An intensive hydrographic sampling (twice a week, 7 depth levels) was conducted in the middle Ría de Arousa (water depth 55 m) during the upwelling- favourable season (May- October) of 1989. This high sampling frequency allows to study the short- time- scale response of microplankton species to recurrent wind- driven upwelling events. Cell numbers above the standard baseline concentrations in the study area of the red forming species Gyrodinium cf. aureolum and Heterosigma akashiwo (eye catching red tide); the ichthyotoxic species Dictyocha fibula and D. speculum (>104 cells l- 1); the ASP species Pseudo- nitzschia spp. (>106 cells l- 1), the PSP species Gymnodinium catenatum (>103 cells l- 1) and the DSP species Dinophysis acuta and D. acuminata (>103 cells l- 1) were episodically recorded during the 6 months period. The contrasting causes of such accumulations ² net in situ‚ growth versus import from the adjacent shelf are quantitatively assessed by solving the classical 2- D non- stationary equation: where N is either cell or biovolume concentration, VX and VZ are the convective horizontal and vertical velocity fields, KZ is the coefficient of vertical turbulent diffusion and NEP is the net cell growth rate (production minus all losses). Cell concentration numbers have been obtained from counts on sedimentation chambers with the light microscope technique. Species biovolumes were estimated from direct size measurements and approximation to geometric forms. The values of VX, VZ and KZ have been estimated with a 2- D non- stationary box- model.