AGGREGATION OF PHAEOCYSTIS DURING PHYTOPLANKTON SPRING BLOOMS IN THE SOUTHERN NORTH-SEA

The role of aggregation during the decline of 2 phytoplankton spring blooms dominated by the Prymnesiophycean Phaeocystis sp. was studied in the southern North Sea in 1989 and 1991. While marine snow aggregates were devoid of Phaeocystis in 1989, a large fraction of the Phaeocystis biomass was associated with aggregates 2 yr later. This discrepancy corresponds to a significant difference in aggregate size between the 2 years studied, which is interpreted to be a consequence of different levels of turbulent mixing. Phaeocystis colonies remained freely suspended during 1989 when aggregates were small, and adhered loosely to the large aggregates observed forming during 1991. Overall, the aggregation potential of Phaeocystis was low in comparison to diatoms. Independent of the degree of aggregation, sedimentation was the dominant loss factor of Phaeocystis biomass from the upper layer. The significance of a low aggregation potential for the role of Phaeocystis in massive foam formation along the North Sea coast is discussed.