Community structure of near-surface zooplankton in the northern California Current in relation to oceanographic conditions

We analyzed how near-surface zooplankton are distributed in the northern California Current, not only in space and time but also with reference to species assemblages, habitat characteristics, and environmental factors. The community structure, spatial distribution patterns, and environmental associations of near-surface zooplankton from June and August 2000 GLOBEC cruises were examined. Crab megalopae, hyperiid amphipods, euphausiids, and chaetognaths dominated the near-surface zooplankton community during both cruises. A geostatistical approach was used to determine that near-surface zooplankton concentrations differed spatially and quantitatively between June and August. Near-surface zooplankton concentration was spatially patchy during June, with the highest levels occurring nearshore in various locations throughout the study region. In August, zooplankton concentration was more uniformly distributed; however, higher concentrations were observed north of Cape Blanco. During June, zooplankton species richness was spatially patchy, whereas in August, species richness was more uniformly distributed throughout the study area. Highest values were observed during June in localized areas along the coast, with lowest values observed further offshore near the shelf break at 200 m. During August, species richness was highest nearshore south of Cape Blanco; yet in the north, higher values extended across the shelf and further offshore. To explore patterns in community structure, we applied cluster analysis, indicator species analysis, and nonmetric multidimensional scaling (NMS) ordination to both data sets. Cluster analyses differentiated taxa by inshore and offshore location in the first division and by latitude in the second division. Results from NMS confirmed the cross-shelf zonation of near-surface zooplankton, with sea-surface temperature the most consistent environmental parameter explaining the distributions. An offshore assemblage of zooplankton was entrained in an eddy and transported onto the shelf in August.