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Marine Ecology Progress Series

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MEPS 180:111-120 (1999)  -  doi:10.3354/meps180111

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

M. J. Gibbons1,*, N. Gugushe1, A. J. Boyd2, L. J. Shannon2, B. A. Mitchell-Innes2

1Zoology Department, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
2Sea Fisheries Research Institute, Private Bag X2, Roggebaai 8012, South Africa

ABSTRACT: Marked changes in the composition and structure of the non-copepod components of a zooplankton assemblage were observed during the course of a 6 d drogue study in the southern Benguela ecosystem. These changes reflected in part the spiraled nature of the drogue and were linked to the settlement of meroplankton from the surface waters and to the nocturnal recruitment, and subsequent accumulation, of migratory holoplankton and zoobenthos from depth. The presence of large numbers of benthic species in the plankton is hypothesised to reflect their upward migration at night owing to food limitation, since little of the overlying diatom bloom was sedimenting out. The presence of low-oxygen bottom water is hypothesised to account for the failure of the detritivorous zoobenthos to return to the seafloor during the day. The results suggest that the composition of zooplankton assemblages in upwelling areas is not only driven by the dynamism of the physical environment, but that, under certain circumstances, a more subtle interplay between the biological and physical environment may result in strong changes in assemblage composition.

KEY WORDS: Zooplankton · Assemblage composition · Southern Benguela · Zoobenthos · Cumacea · Phytoplankton aggregates · Low-oxygen water

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