The onset, development and decay of a winter bloom of the marine diatom Skeletonema costatum was monitored during a 10 d period in the coastal upwelling system of the Ría de Vigo (NW Spain). The succession of upwelling, relaxation and downwelling-favorable coastal winds with a frequency of 10–20 d is a common feature of the NW Iberian shelf. The onset of the bloom occurred during an upwelling-favorable ½ wk period under winter thermal inversion conditions. The subsequent ½ wk coastal wind relaxation period allowed development of the bloom (gross primary production reached 8g C m−2 d−1) utilizing nutrients upwelled during the previous period. Finally, downwelling during the following ½ wk period forced the decay of the bloom through a combination of cell sinking and downward advection.