Written by Quentin Septer

Some years back, citizen scientists set out across America’s National Parks in search of dragonfly larvae. Over the course of a decade, the amateur researchers collected thousands of larvae from 100 National Park units across the nation. Then they sent the larvae back to labs at the United States Geological Survey, the University of Maine, and Dartmouth College, where chemists tested the developing dragonflies for mercury contamination. The results were published in Environmental Science & Technology. The nation’s National Parks, the study found, are wildly polluted with mercury.

Mercury occurs naturally in the environment. Forest fires, volcanic eruptions, the weathering of rocks; all release the element into the atmosphere. But human activities emit mercury on a much grander scale. In the last 500 years, since human beings started mining for gold and other precious metals in numbers sufficient to be recorded in the geological records of earth, we’ve tripled the amount of mercury in the planet’s airways. Gold mining, the burning of coal and fossil fuels, the production of metal and cement; these are among the most potent mercury-emitting activities that we humans have devised. Globally, humanity releases an estimated 2,200 metric tons of mercury into the atmosphere every year. By 2010, humans had emitted roughly 1,540,000 metric tons of mercury, most of which since the year 1850. (For scale, that’s about the weight of 220,000 African bush elephants. Imagine, if you will, a couple hundred thousand elephants made of mercury afloat in the planet’s atmosphere. It’s a lot of mercury.)

But what goes up must come down, so the saying goes, and eventually, this airborne mercury descends to the surface of earth, contaminating soil and water, flora and fauna, fungi and bacteria. In aquatic environments, some of these bacteria react with fallen mercury atoms to form methylmercury—a molecule with damaging neurological, immunological, reproductive and endocrine-disruptive effects in wildlife and human beings alike. And once methylmercury enters the food chain, it bioaccumulates, concentrating in the tissues of whatever unfortunate organisms happen to encounter and consume the stuff.

“The major route of human exposure to methylmercury,” a 2014 study published in the Journal of Preventive Medicine & Public Health states, “is largely through eating contaminated fish, seafood, and wildlife which have been exposed to mercury through ingestion of contaminated lower organisms.” 

Nearly every human being on the planet has detectable levels of methylmercury within their bodies. It’s toxic to the heart, lungs and liver, among other organs. It’s an endocrine disruptor that adversely affects the adrenal glands and the thyroid gland. It causes oxidative stress and damage to mitochondria, among other toxicological effects too numerous to list here. The metal is neurotoxic, and in human beings, methylmercury readily crosses the blood-brain barrier, where it has been shown to damage brain cells, disrupt gene expression, mutate DNA and kill neurons. It crosses the placental barrier, too, and in the United States alone, as many as 75,000 babies are born with learning disabilities every year as a direct result of methylmercury exposure in utero, according to data provided by the Environmental Protection Agency (EPA). 

Simply put, there’s no “safe” level of mercury within the human body—a particularly concerning fact in light of the metal’s prevalence in the environment.

Mercury is a global pollutant, impacting the lives of billions of people around the world. But open questions of mercury pollution remain. What environments are most threatened by mercury pollution? Why are some environments more threatened than others? How can we better monitor and regulate mercury pollution on local, national, and global scales?

Of all things, a few thousand dragonfly larvae and a small army of citizen scientists just might provide some answers to these questions.

“We set out with a couple of goals,” Dr. Collin Eagles-Smith, research ecologist at the United States Geological Survey and lead author of a recent study published in Environmental Science & Technology said in an interview. “One was to identify an indicator that can be used to demonstrate the disconnect between the amount of mercury coming into the environment and the amount posing risks to wildlife and humans.” Another goal of the research was to develop a method of measuring mercury pollution that can be standardized and compared across many different ecosystems, Dr. Eagles-Smith continued. “The only way to really do that at a big scale is to sample a lot of locations that have very different types of environments.”

For years, scientists have been searching for a proper “biosentinel” to evaluate and compare the scale and severity of mercury pollution in the global environment. Ecologists and wildlife biologists have long used fish as a proxy to monitor mercury pollution, with the practical benefit that fish pose direct mercury exposure risks to humans. Fish bioaccumulate mercury in their muscle tissue, and when people eat mercury-tainted fish, we can consume dangerously high levels of the metal. But the lack of identical fish species swimming in America’s rivers, lakes, streams, and other waterways proves a hurdle in the study of nationwide mercury pollution. On top of that, different fish in different waters accumulate pollutants like mercury at different rates.

“It’s not an apples to apples comparison,” Dr. Eagles-Smith says of mercury bioaccumulation in various fish species. 

Dragonfly larvae, however, may have more to tell us about mercury pollution.

Dragonfly larvae are exposed to mercury in much the same way that fish and human beings are—through the foods they eat. As a result, mercury, most of which is stored in the form of methylmercury, swells in concentration within the tissues of these developing insects. And their tissues, in the words of the study, “reflect the localized food web MeHg [methylmercury] availability.” Dragonfly larvae, in plain English, can be used as a tool—in ecology jargon, a “biosentinel”—to measure mercury pollution. 

The hard part is finding all those larvae to test.

That’s where the citizen scientists come in. The Dragonfly Mercury Project, a collaborative effort between National Park Service scientists and plain ole’ civilian folks, began this research back in 2009. Thousands of volunteers—equipped with sampling kits and training from professional scientists in how to identify and collect dragonfly larvae—wandered near and far across America’s public lands. Over the better part of a decade, they collected some 15,000 dragonfly larvae from nearly 500 locations across a hundred National Park units. And in the process, the project provided ecologists like Dr. Eagles-Smith and his colleagues with a lot of data.

About half of the collected dragonfly larvae belong to the Aeshnidae family—a family of dragonflies that inhabit a diverse array of environments throughout North America and around the world. Using these widespread dragonfly larvae, Dr. Eagles-Smith and his team were able to measure and compare mercury pollution levels across an array of ecosystems in America’s National Parks.

Not only are U.S. National Parks wildly polluted with mercury, but the severity of mercury pollution varies wildly across the country, as well—sometimes paradoxically so. Dragonfly larvae collected at the Colorado National Monument, for example, out on the Colorado plateau, where the Colorado River cuts a deep, spectacularly scenic gorge through 1.7 billion year-old red sandstone, contained methylmercury levels 135-times greater than larvae sampled at Bear Creek Lake Park, a city park in a suburb on the southwestern edge of Denver, the state’s most densely populated and thoroughly developed city. The Colorado National Monument, in fact, showed the highest concentrations of methylmercury measured at any site included in the study, and this particular finding highlights a larger trend in the data. Methylmercury concentrations detected in North American Desert ecosystems were, on average, about four times higher than those measured on the Great Plains—results supported by a 2016 study carried out by Dr. Eagles-Smith and colleagues, published in Science of the Total Environment.

Mercury levels even vary between different sites in the same National Park. Across 13 sites in Washington state’s Olympic National Park, mercury levels varied 33-fold. At Mississippi’s Gulf Islands National Seashore, mercury levels differed 32-fold. At Mount Rainier National Park in Oregon: 13-fold. Maine’s Acadia National Park: 12-fold. Yellowstone National Park: 11-fold. Additionally, and somewhat counter intuitively, the highest levels of mercury contamination detected in the study were found in river and stream habitats, as compared to slower-moving and completely still bodies of water like lakes, ponds and wetlands. 

Within National Park boundaries and across the country at large, mercury levels are in flux. However, the mechanisms explaining why these fluctuations occur remain unknown.

“What we can’t say yet is what is responsible for these broad-scale patterns that we see in different types of ecosystems,” Dr. Eagles-Smith said.

But such patterns may have important implications for human and ecological health. So, Dr. Eagles-Smith and his team turned their attention to nearby aquatic life. They compared mercury levels in dragonfly larvae with those found in fish living in the same ecosystems. Methylmercury levels in dragonfly larvae, the team discovered, can actually predict the severity of mercury pollution in the surrounding environment. 

The researchers developed a quantitative model mapping methylmercury concentrations found in dragonfly larvae to those found in nearby fish species—species for which the Environmental Protection Agency has developed human risk exposure guidelines. About 12 percent of the sites examined in the study were found to exceed “high or severe benchmarks of fish, wildlife, or human health risk,” as defined by the EPA’s regulatory criteria. About 56 percent of sites posed moderate health risks, 22 percent met low risk standards, and only 10 percent of sites examined in the study “were below any of the deleterious effect benchmarks.” Roughly 90 percent of the sites studied, in other words, among hundreds sampled across some of the most wild and pristine ecosystems in America, were polluted with mercury to such an extent that the health of humans, fish and other wildlife may be threatened, to varying degrees, in and around those ecosystems. 

While the widespread problems of mercury pollution have been known for decades, the work conducted by Dr. Eagles-Smith and his colleagues—along with an extended network of citizen scientists affiliated with the Dragonfly Mercury Project—has revealed the scale of mercury pollution on U.S. public lands in greater clarity. The work provides researchers with new tools, as well. These tools may help scientists to better monitor and mitigate mercury contamination across the country, and may help to inform new regulatory action on mercury pollution on local and national scales. 

“Our work demonstrated that this variation on the landscape is an important thing to characterize and when we can understand what drives the variation, then it can provide us with tools to begin to address that variation.” Dr. Eagles-Smith said of the fluctuating mercury levels discovered throughout America’s National Parks. “If we understand what is making mercury high within a certain water body, then we might have the ability to manage the landscape in such a way that we can reduce mercury.”