Toxic Fog Is Bringing Dangerous Levels of Mercury to California's Mountain Lions

Mountain lions in areas of the Santa Cruz Mountains affected by marine fog are showing concentrations of mercury three times as high as those in inland areas, scientists have found.

This toxicity level could damage reproduction and even threaten the survival of the lions, say researchers writing in Nature.

Mercury is a naturally occurring element and just one of several toxins released by human activities such as coal burning or mining.

Particles ejected into the atmosphere via these processes fall to the ocean in rain, where bacteria turns mercury into methylmercury, an extremely poisonous substance. Methylmercury brought to the surface of the water returns to the atmosphere as fog, where it can enter terrestrial food chains.

According to the researchers, this is the first study to follow the journey of methylmercury up the food chain to an apex predator—in this case, a mountain lion.

Abnormally high concentrations of mercury were found at each stage of the food chain from lichen to lion, increasing 1,000-fold (or more) with each step, the study's authors say.

"Lichen don't have any roots so the presence of elevated methylmercury in lichen must come from the atmosphere," Peter Weiss-Penzias, an environmental toxicologist and co-author of the paper, said in a statement.

"Mercury becomes increasingly concentrated in organisms higher up the food chain."

While exposure to mercury can cause neurological damage, problems associated with the nervous, digestive and immune systems and even death, Weiss-Penzias and colleagues were keen to stress that the concentrations of mercury found in the fog do not pose a risk to human health.

However, they do pose a problem for California's mountain lions, who are already having to deal with various other (predominantly human-related) threats, including hunting, habitat loss and dangerous driving. According to The San Francisco Chronicle, roughly 100 lions die on Californian motorways each year.

"These mercury levels might compound the impacts of trying to make it in an environment like the Santa Cruz Mountains, where there is already so much human influence, but we don't really know," said senior author Chris Wilmers, a professor of environmental studies and the director of the Puma Project, adding the situation is likely to only get worse.

"Levels will be higher 100 years from now, when the Earth's mercury budget is higher because of all the coal we're pumping into the atmosphere."

Mountain Lion California
Mountain lions in California's coastal regions show concentrations of mercury three times as high as those further inland. Sebastian Kennerknecht

Weiss-Penzias, Wilmers and others measured levels of concentrations in mountain lions using fur and whisker samples collected from 94 coastal animals and 18 inland animals.

In the first group, mercury levels averaged out at around 1,500 parts per billion (ppb)—a concentration three times as high as those further inland, which averaged less than 500 ppb.

Individual lions were found to show concentrations that would be toxic to a smaller predator, such as mink or otters. Others had levels of mercury high enough to impair their ability to produce viable offspring, threatening the future of California's already vulnerable mountain lions.

According to a paper published in Ecological Applications earlier this year, California's Santa Monica and Santa Ana lions could be at risk of local extinction within the next 50 years because of low genetic diversity and mortality exacerbated by isolation driven by human development.

According to the study's authors, a "relatively modest" increase in connectivity could be enough to save them.