Experimental evidence for the recovery of mercury-contaminated fish populations

Anthropogenic releases of mercury (Hg) are a human health
issue because the potent toxicant methylmercury (MeHg), formed
primarily by microbial methylation of inorganic Hg in aquatic ecosystems,
bioaccumulates 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 the production
and bioaccumulation of MeHg. Here we conducted a 15-year whole-ecosystem,
single-factor experiment to determine the magnitude and timing of
reductions in fish MeHg concentrations following reductions in Hg
additions to a boreal lake and its watershed. During the seven-year
addition phase, we applied enriched Hg isotopes to increase local Hg wet
deposition rates fivefold. The Hg isotopes became increasingly
incorporated into the food web as MeHg, predominantly from additions to
the lake because most of those added to the watershed remained there.
Thereafter, isotopic additions were stopped, resulting in an approximately
100% reduction in Hg loading to the lake. The concentration of labelled
MeHg quickly decreased by more than 85% in lower trophic level organisms,
initiating rapid decreases of 38–76% of MeHg concentration in large-bodied
fish populations in eight years. Although Hg loading from watersheds may
not decline in step with lowering deposition rates, this experiment
clearly demonstrates that any reduction in Hg loadings to lakes, whether
from direct deposition or runoff, will have immediate benefits to fish
consumers.