Synthesis of Mercury Distribution and Bioaccumulation Across Western North America

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The unique combination of natural processes and human activities present in western North America can strongly affect the exposure of fish and birds to methylmercury, the most toxic form of mercury (Hg).

Foster's Tern (Sterna forsteri) While Hunting in Flight

Photo of Foster's Tern (Sterna forsteri) while hunting. Forster's terns are among the most at-risk wildlife species to mercury toxicity in western North America. Photo Credit: Joshua Ackerman, USGS

The environmental health risk of Hg is strongly associated with landscape-scale processes that facilitate the conversion of inorganic Hg to methylmercury (MeHg). Inorganic Hg is naturally present in the landscape of western North America due to the geology of the region and its introduction through a combination of historic and contemporary human activities. This combination of distinct patterns in landscape structure, climate, and ecosystems can differentially affect the occurrence, transport, and food web bioaccumulation of Hgand MeHg from the wide variety of sources that exist in western North America.

This synthesis project resulted in the compilation, analysis, and interpretation of spatial and temporal patterns of Hg and MeHgoccurrence in air, soil, vegetation, sediments, fish, and wildlife across western North America, which provided the most comprehensive evaluation to date (2016) on the distribution of Hgand MeHg in this region. Scientists evaluated the connections between landscape variables and concentrations of Hg and MeHg. Scientists also conducted time series analyses over several decades and assessed how resource management activities, such as reservoir management, affected Hg cycling. Additionally, Hg concentrations in fish and birds were evaluated relative to a series of toxicity benchmarks to evaluate potential risk to fish, wildlife, and humans. Results of this synthesis are published in a special issue of Science of the Total Environment.

Key Findings

  • MeHg contamination in fish and birds is common in many areas throughout western North America, and many areas were found to have Hg concentrations above levels associated with toxicity.
  • Patterns of MeHg exposure in fish and wildlife across western North America do not overlap with patterns of inorganic Hg on the landscape, which indicates a disconnect between inorganic Hg distribution and bioaccumulation in fish and wildlife. For example, forest soils contain more Hg than soils from semi-arid lands, yet the highest levels of MeHg in fish and wildlife tended to occur in semi-arid areas.
  • Vegetation patterns strongly influence the amount of Hg emitted to the atmosphere from soils due to the effect of shading and plant uptake, thus forested areas retain atmospherically deposited Hg, whereas less-vegetated areas tend to release Hg to the atmosphere.
  • Land disturbance, such as urban development, agriculture, and wildfires, are important factors in releasing stored Hg from the landscape, potentially making it available for biological uptake.
  • Land and water management activities can strongly influence how inorganic Hg transforms to MeHg and can be transferred to fish, wildlife, and humans. Fish Hg concentrations are 1.5–2.6 times higher in reservoirs than in natural lakes, and the magnitude to water-level fluctuations was strongly correlated with fish Hg concentrations.

These studies highlight that environmental health risks associated with bioaccumulation in fish and birds in western North America are controlled by processes associated with MeHg production rather than the location of inorganic Hg sources or deposition. Developing strategies to mitigate MeHg production in the West may be particularly beneficial for reducing wildlife exposure.

This study was supported by the U.S. Geological Survey's (USGS) Powell Center for Analysis and Synthesis, USGS Ecosystem Mission Area's Environmental Health Program (Contaminant Biology Program and Toxic Substances Hydrology) as well as The National Park Service Air Resources Division, and the U.S. Environmental Protection Agency Region 10 Regional Applied Research Effort (RARE) Program.

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