PIGMENT SIGNATURES OF THE PHYTOPLANKTON COMPOSITION IN THE NORTHEASTERN ATLANTIC DURING THE 1990 SPRING BLOOM

Pigment signatures were used to track the development and composition of a phytoplankton bloom in the northeastern Atlantic during May/June 1990 using reversed-phase high-performance liquid chromatography. Chlorophyll a concentrations at 5 m increased from 1.2 to 3.7 mug l-1 during the first half of May, and decreased progressively thereafter in the post-bloom stage. Multiple regression analysis of chlorophyll a and selected accessory pigments indicate that diatoms (fucoxanthin; 23-70%) and prymnesiophytes (19'-hexanoyloxyfucoxanthin; 40-20%) dominated the chlorophyll a biomass in the development phase, with prymnesiophytes dominating the post-bloom stage (45-55%). Dinoflagellates (peridinin; 5-25%) and ''green'' algae (chlorophyll b; 5-10%) were secondary components of the microalgal community. Depth distributions revealed that the pigment maxima occurred near the surface at 5-15 m, with concentrations decreasing rapidly below 15 m. At the peak of the bloom, diatoms (fucoxanthin) were dominant throughout the water column down to 300 m, while in the post-bloom phase, prymnesiophytes (19'-hexanoyloxyfucoxanthin) dominated the community in the upper 20 m with diatoms accumulating in deeper water. Concomitant measurements of nutrients and downwelling irradiance suggest that nitrate availability limited the growth of the phytoplankton in the upper 15 m and below this depth limitation was due to low irradiance levels.