Dynamics of bacterioplankton, phytoplankton and mesozooplankton communities during the development of an upwelling plume in the southern Benguela

Abstract

Primary production was highest (1 g C m-2h-1) at the start of the study and decreased as the plume moved offshore. The planktonic community was initially characterised by high biomass of bacteria (40-60 mg C m-3), a diatom-dominated phytoplankton community (>5 mg chl m-3), and a mesozooplankton community (30-86 mg C m-3) dominated by the copepod Calanoides carinatus. At the start of the second drogue, phytoplankton production and biomass were lower (0.5 g C m-2h-1, 2-5 mg chl m-3), and the phytoplankton community was dominated numerically by nanoplanktonic flagellates (2-20 μm). Bacterial bioamss estimates (190 mg C m-3) were the highest recorded thus far in situ in southern Benguela. Further offshore, primary production rates were <0.2 g C m-2h-1 and the biomass of phytoplankton and bacteria decreased to <2 mg chl m-3 and 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. 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. -from Authors