Dragonflies as Metal Detectors

A clear plastic bag holding a tiny dragonfly larva on the edge of Calf Creek at Buffalo National River.

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Mercury is a chemical element that occurs naturally in rock in the earth’s crust. When released from rock, mercury is a toxic pollutant that can harm humans and wildlife. It enters the atmosphere through natural events like volcano eruptions and forest fires but more commonly through human activities like mining and burning fossil fuels. Airborne mercury can spread via wind, rainfall, and gravity into soils and waterways. Mercury is hard to detect, as it is odorless. However, scientists can use other indicators to detect mercury pollution in the environment.

Spending years underwater eating insects and even small fish, dragonfly larvae are high on the aquatic food chain. Each time a predator eats prey with mercury inside, that toxin builds up in the predator’s body. The build up of mercury in dragonfly larvae gives researchers an understanding of air and water quality.

The bioaccumulation of mercury through the aquatic food chain, starting with microscopic organisms, then macroinvertebrates, then dragonfly larvae, then fish.

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Why are dragonfly larvae such good indicators of mercury in waterways?

• They spend most of their lives in aquatic environments

• They feed on small underwater organisms during their larval stage

• Their diet causes mercury to build up in their bodies over time

• They’re easy to collect and identify

Students and scientists hold collection nets and smile at us from the bank of Calf Creek at Buffalo National River.

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The Dragonfly Mercury Project is a nationwide citizen science effort that helps researchers better understand the amount and distribution of mercury pollution across North America. This project is a partnership between the University of Maine, National Park Service, United States Geological Survey (USGS ), and other partners.

From 2009 to 2018, around 460 sites were sampled nationwide. More than 100 of these sites were at national parks. In 2018, volunteers and researchers collected dragonfly larvae and recorded site conditions at three locations at Buffalo National River . The samples were shipped to a lab to determine the amount of mercury present in the larvae.

Results showed that although some mercury was present at all three sites, the amount was below the benchmark of potential toxicological risk to fish and wildlife. However, mercury contamination may be higher than recorded because the estimated data only addressed potential risk to wildlife that directly consume dragonfly larvae. The data did not consider the build up of mercury higher in the food chain (example: fish eating fish). Even so, the fact that mercury was present at all three sites gives researchers insight into the air and water quality at Buffalo National River.

Learn more at: https://www.nps.gov/articles/dragonfly-mercury-project.htm