The Not-So-Good News of Polar Bears Consuming Less Mercury Pollution

Polar bears may be exposed to lower levels of mercury in their diets than in the past, but it appears to be because climate change is changing what — and where — they eat.

For Alaska’s polar bears, new research has good news and bad news.

Scientists have found less mercury in the bodies of a group of bears they studied in recent years. The catch is, it’s likely because they’re having to find new sources of food as their normal hunting grounds — the ice of the southern Beaufort Sea — melt away. So they’re coming ashore, scavenging the carcasses of bowhead whales and hunting different species of seals than normal.

“In years when the ice conditions were poor, the polar bears in northern Alaska are feeding more on these bowhead carcasses and bearded seals and less on their sort of traditional prey, which are the ringed seals,” said Melissa McKinney, an ecotoxicologist at the University of Connecticut.

Mercury, a heavy metal that damages the nervous system, is released by burning coal and other industrial processes. It circulates in the atmosphere and collects in the Arctic, where it builds up in the bodies of animals at the top of the food chain — like ringed seals and the polar bears that eat them. Bowhead whales, which are hunted by native Alaskans, and bearded seals aren’t as high on the food chain.

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Chemical analysis of hair samples collected from 198 bears during studies from 2004 to 2011 found mercury levels dropped by as much as 65 percent in that period, McKinney said.

About 20 percent of the southern Beaufort Sea bears appear to be taking advantage of the bowhead carcasses, which are left behind after being harvested. The rest remain offshore on the ice, which has been dwindling in the summers as the Arctic warms at twice the rate of the rest of the globe. The bears with lower mercury levels also tended to be bigger and be in better shape than bears that remain on the ice.

“The diet shift to coastal food is a shift to prey that are lower on the food chain, and therefore are a less mercury-contaminated food resource,” McKinney said. And those bears’ bodies may be burning less of the stored fat and protein reserves where mercury concentrates, resulting in less of the substance getting into their bloodstreams, she said.

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The findings were published Wednesday in Environmental Science & Technology, a research journal of the American Chemical Society. The study involved researchers from Connecticut and from the US Geological Survey, which has been tracking polar bear populations in Alaska and helped fund the work.

McKinney cautioned that the population she and her colleagues studied was a small one, and the results may not apply to bears in other parts of the Arctic, where bears may find different sources of food onshore. But she said the findings provide some insight into how global warming is affecting other risks to the far North’s ecosystem.

“I think the study shows us that either positively or negatively, we need to consider how climate change can shape the risks that are posed by other environmental stressors,” McKinney said. “In this case, we’re talking about toxic pollutants, but there are a whole host of other environmental stressors.”

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