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 15N 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 6 SD: 2.1 6 0.1 and 2.9 6 0.1, respectively). Comparison of AA 15N enrichment patterns in different copepod species suggests that trophic 15N 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. The ability to position and compare individual species in trophic networks is critical to understanding energy flow and ecological relationships. Assignment of trophic position within complex aquatic food webs has been facilitated in recent years by stable isotope techniques (Kling et al. 1992; Post 2002). Stable nitrogen (N) isotopes, in particular, can be used to determine trophic structure because the 15N content of consumers increases with transfer path length through the food web. On average, N isotopic composition, measured as d15N, increases by ,3.4% in a consumer relative to its diet (DeNiro and Epstein 1981; Vander Zanden and Rasmussen 2001; Post 2002). Thus larger consumer d15N values should reflect higher trophic status within a community. However interpretations of d15N-based indices of trophic position are often complicated by changes in isotopic composition of prey resources. For example, spatial heterogeneity or seasonal cycles in N dynamics can drive significant and rapid fluctuations in d15N values at the base of the food web (Rolff 2000; Syväranta et al. 2006). To determine animal trophic position accurately, temporal or spatial isotopic variability in N sources at the base of the food web must be standardized relative to a baseline indicator (e.g., a long-lived primary consumer such as a mussel or snail [Post 2002]). For many ecosystems, finding an appropriate baseline indicator can be difficult. Zooplankton in oligotrophic systems, for example, may feed 1 Corresponding author ([email protected]). 2 Present address: Energy, Environment and Water Research Center, The Cyprus Institute, Guy Ourisson Building, 20 Kavafi Street, P.O. Box 27456, 1645 Nicosia, Cyprus.