Flow visualization around single- 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 motion generated by the seaweeds themselves. For single-bladed specimens of the kelps Laminaria setchellii, Costaria costatum, Macrocystis integrifolia, and Alaria marginata, the transition from a laminar to turbulent 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 gardneri occurred at 2.5–3 cm·s⁻¹ and at 5 cm·s⁻¹ for the coarsely branched red seaweed, Gelidium coulteri. Flow features such as separation, recirculating eddies, and Von Kánncán vortex streets were observed around various morphologtcal features. We suggest that in the field, flow around larger macroalgae such as kelp is mostly turbulent and that many seaweeds will lie within the wakes of neighboring macroalgae. For small, branched seaweeds such as G. coulteri, however, the meshlike structure may damp turbulence within the thallus, thereby increasing the mainstream velocity at which the transition to turbulence occurs.