Woods to Waters: Wildfires amplify mercury contamination in fish

Forest fires mobilize mercury from the soil and, according to new research, can send the toxic metal into nearby streams and lakes where it accumulates in fish.

The finding suggests that ecological and health dangers associated with mercury-contaminated fish could grow if, as researchers predict (SN: 7/8/06, p. 19: The Long Burn: Warming drove recent upswing in wildfires), North American wildfires become larger and more frequent. Even now, “mercury contamination is the most frequent reason for fish-consumption advisories,” says Erin N. Kelly of the University of Alberta in Edmonton, who led the latest study.

The pollutant is toxic to brain cells and builds up in the food chain, so fish eaters such as bears and people face potential harm.

“Fish can have a huge amount of mercury, about 1 million times higher than the concentration found in the water [or] algae,” says Edenise Garcia, an aquatic ecotoxicologist with the Jacques Whitford environmental consulting group in Burnaby, British Columbia.

Mercury is commonly found in topsoil. Intense forest fires burn off topsoil, lofting its mercury contents into the atmosphere, where the metal drifts long distances (SN: 8/26/06, p. 134: Available to subscribers at Mercury Rising: Natural wildfires release pollutant).

Last year, however, Garcia and Richard Carignan of the University of Montreal suggested that incomplete burning redistributes mercury not only in the air but also via water runoff. In the March 2005 Environmental Toxicology and Chemistry, they reported higher mercury concentrations in fish living downstream of partially burned woods than in fish in watersheds that had had no fires. Garcia says that fires destroy the vegetation that holds soil in place, and runoff then washes both toxic contaminants and plant nutrients into waterways.

In their new study, Kelly and her colleagues duplicate Garcia and Carignan’s finding and provide new insight into processes by which fires boost mercury contamination in fish.

Before 2000, Kelly’s team had gathered data in Alberta’s Moab Lake on fishes’ bodily mercury concentrations and stomach contents. After an extensive fire in the area during July and August 2000, the researchers returned and collected more data. They found that in the fall of 2000, mercury contamination spiked in Moab Lake and was higher in two nearby creeks that drained burned watersheds than in two creeks unaffected by the fire.

Furthermore, mercury concentrations in several fish species in the lake rose to several times their prefire concentrations, the researchers report in the Dec. 19 Proceedings of the National Academy of Sciences.

The study’s before-and-after comparison bolsters the idea that “fires accentuate mercury accumulation in fish,” says Garcia.

The lake’s fire-generated influx of plant nutrients contributed to rising mercury contamination in fish because it increased the lake’s biological productivity, Kelly says. The analysis of stomach contents indicated that large fish that had previously subsisted on crustaceans had after the fire feasted on increasingly abundant small fish.

“There was a layer added on to the food chain,” Kelly says. Since mercury concentrations in animals tend to increase with each step up the chain, the extra layer boosted the pollutant’s concentrations in the top predators.