Ml. Villarino et al., EVIDENCE OF IN-SITU DIEL VERTICAL MIGRATION OF A RED-TIDE MICROPLANKTON SPECIES IN RIA DE VIGO (NW SPAIN), Marine Biology, 123(3), 1995, pp. 607-617
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.