Dragonfly Larvae are Effective Bioindicators of Mercury Exposure in Fish and Amphibians—Results of Citizen Science in 100 National Parks and Protected Places

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Mercury concentrations were measured in dragonfly larvae across more than 450 sites in 100 national parks and protected places as part of a partnership among Federal agencies, academic researchers, and more than 4,000 citizen scientists.  Mercury concentrations in dragonfly larvae were positively correlated with mercury concentrations in fish and amphibians living in the same aquatic environments, thus providing a tool to predict mercury exposure in other wildlife. This study also provides a framework for engaging citizen science as a component of research and monitoring programs.


Citizen Scientists identifying invertebrates collected in  Yellowstone National Park

Citizen scientists along with National Park Service personnel identifying invertebrates as part of the Dragonfly Mercury Project. (Credit: National Park Service. Public domain).

Environmental mercury in its organic form (methylmercury) biomagnifies through food webs, posing potential health risks to fish, wildlife, and humans. Methylmercury in fish tissue is currently (2020) a leading cause of fish consumption advisories in North America. Mercury trend detection in environmental media is needed for understanding the success of mitigation efforts to reduce mercury exposure. Mercury measurements made in air, water, sediment, and soil help document environmental sources and emissions, but are either poor or inconsistent predictors of mercury concentrations within biota of associated food webs.

Therefore, the U.S. Geological Survey (USGS) Ecologically-Driven Exposure Pathways Science Team implemented  a national scale study, supported by the USGS’s Environmental Health Programs (Contaminant Biology and Toxic Substances Hydrology), the National Park Service (NPS), University of Maine, and more than 4,000 citizen scientists, to determine if dragonfly larvae could be useful bioindicators to help predict and understand wildlife mercury exposure.

Dragonflies were selected as bioindicators because they address many of the challenges associated with mercury measurements in other wildlife (including the lack of consistency in species among sites; movements and migratory behavior; and variations in factors such as body size, trophic position, and growth rates), which can contribute to variation and uncertainty. Samples were collected from 450 sites in 100 national parks and other protected places by citizen scientists paired with trained NPS staff. 

Dragonfly at Bowman Lake, Glacier National Park, Montana

Dragonfly at Bowman Lake, Glacier National Park, Montana. Dragonfly larvae were selected as bioindicators because they are widely distributed and abundant, are key components of aquatic food web energetic pathways, are relatively long-lived, and show high site fidelity (National Park Service. Public domain). 


Variation in mercury concentrations among various families of dragonfly larvae necessitated a conversion of concentrations to one common unit. The scientists developed Aeshnid-equivalent concentrations using the relation between mercury concentrations in Aeshnidae (the most commonly collected dragonfly family in the study) and other dragonfly families.  The Aeshnid-equivalent mercury concentrations were positively correlated with mercury concentrations in four different fish guilds (groups of species that exploit the same resources) and two types of amphibians (frogs and salamanders).

As an additional step, the team developed a tool (Integrated Risk Impairment Index) to inform potential wildlife health risks using Aeshnid-equivalent mercury concentrations and published information on mercury toxicity. Based on this exercise, 10 percent of sites were below any of the estimated impairment categories, whereas 22, 56, 11, and 1 percent of sites exceeded low, moderate, high, and severe estimated impairment categories, respectively.

This study demonstrates the use of dragonfly larvae as effective bioindicators of mercury exposure in several freshwater taxa and established a sampling network for mercury on protected lands across the United States. These findings highlight the efficacy of carefully designed citizen science efforts to facilitate studies that would be otherwise difficult to conduct because of large resource requirements.  More information on this study can be found in an associated geo-narrative.

The USGS Ecologically-Driven Exposure Pathways Science Team and their collaborators are continuing to develop predictive tools to understand the environmental pathways of contaminant exposure and to understand the effects of exposure on individuals, populations, communities, and entire ecosystems. Future applications of dragonfly larvae monitoring could be useful for understanding the drivers of contaminant availability to aquatic food webs; predicting the potential risk to vertebrates; and evaluating the effectiveness of mitigation actions to reduce contaminant exposure.