From 23 March to 7 April 1984, we studied the responses of
zooplankton populations to the 1982-1983 El Nino event in the Gulf of
California. Twenty six stations were sampled for zooplankton distribution and
abundance. Mean displacement volume was 388 cm3/lOOO m3
, a value higher
than biomass values reported for the California Current and the eastern tropical
Pacific. Maximum values (up to 60 mg/m3 of dry weight) were registered on the
eastern shore of the gulf. At other stations biomass values ranged from 11.2 to
39.2 mg/m3
. No significant differences were observed in the mean biomass of the
central gulf between spring 1983 and spring 1984. However, biomass increased
in the southern gulf during spring 1984. Mean total abundance of zooplankton
was 13% higher than in 1983. Coastal stations registered up to 31% of the total
abundance. Copepods and cladocerans represented over 65% of mean total
abundance and community structure differed from that in 1983: there were
more copepods, euphausiids, tunicates, and siphonophores in 1984, and fewer
cladocerans, ostracods, and red crabs (Pleuroncodes planipes). Similarity analysis
showed two main assemblages: one in the central gulf (temperate zone), the other
near the mouth (tropical zone). These two regions correspond to zones with
different physical and primary production characteristics. The different responses
seen in the central and southern gulf may reflect phytoplankton biomass
and primary productivity dynamics. The El Nino event reduced the biomass in
the southern part of the gulf, although the entire gulf in 1983 and 1984 showed
higher biomass values than other eastern Pacific systems. Year-to-year differences
may be less important than seasonal changes. The EI Nino phenomenon
mainly affected the relative abundances of different taxa
