Interannual variability in copepod community composition at a coastal station in the northern California Current: a multivariate approach

We sampled a single station in the coastal zone off Newport OR (9 km from shore; 60 m water depth) on 206 occasions during 12 years: 1969–1973, 1983 and 1996–2001. We used cluster analysis, ordinations, and indicator species analysis (ISA) to describe temporal variations in copepod community composition. Copepod community structure during summer was distinctly different from winter. Cluster analysis showed that the transition between winter and summer communities occurred early March/April in the 1970s, late (May/June) in the late 1990s, but in (March/April) since spring 2000. Seven copepod assemblages were identified: four were found during the summer upwelling season, two during large El Niño events, and one during winter. Interannual variations in the composition of the summer assemblages was seen: most sampling dates from the summers of 1970, 1973, 2000 and 2001 clustered into one group, and dates from 1971, 1972, and 1999 clustered into a second group. The 1983 and 1998 El Niño events clustered together, but subdivided into “early El Niño” and “late El Niño” communities. The summer of 1969 corresponded with a weak El Niño event but clustered differently from both the other El Niño events and other summer clusters. Samples collected during the 1972 El Niño event clustered with “normal” summers. Non-metric multidimensional ordination analysis showed that two axes accounted for 87% of the variability in community composition; Axis 1 was associated with the influence of El Niño events and seasonal downwelling, and Axis 2 was associated with upwelling-induced productivity. ISA showed Centropages abdominalis, Acartia longiremis, and Microcalanus pusillus as indicators of upwelling; Corycaeus anglicus, Calanus pacificus, and Ctenocalanus vanus as good indicators of El Niño; and Ctenocalanus vanus, Clausocalanus, and Calocalanus styliremis as good indicators of winter conditions.