Allochthonous brown algae are the primary food source for consumers in a temperate, coastal environment

In several parts of the world, detached seagrass and macroalgae accumulate in the surf zone where in situ primary production can be low. This allochthonous resource is therefore likely to be important to consumers, but the various components of the resource likely play different roles in the food webs. We traced sources of production for the abundant amphipod Allorchestes compressa and 2 key predatory fishes in the surf zone of southwestern Australia using multiple stable isotopes (δ13C, δ15N and δ34S) and fatty acids. Seagrasses had higher δ13C than macroalgae, while δ13C of red and brown macroalgae were similar. Mixing models based on δ13C were ambiguous and indicated that brown algae and seagrasses were both feasible carbon sources for A. compressa. δ15N varied little between seagrasses and macroalgae, while δ15N of amphipods was higher and δ15N of fish higher again. δ34S was not useful in distinguishing between macrophytes or trophic levels. The fatty acid composition of brown and red algae and seagrasses were distinct. The composition of essential fatty acids (i.e. fatty acids that the consumer must obtain through its diet) of A. compressa was most similar to that of brown algae. The combination of stable isotope and fatty acid analyses indicates that, although brown algae comprises a lower proportion of wrack biomass than seagrass (17 to 28% vs. 52 to 58%), brown algae, particularly the kelp Ecklonia radiata, contributes disproportionately to the surf-zone food web in southwestern Australia