Abstract
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. This 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, mortality and grazing). The first period was characterised by a downwelling event which produced a reversal of the estuarine circulation pattern (horizontal velocity = 4 km d-1 and downward velocities up to 9 m d-1). This provided suitable conditions for the blooming of Gymnodinium catenatum favoured by its capacity to swim, while the diatom community almost disappeared. The second period corresponded to an upwelling AE relaxation sequence. During upwelling (horizontal velocity = 6 km d-1; upward velocities up to 12 m d-1), Thalassiosira spp. was dominant. It is suggested that the less energetic conditions associated with the relaxation event (horizontal velocity=1 km d-1; upward velocities < 1 m d-1) favoured the selection of Chatoceros spp. due to its lower sinking rate.
Highlights
The hydrodynamic regime has a direct effect on the distribution of populations of phytoplankton since it implies a biomass advection
FIG. 3. – Ekman transport components, wind speed and direction calculated at 43°N 11°W and continental runoff in the Ría de Vigo
The associated alongshore transport first caused the slow-down of the positive residual circulation pattern and favoured its reversal and the introduction of relatively warmer, nutrient-depleted coastal water into the Ría
Summary
The hydrodynamic regime has a direct effect on the distribution of populations of phytoplankton since it implies a biomass advection (the so-called ‘physical dispersion’ in Painchaud et al, 1996). In addition to the hydrodynamic factor, the distribution is determined by many other processes, which we will hereafter call “biogeochemical”. These processes include growth, mortality and grazing, passive sedimentation and swimming, which are highly depen*Received October 4, 1999. Vigo (NW coast of Spain, Fig. 1) constitutes a good example of this connection since its high biological productivity is remarkably affected by estuarine hydrodynamics (Prego, 1993; Nogueira et al, 1997; Tilstone et al, 1999). According to Dyer (1973) and
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