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Comparison of mechanical properties of four large, wave-exposed seaweeds¹

²Plant Biomechanics Group, University of Freiburg, Botanic Garden, Schänzlestrasse 1, 79104 Freiburg i. Br., Germany; ³Botany Department, University of Otago, P.O. Box 56, Dunedin, New Zealand

ABSTRACT

Seaweeds have a simple structural design compared to most terrestrial plants. Nonetheless, some species have adapted to the severe mechanical conditions of the surf zone. The material properties of either tissue sections or the whole stipe of four wave-exposed seaweeds, Durvillaea antarctica, D. willana, Laminaria digitata, and L. hyperborea, were tested in tension, bending, and torsion. Durvillaea has a very low modulus of elasticity in tension (E_tension = 3–7 MN·m^–2) and in bending (E_bending = 9–12 MN · m^–2), torsion modulus (G = 0.3 MN · m^–2) and strength (_brk = 1–2 MN · m^–2), combining a compliable and twistable stipe "material" with a comparatively high breaking strain (_brk = 0.4–0.6). In comparison, the smaller stipes of Laminaria have a higher modulus of elasticity in tension (E_tension = 6–28 MN·m^–2) and in bending (E_bending = 84–109 MN·m^–2), similar strength (_brk = 1–3 MN·m^–2), and a higher torsion modulus (G = 0.7–10 MN·m^–2), combined with a lower breaking strain (_brk = 0.2–0.3) than Durvillaea. Time-dependent, viscoelastic reactions were investigated with cycling tests. The tested species dissipated 42–52% of the loading energy in tension through plastic-viscoelastic processes, a finding that bears important ecological implications. Overall, there seems to be no correlation between single material properties and the size or habitat position of the tested seaweed species.

Key Words: biomechanics • Durvillaea • Laminaria • modulus of elasticity • Phaeophyceae • tension tests • wave exposure

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