Isotopic Determination of Food Sources ofCrassostrea gigasAlong a Trophic Gradient in the Estuarine Bay of Marennes-Ol éron

Along a trophic gradient from the head of an estuary to the open ocean, carbon isotope ratios were determined both in tissues of the Pacific oysterCrassostrea gigasand in different components of the oysters ' potential food resource. In the study area, the Marennes-Ol eron Bay (France), a gradient in δ13C occurred in the particulate organic carbon (POC) from riverine ( −29 ·2 – −27 ·4 ‰) to open sea littoral ( −21 ·8 – −19 ·5 ‰). Isotope composition of oysters collected along this estuarine gradient followed a similar trend, indicating a predominant contribution of organic matter derived from terrestrial detritus to the diet ofC. gigasinhabiting the upper estuarine reaches. A remarkable departure from this general pattern was noted in the isotope composition of oysters sampled from a reef, at the mouth of the Charente river. These oysters exhibited enriched13C composition, suggesting that they were largely feeding upon benthic microalgae ( −16 ‰) from an adjacent wide mudflat. The reported results suggest thatC. gigasis able to preferentially ingest and/or assimilate particular food sources (i.e. benthic diatoms) among diverse sources within the total organic matter pool.