Wounded kelps: patterns and susceptibility to breakage

Kelps are highly productive seaweeds predominantly found in wave-exposed environments. Physical and biological processes such as sand abrasion, whiplash and grazing can wound kelp tissue, potentially weakening its ability to withstand wave forces. Despite the ecological importance of kelp tissue loss, few studies have quantified wounding patterns in kelps or how wounding might affect the biomechanical properties of kelps. We quantified the prevalence of wounds on 360 Ecklonia radiata kelps collected across 3 different reef lines (levels of wave exposure) in April (autumn), June (early winter), August (winter) and October (early spring) (i.e. before, during and after peak wave activity). Small holes in the centre of laterals dominated the wounds. Wounding pattern did not differ between reef lines but changed over time, with most in early winter and least in early spring (from 285 to 71 wounds kelp(-1)). This pattern suggests that wounds accumulate over summer and that highly wounded parts of the frond (but not the entire kelp) break off when encountering the first storms. Biomechanical break-force tests assessed the effect of experimental wounds on the forces required to break kelp tissue. These tests showed that holes or horizontal cuts to the edge of the kelp frond dramatically reduced the strength, extensibility, toughness and stiffness of the tissue. We conclude that wounds are common in E. radiata and that the resulting loss of tissue integrity, caused by even small damages, can dramatically increase the susceptibility to breakage. Our findings may have important ecological implications; the peak in wounds in early winter is likely to increase fragmentation of the kelp, thereby reducing its size and hydrodynamic drag, and, paradoxically, reducing the risk of fatal dislodgment during severe winter storms.