Contrasting patterns of population structure in two habitat-forming kelp species in southeastern Australia.

The distribution and connectivity of species around the globe are changing at a rapid pace. Increasing sea temperatures are a driving factor of changes in temperate macroalgal distributions. Southeast Australia is considered a global ocean-warming hotspot, where macroalgal populations are predicted to decline significantly by 2100. We used genotyping by sequencing and Lagrangian particle modeling to compare the genetic population structure and connectivity of two habitat-forming macroalgae, Macrocystis pyrifera and Durvillaea potatorum, in southeastern Australia. Both species showed regional population structures, although this was greatest in D. potatorum, as which populations showed greater dissimilarity than in M. pyrifera. Particle modeling suggested that self-recruitment and connectivity among populations were highest in northeast Tasmania for both species, with particles often stranding along the nearby coast. Intriguingly, the southernmost M. pyrifera population in Tasmania shared more recent ancestry with a mainland Australia population. Although uncommon, simulations indicated that it is possible for rafts of M. pyrifera from mainland Australia to reach far-southern Tasmania. Hindcast simulations indicated those rafts are likely to come from mainland Australia via the Zeehan Current and along the western Tasmanian coast; unsampled western Tasmanian populations might therefore also share close ancestry with western mainland populations. With genetically distinct populations and low connectivity among areas, southeastern Australian kelp populations are vulnerable to losses of genetic diversity. This study provides insights into contemporary population structure and connectivity processes in two kelp species that form important habitats for biodiversity and fisheries on the Great Southern Reef and which are both undergoing range contractions with rapid environmental change.