Isotopic determination of food sources of Crassostrea gigas along a trophic gradient in the estuarine bay of Marennes-Oleron

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 oyster Crassostrea gigas and in different components of the oysters' potential food resource. In the study area, the Marennes-Oleron Bay (France), a gradient in delta(13)C occurred in the particulate organic carbon (POC) from riverine (-29.2- -27.4 parts per thousand) to open sea littoral (-21.8- -19.5 parts per thousand). 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 of C. gigas inhabiting 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 enriched C-13 composition, suggesting that they were largely feeding upon benthic microalgae (-16 parts per thousand) from an adjacent wide mudflat. The reported results suggest that C. gigas is able to preferentially ingest and/or assimilate particular food sources (i.e, benthic diatoms) among diverse sources within the total organic matter pool. (C) 1996 Academic Press Limited