Image credit: thelearnr/Flickr
Images of polar bears hopelessly adrift on ice rafts are compelling, but the species faces other more subtle threats as well. It has long been known that mercury, which finds its way into snow and water from such human sources as coal-burning power plants, incinerators, and chlorine-producing plants, can travel up the food chain.
Now, new research is showing exactly where mercury enters the polar bear diet, with findings that suggest much more severe exposure in the future.Mercury is a naturally occurring element but an estimated 150 tons are dumped into the environment each year by power and industrial plants. Deposits are eaten or absorbed by microorganisms, which convert it to a potent and toxic chemical: methylmercury. This compound increases in concentration as it moves up the food chain, via a process known as bioaccumulation, leaving animals at the top of the system, like polar bears, with a highly toxic meal.
A joint study between researchers at University of Canterbury and the University of Michigan has pinpointed two distinct food-webs that make up polar bear diets. One is a mercury-laden chain that extends from phytoplankton. The other is based on algae that grow along arctic ice.
The obvious leap is to imagine that as arctic ice is diminished, polar bears will be forced to rely on the more mercury-laden phytoplankton food web.
Though the study did not directly address this trend, Joel Blum, one of the authors, pointed out that:
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...This work provides background information that will be important in our in-depth understanding of mercury bioaccumulation in polar bears.
Melting ice, it seems, means more to polar bears than a diminished habitat.