SHORT‐TIME SCALE DEVELOPMENT OF A GYMNODINIUM CATENATUM POPULATION IN THE RIA DE VIGO (NW SPAIN)¹

Abstract

Wind direction and fresh water runoff determine the circulation pattern of the Ría de Vigo (NW Spain), which in turn influence the selection and distribution of its phytoplankton populations. Coastal winds with a south–southwesterly component reverse the positive estuarine circulation in the Ría, causing an off‐shore to in‐shore flow of surface waters and, consequently, the outflow of inner waters via deeper layers. We found that this reversal imposed a selective force on the phytoplankton population: diatoms, which could not counteract the sinking movement of the surface waters, were diminished, while dinoflagellates remained in the water column. From the end of September to the beginning of October 1993, an accumulation of Gymnodimium catenatum Graham was observed coinciding with an intrusion of coastal water induced by westerly winds which provoked a reversal in the circulation of the Ría. The slow reestablishment of the positive estuarine circulation pattern, which was due to a weak coastal upwelling and considerable fresh water runoff, allowed the population of G. catenatum to flourish.

Role of across‐shelf currents in the dynamics of harmful dinoflagellate blooms in the northwestern Iberian upwelling

“…This resulted in large differences (.30 times) in Chl a concentration between the Ría de Vigo and shelf. High Chl a concentrations in the interior of the Ría de Vigo that coincide with upwelling relaxation and downwelling have been previously reported Fermín et al 1996;Tilstone et al 2000) and attributed to the reduced positive estuarine circulation under upwelling relaxation and the reversal circulation imposed by downwelling. Upwelling relaxation blocks the export of phytoplankton to the shelf (Tilstone et al 2000), while transport of phytoplankton from the shelf to the interior of the Ría de Vigo can occur under downwelling conditions Fermín et al 1996).…”

“…9A, B). Diatoms, which were also necessarily accumulated, probably were not able to counteract the intense downward velocities generated by downwelling in the Ría, and, as suggested by other observations Fermín et al 1996;Crespo et al 2006), they would be rapidly removed from the water column before dinoflagellates accumulated. Dinoflagellates, owing to their vertical swimming capability, can in part neutralize these downward velocities (Figueiras et al 1995), but under strong downwelling, they can also be removed from the water column , as apparently was the case on this occasion (Fig.…”

Diatom dynamics in a coastal ecosystem affected by upwelling: coupling between species succession, circulation and biogeochemical processes

“…pelagica during spring (Figs. 3 & 7, Table 2) is the same as the sequence in autumn that selects red tide species such as Gymnodinium catenatum (Tilstone et al 1994, Fermín et al 1996. When the box model was used to study nutrient dynamics, the results highlighted that SiO 4 H 4 could be the limiting nutrient for diatoms, since HPO 4 2 -was almost in balance and NO 3 -was in excess during upwelling (Figs.…”

Physical–biological coupling in the coastal upwelling system of the Ría de Vigo (NW Spain). II: In vitro approach

“…By contrast, Cryptophyceae sp., marked by the surface alloxantin maximum during the autumn downwelling, sank to the bottom during the autumn relaxation and was reintroduced in the ría by the bottom flow during the autumn upwelling. The ability of dinoflagellates for vertical migration (Figueiras et al 1994, Fermín et al 1996 could keep them at mid-water depths during the autumn relaxation, unlike cryptophytes, that sank to the bottom. Olli et al (2001) over the shelf and 50 to 180 mg C m -2 d -1 in the adjacent ocean.…”