Dragonflies reveal mercury pollution levels in national parks

In a study that started out as a local project to collect dragonfly larvae, researchers have found that the young dragonflies can be used as an indicator of mercury pollution. The discovery will be useful for estimating mercury levels in birds, fish, and amphibians.

Study co-author Celia Chen is the director of the Toxic Metals Superfund Research Program at Dartmouth College.

“Researchers needed a proxy for fish since that is what people and animals eat,” said Chen. “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.”

Dragonflies are found in diverse freshwater habitats across six continents. They have tissues that take up mercury in its toxic form, or methylmercury, which is formed by microbial activity in aquatic environments. 

From 2009 to 2018, more than 4,000 citizen scientists helped to collect almost 15,000 dragonfly larvae in 100 national parks across the United States. The researchers analyzed the samples for total mercury and methylmercury. 

“The support of citizen scientists around the country created the opportunity for this study to have such significance. This is a terrific example of how public outreach around science can bring results that help the entire country,” said Chen.

Elevated mercury concentrations were found at several parks, but these levels varied by more than a hundredfold among different sites throughout the parks.

Mercury concentrations were found to be higher in larvae from rivers and streams than in larvae from lakes and ponds.

The experts also determined that mercury concentrations correlated with those in fish and amphibians at the same sites, which led to the development of a tool to predict mercury in other species using dragonfly larvae. 

Overall, the research showed that about two-thirds of the aquatic sites sampled were polluted with moderate-to-extreme levels of mercury.

Since the parks examined for the study span the entire United States, including Alaska and Hawaii, the findings reflect mercury levels throughout the country.

“To date, we have not conducted such a broad scale survey on mercury in the U.S. The beauty of the dragonfly data set is that it is national, covers a huge area with different systems, and has the potential to create a national baseline of mercury pollution information,” said Chen.

The study is published in the journal Environmental Science & Technology.

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By Chrissy Sexton, Earth.com Staff Writer