Molting elephant seals may pollute coastal seawater with toxic mercury: study

Molting elephant seals might contribute significant amounts of toxic mercury to coastal ecosystems, according to a study published Monday that looked at a California elephant seal rookery.

The findings, published in the U.S. journal Proceedings of the National Academy of Sciences, revealed a previously unrecognized source of mercury contamination in coastal waters.

Mercury is a problem in the marine environment because the most toxic form, methyl mercury, is readily absorbed and accumulates in the bodies of marine organisms.

In a process known as "biomagnification," the toxin becomes more and more concentrated as it passes up the food chain. Thus, mercury concentrations in top predators can be one million to 10 million times higher than the levels found in seawater.

"Many studies have looked at biomagnification up the food chain, and we took that a step further to see what happens next," said first author Jennifer Cossaboon, who led the study as an undergraduate at University of California (UC) Santa Cruz and is now a graduate student in environmental health at San Diego State University.

"Mercury is an element, so it never breaks down and goes away, it just changes forms."

Previously, high mercury concentrations have been found in mussels near large colonies of seals and sea lions at California's Ano Nuevo and San Miguel Island, suggesting that these mammals might be a significant source of methyl mercury contamination.

In the new study, Cossaboon and colleagues compared methyl mercury concentrations in seawater near the northern elephant seal rookery at Ano Nuevo State Reserve to concentrations found at other sites lacking large marine mammal populations.

Compared to other coastal sites, the concentration of methyl mercury in the seawater at Ano Nuevo was twice as high during the breeding season and 17 times higher during the molting season.

This suggested elephant seal molting contributes a significant amount of methyl mercury to seawater surrounding breeding colonies.

Consistent with this view, the researchers found significant mercury concentrations in molted seal hair.

Industrial emissions, mostly from the burning of coal, have increased the amount of mercury in the marine environment two- to four-fold over preindustrial levels, said study coauthor Russell Flegal, professor of microbiology and environmental toxicology at UC Santa Cruz.

"This internal recycling back into the coastal environment just adds to the problem," Flegal said.