Can the natural isotope 15N be used as a trophic level tracer in the Puget Sound food chain?
Stable isotope 15N increases with each trophic level. Phytoplankton preferentially assimilate 14N, causing the remaining dissolved organic nitrogen (DIN) pool to be increasing higher in 15N (Wada 1987). Animals preferentially excrete 15N-depleted nitrogen, causing a magnification of 15N in the food chain. A fairly constant enrichment of 1 to 2 ppm has been observed at each step of the food web (Libes 1992). Thus the 15N of an organism can be used to infer its trophic level. In this study, 15N values of phytoplankton, Copepods, Aetideus sp., Amphipods, Euphausiids, and Euchaeta sp. were sampled from several different basins of Puget Sound in order to develop a partial food web in the Puget Sound. Samples were collected in both winter and spring to see if there were any consistent differences in the entire food web. Winter 15N values showed food web increases of approximately 0.75 ppm from Aetideus sp. through Euchaeta sp. Spring 15N values put each step of the food web at about 1.4 ppm
