Photo by Emma Oxford
The most glorious of insects
The blue dasher, Pachydiplax longipennis, is one of 70 species of dragonflies found in Great Smoky Mountains National Park.
With 360-degree vision, bright-colored bodies that sparkle jewel-like in the sun, and acrobatic flight patterns reaching speeds of nearly 35 miles per hour, dragonflies are some of the more glamorous members of the insect world.
But they’re also reservoirs of mystery. In just the past five years, Great Smoky Mountains National Park has added five new species to its list of 101 resident members of the Odonata order, which includes 70 dragonflies as well as 31 species of their cousins, the damselflies. Recent research has revealed that some species of these enigmatic creatures migrate more than 400 miles in their search for suitable breeding grounds.
“They’re also an important link in the food chain,” said Becky Nichols, the park’s entomologist. “Other things prey on them, but they prey on smaller things.”
This characteristic makes dragonflies invaluable indicators of mercury pollution. In an expansive study underway since 2011, the Dragonfly Mercury Project, a collaboration between the National Park Service and US Geological Survey, has sampled dragonfly larvae at nearly 1,000 sites scattered from the South Pacific to the US–Canada border, seeking to better understand how mercury contamination affects this varied landscape. But the project started with the cooperation of just four parks in the eastern US—and Great Smoky Mountains National Park was one of them.
The Smokies’ extensive commitment to scientific discovery—through collaboration with both professional scientists and enthusiastic community members—made it a “no brainer” to include the park in the pilot program, said project co-lead Colleen Flanagan Pritz, an ecologist for the NPS Air Resources Division.
Mercury is a heavy metal that acts as a neurotoxin, and high concentrations can cause symptoms ranging from peripheral vision loss to walking, speech, and hearing impairment. If exposed in the womb, children can face disabilities in vital areas such as cognitive thinking and fine motor skills. Though mercury can enter the environment through natural sources like volcanoes and geothermal springs, in the last 150 years human activities such as coal combustion, waste incineration, industry, and mining have caused atmospheric mercury to more than double.
But before it can become harmful to humans, mercury must be converted by microorganisms like bacteria from the inorganic form found in polluted air to methylmercury, an organic molecule that living things can absorb. Once consumed, methylated mercury never leaves an animal’s body, and it accumulates in higher levels up the food chain as predators consume contaminated prey.
Since 2019, the Tennessee Department of Environment and Conservation has placed a precautionary fish consumption advisory on smallmouth bass in Abrams Creek and Little River due to mercury concerns. But regularly testing bass and other game species residing in the 2,900 miles of streams flowing through the park is both expensive and time-consuming. Because they’re so much easier to capture and cheaper to test, dragonfly larvae are gaining recognition as an answer to this monitoring conundrum.
“They’ve emerged as a Rosetta Stone, where we’re able to take mercury in dragonflies and translate that into what it means for other organisms,” said project co-lead Collin Eagles-Smith, a supervisory research ecologist for the US Geological Survey Forest and Rangeland Ecosystem Science Center in Corvallis, Oregon. “That can be particularly effective if you’re worried about species that are threatened or endangered, because you don’t have to actually handle or sample them—you can use dragonflies as a proxy.”
Unlike adult dragonflies, larvae are relatively easy to capture and tend not to move around much. Fierce predators, dragonfly larvae use internal hydraulic pressure and a special prehensile structure called a labium to capture everything from mosquito larvae to small fish. These characteristics make them ideally suited to indicate how much mercury is contained in the creatures sharing their pond or stream segment during this often multi-year stage of development.
The Dragonfly Mercury Project has grown from four parks to include more than 180 national parks and protected places across nearly 1,000 sampling sites, with over 8,500 citizen scientists contributing data. In the Smokies, more than 300 specimens have been submitted from 13 different sites on 25 different occasions, collected by the entomology and fisheries teams as well as by participants in the park’s annual high school internship program.
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Photo courtesy of the National Park Service
Word from the Smokies
High school interns gather dragonfly larvae in Chasteen Creek, located in Great Smoky Mountains National Park near Cherokee, to collect data for the Dragonfly Mercury Project in 2014.
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Photo courtesy of the National Park Service
The most glorious of insects
Nets, identification keys, tubs, and Ziplock bags are the tools of the trade during Dragonfly Mercury Project sampling events.
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Photo courtesy of the National Park Service
The most glorious of insects
A high school intern lifts a dragonfly larva from her net during a 2015 sampling event.
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Photo by Richard Toller via Flickr
The most glorious of insects
A dragonfly emerges from its larval exoskeleton. Within hours, its wings will be capable of carrying its body through fast, complex flight patterns.
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Photo by M. Reilly, courtesy of the National Park Service
The most glorious of insects
Dragonfly larvae like this one are fierce predators, using internal hydraulic pressure and a special prehensile structure called a labium to capture everything from mosquito larvae to small fish.
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Image courtesy of the National Park Service and US Geological Survey
The most glorious of insects
In Great Smoky Mountains National Park, samples taken from 13 sites on 25 different occasions have contributed to the Dragonfly Mercury Project dataset.
“We’ve used this project to look at a lot of different questions,” said Paul Super, science coordinator for the Smokies. “When we had the Chimney Tops 2 fire, we collected dragonflies downstream of areas that burned that year just to see if mercury might peak, which I don’t believe it did. We looked at streams that were EPA-listed as being out of compliance because of atmospheric deposition, so we were curious if the mercury would be higher there. We’ve looked at streams that have had fish advisories. We’ve also looked at some streams over multiple years of this project to see if there is a change.”
Of the 25 different sampling sites in the Smokies, 21 (84 percent) yielded a result of moderate mercury risk, meaning that mercury could harm people who eat fish, fish-eating birds and fish, and insect-eating fish. Three sampling sites (12 percent) showed low contamination, and one sampling site (4 percent) showed a high effect range. Those results indicate a greater risk of contamination in the Smokies than the average across the entire project, but a lower risk than in Maine, New Hampshire, and Vermont, where the other pilot sites are located.
“We don’t have as big a problem as we might, but we’re not free of mercury in our food webs,” Super said. “That’s the message we’re getting, is that we need to keep monitoring, be vigilant, and possibly change fish consumption advisories if conditions call for that.”
However, much remains unknown about mercury risk in the Smokies and how to prevent it. By analyzing data from the National Atmospheric Deposition Program, which monitors mercury deposition in the Smokies, the Dragonfly Mercury Project team found that reductions in inorganic mercury deposition don’t always correspond to a reduction in toxic methylmercury found in dragonflies.
“That could be because there’s some process in the environment that either is enhancing the conversion into methylmercury or preventing it from happening,” Eagles-Smith said. “And so it can be really difficult to determine how much of that incoming mercury ends up within a food web.”
Flanagan Pritz and Eagles-Smith hope to see the Dragonfly Mercury Project continue to gather new data and expand to additional sites, offering improved insight into what drives methylmercury accumulation in the food web.
“By having a vast dataset across the country in different types of environments, we’ve been able to build a model that can actually predict how much mercury is going to be in a habitat based on its environmental characteristics,” said Eagles-Smith.
Adding new data to that model will help scientists better understand how changes in inorganic mercury deposition impacts the prevalence of toxic methylmercury in different habitats. But that’s not their only strategy.
The Dragonfly Mercury Project team also wants to conduct more extensive simultaneous collection of fish and dragonfly larvae samples, producing robust data that will help them better refine models that use dragonfly data to predict fish mercury concentrations. Meanwhile, newly available analytical tools let the team “fingerprint” various forms of mercury, showing how deposition varies in different environments—as rain or snow in dry areas, for example, or as a gas in wet forests—and with potential to indicate whether contamination is coming directly from precipitation, from industrial activity, or due to release following long-term storage in the soil. These insights will offer the clearest picture yet on which actions might be most effective at reducing contamination.
It might be hard to believe so much information could be contained in these glittering, slender creatures that flit around ponds and riversides. But there is more to dragonflies than meets our far inferior human eyes.
“They’re cool organisms, and they’re helping us to better understand this potentially serious pollution issue,” Super said. “More power to dragonflies.”
About the author: Holly Kays is the lead writer for the 29,000-member Smokies Life, a nonprofit dedicated to supporting the scientific, historical, and interpretive activities of Great Smoky Mountains National Park by providing educational products and services such as this column.