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The southern Namaqua shelf of the Benguela upwelling system, downstream of the Cape Columbine upwelling cell, is frequently subjected to a variety of harmful algal bloom (HAB) phenomena. Here, winds dictate most physical processes that are important to the development of HABs. Toxic algal cell concentrations and distributions, in relation to their respective toxin content, are compared over 2 autumn periods in successive years which exhibited clearly different wind patterns and hydrographic responses. During the first study period (15 March to 6 April 2005), several periods of relaxation from upwelling-favourable winds were associated with poleward nearshore currents and increasing levels of stratification. The phytoplankton community was dominated by dinoflagellates, including Dinophysis spp. and Protoceratium reticulatum, responsible for the production of toxins associated with diarrhetic shellfish poisoning and yessotoxins, respectively. In contrast, the subsequent study period (7 to 23 March 2006) was characterised by persistent upwelling-favourable winds, equatorward near-surface currents, and a cooler, moderately mixed water column. The phytoplankton assemblage was co-dominated by Pseudo-nitzschia spp. and Chaetoceros spp.; domoic acid concentrations corresponded closely with cell concentrations of Pseudo-nitzschia spp. Contrasting wind patterns, and their influence on water column structure and mesoscale circulation, led to predictably different assemblages of phytoplankton life-forms and their associated toxins.KEY WORDS: Southern Benguela · Upwelling · Harmful algal bloom · Diatom · Dinoflagellate · Algal toxins 348: 19-31, 2007 upwelling in the southern Benguela is highly seasonal due to shifts in the position of the South Atlantic High; the upwelling season extends from September to March, with upwelling reaching a maximum in the austral spring and summer (Nelson & Hutchings 1983). The present study focuses on the greater St. Helena Bay region, where the shelf broadens downstream of the Cape Columbine upwelling cell at 33°S (Fig. 1). This area is characterised by high phytoplankton biomass, as identified by remotely sensed chlorophyll data (Pitcher & Weeks 2006), and is particularly susceptible to the development of HABs during the latter half of the upwelling season, when the contribution of dinoflagellates to the phytoplankton community is greatest. The high productivity is a result of: (1) the periodic resupply of nutrients to the euphotic zone through upwelling; (2) the relative stability of the physical environment; and (3) retentive circulation in the region (Pitcher et al. 1992). Resale or republication not permitted without written consent of the publisherMar Ecol Prog SerA conceptual model of red tide formation in St. Helena Bay has been presented by Pitcher & Nelson (2006). During upwelling, northward flow dominates and an equatorward coastal jet forms off Cape Columbine, where the shelf is narrow and steep (Nelson & Hutchings 1983). Under these conditions, a plume of cold water...

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Sinking rates of phytoplankton assemblages with &verse and variable taxonomic composition, growing under a variety of environmental conditions, were measured In St. Helena Bay (S. Africa) using the homogenous sample method SETCOL. These measurements and flux estimates from sediment trap recovenes enabled the role of sinking phytoplankton in the flux of organic material from the euphotic zone to b e determined. Sinking rates were found to b e influenced by the duration of the settling experiments. Chlorophyll a sinking rates ranged from 0 to 0.91 m d-' but were poorly correlated to phytoplankton carbon sinking rates which ranged from 0 to 0.78 m d-' Sinking rates of phytoplankton populations were significantly correlated not to any of the environmental parameters measured but to taxonomic properties of the assemblages, which were in turn governed by prevailing environmental conditions. Phytoplankton carbon flux estimates, made from both the SETCOL measurements and the sediment trap recovenes, were in general agreement and did not exceed 6.1 % of primary production. Losses from the euphohc zone during the study period resulting from the sinking of intact phytoplankton were therefore of limited importance to the time-dependent changes of phytoplankton biomass. Phytoplankton sinking was, however, considered to have an impact on phytoplankton composition due to the variable sinking rates of the different assemblage components.