Abstract
The southern Namaqua shelf, north of the Cape Columbine upwelling center, is particularly prone to red tide. Two influences of the surface boundary layer on the development of red tide in this area were investigated—the dynamics of the upper mixed layer in determining phytoplankton community composition and the role of mesoscale circulation in bloom concentration and transport. Two survey periods (18 February‐11 March 2000 and 13‐30 March 2001) are reported during which red tide, dominated by the dinoflagellate Gyrodinium zeta , formed in the area. A high degree of concordance between characteristics of stratification and groupings of phytoplankton demonstrated the importance of the upper mixed layer in determining species or life‐form selection and development. Ordination of across‐shelf phytoplankton samples revealed a consistent banded pattern, created by a wind‐induced upwelling plume, tending to isolate a nearshore zone from an offshore domain, thereby creating an area of retention on the coastal side of the plume, favoring development of dinoflagellate blooms. The appearance of red tide was clearly associated with increasingly stratified conditions driven by alongshore flow from the north, following periods of wind relaxation and consequent reversal of surface currents.