Death by small forces [electronic resource] : failure by fatigue in wave-swept macroalgae
Keywords: Biological Sciences.

Along the margin of an ocean, the tides alternately cover and uncover a band of shore. The vertical extent of this region varies with latitude and geographical features, but in most cases the intertidal region supports rich communities of organisms, the compositions of which depend on substratum type and location. Rocky shores in particular have served as model systems for ecological studies, given their dense and diverse assemblages of organisms and their experimental tractability. In this inherently competitive environment, macroalgae are major occupiers of space. Frequent disturbance due to physical and physiological stresses typifies wave-swept shores. As a result, not only biological interactions such as competition and herbivory but also the physical environment determine community dynamics. For seaweeds, breakage due to wave-imposed forces represents a primary source of disturbance and mortality, influencing community composition as well as natural selection. Macroalgal survival in the harsh physical environment of rocky shores has been evaluated through structural, cellular, and molecular analyses, and to rigorously evaluate breakage, mechanics of organisms must be considered. Although breakage of wave-swept macroalgae is central to ecology and evolution in algal populations, a seemingly straightforward question--how do wave-swept macroalgae break?--has remained poorly understood. Biomechanical studies comparing macroalgal material properties to maximal hydrodynamic forces have generally predicted lower rates of breakage and dislodgment than actually occur. This dissertation assumes an alternate approach, considering macroalgal breakage due not just to individual wave-imposed forces but also to repeated wave-imposed forces through the process of fatigue. Through laboratory tests employing standard techniques from fracture mechanics and fatigue analysis, I found that cracks introduced into flat-bladed macroalgae grow in length in conditions of repeated loading, to the point of specimen rupture. Furthermore, the entire process of fatigue--crack initiation, crack growth, and eventual specimen rupture--occurred predictably in the flat-bladed macroalga Mazzaella. Propensity for fatigue failure varied with life history phase and species, and cracks frequently formed near endophytes and some reproductive structures. Extrapolation of laboratory results suggested that fatigue may cause failure in field hydrodynamic conditions as well. To determine the importance of fatigue in natural populations, I created a numerical model of failure by fatigue in intertidal Mazzaella and assessed model predictions by measuring macroalgal breakage and wave-imposed forces in the field. The fatigue-breakage model, incorporating effects of repeated wave-induced forces, predicted a substantially greater fraction of observed breakage than did traditional predictions based only on maximal individual hydrodynamic forces. Accuracy of model predictions varied with life history phase, frond attributes such as temperature stress, and exposure to wave-imposed forces. Combining laboratory mechanical testing, numerical modeling, and field measurements based on ecological techniques, this dissertation improves understanding of how hydrodynamic forces break seaweeds. Repeated forces not individually sufficient to cause breakage can result in accumulated damage and breakage due to fatigue. In the laboratory and in the field, macroalgae fail by fatigue.