For a 6 wk period covering the time before, during, and after the phytoplankton spring bloom, macroscopic aggregates (2 0.5 mm diameter) were repeatedly collected and water column properties simultaneously measured at a fixed station in the Southern North Sea. Distinct changes in aggregate structure and composition were observed during the study. Predominantly detrital aggregates d u r ~ n g the early phase of the study were followed by diatom-dominated algal flocs around the peak of the bloom. Mucus-rich aggregates containing both algal and detrital components and with large numbers of attached bacteria dominated the post-bloom interval. The phytoplankton succession within the aggregates closely reflected the succession in the water column with a time delay of a few days. Algal flocculation did not occur as a simultaneous aggregation of the entire phytoplankton community, but as a successional aggregat~on of selected diatom species. Although the concentrations of ~norganic nutrients diminished considerably during the development of the phytoplankton bloom, the termination of the bloom appeared to be mostly controlled by physical coagulation processes. The importance of biologically-controlled factors for physical coagulation is discussed.