Quantification of zooplankton trophic position in the North Pacific Subtropical Gyre using stable nitrogen isotopes

We quantify the trophic positions of subtropical open-ocean zooplankton species using amino acid-specific (AA) stable nitrogen isotopic compositions. We model animal trophic position by computing trophic N-15 enrichment of glutamic acid relative to phenylalanine, and find that trophic position for primary copepod consumers (Oithona spp., Neocalanus robustior) and secondary copepod consumers (Pleuromamma xiphias and Euchaeta rimana) varied little over a 5-10-yr period in the North Pacific Subtropical Gyre (NPSG; mean +/- SD: 2.1 +/- 0.1 and 2.9 +/- 0.1, respectively). Comparison of AA N-15 enrichment patterns in different copepod species suggests that trophic N-15 enrichment is most consistent in glutamic acid, aspartic acid, and alanine, "trophic" AAs that are intimately involved in the citric acid cycle and energy production. We further test equations involving these trophic AAs and "source" AAs ( which appear to retain the nitrogen isotopic composition of the food-web base), and find that such compound-specific models give results that are identical to those calculated using whole-animal (bulk) stable isotopic compositions. However the benefits of our AA-based approach (i.e., the relatively few samples needed for precise TP estimation, elimination of the need for concurrent prey isotopic analyses, and the ability to utilize formalin-preserved specimens from archived collections), make this a powerful technique for the quantitative assessment of trophic position within the pelagic food web. We further discuss how stable isotopic analyses provide a new perspective on the structure of open-ocean food webs and can be used to trace large seasonal fluctuations in nitrogen source in the NPSG.