After a Fire, Mercury Rises in a Lake’s Food Chain

After a Fire, Mercury Rises in a Lake’s Food Chain

5 December 2006

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Jasper National Park, Alberta, Canada — Scientific studies seldom go completely as planned. There can be twists and turns along the way. Or fires, in Erin N. Kelly’scase.

In 2000, Ms. Kelly, a doctoral student at the University of Alberta, was working on her thesis, a study of how mercury in water, invertebrates and fish in the Canadian Rockies varies with elevation. The question has large implications: mercury accumulates through the food chain and can harm people if they eat fish containing too much of it.

“I went to go and sample my lakes over the elevation gradient,” Ms. Kelly said, “and there was this fire.” A forest fire that summer burned nearly three-quarters of the watershed of Moab Lake in Jasper National Park. So with her adviser, David W. Schindler, she decided to add another facet to her research. “Dave and I came up with the idea that it might be interesting to see what happened with mercury concentrations in the lake,” she said.

Their findings are now in: the fire resulted in a fivefold increase in the concentration of mercury in rainbow trout. But what was even more interesting was the mechanism by which the mercury levels increased, Ms. Kelly, Dr. Schindler and others report in The Proceedings of the National Academy of Sciences. The forest fire altered the lake’s food chain.

Ordinarily, one might expect that after a fire, erosion caused by loss of vegetation would result in a rush of mercury (bound in the soil) into the water. And the researchers did find a spike in mercury concentration in the lake after the fire.

But their experiments showed that the real culprits in increasing the mercury levels in fish were nutrients like nitrogen and phosphorus that washed into the lake. The nutrients increased algal productivity, which in turn led to more young fish, particularly rainbow trout. That caused the adult fish to shift from a largely invertebrate diet to one that consisted of young fish, which had accumulated a great deal of mercury in their own diet.

This “lengthening” of the food chain resulted in a greater accumulation of mercury in the adult fish. “It’s possible that forest fires could cause concentrations in fishes to go over consumption guidelines,” Ms. Kelly said. Because global climate change is expected to increase the number of forest fires, the findings could have longer-term significance for mercury levels in freshwater fish, she said.

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