Different dispersal mechanisms shape multi-scale genetic diversity in habitat-forming Sargassum along the Ningaloo coast, Western Australia

Sargassum is a ubiquitous habitat-forming macroalgal genus in tropical and sub-tropical seascapes, supporting primary and secondary production that underpins biodiversity and ecosystem function. Periodic canopy growth-decay cycles coupled with sexual reproduction allow Sargassum to engage in multi-scale vectors of propagule dispersal ranging from local spore shedding of attached adults to detached Sargassum wracks and rafts that can potentially travel long distances on prevailing currents. We used genetic and genomic methods to reveal the potential implications of these dispersal modes for Sargassum distributed along the Ningaloo World Heritage Area (Western Australia) in terms of connectivity along the known oceanographic features of the Leeuwin and Ningaloo currents. DNA extracted from 500 Sargassum tissue samples across 10 sites were explored with traditional approaches (single gene sequencing of cox3 and rbcL) and higher-resolution genomic tools (SNPs from genotyping-by-sequencing: GBS) to assess population genetic structure across 2º latitude. We found evidence for potential admixture among two putative species ( S. polycystum and S. plagiophyllum ), and some degree of connectivity was inferred among all sampling sites. Homogeneity north and south of Point Cloates indicated particularly strong connectivity across these regions, and/or recent disturbance and recolonisation events. Our findings indicate that Sargassum communities on the west-coast of Ningaloo are likely to be relatively resilient over medium timescales to local-scale disturbances.