Winter community structure of the pelagic copepods in El Niño and non-El Niño conditions off Baja California, Mexico

El Niño probably is the main oceanographic phenomenon in the California region. Yet, little has been investigated about changes in the epipelagic copepod communities related to El Niño conditions, particularly off the Baja California area. Thus, a survey is presented here that compares the structure of the local copepod winter community off Southern Baja California, during the 1982-83 El Niño (EN) and in a non-El Niño (NEN) period (1984). In the NEN survey, the community was dominated by Pleuromamma abdominalis, Euchaeta marina, and Paracalanus parvus, which together constituted 43% of overall copepod numbers. The proportion of tropical species was 87%. In contrast, during the EN period species of Subeucalanus were dominant, and the tropical species represented up to 95% of total species diversity. The effect of EN showed latitudinal variations that would depend largely on the reaches of the warm water intrusion to the north. As compared with the NEN period, El Niño conditions showed a higher overall density and a higher proportion of tropical species along the N-S gradient. The copepod community structure off the Baja California area seems to respond sharply to the conditions set by EN. Density, species richness, or diversity alone might be misleading in identifying EN-NEN changes in the Baja California copepod community.