Long-term changes in zooplankton volumes in the California Current System - the Baja California region

A retrospective analysis of zooplankton volumes (1951 to 1996) was performed for the area between Punta Baja (30 degrees N) and Punta Abreojos (26.7 degrees N) in relation 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 high values from June to October (median monthly volumes between 86 and 108 ml/1000 m(3)) and low values from November to May (58 to 77 ml/1000 m(3)). 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 Nino 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 during the 1957-1958 ENSO. Nonseasonal anomalies for zooplankton and environmental variables were significantly different (p < 0.001) among decades but not between the northern (30 degrees to 28 degrees N) and southern (28 degrees to 26 degrees N) areas. The decrease in zooplankton volume in this region over the last 2 decades is less than that reported for the Southern California Eight. This may be partly caused by seasonal northward movements of tropical zooplankton species along the Baja California coast, Mexico.