Productivity of dinoflagellate blooms on the west coast of South Africa, as measured by natural fluorescence

  • BA Mitchell-Innes
  • GC Pitcher
  • TA Probyn

Abstract

The biomass and productivity of phytoplankton populations inshore on the west coast of South Africa were investigated towards the end of the upwelling season, a period when high-biomass dinoflagellate blooms are
common. Productivity was estimated from natural fluorescence measurements (PNF), using photosynthesis (P) v. irradiance (E) relationships (PE) and by means of the in situ 14C-method (PC). A linear regression of PNF productivity against PC and PE productivities yielded a slope of 0.911 and an r2 of 0.83 (n = 41). Physical and biological variability was high inshore, reflecting alternating periods of upwelling and quiescence. Mean chlorophyll inshore (within a 12 m water column) ranged from 0.7 to 57.8 (mean = 8.9) mg.m-3, mean PNF productivity ranged from 8.4 to 51.0 (mean = 24.6) mgC.m-3.h-1 and daily integral PNF productivity from 0.8 to 4.8 (mean = 2.3) gC.m-2.day-1. Transects sampled during active and relaxation phases of upwelling had different chlorophyll distributions. High chlorophyll concentrations (sometimes >50 mg.m-3) were associated with surface blooms within the region of the upwelling front. Estimates of daily water-column PNF productivity within these frontal blooms ranged from 4.0 to 5.6 gC.m-2.day-1. With relaxation of wind stress, blooms dominated by dinoflagellates flooded shorewards and often formed red tides. Chlorophyll concentrations of
>175 mg.m-3 and productivity rates > 500 mgC.m-3.h-1 and 12 gC.m-2.day-1 were measured during a particularly intense red tide. Offshore, the water column was highly stratified with a well-defined subsurface chlorophyll maximum layer within the pycnocline region. Estimates of daily water-column PNF productivity ranged from
2.4 to 4.0 gC.m-2.day-1 offshore. The high productivity of shelf waters on the West Coast in late summer can be ascribed largely to dinoflagellate populations and their success in both upwelling systems and stratified conditions.
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eISSN: 1814-2338
print ISSN: 1814-232X