Trophic-level interpretation based on δ15N values: implications of tissue-specific fractionation and amino acid composition

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 δ15N of consumers, but few studies have explored such factors in detail. For a better understanding of tissue-specific differences in δ15N, we examined postlarval euphausiids across a variety of seasons and regions in the Southern Ocean. The concentration and δ15N of individual amino acids were analysed to account for both the biochemical and physiological underpinnings of the observed bulk δ15N. Euphausiids showed consistent d15N differences of 1 to 2 ‰ between the digestive gland and abdominal segment, and between reproductively active males and females. These differences in bulk δ15N were accompanied by variations in relative proportions of amino acids (up to 5 mol %) and their δ15N (up to 11‰). Aspartic acid and glutamic acid had the strongest influence on bulk δ15N, due to their high abundance and variable δ15N values. Differences in relative proportions and/or δ15N of glycine and alanine were also important for bulk δ15N values. Isotopic variations in amino acids between gender and tissues were explained by dominant internal processes such as protein synthesis or degradation for energy supply, and by differences in amino acid pool sizes. Despite the offset in bulk δ15N between females and males, several lines of evidence suggested that their trophic levels were similar. Thus, specific amino acid composition and metabolism may confound trophic level interpretations of bulk δ15N values. Micronekton are normally analyzed whole in isotopic studies, and we suggest that their analyses should be restricted to comparable tissues such as muscles.