The development of phytoplankton blooms in upwelled waters of the southern Benguela up welling system as determined by microcosm experiments

Abstract

Phytoplankton bloom development was examined in newly upwelled waters collected off the Cape Peninsula during active upwelling and incubated in the laboratory under conditions approximating those in the ocean. Of interest was the variability of the phytoplankton developmental processes following upwelling. Results were compared to those of drogue studies which shared the same objectives but utilized drogues to tag and follow parcels of upwelled water in order to monitor bloom development. Differences in the time required for bloom development and in the peak population size were attributed to differences in the duration of the lag period, in the size of the seed population, in grazing pressures and in the total amount of nutrient available in the source water. Bloom development was measured in terms of chlorophyll a, particle volume, phytoplankton species and phytoplankton carbon changes. Specific growth rates were initially low prior to the initiation of exponential growth during which maximum growth rates were recorded. Microflagellates were found to be capable of higher growth rates than diatoms. In terms of carbon the microflagellate fraction varied within narrow limits. The variation and peaks in the phytoplankton biomass were caused by the diatom fraction of the total carbon. Following the bloom peak, negative growth rates were attributed to cell death with the rapid autolysis of cell pigments. Changes in the nitrate, silicate and phytoplankton carbon concentration s relative to phosphate were similar to the ratios of Redfield et al. (1963). Growth of the bloom was terminated by either nitrate, silicate or phosphate limitation. Phytoplankton productivity was assessed in terms of changes in phytoplankton carbon, nitrate concentration and C 14 uptake. Based on the results of the microcosm studies simple equations were formulated for the prediction of phytoplankton growth in terms of chlorophyll a following upwelling. Predicted maximum chlorophyll a yield and duration of growth are compared to observed chlorophyll a concentrations during the drogue studies.