MEPS 379:91-107 (2009)  -  DOI: https://doi.org/10.3354/meps07898

ABSTRACT: A study was carried out in the southern Benguela upwelling system in 2006 and 2007 to establish how the nutrient regime determines community succession and the selection of harmful algal 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) reached 57 µg l^–1. High rates of NO₃^– uptake (ρ_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 µg l^–1. NO₃^– concentrations were high (10 to 17 µM), sustaining high ρ_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% phytoplankton biomass), 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 maximum specific uptake rates (15.0 × 10^–3 and 18.0 × 10^–3 h^–1 for NO₃^– and NH₄⁺, respectively). D. acuminata displayed the highest affinity for NH₄⁺, as shown by its higher α (slope of the nutrient uptake vs. concentration curve) of 20.7 × 10^–3, compared to 13.4 × 10^–3 and 5.9 × 10^–3 h^–1 (µmol N l^–1)^–1 for Pseudo-nitzschia spp. and A. catenella, respectively. D. acuminata was an affinity strategist, successful in nitrogen-depleted waters, whereas both Pseudo-nitzschia spp. and A. catenella were velocity strategists, better adapted to utilising high NO₃^– concentrations during upwelling pulses.

KEY WORDS: Benguela upwelling system · Harmful algal blooms · Nutrient uptake · Nutrient kinetics   ·   Dinoflagellates   ·   Diatoms