Environmental health faces significant risks from mercury (Hg), particularly as landscape-scale processes facilitate the transformation of inorganic mercury into methylmercury (MeHg). Western North America’s geology and historical human activities contribute to the natural presence and introduction of inorganic Hg. This region’s diverse landscape, climate, and ecosystems influence how Hg and MeHg from various sources occur, travel, and accumulate in food webs.
A synthesis project was undertaken to compile and analyze spatial and temporal patterns of Hg and MeHg in air, soil, vegetation, sediments, fish, and wildlife across western North America. This extensive evaluation, completed in 2016, offers the most comprehensive understanding to date of Hg and MeHg distribution in the region. Scientists investigated the relationship between landscape variables and Hg and MeHg concentrations. They also conducted long-term analyses and assessed the impact of resource management activities, such as reservoir management, on Hg cycling. Furthermore, mercury levels in fish and birds were compared against toxicity benchmarks to evaluate potential risks to fish, wildlife, and human health. The findings of this synthesis are available in a special issue of Science of the Total Environment.
Key Discoveries on Mercury and Wildlife
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Widespread Methylmercury Contamination: Methylmercury (MeHg) contamination is prevalent in fish and bird populations throughout western North America. Numerous areas exhibit mercury concentrations exceeding toxicity thresholds, posing a significant threat to wildlife health.
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Disconnect Between Inorganic Mercury and Bioaccumulation: Intriguingly, the patterns of MeHg exposure in fish and wildlife do not align with the distribution of inorganic mercury across the landscape. This indicates that the presence of inorganic mercury is not a direct predictor of bioaccumulation in food webs. For instance, while forest soils contain higher levels of inorganic mercury compared to semi-arid lands, the highest MeHg concentrations in fish and wildlife are often found in semi-arid environments. This suggests that the processes converting inorganic mercury to methylmercury are more critical drivers of risk than the initial location of inorganic mercury.
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Vegetation’s Role in Atmospheric Mercury Release: Vegetation cover significantly affects the amount of mercury released from soils into the atmosphere. Forested areas tend to retain atmospherically deposited mercury due to shading and plant uptake. Conversely, less vegetated areas are more likely to release mercury back into the atmosphere, influencing mercury cycling at a landscape scale.
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Impact of Land Disturbance on Mercury Mobilization: Land disturbances such as urban development, agricultural activities, and wildfires play a crucial role in releasing stored mercury from the landscape. This mobilized mercury can then become available for biological uptake, increasing the risk of bioaccumulation in local ecosystems.
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Water Management Influences Methylmercury Production: Land and water management practices have a substantial influence on the transformation of inorganic mercury to methylmercury and its subsequent transfer to fish, wildlife, and potentially humans. Notably, mercury concentrations in fish are significantly higher in reservoirs (1.5–2.6 times) compared to natural lakes. The degree of water-level fluctuation in reservoirs was also found to be strongly correlated with fish mercury concentrations, highlighting the impact of water management on mercury cycling and bioaccumulation.
These findings underscore that the environmental health risks associated with mercury bioaccumulation in fish and birds in western North America are primarily governed by the processes that drive methylmercury production, rather than solely by the location of inorganic mercury sources or deposition patterns. Effective strategies to mitigate methylmercury production in the West are essential for reducing mercury exposure in wildlife and protecting ecosystem health.
This research was supported by the U.S. Geological Survey’s (USGS) Powell Center for Analysis and Synthesis, the USGS Ecosystem Mission Area’s Environmental Health Program (including the Contaminant Biology Program and Toxic Substances Hydrology), the National Park Service Air Resources Division, and the U.S. Environmental Protection Agency Region 10 Regional Applied Research Effort (RARE) Program.
Further Information
- USGS Featured Story: Comprehensive Study finds Widespread Mercury Contamination Across Western North America
- Biodiversity Research Institute: Mercury Cycling, Bioaccumulation, and Risk across Western North America: A Landscape Scale Synthesis Linking Long-Term Datasets
- Science of the Total Environment Special Issue: Mercury in Western North America—Spatiotemporal Patterns, Biogeochemistry, Bioaccumulation, and Risks
