Amphibians threatened by mercury, with unknown health impacts
The first wide-scale assessment of methylmercury, a very poisonous form of mercury, in adult amphibians in the U.S. to date has found that this toxic compound is common, widespread and, in some cases, can reach very high levels.
Studying amphibians and mercury
The study, titled “Broad-scale Assessment of Methylmercury in Adult Amphibians,” and published in the journal Environmental Science and Technology, encompasses data from over 3,200 amphibians spanning 14 species across 26 populations. This collaboration involved scientists from various regions of the country.
“Amphibians are the most endangered group of vertebrates worldwide, but until this study, we knew relatively little about the variability of mercury bioaccumulation in amphibians,” said Anne Kinsinger, the U.S. Geological Survey (USGS) Associate Director for Ecosystems.
“Trailblazing USGS science, like this study, provides a solid foundation for research and helps managers address the most pressing issues facing fish and wildlife conservation.”
Methylmercury concentrations
The investigation unveiled a range of methylmercury concentrations in amphibians, influenced by factors including the specific site, as well as the life history characteristics of the amphibians, such as their diet, size, and sex.
Some locations revealed barely detectable levels, while others demonstrated concentrations exceeding wildlife health benchmarks.
Mercury levels vary in amphibians
While the study noted substantial variation in methylmercury levels across amphibians, the range was less extensive than that reported for other species like dragonflies, fishes, and birds.
This lesser variation in amphibians is thought to be due to the current focus on sampling from wetlands, in contrast to the diverse habitats considered in studies on other animal species.
Bioaccumulation
Mercury, recognized as a global contaminant of concern, is suspected to be a contributing factor to the declining numbers of amphibians, though its specific role in this decline is yet to be fully understood.
Produced by aquatic microbes, methylmercury is the most bioavailable and toxic form of mercury for vertebrates. It enters the food chain and accumulates within animals, a process referred to as bioaccumulation.
Unknown health effects
“Despite its toxicity, scientists only have a limited understanding of methylmercury’s effects on amphibians,” said lead author Brian Tornabene, a postdoctoral fellow at USGS. “The results from this study can be used to inform future research on the health effects of methylmercury exposure on amphibians, which for some was very high.”
The study also provides new methodologies and baseline data critical for assessing mercury risk, particularly for species under conservation concern, including those listed under the Endangered Species Act, according to co-author Michael Adams, the leader of the USGS Amphibian Research and Monitoring Initiative.
A novel approach proposed by the study involves using dragonfly larvae as proxies for assessing methylmercury bioaccumulation in amphibians, especially for species that are challenging to sample directly. This method is currently being implemented in a nationwide project by the USGS/National Park Service.
Study implications
The current findings highlight the particular vulnerability of amphibians to environmental contaminants like mercury, owing to their reliance on aquatic habitats. It also acknowledges the need for a deeper understanding of how exposure to contaminants, in conjunction with other threats such as diseases, impacts amphibian populations.
By providing a comprehensive view of methylmercury variation in amphibians, this study marks a significant step forward in unraveling the complex interplay between environmental contaminants and amphibian health and conservation.
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