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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.

[1] We quantify the wind contribution to the development of interannual sea surface temperature (SST) anomalies along the shelf of southern Africa. We compare numerical simulations that differ only in the amount of variability kept in the ERS1/2-derived surface wind forcing. Surprisingly, most of the cold and warm episodes over the Agulhas Bank are strictly related to local fluctuations of the forcing, whereas the shelf of the west coast extending 400 km north of Cape Columbine is equally sensitive to open-sea wind fluctuations. We diagnose the respective role of mesoscale eddy activity and of low frequency and intra-monthly wind fluctuations in generating interannual SST variability. The fair degree of correlation obtained at a few locations between the model and concomitant observations confirms the interest of a regional numerical tool to study anomalous events in the Benguela system.

Changes that occurred in sea surface and some deeper temperatures, surface winds, currents, plankton and some dominant crustacean (rock lobster Jasus lalandii), molluscan (squid Loligo vulgaris reynaudii and abalone Haliotis midae), fish (anchovy Engraulis capensis, pilchard Sardinops oce/latus, Cape hakes Merluccius spp., sole Austroglossus pectoralis, kingklip Genypterus capensis and various species of linefish) and seabird (Cape gannet Morus capensis) resources in the Benguela ecosystem in the 1980s are documented. Although fishing had a clear influence on changes in some of the resources, a wide spectrum of organisms, from plankton to top predators, were influenced directly or indirectly by the abiotic environment. Concurrent changes in many of the resources suggest that the same environmental change may impact on more than one. Simultaneous change throughout the Benguela system indicates that environmental forcing sometimes takes place over wide geographic areas. At the northern and southern extremities of the Benguela system, responses to environmental change may be different because processes through which the change is mediated are different.Verandering wat plaasgevind het in die see-oppervlak-en sommige dieper temperature, oppervlakwinde, strome, plankton en sommige hulpbronne soos skaaldiere (kreef Jasus lalandii), weekdiere (tjokka Loligo vulgaris reynaudii en perlemoen Ha/iotis midae), vis (ansjovis Engraulis capensis, sardyn Sardinops oce/latus, Kaapse stokvis Merluccius spp., tongvis Austroglossus pectoralis, koningklip Genypterus capensis en verskeie lynvisspesies) en seevoels (witmalgas Morus capensis) in die Benguela-ekostelsel in die 1980s word gedokumenteer. Ofskoon die vissery 'n duidelike invloed op veranderinge in somrnige van die hulpbronne gehad het, is 'n wye spektrum organismes, van plankton tot spitsroofdiere, regstreeks of onregstreeks be"invloed deur die nie-Iewende omgewing. Gelyktydige veranderinge in baie van die hulpbronne dui daarop dat dieselfde omgewingsverandering op meer as een van hlllle 'n uitwerking mag he. Gelyktydige verandering dwarsdeur die Benguelastelsel laat dink dat omgewingsforsering soms oor wye geografiese gebiede plaasvind. By die noorder-en suider-uiteindes van die Bengllelastelsel kan die reaksies op omgewingsverandering verskil omdat die prosesse waardeur die verandering teweeggebring word, verskil. 1992 4 J 2 J