DYNAMICS OF BACTERIOPLANKTON, PHYTOPLANKTON AND MESOZOOPLANKTON COMMUNITIES DURING THE DEVELOPMENT OF AN UPWELLING PLUME IN THE SOUTHERN BENGUELA

Temporal changes in the biomass relationships and community structure of the planktonic food web during the development of a plume of upwell ed water in the southern Benguela were investigated during 2 consecuti ve drogue studies. Three distinct water masses of increasing age were encountered. Primary production was highest (1 g C M-2 h-1) at the sta rt of the study and decreased as the plume moved offshore. The plankto nic community was initially characterised by a high biomass of bacteri a (40 to 60 mg C m-3), a diatom-dominated phytoplankton community (> 5 mg chl m-3), and a mesozooplankton community (30 to 86 mg C m-3) domi nated 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-1, 2 to 5 mg Chl M-3), and the phytoplankton community was dominat ed numerically by nanoplanktonic flagellates (2 to 20 gm). Bacterial b iomass estimates (190 Mg C M-3) were the highest recorded thus far in situ in the southern Benguela. Further offshore, primary production ra tes were < 0.2 g C m-2 h-1 and the biomass of phytoplankton and bacter ia decreased to < 2 mg chl M-3 and ca 20 mg bacterial C m-3. C carinat us dominated the copepod biomass throughout the study period, and show ed a general inverse relationship with phytoplankton biomass. Grazing impact may have contributed significantly to the decline of the bloom, with copepods ingesting 5 to 10 % of phytoplankton biomass in maturin g upwelled water, and up to 38 % towards the end of the bloom. Results suggest that herbivorous copepods become food-limited during the quie scent phase of the upwelling cycle or when the phytoplankton community is dominated by small nano- and picoplanktonic cells. Microheterotrop hic pathways appear to be an important component of the pelagic food w eb in the southern Benguela. The relative dominance of the classical d iatom-mesozooplankton food chains versus longer microbial food webs ma y have important implications for the population dynamics of pelagic f ish.