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 (F rance), a gradient in delta(13)C occurred in the particulate organic c arbon (POC) from riverine (-29.2- -27.4 parts per thousand) to open se a littoral (-21.8- -19.5 parts per thousand). Isotope composition of o ysters collected along this estuarine gradient followed a similar tren d, indicating a predominant contribution of organic matter derived fro m terrestrial detritus to the diet of C. gigas inhabiting the upper es tuarine reaches. A remarkable departure from this general pattern was noted in the isotope composition of oysters sampled from a reef, at th e mouth of the Charente river. These oysters exhibited enriched C-13 c omposition, suggesting that they were largely feeding upon benthic mic roalgae (-16 parts per thousand) from an adjacent wide mudflat. The re ported 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 Academ ic Press Limited