Nitrogen nutrition in assemblages dominated by Pseudo-nitzschia spp., Alexandrium catenella and Dinophysis acuminata off the west coast of South Africa

A study was carried out in the southern Benguela upwelling system in 2006 and 2007 toestablish how the nutrient regime determines community succession and the selection of harmfulalgal bloom species. In March 2006, Pseudo-nitzschia spp. reached concentrations of 13 × 106 cells l–1,representing 80% of the total estimated phytoplankton biomass, while chlorophyll a (chl a) reached57 µg l–1. High rates of NO3– uptake (?NO3–, maximum 0.56 µmol N l–1 h–1) led to NO3– depletion(<0.1 µM). However, cell numbers remained high for several days, sustained by regenerated nitrogen,with ƒ-ratios dropping from 0.79 to 0.12. In March 2007, a bloom of Alexandrium catenella (4.5 ×105 cells l–1, 77% of the phytoplankton biomass) occurred, with surface chl a concentrations of 26 µgl–1. NO3– concentrations were high (10 to 17 µM), sustaining high ?NO3– (maximum 0.61 µmol N l–1 h–1)and ƒ-ratios up to 0.87. In April, Dinophysis acuminata reached 3.1 × 104 cells l–1 (91% phytoplanktonbiomass), with NO3– concentrations <0.5 µM and ƒ-ratios <0.1, indicative of regenerated production.Nutrient uptake kinetics showed that Pseudo-nitzschia spp. displayed the highest maximumspecific uptake rates (15.0 × 10–3 and 18.0 × 10–3 h–1 for NO3– and NH4+, respectively). D. acuminatadisplayed the highest affinity for NH4+, as shown by its higher ? (slope of the nutrient uptake vs. concentrationcurve) of 20.7 × 10–3, compared to 13.4 × 10–3 and 5.9 × 10–3 h–1 (µmol N l–1)–1 for Pseudonitzschiaspp. and A. catenella, respectively. D. acuminata was an affinity strategist, successful innitrogen-depleted waters, whereas both Pseudo-nitzschia spp. and A. catenella were velocity strategists,better adapted to utilising high NO3– concentrations during upwelling pulses