The size-structure of plankton assemblages across the continental shelf based on the data collected during the pelagic Spawner Biomass cruise in the southern Benguela upwelling ecosystem

Abstract

Abstract by author:

It is generally thought that the size structure of coastal plankton assemblages changes with distance from the coast in upwelling areas. Literature reviews indicate that closer to the coast, there are large phytoplankton cells (mostly diatoms), large species or adult stages of zooplankton, and adult stages of pelagic fishes. By contrast, small phytoplankton (mostly naked flagellates), small species or larval stages of zooplankton, and larval stages of pelagic fishes are to be encountered offshore. These observations in the literature were summarized by Hutchings (1992), in his hypothesis, which stated that the size structure of plankton assemblages across the shelf along the west coast of South Africa allows efficient transfer of primary production to larval and juvenile pelagic fish. We set out to test Hutchings' hypothesis using a series of plankton samples collected across the shelf in the southern Benguela upwelling ecosystem during the pelagic spawner biomass survey of 1994. An analysis of the size composition of the phytoplankton and zooplankton samples collected revealed that large phytoplankton cells and small zooplankton mostly dominated the inshore waters, while in offshore waters there were small phytoplankton cells and large zooplankton. The Box and Whisker plots indicated "cleaner" decreasing trends in sizes of large phytoplankton and small zooplankton across the shelf along the west coast than the south coast. Similarly, "cleaner" increasing trends in sizes of small phytoplankton and large zooplankton across the shelf were observed along the west coast that the south coast. This deviates from Hutchings1 (1992) hypothesis, and can be considered to have implications on fish recruitment. However this must be examined further so no conclusive evidence can be drawn from the present data