Dynamics of stable isotopes ratios (D13C and D15N) in different organs of Crassostrea gigas at two contrasted ecosystems: insights from growth and food sources

We studied the influence of food availability on the growth (whole body and organs) of the oyster Crassostrea gigas and on the dynamics of their d13C and d15N respective values. Juvenile oysters originating from Arcachon Bay were transplanted to two contrasting ecosystems, Baie des Veys (BDV) and Rade de Brest (RDB), for a 1-yr growth survey. In BDV, chlorophyll-a concentrations ([Chl-a]) was 3 times higher than in RDB on average, which accounts for the differences in oyster growth between BDV and RDB. Differences in trophic conditions could also explain the differences in C/N ratios and 13C values between sites; these differences widened when lipid normalised 13C values were used. Oysters clearly selected microalgae as the main food source, and especially phytoplankton. Gi (Gills), Mu (Muscle) and Re (remaining tissues) clearly exhibited different isotopic enrichment levels, with dMu > dGi > dRe regardless of C or N elements, culture sites and seasons. Isotopic discrimination between organs was rather equivalent between sites. This unexpected result would benefit from an experiment under condtions along a calibration of the lipid normalisation to correct 13C values in bivalves