Comparative Analysis of Shelf-Slope-Oceanic Ecosystems in an Upwelling Region: Implications for Juvenile Salmon

We sampled hydrography, chlorophyll, copepods and euphausiids during biweekly cruises at seven hydrographic stations across the continental shelf off Newport Oregon (44º40’N) from 1996 until present. Alongshore current velocities were estimated from gridded AVISO altimeter data and water level data from NOAA tide gauges (1993 – 2010) and cross-shelf transports were approximated using an upwelling index. Copepods from those samples were identified to species then pooled into one of four categories based on water mass affinities. We compared zooplankton communities between a nearshore station, NH05 ~8km offshore, and an offshore station, NH25 ~42km offshore. Cold neritic copepods (Calanus marshallae, Pseudocalanus mimus and Acartia longiremis) were the dominant group at both stations. At the nearshore station NH05, the alongshore current exhibited a strong seasonal pattern with the biomass of cold neritic copepods fluctuating in opposite phase: positive alongshore current (northward) corresponding to low biomass in winter and negative alongshore current (southward) corresponding to high biomass in summer. At the offshore station NH25, the biomass of cold neritic copepods appeared to be related to cross-shelf transport. When alongshore transport was compare to the Pacific Decadal Oscillation (PDO), we found that when PDO is positive, i.e., warm conditions, there was more water from south during the winter season, and when PDO is negative, i.e., cold conditions, there was more water from the north during the summer season. The cumulative alongshore transport showed significant correlation with juvenile salmon survival in this region and helps explain how large scale forcing influences local zooplankton communities and upper trophic levels in the upwelling zone off Oregon.