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

During an intense (up to 33 X 10 6 cells L −1 ) Alexandrium minutum bloom in the Penzé estuary (France), total NO 3 , NH 4 , and PO 4 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 NH 4 and PO 4 were far short of this peak demand, whereas those of NO 3 were far in excess, indicating that PO 4 supply is important for sustaining the bloom. Comparison of the measured NO 3 uptake rates with advective fluxes indicates that a reduction of NO 3 concentrations in river waters to <200 µmol L −1 would be necessary to contain the bloom in the Penzé estuary. The role of NO 3 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.