Linking nitrogen dynamics to climate variability off central California: a 51 year record based on N/N in CalCOFI zooplankton

Long-term variability in zooplankton N/N was investigated in two species of calanoid copepods (Calanus pacificus and Eucalanus californicus) and two chaetognaths (Sagitta bierii and Sagitta euneritica) sampled in the spring of selected years from 1951 to 2001 off the central California coast. No statistically significant trend in N/N was detected for any of the four species, with isotopic ratios in 2001 resembling those in copepods and chaetognaths sampled five decades earlier. Zooplankton body lengths also showed no long-term trends. With respect to proposed regime shifts in this region, heterogeneity in N/N was detected only for S. bierii when comparing the periods 1951–1975, 1978–1998, and 1999–2001. In this species the N/N in the most recent, brief period (1999–2001) averaged slightly lower than in the previous period. Three of the four species (C. pacificus, S. bierii, and S. euneritica) showed significant increases in N/N during major El Niños. El Niño-related enrichment in N could arise as a consequence of increased nitrate demand:supply at the base of the food web or advection of N-enriched nitrate from more southerly waters. While a range of physical and climate indices were evaluated, anomalies of N/N from the longterm mean were found to be significantly related only to: (i) the Southern Oscillation Index in the case of both chaetognath species, (ii) a regional surface water temperature record (S. bierii only), (iii) an index of wind-driven coastal upwelling for the surface-dwelling C. pacificus, and (iv) variability in the Pacific Decadal Oscillation for the somewhat deeper-dwelling E. californicus. The relationships among each species’ N/N averaged over the total sampling period was: E. californicusEC. pacificus5S. euneritica o S. bierii, consistent with trophic N biomagnification and the predatory nature of Sagitta. r 2003 Elsevier Ltd. All rights reserved.