California’s Central Valley and San Francisco Bay Estuary have a long history of mercury contamination from past mercury mining and gold extraction. The USGS Davis Field Station Mercury Lab studies mercury bioaccumulation in birds and fish, and the ecotoxicological effects of mercury and other contaminants, such as selenium, on avian reproduction.

Waterbirds are particularly susceptible to mercury because of their use of wetland habitats where methylmercury (the most toxic and biologically available form) is produced and relatively low methylmercury exposure can reduce reproductive success. Recent research by USGS WERC has shown that several waterbird populations within the San Francisco Bay Estuary are at high risk to mercury contamination.

In particular, about half of the Forster’s Tern population breeding within the Estuary is considered to be at high-risk to mercury contamination due to their present-day mercury concentrations. Upon arrival within the Estuary, terns rapidly accumulate mercury prior to and during the breeding season. In fact, Forster’s Tern mercury concentrations increase nearly 3-fold within just 2 months of arriving in the Estuary to breed. USGS WERC also has found sublethal effects of mercury on adult birds, including demethylation of mercury in bird livers.

Methylmercury in adults is then transferred proportionately to offspring, potentially impairing reproduction. In fish-eating Forster’s Terns, USGS WERC has found that: 1) failed-to-hatch eggs and abandoned eggs have higher mercury concentrations than randomly sampled eggs, 2) the likelihood of an embryo being malpositioned increases with egg mercury concentrations, 3) the probability of an egg successfully hatching decreases with egg mercury concentrations, and 4) the probability of nest survival decreases with egg mercury concentrations.

For those eggs that are able to hatch, the resulting chicks are still vulnerable to the effects of residual mercury exposure from the egg, especially shortly after hatching when mercury deposited by the mother is still relatively high. In invertebrate-eating Black-necked Stilts, USGS WERC found that dead chicks had higher mercury concentrations than live, randomly sampled chicks of similar young age.

Altogether, these results suggest that mercury contamination may currently be impairing bird reproduction and, perhaps, population growth within the San Francisco Bay Estuary.

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 wildlife, and quantify the individual, population, and ecosystem scale effects of bioaccumulative contaminants in the environment. Researchers apply their integrated expertise in avian ecology, reproductive ecology, food web ecology, aquatic ecology, toxicology, and environmental chemistry to answer complex questions related to the management of aquatic habitats.

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