Mercury is a common environmental contaminant found in many ecosystems across the United States. More commonly, researchers track mercury levels by analyzing its presence in fish and birds. A recently published study in the journal Environmental Science & Technology evaluated using dragonfly larvae to provide a more effective method for monitoring environmental mercury.
Benefits of using dragonfly larvae to track mercury
Fish are not always present in all ecosystems, especially in regions where water sources are intermittent or scarce, such as deserts. These arid environments may have water bodies that dry up seasonally or only appear after rare rain events, making them unsuitable habitats for fish.
Dragonflies are far more widespread and adaptable. They can be found in a diverse range of aquatic habitats, including those that are temporary or highly variable, such as ponds, streams, wetlands, and even small, ephemeral pools in deserts.
Dragonfly larvae, which live in water, can survive in these fluctuating environments due to their shorter life cycles and ability to thrive in conditions that would be challenging for fish.
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Dragonfly larvae are effective bioindicators of environmental mercury
The widespread presence of their larvae makes dragonflies excellent bioindicators for monitoring environmental health, including mercury contamination. The ability of dragonflies to inhabit a wide variety of freshwater ecosystems allows researchers to gather data from a broader range of locations, providing a more comprehensive understanding of mercury distribution and its impact on different environments. Research has shown that mercury levels in dragonfly larvae correlate well with those in fish and other wildlife.
Dragonflies have high site fidelity, meaning they stay in one area. This means that they are reliable for tracking local mercury levels. Their long larval stage, narrow range of prey, and prevalence across various freshwater habitats make them ideal for monitoring mercury over time and space.
How mercury enters the environment
Mercury (Hg) enters the environment through both natural processes and human activities. Natural sources of mercury include volcanic eruptions, weathering of rocks, and forest fires, which release mercury stored in the Earth’s crust into the atmosphere. Mining and coal burning are the two largest sources of mercury from human activities in the United States.
Once in the atmosphere, mercury can travel long distances before settling back to the ground through wet deposition (rain and snow) and dry deposition (direct deposition onto surfaces).
Environmental mercury can be toxic to humans and wildlife
Once deposited, mercury can be transformed by microbes into methylmercury, a more toxic form. This conversion depends on specific environmental conditions, meaning high levels of methylmercury can occur even with low mercury inputs and vice versa. Methylmercury poses a significant risk to fish, wildlife, and humans because it is readily absorbed from food and accumulates in tissues.
Methylmercury also magnifies through the food chain, leading to potentially harmful levels even from small environmental concentrations, causing issues like altered behavior and reduced reproductive success in various species.
Mercury Integrated Impairment Index
The Mercury Integrated Impairment Index classifies mercury concentrations to estimate the potential toxicological risks to fish, wildlife, and humans. It uses Aeshnid-equivalent total mercury (THg) concentrations, which correlate mercury levels in dragonfly larvae with benchmarks for health risks across various fish groups. These benchmarks span a range of potential hazards:
- Sub-impairment (below 60 ng/g dw): Only piscivorous fish (fish that eat other fish) pose a low health risk as prey.
- Low impairment (60–100 ng/g dw): Piscivorous fish and sunfish exceed low-risk dietary benchmarks for birds and fish.
- Moderate impairment (100–300 ng/g dw): Several fish groups and dragonfly larvae exceed moderate-risk health benchmarks and the EPA’s human health criterion for methylmercury.
- High impairment (300–700 ng/g dw): Piscivorous fish and sunfish exceed high-risk dietary and health benchmarks for fish and birds, with several other fish groups also exceeding moderate-risk benchmarks.
- Severe impairment (above 700 ng/g dw): High-risk benchmarks for fish health and dietary intake are exceeded across multiple fish groups, posing significant ecological and health risks.
Analyzing geographic variation in mercury in the United States
The study analyzed 161 unique testing sites within 73 National Park Service locations in order to look at regional variation among mercury levels within the United States. 109 of the 179 samples were taken from the dragonfly family Aeshnidae.
The study categorized data by ecoregions and habitat types to understand mercury isotope patterns within and between parks. This categorization helped in analyzing the differences in mercury deposition and bioaccumulation across various landscapes and water bodies.
The ecoregions included:
- Alaska Interior
- Eastern Coastal Plains
- Eastern Temperate Forests
- Great Plains
- Hawaii Islands
- Mediterranean California
- Mountain West Coast Forest
- North American Desert
- Western Cordillera
Each study site was categorized into one of three habitat types:
- Lentic (lakes and ponds)
- Lotic (rivers and streams)
- Wetland
Differences in mercury isotopes between the eastern and wester U.S. parks
Research involving dragonfly larvae from 73 U.S. national parks revealed several important insights:
Deposition pathways: Differences in mercury isotope values helped identify the relative importance of wet and dry deposition. For example, arid regions showed more influence from wet deposition, while forested areas indicated significant dry deposition inputs.
Geographic patterns: Mercury isotope values varied across different regions and habitat types. Eastern parks, influenced by numerous domestic emission sources, showed different isotope signatures compared to western parks, which are more affected by global emissions and wildfires.
Environmental factors: Factors like dissolved organic carbon (DOC), total phosphorus, and vegetative shading (measured by the normalized difference vegetation index, NDVI) significantly influenced mercury isotope values. Higher DOC levels in lotic (flowing water) systems were associated with lower photochemical degradation of mercury.
Implications for mercury management
Understanding how mercury is deposited and cycles through different ecosystems is crucial for effective management and mitigation. By using dragonfly larvae as bioindicators, researchers can track changes in mercury contamination over time and across large geographic areas. This approach helps identify ecosystems that are more sensitive to mercury inputs and those that may respond more quickly to reductions in atmospheric mercury emissions.
References
Janssen, S. E., Kotalik, C. J., Willacker, J. J., Tate, M. T., Pritz, C. M. F., Nelson, S. J., … & Eagles-Smith, C. A. (2024). Geographic Drivers of Mercury Entry into Aquatic Food Webs Revealed by Mercury Stable Isotopes in Dragonfly Larvae. Environmental Science & Technology. DOI: 10.1021/acs.est.4c02436.
Eagles-Smith, C. A., Willacker, J. J., Nelson, S. J., Flanagan Pritz, C. M., Krabbenhoft, D. P., Chen, C. Y., … & Pilliod, D. S. (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, 54(14), 8779-8790. DOI: 10.1021/acs.est.0c01255
Fonte : National Geographic
