Corals and especially the Great Barrier Reef (GBR) is at a much higher risk due to ocean acidification, a new study has found. The finding highlights the most vulnerable regions to the phenomenon, something which is essential for future conservation measures.

Simulated color of the ocean over the Great Barrier Reef. Images are constructed by combining the optically active constituents in the water simulated by the eReefs models. Green hues represent phytoplankton blooming, dark blues the deep ocean waters, and browns the sediments. Credit: Mathieu Mongin.

Some call it “the evil twin of global warming”, while others call it “the other CO2 problem” – ocean acidification is often underlooked, although it threatens to do massive damage to the ocean’s inhabitants.

How it happens

When we emit carbon dioxide (CO2) into the atmosphere, not all of it stays there as a greenhouse gas. An estimate 30-40% of the carbon dioxide from human activity released into the atmosphere dissolves into oceans, rivers, and lakes. Due to its chemical characteristics, much of it interacts with water to form carbonic acid. Some of these extra carbonic acid molecules react with a water molecule to give a bicarbonate ion and a hydronium ion, thus increasing ocean acidity (H+ ion concentration).

Since the industrial revolution began, it is estimated that surface ocean pH has dropped by slightly more than 0.1 units on the logarithmic scale of pH, representing about a 29% increase in H+. This is already causing massive problems, and researchers believe the worst is yet to come.

Why this matters – a lot

There’s not a creature in the oceans that is unaffected by this increase in acidification. Depressing metabolic rates in jumbo squid, depressing the immune responses of blue mussels, and coral bleaching are just a few in a sea of problems. But the largest issue is with calcifying organisms.

Many of the ocean’s creatures have carbonate shells – they rely on synthesizing the calcium and carbon shell. The thing is, the more acidic the environment is, the higher the concentration of calcium needed to synthesize the shell. In other words, the more acidification in the oceans, the harder it is for animals to create their shells.

Corals typically build their skeletons with aragonite, a form of calcium carbonate (CaCO3) so they fall into this category. Atmospheric CO2 is altering the chemistry of the sea water, so 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.

What the study found

Like most processes that happen on a global scale, there is a great deal of heterogeneity between different areas. Mathieu Mongin and his colleagues from CSIRO Oceans and Atmosphere in Australia combined modeled data with field observations to determine how this variability is expressed for the GBR. 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). Southern reefs are at a much higher risk than previously anticipated.

However authors warn that this is just the picture painted by the acidification – there are also other factors to be taken into consideration, especially human pressures. Still, this is very concerning news. More variability doesn’t bode well when they factored in models of climate change over the next century. Even the best possible future carbon emissions scenario may produce significant losses on the Great Barrier Reef, the researchers write.