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Dissolved CO2 and Oxygen Dynamics on Coral Reefs: from Natural Variability and Impacts on Calcification to Projections under Warming


Coral reefs globally are facing impacts from ocean warming, acidification, and oxygen loss as a result of anthropogenic climate change. Understanding the spatiotemporal patterns of reef carbonate chemistry and oxygen variability, as well as how low pH or oxygen conditions might affect coral physiology, is key to predicting how global reefs will be impacted in the future. In this dissertation, I leveraged dissolved oxygen data from autonomous sensors deployed at 32 sites around the world to explore present-day oxygen variability and project changes in hypoxia exposure under modeled ocean warming. I show that hypoxia is pervasive on global coral reefs, with 84 % of the reef habitats surveyed experiencing weak to moderate hypoxia and 13 % experiencing severe hypoxia under present-day conditions. Calculations of reef oxygen loss under 5 warming scenarios reveal that warming will increase the duration, intensity, and severity of hypoxic events on reefs, leading to severely hypoxic conditions on more than a third of these reef habitats by 2100. In case studies of reefs in Bermuda and Taiwan, I examined multidimensional variability in carbonate chemistry and oxygen across a reef and assessed the potential for seagrass beds to serve as refugia for corals from ocean acidification and deoxygenation. In Bermuda, data from spatial seawater surveys and a suite of autonomous sensors at the surface and benthos revealed strong signals of both benthic and water column productivity that interacted with local geomorphology and hydrodynamics to create the observed patterns in carbonate chemistry and oxygen across the reef. In Taiwan, strong gradients in temperature, pH, and oxygen across the seagrass bed were associated with significant differences in coral skeletal extension rate, density, and ∂13C isotopic composition measured from coral cores. However, there was no evidence that the presence of seagrass significantly impacted coral calcification rates along this gradient. Altogether, this dissertation provides projections of coral reef oxygen loss under rapid climate change and highlights the contributions of local conditions to observed variability in seawater chemistry with complex impacts on coral growth.

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