Multi-decadal variations in stable N isotopes of California Current zooplankton

We analyzed variations in naturally occurring N in four species of zooplankton as an index of climate influences on pelagic food web structure in a major eastern boundary current ecosystem. Our analyses focused on two species of particle-grazing copepods (Calanus pacificus and Eucalanus californicus) and two species of carnivorous chaetognaths (Sagitta bierii and S. euneritica), drawing on the CalCOFI zooplankton time series from both the southern and central sectors of the California Current System. We detected a significant difference between regions in average stable N isotope content of the two species of copepods, with N elevated by 0.5-1.1 per mil in the southern region, but no regional differences in the isotopic content of the chaetognaths. We address climate influences over a 54-year time period, on three different time scales: interannual (dominated by ENSO), decadal, and multi-decadal. Three of four species showed evidence of an ENSO-related isotopic shift toward increased N during El Niño conditions. In addition, in Southern California waters, C. pacificus and S. euneritica showed elevated N in the warm phase of the NE Pacific between 1978-1998 relative to the preceding and following time periods. When considered over the entire 51⁄2 decades treated here, for most species there was remarkable long-term stability in stable isotope content in both southern and central California waters, despite interannual and decadal perturbations. Only E. californicus in the southern sector showed a significant downward secular trend in N. Variability of N in 3 species covaried with the average nitrate concentration in the mixed layer, suggesting altered nitrate utilization at the base of the food web as a primary mechanism underlying interannual changes in zooplankton isotopic content.