Member Highlight: Toxic Mercury Levels Are Actually Declining In Alaskan Polar Bears—But That’s Not As Great As It Sounds

2018-01-02T13:55:29+00:00 June 19, 2017|
The mercury might be rising in the thermometer, but not in these bears. (Credit: Alan D Wilson)

(Click to enlarge) The mercury might be rising in the thermometer, but not in these bears. (Credit: Alan D Wilson)

Imagine: vast expanses of frozen sea, stretching from the northern coast of Alaska into the Arctic horizon. Welcome to the Southern Beaufort Sea—or at least, the Southern Beaufort Sea as it used to be.

(From Popular Science / by Aparna Nathan) — This icy Arctic ecosystem is dominated by the majestic polar bear, but warmer temperatures are changing both the landscape and its inhabitants.

In a recent study in Polar Research, scientists at the University of Connecticut and the United States Geological Survey found that mercury levels measured in Southern Beaufort polar bears’ hair have actually declined significantly in recent years, particularly in male polar bears. Surprisingly, despite worries about increasing pollution, mercury levels dropped by about 13 percent per year in samples collected from 2004 to 2011. The decrease was insignificant in female bears (4.4 percent), but much higher in male bears (15 percent). “We see this very substantial drop in polar bear mercury concentration over a relatively short period of time,” says Melissa McKinney, first author of the study and assistant professor at the University of Connecticut.

At first glance, this is promising; after all, mercury is toxic and can cause problems in the brain, kidney, liver, and immune system. And since mercury is found in power plant emissions, man-made pollution has been causing spikes in environmental levels.

But don’t get too excited—these declines are not necessarily due to environmental clean-up. Instead, the researchers found that the declines were driven by changing food sources changing habitats. The Southern Beaufort Sea used to be frozen nearly year-round, with the exception of the warmer months between July and October. As Arctic temperatures have risen over recent years, the sea ice has receded and the polar bears’ traditional habitat has slipped out of reach—and along with it, their food sources. In the early 2000s, Southern Beaufort polar bears fed on fellow ice-dwelling ringed seals and only a handful of bears wandered onshore, usually for no more than two weeks. But with the ice sometimes nearly 400 miles away from shallow waters, polar bears are now spending almost two months on coastal land and have turned to new food sources.

Enter humans: subsistence hunters from Arctic communities leave bowhead whale carcasses on shore, and these ready-to-eat meals have become popular among polar bears. And they might actually be helping decrease the bears’ mercury levels, at least among males who aren’t deterred by the massive size of the whales. Prior studies have connected the blubbery diet to polar bears’ improved body condition (high BMIs). This is in turn associated with lower mercury levels. That could be because the food itself contains less mercury, or because the mercury is being stored in tissues and isn’t released until the bear metabolizes them for extra energy—like when food is scarce, or when the bear has a lower BMI. That’s less likely on a whale diet, says study author Todd Atwood, a wildlife biologist at the United States Geological Survey’s Alaska Science Center.

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