Connectivity among fragmented populations of a habitat-forming alga, Phyllospora comosa (Phaeophyceae, Fucales) on an urbanised coast

Despite a growing body of knowledge on the ecological consequences of loss and fragmentation of habitat-forming macroalgae, little is known about the genetic implications of such losses. Here, we investigate the genetic consequences of fragmentation caused by the loss of the habitat-forming macroalga Phyllospora comosa from 70 km of urbanised coastline in Sydney, Australia. Contrary to predictions, there appeared to be substantial connectivity among fragmented populations, although spatial autocorrelation analysis revealed that this may be an artifact of allele size homoplasy beyond scales of similar to 80 km. Genetic differentiation was not related to geographic separation of populations. This may be explained by the nature of prevailing currents (East Australian Current) that promote nonlinear dispersal in 'leaps', sourcing propagules from one area and depositing them via eddies that either come ashore or disperse. Populations that were tens of kilometers apart were often genetically different, which was likely due to barriers to dispersal, such as sandy beaches and mouths of estuaries, or rapid fertilization and recruitment of zygotes on small spatial scales. Our research provides a basis for designing a rehabilitation program for populations of Phyllospora comosa, with appropriate consideration of genetic diversity and structure.