Phytoplankton extracellular release of organic substances was studied in the Southern Bight of the North Sea in relation to changes in growth conditions (light intensity and inorganic nitrogen concentration) and in taxonomic phytoplankton composition. Light intensity had no effect on percent extracellular release (PER). A high negative correlation between PER and mineral nitrogen concentration was found during spring bloom when phytoplankton were dominated by flagellates (Phaeocystis poucheti in the Southern Bight; dinoflagellates in the oligotrophic English Channel). The physiological response of the flagellate populations to shortage of nutrients differed. High PER was measured during the decline of the bloom with lower values in oligotrophic (46 %) than eutrophic waters (70 to 80 %). On the other hand, no significant correlation was found between PER and mineral nitrogen for diatoms which dominate the phytoplankton population outside the spring period. PER values were unchangeably low (0 to 14 %) over a large range of mineral nitrogen concentrations (0 to 50 µg at N l-1).
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