Mercury Accumulation in Waterbirds (Black Rails) Related to Sediment Chemistry in San Francisco Bay Wetlands

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The U.S. Geological Survey (USGS) assessed the effect of sediment chemistry, food web structure, and diet on mercury bioaccumulation in black rails in the San Francisco Bay watershed. Differences in mercury accumulation in the birds were related to differences in sediment chemistry in the wetlands.  



WERC Black Rail in marsh

State-threatened California Black Rail.

Methylmercury is a globally pervasive contaminant that is a concern in San Francisco Bay, California, because hydraulic mining during the Gold Rush resulted in mercury-contaminated sediments in the bay. Mercury-contaminated sediments have put several species of waterbirds, including the State-threatened California black rail (Laterallus jamaicensis coturniculus), at risk of exposure and adverse effects from methylmercury.

Black rails are year-round residents of San Francisco Bay, and previous studies demonstrated that a portion of the black rail population had methylmercury concentrations that could result in reproductive toxicity. Black rails from different wetlands had different concentrations of methylmercury; however, little is known about the factors that contribute to spatial differences in bioaccumulation in birds inhabiting the San Francisco Bay watershed. Understanding these drivers could provide clues to potential management strategies to mitigate exposure.

Sediment methylmercury concentrations differed among three tidal wetlands in the Petaluma River watershed in northern San Francisco Bay. Moreover, 3 of 26 measurements were identified that explained differences in sediment chemistry among wetlands: the porewater concentration of ferrous iron, the percent organic matter, and the sediment methylmercury concentration.

Without harm to the birds, the researchers measured stable carbon and nitrogen isotopes in feather samples and invertebrate prey to evaluate methylmercury biomagnification in food webs, and regurgitated material was analyzed to study the black rail diet. Food web structure and biomagnification were similar among wetlands. In addition, regurgitation samples indicated that black rails had similar diets among wetlands. Given the similarities in diet, food web structure, and methylmercury biomagnification among wetlands, the scientists concluded that the variation in sediment chemistry and associated sediment methylmercury production was the primary driver of differences in methylmercury concentrations among black rails from different wetlands.

Petaluma March

Typical San Francisco Bay wetland habitat dominated by pickleweed (Salicornia pacifica).

(Credit: Kyle Spragens, U.S. Geological Survey. Public domain.)

Determining which regions and species are at risk of methylmercury toxicity remains challenging because methylmercury production and availability are controlled by complex and variable factors in tidal wetlands. Studies, such as this one, that consider the effect of sediment chemistry, food web structure, and diet on methylmercury bioaccumulation, provide the science to support management decisions and mitigation strategies for methylmercury contamination.

This research was supported by the USGS Environmental Health Program (Contaminants Biology and Toxic Substances Hydrology) of the U.S. Geological Survey Ecosystems Mission Area, the USGS Western Ecological Research Center, a Calfed Bay-Delta Program grant (ERP02D-P62) to the San Francisco Estuary Institute, the U.S. Fish and Wildlife Service San Francisco Estuary Program, and the University of California Davis John Muir Institute for the Environment.