SHORT-TIME SCALE DEVELOPMENT OF A GYMNODINIUM-CATENATUM POPULATION INTHE RIA-DE-VIGO (NW SPAIN)

Wind direction and fresh water runoff determine the circulation patter n of the Ria de Vigo (NW Spain), which in trm influence the selection and distribution of its phytoplankton populations. Coastal winds with a south-southwesterly component reverse the positive estuarine circula tion in the Ria, causing an off-shore to in-shore flow of surface wate rs and, consequently she outflow of inner waters via deeper layers. We found that this reversal imposed a selective force on the phytoplankt on population: diatoms, which could not counteract the sinking movemen t of the surface waters, were diminished, while dinoflagellates remain ed in the water column. From the end of September to the beginning of October 1993, an accumulation of Gymnodimium catenatum Graham was obse rved coinciding with an intrusion of coastal water induced by westerly winds which provoked a reversal in the circulation of the Ria. The sl ow reestablishment of the positive estuarine circulation pattern, whic h was due to a weak coastal upwelling and considerable fresh wafer run off allowed the population of G. catenatum to flourish.