δ15N of organic matter sources and benthic invertebrates along an estuarine gradient in Marennes-Oléron Bay (France):implications for the study of trophic structure

Abstract

This study examined the delta(15)N of suspended and sedimented particulate organic matter, primary sources of organic matter and benthic invertebrates 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 benthic food web structure in estuarine environments. delta(15)N values indicated that there was direct utilisation of benthic diatoms as a food source by oyster Crassostrea gigas near intertidal mudflats but suggested trophic mediation between terrestrial detritus and oysters in the upper estuarine reaches. delta(15)N data showed that the trophic position of oysters may vary in the estuarine bay. The delta(15)N values for the other invertebrates investigated revealed that apparent discrepancies may occur concerning the correspondence between delta(15)N, trophic level and feeding mode of invertebrates in estuarine ecosystems. In fact, these results suggest that delta(15)N can be a useful tool to characterise trophic transfers and to establish an isotopic food web model, provided that one considers the different feeding habitats along the estuarine gradient.