Sarg_Ningaloo_Supp.materials

<i>Sargassum</i> 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 <i>Sargassum</i> to engage in multi-scale vectors of propagule dispersal ranging from local spore shedding of attached adults to detached <i>Sargassum</i> wracks and rafts that can potentially travel long distances on prevailing currents. We used genomic methods to reveal the potential implications of these dispersal modes for <i>Sargassum</i> 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 <i>Sargassum</i> tissue samples across 10 sites (varying from 2 to 164 km apart) were explored with traditional approaches (single gene sequencing of cox3 and <i>rbc</i>L) 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 (<i>S. polycystum </i>and <i>S. plagiophyllum</i>), 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 <i>Sargassum</i> communities on the west-coast of Ningaloo are likely to be relatively resilient over medium timescales to local-scale disturbances.