Zn Isotope Fractionation in the Oyster Crassostrea hongkongensis and Implications for Contaminant Source Tracking

Variations in stable isotope ratios have been used to trace sources of contaminants as well as their biogeochemical pathways in the environment. In this study, we investigated the influences of internal redistribution among tissues and ambient water conditions on Zn isotope fractionation in oysters. There was no significant difference in Zn isotope ratios during in vivo Zn transportation among various oyster tissues. Estuarine oysters were exposed to additional Zn either at different salinities or at different Zn concentrations, following which the Zn isotope ratios in the oysters were measured. Results showed no significant difference in δ^66/64Zn values in the oysters exposed at different salinities. Tissue Zn accumulation increased with increasing Zn levels in water over the 30 day exposure. Within this period, there was a nearly 0.3‰ difference in averaged δ^66/64Zn values in the exposed oysters compared to the initial δ^66/64Zn values in the oysters prior to exposure. However, there was no evidence of significant difference in δ^66/64Zn values in oysters exposed at different Zn levels, with postexposure signatures similar to the δ^66/64Zn values of the Zn solution added. Our results suggested that the δ^66/64Zn values measured in the oysters were approaching the δ^66/64Zn values of the "source" faster with increasing Zn concentrations added in the water. This study highlighted the absence of Zn isotope fractionation during Zn internal distribution and in vivo transport in oysters. The calculation of the contributions of different Zn sources demonstrated that oysters can be a sentinel animal for Zn source tracking in marine environments.