Characterisation of wave-influenced macroalgal propagule settlement

Successful attachment of macrophyte propagules in the rocky intertidal zone requires that propagules actually arrive at the substrate (settlement). The analysis first considers the small-scale processes occurring at the propagule scale then places these results in the context of long timescale variability in wave forcing due to weather and climate. The non-dimensional scaling is developed in terms of a Particle Reynolds number, Re⁎p, that depends on both flow and particle scales. Scaling for size, inertia and buoyancy are described in terms of the Re⁎p. Settlement mechanics of several macrophyte species (Durvillaea antarctica, Hormosira banksii and Cystophora torulosa) are compared using direct velocity measurements and placed in an environmental context using a twenty-year wave hindcast. The forcing is shown to be more closely linked to wave orbital speeds rather than breaker-driven phase speeds, and (negative) buoyancy has the greatest influence, identified through a Buoyancy Travel Ratio (BTR). Differences in these physical scales between the species are only moderate, suggesting supply, mucous and stickiness effects must be considered to fully resolve difference in success.