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Coral reefs are the result of long-term net calcium carbonate (CaCO3) accretion and the balance between carbonate production and erosion, referred to as the carbonate budget. Coral calcification is influenced by environmental conditions (e.g. temperature, depth and ocean acidification) and depends on a symbiotic partnership between the coral and its dinoflagellate algae in the family Symbiodiniaceae. When a coral encounters stress, the symbiosis breaks down resulting in coral bleaching which impacts reef calcification. To effectively monitor reef health, it is essential to determine the current status of reef environments and whether reef frameworks are actively accreting CaCO3 or being eroded away. This study aims to determine reef accretional health in three bioregions: Bahrain (located in the hottest sea on the planet – the Arabian Gulf), the Seychelles (with an environment susceptible to regular El Nino disturbances - Western Indian Ocean) and Indonesia (located within the epicenter of marine biodiversity, the Coral Triangle - Central Indo-Pacific). In addition, genetic diversity of selected coral symbionts is investigated. In light of predicted warming trends, which present a threat to the structural integrity of reefs worldwide, questions arise whether reefs will be able to maintain a positive carbonate budgetary state to keep pace with future sea level rise and maintain the stability of island-nations such as Bahrain, the Seychelles and Indonesia. This thesis explores the past and present trajectories of reefs within these bioregions whilst providing important insights for consideration to model future trajectories of these reefs.