Seasonal and interannual variability in phytoplankton biomass on the southern African continental shelf: evidence from satellite-derived pigment concentrations

Surface ocean phytoplankton biomass variability on the southern African continental shelf area is investigated using coastal zone colour scanner data for the period 1979–1986. Coherent interannual trends in surface ocean chlorophyll for both the west coast Benguela and the south coast Agulhas Bank areas correspond with sea surface temperature records, rather than with local records of upwelling-favourable winds. This finding implies that interannual variability in phytoplankton biomass, in contrast to the well-established seasonal cycle, is associated with large-scale oceanic circulation features and forcing mechanisms, rather than with localized upwelling events. The most probable causative mechanism is suggested to be anomalous advective fluxes of warm surface water into areas typically of high biomass deriving from the Agulhas Current retroflection in the south. Studyingvariability in phytoplankton biomass on the continental shelf in the context of large-scale oceanic circulation features is important to understanding long-term trends in productivity and fisheries resources