It’s a wilderness home to otters and sea lions, coyotes and cormorants. But the water gets more contaminated than industrialised San Francisco Bay, 70km away.
Now scientists have identified the cause: moulting elephant seals.
Researchers from the University of California, Santa Cruz, have worked out why the water around an isolated seal rookery is being poisoned by mercury.
The findings, outlined in the journal PNAS, reveal a new step in the life cycle of a toxic metal that just won’t go away.
Co-author Jennifer Cossaboon said plenty of researchers had investigated “biomagnification” of mercury up the food chain.
“We took that a step further to see what happens next,” Ms Cossaboon said.
“Mercury is an element, so it never breaks down — it just changes form.”
Emissions from coal-fired power stations have more than doubled mercury concentrations in the sea since the industrial revolution. Scientists expect the build-up to continue, with a recent study of Hawaiian yellowfin tuna finding mercury concentrations were increasing by at least 3.8 per cent a year.
In its most toxic form, known as methylmercury, the metal accumulates inside marine organisms as it climbs the food chain.
Concentrations in top predators such as tuna and sharks are up to 10 million times the levels in seawater, and can damage the sensory organs of people who eat these creatures.
The new study has found a feedback loop that could accelerate the problem. The scientists identified massive fluctuations in methylmercury concentrations at Ano Nuevo State Reserve, where up to 10,000 northern elephant seals congregate every year.
Mercury concentrations spiked 17 times higher than in other coastal waters during the summer moulting season, when the seals shed their entire outer layer of skin and hair.
The researchers are now trying to determine whether this triggers a domino effect, with moulted mercury reabsorbed by other marine creatures.
“No one has studied how mercury in degrading moult behaves in the marine environment,” said Ms Cossaboon, who led the study as an undergraduate student. “(We do not know) whether it would end up being retained in the tissues of organisms that consumed degraded hair.”
A 1981 study by co-author Russell Flegal found elevated mercury concentrations in mussels near seal and sea lion colonies at Ano Nuevo and at San Miguel Island, near Los Angeles. “At that time we didn’t have the analytical instruments to detect mercury at the concentrations found in seawater,” Professor Flegal said.
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