Dislodgment and attachment strength of the intertidal macroalga Hormosira banksii (Fucales, Phaeophyceae)

<i>Hormosira banksii</i> is a dominant intertidal alga characterised by limited dispersal of propagules, yet it has a broad distribution throughout temperate Australasia. However, the high abundance of beach wrack of <i>H. banksii </i>on sandy beaches along southwest Victoria, the frequency of frond dislodgment and the force required to break it from the substratum remain unknown. Dislodgment of macroalgae has been shown to influence dispersal processes; therefore, we tested the model that long-distance dispersal of <i>H.</i> <i>banksii</i> is facilitated by weak attachment to the substratum and frequent dislodgment of fertile fronds. We monitored the dislodgment of <i>H. banksii </i>individuals and conducted <i>in situ </i>pull tests to determine the attachment strength of fronds of <i>H. banksii</i>. We further tested whether thallus size or pull direction influenced attachment strength of <i>H. banksii</i>. We found that breakage of vesicles and fronds was a regular event during the survey period and that the mean attachment strength of fronds of <i>H. banksii</i> was weak compared to other intertidal algae. Furthermore, we found that thallus size did not influence the force required to break fronds of<i> H.banksii </i>from the substratum but that the direction that fronds were pulled from the substratum did. We suggest that fronds of <i>H. banksii</i> do drift frequently and that long-distance dispersal is likely to be an important mechanism for the distributional success of this species.