Changes in dominance of copepods off Baja California during the 1997-1999 El Nino and La Nina

Changes in copepod species composition were analyzed from September-October 1997 to April 1999. The community changed substantially between El Nino 1997-1998 and La Nina 19981999, following trends in oceanographic conditions. The warm period was characterized by a community rich in equatorial, tropical/subtropical and warm-temperate cosmopolites, but almost lacking in subarctic copepods. The widespread presence of Undinula vulgaris appears to be due to the strong poleward jet that developed in the coastal region during autumn 1997. Although the coastal jet dispersed in early 1998, the strong influence of equatorial water continued, as indicated by the record abundance of Subeucalanus subtenuis, an equatorial species in oceanic eutrophic waters. The community during the cold period comprised very few copepods with equatorial affinity, few tropical/subtropical and warm-temperate cosmopolites, and an increased relative importance of subarctic species. Copepods typical for the California Current (transition zone group) displayed no significant differences between the extremes of El Nino/La Nina during the fall and winter seasons. A strong La Nina increase in the transition zone group (particularly in Calanus pacificus) was observed until spring 1999, and was probably related to a record coastal upwelling during the La Nina conditions at this time. El Nino 1997-1998 had a greater influence on equatorial species in the region than the previous El Nino event of 1957-1959.