How do ocean currents interact with ocean islands across scales to controldispersal and other biophysical processes?

Starting with the largest scales, my first study will focus on inter-island connectivity within Seychellesthrough ocean currents. Whilst ocean currents are often thought about in terms of their Eulerian proper-ties, understanding the transport of particles embedded within ocean flow is hugely important for a broadspectrum of environmental problems (van Sebille et al., 2018). One such application is coral reef connec-tivity. Corals, along with many other invertebrates, reproduce by producing large numbers of larvae whichpassively drift with ocean currents. If currents consistently transport larvae from one reef to another, theformer ('source' reef) acts as a supply of new larvae to the latter ('sink' reef) and the two reefs are said tobe 'connected'. Understanding these connections underpins modern marine conservation planning (Treml& Halpin, 2012). As tracking the movement of billions of real coral larvae is unfeasible, these larvae caninstead be simulated as virtual larvae in a numerical model (Siegel et al., 2003; Mitarai et al., 2009). Anotherapplication is the tracking and source attribution of marine plastic debris, the accumulation of which is agrowing problem in Seychelles (Raguain & Burt, 2019) and other ocean islands (Lavers et al., 2019), whichcan also be investigated through numerical models (e.g. van Sebille et al., 2019).Whilst this approach can capture long-distance dispersal through mesoscale flows, submesoscale flows dom-inate coast-proximal environments and the best achievable resolution in modern regional-scale dispersalmodels (e.g. Medel et al., 2018; Thompson et al., 2018; Uchiyama et al., 2018) is incapable of adequatelyrepresenting these flows (Dauhajre et al., 2019). This gap in understanding how particles escape coasts andenter the large-scale circulation has therefore been identified as a priority problem (Edmunds et al., 2018).Islands in the path of significant ocean currents can generate a type of fluid instability known as 'wake eddies'(Wolanski et al., 1984; Heywood et al., 1990), which are known to be sensitive to the Rossby and Ekmannumbers (Heywood et al., 1996). These wake eddies are thought to significantly affect how dispersive orretentive reefs are (Largier et al., 2004) but to date this has only been investigated in idealised configurations(Coutis & Middleton, 2002; Cetina-Heredia & Connolly, 2011). A parameterisation for reef retention as afunction of an island's fluid dynamical regime would significantly improve regional-scale dispersal models,but no such parameterisation currently exists. This will be the subject of my second study.The final study relates to Aldabra Atoll, Seychelles, a highly remote island with limited human interventionand a UNESCO World Heritage Site due to its ecological importance (Claudino-Sales, 2019). The atoll isdominated by a large lagoon, connected to the ocean only through several narrow channels (Stoddart et al.,1971). The tidal range at Aldabra is large compared to many atolls, so particularly in the sheltered lagooninterior, corals are within a tide-dominated regime which is regarded as being poorly understood (Lowe& Falter, 2015). This is complicated by the highly unusual tides observed within the lagoon due to therestricted connectivity between the lagoon interior and open ocean (Pugh, 1979; Farrow & Brander, 1971).Our understanding of these tides remains highly limited, yet it may be valuable for understanding manyaspects of the lagoon ecology (Hamylton et al., 2012).