Mesoscale heterogeneities of the phytoplankton distribution in St Helena Bay, South Africa, following an upwelling event

Abstract

The effect of the variability of the hydrographic field following an upwelling event on the phytoplankton composition and distribution within the St Helena Bay region is investigated. Phytoplankton samples transecting newly upwelled, maturing and aged upwelled water were examined. Both cell concentrations and species diversity were found to increase with the age of the upwelled water. Several species exhibited intense concentration gradients, with stations of similar hydrological conditions having similar populations. All stations were numerically dominated by the microflagellates. In terms of carbon, different components of the phytoplankton were found to be associated with different stages of the upwelling circulation. The variation and peaks in the biomass were primarily attributable to the diatom fraction of the total carbon, with the microflagellates and dinoflagellates maintaining a relatively stable background level. Phytoplankton carbon: chlorophyll a ratios exhibited a coherent spatial pattern. Chlorophyll a values from different locations and depths were therefore not a comparable index of biomass. Changes in the species composition between newly upwelled and aged upwelled water are thought to have been determined to a large degree by the varying histories of these water masses.