Toxic Coastal Fog Linked to Dangerously High Levels of Mercury in Mountain Lions

A mountain lion.
A mountain lion.
Image: Sebastian Kennerknecht

An unlikely culprit is being blamed for elevated mercury levels found in mountain lions living along the California coast: marine fog.


Marine fog appears to be responsible for elevated levels of mercury in coastal terrestrial food webs, and it’s trickling all the way to the top, according to new research published this week in Scientific Reports. Pumas living in the fog belt of the Santa Cruz Mountains have three times the amount of mercury in their systems compared to their cohorts living outside of the fog zone. It’s yet another threat to a species already at risk.

Environmental toxicologist Peter Weiss-Penzias from UC Santa Cruz led the new research, and it’s the first time scientists have tracked the neurotoxin from its presence in the air through to its presence in an apex predator. Delivered by marine fog, the mercury first contaminates plants, namely lichen. These plants are then eaten by herbivores such as deer, who are in turn consumed by mountain lions. It’s the circle of life, but with a dash of despair in the form of methylmercury—a particularly toxic water-soluble form of the chemical element.

“Lichen don’t have any roots so the presence of elevated methylmercury in lichen must come from the atmosphere,” said Weiss-Penzias in a UC Santa Cruz press release. “Mercury becomes increasingly concentrated in organisms higher up the food chain.”

Indeed, by the time it’s at the level of the mountain lions, the methylmercury concentrations have increased by as much as 1,000 times, according to the study.

Infographic showing how mercury makes its way into the terrestrial food web.
Infographic showing how mercury makes its way into the terrestrial food web.
Image: UC Santa Cruz

Mercury is released into the environment via both natural and artificial processes, the latter being human activities such as mining and coal-fired plants. After falling into the ocean, anaerobic bacteria in deep waters convert mercury to methylmercury. From there it returns to the atmosphere, where it gets scooped up and carried by fog, which stabilizes methylmercury. Once over land, it falls down in microdroplet form, collecting on plants and the ground, kickstarting the process of bioaccumulation and its dissemination through the terrestrial food web.


Mercury from marine fog poses no risk to humans, but the same cannot be said for terrestrial mammals. Methylmercury can cause neurological damage to mammals at high concentrations, including impaired memory and motor function. It can also result in unhealthy offspring.

For the study, Weiss-Penzias and his colleagues analyzed fur and whisker samples from nearly 100 coastal pumas and 18 pumas living away from the coast. The results were as clear as they were alarming; the mountain lions in the fog zones had methylmercury levels at 1,500 parts per billion (ppb), compared to the 500 ppb seen in mountain lions living outside the fog zone. One lion had levels that would be toxic to a mink or otter, while two lions had levels known to reduce fertility. Deer living inside the fog zone were also found to exhibit higher mercury levels than those living outside the zone.


This is upsetting news given the plight of mountain lions, who are already threatened by loss of habitat and human encroachments.

“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 Chris Wilmers, a co-author of the study and the director of the Puma Project. “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.”


To which Weiss-Penzias added: “We need to protect the top predators in the environment. They’re keystone species. They perform ecosystem services. When you change one thing, it has cascading effects through the system.”

Senior staff reporter at Gizmodo specializing in astronomy, space exploration, SETI, archaeology, bioethics, animal intelligence, human enhancement, and risks posed by AI and other advanced tech.



It’s regrettable to see yet another threat to cougars, but it sounds as though this form of mercury pollution is entirely natural. If it is, it’s been going on ever since coastal fogs began delivering significant amounts of water to the mountains, and the wildlife populations there have been dealing with elevated mercury levels for a long time. Any evidence for them adapting? More to the point, what can the human population be expected to do about this?