“Cracking the code” on mercury bioaccumulation

A new model will allow scientists to estimate mercury risk across the landscape, including in places without prior data.

NPS Photo

Mercury (Hg) is a toxic pollutant that can pose significant risks to ecosystem and human health. Hg is converted into methylmercury before entering freshwater food webs. Methylmercury is the form that bioaccumulates and can have negative effects on wildlife and humans. So, it is important to figure out what factors influence Hg production and bioaccumulation, and how we can limit the impacts.

Previous work has shown that Hg levels can be influenced by factors like acidity (pH), land cover, soil organic matter, and dissolved organic carbon (DOC). But the relationships vary across different locations and environmental conditions, like climate and geography. This makes it difficult to predict Hg risk across the landscape. In a new study, scientists have begun to “crack the code” on what drives Hg bioaccumulation. Their findings have important implications for how we manage national park lands.

Scientists from the U.S. Geological Survey and the National Park Service built a model to provide estimates of Hg risk across the U.S., based on data collected primarily in national parks. They used more than 21,000 measurements of Hg in dragonflies. Then, they added a suite of chemical and landscape factors to determine different drivers of Hg levels across the country. Model predictions explained 85% of the variation in dragonfly Hg levels across the U.S. (That’s shockingly good!) Certain factors were more important than others, but their relative importance varied among sites, habitats, and ecoregions. These results highlight the context of environmental conditions in determining how Hg methylation and bioaccumulation differ among locations. And that matters when we think about ecological and health impacts.

This groundbreaking model will serve as the engine behind a new tool that will help park managers estimate Hg risk across all waterbodies, including previously unsampled ones. The data will help inform research and monitoring, as well as mitigation and management decisions in national parks.

Read more from the paper below.

Ecosystem drivers of freshwater mercury bioaccumulation are context-dependent: insights from continental-scale modeling

Abstract

Significant variation in mercury (Hg) bioaccumulation is observed across the diversity of freshwater ecosystems in North America. While there is support for the major drivers of Hg bioaccumulation, the relative influence of different external factors can vary widely among waterbodies, which makes predicting Hg risk across large spatial scales particularly challenging. We modeled Hg bioaccumulation by coupling Hg concentrations in more than 21,000 dragonflies collected across the United States from 2008-2021 with a suite of chemical (e.g., dissolved organic carbon (DOC), pH, sulfate) and landscape (e.g., soil characteristics, land cover) variables representing external drivers of Hg methylation, transport, and uptake. Model predictions explained 85% of the variation in dragonfly Hg concentrations across the United States. Certain predictor variables were more important than others (e.g., DOC, pH, percent wetland), and varied among waterbodies. Variation in Hg bioaccumulation was explained by including habitat and ecosystem type in a hierarchical modeling framework, which confirms the context-dependency of external factors in explaining Hg bioaccumulation across disparate freshwater ecosystems. This continent-scale model provides valuable insights into the processes underlying landscape-scale patterns in Hg exposure risk and demonstrates that drivers of Hg methylation and bioaccumulation are habitat and ecosystem dependent.

Kotalik, C.J. et al. 2025. Ecosystem drivers of freshwater mercury bioaccumulation are context-dependent: insights from continental-scale modeling. Environmental Science & Technology. DOI: 10.1021/acs.est.4c07280

Also see:

• New USGS Model Uses Dragonflies to Estimate Freshwater Mercury Risk on Protected Lands in the U.S., USGS News Release, January 16, 2025