In situ variations of the xanthophylls diatoxanthin and diadinoxanthin: photoadaptation and relationships with a hydrodynamical system in the eastern English Channel ABSTRACT Photosynthetic pigments from phytoplankton were measured by high performance liquid chron~atography (HPLC) along inshore-offshore transects and vert~cal profiles in the southeastern Enghsh Channel An inverse relatlonshp between the (Dd+Dt)/chl a and Dt/(Dd+Dt) ratios (Dt = diatoxanthin, Dd = dladinoxanthin, chl a = chlorophyll a) and light penetration was found, indicating a photoadaptative response of phytoplankton to h g h hght irradiances High ratios are present in less turbid, clearer offshore waters in companson to coastal waters influenced by river inputs T h s response can be caused both by seasonal changes in mean irradiances or by a f e r e n t amounts of turbidity, which affect light penetration The lack of variation along the vertical proflles with respect to the surface is interpreted as an indicator of a recent vertical transport of phytoplankton by the highenergy tidal currents present in the study area In addition, a diel cycle was noted, w t h a decrease in the concentrations of diatoxanthin at nlght The use of this plgment a s a marker of water masses must be cautious, since pigment concentration and composition are dependent on the physiological state of the cell, on the environmental conditions and dynamics, and on seasonal and diel cycles

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