Experimental evidence for recovery of mercury-contaminated fish populations

Anthropogenic releases of mercury (Hg) are a human health issue because methylmercury (MeHg), formed primarily by microbial methylation of inorganic Hg in aquatic ecosystems, biaccumulates to high concentrations in fish consumed by humans. Predicting the efficacy of Hg pollution controls on fish MeHg concentrations is complex because many factors influence MeHg production and bioaccumulation. Here, over a 15-year whole-ecosystem, single-factor experiment, we quantified the magnitude and timing of reductions in fish MeHg concentrations following reductions in Hg additions to a boreal lake and its watershed. During a seven-year addition phase, enriched Hg isotopes increased local wet deposition fivefold and became increasingly incorporated into the food web as MeHg, elevating fish concentrations. After ceasing Hg loading, biogeochemical records of labelled Hg decreased by up to 91% over ten years, initiating rapid decreases of 38–76% in MeHg concentrations in key fish populations within eight years. Although watershed-derived Hg loading may not decline in parallel with atmospheric deposition, this experiment demonstrates that any reduction in Hg loadings to lakes—via direct deposition or runoff—yields immediate benefits to fish consumers.