Stable nitrogen isotopes in zooplankton: Some geographic and temporal variations in the North Pacific1

Abstract
Copepods and chaetognaths from the Southern California Bight had higher 15N:14N ratios than did those from the North Pacific Central Gyre. Within the bight, zooplankters were more enriched in 15N when NO3 was the major nitrogenous nutrient for phytoplankton than at other times. The 15N enrichment is related to the degree of exchange of nitrogen between the euphotic zone and deeper water, and the low 15N:14N ratio in the central gyre is probably also related to the importance of nitrogen fixation. In contrast, little geographic or temporal variation was observed in zooplanktonic δ13C.