Western Australia has a rich diversity of seagrasses, many of which are meadow-forming species with a high diversity of associated epiphytes. Potential food sources and dominant invertebrates and fishes were collected in a non-quantitative sampling programme designed to examine the variability in naturally occurring isotopes (C-13/C-12 and N-15/N-14) within an Amphibolis griffithii dominated seagrass bed in Western Australia. The aims of this study were to determine the isotopic composition of the organisms, and to determine the sources of carbon available to consumers using the variations in the ratio of N-15/N-14 and C-13/C-12 among organisms in the seagrass assemblage. Autotrophs showed a wide distribution of delta C-13 values, with seagrass material significantly enriched in C-13 relative to macroalgal sources by > 10 parts per thousand. This variation allowed us to successfully identify macroalgae as the main contributor of carbon to the trophic structure. delta N-15 ratios did not vary to the degree that would make it useful as tracer, but it was applied to estimating the total number of trophic transfers of nitrogen. Analysis of delta N-15 values suggested that four trophic positions were present, with fishes (Acanthallitcres vittiger, Scobonichthys granulatus and Siphonognathus radiatus, Pelsartia humeralis, Pelates sexlineatus, Leviprora inops, Odax acroptilus and Notolabrus parilus) occupying the top two levels. delta C-13 of seston (20-200 mu m) and sedimentary organic matter indicate that seagrass material is the main contributor to these two carbon pools, and that very little of it is incorporated into the trophic structure. (c) 2005 Elsevier Ltd. All rights reserved.