Controls on isotopic chemistry of the American oyster, Crassostrea virginica: implications for growth patterns

The American or eastern oyster, Crassostrea virginica, can potentially serve as a recorder of environmental change in estuarine habitats. However, before oysters can confidently be used to reconstruct past variation in salinity and temperature in coastal environments, relations between modern shell chemistry and ambient salinity and temperature must be understood. Samples from the resilifer on the surface of the hinge (foliated calcite) and in cross-section (foliated and chalky layers) have very similar isotopic composition, implying that all three areas respond identically to ambient water conditions. More detailed sampling focused on the chalky layer because this region exhibits maximum shell extension which allows for high-resolution sampling. Water temperature, salinity, δ18OWATER, and the δ13CDIC (dissolved inorganic carbon) were measured in Blackwater River, near Naples, Florida, from June 1997 to November 1999 to construct a predicted model shell against which the observed isotopic data from live-collected oysters were compared. From these data, it was determined that C. virginica precipitated its shell near isotopic equilibrium. Values of δ13CSHELL follow the general trend of the predicted equilibrium model with a positive offset of ∼1‰ during cold winter months. Variation in growth rate or incorporation of a higher percentage of metabolically derived carbon may explain the degree of offset. C. virginica from southwest Florida does not appear to deposit shell when water temperature is above ∼28(±2)°C, though the influence of temperature versus gametogenesis attributing to the slowing down of shell accretion cannot be deconvoluted. Importantly, there is no consistent pattern between the location of dark, translucent growth bands and peaks or valleys on the profile of isotopic composition of the shell. Therefore, these growth structures cannot be used as independent proxies of growth season, tides, lunar phase, or shell age.