Marine Fog Brings Mercury Pollution to California Mountain Lions

Potent neurotoxin joins the growing list of threats these apex predators face.

Peter Weiss-Penzias was biking to work one foggy July morning in 2011 when he came up with the question that would drive the next stage of his research. “I was getting soaking wet from the fog, and it just dawned on me,” he says. “What’s in this stuff?”

An environmental toxicologist at the University of California, Santa Cruz, Weiss-Penzias knew that the ocean leaked dimethylmercury, one of the most potent known neurotoxins. His colleagues had recently demonstrated that seasonal upwelling in Monterey Bay brought the organomercury compound from the ocean’s depths to its surface. He also knew that the fog is essentially an extension of the ocean.

lone mountain lion p-35
A new study shows that mountain lions in coastal ecosystems, like the fog-dependent coniferous forests of the Santa Cruz Mountains, have elevated levels of mercury in their bodies — on average three times higher than mercury levels found in mountain lions in the Sierra Nevada, outside of the fog belt. Photo of a a Southern California mountain lion courtesy of National Park Service.

“Since the fog layer is sitting right over the ocean, and the ocean is leaking dimethylmercury, could it be picked up by the fog?” he wondered.

This past November, Weiss-Penzias published research showing that marine fog is indeed a vehicle for transporting dimethylmercury, with mountain lions and other mammals in coastal ecosystems on the receiving end of this toxic metal exposure. The study, published in Scientific Reports, shows that mountain lions in coastal ecosystems, like the fog-dependent coniferous forests of the Santa Cruz Mountains, have elevated levels of mercury in their bodies — on average three times higher than mercury levels found in mountain lions in the Sierra Nevada, outside of the fog belt.

“This is a pretty unpolluted environment in the Santa Cruz Mountains,” says Weiss-Penzias. “If they’re getting this excess mercury just from fog, that’s of some concern.”

For all the benefits fog brings to coastal ecosystems, this research demonstrates how toxic mercury infiltrates that process. Weiss-Penzias calls mercury a “global pollutant,” which means that it is uniformly distributed across much of the northern hemisphere no matter its source. A natural element found in the Earth’s crust, mercury is released into the atmosphere through industrial activities like gold-mining and coal-fired power plants. Atmospheric mercury then reaches the ocean, where deep-water bacteria converts it into the highly toxic dimethylmercury and upwelling brings it back to the surface. There, the fog picks it up and transports it to land.

Lichen, a rootless plant that depends on the atmosphere for water and food, picks up that dimethylmercury, which then bioaccumulates and starts to make its way up the food chain — from lichen to mule deer to mountain lions. According to the study, the mercury concentrations can increase 1,000 times with each step.

Mercury poisoning in mountain lions is known to reduce fertility, thus hindering mountain lions’ ability to reproduce. Of course, this only adds to the stress mountain lions already face in California.

“They’re more likely to die young because of depredation permits or road fatalities,” says Chris Wilmers, a mountain lion biologist at UCSC and coauthor of the study. “They’re also exposed to other toxins and poisons in the environment, and their genetics are much less variable because of the isolation roads and development create.”

In California, mountain lion hunting persists as a form of livestock management via depredation permits, issued by the state to landowners whose livestock or pets have been killed by the predator. Of course, research has questioned the effectiveness of such lethal predator control. In a 2013 study published in PLOS One, a team in Washington State showed that remedial hunting actually increased the rate of conflict between cougars and livestock by disrupting cougar social structures. In other words, killing an adult male in his territory allows younger, less mature males to enter the area looking to claim the territory — thus increasing the chance of further livestock depredation. Notably, California is the only state in the West with a permanent ban on mountain lion sport hunting, but depredation permits account for around 100 mountain lion deaths each year.

Another 100 or so mountain lions are killed each year on highways around the state, while in Northern California in particular, rodenticide and other commercial insecticides used on illegal pot farms contaminate forests and streams, threatening mountain lions and other wildlife.

But back on the coast, Wilmers notes another growing cause for concern. Urban, exurban, and highway development have bisected habitats, marooning mountain lions into genetically isolated populations. Wilmers’ study area in the Santa Cruz Mountains, for instance, is surrounded on all sides by the San Francisco Bay Area to the north, the coastline to the west and south, and an urbanizing Highway 101 corridor to the east.

A recent paper in Conservation Genetics identified ten distinct mountain lion populations in California, their boundaries coinciding with interstates and development. Some of those populations have such a low effective population — a term Wilmers’ defines as a “gene’s-eye view” of population viability — that they face potential extinction in the next 50 years.

Mercury poisoning is evidently the least of mountain lions’ worries in California. But according to Weiss-Penzias, this problem is expected to increase over time. “We expect more mercury to be entering the atmosphere due to climate change,” he says. Like coal, the peat moss in permafrost naturally contains mercury, he explains. As permafrost warms due to climate change, that mercury will be released into the atmosphere, alongside methane.

Researchers have long known that mercury compounds can have adverse effects on human health. The gaseous dimethylmercury released by the ocean takes a few days to metabolize into methylmercury, which has been shown to enter the brain and cause neurodevelopmental disorders and death.

Weiss-Penzias’s research on the West Coast has not detected dimethylmercury levels high enough to pose these direct health risks to humans, but a next step in his research will focus on indirect pathways to human exposure. He plans to take a closer look at coastal agriculture in particular, including animal husbandry industries like cow and goat milk, that may be contaminated by fog-borne mercury.

This research is the first step in pinpointing how mercury pollution and oceanic processes combine to affect a coastal terrestrial environment, an ecosystem where mountain lions already feel the symptoms of human impact — which, in turn, has a cascading effect on the ecosystem at large. “Mountain lions are keystone species,” Weiss-Penzias says. “A functioning ecosystem will have top predators that are doing well and not stressed.”

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