Trophic interaction among organisms in a seagrass meadow ecosystem as revealed by bulk δ13C and amino acid δ15N analyses

Abstract Numerous studies have used stable isotope analysis (SIA) of carbon and nitrogen within the bulk tissues of organisms to determine the trophic structure among organisms in a food web. Recently, SIA has evolved to compound‐specific stable isotope analysis (CSIA) of nitrogen within amino acids to significantly reduce the uncertainty in the estimated trophic position (TP) of organisms based on the isotopic difference between glutamic acid and phenylalanine within a single organism. However, because the initial offset ( β ) between glutamic acid and phenylalanine differs between aquatic algae (ca. +3.4‰) and vascular plants (ca. −8.4‰) in food webs that rely on both resources, β should be replaced by a value adapted to the admixture of primary producers for each specimen. In this study, we established a new method involving the β value ( β mix ) of each consumer specimen determined based on its bulk tissue δ 13 C value and successfully obtained realistic TPs (TP mix ) for organisms in a complex seagrass meadow food web. Remarkable differences between the TP mix and traditional TP algal values were found in deposit feeders due to the large contribution of seagrass to their basal resources. The estimated TPs of organisms increased by up to 1.5 units (from TP algal to TP mix ), in terms of trophic transfer, when their diets included substantial seagrass‐derived contributions. Thus, combinatorial analysis of the amino acid δ 15 N and specimen‐specific β mix values provides better understanding of the trophic interactions in food webs, even in complex seagrass meadow ecosystems.