The Link Between Marine Fog and High Levels of Mercury in Mountain Lions

A mountain lion in the Santa Cruz Mountains, California.

A mountain lion in the Santa Cruz Mountains, California. Credit: Sebastian Kennerknecht

Scientists have discovered that marine fog in California carries with it high levels of mercury, which they say is being deposited on land and making its way up the food chain. Pumas living in the coastal Santa Cruz Mountains, an area often draped in fog, have mercury levels three times higher than mountain lions that live outside the fog zone. Lichens and deer also had significantly higher levels of mercury in the fog belt.

The research, published this week in the journal Scientific Reports, found that mountain lions had mercury concentrations approaching toxic thresholds that could cause neurological damage, impact reproduction, and threaten survival.

Mercury is released into the atmosphere through a variety of industrial processes, including mining and coal-fired power plants. Once airborne, the element can travel internationally, crossing countries and oceans. As explained in a press release, “as atmospheric mercury rains down on oceans, it is converted by anaerobic bacteria in deep waters to methylmercury, the most toxic form of mercury. Upwelling brings some methylmercury to the surface, where it is released back into the atmosphere and carried by fog.” The presence of elevated levels of methylmercury in lichens, which don’t have roots, is evidence that this contaminate is coming from the atmosphere.

“Fog is a stabilizing medium for methylmercury,” Peter Weiss-Penzias, an environmental toxicologist at the University of California, Santa Cruz, and lead author of the new research, said in a statement. “Fog drifts inland and rains down in microdroplets, collecting on vegetation and dripping to the ground, where the slow process of bioaccumulation begins.”

This mercury-tainted fog poses no health risks to humans, Weiss-Penzias said. But as it moves up the food chain, from lichen to deer to mountain lions, concentrations can increase 1,000-fold. The study examined mercury concentrations in fur and whisker samples from 94 coastal pumas and 18 non-coastal ones. The samples from coastal lions had average mercury concentrations of 1,500 parts per billion (ppb), compared to nearly 500 ppb for non-coastal lions.

Scientists stress that this mercury contamination threatens a species already struggling with rampant habitat loss and fragmentation.

“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 biologist at UC Santa Cruz, director of the Puma Project, and a coauthor of the new study. “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.”