations were a significant predictor of MMHg in benthic macroinvertebrates in flooded sectors
- Trophic magnification slopes: no significant differences between flooded and unflooded sectors
- Trophic magnification intercepts: higher in flooded sectors, indicating elevated base-of-food-web MMHg exposure
Study sites: nine sectors along ~75 km of St. Maurice River, including two ROR pondage areas (Chute-Allard: 2 km² flooded; Rapides-des-Coeurs: 3.7 km² flooded) and reference sectors.
Species analyzed: periphyton (on macrophytes and wood substrates); macroinvertebrates from 33 families and 2 subclasses; juvenile fish (primarily fallfish Semotilus corporalis and yellow perch Perca flavescens, length < 5 cm).
Methods (brief)
Field work late August 2017, 2018, 2019. Total Hg by EPA Method 7473 (DMA-80, Milestone). MMHg by distillation-ethylation-CVAFS (EPA Method 1630, Tekran 2700). Detection limit 0.1 ng g⁻¹ (DW) for both THg and MMHg. Quality control: TORT-2 certified reference material; CALA intercalibration criteria met (THg recovery 91–109%, MMHg recovery 87–104%). Stable isotope analysis (δ¹³C, δ¹⁵N) for trophic position. All concentrations reported as dry weight.
Implications
Certification: River fish and invertebrates from watersheds with ROR hydroelectric development may have elevated MeHg even absent the massive reservoir flooding associated with large dams. This is relevant to sourcing freshwater fish from Canadian rivers and potentially other boreal river systems.
Courses: Illustrates that periphytic biofilms (not just anoxic sediments) are an underappreciated gateway for MeHg entry into food webs; the ROR dam angle is a timely mitigation context.
App: Freshwater fish from boreal rivers with hydroelectric development warrant an elevated MeHg flag; the effect is distinct from the large-reservoir flooding signal that the literature documents more frequently.