Zonal and seasonal variations in zooplankton community structure off the central Oregon coast, 1998–2000

Previous studies of zooplankton communities off the central Oregon coast have been primarily descriptive or have focused on only a few taxa. To more formally explore patterns in zooplankton community structure, we used cluster analysis, Non-metric Multidimensional Scaling (NMDS) ordination, and Indicator Species Analysis to examine seasonal, El Niño/La Niña, and onshore/offshore differences in community composition. Hydrographic and zooplankton data were collected off Newport, Oregon (44.7°N) at eight stations between 8 and 105 km from shore (60-2900 m water depth), on 15 cruises between Jan 1998 and Sept 2000 as part of the US GLOBEC Northeast Pacific Long-Term Observation Program. Zooplankton were collected with a 202 μm, 0.5 m diameter ring net lifted vertically at 30 m min−1 from 100 m, to the surface. 97/1998 El Niño played an important role in structuring the zooplankton community from the beginning of our sampling in January 1998 through November 1998. The “El Niño” group identified by cluster analysis and NMDS ordination was highly diverse and was characterized by taxa with offshore and subtropical neritic affinities. When the El Niño group was present, there was no onshelf-offshelf gradient in community composition as was seen during “normal” summers. The “El Niño” group was preceded and followed by a “Transitional” group with slightly lower diversity that existed through the spring of 1999. By summer 1999, conditions had returned to “normal.” upwelling strongly affected the structure of zooplankton assemblages in the “normal” years of 1999 and 2000, leading to differences between nearshore and offshore zooplankton groups. The zooplankton assemblages sampled on the continental shelf during these summers were unique, composed of boreal neritic species (such as the copepods Calanus marshallae, Pseudocalanus mimus, and Acartia longiremis) and larvae of several meroplanktonic taxa, whereas the off-shelf assemblages resembled the winter zooplankton assemblages which were comprised of subtropical neritic and Transition Zone species. The spatial extent of the continental shelf group present during the summer upwelling season varied between early and late summer; differences may be related to the integrated strength and duration of the north winds over the upwelling season.