Assessing the risk of ocean acidification for scleractinian corals on the Great Barrier Reef
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Ocean acidification will impact the photo-physiology of reef-building corals as it can lead to dysfunction of the symbiosis and loss of productivity. The major objective of this thesis was to provide insight into the mechanism of CO₂₋induced bleaching and productivity loss across multiple life-history stages and interpret these findings in an ecological context. Chapter 1 provides a review of the literature investigating the photo-physiological impact of ocean acidification, emphasizing the experimental conditions in studies that observed Symbiodinium dysfunction and productivity loss. Chapter 2 presents a working hypothesis to describe the fundamental physiological aspects of coral bleaching under ocean acidification. This research investigates the response of Acropora aspera using pulse amplitude modulation (PAM) fluorometry and oxygen respirometry under increased pCO₂ with concomitant high light conditions. The dinoflagellate density and HPLC pigment analysis are utilised to characterise the CO₂₋induced bleaching response. We present a conceptual model linking photorespiration to CO₂₋induced bleaching and productivity loss. The impact of ocean acidification on coral reef ecosystems is likely to deviate from oceanic climate models due to diel modification of carbonate chemistry by community metabolism. Chapter 3 characterises the diurnal variation in carbonate chemistry at sites around Lizard Island and links this to the ocean acidification response of Acropora millepora collected from these sites. Furthermore, we utilise permutational multivariate statistical analyses to partition the variation in carbonate chemistry attributable to community composition at these sites. It was hypothesized that greater diurnal variation in carbonate chemistry may improve resilience of scleractinian corals to future ocean acidification conditions. This chapter highlights that site-specific physiological trade-offs may influence the response of reef-building corals to future ocean acidification scenarios. Chapter 4 reports a visual bleaching response in A. millepora juveniles under future ocean acidification conditions. The effect of ocean acidification on coral juveniles is hypothesised to impact Symbiodinium uptake and photochemical efficiency. We utilised the iPAM to align the photochemistry in the juveniles with their visual bleaching response and Symbiodnium type, as assessed by denaturing gradient gel electrophoresis (DGGE) of the internal transcribed spacer region 1 (ITS1) of the ribosomal genes. This study links the bleaching response with recruits containing a dominant population of Symbiodinium type D1 or D1-4, with potential implications for post-settlement survivorship and population dynamics. Lastly, in Chapter 5 the key findings of this thesis are discussed in light of the ecological implications for the Great Barrier Reef. The synopsis outlines the effect of ocean acidification on the photo-physiology, productivity, calcification, reproduction and symbiont acquisition of reef-building corals. Future avenues for research are suggested based on new research gaps revealed by this thesis with the aim to continue to provide up-to-date scientific information to policy makers and reef managers.
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