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

Abstract

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.