SIMULATION AND FIELD-MEASUREMENTS OF PHYTOPLANKTON-BACTERIA-ZOOPLANKTON INTERACTIONS IN THE SOUTHERN BENGUELA UPWELLING REGION

A general size-based model is used here to predict the complex tempora l successions observed in planktonic communities after an upwelling ev ent in the southern Benguela, and to resolve a number of fundamental q uestions regarding the trophic dynamics of the pelagic food web. The m odel was not based on field data, and simulation results do not exactl y mimic field and laboratory results. Nonetheless, the simulation mode l makes a major contribution towards our understanding of the dynamics of the planktonic food web after upwelling. The model predicts rapid growth of a phytoplankton community dominated by nanophytoplankton-siz ed cells and a later netphytoplankton bloom. After nitrate depletion t he bloom is dominated by nanophytoplankton dependent upon regenerated nitrogen. Analysis of C and N flows showed that respiration and grazin g were largely responsible for the decline of the phytoplankton bloom, accounting for 47 and 44 % respectively of the total C fixed by phyto plankton over the 20 d period. Mesozooplankton grazed 62 % of the decl ining bloom (Days 10 to 20), but only 18 % of the total C fixed. This was due to the dominance of nanophytoplankton primary producers, which were unavailable to the larger mesozooplankton, but were consumed by microzooplankton. The microbial food web played dn important role in N cycling and in the production of mesozooplankton throughout the simul ation period. To determine the relative importance of the classical di atom-dominated food chain versus the microbial food web during one upw elling event, network analysis was used to assess C and N flows in the model foodweb. The total dependency coefficients showed that mesozoop lankton depend mainly on netphytoplankton (76 %) for C during the firs t 10 d of the phytoplankton bloom, but depend equally on netphytoplank ton (70 %) and microzooplankton (69 %) for N. During the last 5 d of t he bloom, the biomass of both maj or prey items is low, and mesozoopla nkton depend equally (21 to 22 %) on netphytoplankton and microzooplan kton for both C and N. Food chains are longer and trophic efficiency d ecreases. The frequency of upwelling in the southern Benguela may thus be an important factor determining the relative dominance of short di atom-based food chains versus the microbial food web, and therefore th e annual yield of pelagic fish.