Flow-seaweed interactions of Saccharina latissima at a blade scale: turbulence, drag force, and blade dynamics

Physical interactions between seaweed blades of Saccharina latissima and unidirectional turbulent fow were examined in an open-channel fume, focussing on fow velocities, drag force acting on a blade, and blade reconfguration. The data reveal that seaweed blades adjust to high-energy fow conditions relatively quickly, efciently reducing fow-induced drag via compaction, a mechanism of blade reconfguration. The drag coefcient of blades of S. latissima varied between 0.02 and 0.07 over a range of mean fow velocities from 0.1 to 0.55 m/s. Both fow action and blade biomechanical characteristics infuenced the blade dynamics, with the fow role being predominant in highly energetic conditions. The interaction mechanisms and their strength were found to be scale-dependent, with the combined efect of reduced mean fow velocity and enhanced turbulence in blade wakes. The thickness of the difusive boundary layer, an important factor in nutrient uptake from the surrounding water, was estimated to be in the range from 0.010 to 0.067 mm. Mechanisms of blade adjustment to the fow and scale-dependent dynamic interactions between blades and turbulent eddies have direct implications for seaweed growth, acclimation, and survival. The estimates of the drag coefcient and the thickness of the difusive boundary layer will be useful for the development of bio-physical models, environmental assessments, and design of seaweed farms.