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. N-15-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 delta(15)N signature between the 2 food sources. After adding the enriched substrate delta(15)N values of epiphytes ranged from 87 to 713 parts per thousand, and were consistently higher than the delta(15)N Values of seagrass, which ranged from 25 to 90 parts per thousand. Enriched substrate additions every 3 d resulted in the greatest sustained separation between seagrass and epiphytes over 18 d. The results demonstrate that enriched N-15 tracers are useful for separating the delta(15)N signatures of previously difficult to distinguish primary sources, and that this technique has the potential to resolve ambiguous natural abundance isotope results.
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