Biomechanical analysis of wave-induced mortality in the marine alga Pterygophora californica

Abstract

A biomechanical analysis was carried out on the stipe of the marine brown alga Pterygophora cahfornica (Ruprecht). Tensile and compressive mechanical tests on the cortical material revealed that the plant is constructed from a reasonably stiff (tensile modulus of elasticity = 60 MN m-2), strong (tensile strength = 9 MN m-2) and extensible (f d u r e strain = 0.49) material. Consequently, the stipe is a very robust structure that is extremely difficult to break. However, the work of fracture of the cortical material is only about 400 J m-', making it a relatively brittle material. Thus, it is expected that small notches or flaws at the stipe surface will dramatically reduce the ability of the stipe to resist bending forces caused by waves. Fracture mechanics analysis suggests a critical flaw size for the cortical material of about 0.2 mm, and any surface defect larger than this size will degrade the mechanical strength of the plant. The red urchin Strongylocentrotus franciscanus is known to graze on the stipe of P. californica. This grazing is known to mtroduce surface flaws an order of magnitude or more larger than the critical flaw size, and therefore it IS very likely that such grazlng will make a substantial contribution to wave-induced mortahty of these plants.