DIEL MIGRATION OF PHYTOPLANKTON AND SPECTRAL LIGHT-FIELD IN THE RIA DE VIGO (NW SPAIN)

Diel migration of Mesodinium rubrum, Eutreptiella sp., Scrippsiella tr ochoidea, Dinophysis acuminata and Ceratium furca throughout a 24 h cy cle is described for a stable, well-stratified estuary (Ria de Vigo, N W Spain). Daily changes in light quantity and in spectral light ratios i.e. red:far-red, blue:red, green:red and blue:green have been analys ed. The spectral light ratios changed at twilight and around noon at v arious depths. Some of the downward migrations were well predicted by Stokes' law, while other migrations were faster and deeper than calcul ated. The coincidence of these movements with abrupt changes in red:fa r-red, green:red and blue:green light ratios is discussed. Some specie s are able to migrate through the pycnocline, whereas others do not se em to be able to do so. Several species are present in maximum numbers at depth at night, while others display upward migration independent of light, suggesting the existence of endogenous rhythms. Upward migra tion at dusk began with dispersal of populations, with renewed aggrega tion at the sea surface coincident with an increase in the red:far-red ratio at 6 m and the green:red ratio at 6 and 10 m. Based on direct e vidence for the control of flagellar mobility by light quality reporte d by other authors from laboratory studies, it is suggested that, toge ther with other cues, spectral light ratios of different light qualiti es modulate vertical phytoplanktonic migration.