SHORT-TERM VARIABILITY DURING AN ANCHOR STATION STUDY IN THE SOUTHERN BENGUELA UPWELLING SYSTEM - A SIMULATION-MODEL

A stratified one-dimensional model was constructed after the completion of an anchor station study in St Helena Bay on the west coast of South Africa. The objectives of the model construction were to synthesise the results of die separate investigations making up the whole study, to highlight critical processes and to reveal any major gaps or inconsistencies in current understanding of the ecosystem's functioning. The model included twelve state variables, and the phytoplankton and nitrogen variables were stratified into 31 lm layers from the surface to 31m depth and a single deep water layer. The other state variables were modelled as total mass in the water column. The model was driven by daily input of mean euphotic zone temperature, water column stability and input of new nitrogen by upwelling events. Photosynthesis was also driven by light but, in the absence of observed values, daily solar radiation was assumed to be constant. The iteration period was 12 hours, with separate day and night routines. Observed starting values and rates were used in the model whenever available. In general the major trends observed were simulated and the root mean square percent error varied from 0.07 for new nitrogen to 15.12 for diatoms. The model could be improved with better knowledge of die role of light limitation in diatoms and the role of grazing, particularly on autotrophic microflagellates. Patchiness and advection were found to be important factors affecting meso- and macrozooplankton abundance. The model indicated that pelagic fish were unlikely to have suffered from food limitation at the time of the study.