CHANGES IN SUSPENDED-MATTER FLOC SIZE DURING THE TIDAL CYCLE IN THE DOLLARD ESTUARY

Measurements of in situ particle size of suspended matter in a tidal c hannel in the Dollard (Dutch Wadden Sea) indicated systematic variatio ns in floc size during the tidal cycle that can be explained by assumi ng settling during slack tide, resuspension during the early ebb and e arly flood, flocculation of fine particles into large ones during most of the tide, and deflocculation of large flocs into smaller particles during or after settling to the bottom. There was a characteristic di fference in floc-size variation during ebb and during flood. During th e ebb maximum floc size coincided with maximum suspended-matter concen tration (maximum collision frequency); during the flood maximum floc s ize continued to increase towards high-water slack tide. The adjacent tidal flats had a marked influence on floc size: where the flats were small, the highest percentages of large flocs (>128 mum) occurred arou nd slack tide when current velocities are low. Where the adjacent flat s were broad and extensive, high percentages also occurred during inte rmediate periods. Flocs of maximum size are probably not in equilibriu m with the bulk of the suspended matter: they are mainly formed during periods of high suspended-matter concentrations of short duration.