UC San Diego

Coral reefs are under threat from global environmental perturbations including ocean warming, acidification, and deoxygenation. The degree to which these perturbations will affect coral reefs is dependent on a range of factors including the local hydrodynamics and biogeochemical processes, both which vary widely across space and time. Consequently, not all coral reefs will be affected equally due to differences in local properties and processes. To develop predictive capacity for how coral reefs will be affected by ocean acidification (OA) it is necessary to first understand the current seawater CO2 chemistry variability and drivers. In this dissertation, autonomous sensors and discrete seawater samples were used to characterize the natural spatial and temporal variability of seawater CO2 chemistry across different reef scales and habitats in Australia and South East Asia (i.e., Heron Island, Great Barrier Reef; Dongsha Atoll and Taiping Island, South China Sea; and Onna-son Reef, Okinawa). In Heron Island, the largest spatial and temporal variability in seawater chemistry was associated with the most shallow, western region of the platform that also had the longest residence time. Interactions between reef geomorphology and timing of the tidal cycle greatly influenced chemical gradients and variability leading to some unexpected trends and patterns. On Dongsha, elevated pH and aragonite saturation state (ΩAr) were observed inside a semi-enclosed lagoon during both day and night. Future projections showed that this environment will not cross, detrimental aragonite thresholds (e.g., ΩAr < 2.92) as frequently as patch reefs and the large-scale lagoon of Dongsha. However, concurrent high water temperature and hypoxia indicated that this environment will not offer respite to taxa sensitive to OA. In Onna-son, the influence of seaweed cultivation on seawater chemistry during spring was compared to times of no cultivation during fall and winter. pH elevation was observed during both spring (+0.13 units) and fall (+0.10 units), but it was not possible to separate the role of cultivated seaweed from natural taxa. This dissertation demonstrates current coral reef seawater CO2 chemistry conditions and biochemical function, information that will be critical for making rigorous projections for the future.