ABSTRACT: To assess the consequences of very-high-nitrate freshwater inputs on phytoplankton community structure a complete 1993-1994 annual cycle is described for the Bay of Brest (France). In contrast to other nitrate-enriched coastal ecosystems where small andnon-siliceous species dominate in summer, in this well-mixed macrotidal ecosystem diatoms >10 µm dominated bloom successions in surface waters from March to September 1993. Following a small bloom of Skeletonemacostatum in March, anintensive spring bloom (biogenic silica, BSi = 1.8 to 2 µmol Si l^-1) observed in April-May (first dominated by Thalassiosira spp. and then by Rhizosolenia spp. by late May) collapsed due to Si limitation. Although dinoflagellatesdeveloped during the decline of this spring bloom and remained abundant until late September, a new diatom-dominated population (Chaetoceros spp.) developed from the end of May until late September (BSi decreased from about 1 to 0.5 µmol Sil^-1). Finally, at the end of the productive period (September), a bloom dominated by the 10 µm size class, essentially due to cryptophyceans, occurred. On an annual basis, the >10 µm size class accounted for 73 to 74? of the time-weightedaverages of chlorophyll a, of primary production, and of biogenic silica in surface waters. Finally, there is no evidence that the phytoplankton community structure of the Bay of Brest was dramatically affected by nitrate enrichment, either interms of size structure or of diatom abundance relative to dinoflagellates (although the emergence of some undesirable species among the dinoflagellates was observed). A silicic acid (i.e. silicate) pump, specific to well-mixed coastal ecosystems, isinferred to explain the dominance of diatoms during the periods following the spring bloom. It provides a mechanism that prevents silicon loss out of the system by trapping it at the shallow water-sediment interface at the end of the spring bloom. Thetidally induced vertical mixing allows silicic acid from biogenic silica dissolution to become readily available for new diatom developments in surface waters.