Zooplankton of the Gulf of California after the 1982-1983 El Nino Event: Biomass Distribution and Abundance

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

Long-term Changes of Zooplankton Volumes in the California Current System. The Baja California Region.

A retrospeciive anaiysis oi zooplankton volumes (1951 to 1996) was performed for the area between Punta Baja (30°N) and Punta Abreojos (26 ?ON) in relatlon to the warming anomaly that has taken place in the California Current System during the last 2 decades. The seasonal cycle of median standing stock of zooplankton In this area showed a moderate alternation between h~ghva lues from June to October (median monthly volumes between 86 and 108 m1/1000 m") and low values from Kovember to May (58 to 77 m1/1000 m3). The quarterly long-term means of zooplankton volumes were the lowest in winter, as were wind speeds. The standard deviations associated with the long-term means indicated interannual variability was higher than seasonal variability. The time series showed an interval of high zooplankton volume between 1952 and 1957. Following the strong ENSO (El Nifio Southern Oscillation) of 1957-1958, a period of low values occurred which extended into the early 1960s. There was a slow recovery of zooplankton biomass through the rest of the 1960s, but it did not reach the earlier high values. Available data suggest the increasing trend reached a peak in 1975. Subsequently, from 1976 to the ENSO of 1982-1983, the biomass decreased. For the remainder of the 1980s, the few existing data showed an erratic behavior of the biomass. In the 1990s, there has been a decrease to values even lower than those observed durlng the 1957-1958 ENSO. Nonseasonal anomalles for zoop1.ankton and environmental variables were significantly different (p < 0.001) anlong decades but not between the northern (30" to 28" N) and southern (28" to 26" N) areas. The decrease in zooplankton volume in this region over the last 2 decades is less than that reported for the Southern California Bight. This may be partly caused by seasonal northward movements of tropical zooplancton species along the Baja California coast, Mexico