Onshore-offshore variations in copepod community structure off the Oregon coast during the summer upwelling season

Abstract

During the summer of 1994, 1996, and 1997, we conducted hydrographic and plankton surveys of the upper 70 m in the nearshore, the continental shelf, and off-shelf oceanic waters off Oregon, USA. Copepod densities and biomass were estimated along 4 transects from each cruise. The on-shelf copepod biomass was 2.8 times greater than off-shelf biomass. Nonmetric multidimen- sional scaling identified an on-shelf and an off-shelf copepod community. The change in community composition usually occurred at or slightly offshore of the continental shelf break, defined here as 180 m water depth. Indicator-species analysis identified the subarctic neritic species, Calanus mar- shallae, Pseudocalanus mimus, and Acartia longiremis, as good indicators of continental shelf waters. 'Warm water' species Mesocalanus tenuicornis, Calocalanus styliremis, Clausocalanus spp., and Ctenocalanus vanus were indicators of off-shelf waters. The copepod communities off the coast of Oregon during the summer upwelling season reflect the origins of the dominating currents within each habitat. The coastal copepod community is subarctic neritic in origin, consistent with southward coastal flows. The offshore copepod community is a mixture of species with origins in the Transition Zone as well as species that are typical of the coastal region of the California Current off central and southern California. We speculate that the central and southern California Current species are pre- sent in offshore waters of Oregon in summer because they are transported north with the Davidson Current in winter but, with the initiation of the upwelling season in spring, they are transported into offshore waters where they establish viable populations.