SUBSURFACE DINOFLAGELLATE POPULATIONS, FRONTAL BLOOMS AND THE FORMATION OF RED TIDE IN THE SOUTHERN BENGUELA UPWELLING SYSTEM

The West Coast of South Africa is often subjected to problems associat ed with red tides which are usually attributed to blooms of migratory dinoflagellates. This study investigates the coupling between the phys ical environment and the biological behaviour and physiological adapta tion of dinoflagellates in an attempt to understand bloom development, maintenance and decline. Widespread and persistent subsurface dinofla gellate populations dominate the stratified waters of the southern Ben guela during the latter part of the upwelling season. Chlorophyll conc entrations as high as 50 mg m(-3) are associated with the thermocline at approximately 20 m depth but photosynthesis in this region is restr icted by low Light. The subsurface population is brought to the surfac e in the region of the upwelling front. Here increased Light levels ar e 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 curr ent conditions these frontal blooms are transported and accumulated in shore and red tides are formed.