Whereas zooplankton in most eastern boundary current systems (EBCSs) studied to date has declined in terms of biomass over the past 5 decades, it has increased 100-fold in terms of numerical abundance in the southern Benguela region, thereby undergoing a displacement in species composition through time. These long-term changes in coastal zooplankton off South Africa have previously been hypothesized to be mediated by both “bottom-up” and “top-down” control mechanisms, in contrast to other EBCSs and marine ecosystems in general which have traditionally been viewed to be structured from below. From a “bottom-up” perspective, the increase in zooplankton abundance is a long-term biological response to intensified coastal upwelling, common to EBCSs, and its associated processes of nutrient enrichment and increased phytoplankton production and biomass. On the other hand, such increase in zooplankton and the accompanying shift in its community structure are thought to result from a “cascading” trophic effect of predator-prey interactions between apex predators and their small pelagic forage species, and between the latter and their zooplanktonic prey. This study provides new information in terms of decadal-scale increasing trends in nutrient concentration and phytoplankton biomass off the West Coast of South Africa, lending support to the above “bottom-up” hypothesis. In addition, published data on long-term, density-dependent increases in pelagic fish condition and maturity are interpreted within the context of the above “top-down” mechanism which is postulated to control zooplankton populations in the Benguela region concurrently with “bottom-up” mechanisms.
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Vol. 29 • No. 1