Using Stable Isotopes to Infer the Relative Importance of Biogeochemical and Food Web Processes on Mercury Bioaccumulation at Multiple Spatial Scales in Estuarine Wetland Fishes

Estuarine wetlands are transitional habitats that are characterized by diverse gradients of biogeochemical processes and dynamic food webs.  Biogeochemical and food web processes each have unique influences on mercury bioaccumulation, but the relative importance of each process on mercury bioaccumulation is not clear.  In this study, we employed carbon, nitrogen, and sulfur stable isotopes to examine the trophic and biogeochemical drivers of mercury bioaccumulation into fishes at a range of spatial scales across a gradient of estuarine wetlands in San Francisco Bay, California.  Preliminary data indicate that Hg concentrations in fishes were most strongly correlated with sulfur stable isotope ratios in impounded wetland habitats, but not tidal habitats, suggesting that biogeochemical drivers of mercury methylation may dominate bioaccumulation in fishes in those habitats.  However, our preliminary results also indicate that foraging habitat and trophic position are also important determinants of fish mercury concentrations in low salinity wetlands, but not moderate or high salinity wetlands.  These results indicate the importance of simultaneously considering biogeochemical and food web processes in evaluating drivers of fish mercury concentrations in estuarine wetlands.