Long-term changes in the species composition of calanoid copepods off southern California

Much of the lower frequency (interannual to interdecadal) variability in macrozooplankton biomass in the California Current System (CCS) is thought to be driven by climatic fluctuations. The objective of this study is to understand the effects of climate on copepod populations in the CCS. The approach is to describe the low-frequency fluctuations in the densities of selected species of calanoid copepods, and the relationships between those fluctuations and climate variability.; Copepods collected off southern California between 1951 and 1999 by the California Cooperative Oceanic Fisheries Investigations (CalCOFI) were enumerated. Samples (7 to 19 stations per cruise) from 41 spring and 32 fall cruises were analyzed for 40 species, subspecies and stages of calanoid copepods. An analysis of a sampling gear change in the mid-1970s revealed no evidence for an effect of sampling gear on estimated densities, although power to detect a difference was low in many cases.; In spite of large environmental changes in the CCS, dominance structure of the calanoid copepods has remained stable over 49 years. Except for six anomalous years, the rank order of abundance was similar in all spring cruises. Five of the six anomalous years occurred after the 1976–77 climatic regime shift. Three of these years coincided with strong El Niño events, while two occurred during warm water years in the CCS immediately following the regime shift.; Many species apparently responded to observed or hypothesized climatic regime shifts in 1976–77 and 1989. 28% of the species examined increased in abundance in the mid-1970s, while another 28% underwent abrupt changes, mostly declining, around 1990. Surface waters warmed and stratification intensified after the mid-1970s, but there is little evidence in the CalCOFI hydrographic data for a climatic regime shift around 1989.; Effects of climatic variability on the two dominant species were investigated. Metridia pacifica s.l. abundance in spring was strongly influenced by the El Niño-Southern Oscillation (ENSO), while Calanus pacificus abundance was inversely related to temperature at 150 m. The different life histories of the two species must be considered in explaining their different responses to environmental variability.