| Summary: | The diurnal fluctuations in seawater carbon chemistry that occur at sites within a coral reef system are due to water circulation, air-sea heat and gas exchange, and biogeochemical processes. The daily changes in dissolved carbon ion speciation and pHT on reefs can be larger than the century-scale shifts predicted for open ocean waters under climate change scenarios. We implement a 167m resolution 3D hydrodynamic model of a coral reef (Heron Island reef, southern Great Barrier Reef, Australia) and couple it to a carbon biogeochemistry model. The model is forced by benthic fluxes calculated using a detailed habitat map and light and habitat-dependent parameterisations of calcification/photosynthesis developed from flume studies on Heron Island. During a two month simulation the model is able to reproduce the observed variability in the water temperature at 8 locations within the reef, demonstrating an ability of the model to capture the circulation. The simulation shows that the dominant processes driving the variability in carbonate chemistry at a location on the reef are the location of the different benthic communities and the path the water has taken to arrive at that location. A spatially-resolved age tracer indicates that the residence time of water over the reef varied between 16 and 60h, depending on tides, winds and location. The longer transit times over the reef reduced the aragonite saturation state, Ωa, in the overlying water to as low as 2. In the model simulation, the reef ecosystem reduces Ωa at a rate of 0.018 per hour on the reef. A scenario in which we removed the non-calcifying benthic microalgae from the reef showed a reduction in the Ωa in some regions from 5 to 2.5, and an average reduction of the coral calcification by 15%. This demonstrates the importance of processes that can alter the photosynthesis/respiration and calcification/dissolution balance when considering climate change impacts due to ocean acidification on corals at the reef scale. Carbon chemistry; Coral reef; ...
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