Decadal-scale Trends Across Several Marine Trophic Levels in the Southern Benguela Upwelling System off South Africa

Abstract

Abstract 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 e...