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 Hedophyllum sessile to investigate how macroalgal biomechanical attachment properties vary with developmental stage, substratum-type, season, and wave-exposure. Hedophyllum sessile is an intertidal kelp species that is able to survive in wave-exposed areas in the Northeast Pacific. For both juveniles and adults, holdfast attachment force and strength were measured at a wave-exposed and wave-protected site in Barkley Sound, British Columbia, Canada. Substratum and wave-exposure effects on attachment properties were tested in juvenile populations. Adult populations were sampled prior to (in July 1996) and after (in November 1996) a series of storms. Site and seasonal wave-exposure effects on attachment properties were tested in these adult populations. Comparisons to known attachment properties of other temperate macroalgal species were also made. Causes for these patterns are discussed but were 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 ( approximately 100 N), but relatively weak ( approximately 0.07 MNm(-2)). Adult attachment did not vary with site wave-exposure, but there was a shift within each site to more resistant holdfasts after a series of early winter storms. Seasonal increases in storm swells correlated to more thallus tattering and selected against large, loose holdfasts. The data presented here suggest that results from holdfast attachment field studies in one season cannot be extrapolated to another due to a complex set of dynamics. This is the first documentation of seasonal patterns in macroalgal attachment properties.