In the recent past it has been suggested that downwelling is the oceanographic process responsible for the initiation of harmful dinoflagellate blooms in the Rias Baixas, Northwest Spain. The studies that supported this hypothesis came from short-time observations during red tide events in the region. It appears necessary to confirm or reject the hypothesis using a more intensive sampling frequency in the area. We present here the results obtained from a twice weekly sampling conducted in a station located in the middle of Ria de Vigo over an annual cycle. The Ria de Vigo is located in an area subject to seasonal upwelling and it has been found that the seasonal evolution of total microplankton abundance follows that of diatom abundance except during strong downwelling in autumn. During downwelling diatoms disappeared from the water column and motile forms such flagellates and large dinoflagellates almost exclusively dominated the microplankton population. The results therefore confirm the previous hypothesis that downwelling events are necessary to remove diatoms from the surface layer, which creates a niche for motile forms, which can grow if environmental conditions (mainly nutrients and light) are favourable. It may be concluded that downwelling is the general mechanism that promotes dinoflagellate red tides in the region as opposite to the dualism upwelling-diatom blooms. A similar mechanism could operate in other semi-enclosed basins under the influence of upwelling-downwelling cycles.