PARTICLE AGGREGATION DURING A DIATOM BLOOM .1. PHYSICAL ASPECTS

Aggregate size and abundance were monitored in situ at a fixed station in the southern North Sea during a 6 wk investigation which covered the phytoplankton spring bloom and the pre- and post-bloom periods. Particle aggregates were abundant during the entire period of study. Biologically derived material, such as algal cells, played a central role in aggregate formation. Maximum total aggregate volume coincided with the peak of the bloom. Maximum aggregate size did not correlate with either phytoplankton biomass or total suspended matter. Despite a distinct increase in the amount of aggregated material during the development of the bloom, maximum aggregate size remained at about 1 mm diameter during most of the investigation. The formation of large, marine-snow-sized aggregates up to 5 cm in longest dimension, which was restricted to a short period following the decline of the phytoplankton bloom, coincided with comparatively low shear rates. Results are consistent with physical coagulation models. Aggregate formation can be described by a 2-state system in which the amount of aggregated matter is low during the development and following the decline of a bloom, and high during the peak of a bloom.