Stable isotope analysis of food webs is of limited use where there is little or no difference in the natural abundance isotopic ratios of potential food sources. 15N-enriched potassium nitrate was used to differentially label 2 potential food sources for seagrass fauna: seagrass and its attached epiphytes. Different combinations of exposure time to the enriched substrate and different concentrations of enriched substrate were used to maximise the difference in δ15N signature between the 2 food sources. After adding the enriched substrate δ15N values of epiphytes ranged from 87 to 713‰, and were consistently higher than the δ15N values of seagrass, which ranged from 25 to 90‰. Enriched substrate additions every 3 d resulted in the greatest sustained separation between seagrass and epiphytes over 18 d. The results demonstrate that enriched 15N tracers are useful for separating the δ15N signatures of previously difficult to distinguish primary sources, and that this technique has the potential to resolve ambiguous natural abundance isotope results.