Ocean Acidification

Rising carbon dioxide emissions from human activities poses another threat to coral reefs in the form of ocean acidification. More than 30% of anthropogenic carbon dioxide emitted to the atmosphere is taken up by the ocean, a process which lowers the pH Sabine et al (2004). Increased ocean acidity affects coral reefs by reducing the calcification rate of reef builders and making coral susceptible to dissolution. The ability of coral reefs to produce calcium carbonate defines these ecosystems, and ensuring that a net surplus is produced enables reefs to build up. This documentary by the Natural Resource Defence Council provides a good deal of information regarding ocean acidification, and the potential impact to coral reef ecosystems:







Inorganic carbon, which is dissolved in the oceans, is used by coral organisms to deposit calcium carbonate. Increasing uptake of atmospheric carbon dioxide by the oceans requires carbonate ions that would otherwise be used by marine organisms to build shells or skeletons, as in the case of coral reef ecosystems. Decreasing carbonate ion concentrations is therefore likely to cause weak and brittle coral skeletons and slow growth rates. The chemistry of this process is relatively well understood, but the impact it will have on the global coral resource is heavily debated. Gattuso et al (1999) reviewed the carbon and carbonate cycle in coral reefs and the potential effects of human induced environmental change. 

  

Crustose coralline algae are an important calcifying organism in most marine habitats. In coral reefs they produce carbonate sediments and form coral reef structures out of carbonate fragments. Kuffner et al (2007) set up an eight week mesocosm experiment to quantify changes to calcifying components as a result of decreasing calcium carbonate saturation, which occurs with increasing carbon dioxide uptake by the oceans. They showed that the recruitment rate and growth of crustose coralline algae was significantly inhibited in mesocosms that had higher carbon dioxide levels. This study shows that ocean acidification has the potential to cause severe changes to benthic community structure in coral reefs, which will threaten coral reef ecosystems. 

 

This problem is not just restricted to tropical coral reefs, cold-water and deep water corals are severely threatened by ocean acidification. Their slow growth rate and limited ability to recover from disturbances make them more vulnerable to the effects of human activities. Turley et al (2007)  review the potential impact of ocean acidification on cold water corals, highlighting the lack of knowledge regarding these inaccessible ecosystems. They refer to a study by Guinotte et al (2006)  which predicts that 70% of known cold water coral ecosystems will be in water that has very low carbonate ion saturation levels, meaning it is unlikely that the corals will be capable of calcifying.
 


The ability of coral reefs to form calcium carbonate rapidly ensures that they are able to migrate upwards in order to adapt to changing sea levels. The threat posed to coral reef ecosystems by ocean acidification will limit their ability to adapt and could result in significant global losses of coral reef ecosystems. However, the impact of ocean acidification is not restricted to the reef building organisms. Many coral reef species produce calcium carbonate shells or skeletons that are central to their survival. Although there has been little research carried out on these species, Klepyas and Yates (2009) summarise knowledge regarding the effects of ocean acidification on these taxa in a special feature in the journal Oceanography. Deep sea sediment cores have provided a lot of information regarding past ocean acidification events, and have enabled an understanding of how these important coral species may be affected. Kump et al (2009) compares previous ocean acidification events to the human induced changes that are occurring today. Once again it appears the environmental changes that are taking place in our oceans are occurring at an unprecedented rate, a fact which questions the ability of coral reef ecosystems to adapt and survive.