Copepod communities in Monterey Bay during the 1997–1999 El Niño and La Niña

The abundance and biomass of the copepod assemblage in the upper 200 m of Monterey Bay was assessed on 43 occasions between March 1997 and December 1999 using 200 and 500 μm meshed nets. Based on the 500 μm net collections, the copepod assemblage averaged 36.3 copepods m−3 with biomass of 2.5 mg AFDW m−3, and was dominated by Calanus, Metridia, Eucalanus, Pleuromamma, Nannocalanus, and Rhincalanus. Composite data from the combined 200 and 500 μm nets indicated a mean abundance of 1267 copepods m−3, ranking the Clausocalanids, Oithona, Acartia, Paracalanus, Metridia, and then Calanus as the numerically dominant taxa. The mean biomass was 8.6 mg AFDWm−3, and in terms of biomass, the top ranking taxa were Calanus, the Clausocalanids, Metridia, Acartia, Oithona, and Paracalanus. There was a brief, but strong depression of zooplankton following the thermal peak of the El Niño, most notable in the large-bodied copepods including Calanus, Eucalanus, Metridia, and Rhincalanus. The typical springtime dominance of large-bodied copepods was not observed during the 1999 La Niña. Smaller-bodied copepod taxa appeared less influenced by either the El Niño or La Niña in terms of both their abundance and biomass, although responses at the species level did occur within some of these taxa. We propose that the strongest copepod depression during the El Niño was largely as a result of the displacement of the copepods by inflows of more southern, offshore waters. In contrast, the depression of large-bodied forms during the La Niña was the result of washout from anomalously strong upwelling, while smaller-bodied forms were being more readily reseeded from nearshore populations. Overall, the total abundance and biomass of copepods within Monterey Bay were surprisingly stable over the study period. We caution that regional expectations of climate related perturbation may not translate to all local scales.