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Scientists Identify Processes that Affect Fish Mercury Concentrations in Estuarine Wetlands

Landsat image of the south end of San Francisco Bay Estuary and associated wetlands
Landsat image of the south end of San Francisco Bay Estuary and associated wetlands. U.S. Geological Survey (USGS) scientists studied the relationships between mercury concentrations in fish and measures of physical habitat from 31 wetlands in the San Francisco Bay Estuary. National Aeronautics and Space Administration (NASA) Earth Observatory images by Joshua Stevens (NASA) using Landsat data from the USGS. Image acquired October 31, 2015.

In a study designed to help resource managers when considering mercury exposure risk, scientists determined that sulfur cycling—a process closely related to mercury methylation rates—and ecological conditions that influence exposure are important factors affecting fish mercury concentrations in estuarine wetlands.

Mercury is widespread in the environment. In its organic form, methylmercury, it is toxic to fish, wildlife, and humans and biomagnifies through food webs. The bioaccumulation of mercury is closely tied to the biogeochemical processes that control methylmercury production and availability to biota; however, these processes are complex, making it challenging to determine their respective roles in mercury bioaccumulation. This information gap presents a challenge to resource managers when considering steps to mitigate exposure risk.

U.S. Geological Survey (USGS) scientists measured multiple biogeochemical and ecological processes to provide insight into the factors affecting mercury bioaccumulation in fish within the San Francisco Bay Estuary. They examined the relationships between mercury concentrations in fish, multiple stable isotope ratios (nitrogen, carbon, and sulfur) in fish, and measures of physical habitat from 31 wetlands in the San Francisco Bay Estuary.

Scientists determined that carbon and nitrogen isotope ratios were not clear indicators of fish mercury concentrations because of their substantial variation among habitats and among individual wetlands. However, sulfur isotope ratios were important predictors of fish mercury concentrations at the scales examined.

Results from this study improve the understanding of the processes affecting mercury exposure risk in the San Francisco Bay and other estuarine ecosystems throughout the United States. Findings indicate that sulfur cycling, which is closely related to production of methylmercury, plays an important role in affecting mercury concentrations in estuarine fish at the scales examined. The findings also indicate that the effect of sulfur cycling on mercury bioaccumulation in fish varies among habitats and individual wetlands with different ecological conditions.

This study builds on the long-term goals of USGS Environmental Health Programs, which provide the science to minimize the risks of contaminant exposure and adverse effects, if any, to fish, wildlife, livestock, companion animals, and humans resulting from bioaccumulation and biomagnification in food chains.

This study was funded by the USGS Contaminant Biology Program, Ecosystems, CALFED Ecosystem Restoration Program, and the U.S. Fish and Wildlife Service.


Willacker, J.J., Eagles-Smith, C.A., and Ackerman, J.T., 2017, Mercury bioaccumulation in estuarine fishes--Novel insights from sulfur stable isotopes: Environmental Science and Technology, v. 51, no. 4, p. 2131-2139, doi:10.1021/acs.est.6b05325.

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