During an intense (up to 33 x 10 sup(6) cells L sup(-1)) Alexandrium minutum bloom in the Penze estuary (France), total NO sub(3), NH sub(4), and PO sub(4) requirements of the bloom were, respectively, 184, 25, and 20 mu mol L sup(-1), with peak uptake rates of 43, 6, and 4.8 mu mol L sup(-1) d sup(-1). The measured ambient concentrations of NH sub(4) and PO sub(4) were far short of this peak demand, whereas those of NO sub(3) were far in excess, indicating that NO sub(3) supply is important for sustaining the bloom. Comparison of the measured NO3 uptake rates with advective fluxes indicates that a reduction of NO sub(3) concentrations in river waters to less than 200 Mu mol L sup(-1) would be necessary to contain the bloom in the Penze 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