Determination of aquatic food‐web structure based on compound‐specific nitrogen isotopic composition of amino acids

The nitrogen isotopic composition (δ15N) of amino acids is potentially useful as an alternative method for estimating the trophic levels of organisms in food webs. However, because this “amino acid method” has been constructed from the observations of only a few case studies of food‐consumer combinations in previous studies, the universality of the approach remains unclear. In this study, we investigated the δ15N signatures of amino acids in 17 photoautotrophs and the trophic relationships during four controlled feeding experiments using green algae, zooplankton, and fish. The results are consistent with those reported in previous studies, implying that the amino acid method can be applied to a variety of organisms. From these and previously published data, we estimate the two factors (β, isotope differences among amino acids in primary producers; Δ, the 15N‐enrichment factor for each trophic level) required to calculate the trophic level. Based on the lowest error (1σ = 0.12) in the estimated trophic level, we conclude that a comparison of the δ15N values for glutamic acid and phenylalanine is most useful in calculating precise estimates of the trophic level, using the following equation: trophic level (TLGlu/Phe) = (δ15NGlu − δ15NPhe − 3.4)/7.6 + 1.