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

Abstract

Dyck and DeWreede, 2006b). For most seaweeds, such breakage soon translates into death for dislodged portions of thalli. Indeed, dislodged thalli and frond fragments form important carbon and nitrogen sources in coastal and nearshore environments (e.g. Rossi and Underwood, 2002;Dugan et al., 2003;Orr et al., 2005;Liebezeit et al., 2008;Lastra et al., 2008).Nonetheless, biomechanical models of macroalgal breakage have frequently underestimated, and sometimes greatly underestimated, breakage of seaweeds due to wave-imposed forces. 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. Recent laboratory testing has established that some seaweeds fail by fatigue, accumulating damage over a series of force impositions. Failure by fatigue may thus account, in part, for the discrepancy between prior breakage predictions, based on individual not repeated wave forces, and reality. Nonetheless, the degree to which fatigue breaks seaweeds on waveswept shores remains unknown. Here, we developed a model of fatigue breakage due to wave-induced forces for the macroalga Mazzaella flaccida. To test model performance, we made extensive measurements of M. flaccida breakage and of wave-induced velocities experienced by the macroalga. The fatigue-breakage model accounted for significantly more breakage than traditional prediction methods. For life history phases modeled most accurately, 105% (for female gametophytes) and 79% (for tetrasporophytes) of field-observed breakage was predicted, on average. When M. flaccida fronds displayed attributes such as temperature stress and substantial tattering, the fatigue-breakage model underestimated breakage, suggesting that these attributes weaken fronds and lead to more rapid breakage. Exposure to waves had the greatest influence on model performance. At the most wave-protected sites, the model underpredicted breakage, and at the most wave-exposed sites, it overp...

The extraordinary joint material of an articulated coralline alga. I. Mechanical characterization of a key adaptation

“…Using this mathematical framework, we then draw from the distribution of u max recorded at our site by Mach et al (2011) to sum these strain increments through time beginning at ε=0. From a random selection of 86,400 u max values recorded by Mach et al (2011), we draw the u max of one wave at random and calculate Δε, the increase in strain after one wave (10 s).…”

“…From a random selection of 86,400 u max values recorded by Mach et al (2011), we draw the u max of one wave at random and calculate Δε, the increase in strain after one wave (10 s). We then choose another u max at random, calculate a new Δε, and add it to the previous strain to give ε after 20 s. This procedure is then iterated until t=1.892×10 8 s (1.892×10 7 waves), the estimated 6 year maximum lifetime of C. cheilosporioides (Martone, 2010).…”

“…Because joints do not weaken appreciably through repeated loadings, whether a frond will survive or not is determined by the largest force it encounters. (This is in contrast to fleshy red algae, in which the accumulated damage from small forces limits size; Mach, 2009;Mach et al, 2007Mach et al, , 2011 Thus, from easily acquired records of maximum wave force (Denny and Wethey, 2001), one can accurately estimate the maximum size to which C. cheilosporioides' fronds can grow in a given wave environment (Martone and Denny, 2008a;Denny, 2016). Because C. cheilosporioides is a major competitor for space in the low intertidal zone, these estimates of maximum size can be valuable when considering the ecological consequences of a changing wave climate.…”

Indefatigable: an erect coralline alga is highly resistant to fatigue

“…Individual fronds of C. cheilosporioides were collected at Hopkins Marine Station (Pacific Grove, CA, USA), from the same site used in several previous experiments (Hale, 2001;Martone, 2006;Martone and Denny, 2008a;Martone and Denny, 2008b;Mach et al, 2011). Fronds were held in chilled seawater until tested, usually within 5days.…”

“…Breakage can have serious consequences. Fatigue failurerather than the imposition of extreme hydrodynamic forces -controls the rate of survival, and thereby the maximum size, of the common intertidal red alga Mazzaella flaccida (Mach et al, 2011).…”