Linking nitrogen dynamics to climate variability off central California: a 51 year record based on 15N/14N in CalCOFI zooplankton

Abstract

Abstract Long-term variability in zooplankton 15N/14N 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 15N/14N 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 15N/14N was detected only for S. bierii when comparing the periods 1951–1975, 1978–1998, and 1999–2001. In this species the 15N/14N 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 15N/14N during major El Ninos. El Nino-related enrichment in 15N could arise as a consequence of increased nitrate demand:supply at the base of the food web or advection of 15N-enriched nitrate from more southerly waters. While a range of physical and climate indices were evaluated, anomalies of 15N/14N from the long-term 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’ 15N/14N averaged over the total sampling period was: E. californicus≈C. pacificus⪡S. euneritica