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

Copepods and chaetognaths from the Southern California Bight had higher 15 N: 14 N ratios than did those from the North Pacific Central Gyre. Within the bight, zooplankters were more enriched in 15 N when NO 3 − was the major nitrogenous nutrient for phytoplankton than at other times. The 15 N enrichment is related to the degree of exchange of nitrogen between the euphotic zone and deeper water, and the low 15 N: 14 N 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 δ 13 C.