Controls on floc size in a continental shelf bottom boundary layer

Simultaneous in situ observations of flee size, waves, and currents in a co ntinental shelf bottom boundary layer do not support generally accepted fun ctional relationships between turbulence and flee size in the sea. In Septe mber and October 1996 and January 1997, two tripods were deployed in 70 m o f water on the continental shelf south of Woods Hole, Massachusetts. On one a camera photographed particles in suspension 1.2 m above the bottom that had equivalent circular diameters larger than 250 mum, and on the other, th ree horizontally displaced acoustic current meters measured flow velocity 0 .35 m above the bottom. The tripods were separated by similar to 150 m. Typ ically, maximal flee diameter stayed relatively constant, around 1 mm, and it showed a dependence on turbulence parameters that was significantly weak er than that predicted by any model that assumes that turbulence-induced st resses limit flee size. Occasionally, when waves and currents generated int ense near-bed turbulence, flocs were destroyed. These precipitous decreases in maximal flee size also were not predicted by conventional models. The c orrelation in time between episodes of flee destruction and elevated combin ed wave-current stresses provides the first quantitative support for the hy pothesis that flee size throughout bottom boundary layers can be controlled by breakup in the intensely sheared near-bed region. These observations de mand a reassessment of the forces limiting flee size in the sea, and they i ndicate the potential for significant simplifying assumptions in models of flee dynamics.