Succession of phytoplankton assemblages in relation to estuarine hydrodynamics in the Ria de Vigo: A box model approach

Two phytoplankton succession events (September-October 1993 and March 1994) in the Ria de Vigo were studied in relation to their hydrodynamic context. By means of a circulation box model we calculated horizontal and vertical residual fluxes as well as budgets of phytoplankton associated with them. T his allowed us to assess the importance of the two factors involved in the selection processes: one hydrodynamic (dependent upon physical dispersion) and one biogeochemical (involving all other processes such as growth, morta lity and grazing). The first period was characterised by a downwelling even t which produced a reversal of the estuarine circulation pattern (horizonta l velocity = 4 km d(-1) and downward velocities up to 9 m d(-1)). This prov ided suitable conditions for the blooming of Gymnodinium catenatum favoured by its capacity to swim, while the diatom community almost disappeared. Th e second period corresponded to an upwelling --> relaxation sequence. Durin g upwelling (horizontal velocity = 6 km d(-1), upward velocities up to 12 i n d(-1)), Thalassiosira spp. was dominant. It is suggested that the less en ergetic conditions associated with the relaxation event (horizontal velocit y = 1 km d(-1); upward velocities < 1 m d(-1)) favoured the selection of Ch atoceros spp. due to its lower sinking rate.