The significance of silicon in the diatom bloom dynamics of eutrophicated Belgian coastal waters (Southern Bight of the North Sea) was assessed by investigating diatom species succession and silicification level (Si:C) in relation with environmental control during an annual cycle from early February to mid-December 1995. The Si:C of natural diatom communities was determined by 2 methods. The measurement of biogenic silica after alkaline digestion of particulate material and the estimate of diatom carbon biomass derived from microscope observations were performed on weekly samples from Stn 330 in Belgian coastal waters. Alternatively, parallel 24 h kinetics experiments of 32Si uptake and 14C incorporation into proteins, as an index of phytoplankton biomass production, into diatom-dominated natural communities were conducted in simulated natural conditions. The seasonal pattern of diatom growth was characterised by 3 diatom assemblages dominated by respectively small colonial species, Chaetoceros spp.-Schroederella sp. and Rhizosolenia spp. The colonial haptophyte Phaeocystis bloomed for 2 mo, co-occurring with the 2 latter diatom communities. The 3 diatom assemblages were characterised by distinct Si:C with values varying from 0.80 to 0.05 during the growing season. The Si:C of the blooming assemblages are discussed with respect to inter- and intra-specific variations based on changes in ambient controlling factors (nutrient, light and temperature) and compared with information available from the literature for individual diatom species. The results suggest that silicic acid availability controls the Si:C of the main diatom assemblages. This is supported by the positive relationship between the diatom Si:C and silicic acid concentrations. We conclude that silicic acid availability is an important factor in the selection of diatom species, shaping the diatom succession in temperate coastal waters.