Variations in holdfast attachment mechanics with developmental stage, substratum-type, season, and wave-exposure for the intertidal kelp species Hedophyllum sessile (C. Agardh) Setchell

Biomechanical models that describe physical and biological interactions on wave-exposed shores typically assume that a species' attachment properties are similar between seasons and sites. We tested this assumption using Hedo phyllum sessile to investigate how macroalgal biomechanical attachment prop erties vary with developmental stage, substratum-type, season, and wave-exp osure. Hedophyllum sessile is an intertidal kelp species that is able to su rvive in wave-exposed areas in the Northeast Pacific. For both juveniles an d adults, holdfast attachment force and strength were measured at a wave-ex posed and wave-protected site in Barkley Sound, British Columbia, Canada. S ubstratum and wave-exposure effects on attachment properties were tested in juvenile populations. Adult populations were sampled prior to tin July 199 6) and after tin November 1996) a series of storms. Site and seasonal wave- exposure effects on attachment properties were rested in these adult popula tions. Comparisons to known attachment properties of other temperate macroa lgal species were also made. Causes for these patterns are discussed but we re not isolated in these studies. Juveniles' attachment properties differed on different substrata types and between wave-exposures, with the highest attachment forces and the most attached juveniles in articulated coralline algal turfs. Adult attachment is firm (similar to 100 N), but relatively we ak (similar to 0.07 MN m(-2)). Adult attachment did not vary with site wave -exposure, but then was a shift within each site to more resistant holdfast s after a series of early winter storms. Seasonal increases in storm swells correlated to more thallus tattering and selected against large, loose hol dfasts. The data presented here suggest that results from holdfast attachme nt field studies in one season cannot be extrapolated to another due to a c omplex set of dynamics. This is the first documentation of seasonal pattern s in macroalgal attachment properties. (C) 2000 Elsevier Science B.V. All r ights reserved.