Impact of the local environmental variability on the patterns of coral recruitment on Indo-Pacific reefs
Authors:   , Boschetti Simona Tiziana 
Published: November 2021

<p>Coral reefs are threatened by a range of human activities at both local and global scales. The result of these impacts has resulted in a worldwide decline in the coral reef ecosystems. Corals are the principle reef builders and the maintenance of their populations is fundamental for healthy reef ecosystems. Local environmental factors are critically important in shaping coral populations, particularly at the post-settlement phase, when young coral colonies are most vulnerable to disturbances. In this context, understanding the environmental factors that drive coral recruitment and affect coral survivorship in the early life history stages is vital to effectively manage coral reefs.  In this thesis I began by investigating the effect of abiotic and biological factors on coral recruitment and juvenile coral life history stages using settlement panels deployed in the Wakatobi Marine National Park (SE Sulawesi, Indonesia). My objectives were to assess the spatio-temporal variability in coral recruitment rates and juvenile abundance. I used a modelling approach to identify the environmental factors that affected the distribution and abundance patterns of corals. Then, I focused on the main environmental factors, identified from previously published research, affecting coral recruitment. I conducted a caging experiment to assess the impact of fish predation on coral juveniles. Finally, I analysed the development of the benthic community and the interactions between corals and benthic organisms in the first two years of colonisation of artificial bare surfaces.  I found high spatial and temporal variability in recruitment rates over seven years of data, values were lower than on other Indo-Pacific reefs and ranged from 9.6 (±8.21 SE) to 317.19 (±12.76 SE) rec. m⁻²; while juvenile abundance ranged from 4.2 (±1.49 SE) to 33 (±6.36 SE) juv. m⁻². The local characteristics of the sites, such as coral cover, influenced the distribution of coral colonies in early life history stages; furthermore differences in coral density between the two life history stages (juvenile and recruits) were consistent over time. However, no single or combination of factors adequately explained abundance patterns for either recruits or juveniles. Fish predation did not appear to be the main cause of coral post-settlement mortality in the Wakatobi and it affected only 10.8% of the coral juveniles in the experiment. In contrast, 58.51% of the coral juveniles were found to be overgrown by algae and other invertebrates, however only turf and green encrusting algae affected coral survivorship. Coral colony abundance and the number of interactions with other benthic organisms, especially crustose coralline algae (CCA) and sponges, increased over time on panels and they were different between the front and back side of the panels, which was attributed to differences in light and predation regimes. Coral recruitment was higher on older benthic communities, although none of the known coral recruitment promoters, such as CCA, or competitors, such as turf algae, were correlated with coral abundance.  My results show that changes in coral populations between the recruit and juvenile stages are likely driven by small-scale processes. The site characteristics determine the final patterns, which vary over time following temporal fluctuations in environmental factors. The effect of the interactions between algae and sponges with coral recruits and their influence on juvenile survivorship suggests these organisms having a role in coral recruitment success and highlight their importance as a focus for reef management. Furthermore, the use of long term studies allowed a better understanding of the high variability present in coral recruitment and the trends of the recruitment process, which are useful information for conservative purposes.</p>