In collaboration with the San Francisco Estuary Institute, UC Davis researchers have made headway in understanding the sources of toxic mercury in the San Francisco Bay and Delta. The findings, revealed in SFEI’s annual report of the Regional Monitoring Program for Water Quality (RMP), indicate that changes in current habitat management techniques could prevent mercury from entering the food web and threatening the health of wildlife and humans.
A small fraction of elemental mercury is converted to the organic form methylmercury by bacteria in the sediments of aquatic environments. Metyhylmercury poses a health concern if it enters the aquatic food web and builds up to toxic levels further up the food chain.
According to SFEI’s annual report, this threat is a reality. Researchers at the U.S. Geological Survey documented a higher risk of hatch failure for the eggs of exposed Bay water birds, and mercury concern is the main incentive for fish consumption advisories in the Bay.
Insights about the controlling factors and areas where methylmercury is most problematic may indicate what can be done in terms of management, said Richard Looker, an engineer for the San Francisco Bay Regional Water Quality Control Board.
Darell Slotton, a research ecologist with the department of environmental science and policy at UC Davis, developed a sensitive approach to trace and record sources of mercury contamination using small, young fish that tend to stay in the same area.
“We can use these little fish that are out there in the environment, like the canary in the goldmine, as an indicator of the levels of mercury exposure in different places around the Bay and the Delta, and at different times of year,” Slotton said.
His method was refined over a period of 20 years, and uses a technique to determine mercury levels in individual fish.
Slotton monitored fish samples collected every four months from about 25 sites scattered around the Sacramento-San Joaquin Delta and the Central Valley. The RMP used a similar approach in a smaller study with fish collected around the Bay from 2005 to 2007. Both efforts were part of the Fish Mercury Project, a three-year CALFED-funded program to examine and monitor potential changes in the amount of mercury in fish in the Bay-Delta watershed.
One highlight of Slotton’s findings linked elevated mercury contamination to lands that underwent periodic flooding. These included the Yolo Bypass, which is deliberately flooded during the rainy seasons in order to divert water from the Sacramento River away from many communities, as well as normally dry areas in the Central Valley that flooded during 2006.
Other potential sources for contamination are the vast wetland restoration projects that have been created in the Bay-Delta watershed.
The conventional knowledge 10 years ago was that any wetland habitat is a suspected methylmercury production source, Slotton said.
Instead, he and other groups found that many sites that are often submerged in water, including the restored wetlands, are not problem areas for food web contamination.
“The biggest implication is focusing our management efforts on … these wetlands that go dry and are flooded,” Slotton said. “A lot of places that undergo that kind of cycle are manmade.… We can keep these places wet rather than letting them go dry in the first place, and the fish will tell us if it’s working or not.“
No management solutions have been implemented as a result of the project, according to Looker.
“We can’t really draw conclusions until we have a sufficient amount of data to know that the patterns we’re seeing are not just random but that they’re consistent and repeated through time,” Looker said.
Although the Fish Monitoring Project ends this year, Slotton will continue working with the RMP to annually monitor small fish around the Bay.
“Mercury remains an important focus for the RMP,” said SFEI senior project manager Meg Sedlak. “More extensive small fish monitoring is a key component of the RMP Mercury Strategy.“
ELAINE HSIA can be reached at email@example.com