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

  • R Barlow Marine & Coastal Management, Department of Environmental Affairs and Tourism, Private Bag X2, Rogge Bay 8012, Cape Town, South Africa
  • H Sessions Marine & Coastal Management, Department of Environmental Affairs and Tourism, Private Bag X2, Rogge Bay 8012, Cape Town, South Africa
  • M Balarin Marine & Coastal Management, Department of Environmental Affairs and Tourism, Private Bag X2, Rogge Bay 8012, Cape Town, South Africa
  • S Weeks OceanSpace CC, University of Cape Town, Private Bag, Rondebosch 7700, South Africa
  • C Whittle OceanSpace CC, University of Cape Town, Private Bag, Rondebosch 7700, South Africa
  • L Hutchings Marine & Coastal Management, Department of Environmental Affairs and Tourism, Private Bag X2, Rogge Bay 8012, Cape Town, South Africa
Keywords: pigment indices, satellite imagery, seasonal variability, temperature

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.

African Journal of Marine Science 2005, 27(1): 275–287
Published
2005-06-30
Section
Articles

eISSN: 1814-2338