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The ecology and anchorage mechanics of kelp holdfasts Boizard, Sophie Marie Dominique Sandrine Valerie

Abstract

The intertidal zone on wave-swept shores is among the most stressful environments on earth. The ability of organisms to survive and thrive in such environments depends on their ability to withstand breakage and wave dislodgement. The research presented in this thesis investigated two aspects of the biology of kelp holdfasts. I first examined whether the recruitment of the kelp Hedophyllum sessile was facilitated by the presence of holdfasts of adult conspecifics and how canopy cover and wave-exposure mediated this interaction. Field experiments indicated that adult holdfasts and substrata of high structural complexity, such as articulated coralline algae, enhanced recruitment. However, the ability of structurally complex substrata to facilitate recruitment depends largely on the extent of canopy cover and to a lesser extent on wave-exposure. Mechanisms by which canopy cover mediates substratum-specific recruitment processes may hold significant implications for population persistence and successful recruitment, especially following periods of high disturbance. Secondly, I investigated the functional morphology of the holdfast of the kelp Laminaria setchellii in relation to its role in providing attachment to the substratum and resistance against wave dislodgment. Results of field investigations indicated that the thallus of L. setchellii responds to increased wave exposure by decreasing blade size and increasing holdfast size; a concomitant increase in holdfast attachment force was not observed. At high wave exposure sites, the integrity and attachment of L. setchellii holdfasts is jeopardized by the presence of barnacles and burrowing crabs. Field investigations further showed that L. setchellii exhibits asymmetry with respect to the prevailing wave direction. The blade and upper part of the stipe are oriented with their longer axis perpendicular to the flow so that reconfiguration and bending is facilitated. The lower part of the stipe and holdfast are oriented with their longer axis parallel to the flow, resisting bending. By exhibiting such pattern of asymmetry along it thallus, L. setchellii avoid large hydrodynamic forces while still avoiding large swaying and remaining off the substratum in water of higher light intensities and velocities.

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