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

Seeyave, S., Probyn, T.A., Pitcher, G.C., Lucas, M.I. and Purdie, D.A. (2009) Nitrogen nutrition in assemblages dominated by Pseudo-nitzschia spp., Alexandrium catenella and Dinophysis acuminata off the west coast of South Africa. Marine Ecology Progress Series, 379, 91-107. (doi: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 (?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 ?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 ? (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 Pseudonitzschia
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.

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Contributors

Author: D.A. Purdie

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