Copepod species composition linked to ocean transport in the Northern California Current Keister, J.E., N. Mariani, C.A. Morgan, E. Di Lorenzo, V. Combes and W.T. Peterson In the Northern California Current, copepod communities show interannual and multi-year shifts in the dominance of species that track survival of fish populations. These zooplankton community changes are correlated with the Pacific Decadal Oscillation (PDO) index. A “warm-water” copepod species group is more abundant during warm (positive) phases of the PDO and less abundant during cold (negative) phases; the reverse occurs for a “cold-water” species group. This led to the hypothesis that the relative dominance of warm/cold copepod communities in the Northern California Current is associated with changes in the horizontal advection of surface water over different phases of the PDO. A Regional Ocean Modeling System (ROMS) and passive tracer simulations were used to investigate variation in surface water advection to coastal regions of the northern California Current over the period of 1950 to 2007. The results of the simulations were compared to observations of the species composition of zooplankton collected off Newport, Oregon (44.7°N 124.2°W) since 1996. Horizontal surface water movements change with the phase of the PDO and were related to regional changes in copepod species dominance. The surface current anomalies during the positive phase of the PDO showed average downwelling conditions (west to east transport) and stronger northward currents at the coast. These anomalies tended to transport warmer waters and the associated copepod species into the region. During the negative phase of the PDO, an increase in surface currents originating from the north enters the study area, and intrusion from the south and west declines. Overall, the modeled transport anomalies explained a significant part of the variance in copepod communities. Elucidating the mechanisms of variability in copepod community composition furthers our understanding of the physical controls on ecosystem processes in the Northern California Current. POBEX_COP_TRANS