Fg. Figueiras et al., RED TIDE ASSEMBLAGE FORMATION IN AN ESTUARINE UPWELLING ECOSYSTEM - RIA-DE-VIGO, Journal of plankton research, 16(7), 1994, pp. 857-878
Red tides are conspicuous in the upwelling system of Galicia (NW Iberi
an 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 wat
ers. A study in the Ria de Vigo, carried out during late September 199
0, showed the development of a red tide assemblage. composed of Alexan
drium affinis, Ceratium fusus and Gymnodinium catenatum, during a 2 we
ek upwelling-downwelling cycle. Growth occurred at the bottom of the t
hermocline-top of the nutricline. Above this assemblage, a diatom asse
mblage (large diatoms) was blooming. Prior to the formation of the red
tide, a subsurface chlorophyll maximum made up of small diatoms (Nitz
schia f. seriata, Chaetoceros socialis), small flagellates (<30 mum) a
nd small gymnodinid forms (<30 mum) was observed. In the nutrient-depl
eted upper layer, several autotrophic and large heterotrophic dinoflag
ellates 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 velocitie
s of approximately 2.5 m day-1 and a stratified upper layer of 10 m de
pth (flushing rate 0.25 day-1) are the main physical constraints for r
ed tide development.