RED TIDE ASSEMBLAGE FORMATION IN AN ESTUARINE UPWELLING ECOSYSTEM - RIA-DE-VIGO

Red tides are conspicuous in the upwelling system of Galicia (NW Iberian Peninsula). At present, there are conflicting hypotheses about the generation site of these phytoplankton assemblages. It is interesting to know whether the rias can be sites of red tide formation or if they act only as accumulation sites of populations advected from shelf waters. A study in the Ria de Vigo, carried out during late September 1990, showed the development of a red tide assemblage. composed of Alexandrium affinis, Ceratium fusus and Gymnodinium catenatum, during a 2 week upwelling-downwelling cycle. Growth occurred at the bottom of the thermocline-top of the nutricline. Above this assemblage, a diatom assemblage (large diatoms) was blooming. Prior to the formation of the red tide, a subsurface chlorophyll maximum made up of small diatoms (Nitzschia f. seriata, Chaetoceros socialis), small flagellates (