Techniques for predicting the lifetimes of wave-swept macroalgae: a primer on fracture mechanics and crack growth

Abstract

SUMMARY Biomechanical analyses of intertidal and shallow subtidal seaweeds have elucidated ways in which these organisms avoid breakage in the presence of exceptional hydrodynamic forces imposed by pounding surf. However, comparison of algal material properties to maximum hydrodynamic forces predicts lower rates of breakage and dislodgment than are actually observed. Why the disparity between prediction and reality? Most previous research has measured algal material properties during a single application of force, equivalent to a single wave rushing past an alga. In contrast, intertidal macroalgae may experience more than 8000 waves a day. This repeated loading can cause cracks– introduced, for example, by herbivory or abrasion – to grow and eventually cause breakage, yet fatigue crack growth has not previously been taken into account. Here, we present methods from the engineering field of fracture mechanics that can be used to assess consequences of repeated force imposition for seaweeds. These techniques allow quantification of crack growth in wave-swept macroalgae, a first step towards considering macroalgal breakage in the realistic context of repeated force imposition. These analyses can also be applied to many other soft materials.

Production and fate of kelp detritus

“…Estimated rates of productivity, blade erosion, dislodgement of whole fronds and thalli (g dry mass m Hayashida (1977) theless, dis lodgement and erosion are typically greater at sites with high wave exposure than at more protected sites (Duggins et al 2003, Krumhansl & Scheibling 2011a). The measured force to break kelp stipes and blades often exceeds the predicted hydrodynamic force acting on kelps (Utter & Denny 1996, Denny et al 1997, Mach et al 2007), suggesting that breakage and dislodgement are uncommon. For example, Utter & Denny (1996) High erosion and dislodgment rates occur as a result of damage to kelp holdfasts, stipes, and blades.…”

Burrowing in marine muds by crack propagation: kinematics and forces

“…Relevant fracture mechanics have been recently reviewed (cf. Dorgan et al, 2006;Mach et al, 2007) and can be found in textbooks (e.g. Anderson, 1995).…”

Failure by fatigue in the field: a model of fatigue breakage for the macroalga Mazzaella, with validation

“…The traditional approach has involved determination of a seaweed's breaking strength and comparison of this strength with maximal wave-induced force, with breakage often under-predicted (Koehl and Alberte, 1988;Gaylord et al, 1994;Johnson and Koehl, 1994;Friedland and Denny, 1995;Utter and Denny, 1996;Denny et al, 1997;Johnson, 2001;Kitzes and Denny, 2005). Researchers have suggested that other factors, such as damage due to herbivory, abrasion or physiological stressors, may account for observed breakage rates, weakening fronds that individual waves then break (Friedland and Denny, 1995;Utter and Denny, 1996;Kitzes and Denny, 2005;Denny, 2006).Additionally, researchers have investigated the possibility that seaweeds break not just from large individual wave-imposed forces but from damage accumulated over a series of wave-imposed forces (Hale, 2001;Mach et al, 2007a;Mach et al, 2007b;Mach, 2009 Accepted 24 January 2011 SUMMARY Seaweeds inhabiting the extreme hydrodynamic environment of wave-swept shores break frequently. However, traditional biomechanical analyses, evaluating breakage due to the largest individual waves, have perennially underestimated rates of macroalgal breakage.…”

“…Additionally, researchers have investigated the possibility that seaweeds break not just from large individual wave-imposed forces but from damage accumulated over a series of wave-imposed forces (Hale, 2001;Mach et al, 2007a;Mach et al, 2007b;Mach, 2009 Accepted 24 January 2011 SUMMARY Seaweeds inhabiting the extreme hydrodynamic environment of wave-swept shores break frequently. However, traditional biomechanical analyses, evaluating breakage due to the largest individual waves, have perennially underestimated rates of macroalgal breakage.…”