Bivalve tissue as a carbon and nitrogen isotope baseline indicator in coastal ecosystems

Pinctada fucata martensii mantle tissue and gut contents were examined as baseline indicators of carbon and nitrogen isotope composition at six stations in the Uwa Sea, Japan. Substantial variations in δ13C and δ15N values of oysters among stations were observed, with δ13C being consistently lower at Hiburi Island (−18.1‰) than at other stations (−17.2‰). Oysters from fish farm sites were enriched in δ15N (8.1‰) relative to those from unaffected sites (6.8‰), suggesting that fish farming tends to increase baseline δ15N values. The mean Δδ13C (0.8‰) was consistent over space and time, whereas the average Δδ15N slightly increased in summer. The relatively low δ15N enrichment compared to the theoretical isotope fractionation factor (3.4‰) may be due to oyster-specific physiological attributes. Carbon and nitrogen isotope turnover rates were roughly similar within a tissue, and mantle tissue turnover rate was estimated to be 120–180 days. These results indicated that oysters are long-term integrators of δ13C and δ15N from their diet and that δ13C of oysters is a more accurate bioindicator of isotopic baselines than δ15N for marine ecological studies.