Seasonal and interannual variability of the calanoid copepod community structure in shelf waters of the Eastern Tropical Pacific

The response of the calanoid copepod community structure and diversity to the variability of the oceanographic conditions was analyzed from January 1996 to December 1998 in the central Mexican Pacific. Interannual variability in the water column was marked by the influence of the 1997-98 El Niño (EN). However, seasonal variability was principally linked to the periodicity of coastal upwelling processes. Two species, Subeucalanus subcrassus and Centro - pages furcatus, contributed 62% to the copepod abundance over the 3 yr period, with 16 species making up 95% of the copepod community. Most species were subtropical-tropical (88%), but equatorial (8%) and cosmopolitan (2%) species were also present. Up to 17 species were recorded in 50% or more of samples, of which 10 were coastal or neritic forms. The calanoid copepod community structure varied seasonally, with increased abundance and reduced species richness during upwelling conditions. During upwelling relaxation, the abundance declined but the species richness increased. Additional community variations were related to the EN event, when copepod abundances were significantly lower than during non-El Niño (NEN) periods (F = 20.387, p < 0.05), but seasonal changes were still detectable. There was no increase of species richness in the area related to the influence of EN. These data suggest that the response of this tropical neritic copepod community diverges from that of other zooplankters and that the community is more stable than expected, keeping a seasonal pattern even during the influence of a strong EN.