Nitrogen and phosphorus requirements of an Alexandrium minutum bloom in the Penzé Estuary, France

Abstract

During an intense (up to 33 X 106 cells L−1) Alexandrium minutum bloom in the Penzé estuary (France), total NO3, NH4, and PO4 requirements of the bloom were, respectively, 184, 25, and 20 µmol L−1, with peak uptake rates of 43, 6, and 4.8 µmol L−1 d−1. The measured ambient concentrations of NH4 and PO4 were far short of this peak demand, whereas those of NO3 were far in excess, indicating that PO4 supply is important for sustaining the bloom. Comparison of the measured NO3 uptake rates with advective fluxes indicates that a reduction of NO3 concentrations in river waters to <200 µmol L−1 would be necessary to contain the bloom in the Penzé estuary. The role of NO3 was restricted to sustenance of the bloom, whereas warm conditions resulting in a water column stability seem to have triggered the bloom, and a self‐shading, probably coupled with a phosphorus limitation, caused its decline.