Wetlands provide numerous ecosystem services, but also can be sources of methylmercury production and export. Wetlands often have increased methylmercury production compared to other aquatic habitats, because biogeochemical conditions common within wetlands facilitate methylation of inorganic mercury, to the more bioaccumulative and toxic form of mercury (methylmercury). Rice agricultural wetlands in particular may be important sites for methylmercury bioaccumulation due to their worldwide ubiquity, periodic flooding schedules, and high use by wildlife.

USGS WERC is using wild and caged biosentinel fish species to monitor mercury bioaccumulation in wetlands over short time scales. Recent USGS WERC studies have shown that western mosquitofish caged for 60 days in agricultural wetlands (white rice and wild rice) bioaccumulated mercury to substantially higher concentrations than mosquitofish caged in nearby permanent wetlands in the Yolo Bypass within California’s Central Valley.

USGS WERC also is using wild and caged biosentinel fish to monitor mercury exposure at Cosumnes River Preserve, Twitchell Island in the Delta, and Cache Creek Settling Basin. Goals of these projects are to determine management practices that limit or reduce methylmercury production and bioaccumulation in seasonal wetlands and rice fields. Together with local rice farmers, we are studying farming techniques such as discing rice straw stubble into the soil or removing rice straw residue through bailing prior to fields being flooded in winter.  These farming techniques could reduce residual rice straw and, in turn, the available dissolved organic matter which is the food of iron and sulfate-reducing bacteria that produce the toxic form of mercury – methylmercury.  Additionally, we are working with wetland managers to test strategies for managing seasonal wetlands, such as changing the timing and duration of flooding, to reduce the wet-dry cycles that reset the redox conditions that promote the iron and sulfate-reducing bacteria’s activity that causes mercury to be methylated into its toxic form.

USGS WERC has used wild fish species, such as mudsuckers, sticklebacks, and silversides, to show that mercury concentrations in San Francisco Bay salt ponds and associated wetlands and tidal marshes change dramatically over short time periods which have important implications for wildlife risk and mercury monitoring programs.

The overarching goals of the mercury research program at USGS WERC Davis Field Station are to evaluate the ecological factors the drive contaminant bioaccumulation in wetland-dependent fish and wildlife, and quantify the individual, population, and ecosystem scale effects of bioaccumulative contaminants in the environment.

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