ABSTRACT: This study examined the d¹⁵N of suspended and sedimented particulate organic matter, primary sources of organic matter and benthic invertebrates along an estuarine gradient in Marennes-Oléron Bay, France. Particular emphasis was given to the use of d¹⁵N as a tracer of the origin of organic matter and as a means of determining the benthic food web structure in estuarine environments. d¹⁵N 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. d¹⁵N data showed that the trophic position of oysters may vary in the estuarine bay. The d¹⁵N values for the other invertebrates investigated revealed that apparent discrepancies may occur concerning the correspondence between d¹⁵N, trophic level and feeding mode of invertebrates in estuarine ecosystems. In fact, these results suggest that d¹⁵N 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.

KEY WORDS: Nitrogen isotope ratio · Particulate organic matter · Crassostrea gigas · Benthic food web · Estuary · Marennes-Oléron Bay

Published in MEPS Vol. 166 (1998) on May 28
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998