Between March 1992 and April 1993, a n intensive sampllng p r q g a m was carried out at a coastal station in the Northwestern Mediterranean Sea to study the relations9:p between phytoplankton distributions, as evaluated by taxonomic pigments, and the hydrographx structures of the water column. The study period covered the range of hydrographlc conditions whlct prevail in the Mediterranean Sea. The 0 to 75 m integrated chlorophyll a concent]-ation averaged 23 3 m g m 2 , with the highest values

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... (above 45 mg m-') restricted to semi-mixed penods. The major ph floplankton signature and water column structure r e l a t~o n s h~p s were: (1) phytoplankton~c prokaryotes (ryanobacteria and prochlorophytes] appear sensitive to water column mixing with prochlorophylc~ being the most sensitive group a s strong stratification is associated with the highest biomass found mainly in deeper waters; (2) prymnesiophytes and chrysophytes (19'-BF and 19'-HF) appear the most abundant under a variety of conditions and therefore seem able to adapt to various water column structures; (3) diatoms bloom in semi-mixed conditions, but while these conditions are necessary, they are not sufficient for bloom formation; and (4) green chlorophyll b-containing flagellates appear to requlre sI.jsng mixing. During the stratification period, 2 noticeable wlnd-induced mixing events occurred, arc while the first d~d not have any marked influence on the phytoplankton community, the second was i-~llowed by a subsurface development of green flagellates and diatoms. This second wind-mixlng e v e 3 #also altered the vertlcal prokaryote distribution, but 1 wk after this perturbation vert~cal segregatitm of prochlorophytes and cyanobacteria was reestablished. The results suggest that, while different ~h~r~l p l a n k t o n taxa are generally adapted to speciflc water column structures, this is not always the case l r p e c~a l l y at small scales where specific lighthutrient requirements may have to be met.