Date of Award
2024
Document Type
Thesis
Publisher
Edith Cowan University
Degree Name
Doctor of Philosophy
School
School of Science
First Supervisor
Kathryn McMahon
Second Supervisor
Paul Lavery
Third Supervisor
Elizabeth Sinclair
Abstract
Seagrass ecosystems are highly productive and fulfil vital roles in the coastal environment as ecosystem engineers and keystone species. Unfortunately, anthropogenic pressures have resulted in a significant loss of seagrass habitat and associated ecosystem services. The complex and unpredictable nature of these pressures, and their increasing extent and intensity globally, makes resilience-based ecosystem management a valuable approach. Resilience-based management requires knowledge on processes enhancing ecosystem resilience and on actions supporting these processes. Genetic diversity, which is the allelic variation among individuals of a population, has been recognised as one of the pillars of resilient ecosystems. Genetic diversity within a population can contribute to its capacity to resist and recover from disturbances and to adapt to changing conditions. Thus far, however, our understanding of the spatial and temporal variation in genetic diversity in seagrasses has been very limited, hindering the use of genetic data in seagrass management. This knowledge gap was addressed within this thesis, through three separate approaches: a global review on population genetic diversity of seagrass populations and two field studies which focused on variation in diversity in two seagrass species across different spatial and temporal scales. The review aimed to assess global variation in the genetic diversity of seagrasses across key biological attributes and geographical distributions. The first field study aimed to investigate temporal variation in clonal and genetic diversity and clone composition of the widely distributed, colonising tropical seagrass Halodule uninervis. The second field study aimed to characterise the spatial patterns in genetic diversity and population genetic structure of Amphibolis antarctica across its distribution along a strong salinity gradient in Shark Bay, Western Australia.
The review explored how commonly reported clonal and genetic diversity metrics varied across family, life history strategy, reproductive mode, bioregion, and latitude. This was achieved through a global systematic literature review focused on publications with data on genetic diversity of natural seagrass populations. A total of 154 articles were found with genetic diversity data from 1622 populations, of which the majority (1483 populations) used microsatellite markers. There was large variation in diversity across populations and weak spatial patterns, indicating that local conditions appear to strongly influence genetic diversity as they overwhelm global patterns. There were also significant effects of life history strategy, reproductive mode, and family on clonal and genetic diversity metrics. Lower clonal richness and higher inbreeding coefficient were found in colonising compared to opportunistic and persistent taxa and higher clonal richness was observed in monoecious compared to dioecious and hermaphroditic taxa.
The first field study assessed temporal variation in clonal and genetic diversity, and clone composition of the colonising seagrass H. uninervis in the short (6 months) and long-term (4-6 years) along ~1000 km of West Australian coastline at 15 sites and across a range of disturbance conditions. A total of 44 Single Nucleotide Polymorphisms (SNPs) were used to estimate population genetic diversity. Diversity varied among sites, but varied more for clonal than genetic diversity metrics, e.g. clonal richness ranged from 0.00-0.83 and observed heterozygosity ranged from 0.27-0.34. Clonal and genetic diversity showed unexpected temporal stability over both timescales for all populations, despite temporal changes in clone composition. Furthermore, 19% of clones persisted over 4-6 years, and a maximum clonal age up to 3905 years was estimated.
The second field study characterised genetic diversity and structure, and tested for evidence of local adaptation, in the temperate seagrass A. antarctica across an extreme environmental salinity gradient (> 20) in the Shark Bay World Heritage Site, Australia. In addition, this salinity gradient was characterised and hydrodynamic connectivity among sites was explored through modelling. Amphibolis antarctica was genotyped at 10 sites across this gradient using Double digest Restriction site Associated DNA sequencing (ddRADseq), resulting in 6523 loci. Intermediate levels of genotypic diversity (global R = 0.50 ± 0.06 SE) and observed heterozygosity (global HO = 0.30 ± 0.00 SE), and high genetic connectivity among most sites indicates potential for genetic resilience at a population level with pathways for recovery. Modelled salinity patterns showed extreme differences in mean monthly salinity among (> 20) and within (> 15) sites. Only 11 of the 6523 loci identified as potentially adaptive, but were not associated with salinity, providing no evidence of local adaption, and suggesting phenotypic plasticity to salinity variation instead.
These studies provide important insights into the genetic diversity, population dynamics and longevity of seagrasses and the potential to contribute to population resilience, with implications for seagrass management. First, the finding of large variation in genetic diversity measures and population dynamics, like genet persistence, among populations within species indicate that it is important to collect site-specific genetic data and to understand local conditions when incorporating genetic diversity into seagrass management. Second, the unexpected longevity in a colonising taxa rivalled that found in persistent seagrasses. Since longevity contributes to the stability and persistence of populations over time, these findings indicate that a re-evaluation of the persistence of genets and populations in seagrasses across life history strategies is needed. Third, the temporal stability in diversity observed in H. uninervis indicates it is feasible to include clonal and genetic diversity in seagrass monitoring since genetic assessments can provide an estimate of genetic diversity that is valid for multiple years. Fourth, the persistence of individual genets observed in a colonising taxa means there are opportunities for the assisted recovery of colonising seagrass meadows through the introduction of new clones. Finally, this research showed that the estimated natural recovery period for seagrass can be tens to thousands of years, which indicates that assisted recovery could be a valuable tool within seagrass management.
DOI
10.25958/btdh-0d30
Recommended Citation
Frouws, A. M. (2024). Spatial and temporal variation in population genetic diversity in seagrasses and the implications for resilience. Edith Cowan University. https://doi.org/10.25958/btdh-0d30