Title: Long term Changes in California Current Zooplankton A Retrospective Analysis Project Summary: Marine zooplankton are one of the primary pathways through which physical climate signals propagate to marine fish populations. Evidence now shows geographically extensive changes of zooplankton biomass in concert with variations in the atmospheric and oceanic circulation in the NE Pacific. However, such analyses of bulk zooplankton biomass do not distinguish among taxa with diverse life histories, some of which are important trophic links to planktivorous fishes and others of which are not. The species composition of the zooplankton can strongly influence the intensity of zooplankton fish linkages, and consequently alter recruitment success. Mechanistic understanding and quantitative modeling of climate linkages to planktivorous fishes will depend upon specific knowledge of the zooplankton fauna and the differential responses of different zooplankton taxa to variations in circulation and productivity of the NE Pacific. We propose a retrospective analysis of the past 43/a decades of the California Current System (CCS). We will use the high quality CalCOFI zooplankton collection, together with associated hydrographic data and indices of atmospheric forcing, to understand the causes of changes in the zooplankton from 1951 to the present. Three aspects of the zooplankton composition will be investigated: changes in the high level taxonomic composition of all holozooplankton taxa, including gelatinous and crustacean forms; changes in the species composition of copepods and selected other taxa; and changes in trophic structure and nitrogen economy as inferred from the N stable isotope composition of two species of particlegrazing copepods. We hypothesize that there have been differential, taxon specific responses to: ª Decadal scale changes in the climate of the NE Pacific, including the 1976 77 warming event. . El Nifio and other interannual variations in flow from the equatorial region and from the Subarctic Pacific. Regional differences in the intensity of coastal upwelling and cross shore transport. Our studies will provide, for the first time, an understanding of multi decadal zooplankton species changes in the Pacific. We will uncover the taxa responsible for the longterm 70% decline in CCS zooplankton biomass. We will establish the temporal coherence of population changes in the central and southern sectors of the CCS, with which to analyze the covariation with related zooplankton species in the Subarctic Pacific. These studies will form the foundation for the design of new GLOBEC field studies and the development of NE Pacific pelagic ecosystem models.
