Preliminary evaluation of potential stable isotope and trace element productivity proxies in the oyster Crassostrea gigas

As most bivalve mollusks are filter feeders or deposit feeders—predatory septibranchs (Poromyacea) being the notable exception—they are closely linked with the activity of primary producers. Perhaps this is nowhere more evident than in shallow water habitats influenced by seasonal phytoplankton blooms. In these systems, photoautotrophs are capable of changing the physical and chemical properties of the water column. In turn, clams, ingest large quantities of phytoplankton and precipitate their shells in these chemically altered waters. Thus, clam shells likely archive valuable information about phytoplankton dynamics in shallow water habitats. To better understand this relationship, we investigated geochemical records from two potential productivity proxy systems archived in shells of the oyster Crassostrea gigas that live in the San Francisco Bay estuary, California, USA. Here we present multi-year records of shell carbon isotope (δ13Ccarb) and barium/calcium (Ba/Cacarb) variation. These archives are compared with records of environmental variation using oxygen isotopes (δ18Ocarb) as a temporal marker. We also present high-resolution δ13Ccarb, δ18Ocarb and Ba/Ca profiles associated with the 2003 spring phytoplankton bloom. Our results suggest that the prominent positive δ13Ccarb excursion, present in shell material deposited in the spring of 2003, most likely reflects photosynthetic enrichment of the dissolved inorganic carbon pool. Calibration of stable isotopes with calculated chlorophyll a concentrations, measured during the bloom, suggests that our δ13Ccarb sample variation reflects phytoplankton abundance with 3 to 4 day resolution. These findings highlight the potential utility of stable isotope records as tools for reconstructing patterns of primary productivity. Elevated Ba/Ca values are recorded each year in the spring. Thus, like stable isotope profiles, Ba/Ca profiles from these specimens demarcate individual years of shell growth and establish ontogenetic age. Low- and high-resolution Ba/Ca profiles do not, however, correlate with the 2003 spring phytoplankton bloom, casting doubt on the relationship between Ba/Ca ratios and phytoplankton blooms. Although the Ba/Ca peak timing loosely correlates with freshwater discharge events, peak amplitude does not, leaving the environmental cause/trigger of these peaks unknown. Nevertheless, the San Francisco Bay estuary seems like an ideal environment in which to further investigate the relationship between skeletal geochemical variation and patterns of primary productivity.