Trophic discrimination of nitrogen stable isotopes in amino acids varies with diet quality in a marine fish

Analysis of individual amino acid (AA) delta N-15 values is increasingly common in studies of food web architecture, movement ecology, and biogeochemical cycling. However, observations that nitrogen isotope fractionation of AAs may vary across species and trophic positions (TP) complicate the application of compound-specific stable isotope analysis (CSIA) in studies of aquatic food webs. We reared a common estuarine fish (Fundulus heteroclitus) on four diets to test the hypothesis that diet composition significantly affects AA trophic discrimination factors (TDFs) derived from normalizing fractionation of trophic AAs to the canonical source AA phenylalanine. Phenylalanine showed little trophic fractionation regardless of diet composition, confirming its use as a proxy for delta N-15 values at the base of food webs. However, TDF values showed significant negative relationships with two important measures of diet quality, protein content and AA imbalance between diet and consumer. As a result, consumers feeding on high protein diets with AA compositions matching their own had significantly lower TDF values than previous literature values, while consumers feeding on contrasting low quality diets had significantly higher TDF values than previously reported. Our results provide an explanation for the apparent relationship between TDF value and TP, and highlight a significant limitation when using a single TDF value among all trophic transfers within a food web. Accurate determination of TPs using CSIA will likely require multiple TDF values across trophic levels or further exploration of other trophic AAs, such as proline, that may show less variation in TDF with diet composition than glutamic acid.