The effect of a large cape on distribution patterns of coastal and oceanic copepods off Oregon and northern California during the 1998–1999 El Niño–La Niña

Hydrographic and ocean drifter measurements made along the Oregon coast indicate that the spatial structure of the coastal upwelling system differs in waters to the north and the south of Cape Blanco, Oregon. North of the Cape, a 10–30 km wide zone of coastal upwelling parallels the coast, but south of the Cape, increased wind stress leads to a seaward expansion of the upwelling system and cold upwelled water extends 50–100 km offshore. Because the hydrography and the transport differ, we hypothesize that zooplankton distributions will differ as well. In this paper we investigate differences in copepod distributions and copepod community composition between the waters north and south of Cape Blanco. Five cruises were conducted in 1998 and 1999, which were years of contrasting ocean conditions; there was a strong El Niño in 1998, which was followed by a strong La Niña in 1999. Copepod biomass did not differ between the El Niño and La Niña periods; however, species composition of the copepod assemblages differed vastly. During the 1998 El Niño, the copepod community was dominated by subtropical neritic and warm-water offshore species. During the 1999 La Niña, the zooplankton community was dominated by cold water boreal neritic species. The warm water species were widely distributed in shelf and slope waters in 1998, whereas in 1999, they were found primarily offshore of central Oregon, but over the shelf off northern California. During the summer upwelling season of both years, copepod community composition in shelf waters differed significantly from slope waters in the region to the north of the Cape, however, community composition was the same in shelf and slope waters in the region south of the Cape. These results lead us to suggest that offshore transport by the upwelling jet may be an important mechanism controlling copepod community structure south of Cape Blanco. When we examined these patterns in community composition on a species-by-species basis, among the dominant boreal copepod species, Pseudocalanus mimus and Acartia longiremis were displaced offshore and maintained high population densities in the waters south of Cape Blanco whereas densities of Calanus marshallae and Centropages abdominalis declined in the waters south of the Cape. Thus, the interaction between the boreal copepods and the waters north versus south of Blanco is species-specific. Species may be either lost or retained depending upon interactions between vertical current shear and their vertical distributions. Alternatively, there may be a differential ability among species to survive and reproduce in waters offshore and south of Cape Blanco.