Interannual changes in the calanoid copepod community off southern Baja California, Mexico

The calanoid copepod community was studied from zooplankton samples collected along transect 570 (off Bahía Magdalena) of the CICIMAR sampling grid during four spring cruises carried out during 1983-86. The null hypothesis postulating no interannual differences of the copepod community was tested. Calanoids represented between 84% and 96% of the total copepod numbers. Interannual variability of abundance was high: the overall abundance was lowest in 1985 and highest in 1984. A characteristic pattern of calanoid species abundance occurred each year; each arrangement could be considered as the result of local resource partitioning in which the abundance of a given species is in some way equivalent to the portion of niche space it occupies. In 1983 three species showed high abundance but a reduced distribution: Subeucalanus subcrassus, Euchaeta rimana, and Pleuromamma abdominalis. Between 1984 and 1986, Calanus pacificus was the most abundant and widely distributed, and was considered to be the representative species of the Copepoda during this period. Results of this study allowed rejection of the null hypothesis: interannual changes in oceanographic conditions are reflected in the copepod community, which shifted from a low-diversity, high-dominance stage during non-El Niño conditions to a more tropical, diverse community during the 1983 El Niño event. The southern boundary of the California Current system off Baja California varies, with changes identifiable through analysis of the structure of the calanoid copepod community.