Ecologie alimentaire de l'huitre Crassostrea gigas : dynamiques des compositions isotopiques naturelles

Trophic transfers of particulate organic matter sources to the Pacific oyster Crassostrea gigas were investigated in the oyster farming bay of Marennes-Oléron for two years. Stable isotope ratios of carbon and nitrogen were used to trace the origin and fate of primary production and organic matter inputs into the bay, and to depict trophic transfers to oysters that were experimentally reared according to traditional methods. Characterizing particulate organic matter sources available for oysters utilized a combination of hydrobiological parameters and stable isotope ratios. Processes of food incorporation by oysters were described for five organs such as gills, digestive gland, mantle, muscle and gonads, and were interpreted taking into account experimental estimates of tissue turnover rates and trophic enrichments. In summer and depending of the organs, isotopic turnover rates of carbon vary between 1 and 4 months although isotopic turnover rates of nitrogen were 3 to 5 times faster than those of carbon, in relation to the metabolism of proteins in herbivorous species. Spatial and temporal variability of the source contributions (extrinsic factor) to the oyster diet and the gametogenic cycle (intrinsic factor) jointly influenced tissue isotopic signals. Then the isotopic composition of oyster corrected for its trophic enrichment allows its diet to be determined on a monthly basis. The faster turnover rate of the digestive gland gave information about the temporal dynamics of the available food sources, whereas muscle tissues, with longer turnover, revealed spatial dynamics of available food sources during periods of growth. To conclude, this study demonstrated that the Pacific oyster, as a trophoindicator, is an efficient tool to highlight nutritional dynamics in estuarine macrotidal environment.