ABSTRACT: Between March 1992 and April 1993, an intensive sampling program was carried out at a coastal station in the Northwestern Mediterranean Sea to study the relationship between phytoplankton distributions, as evaluated by taxonomic pigments, and the hydrographic structures of the water column. The study period covered the range of hydrographic conditions which prevail in the Mediterranean Sea. The 0 to 75 m integrated chlorophyll a concentration averaged 23.3 mg m^-2, with the highest values (above 45 mg m^-2) restricted to semi-mixed periods. The major phytoplankton signature and water column structure relationships were: (1) phytoplanktonic prokaryotes (cyanobacteria and prochlorophytes) appear sensitive to water column mixing with prochlorophytes being the most sensitive group as 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 require strong mixing. During the stratification period, 2 noticeable wind-induced mixing events occurred, and while the first did not have any marked influence on the phytoplankton community, the second was followed by a subsurface development of green flagellates and diatoms. This second wind-mixing event also altered the vertical prokaryote distribution, but 1 wk after this perturbation vertical segregation of prochlorophytes and cyanobacteria was reestablished. The results suggest that, while different phytoplankton taxa are generally adapted to specific water column structures, this is not always the case, especially at small scales where specific light/nutrient requirements may have to be met.

KEY WORDS: Cyanobacteria . Chemotaxonomy . Hydrography . HPLC . Mediterranean Sea . Oligotrophic sea . Pigments . Phytoplankton . Prochlorophytes

Published in MEPS Vol. 124 (1995) on August 10
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1995