The exposure of the Great Barrier Reef to ocean acidification

Chair: James Orr

Mathieu Mongin(1), Mark E. Baird (1), Bronte Tilbrook (1, 2), Richard J. Matear (1), Andrew Lenton (1), Mike Herzfeld (1), Karen A. Wild-Allen (1), Jenny Skerratt (1), Nugzar Margvelashvili (1), Barbara J. Robson (3), Carlos M. Duarte (4), Malin S. M. Gustafsson (5), Peter J. Ralph (5), Andrew D. L. Steven (1)

1 CSIRO Oceans and Atmosphere Flagship, Hobart, Australia
2 Antarctic Climate and Ecosystems Co-operative Research Centre, Hobart, Australia
3 CSIRO Land and Water Flagship, Canberra, Australia
4 Red Sea Research Center, King Abdullah University of Science and Technology, Kingdom of Saudi Arabia
5 Plant Functional Biology and Climate Change Cluster (C3), Faculty of Science, University of Technology Sydney, Australia

The Great Barrier Reef (GBR), one of the world’s iconic marine ecosystems, is founded on reef-building corals1. Corals typically build their exoskeleton with aragonite – a crystalised form of calcium carbonate – but ocean acidification is lowering the pH and aragonite saturation state of seawater (Ωa)2. The downscaling of ocean acidification projections from the global scale to the GBR requires that the diverse set of coastal-shelf-open ocean drivers controlling Ωa be resolved3. In a novel approach, we used observations4 and a hindcast of a regional coupled circulation–biogeochemical model to estimate the mean Ωa experienced by 3,581 individual reefs of the GBR, and to apportion the relative contributions of the hydrological cycle, regional hydrodynamics and metabolism on Ωa variability. The coupled model shows a clear spatial structure in Ωa, with large gradients across and along the GBR shelf. We find more detail, and a greater range (1.43), than found in previously-compiled coarse model-generated maps of Ωa of the region (0.4)5, or in sparse observations (0.51)4. Most of the variability in Wa was due processes occurring upstream of the reef in question. By quantifying the relationship between the open-ocean and the GBR we see that the critical thresholds for coral health are likely to be passed sooner than currently projected by the IPCC assessment report.