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 (<30 mum) and small gymnodinid forms (<30 mum) was observed. In the nutrient-depleted upper layer, several autotrophic and large heterotrophic dinoflagellates dominated. It is suggested that the ratio between the velocity of upward water movement and the depth of the stratified upper layer (flushing rate, day-1) is the critical parameter which triggers active phytoplankton growth. It can be concluded that upward water velocities of approximately 2.5 m day-1 and a stratified upper layer of 10 m depth (flushing rate 0.25 day-1) are the main physical constraints for red tide development.