Seasonal variation in phytoplankton in the southern Benguela: pigment indices and ocean colour

Abstract

Pigment indices and satellite imagery were used to investigate the seasonal variation in phytoplankton in the southern Benguela, with the aid of monthly monitoring on two transect lines. Sea surface temperature images revealed the similarity in thermal conditions during autumn, winter and spring, with a broad zone of 11–14°C water extending northwards from Cape Columbine and a narrow zone extending south along the coast to the Cape Peninsula. In summer, warm water (18–20°C) intruded from the South Coast into the West Coast offshore region and cold water (10–12°C) prevailed inshore at particular upwelling centres. The phytoplankton response to these thermal conditions resulted in elevated chlorophyll a (2–15mg m –3) in the 11–14°C waters during all four seasons. The zones of elevated chlorophyll were generally broader from Cape Columbine northwards and narrower south of Cape Columbine to the Cape Peninsula. Pigment indices indicated that phytoplankton biomass was high in summer, but also quite extensive in the autumn and spring, with diatoms generally dominating inshore and small flagellates offshore. Diagnostic indices indicated that flagellates were prominent at inshore localities during all seasons, however, suggesting that this group contributed significantly to phytoplankton communities on the shelf.