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

Hormosira banksii 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 H. banksii 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 H. banksii is facilitated by weak attachment to the substratum and frequent dislodgment of fertile fronds. We monitored the dislodgment of H. banksii individuals and conducted in situ pull tests to determine the attachment strength of fronds of H. banksii. We further tested whether thallus size or pull direction influenced attachment strength of H. banksii. 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 H. banksii was weak compared to other intertidal algae. Furthermore, we found that thallus size did not influence the force required to break fronds of H.banksii from the substratum but that the direction that fronds were pulled from the substratum did. We suggest that fronds of H. banksii do drift frequently and that long-distance dispersal is likely to be an important mechanism for the distributional success of this species.

Restoration of the habitat-forming fucoid alga Hormosira banksii at effluent-affected sites : competitive exclusion by coralline turfs

Fucoid algae often dominate intertidal rocky shores, providing habitat and modifying ecosystem resources for other species, but are susceptible to discharge of sewage effluent. In this study we assessed the potential for competition from coralline turfs to inhibit restoration of the intertidal fucoid macroalga Hormosira banksii at sites associated with an ocean outfall a scenario of improving following water quality in the nearshore coastal environment. The percentage cover and number of individuals of H. banksii were negatively correlated with both the percentage cover and turf height of Corallina officinalis. In contrast, H. banksii was positively associated with rocky substrata and recruited well to rock-surface substrata. Importantly, there appears to be a threshold abundance where the percentage cover of H. banksii rarely reaches above 20% cover amongst coralline turfs with >40% cover. These data support a model of alternative community states: H. banksii dominated canopy on rocky substrata versus C. officinalis turf. In field and laboratory experiments, extensive coralline turfs (up to 4 cm thick) were shown to inhibit recruitment of H. banksii. This study shows competitive exclusion by coralline turfs may limit the successful restoration of habitat provided by H. banksii to shores that have been affected by sustained discharge of secondarily treated sewage effluent. We suggest potential strategies for management authorities to consider when seeking ways of restoring fucoid communities affected by anthropogenic disturbances such as wastewater disposal.