The Dragonfly Mercury Project
A citizen science framework for monitoring mercury pollution in U.S. national parks using dragonfly larvae as biosentinels.
The Dragonfly Mercury Project Story
A large network of people has been monitoring mercury in dragonflies as part of a study led by the U.S. Geological Survey (USGS) and National Park Service (NPS). These insects provide valuable information for understanding the potential health risks of mercury. Scroll down to discover why and how mercury is an environmental health concern. Keep scrolling to see learn about how USGS, NPS, and others have engaged citizen scientists to assist in the understanding of mercury contamination in our natural areas. Then take a deep dive into USGS findings through exciting interactive maps and charts. And explore the relationships between mercury, dragonflies, and other wildlife and humans.
Photo of citizen scientists inspecting dragonfly larvae at Acadia National Park (ME), from National Park Service.
Mercury Overview
Mercury (Hg) is a toxic pollutant spread around the world. It poses health risks to humans and wildlife. Atmospheric emissions of mercury can enter food webs in even the most remote and pristine settings.
Most of the mercury that results from human activities comes from mining and coal burning. Photo credit, Ella Ivanescu, Unsplash.
Mercury released into the air can travel thousands of miles before settling in even the most remote and pristine locations. After landing, naturally occurring microbes can convert the atmospheric mercury into a more toxic form, called methylmercury.
But this conversion only happens if certain environmental conditions are met. That means it’s possible to have high levels of methylmercury even though mercury inputs are low. It’s also possible to have low methylmercury levels even in areas where mercury inputs are high.
Methylmercury can be dangerous for fish, wildlife, and humans because it’s easily absorbed from food. Methylmercury also bioaccumulates – or builds up in the tissues of an organism. And it biomagnifies – or increases with each step up the food chain. So even small amounts of methylmercury in the environment can result in levels high enough to cause serious toxic effects, such as altered behavior and reduced reproductive success in fish, birds, and even humans.
Mercury biomagnifies, or increases in concentrations with each successive step up the food chain, schematic from National Park Service.
Schematic of Sources and Paths of Mercury in the Environment, from National Park Service.
Citizen Science
The Dragonfly Mercury Project ( DMP ) is a national surveillance, monitoring, and research program jointly run by USGS and NPS, in cooperation with Appalachian Mountain Club, Dartmouth College, and others. It brings together citizen engagement and education with scientific efforts to understand mercury risks to protected areas.
The DMP learns about mercury risks by collecting samples of dragonfly larvae – the water-dwelling, immature form of the adult insect. The dragonfly larvae serve as mercury biosentinels (indicators of ecosystem risk to mercury). The DMP informs mercury risk to protected lands, while also engaging more than 4,500 citizen scientists through over 20,000 hours of stewardship to this project.
The DMP works with a network of park staff and local citizen scientists to sample dragonfly larvae from various locations within national parks. Data are collected on each specimen in the field. Then the larvae are handled and preserved using clean protocols prior to shipment to the USGS laboratory. The samples then undergo the technical process of mercury analysis.
Dragonfly larva, family Aeshnidae, collected for the study from Olympic National Park (WA), photo from U.S. Geological Survey.
Dragonfly larvae are excellent mercury biosentinels for a number of reasons:
- Eat other insects so contain higher concentrations of mercury
- Found in most types of waterbodies
- Found on 6 continents
- Live in a variety of habitat types (tundra streams to desert oases)
- Easy and cost-effective to monitor
- Provide high-quality data on mercury condition at collection sites
- Reflect concentrations of mercury in fish and other wildlife
Photo of high school student identifying a dragonfly larva for mercury analysis as part of a "BioBlitz" Rocky Mountain National Park (CO), from National Park Service.
Study Findings
Explore the Data Through Interactive Maps and Charts
- Click on drop-down boxes to filter or highlight data
- Hover over map points for detailed park and site information
- Zoom in or out on maps to narrow or expand view
- Click on map points, legends, graph data points and bar charts to filter data
Sampling dragonfly larvae occurs across the country, even in places as remote as Katmai National Park & Preserve (AK), photo from National Park Service.
The interactive map shows the participating U.S. national parks and each year they have collected dragonfly larvae for the DMP project.
View the Participating Parks Here
As of 2018, the DMP has sampled more than 500 waterbodies in more than 100 U.S. national parks and other protected areas. Parks and waterbodies are added each year.
Click the play button in the interactive map to show the participating U.S. national parks time series map.
Dragonfly larvae are voracious predators. This is evident from the mouthparts on this Cordulegastridae larva, which can extend outward to capture prey as they pass by. Photo credit, Jan Hamrsky.
The interactive map shows the participating U.S. national parks in a time series map.
Mercury Concentrations in Dragonfly Larvae Range Widely Across the Landscape
Total mercury concentrations, or THg, in dragonfly larvae vary widely both among park units as well as among sites within parks. THg in larvae even vary among sites that are very close to one another. Average mercury concentrations at sites are spread across a wide range. In fact, concentrations from the highest site were 135 times higher than those from the lowest site.
This extreme variation highlights the importance of site characteristics in determining dragonfly larvae mercury exposure. It also stresses the need for sampling across many sites to gain a better picture of relative mercury risk.
A group of citizen scientists preparing to sample dragonfly larvae at Pinnacles National Park (CA), photo from National Park Service.
The interactive figure shows the total mercury (THg) concentrations in dragonfly larvae collected from National Park units across the conterminous U.S., Alaska, and Hawaii. Bars represent park unit geometric means, and circles represent geometric mean THg concentrations for individual sampling sites (populations) within each unit. THg concentrations are in nanograms/grams dry weight (ng/g dw).
Environmental Conditions Can Influence Mercury Bioaccumulation
Dragonfly larvae mercury concentrations also vary among the types of waterbodies from which they were collected.
Dragonfly larvae from rivers and streams had higher mercury concentrations than those from ponds and lakes. Interestingly, mercury in dragonflies sampled directly from wetlands was relatively low compared to other habitats. Yet rivers, streams, lakes, and ponds surrounded by wetlands had higher dragonfly mercury concentrations than those without wetland influences.
These findings highlight the complex relationships between environmental conditions and mercury bioaccumulation.
The interactive figure shows the least squares mean total mercury concentrations (ng/g dw) in dragonfly larvae among habitat types in national park units across the United States.
Mercury Concentrations in Dragonfly Larvae Vary Among Ecoregions
Ecoregions are areas of the country with similar climate and landscape features. By looking at how dragonfly mercury differs among these ecoregions, we can learn how landscape features influence mercury bioaccumulation.
Sampling locations spanned 13 of the 15 North American ecoregions. We found that dragonfly mercury concentrations differed among many of them. After accounting for the influence of habitat type, dragonfly larvae from the North American Deserts ecoregion had the highest average mercury concentrations. Those collected in the Great Plains ecoregion had the lowest average mercury concentrations.
Scientists have only recently begun to uncover the potential for desert ecosystems to have elevated mercury in fish. Our study suggests the processes leading to higher mercury bioaccumulation in these ecosystems also affect dragonflies.
Desert habitat sampled for dragonflies at Capital Reef National Park (UT), photo from National Park Service.
The interactive figure shows the least squares mean dragonfly larvae total mercury concentrations (ng/g dw) among level I ecoregions within the continental United States and Alaska.
Mercury Accumulation Differs Among Dragonfly Families
There are seven families of dragonflies in North America. Each has different life histories and foraging strategies that can influence how much mercury they accumulate. It’s important to understand how mercury concentrations differed among these families to make sure data are comparable when different families are sampled from different locations.
Dragonfly larvae families have very different body types as shown in this photo of the exuviae from two families, a Gomphidae (left) and an Aeshnidae (right). Exuviae are the cast-off skins left aside when the dragonfly emerges from the larva. Photo from National Park Service.
Mercury concentrations did vary among the five families sampled. The relationships between families were predictable and consistent. So, we developed equations to translate mercury concentrations from one family to comparable concentrations in other families. This allowed us to develop a single mercury index across all sites.
We found that 80% of the mercury in dragonfly larvae is methylmercury. This allowed us to use faster and less costly total mercury analyses to represent the risks posed by methylmercury. Methylmercury poses the greatest health risks, but measuring it is time-consuming and expensive.
The interactive figure shows the least squares mean total mercury (THg) concentrations (ng/g dw) in larvae of five dragonfly families collected from 457 sites across the United States, and the relationships between THg concentrations among families. Libellulidae and Corduliidae are combined as a single family group because their distinction can be uncertain without the identification of the genus.
Dragonfly Larvae Predict Mercury in Fish and Wildlife
We paired dragonfly mercury concentrations with those of fish or amphibians sampled from the same sites. We found that mercury in dragonfly larvae predicted mercury in these other sentinel species. That means we can use dragonfly mercury levels to estimate exposure in other members of the same ecosystem.
Dragonfly larvae are easier to collect and more widespread than fish and amphibians. That allows us to cost effectively assess more waterbodies throughout the nation for mercury risk.
Brook trout sampled for mercury from Mount Rainier National Park (WA), photo from U.S. Geological Survey. Dragonfly larvae mercury can serve as a surrogate for fish mercury concentrations, and represent the risk of mercury to aquatic ecosystems.
The interactive figure shows the relationship between Aeshnid-equivalent dragonfly larvae total mercury (THg) concentrations and THg concentrations in other fish and wildlife. Fish THg concentrations are in micrograms/gram wet weight (µg/g ww) and Aeshnid-equivalent and amphibian concentrations are in ng/g dw.
Developing an Index of Integrated Impairment for Fish, Wildlife, and Humans
Impairment benchmarks for mercury are the concentrations when an organism’s health may be harmed. Scientists can use the benchmarks to gauge the threat mercury poses to a given species at a particular location, but mercury concentrations are not the same for all fish and wildlife species in a location. Therefore, information from one species is not always helpful for understanding risk to others.
We can use mercury concentrations in dragonfly larvae to estimate concentrations in various fish and wildlife species. We developed a series of integrated impairment indices that estimate the potential threat of mercury to a range of taxa within an ecosystem. These index categories are based on both the number of taxa exceeding various benchmarks, as well as the severity of the benchmarks themselves. The index we developed provides important information on potential mercury risks in all the study’s parks and sites. In fact, an index of moderate impairment or higher suggests some fish species may exceed the US EPA benchmark for protection of human health.
Mercury is the leading cause of fish consumption advisories in the United States. Dragonfly larvae can be used as a simple and effective tool for assessing the status of mercury contamination in waterbodies and informing the likelihood that fish may exceed health guidelines. Photo credit, David Boozer, Unsplash.
This figure shows the integrated impairment indices for potential ecosystem risk to mercury. Integrated impairment index categories (specified by different colored boxes) represent concentrations corresponding to exceedances of a range of individual published toxicity benchmarks across several fish guilds. The symbols and lines represent the modeled Aeshnid-equivalent total mercury (THg) concentrations associated with the THg concentrations of each impairment benchmark for each fish guild. Circles represent fish dietary benchmarks, triangles represent fish health benchmarks, diamonds represent avian dietary benchmarks, and squares represent US EPA MeHg criterion. White, gray, and black symbols reflect low, moderate, and high-severity benchmarks, respectively. See the publication for more information.
Impairment Risk Varies Across the Country
Overall, most sites were in the moderate to low risk category. However, 12% of sites, distributed across all regions of the U.S., were at high or severe risk for mercury toxicity.
These sites with elevated risk for mercury toxicity in protected areas show the widespread nature of mercury contamination. They also show the value in ongoing monitoring within the National Park Service system.
The interactive map shows the integrated impairment indices for all 457 sites sampled between 2009 and 2018. The inset for the cumulative frequency distribution illustrates the proportion of sites and years that fall into each of the five categories.
Next Steps
The DMP advances mercury science and management in several ways. A surveillance network has been established that covers the United States. Much information has been gained about the environmental conditions that cause increase mercury risk. And an integrated impairment index has been developed that informs wildlife and human health risk from mercury based on dragonfly concentrations.
As the DMP continues, ongoing data collection will allow a greater understanding of how mercury is distributed across the landscape and how it changes over time and in response to management actions. As a result, efforts continue that will better inform how physiological and ecological conditions influence how dragonfly mercury concentrations are interpreted. The long time series of data will help inform how environmental mercury concentrations respond to reductions in global mercury emissions. Additional sites will facilitate modeling to learn which landscape drivers influence mercury risk.
Photo of Azure Darner adult dragonfly from Lake Clark National Park & Preserve (AK), from National Park Service.
More Information
Contact us if your organization is interested in participating in the Dragonfly Mercury Project!
For more information about this study, please see the following USGS publication : Eagles-Smith, C., J. Willacker, S. Nelson, C. Flanagan Pritz, D. Krabbenhoft, C.Y. Chen, J. Ackerman, E. Grant, and D. S. Pilliod. 2020. A National-Scale Assessment of Mercury Bioaccumulation in United States National Parks Using Dragonfly Larvae as Biosentinels Through a Citizen Science Framework. Environmental Science & Technology.
To view or download the data, see the USGS Data Release .
For more information about the Dragonfly Mercury Project, see the Project Web Page .
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Photo of citizen scientists after collecting dragonfly larvae from Rocky Mountain National Park (CO), from National Park Service.
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