by Bill Schlesinger
Legacy pollution occurs when emissions of a pollutant from long ago persist in the environment and return to impact us at a later time. About 8 months ago, I blogged about the legacy of phosphorus pollution that is retained in agricultural soils after they are fertilized. See: Legacy Phosphorus – Translational Ecology (duke.edu). Today, I want to talk about the legacies of mercury deposited from the atmosphere, where it is derived from the burning of fossil fuels and the smelting associated with the extraction of gold, using mercury as a carrier.
Mercury is carried globally in the lower atmosphere, and it is often deposited far from its original source. Mercury deposited from the atmosphere has been recognized as a source of pollution in boreal lakes, which occur in otherwise pristine environments. The mercury that accumulates in lake sediments is a legacy of past mercury pollution.
Mercury pollution in fish stems from the conversion of mercury deposited from the atmosphere to methylmercury, which accumulates in tissues. Microbes in lake sediments convert the elemental mercury to methylmercury. The methylmercury is first taken up by the small organisms of a lake (bacteria, phytoplankton, zooplankton etc.) and later transferred to larger fish that feed on them. At each stage of the food chain, the amount of methylmercury accumulates, so the largest fish get the biggest dose. In some lakes, the concentration of mercury in fish is high enough to poison iconic birds, such a loons, that feed on fish, as well as the fish that we might catch on a wilderness expedition.
Various policy makers have questioned whether the elimination of mercury from smokestacks is worth the effort, since a lot of mercury is already dispersed in the environment, where it remains as a legacy.
New work by a large team of researchers mostly from Canada suggests otherwise. For 7 years, these workers applied a special isotope of mercury to an experimental lake, which “labelled” the mercury in the various organisms—small to large—in the lake. Then they withheld further additions of mercury and found that within 8 years, the smaller organisms had lost 91 percent of the labelled methylmercury. They had flushed their biomass of the pollutant. At the end of the 8-year-period, large fishes had lost 38 to 76 percent of the labelled mercury in their biomass. Of course, the mercury is still out there somewhere, but at least it’s not in the food chain.
The experiment shows that the legacy of mercury pollution can flush itself out of an aquatic ecosystem within a decade, so the benefits of reducing mercury pollution deposited from the atmosphere are quickly reflected throughout the ecosystem. In some cases, nature can clean up our mess remarkably rapidly, making pollution regulations doubly important. For once, this week, we have a “feel-good” story.
Blanchfield, P.J. and 25 others. 2022. Experimental evidence for recovery of mercury contaminated fish populations. Nature 601: 74-78.
Driscoll, C.T. and 7 others. 2007. Mercury contamination in forest and freshwater ecosystems in the northeastern United States. BioScience 57: 17-28.
Selin, N.E. 2009. Global biogeochemical cycling of mercury: a review. Annual Review of Environment and Resources 34: 43-63.