Browsing by Author "Mokumo, Mosihla Frederick"

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    Linking physiological, genomic, and ecological functioning in the seagrass, Zostera capensis
    (Stellenbosch : Stellenbosch University, 2024-03) Mokumo, Mosihla Frederick; Von der Heyden, Sophie ; Midgley, Guy F. ; Adams, Janine Barbara; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: In South Africa, the most abundant seagrass, Zostera capensis, occurs in predominantly open and sheltered estuaries and carpets intertidal and subtidal zones. Zostera capensis exhibit a wide yet discontinuous distributional range, limited to estuaries in the cool-temperate biogeographic region along the west coast as well as estuaries in the warm-temperate south coast and the sub-tropical east coast of South Africa. Due to its continued population decline, small area of occupancy and its extirpation status in Durban Bay and St Lucia, Z. capensis is now categorised as Endangered. In South Africa, Z. capensis serves as nursery and foraging grounds for organisms of high conservation importance such as the seahorse Hippocampus capensis, pipefish Syngnathus watermeyeri and klipfish Clinus spatulatus, among others. With over 90 years of research, however, Z. capensis remains poorly protected and severely fragmented, with continued exposure to anthropogenically induced pressures, where eutrophication (leading to agal bloom) and effects of heavy exploitation for bait considered the main pressures in systems such as the Knysna Estuarine Bay. Currently, less is known regarding intertidal and subtidal differences and their responses to global climate change. South Africa is also trailing behind in terms of restoration and rehabilitation trials. Considering these scientific gaps and the continued decline in Z. capensis, this PhD took a broad experimental approach to investigate aspects around morpho-physiological variations between intertidal and subtidal plants, their photophysiological and genotypic responses to thermal stress, as well as investigating transplantation as a mechanism for seagrass conservation. To understand the morpho-physiological responses of plants to their local environmental conditions, an in situ study was conducted in a permanently open estuary (Knysna Estuarine Bay) and a temporarily closed estuary (TCE), Klein Brak Estuary. Generally, seagrasses with ecotypes spanning across bathymetric cline exhibit intertidal ecotypes (exposed twice daily), which are exposed to fluctuating irradiance and temperature regime, and subtidal ecotypes (always submerged) which experience relatively narrower temperature changes but affected by light attenuation, especially in eutrophic systems. Therefore, both ecotypes or plants show morphological and photophysiological differences, with intertidal ecotypes expected to show narrower and shorter leaf width and length (to decrease leaf surface area for light absorption), decreased photophysiological responses (Fv/Fm) but increased shoot and leaf density and the subsequent seagrass %cover (to induce self-shading). These response mechanisms are important for seagrass meadows exposed to high temperatures and light irradiance. For subtidal ecotypes in light attenuated areas, meadows usually show increased leaf length and width as a mechanism to capture light needed for photochemistry. In addition, ecotypes show decreased leaf and shoot density to reduce competition for light, this also affect their %cover which is then decreased in these ecotypes. Seagrass responses occur at leaf-scale (Fv/Fm, and all photosynthesis-related changes), shoot-scale (leaf length and width) and meadow-scale (shoot and leaf density and %cover). To investigate these differences in Knysna, leaf-, shoot-, and meadow-scale responses were determined in each tidal zone at the upper, middle, and lower reaches. Results showed that seagrass responses are localised due to topographical attributes of the estuary and that leaf-scale responses, which are modulated in seconds, were not significantly different at upper estuary, with Fv/Fm values in intertidal ecotypes decreased due to higher light intensities measured at the time of sampling. Results also showed that both intertidal and subtidal plants at the middle estuary suffered mechanical damage, with intertidal plants at the lower estuary showing significantly lower %cover due to anthropogenic effects and sandy and mobile sediment. In Klein Brak Estuary, the same methods were used, however, the estuary was experiencing a drought event and closed estuary moth, therefore, there was no tidal influences. However, exposed plants showed higher Fv/Fm than submerged plants. Nonetheless, shoot- and meadow-scale responses were consistent with other studies. However, the seagrass in this estuary is highly variable due to drought and flooding events associated with South African estuaries, especially TCEs. Collectively, these results formed the basis and highlighted the need for spatiotemporal and seasonal monitoring of Z. capensis to identify meadows that are more permanent and can be used as donor sites for restoration trials. This PhD also investigated the use of different planting patterns (straight-line, compact, and star) and core sizes (11, 18, and 25 cm Ø) in transplanting intertidal and subtidal plants in the Knysna and Klein Brak estuaries in South Africa. Planting patterns, ecotypes and size did not influence persistence; however, smaller cores should be used as these have less impact on donor meadows. Although transplants did not persist for more than three months in both estuaries, our study demonstrated that seagrass restoration in South Africa is challenging due to limited suitable habitats (in Knysna) and strong environmental variability (in Klein Brak) in estuarine ecosystems. Results also showed that restoration trials be prioritised in predominantly open estuaries and that monitoring of donor sites is an important aspect to consider in restoration studies. This PhD also investigated the transcriptomic and photophysiological (Fv/Fm) responses of intertidal and subtidal plants to a simulated acute MHW (36 °C), with results showing that plants respond in a similar manner as no differentially expressed genes (DEGs) were detected, though Fv/Fm showed significant differences between plants during the MHW. However, DEGs were observed during the MHW and after the seven-days long recovery phase. Generally, Z. capensis show a downregulation of DEGs than upregulation following a heat stress. During the recovery phase, Fv/Fm was significantly decreased in both plants compared to control condition. This was complemented by the DEGs which showed downregulation of genes associated with Photosystem I and II, the most sensitive yet important machinery in photochemistry. These results showed that an acute 36 °C is above the tolerant threshold for Z. capensis, and that the species will disappear under these conditions.