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

The biomass and productivity of phytoplankton populations inshore on the we st coast of South Africa were investigated towards the end of the upwelling season, a period when high-biomass dinoflagellate blooms are common. Produ ctivity was estimated from natural fluorescence measurements (P-NP) using p hotosynthesis (P) v. irradiance (E) relationships (P-E) and by means of the in situ C-14-method (P-C). A linear regression of P-NF productivity agains t P-C and P-E productivities yielded a slope of 0.911 and an r(2) of 0.83 ( n = 41). Physical and biological variability was high inshore. reflecting a lternating periods of upwelling and quiescence. Mean chlorophyll inshore (w ithin a 12 m water column) ranged from 0.7 to 57.8 (mean = 8.9) mg m(-3), m ean P-NF productivity ranged from 8.4 to 51.0 (mean = 24.6) mgC.m(-3).h(-1) and daily integral P-NF productivity from 0.8 to 4.8 (mean = 2.3) gC.m(-2) day(-1). Transects sampled during active and relaxation phases of upwellin g had different chlorophyll distributions. High chlorophyll concentrations (sometimes > 50 mg.m(-3)) were associated with surface blooms within the re gion of the upwelling front. Estimates of daily water-column PNF productivi ty within these frontal blooms ranged from 4.0 to 5.6 gC.m(-2) day(-1). Wit h relaxation of wind stress, blooms dominated by dinoflagellates flooded sh orewards 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 wate r column was highly stratified with a well-defined subsurface chlorophyll m aximum layer within the pycnocline region. Estimates of daily water-column P-NF productivity ranged from 2.4 to 4.0 gC(.)m(-2)day(-1) offshore. The hi gh productivity of shelf waters on the West Coast in late summer can be asc ribed largely to dinoflagellate populations and their success in both upwel ling systems and stratified conditions.