Chemistry:Impacts of ocean acidification on the Great Barrier Reef

Ocean acidification threatens the Great Barrier Reef by reducing the viability and strength of coral reefs. The Great Barrier Reef, considered one of the seven natural wonders of the world and a biodiversity hotspot, is located in Australia. Similar to other coral reefs, it is experiencing degradation due to ocean acidification. Ocean acidification results from a rise in atmospheric carbon dioxide, which is taken up by the ocean. This process can increase sea surface temperature, decrease aragonite, and lower the pH of the ocean.

Calcifying organisms are under risk, due to the resulting lack of aragonite in the water and the decreasing pH. This decreased health of coral reefs, particularly the Great Barrier Reef, can result in reduced biodiversity. Organisms can become stressed due to ocean acidification and the disappearance of healthy coral reefs, such as the Great Barrier Reef, is a loss of habitat for several taxa.

Map of the Great Barrier Reef

Background

Atmospheric carbon dioxide has risen from 280 to 380 ppm since the industrial revolution. This increase in carbon dioxide has led to a 0.1 decrease in pH, and it could decrease by 0.5 by 2100. When carbon dioxide meets seawater it forms carbonic acid, which then dissociates into hydrogen, bicarbonate, and carbonate and lowers the pH of the ocean. Sea surface temperature, ocean acidity, and dissolved inorganic carbon are also positively correlated with atmospheric carbon dioxide. Ocean acidification can cause hypercapnia and increase stress in marine organisms, thereby leading to decreasing biodiversity. Coral reefs themselves can also be negatively affected by ocean acidification, as calcification rates decrease as acidity increases.

Aragonite is impacted by the process of ocean acidification, because it is a form of calcium carbonate. It is essential in coral viability and health, because it is found in coral skeletons and is more readily soluble than calcite. Increasing carbon dioxide levels can reduce coral growth rates from 9 to 56%. Other calcifying organisms, such as bivalves and gastropods, experience negative effects due to ocean acidification as well.

As a biodiversity hotspot, the many taxa of the Great Barrier Reef are threatened by ocean acidification. Rare and endemic species are in greater danger due to ocean acidification, because they rely upon the Great Barrier Reef more extensively. Additionally, the risk of coral reefs collapsing due to acidification poses a threat to biodiversity. The stress of ocean acidification could also negatively affect biological processes, such as photosynthesis or reproduction, and allow organisms to become vulnerable to disease.

Coral health

Calcification and aragonite

Coral is a calcifying organism, putting it at high risk for decay and slow growth rates as ocean acidification increases. Aragonite, which impacts the ability of coral to take up CaCO₃, decreases when pH decreases. Levels of aragonite have decreased by 16% since industrialization, and could be lower in some portions of the Great Barrier Reef because the current allows northern corals to take up more aragonite than the southern corals. Aragonite is predicted to reduce by 0.1 by 2100. Since 1990, calcification rates of Porites, a common large reef-building coral in the Great Barrier Reef, have decreased by 14.2% annually. Aragonite levels across the Great Barrier Reef itself are not equal; due to currents and circulation, some portions of the Great Barrier Reef can have half as much aragonite as others. Levels of aragonite are also affected by calcification and production, which can vary from reef to reef. If atmospheric carbon dioxide reaches 560 ppm, most ocean surface waters will be adversely undersaturated with respect to aragonite and the pH will have reduced by about 0.24 units – from almost 8.2 today to just over 7.9. At this point (sometime in the third quarter of this century at current rates of increase) only a few parts of the Pacific will have levels of aragonite saturation adequate for coral growth. Additionally, if atmospheric carbon dioxide reaches 800 ppm, the ocean surface water pH decrease will be 0.4 units and total dissolved carbonate ion concentration will have decreased by at least 60%. At this point it is almost certain that all reefs of the world will be in erosional states. Increasing the pH and replicating pre-industrialization ocean chemistry conditions in the Great Barrier Reef, however, led to an increase in coral growth rates by 7%.

Temperature

Ocean acidification can also lead to increased sea surface temperature. An increase of about 1 or 2 °C can cause the collapse of the relationship between coral and zooxanthellae, possibly leading to bleaching. Average sea surface temperature in the Great Barrier Reef is predicted to increase between 1 and 3 °C by 2100. This breakdown of the relationship between the coral and the zooxanthellae occurs when Photosystem II is damaged, either due to a reaction with the D1 protein or a lack of carbon dioxide fixation; these result in a lack of photosynthesis and can lead to bleaching.

Reproduction

Ocean acidification threatens coral reproduction throughout almost all aspects of the process.Gametogenesis may be indirectly affected by coral bleaching. Additionally, the stress that acidification puts on coral can potentially harm the viability of the sperm released. Larvae can also be affected by this process; metabolism and settlement cues could be altered, changing the size of population or viability of reproduction. Other species of calcifying larvae have shown reduced growth rates under ocean acidification scenarios. Biofilm, a bioindicator for oceanic conditions, underwent reduced growth rate and altered composition in acidification, possibly affecting larval settlement on the biofilm itself.

Biodiversity

The Great Barrier Reef is a biodiversity hotspot, but it is threatened by ocean acidification and its resulting increased temperature and reduced levels of aragonite. Elasmobranchs in the Great Barrier Reef are vulnerable to ocean acidification primarily due to their reliance on the habitat and ocean acidification's destruction of coral reefs. Rare and endemic species, such as the porcupine ray, are at high risk as well. Larval health and settlement of both calcifying and non-calcifying organisms can be harmed by ocean acidification. A predator to coral reefs in the Great Barrier Reef, the Crown of Thorns sea star, has experienced a similar death rate to the coral on which it feeds. Any increase in nutrients, possibly from river run-off, can positively affect the Crown of Thorns and lead to further destruction of the coral.

Coralline algae holds together some coral reefs and is present in multiple ecosystems. As ocean acidification intensifies, however, it will not respond well and could damage the viability and structural integrity of coral reefs. Ocean acidification can also indirectly affect any organism; increased stress can reduce photosynthesis and reproduction, or make organisms more vulnerable to disease. Additionally, as coral reefs decay, their symbiotic relationships and residents will have to adapt or find new habitats on which to rely.

Organisms have been found to be more sensitive to the effects of ocean acidification in early, larval or planktonic stages. As ocean acidification does not exist in a vacuum, the multiple problems facing the Great Barrier Reef combine to further stress the organisms. Not only can ocean acidification affect habitat and development, but it can also affect how organisms view predators and conspecifics. Studies on the effects of ocean acidification have not been performed on long enough time scales to see if organisms can adapt to these conditions. However, ocean acidification is predicted to occur at a rate that evolution cannot match. Increasing temperature is also affecting the behavior and fitness of the common coral trout, a very important fish in sustaining the health of coral reefs.