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Effects of wave-exposure on an intertidal kelp species hedophyllum sessile (c. agardh) Setchell : demographics and biomechanics

University of British Columbia

Our understanding of how wave-exposure affects a macroalgal species' ecology is not predictive since it is unknown how the species interacts with its physical environment and the ultimate effects on recruitment or mortality rates. This thesis investigates wave-exposure effects on the population ecology of the intertidal kelp species Hedophyllum sessile by a series of demographic and biomechanical field experiments and physical modeling of survivorship. Sporophyte recruitment, survivorship, and reproductive output were measured at sites of different exposure in Barkley Sound, British Columbia. Site exposures were different when offshore daily significant wave heights exceeded 2.1 m, usually during fall and winter months. Important conclusions are: (1) Recruitment is not dependent on exposure, but is different between years and substratum types, with more recruitment on articulated coralline algae; (2) The theoretical trade-off between thallus size-minimization to reduce wave-induced force and maximization to increase reproductive output was not supported; despite lower thallus surface area in the exposed site, these individuals had larger soral surface area, and ; (3) Adult mortality is wave-induced; survivorship and life expectancy were lower in the exposed site. Holdfast attachment mechanics were measured. Juveniles have differential attachment properties on different substrata types and exposures; adult attachment is firm (-100 N), but relatively weak (-0.07 MN- m"2). Site exposure did not affect adult attachment but there was a shift within each site to more resistant holdfasts after a series of early winter storms. Drag experiments and model approaches demonstrated that thallus size, not shape, effectively minimizes drag force and survival. H. sessile's size is reduced by tattering. Model simulations predicted adult survival for exposed and protected sites for 3-month intervals during peak wave seasons. Results using only drag forces were not predictive whereas accelerational plus drag forces approximated real survivorship. This thesis has shown that populations in more exposed sites will be more reliant on successful recruitment to persist because of higher adult mortality. Individuals will be at highest dislodgment risk during storms in summer and early fall months when thalli have not tattered sufficiently to reduce hydrodynamically-induced forces and holdfasts are weak.

Thesis/Dissertation

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2009-06-24

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http://hdl.handle.net/2429/9574

Botany

University of British Columbia

UBCV

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