This study presents (1) the first annual time-series recorded for any marine system of the weekly variability in rates of production (P) and dissolution (D) of biosilica (BSiO2) and (2) the first evidence of the end of the year-round dominance of diatoms in the Bay of Brest, a well-mixed anthropogenically nitrate-enriched macrotidal ecosystem, typical of western Europe. In coastal waters, silicic acid (Si(OH)4) availability depends on both external inputs and internal recycling. We quantified the contribution of internal recycling through experiments conducted from April 2001 to April 2002 in the surface waters of SOMLIT-Brest station at the outlet of the bay. From spring to mid-summer, diatoms dominated successive microphytoplankton blooms and, since they require Si(OH)4 to construct their frustules, the concentration of Si(OH)4 in surface waters was severely reduced at this time, relative to winter (mean 1.62 µM); BSiO2 concentration and production and dissolution rates were high, averaging 1.26 µmol l-1, 0.96 µmol l-1 d-1 and 0.40 µmol l-1 d-1, respectively. From mid-summer to mid-fall, non-siliceous phytoplankters predominated, and the mean surface-water concentration of Si(OH)4 was 4.67 µM. BSiO2 concentration and production and dissolution rates were low, averaging 0.69 µmol l-1, 0.10 µmol l-1 d-1 and 0.04 µmol l-1 d-1, respectively. The summer shift from diatom to dinoflagellate dominance was likely under bottom-up control. The Bay of Brest, with D:P ratios averaging 0.43 during the productive period, is intermediate between coastal upwelling systems, where new production usually predominates (0.1 < D:P < 0.25), and oceanic oligotrophic systems, in which regenerated production usually predominates (D:P ~ 1.0).