Red tides caused by the toxic dinoflagellate Alexandrium minutum Halim were observed in the Penzé River estuary (Morlaix Bay, NW France) in the early summers of 1996 and 1997. During these events, maximum motile cell concentrations reached 18.106 and 44.106 cell.l-1 respectively, corresponding to 56% and 97% of the total phytoplankton population, also constituted with diatoms (particularly Nitzschia species) and other dinoflagellates (Heterocapsa triquetra and Scrippsiella spp.).

Results concerning the environmental conditions including hydrology, dissolved oxygen and nutrient concentrations obtained in 1997 on four transects of 7 stations along the salinity gradient in the estuary will be presented; the environmental changes prior, during and after the bloom will be detailed. During Alexandrium minutum proliferations nitrogen uptake (ammonium and nitrate) was measured using 15N tracer techniques. Nitrate and ammonium uptake reached respectively 292 and 95 nmol.l-1h-1 and nitrate constituted the main source of nitrogen, representing up to 75% of the nitrogen uptake.