Population connectivity in marine macroalgae

With the worldwide decline of temperate marine macroalgal populations, it is becoming important that efforts are made to reduce the impact of anthropogenic stressors faced by these communities through implementing networks of Marine Protected Areas (MPAs). The degree to which macroalgal communities will be affected by future climate change and anthropogenic disturbance will not only impact macroalgal populations directly, but the diverse suite of marine organisms they support. The aims of this thesis were to establish general dispersal capabilities of macroalage, as well as to quantify the geographic spacing of over which gene flow is maintained. Furthermore, through genetic analysis I investigated the historical and contemporary processes that have shaped the genetic structure of key southeast Australian macroalgal populations. These results will help to determine ideal configurations of MPAs intended to conserve macroalgae, as well as highlight historical and contemporary dispersal barriers that may impact population connectivity and determine the placement of marine reserves. A meta-analysis was performed using a collection of published studies spanning a variety of macroalgal species and geographic regions (Chapter 2), to determine whether isolation by distance (IBD) relationships were generally seen amongst macroalgae. Although individual studies report IBD regression coefficients, a review such as this is lacking within the literature. A general trend of IBD was found across all studies, and was consistent across species life histories, habitat and genetic marker type. Additionally, an optimal spacing of 50–100 km between populations would facilitate a suitable level of gene flow and maintain connectivity. This chapter is published in Conservation Biology. Chapter 3 is published in Applications in Plant Sciences and describes the development of microsatellite loci for the Tasmanian endemic macroalgae Lessonia corrugata. Microsatellites were identified using next generation sequencing techniques and 29 loci were screened for polymorphism; seven loci were polymorphic and optimised for future use in population genetic assessments. Samples from 14 spatially discrete populations of this species were analysed from the Derwent Estuary, Tasmania (Chapter 4). The proportion of intervening habitat types (sand, rock and open water) and marine distance, between each of these locations, was quantified and incorporated into Linear Mixed Effects models to determine their relative influence on population genetic structure. Dispersal between populations of L. corrugata was limited by increases in intervening marine distance, as well as by larger proportions of open water (deeper than the euphotic zone) between sites. Proportions of intervening benthic habitat (rock/sand) also influenced genetic structure, with sand reducing gene flow. Chapter 5 assesses the phylogeography of four key southeast Australian habitat forming macroalgal species across known biogeographic provinces. Mitochondrial and chloroplast genetic markers were used to define the phylogeographic structure of: Ecklonia radiata, Macrocystis pyrifera, Phyllospora comosa, and Lessonia corrugata. Phylogeographic variation of E. radiata and L. corrugata corresponded to historical barriers to dispersal inferred for other species. Both M. pyrifera and P. comosa lacked any phylogeographic structuring and spatial genetic variation across the entire distribution of samples. Shallow genetic variation indicates a potential recent arrival (< 3 Mya) of habitat forming macroalgae in temperate Australia. The shallow genetic variation among these species raises concerns for the future of these macroalgal populations under climate change scenarios. These results also confirmed that sampled populations of L. corrugata in Chapter 4 form a single deme, and therefore observed contemporary population genetic structuring cannot be attributed to historical processes. This chapter is published in Phycologia.