FLOW VISUALIZATION AROUND SINGLE-BLADED AND MULTIPLE-BLADED SEAWEEDS WITH VARIOUS MORPHOLOGIES

Water flow was visualized around 10 seaweeds with various morphologies to determine the onset of turbulence and to estimate the scales of mo tion generated by the seaweeds themselves. For single-bladed specimens of the kelps Laminaria setchellii, Costaria costatum, Macrocystis int egrifolia, and Alaria marginata, the transition from a laminar to turb ulent velocity boundary layer occurred at mainstream velocities of 1.5 cm.s(-1). Transition to turbulence for multiple-bladed specimens of M . integrifolia, Nereocystis luetkeana, Egregia menziesii, and Fucus ga rdneri occurred at 2.5-3 cm.s(-1) and at 5 cm.s(-1) for the coarsely b ranched red seaweed, Gelidium coulteri. Flow features such as separati on, recirculating eddies, and Von Karman vortex streets were observed around various morphological features. We suggest that in the field, f low around larger macroalgae such as kelp is mostly turbulent and that many seaweeds will lie within the wakes of neighboring macroalgae. Fo r small, branched seaweeds such as G. coulteri, however, the meshlike structure may damp turbulence within the thallus, thereby increasing t he mainstream velocity at which the transition to turbulence occurs.