Zooplankton of the Cape Peninsula upwelling region

Includes bibliographical references.The vertical and horizontal distribution of meso-zooplankton in the vicinity of an extremely active upwelling site in the southern Benguela Current is described. Zooplankton samples collected in oceanic, newly-upwelled and mature upwelled water showed that temperature, chlorophyll and time of day had considerable effects on the vertical distribution of different species. Most species underwent limited daily vertical migrations within, the upper mixed layer, while concentrations of animals decreased markedly below the thermocline except where sinking had occurred, when an inverted distribution, particularly of juveniles, was observed. The Bray-Curtis dissimilarity coefficient was used to distinguish sample groups within the data. Differences in environmental parameters between groups were examined using the Mann-Whitney U-test. Species groups were separated using both the Bray-Curtisand McConnaughey coefficients and their relative merits are discussed. Samples from the upper mixed layer in upwelled water contained the greatest zooplankton standing crop in terms of dry weight, with Centropages brachiatus the most abundant organism. The frontal zone, with a strong thermal gradient, contained animals with distinct warm and cold water affinities as well as a group of species closely associated with the temperature gradient .The near-surface distribution of the bulk of the population, combined with the lack of cool-water continuity to the south, poses especial problems for the maintenance of the upwelled water community in the area during the upwelling season. It is suggested that sinking at the oceanic front combined with periodic shoreward flow of the upper layers when the southerly wind stress is relaxed, are sufficient to allow zooplankton to replenish rapidly from the north and offshore