Settlement rates of macroalgal propagules: Cross-species comparisons in a turbulent environment

The ability of propagules (fertilized eggs) of five species of fucoid algae (Hormosira banksii, Durvillaea antarctica, Cystophora torulosa from New Zealand, and Fucus gardneri and Pelvetiopsis limitata from Oregon, U.S.A.) to settle and attach was tested in a turbulent, stirred tank. The time taken to reach a steady state of settlement numbers varied between species and turbulence intensities. Normalized steady-state (NSS) settlement numbers showed differences among species. A settlement model, based on principles invoked in the analysis of motion of bed sediments in rivers, was developed. The model indicates that the NSS settlement number depends on two parameters, a propagule Reynolds number and an entrainment function that represents the relative importance of the shear stress experienced by settled propagules and their submerged weight. The inability of this model to collapse the data for all species suggests that the stickiness of the propagules, due to their mucus coatings, plays a significant role in the settlement process. P. limitata (largest propagules) exhibited the least effective attachment to the substratum, whereas F. gardneri (second largest) and D. antarctica (smallest propagules) were the most effective at withstanding hydrodynamic forces that detach propagules. We also model the boundary layer above a flat-bed, driven by linear water-waves, using a skin-friction drag coefficient and show that this study represents the lower end of the shear velocity u(*) range. However, these experiments capture the main region of variability in long-term propagule attachment, and indicate that most of these fucoid species will have successful settlement only during calm conditions.