Phytoplankton community structure derived from HPLC analysis of pigments in the Faroe-Shetland Channel during summer 1999: the distribution of taxonomic groups in relation to physical/chemical conditions in the photic zone

A study of the phytoplankton community in the Faroe-Shetland Channel was conducted in July 1999. Samples were collected at various depths in the photic zone along three transects (the northern entrance, the center and the southern entrance). Except for a few easterly stations where nitrate and silicate were below 1 muM, all nutrients (phosphate, silicate, ammonium, nitrite and nitrate) were non-limiting for phytoplankton growth. HPLC pigment analysis revealed a pronounced (>50%) dominance of Prymnesiophyceae at all stations. Their pigment ratio of diatoxanthin + diadinoxanthin/Chl a (DDX/Chl a) indicated that the phytoplankton community was controlled by light. Primary production in the delayed spring bloom varied from 1.2 to 1.8 g C m(-2) day(-1) along the northern transect. Along the other two transects, primary production ranged from 1.6 to 3.8 g C m-2 day(-1). Associated with the characteristics indicating the establishment of a bloom, the relative contribution of diatoms and Prymnesiophyceae increased, whereas that of Prasinophyceae, Cryptophyceae, Chrysophyceae and Cyanobacteriaceae decreased. With respect to their vertical distribution, Cyanobacteriaceae, Chrysophyceae and Dinophyceae tended to have a higher abundance, relative to other taxonomic groups, in the surface layers. The relative abundance of diatoms and Chlorophyceae increased with depth. The DDX/Chl a radio of the Prymnesiophyceae decreased with depth, indicating that vertical mixing in the upper 30 m of the photic zone occurred less frequently than the time span of physiological acclimation of cellular pigment composition.