The contribution of single and colonial cells of Phaeocystis pouchetii to spring and summer blooms in the north-eastern North Atlantic

Studies of the phytoplankton ecology in different localities in north-Norwegian fjords, the White Sea and the Barents Sea were carried out in spring and early summer to investigate the contribution of single and colonial stages of Phaeocystis pouchetii to phytoplankton abundance. Three different types of flagellated and four colonial cells were observed in all localities. P. pouchetii was rare under the ice of the Barents and White Seas, but their abundance increased rapidly during ice retreat. Single cell C dominated over colonial cell C, often by 50 times or more. The highest share of colonial cells was encountered in April in northern Norwegian fjords, in May in the Barents Sea and in May–June in the White Sea. At times the single cell dominated the total P. pouchetii biomass in Balsfjord (April 1999, 2001) with hardly any colonies present. In the White Sea colonies of P. pouchetii were less abundant than in the other regions. Cell carbon of P. pouchetii colonies appears never to be as dominating in the north-eastern North Atlantic as P. globosa blooms in coastal regions such as the southern North Sea. However, the lobal matrix of P. pouchetii colonies appears to be less solid than that of P. globosa and partly dissolution of the colony matrix during handling and storage of fixes samples induces uncertainty about the absolute numbers of P. pouchetii colonial cell counts. Despite of that, single cells of P. pouchetii seem to dominate significantly over colonial cell biomass at most sites and during some years and in some regions colonial cells seem rare. We speculate that top-down regulation of Phaeocystis spp. blooms possibly determines the ratio between single and colonial cells.