AbstractsBiology & Animal Science

[Truncated abstract] The biophysical processes responsible for the dispersal of the seagrass Posidonia australis, a widespread species in south-western Australia, were investigated to understand the dispersal mechanisms, pathways and capacity for long distance dispersal (LDD) for this species. In general, seagrasses reproduce both vegetatively and sexually, with most research to date focusing on the dynamics and implications of the former, i.e. vegetative (clonal) growth. This has led to a good understanding of the processes that ultimately drive clonal growth in seagrasses; however, while seagrasses are known to invest significant energy into seed production, the dispersal processes associated with this sexual reproduction still remain largely unknown for many seagrasses. Some recent studies have suggested that sexual reproduction can be important to population growth within existing meadows, but these have largely relied on assumed dispersal pathways from genetic studies, i.e. process-based studies focusing on the mechanisms of seed dispersal have largely been lacking. The first component of this thesis focuses on understanding how the physical properties and life cycle characteristics of P. australis and a contrasting species Halophila ovalis seeds and fruit interact with atmospheric and oceanic forcing to control their rates and patterns of dispersal. A combined laboratory and field study is first described that focuses on quantifying the physical characteristics, transport processes and survival of both fruits and seeds. The results suggest that P. australis is well adapted to LDD during the floating stage of the fruit, where ~2% of the wind velocity was directly transferred onto the fruit (i.e., via windage), thus further augmenting the transport over that due to water motion alone. Following dehiscence, the rapid settlement (~10 cm s-1) then occurs when a seed is released from the fruit, and once it settles to the seafloor, a movement threshold (critical shear stress) of ~100 mPa restricts secondary transport in the bottom boundary layer. In the second component of this thesis, results are presented from a large physical oceanographic field study designed to investigate the ocean circulation patterns in the coastal waters of Perth, Western Australia during summer when P. australis fruit are released. Overall, the predominant northward wind stresses during the summer tended to drive a persistent northward flow on the inner shelf and within nearshore regions; however, these alongshore currents oscillated (and sometimes reversed) due to the transient nature of the local wind forcing (typical forcing period ~7 days) associated with the propagation of synoptic weather patterns through the region. Conversely, on the outer shelf the currents were on-average southward, driven by the mean alongshore pressure gradient along the Western Australia coast that was stronger than the mean northward wind stress; however, the shelf flow episodically reversed due to substantial unsteadiness in the alongshore pressure…