The inner Great Barrier Reef (GBR) is at risk of faster dissolution due to ocean acidification than previously thought, according to a study published in Nature Communications this week. The findings highlight the most vulnerable regions of the GBR to ocean acidification, knowledge of which is essential to effectively target conservation and management strategies.
Corals typically build their skeletons with aragonite, a form of calcium carbonate (CaCO3). However, ocean acidification - the reduction in ocean pH as a result of increasing levels of atmospheric CO2 - is altering the chemistry of seawater such that CaCO3 is less able to be deposited from solution. This implies that the so-called ‘aragonite saturation state’ of seawater, a measure of the ability of CaCO3 to be deposited, is being progressively reduced. As aragonite saturation declines, it becomes more difficult for corals to build their skeletons, and the risk of dissolution increases. Knowledge of aragonite saturation across the GBR has previously been limited to a few individual reefs, and the drivers of any spatial variability have been little-known until now.
Mathieu Mongin and colleagues use a regional, coupled ocean circulation and biogeochemical model, in combination with field observations, to estimate aragonite saturation within the entire GBR, based on data from 3,581 reefs. The authors show that there is up to about 50% more spatial variability in aragonite saturation levels than previously thought, and that this variability is mostly governed by the depletion of carbonate ions through coral growth upstream (mainly in reefs in the north and along the edge of the GBR), indicating that inner and southern reefs are at a greater risk of dissolution. This increased variability suggests that projected changes in aragonite levels, and thus impacts on the GBR, may be greater than anticipated.
Changes in aragonite saturation are just one of many factors that govern the ecological state of the GBR, and accurate projections of GBR vulnerability also require consideration of regional carbon cycles, calcification and dissolution processes, and other human pressures, caution the authors.