Plankton productivity of the Benguela Current Large Marine Ecosystem (BCLME)

• Plankton are ideal indicators of ecosystem change and require continued monitoring in the BCLME.
• A direct link between phytoplankton dynamics and environmental drivers is not evident.
• Zooplankton shows substantial, differential long-term changes in the north and south.
• The relative importance of bottom-up and top-down control of plankton is uncertain.

Environmental drivers that have been observed to cause changes in phytoplankton biomass and production include surface warming, increased wind stress and upwelling, extension of low oxygen zones, changes in nutrient distributions, and increased stratification. While there have been documented variations in phytoplankton biomass and primary production at seasonal and interannual time scales in the BCLME, there appears to be no strong evidence of decade-scale changes or the expected ecosystem-wide increase/decrease in production in response to projected increases/decreases in upwelling-favourable winds. During the past six decades there have been substantial, long-term changes in abundance, biomass, production and species and size composition of neritic zooplankton communities in both the northern and southern Benguela subsystems. Copepods have increased since the 1950s in both subsystems, until a turning point around the mid-1990s in the south and a decade later in the north, after which they have been declining. Both subsystems also experienced a shift from large to smaller species dominating. These major changes reflect patterns of spatial, temporal and size-based heterogeneity in the BCLME and are thought to be mediated locally and differentially through bottom-up and top-down forcing mechanisms. While the relative importance of these control mechanisms remains uncertain, changes in the plankton as observed in the BCLME have fundamental effects on biogeochemical processes, food web structure and ecosystem functioning, as well as on the ecosystem services supported by the plankton. Because plankton are ideal indicators of ecosystem change, continued transboundary monitoring of their communities in the BCLME is warranted in the long term, e.g. using cost-effective technologies such as satellite imagery of ocean colour and the deployment of Continuous Plankton Recorders from ships-of-opportunity.