Trophic-level interpretation based on d15N values: implications of tissue-specific fractionation and amino acid composition

Abstract

Stable nitrogen isotope ratios are routinely used to disentangle trophic relationships. Several authors have discussed factors in addition to diet that might contribute to variability in δ 15 N of consumers, but few studies have explored such factors in detail. For a better understanding of tis- sue-specific differences in δ 15 N, we examined postlarval euphausiids across a variety of seasons and regions in the Southern Ocean. The concentration and δ 15 N of individual amino acids were analysed to account for both the biochemical and physiological underpinnings of the observed bulk δ 15 N. Euphausiids showed consistent d 15 N differences of 1 to 2 ‰ between the digestive gland and abdom- inal segment, and between reproductively active males and females. These differences in bulk δ 15 N were accompanied by variations in relative proportions of amino acids (up to 5 mol %) and their δ 15 N (up to 11‰). Aspartic acid and glutamic acid had the strongest influence on bulk δ 15 N, due to their high abundance and variable δ 15 N values. Differences in relative proportions and/or δ 15 N of glycine and alanine were also important for bulk δ 15 N values. Isotopic variations in amino acids between gen- der and tissues were explained by dominant internal processes such as protein synthesis or degrada- tion for energy supply, and by differences in amino acid pool sizes. Despite the offset in bulk δ 15 N between females and males, several lines of evidence suggested that their trophic levels were simi- lar. Thus, specific amino acid composition and metabolism may confound trophic level interpretations of bulk δ 15 N values. Micronekton are normally analyzed whole in isotopic studies, and we suggest that their analyses should be restricted to comparable tissues such as muscles.