Trophic Relationships and the Nitrogen Isotopic Composition of Amino Acids in Plankton

Stable nitrogen isotope ratios of whole organisms and tissues are routinely used in studies of trophic relationships and nitrogen flow through ecosystems, yet changes underlying increases in δ15N from food source to consumer are not completely understood. In this study, the δ15N of 16 amino acids in marine planktonic consumers and their food sources were examined using gas chromatography/combustion/isotope ratio mass spectrometry of their N-pivaloyl-i-propyl-amino acid ester derivatives. Moderate increases in bulk δ15N with trophic position reflect an averaging of large increases in the δ15N of some amino acids, and little or no change in others. Amino acids showing consistently large increases (e.g., glutamic acid changes by ~7‰ between food and consumer) provide greater scope for defining trophic position than the smaller isotopic changes in bulk material. In contrast, amino acids like phenylalanine show no change in δ15 N with trophic position and therefore preserve information about nitrogen sources at the base of the food web. The ability to acquire information about both trophic level and nitrogen sources at the base of the food web from single samples of consumer tissues offers a powerful new tool for elucidating pathways of N transfer through food webs.