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Mercury-Laced Marine Fog Threatening Mountain Lions

Analysis by Dr. Karen Shaw Becker

mountain lion

Story at-a-glance -

  • Methylmercury bioaccumulation or ocean-derived monomethylmercury are terms for the fog-induced mercury that is damaging the health of endangered species along the California coast, including mountain lions
  • Mountain lions, aka pumas, are apex predators, meaning top of the food chain, so it’s significant to note that mercury concentrations in the fog zone were shown to be three times higher than areas outside it
  • Environmental toxicologists say it’s significant that methylmercury was also found in lace lichen, black-tailed deer and mule deer in the coastal fog zone because the deer eat the lichen, and the mountain lions eat the deer
  • Scientists say mercury may also have a negative impact on predator populations because of lethal blood concentrations in their offspring; “sublethal" levels can lower fertility and reproductive success in such animals
  • Top predators in an ecosystem are the keystone species in an environment because they perform services for the ecosystem, so awareness is key for preserving the delicate ecological balance that humans and modern life continue to threaten

For those concerned that high levels of mercury in the air and water can adversely affect their own health as well as the health of their pets, it can be just as damaging to wildlife, and sometimes it arises from unexpected sources.

One case in point is the coastal fog that frequently envelops the Santa Cruz Mountains, which researchers at the University of California-Santa Cruz (UCSC) say is gradually impacting mountain lions. In fact, one study reveals that mountain lions of that area have mercury in their systems three times higher than that of the mountain lions roaming outside the "fog zone."1

Scientists who tracked the source of the vapor identified it as a particularly toxic variety of mercury; there are even special terms for it: methylmercury bioaccumulation or ocean-derived monomethylmercury (MMHg).

Environmental toxicologist (aka atmospheric chemist) and leader of the study Peter Weiss-Penzias says it's significant that methylmercury was also found in lace lichen and black-tailed deer and mule deer found in the fog zone. According to the study, published in the journal Nature:

"Mercury (Hg) is a globally distributed and ubiquitous pollutant found in terrestrial, freshwater and marine ecosystems. The health threats to exposure are significant for ecosystems and human welfare worldwide, but quantifying these risks is complicated by the many facets of mercury cycling which are inherently complex, especially considering the effects of global change.

Unlike other metals, the atmosphere is the main pathway for Hg to be transported from sources (both natural and anthropogenic) to receptor regions."2

How Toxic Fog Makes Its Way to Mountain Lions

Scientists refer to mountain lions, also known as puma, as apex predators, which is how toxic mercury travels up the food chain. That's significant for two reasons: one, because the mercury concentrations in the fog zone were three times higher than in other areas, and two, because the deer eat the lichen, and the mountain lions eat the deer.

Like big ocean fish such as swordfish and tuna that absorb higher levels of mercury when they eat plankton and other smaller fish (which is how mercury can end up in pet food), larger land animals that eat smaller animals and plants laden with mercury end up with the highest concentrations; mercury absorption is cumulative.

Further, UCSC refers to this type of mercury as a neurotoxin, and notes that the levels found in pumas "are approaching toxic thresholds that could jeopardize reproduction and even survival."3 Additionally:

"Although mercury levels in fog present no health risk to humans, the risk to terrestrial mammals may be significant. With each step up the food chain, from lichen to deer to mountain lions, mercury concentrations can increase by at least 1,000 times."4

Weiss-Penzias and his team of researchers used fur and whisker samples from 94 mountain lions inside the fog zone and 18 of the wild cats outside the fog zones and showed that mercury concentrations averaged approximately 1,500 parts per billion (ppb) inside the problem areas, compared to about 500 ppb in the cats inhabiting noncoastal areas.

But there was more: At least one of the mountain lions suffering from mercury poisoning had levels determined to be toxic to other species of carnivores; namely mink and otters. Nonlethal effects can include memory loss and decreased motor coordination. According to the study, these levels of mercury concentrations:

" … can cause clinical Hg intoxication or subtler neurological impairments that could detrimentally affect survival … Among pumas sampled from coastal regions in this investigation, one individual had a fur sample … a puma found dead with no apparent cause of death known at the time."5

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How Methylmercury Concentrations Threaten Animal Species

Mercury concentrations that adversely affect wildlife along the California coast can cause even more damage than merely killing mountain lions — and lower concentrations than what have been found can prove lethal to them, the study reveals.

It may also have a negative impact on predator populations due to lethal blood concentrations in their offspring. "Sublethal" levels can lower fertility and reproductive success in such animals. UCSC noted that the mercury threat to wild animals is exacerbated by other threats that make life more challenging for them than it was, say, a hundred years ago.

There's the encroachment of humans and modern life on their habitats, but there's also a growing threat in the air due to other factors such as coal emissions, says senior study author Chris Wilmers, director of the Puma Project and a professor of environmental studies at UCSC.

However, while people in the U.S. are exploring how such threats can be lessened for humans, wildlife and the environment at large, it's not possible to control it all. Weiss-Penzias acknowledged that when he noted, "Mercury is a global pollutant … What's emitted in China can affect the United States just as much as what's emitted in the United States."6

But the fog changes forms, just like melting ice turns to water and high temperatures cause condensation, which on the coastal perimeters can change due to any number of factors. According to Weiss-Penzias:

"Fog is a stabilizing medium for methylmercury. Fog drifts inland and rains down in microdroplets, collecting on vegetation and dripping to the ground, where the slow process of bioaccumulation begins."7

In addition:

"Ocean bacteria convert mercury to methylmercury, which is then brought to the surface by upwelling. Released back into the atmosphere, methylmercury is carried inland by fog and drips to the ground, where it begins the slow process of bioaccumulation. 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."8

Other Ways Mercury Can Damage Wildlife

Fog concentrations containing high levels of mercury aren't unique to the Santa Cruz Mountains of coastal California, and mountain lions aren't the only animals affected. Weiss-Penzias is also interested in investigating how top-of-the-food chain animals like coyotes, bobcats and birds are impacted in coastal Chile.

Incidentally, the UCSC scientists compared mountain lions of the California coast with wild panthers found in Florida. Between 1978 and 1991, U.S. Fish and Wildlife Service (FWS) collected samples to test how mercury levels in the Big Cypress Swamp and Everglades ecosystems affect big cats and, subsequently, the food chain.

Updated in late 2019, FWS studies noted, "Panthers with the highest levels of tissue Hg were those which regularly consumed non-ungulates primarily, raccoons, armadillos, rabbits, and alligators."9

Dome Island is a 16-acre preserve considered to be a conservation gem situated in the middle of Lake George in upstate New York. But besides being one of the area's last untouched islands, myriad songbirds, such as black-capped chickadees and red-eyed vireos, have, as far back as 2012, reportedly shown some of the highest mercury levels among Northeastern region songbirds.

According to the National Wildlife Federation (NWF),10 the news startled area residents and scientists alike, as did the discovery that spiders on the island also had high mercury levels, which explains at least in part why their prey, the songbirds, were also compromised by toxicity from the heavy metal.

NWF's director of climate and energy policy, Joe Mendelson, observed that the mercury problem is a lot worse than many had realized because, "It has become clear in recent years that no corner of the food web is untouched by mercury."11

How to Protect Wildlife Where You Can

With all the ways mercury is encroaching on the U.S., whether in cities through smog or on coasts due to fog, there are steps that can be taken to fight threats against wild animals to protect not only their environments but their habitats.

As one writer points out, mercury-laden fog isn't the only threat against the mountain lions living in coastal California. Studies have shown that rat poison killed five of the great cats, according to Smithsonian Magazine.12

Even if such pesticides don't kill these and other wild species, it can irreparably harm them. For instance, the animals have come down with potentially fatal cases of mange, which in two cases caused uncontrolled bleeding, according to reports from LAist.13

Not surprisingly, there are a number of traffic-related incidents that have killed and injured mountain lions and other endangered species along the California coast, as well as across other areas of the U.S.

Awareness is one of the key elements for preserving the delicate ecological balance that humans and modern life affect negatively, and sometimes in ways they're unaware of. According to Weiss-Penzias, top predators are the keystone species in an environment because they perform services for the ecosystem. "When you change one thing, it has cascading effects through the system."14