Eight-legged bags of poison

If you know an old lady who swallowed a spider, tell her to cough it up. Spiders and insects living near a mercury-contaminated river contain unusually high levels of the toxic metal, and it is turning up in area songbirds, a new study finds.

“We think of mercury as an aquatic problem,” comments wildlife toxicologist Tony Scheuhammer of National Wildlife Research Centre in Ottawa. “This study shows a particular way that it can become a terrestrial ecosystem problem.”

Numerous studies have documented widespread mercury contamination of streams, wetlands and lakes, but the details of its journey remain murky. It’s clear that certain bacteria living in low-oxygen environments such as river-bottom mud can convert inorganic mercury to methylmercury, the form that accumulates most easily in the tissues of living things. Then the metal travels up the food chain, its concentration magnifying with each step.

The current study looked beyond the life aquatic, investigating mercury levels in 13 bird species living along the South River, a tributary of the ShenandoahRiver in Virginia. In the 1970s, a serious mercury contamination problem was discovered in that river near the site of a fiber production plant that operated between 1929 and 1950.

Lead author Daniel Cristol of the College of William & Mary in Williamsburg, Va., says that “it was partly a contrarian streak” that prompted him to look into mercury levels of terrestrial birds — loons and other fish-eaters are usually the focus of such investigations. Cristol and a team of his students began sampling in 2005 and were surprised at the mercury levels they found in birds that eat insects for a living. “It was undeniable — across the board — and the only link we saw was the insects, so we started sampling insects,” Cristol says.

The researchers took blood from more than 200 birds living within 50 meters of the river and also sampled more than 100 birds of the same species from sites upriver from the contamination. Fish-eating birds were also sampled for comparison. And the team intercepted food items such as spiders, grasshoppers and caterpillars from three species of birds trying to feed their young.

Spiders made up 20 to 30 percent of these birds’ diets, yet delivered about 75 percent of the mercury, much of it the methylmercury that moves into living tissue, the researchers report in the April 18 Science. And the eight-legged critters had higher overall mercury levels than the local fish-eating kingfishers.

Spiders are predatory, so it makes sense that they have high levels of the toxic metal, comments aquatic ecologist David Schindler of the University of Alberta in Edmonton. “Anything that lengthens the food chain pushes the mercury up, and it is biomagnified by about a factor of 10 with each step,” he says. If spiders are eating mercury-laden insects, the birds eating those spiders are getting a heftier dose than birds eating caterpillars, creatures that eat plants.

It’s not clear how the mercury is getting into the land-dwelling insects to begin with. The spiders might eat insects that spent some part of their life cycle in the water, or mercury may have been deposited on land during past flooding. Schindler hopes to address these questions this summer with experiments that will trace mercury through the food web. He also plans to examine whether the birds’ reproductive success is affected by the poisonous metal.

The work is important, says Schindler, but he is not totally surprised by the findings that the metal has made its way onto land. “Mercury is a particularly slippery customer,” he says.