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

During an intense (up to 33 X 10(6) cells L(-1)) Alexandrium minutum bloom in the Penze estuary (France), total NO(3), NH(4), and PO(4) requirements of the bloom were, respectively, 184, 25, and 20 mumol L(-1), with peak uptake rates of 43, 6, and 4.8 mumol 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, were far in excess, indicating that NO(3) 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 mumol L(-1) would be necessary to contain the bloom in the Penze 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.