Age affects the strain-rate dependence of mechanical properties of kelp tissues.

Premise: The resistance of macroalgae to hydrodynamic forces imposed by ambient water motion depends in part on the mechanical properties of their tissues. In wave-swept habitats, tissues are stretched (strained) at different rates as hydrodynamic forces change. Previous studies of mechanical properties of macroalgal tissues have used either a single strain rate or a small range of strain rates. Therefore, our knowledge of the mechanical properties of macroalgae is limited to a narrow fraction of the strain rates that can occur in nature. In addition, although mechanical properties of macroalgal tissues change with age, the effect of age on the strain-rate dependence of their mechanical behavior has not been documented.

Methods: Using the kelp Egregia menziesii, we measured how high strain rate (simulating wave impingement) and low strain rate (simulating wave surge) affected mechanical properties of frond tissues of various ages.

Results: Stiffness of tissues of all ages increased with strain rate, whereas extensibility was unaffected. Strength and toughness increased with strain rate for young tissue but were unaffected by strain rate for old tissue.

Conclusions: Young tissue is weaker than old tissue and, therefore, the most susceptible to breakage from hydrodynamic forces. The increased strength of young tissue at high strain rates can help the frond resist breaking when pulled rapidly during wave impingement, when hydrodynamic forces are largest. Because breakage of young tissue can remove a frond's meristem and negatively impact the survival of the whole kelp, strain-rate dependence of the young tissue's strength can enhance kelp's survival.