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 delta C-13 and delta N-15 values of oysters among stations were observed, with delta C-13 being consistently lower at Hiburi Island (-18.1 parts per thousand) than at other stations (-17.2 parts per thousand). Oysters from fish farm sites were enriched in delta N-15 (8.1 parts per thousand) relative to those from unaffected sites (6.8 parts per thousand), suggesting that fish farming tends to increase baseline delta N-15 values. The mean Delta delta C-13 (0.8 parts per thousand) was consistent over space and time, whereas the average Delta delta N-15 slightly increased in summer. The relatively low delta N-15 enrichment compared to the theoretical isotope fractionation factor (3.4 parts per thousand) 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 delta C-13 and delta N-15 from their diet and that delta C-13 of oysters is a more accurate bioinclicator of isotopic baselines than delta N-15 for marine ecological studies. (C) 2008 Elsevier Ltd. All rights reserved.