MEPS 172:253-264 (1998)  -  doi:10.3354/meps172253

Subsurface dinoflagellate populations, frontal blooms and the formation of red tide in the southern Benguela upwelling system

G. C. Pitcher*, A. J. Boyd, D. A. Horstman, B. A. Mitchell-Innes

Sea Fisheries Research Institute, Private Bag X2, Rogge Bay 8012, Cape Town, South Africa

ABSTRACT: The West Coast of South Africa is often subjected to problems associated with red tides which are usually attributed to blooms of migratory dinoflagellates. This study investigates the coupling between the physical environment and the biological behaviour and physiological adaptation of dinoflagellates in an attempt to understand bloom development, maintenance and decline. Widespread and persistent subsurface dinoflagellate populations dominate the stratified waters of the southern Benguela during the latter part of the upwelling season. Chlorophyll concentrations as high as 50 mg m-3 are associated with the thermocline at approximately 20 m depth but photosynthesis in this region is restricted by low light. The subsurface population is brought to the surface in the region of the upwelling front. Here increased light levels are responsible for enhanced production, in some instances exceeding 80 mgC m-3 h-1, and resulting in dense dinoflagellate concentrations in and around the uplifted thermocline. Under particular wind and current conditions these frontal blooms are transported and accumulated inshore and red tides are formed.

KEY WORDS: Dinoflagellates · Subsurface populations · Frontal blooms · Red tide · Upwelling systems

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