N-15 enrichment as a method of separating the isotopic signatures of seagrass and its epiphytes for food web analysis

Stable isotope analysis of food webs is of limited use where there is littl e or no difference in the natural abundance isotopic ratios of potential fo od sources. N-15-enriched potassium nitrate was used to differentially labe l 2 potential food sources for seagrass fauna: seagrass and its attached ep iphytes. Different combinations of exposure time to the enriched substrate and different concentrations of enriched substrate were used to maximise th e difference in delta(15)N signature between the 2 food sources. After addi ng 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 Va lues of seagrass, which ranged from 25 to 90 parts per thousand. Enriched s ubstrate additions every 3 d resulted in the greatest sustained separation between seagrass and epiphytes over 18 d. The results demonstrate that enri ched N-15 tracers are useful for separating the delta(15)N signatures of pr eviously difficult to distinguish primary sources, and that this technique has the potential to resolve ambiguous natural abundance isotope results.