Carbon recycling, energy flux, and mercury dynamics along a river affected by multiple disturbances

Abstract Small hydroelectric power plants, such as run‐of‐river dams (RORs), are increasingly promoted as low‐impact renewable energy sources, yet their disturbance on mercury (Hg) cycling remains poorly evaluated in impact assessments. Recent work suggests that ROR impacts may be underestimated, as transient Hg increases were observed in fish following impoundment, and co‐occurring disturbances including forest fires and logging may further alter carbon and Hg fluxes. This study investigates how RORs (impounded in 1959 and 2008), forest fires (in 2010), and logging (up to 2020) influence Hg dynamics along a regulated river (Quebec, Canada). In summer 2023, we sampled six river sectors representing various combinations of natural and anthropogenic disturbances. Methylmercury (MeHg) concentrations in secondary consumers (macroinvertebrates) in sectors combining dams and watershed disturbances remained up to 2.3‐times those of the reference sector, compared to 1.5‐times in single‐disturbance sectors. Despite a 34% decline in fish MeHg in the dam and fire impacted sector between 2018 and 2023, cyprinids downstream of dams still exhibited up to 2‐times higher contamination than undammed sectors. Fish MeHg was strongly predicted by depleted carbon isotope (δ 13 C) values ( R 2 marginal = 0.72), associated with decomposition processes. Hydrogen isotope (δ 2 H) values contributed to MeHg variability in models including end‐members, suggesting that reliance on autochthonous sources contributes to sustained MeHg bioaccumulation, particularly when allochthonous inputs occur. These findings indicate that RORs may have enhanced ecological Hg impacts when combined with watershed disturbances. Improved, flexible planning will be essential (e.g., logging) under climate change scenarios where forest fires are expected to rise.