Dragonfly larvae have long been considered an indicator of good water quality because many species require clear, clean water to thrive. Finding any of those species in a water body suggests that the water is healthy. Now a nationwide citizen science project led by a Dartmouth College researcher has found that the larvae of these predatory insects can also be used to determine the level of mercury pollution in a water body.
The findings will make it easier to conduct mercury research, according to Celia Chen, director of Dartmouth’s Toxic Metals Superfund Research Program, and could lead to a national registry of pollution data on mercury.
Dragonfly larvae can be found in rivers, streams, ponds, and lakes of all sizes, and the larvae typically spend nearly a year in the water before emerging to transform into adult insects. The larvae take up mercury in its toxic form from the water, just as fish, birds, and amphibians do, accumulating the mercury in their body tissues.
“Researchers needed a proxy for fish, since that is what people and animals eat,” said Chen, co-author of a study published in the journal Environmental Science & Technology. “Fish can be hard to work with for a national-level research program, so it’s helpful to be able to focus our research on dragonfly larvae.”
Methylmercury, the organic form of the toxic metal mercury, poses health risks to humans and wildlife through the consumption of fish. Mercury pollution primarily comes from power plant emissions and other industrial uses.
With the help of National Park Service rangers, thousands of citizen scientists from across the country – mostly students and park visitors – collected larval dragonfly specimens from nearly 500 aquatic sites in the National Park System between 2009 and 2018 as part of the Dragonfly Mercury Project. Their results revealed that about two-thirds of the sites were polluted with moderate to extreme levels of mercury. Because mercury is distributed widely in the atmosphere and deposited in water bodies around the world, the findings reflect the levels of mercury throughout the country.
Among the study’s other findings, the researchers concluded that mercury appears in higher levels in faster moving bodies of waters, for example in rivers and streams, than in ponds, wetlands, and other slow-moving water bodies.
“Collectively, this continental-scale study demonstrates the utility of dragonfly larvae for estimating the potential mercury risk to fish and wildlife in aquatic ecosystems and provides a framework for engaging citizen science as a component of landscape mercury monitoring programs,” wrote the researchers.
The project was originally launched in 2007 by Sarah Nelson, associate professor of forest resources at the University of Maine, and expanded by Dartmouth into New Hampshire and Vermont, where citizen scientists in the Upper Valley collect dragonfly larvae each fall.
“The support of citizen scientists around the country created the opportunity for this study to have such significance,” said Chen. “This is a terrific example of how public outreach around science can bring results that help the entire country.”