EVIDENCE OF IN-SITU DIEL VERTICAL MIGRATION OF A RED-TIDE MICROPLANKTON SPECIES IN RIA DE VIGO (NW SPAIN)

The migration capacity of red-tide species in the natural environment was studied at a station in the Ria de Vi,oo (Rias Bajas, NW Spain) ov er a 24 h period in September 1991. The Ria de Vigo, where red tides a re frequent, normally shows a positive estuarine circulation and is su bjected to seasonal upwelling, and downwelling phenomena. A marked die l pattern was observed for five species that are capable of causing re d tides (Ceratium furca, Scrippsiella trochoidea, Dinophysis acuminata , Mesodinium rubrum, and Eutreptiella sp.). Such diel behaviour could be clearly advantageous in a stratified environment where light and nu trients are often in two separate layers. Active movement enables spec ies such as dinoflagellates and some ciliates to exploit high levels o f irradiance at the surface during the day and to take up nutrients in deeper layers at night. Patchy distribution of phytoplankton, shear-i nduced horizontal dispersion and density variations were considered, b ut none of them accounted for the vertical changes observed. Vertical migration is thought to be one of the mechanisms that could promote bl ooms in nutrient-depleted surface layers.