U. MICHIGAN (US)—As concerns grow about the effect melting sea ice may have on polar bears, scientists say there may be another danger lurking—mercury pollution.
Usually being at the top of the food chain is a good thing, but in this case, it may actually work against polar bears, says Joel Blum, professor of ecology and evolutionary biology and the John D. MacArthur Professor of Geological Sciences at the University of Michigan.
Deposited onto land or into water, mercury is picked up by microorganisms, which convert some of it to methylmercury, a highly toxic form that builds up in fish and the animals that eat them.
It is estimated that 150 tons of mercury enter the environment each year from human-generated sources such as coal-burning power plants, incinerators, and chlorine-producing plants.
As bigger animals eat smaller ones, the methylmercury is concentrated—a process known as bioaccumulation.
Sitting at the top of the food chain, polar bears amass high concentrations of the contaminant.
Details of how mercury moves through different food webs— particularly in the Arctic, where snow and ice contribute to mercury deposition—are not well understood.
To tease out that information, Blum and Travis Horton of the University of Canterbury studied polar bear hair samples from museum specimens collected in the late 19th and early 20th centuries, before mercury emissions from human-generated sources began to escalate.
By looking at three chemical signatures—nitrogen isotopes, carbon isotopes, and mercury concentrations—the researchers learned that polar bears get their nutrition (and mercury) from two main food webs.
At the base of one web are microscopic plants that float on the surface of the ocean (known as phytoplankton). The foundation of the second web is algae that live on sea ice.
The study shows that polar bears that get most of their nutrition from phytoplankton-based food webs have greater mercury concentrations than those that participate primarily in ice algae-based webs.
While it’s tempting to speculate that declining sea ice, due to global warming, may force polar bears to depend more on phytoplankton-based webs, thus increasing their mercury exposure, the study doesn’t directly address that issue.
It does, however, provide other useful information, Blum says.
“If you want to understand the potential effects of changing ecosystems on polar bears, you need to be aware of the existence of these two food webs, which may possibly be affected by sea ice,” Blum explains.
“This work provides background information that will be important in our in-depth understanding of mercury bioaccumulation in polar bears.”
Researchers from Yale University and Stanford University contributed to the study, which appears in the December issue of the journal Polar Research.
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