Mercury Isotopic Composition of Young-of-Year Fish in San Francisco Bay

Variations in the stable isotopic composition of mercury (Hg) can provide information about sources and biogeochemical transformations of Hg in the environment. Mass-dependent fractionation of Hg isotopes, reported as δ202Hg relative to NIST 3133, has been observed during multiple processes including volatilization, diffusion, reduction and de-methylation. Large mass-independent fractionation of Hg (>0.2‰) reported as Δ199Hg, has been observed only during photochemical reduction of Hg and MeHg. In this study we analyzed the Hg isotopic composition of sediments and young (<4 months) fish from localized polyhaline habitats throughout San Francisco Bay to investigate sources of Hg to the aquatic food web and Hg cycling. Mississippi silverside have δ202Hg values ranging from -0.25 to +0.60‰, with regional variation among South and Lower South Bay (+0.10 to +0.60‰), San Pablo Bay (-0.25 to -0.03‰), and Suisun Bay (+0.13 to +0.42‰). Fish δ202Hg values roughly correlate with sediment δ202Hg values from the same sites. We suggest that each of the Guadalupe, Petaluma, and Sacaramento-San Joaquin river systems supply a distinct source of Hg to San Francisco Bay, likely associated with contaminated sediment. Mississippi silverside have Δ199Hg values ranging +0.46 to +1.55‰. Unlike the δ202Hg values, fish Δ199Hg values do not vary in a regular pattern throughout the Bay. Rather, fish from approximately one-third of sites analyzed exhibit relatively elevated Δ199Hg (> 0.8‰), possibly associated with high water clarity. In all fish analyzed, the ratio of Δ199Hg to Δ201Hg values have a narrow range of 1.19 to 1.40 (avg = 1.26 ± 0.06). These ratios in fish are consistent with laboratory studies of photochemical MeHg degradation, which yielded Δ199Hg: Δ201Hg ratios of ~1.3 (Bergquist and Blum, 2007). We suggest that Δ199Hg in fish varies with the extent of photochemical MeHg degradation prior to food web uptake. Bergquist, B. A. and Blum, J. D., 2007. Mass-dependent and -independent fractionation of Hg isotopes by photoreduction in aquatic systems. Science 318, 417-420.