Temporal changes in the biomass relationships and community structure of the planktonic food web during the development of a plume of upwelled water in the southern Benguela were investigated during 2 consecutive drogue studles. Three distinct water masses of increasing age were encountered. Primary production was highest (1 g C m-' h-') at the start of the study and decreased as the plume moved offshore. The planktonic community was initially characterised by a high biomass of bacteria (40 to 60 mg C m-"), a diatom-dominated phytoplankton community ( > 5 mg chl m-3), and a mesozooplankton community (30 to 86 mg C dominated by the copepod Calanoides carinatus. At the start of the second drogue, phytoplankton production and biomass were lower (ca 0.5 g C m-2 h-', 2 to 5 mg chl m-3), and the phytoplankton community was dominated numerically by nanoplanktonic flagellates (2 to 20 pm). Bacterial biornass estimates (190 mg C m-3) were the highest recorded thus far in sltu in the southern Benguela. Further offshore, primary production rates were < 0.2 g C m-'h-' and the biomass of phytoplankton and bacteria decreased to < 2 mg chl and ca 20 mg bacterial C m-3 C. carinatus dominated the copepod biomass throughout the study period, and showed a general inverse relationship with phytoplankton biomass. Grazing impact may have contributed significantly to the decline of the bloom, with copepods ingesting S to 10 % of phytoplankton biomass in maturing upwelled water, and up to 38 % towards the end of the bloom. Results suggest that herbivorous copepods become food-limited during the quiescent phase of the upwelling cycle or when the phytoplankton community is dominated by small nano-and picoplanktonic cells. Microheterotrophic pathways appear to be an important component of the pelagic food web in the southern Benguela. The relative dominance of the classical diatom-mesozooplankton food chains versus longer microbial food webs may have important implications for the population dynamics of pelagic fish.