As sea ice melts, polar bears’ mercury levels fall

Climate change hasn’t been kind to the polar bear (Ursus maritimus), which has seen its traditional habitat dwindle as sea ice melts. But the rising mercury in thermometers in the Arctic may have one upside for some Alaskan polar bears: the toxic mercury in their diet is falling. 

“Certainly, this is a short-term good news story that mercury levels are going down,” said TWS member Melissa McKinney, an ecotoxicologist at the University of Connecticut. But it raises other concerns about the effects of climate change on the bears, McKinney said, including the possibility that the bears’ shift to the shore will put them in closer contact with humans.

In a new study published in Environmental Science & Technology, McKinney and a team of researchers studied mercury concentrations in polar bears in Alaska’s southern Beaufort Sea from 2004 to 2011. Over the period, the bears were coming onto land in search of food more frequently due to melting sea ice in their usual foraging habitat. Instead of pursuing their normal prey source, ringed seals (Pusa hispida), they were consuming other species along the shore, such as bowhead whale (Balaena mysticetus) carcasses.

McKinney’s team was interested in what these changes meant for other risk factors to polar bears, including toxic mercury in the environment.

They found that mercury concentrations declined about 65 percent in polar bears — a finding not reflected in other wildlife in the region.

“We know that the reduction we’re seeing in this population we didn’t generally observe in other Alaskan wildlife,” she said.

The researchers measured mercury levels in the polar bears’ hair over the seven-year period and consulted a previous study they published that looked at fatty acid to compare observed prey with what the bears were actually eating and noted the bears’ body mass index over time.

They found that as the bears ate more coastal foods, the mercury concentrations in their hair decreased, and when they ate coastal foods such as bowhead whales, which contain large amounts of meat and blubber, they were in better shape and had a higher BMI.

A new U.S. Geological Survey study has found that rapid global warming has sped up sea ice movements, causing fewer foraging opportunities for the bears. It also has caused the bears to increase their energy in order to follow ice drifts.

McKinney estimates about 20 percent of the polar bear population comes onto land to reap the benefits of larger food sources.

She believes the lower levels of mercury are due to coastal prey such as bowhead whales and bearded seals (Erignathus barbatus) being at a lower trophic position on the food web than ringed seals. Bowhead whales feed on species such as zooplankton, while ringed seals are consuming things like fish that are higher in mercury.

When the bears are in better condition, they also rely less on fat and built-up reserves, McKinney said. When bears burn off fat, they release contaminants such as mercury, but if they are in better condition, they release less mercury.

Header Image: Two polar bears stand on the shore of Alaska’s Beaufort Sea. ©Stewart Breck