PARTICLE AGGREGATION DURING A DIATOM BLOOM .2. BIOLOGICAL ASPECTS

For a 6 wk period covering the time before, during, and after the phytoplankton spring bloom, macroscopic aggregates (less-than-or-equal-to 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 during 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 aggregation of selected diatom species. Although the concentrations of inorganic nutrients diminished considerable 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.