Vulnerability to future environmental conditions and population genetics of the seagrass, Zostera capensis
Thesis (MSc)--Stellenbosch University, 2016.
ENGLISH ABSTRACT: Climate and anthropogenic-associated environmental changes are pressuring coastal ecosystems and their services. Genetic diversity is one aspect providing resilience for species to persist through environmental perturbations and maintain functional roles in ecosystems. Seagrasses are a poorly studied yet vital component of coastal systems, serving as spawning grounds and nurseries for ecologically and commercially important species. As such, this thesis studied the distribution determinants of the seagrass, Zostera capensis, in order to assess the potential impact of environmental change on its distribution in South Africa. Further, I investigated the population genetics of Z. capensis in southern Africa. Generalised additive modelling was applied in chapter one to quantify the relative importance of likely determinants of the contemporary distribution of Z. capensis, then to project its potential future distribution to examine its vulnerability to projected South African environmental conditions. Estuarine mouth state and precipitation were major determinants of contemporary Z. capensis distribution. My model projected the loss of significant areas (30%) of suitable habitat for Z. capensis near current range limits, placing the species at increased risk of contraction into a diminishing range of only a few estuaries on the south-eastern coast. These results align with projections of diminishing ranges for other seagrasses globally and raise concerns about future declines in extent and quality of seagrass habitat, particularly the negative effects on coastal biodiversity, commercially important fish populations with seagrass-dependant juvenile stages and trophic transfers to adjacent marine ecosystems. I recommend that marine spatial and biodiversity management planning be guided by assessing the on-going status of such species through monitoring, while also targeting seagrass protection in southerly sites to optimize future resilience. Chapter two aimed to understand genetic diversity, gene flow and population structure of Z. capensis populations in southern African using microsatellite markers designed for a sister species, ITS and matK regions. Although many seagrasses reproduce largely vegetatively, species such as Zostera marina display high levels of clonal diversity, significant isolation by distance and strong population structure at both regional and fine scales. As Z. capensis is found across a broad range of temperature and environmental conditions, and across South African bioregions, I expected to find similar trends to those observed in Z. marina. Sequences from the matK region confirmed the species identification of all samples and showed little to no variation across all populations. ITS was excluded as multiple fragments of similar size were amplified by this marker. The nine microsatellites markers were unsuccessful at amplifying Z. capensis, and due to time and financial constraints, new microsatellite markers could not be designed. The lack of observed variation suggests a high level of connectivity, gene flow, clonal reproduction or selection, but is more likely due to insufficient marker resolution. In this region genetic structuring of estuarine species often corresponds with the three main biogeographic regions and physical barriers to gene flow. Future research using higher resolution markers, such as next-generation sequencing, may reveal more about the vulnerability, resilience and adaptations of Z. capensis, with implications for its future.
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