Copepod Community Structure at Bahia Magdalena, Mexico during El Niño 1983–84

Changes in the community structure and distribution patterns of 71 species of copepods collected during eight oceanographic surveys at Bahia Magdalena, Mexico were analysed. From March 1983 to January 1985, the responses of the copepod community structure to the seasonal environmental conditions and temporal changes during the peak warming of the El Niño event 1982–83, the relaxation phase in 1984, and the winter 1984–85 cooler period were studied. As in other estuaries and bays, the copepod community had a few dominant species, which have a seasonal succession throughout the year related to seasonal changes in sea-surface temperature. There are two major successional stages in the annual cycle of zooplankton development. During the warming period (July–October of 1983 and 1984),Acartia lilljeborgiiGiesbrecht andAcartia tonsaDana (tropical affinity) were the dominant species in the copepod community. During the cooler period (December–June), the dominant species wasParacalanus parvus(Claus). During Summer and Autumn 1983 (El Niño peak), an unusual increase of tropical-equatorial copepod species were found, that are regularly found offshore. The resident speciesAcartia clausiGiesbrecht was replaced byA. tonsa. Despite these changes in the species composition because of El Niño, seasonal succession is the general rule in Bahia Magdalena and was only enhanced by the presence of several tropical species during the warmest period.