Grant C. Pitcher ¹, Collin S. Roesler ² & Grev Nelson ¹, ¹Marine & Coastal Management, Private Bag X2, Rogge Bay 8012, Cape Town, South Africa, ² Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Maine, USA

Abstract

In the southern Benguela, three upwelling centres may be distinguished, all of which coincide with a narrowing of the shelf: Namaqua (30^oS), Cape Columbine (33^oS) and Cape Peninsula (34^oS). The region between the Namaqua and Cape Columbine cells is particularly susceptible to red tide formation and its negative impacts. During late summer and autumn surface boundary layer characteristics favour phytoplankton life forms typically associated with harmful algae in this region. The habitat preference of these phytoplankton species may be identified by an index of stratification. Accumulation of the bloom and the development of red tide is associated with frontal systems and zones of convergence associated with the complex 3-dimensional characteristics of this region. Areas of potential impact are typically inshore and their exposure to harmful blooms is dependent on the duration and frequency of the transport of blooms into the coastal environment. The simultaneous measurement of currents with observations of blooms, has allowed the role of cross-shelf and longshore currents in the transport of blooms to be understood and quantified. Models of harmful blooms in local areas of potential impact may therefore reduce to the advective term of the population dynamics equation. Moorings providing measures of these specific hydrographic features, supported with in situ measures of ocean colour, show considerable promise in offering short-term prediction of the possible threat of red tide to the coastal environment.