When a forest burns, a lot of mercury goes up in smoke. The neurotoxic element--found naturally in soil and vegetation--blows away, often traveling hundreds or thousands of kilometers. Yet some of this mercury can also stick around by washing into nearby lakes, where it ends up in fish. Now a study has revealed a surprising mechanism through which forest fires can boost the concentration of mercury in fish even higher: by throwing the aquatic ecosystem out of whack.
The research was slightly serendipitous. Erin Kelly, a graduate student at the University of Alberta, Edmonton, was studying mercury in the Canadian Rockies when, in July 2000, a forest fire hit one of her field sites at Moab Lake in Jasper National Park. Her adviser, David Schindler, knew of pre-fire samples of fish and water from the lake, so they and colleagues were able to investigate the effect of the 1120-hectare blaze on mercury in the ecosystem.
Within a month of the blaze, a pulse of mercury entered the lake via soil eroded from the burned hectares. The runoff also doubled the amount of nitrogen in the lake and quadrupled the level of phosphorus. Invertebrates thrived on these nutrients, leading to larger numbers of all members of the food web. Small trout, in particular, experienced a population boom.
These factors altered the food web by adding new predation. The abundant young rainbow trout were consumed by four species of fish that normally subsist on invertebrates, so they accumulated more mercury than they normally did. Adult rainbow trout, for example, had 5 times more mercury in their bodies than before the fire, the team reports this week in Proceedings of the National Academy of Sciences. Lake trout, the top predator, also began to eat a fish called cisco after the fire, which increased its mercury content even more. Mercury in these trout exceed the amount deemed safe by the U.S. Environmental Protection Agency. Nearby lakes with unburned watersheds had no change in their food webs or mercury levels.
The team calculates that 88% of the increase in fish mercury content was due to the change in the food web, with the remainder due to the extra mercury entering the lake. "If that pulse was more sustained or had happened during the most productive time of year, it's likely that the amount of mercury could have increased even more," Kelly says. She and her coauthors note that mercury contamination could become more common as climate change increases forest fires in North America.
"What's exciting is it shows that forest fires have a double whammy on food chains," by adding mercury and then increasing its accumulation in food webs, says Merritt Turetsky, a wetland ecologist at Michigan State University in East Lansing. "There's more mercury flowing through the food chain than we would have expected."