Mercury Found In California Mountain Lions May Be Delivered By Coastal Fog

At least two pumas had "sublethal" levels that reduce fertility and reproductive success. Credit: Sebastian Kennerknecht

A watershed divide in the Santa Cruz Mountains has provided the ideal testing ground for whether or not mercury accumulates in pumas from a subtle yet significant source: coastal fog.

Researchers found that concentrations of mercury were three times higher for pumas that live inside the fog zone than those roaming the other side of the range, which acts as a barrier to penetration. 


Mercury is a pollutant found all around the world and in a variety of terrestrial, freshwater, and marine ecosystems. Unlike many other metals, the atmosphere is the primary pathway for mercury to be transported across distances and is a novel way for land-based animals on the coast to be exposed to mercury, say the authors of a new study.

"The results were a surprise. We did not think that methylmercury in fog could impact a top predator in a coastal terrestrial ecosystem since the amount of water in fog is much less than that in rain," lead author Peter Weiss-Penzias, an environmental toxicologist from UC Santa Cruz, told IFLScience.

Pumas – also known as mountain lions, cougars, and panthers – are a keystone species that provide crucial ecosystem services to their environment. To determine the levels of mercury in the creatures, the team took fur and whisker samples from 94 coastal mountain lions and 18 non-coastal ones, as well as samples from lichen and deer.

"Lichen doesn't have any roots so the presence of elevated methylmercury in lichen must come from the atmosphere," said Weiss-Penzias, whose study is published in Scientific Reports. "Mercury becomes increasingly concentrated in organisms higher up the food chain."

New findings indicate fog-borne methylmercury is making its way all the way up the food chain from lichen to deer and then mountain lions. (Infographic courtesy Peter Weiss-Penzias)

The pumas on the coastal side averaged about 1,500 parts per billion (ppb), whereas the pumas on the other side of the range averaged around 500 ppb. The levels found in deer and humans do not appear toxic, the team note. However, the mercury appears to be bioaccumulating in the pumas, with the lichens absorbing the toxin from the fog, the deers then munching on the lichen, and the pumas preying on the deer.

Mining and coal-burning plants are the primary drivers of the pollution, say the team. "The ultimate source of mercury to the ocean is from atmospheric deposition (rain and particles) that contains the mercury in coal combustion emissions, gold mining emissions, and other processes (some natural ones too: 70 percent man-made, 30 percent natural)," Weiss-Penzias added to IFLScience.

When mercury in the atmosphere rains down on the ocean, it is converted by bacteria into methylmercury – its most toxic version. When upwelling brings the toxin to the surface, it goes back into the atmosphere and is then carried by marine fog. 

"Fog is a stabilizing medium for methylmercury," said Weiss-Penzias. "Fog drifts inland and rains down in microdroplets, collecting on vegetation and dripping to the ground, where the slow process of bioaccumulation begins." 


The effects of mercury toxicity on pumas have not been conducted yet in the pumas, but based on mink and otter research, the team say it's possible that some of the levels found in the pumas are toxic. In particular, the team found one dead puma with a fur sample that exceeded the threshold, with cause of death unknown.

Most of the female pumas had blood concentrations of mercury below the threshold for sub-lethal effects on offspring survival. However, two of their samples returned levels that exceeded the threshold. Future research is now needed to better understand how mercury may influence puma reproduction and health.

In 2013, a global treaty called the Minamata Convention on Mercury was formed to understand the effects of mercury on human health and the environment. "It's important for the future of that treaty to understand all the different ways that mercury impacts the environment," concluded Weiss-Penzias.