In April 1986 a strong intrusion of Angolan waters into the northern part of the Benguela system coincided with an abatement in the upwelling activity that characterizes the region. This advective process facilitated penetration by large numbers of species and individuals of the gelatinous zooplankton, chiefly siphonophores and hydromedusae. The increased densities in turn were reflected in higher concentration levels and greater diversity in the populations of planktonic cnidarians than are usually generated by the region's upwelling activity. The most conspicuous result was a pronounced latitudinal gradient that was clearly detectable above the thermocline, though the effect of the hydrographic event tapered off with depth. The consequences on upwelling regions of interannual events of the scope of that considered here are discussed in relation to the biological characteristics of the siphonophores and rnedusae

11 pages, 5 figures, 3 tables

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