Adaptive mechanical tolerance and dislodgement velocity of the kelp Laminaria japonica in wave-induced water motion

Adaptive mechanical tolerance of Laminaria japonica to water motion was exa mined by field transplant and laboratory flow-tank experiments. L. japonica attaching to plastic plates were obtained at mid-water platforms sheltered from and moderately exposed to wave action. The environmental water veloci ties were determined during the cultivation period and transplant experimen t using ultrasonic flow meters and available offshore wave height records. When transplanted to 3 depth sites with different intensities of wave-induc ed water motion, almost all the 'sheltered' plants were dislodged from the plastic plates by waves within 5 d, while most of the 'exposed' ones remain ed attached even at the shallowest depth 72 d after transplantation. This d ifference in mechanical tolerance was quantified by a flow-tank experiment conducted in sinusoidal oscillatory flows: 50 % of the 'sheltered' plants w ere dislodged at a velocity amplitude of 0.8 to 0.9 m s(-1), while all of t he 'exposed' plants persisted even at 1.1 m s(-1). The maximum water veloci ties encountered by the transplants were estimated using ultrasonic flow me ter records and the period-averaged water speeds, which were determined by correlating water velocity with decreased mass of gypsum blocks. These esti mates coupled with the observed survivorships at the transplant sites were consistent with the results of the now-tank experiment. Attachment strength increased significantly as the holdfast grew for the 'exposed' plants but was constant for the 'sheltered' ones. In addition, the 'sheltered' plants had a short, wide, undulated blade which resulted in higher drag while the 'exposed' plants had a long, narrow, flat blade and thereby lower drag. Des pite the marked difference in tolerance to water motion, the frequency dist ribution of the root-mean-square water velocity experienced by plants durin g the growth period was only 0.05 m s(-1) higher at the exposed culture sta tion than at the sheltered one, suggesting the presence of a critical veloc ity in developing the adaptive mechanical tolerance. The observed dislodgem ent velocity for the 'sheltered' plants was considerably lower than predict ed from general hydrodynamic theories describing attachment strength of a t hallus stretched out in the direction of flow. This indicates that detailed dynamic behaviors of plants should be explored to predict the wave-induced mechanical failure in large, flexible algae such as L, japonica.