DELTA-N-15 OF ORGANIC-MATTER SOURCES AND BENTHIC INVERTEBRATES ALONG AN ESTUARINE GRADIENT IN MARENNES-OLERON BAY (FRANCE) - IMPLICATIONS FOR THE STUDY OF TROPHIC STRUCTURE

This study examined the delta(15)N of suspended and sedimented particu late organic matter, primary sources of organic matter and benthic inv ertebrates along an estuarine gradient in Marennes-Oleron Bay, France. Particular emphasis was given to the use of delta(15)N as a tracer of the origin of organic matter and as a means of determining the benthi c food web structure in estuarine environments. delta(15)N values indi cated that there was direct utilisation of benthic diatoms as a food s ource by oyster Crassostrea gigas near intertidal mudflats but suggest ed trophic mediation between terrestrial detritus and oysters in the u pper estuarine reaches. delta(15)N data showed that the trophic positi on of oysters may vary in the estuarine bay. The delta(15)N values for the other invertebrates investigated revealed that apparent discrepan cies may occur concerning the correspondence between delta(15)N, troph ic level and feeding mode of invertebrates in estuarine ecosystems. In fact, these results suggest that delta(15)N can be a useful tool to c haracterise trophic transfers and to establish an isotopic food web mo del, provided that one considers the different feeding habitats along the estuarine gradient.