Wounding, healing and survivorship in three kelp species

Abstract

Pterygophora californica Ruprecht, Eisenja arborea Areschoug and Laminana setchellii Silva are 3 common kelps (Laminariales, Phaeophyta) found in the shallow subtidal of open coastal British Columbia, Canada. Earlier laboratory studies showed that P. californica stipes had a cr~tical flaw length (CFL, the length of a flaw which, if exceeded, results in structural failure) of 0.2 mm. In this paper w e report on studies to determine whether this CFL is valid in situ, whether all 3 species of kelp respond similarly to stipe damage and whether tissue that regrows in the site of a wound increases the mechanical strength of the stipe. We tested the effect of razor cuts less than the CFL and also razor cuts and cuts imposed by a triangular file that exceeded the CFL. Results were not as predicted. Neither razor cuts nor file cuts greater than the CFL significantly decreased the survivorship for P californica over control thalli. L. setchellii had a significantly lower survivorship when cut by a triangular file but not when cut by a razor Tissue regrowth does not significantly increase mean strength of the stipe. We hypothesize that the low survivorship of L. setchellii a s a result of stipe damage is one factor responsible for preventing successful establishment of adults of this species in sites where sea urchins a r e common. In contrast, the comparatively h~g h e r survivorship of P. calilornica adult plants subjected to similar damage may b e one factor enabling this species to survive in such sites.

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

“…For such materials, strength reduction can be predicted reliably with Eqn·3. For seaweeds, however, large deformations act to round the crack tip and reduce stress concentrations, thereby limiting the utility of linear elastic expressions in predicting strength reduction in the presence of cracks (Biedka et al, 1987;Denny et al, 1989;DeWreede et al, 1992). Cracktip rounding ameliorates the strength reduction predicted by Eqn·3.…”

“…Cracktip rounding ameliorates the strength reduction predicted by Eqn·3. Nonetheless, cracks of various geometries have been demonstrated to increase the likelihood of breakage in several macroalgae (Denny et al, 1989;DeWreede et al, 1992).…”

“…Once a crack has formed in an alga through fatigue, herbivory or abrasion, it can locally amplify stress, thereby decreasing the alga's ultimate strength (where strength is calculated from bulk force applied to a specimen, disregarding local amplifications) and rendering the alga more susceptible to breakage by the imposition of a single large stress (e.g. Black, 1976;Johnson and Mann, 1986;Armstrong, 1987;Biedka et al, 1987;Denny et al, 1989;Lowell et al, 1991;DeWreede et al, 1992). Even if an alga containing a crack does not experience stress sufficient to break it in a single loading, repeated stresses below the alga's ultimate strength may cause a crack to grow to a length at which breakage occurs (Hale, 2001).…”

Death by small forces: a fracture and fatigue analysis of wave-swept macroalgae

“…The presence of such cracks -in addition to any cuts or nicks formed through abrasion or herbivory -reduces breaking strength (the stress that algae can resist before fracturing in two), thereby increasing the probability of breakage by large forces (e.g. Black, 1976;Johnson and Mann, 1986;Armstrong, 1987;Biedka et al, 1987;Denny et al, 1989;Lowell et al, 1991;DeWreede et al, 1992). Furthermore, once cracks are present (regardless of their source), repeated loading may cause them to grow until the alga breaks, even when algae are subjected only to small forces.…”

Mechanisms underlying the effect of the chiton Katharina tunicata on the kelp Hedophyllum sessile:size escapes and indirect effects

“…Simi!or!y, maiine herbivores ai-e well known for their effects on benthic algal population and comn~unity structure and several investigations also have attempted to attribute morphological, structural, chemical and reproductive features of algal thallus forms to their effects (Littler & Littler 1980, Padilla 1984, Paul & Hay 1986, Hay & Fenical1988, Hay 1996. However, recent attention to the indirect relationship between herbivores and macroalgae by way of wave exposure (Biedka et al 1987, DeWreede et al 1992, Padilla 1993, Tegner et al 1995 suggests that determining the relative importance of these factors separately may be inappropriate, if not impossible (i.e. studies examining the effects of wave exposure on algal survivorship in situ must control for the indirect effects of herbivores in order to assess only the effects of hydrodynamic forces).…”