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

The calanoid copepod community was studied from zooplankton samples collect ed along transect 570 (off Bahia Magdalena) of the CICIMAR sampling grid du ring four spring cruises carried out during 1983-86. The null hypothesis po stulating no interannual differences of the copepod community was tested. C alanoids represented between 84% and 96% of the total copepod numbers. Inte rannual 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 r esult of local resource partitioning in which the abundance of a given spec ies is in some way equivalent to the portion of niche space it occupies. In 1983 three species showed high abundance but a reduced distribution: Subeu calanus, subcrassus, Euchaeta rimana, and Pleuromamma abdominalis. Between 1984 and 1986, Calanus pacificus was the most abundant and widely distribut ed, and was considered to be the representative species of the Copepoda dur ing this period. Results of this study allowed rejection of the null hypoth esis: interannual changes in oceanographic conditions are reflected in the copepod community which shifted from a low-diversity, high-dominance stage during non-El Nino conditions to a more tropical, diverse community during the 1983 El Nino event. The southern boundary of the California Current sys tem off Baja California varies, with changes identifiable through analysis of the structure of the calanoid copepod community.