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 temporal successions observed in planktonic communities after an upwelling event in the southern Benguela, and to resolve a number of fundamental questions regarding the trophic dynamics of the pelagic food web. The model was not based on field data, and simulation results do not exactly mimic field and laboratory results. Nonetheless, the simulation model 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-sized cells and a later netphytoplankton bloom. After nitrate depletion the bloom is dominated by nanophytoplankton dependent upon regenerated nitrogen. Analysis of C and N flows showed that respiration and grazing were largely responsible for the decline of the phytoplankton bloom, accounting for 47 and 44 % respectively of the total C fixed by phytoplankton over the 20 d period. Mesozooplankton grazed 62 % of the declining 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 simulation period. To determine the relative importance of the classical diatom-dominated food chain versus the microbial food web during one upwelling event, network analysis was used to assess C and N flows in the model foodweb. The total dependency coefficients showed that mesozooplankton depend mainly on netphytoplankton (76 %) for C during the first 10 d of the phytoplankton bloom, but depend equally on netphytoplankton (70 %) and microzooplankton (69 %) for N. During the last 5 d of the bloom, the biomass of both maj or prey items is low, and mesozooplankton depend equally (21 to 22 %) on netphytoplankton and microzooplankton for both C and N. Food chains are longer and trophic efficiency decreases. The frequency of upwelling in the southern Benguela may thus be an important factor determining the relative dominance of short diatom-based food chains versus the microbial food web, and therefore the annual yield of pelagic fish.