Résumé
The spatial and temporal variability in phytoplankton biomass in the Benguela ecosystem was investigated over an 8-year period using a chlorophyll index (Chi Index) derived from low resolution SeaWiFS ocean colour data. Monthly composite images revealed that the surface chlorophyll a concentration was high near the coast and generally decreased with distance offshore, allowing observation of the spatial extent of phytoplankton distribution in relation to coastal upwelling. The Chi Index was estimated as the integration of the chlorophyll a concentration from the coast to an offshore minimum level of 1mg m(-3). The index captured both the cross-shelf structure and the major part of the chlorophyllrich areas and was used to examine the average seasonality and annual variability in phytoplankton biomass. The data indicated that the Benguela ecosystem may be divided into two major sectors, north and south, with the divide occurring at the Luderitz upwelling cell where the Chi Index, the chlorophyll a concentration, and the offshore extent of phytoplankton were at a minimum. The index revealed a clearer and more contrasting seasonal pattern than simple coastal averaged values of chlorophyll. The latitudinal gradient and seasonal change in the index and wind forcing showed that the northern and southern sectors were out of phase and that the Benguela ecosystem appears to have a dipole structure. The interannual variation in biomass was highlighted and showed a high variability in the dipole, including shorter periods of synchronicity during 2001 and 2002. The simple Chi Index derived for the Benguela appears to be robust and generic enough to be applied to other upwelling systems as a reasonably good index of variability in phytoplankton biomass for comparative studies.