Loss of Sea Ice Poses Mercury Risk

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Even as many residents of northern mid-latitudes hunker down during a persistently cold winter, farther north conditions continue to conspire to create the opposite effect. The minimum Arctic sea ice extent for 2010 was the third lowest recorded, after 2007 and 2008, and the extent for December was the lowest for that month since satellite records began in 1979.

While declining Arctic sea ice is generally equated in the popular imagination with an uncertain future for polar bears and perhaps other marine mammals, the wholesale alteration of this ecosystem inevitably seems likely to have a wide range of profound consequences. For example, in at least some areas of the Arctic Ocean, increased melting of sea ice has resulted in a decline in large marine algae and a growth in smaller, less productive algal species (opens PDF).

Now, a new study in the journal Nature Geoscience suggests that declines in sea ice cover might affect the amount of mercury entering into the Arctic marine food chain.

Mercury pollution in the Arctic has been a concern for many years (opens PDF); although mercury emissions from industrial activities are declining in the West, they appear to be increasing in Asia, and particularly China. The Arctic is especially vulnerable to mercury's impacts for several reasons. Gaseous elemental mercury can remain in the atmosphere for a relatively long time, enabling its transport to polar realms. From the Arctic atmosphere, oxidized mercury can deposit easily on snow and ice. The photodegradation of oxidized mercury on the sunlight-reflecting ice produces a volatile element of oxidized mercury enriched in lighter isotopes. But once the ice melts, anaerobic organisms in the water convert the mercury to a form called methylmercury. Water's high absorption of ultraviolet (UV) light further fractionates the methylmercury, causing an enrichment of heavy isotopes. Methylmercury poses a bigger threat to the food chain as it is more toxic, and, unlike oxidized mercury, bioaccumlates easily in fatty tissues. It is a particular concern for marine mammals that build up layers of blubber that then provide them energy during lean times.

Studies have long shown geographical and seasonal differences in mercury concentrations in the Arctic; in the new paper, a French-American team led by marine geochemist David Point sought to explain some of these differences. They did so by analyzing the isotopic ratio of mercury in murre eggs collected in several Arctic and sub-Arctic locations around the coast of Alaska.

"The more UV radiation in ice free waters lead to heavier isotopic mercury in the water compartment before it is biomagnified in the food chain," Point told Discovery News. His team found high levels of fractionated methylmercury in eggs from areas where there was little or no sea ice cover than from areas where sea ice cover existed most of the year. "Foraging behaviour and geographic variations in mercury sources and solar radiation fluxes were unable to explain the latitudinal gradient," they report. They conclude that further loss of Arctic sea ice will increase the extent to which methylmercury enters the Arctic marine food web.

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Photograph of common murres on ice off Cape Lisburne, Alaska, by D. Roseneau/US Fish & Wildlife Service.