Potential food sources and dominant invertebrates and fishes were collected for the examination of variability in C-13/C-12 and N-15/N-14 to determine the sources of carbon available to consumers within a Western Australian Posidonia sintiosa-dominated seagrass bed. Autotrophs showed a wide distribution of delta(13)C values, with P. sinuosa at -11.3 +/- 0.8 parts per thousand and macroalgae ranging from -16.6 to -31.7 parts per thousand. This variation allowed us to successfully identify rnacroalgae as the main contributor of carbon to the trophic structure, although no distinction could be made between epiphytic rnacroalgae on seagrass, or allochthonous macroalgal sources. The range in delta(15)N ratios among potential food items at the trophic base was too small to make it useful as tracer of nitrogen flow pathways, but it consistently increased from macrophytes and detritus (4.1-6.8 parts per thousand), to invertebrates (5.7-7.4 parts per thousand) located near the middle of the food web, to fishes (8.3-11.9 parts per thousand), with piscivorous species such as Leviprora inops generally having a higher N-15. delta(13)C of seston (-12.8 parts per thousand) and sedimentary organic matter (-8.7 parts per thousand) indicate that seagrass material is the main contributor to these two carbon pools, and that very little of it contributes to animal biomass. (c) 2005 Elsevier B.V. All rights reserved.