ENVIRONMENTAL-CONTROL OF PHYTOPLANKTON ASSEMBLAGES IN NEARSHORE MARINE WATERS, WITH SPECIAL EMPHASIS ON PHOTOTROPHIC ULTRAPLANKTON

The present study assesses conditions that determine the dominance of (i) phytoplankton by cells >5 mu m versus cells <5 mu m, (ii) phototro phic ultraplankton by prokaryotes versus eukaryotes, (iii) prokaryotes by phycocyanin-rich versus phycoerythrin-rich cyanobacteria, and (iv) eukaryotes by cryptophyte versus non-cryptophyte cells. In the Pale d es Chaleurs, Gulf of St Lawrence, Canada, the decrease in phytoplankto n >5 mu m during the summer is related to nitrate and silicate exhaust ion. In contrast, cells <5 mu m are less affected by oligotrophic cond itions and are favoured by temperatures >10 degrees C. Ultraplankton e ukaryotes, because of larger cell volumes, may be selectively grazed b y microzooplankton, and are likely to be more affected by oligotrophic conditions than the dominating cyanobacteria. In the prokaryote assem blage, a relatively high contribution of phycocyanin-rich cyanobacteri a is observed at times of low salinities and high attenuation of irrad iance. Conversely, the dominant phycoerythrin-rich forms thrive in war mer and clearer waters. Finally, the eukaryote assemblage is dominated by non-cryptophyte cells, possibly because cryptophytes, which are la rger in size, may be more affected by grazing than non-cryptophytes. T he contribution of cryptophytes to the eukaryote assemblage increases at high temperatures, high salinities and low nutrient concentrations.