Changes in the composition of the non-copepod zooplankton assemblage in StHelena Bay (southern Benguela ecosystem) during a six day drogue study

Marked changes in the composition and structure of the non-copepod componen ts of a zooplankton assemblage were observed during the course of a 6 d dro gue study in the southern Benguela ecosystem. These changes reflected in pa rt the spiraled nature of the drogue and were linked to the settlement of m eroplankton from the surface waters and to the nocturnal recruitment, and s ubsequent accumulation, of migratory holoplankton and zoobenthos from depth . The presence of large numbers of benthic species in the plankton is hypot hesised to reflect their upward migration at night owing to food limitation , since little of the overlying diatom bloom was sedimenting out. The prese nce of low-oxygen bottom water is hypothesised to account for the failure o f the detritivorous zoobenthos to return to the seafloor during the day. Th e results suggest that the composition of zooplankton assemblages in upwell ing areas is not only driven by the dynamism of the physical environment, b ut that, under certain circumstances, a more subtle interplay between the b iological and physical environment may result in strong changes in assembla ge composition.