Ecological and physiological responses of hard corals to variations in seawater carbonate chemistry

It is predicted that ocean acidification (OA) threatens coral reefs worldwide, by lowering seawater pH which in turn compromises essential metabolic processes such as carbonate genesis of corals. Inshore waters however, experience different spatial and temporal carbonate chemistry variability, raising questions over the future impact of OA within these habitats. It also remains unclear whether local biogeochemical conditions of some marine habitats can buffer, or provide a refuge against OA. The thesis systematically examines the response (ecological abundance, distribution, recruitment, and metabolic expenditure) of corals that have expanded their niche into variable pH habitats, to assess both the potential impact of OA and whether any habitats may act as a refuge against its effects by: (i) establishing robust methods to measure the local carbonate chemistry and the metabolic activity of corals in situ, (ii) characterising the natural carbonate chemistry variability over different temporal and spatial scales, and evaluating the biological versus abiotic control of non-reef habitats, (iii) quantifying the metabolic expenditure of corals living within non-reef habitats and assessing whether there are similarities in the physiological responses of corals existing in different regions to ascertain commonalities, and finally (iv) testing the impact of future predicted changes in temperature and pH on the physiological responses of corals from different variability habitats. The thesis demonstrates that across bioregion sites non-reef habitats exist that have highly variable carbonate chemistry but still house corals. These non-reef habitats have very different carbonate chemistry, influencing both their own susceptibility to future OA and their potential services (buffering versus pre-conditioning) for local coral populations. Future studies can expand on this work by assessing the molecular differences of corals found within these highly-variable habitats to explore further the potential of adaptation and/or acclimatisation of coral species to low pH.