How Ocean Acidification can have long term effect on Corals

 

Ocean water is saturated with calcium carbonate. This calcium is useful for many species like corals and planktons. The calcium in the water contributes mainly to the formation of skeletons of the corals. But the increasing carbon dioxide in the atmosphere, the concentration of carbonates and the pH of ocean water is decreasing eventually decreasing the calcium carbonates. Researchers predict that if the saturation keeps on decreasing then corals will face a great problem in future. The Southern Hemisphere Oceans have shown a decrease in carbonate saturation. By 2100, there might be a chance that corals become extinct or extremely endangered in the Southern Oceans. 

Ocean uptakes atmospheric CO2. This helps to moderate future climate changes. But when CO2 enters the ocean water, it undergoes hydrolysis and releases a hydrogen ion. As the concentration of H+ ions increases, the pH decreases. The pH of surface ocean water has already decreased by 0.1 units in the last few decades. It is predicted that by the end of this century the pH will decrease by 0.3-0.5 units. The numbers might look very small, but a decrease by 0.3 to 0.5 units mean about a 100% to 150% increase in the concentration of hydrogen ions. An increase in pH means a decrease in the rate of formation of biogenic calcium carbonates.

This calcium carbonate is used to form reefs out of aragonite and in planktons, it is used to form shells. In the laws of nature, everything must be in balance. This theory also applies here. If ocean water becomes supersaturated with carbonates, corals will face great damage. Earlier it was predicted that carbonate saturation will decrease in cold surface ocean water when the atmospheric carbon dioxide reaches 1200 p.p.m.v. But recent studies show that the polar regions will be undersaturated when the concentration reaches around 600 p.p.m.v, which means in the next 50 years, there will be a major decrease in the pH of these surface water. It is predicted that the concentration of carbon dioxide will reach 788 p.p.m.v in the next few decades.

According to the survey of WOCE (World Ocean Circulation Survey) and the (JGOFS) Joint Global Ocean Flux Studies, the average concentration of carbonates required for proper growth of corals and other species is 105 microM/kg for southern oceans and 230 microM/kg for tropical ocean waters. The lower concentration in the southern ocean is due to two factors, cold polar water (low surface temperature) and a large amount of upwelled bottom water. Since the last century, the concentration of carbonates in the surface water is already decreased by 10%, which means the decrease of 29 microM/kg in tropical water and 18 microM/kg in the southern ocean. By the end of this century, the concentration of carbonates will be reduced by 50%. As the amount of carbon dioxide in the atmosphere is increasing day by day, the rate of decrement in concentration is constantly accelerating. It is also predicted that by the next 50 years, the entire water column of the Wedell Sea will be under-saturated.

In contrast, the rate of natural increase in the concentration of carbonates is very slow. The seasonal changes in climate also affect the concentration. During winter, the water of the southern ocean becomes even less concentrated due to the cooling of the water. According to the researcher, the surface water carbonates will also slightly increase due to 21st-century climate change (as oceans are getting warmer than they were before). 

The exact study of calcification rates is very difficult for the researchers because these rates are affected by multiple factors like light, nutrients, change in temperature etc. The calcification rates in corals are strongly bonded by the change in temperature. The rate of calcification in corals is also believed to be related to photosynthesis. It is assumed that the removal of a small amount of carbon dioxide during photosynthesis increases the rate of calcification. Some researchers also believe that photosynthesis and calcification are not related. Also, researchers were not able to prove the inter-relationship.  But if we consider only the hydrolysis of carbon dioxide in water, then the rate of calcification is decreased in large proportion. 

With the decreased rate of calcification, it is believed that the corals might face metabolic and reproductive problems. The decreased rate may also disturb the growth of corals. A decrease in the rate of calcification means less calcium inside the body of corals, this makes their body even more fragile and weak. The decrease in carbonate concentrations has decreased the growth rate of many algae and plankton. Increased carbon dioxide in water results in the dissolution of calcium shells of these species. Also with decreasing calcification, the corals might show a change in growth location and depth. As all the species are interrelated in the food chain, any damage to corals or planktons or algae might disturb the entire marine food chain. 

There is a major misbelief that the increased concentration of carbon dioxide will help in the photosynthesis of corals and will eventually increase their growth. However, the corals and most of the marine species does not use direct carbon dioxide, unlike land plants. Instead, they use hydrogen carbonate ions for photosynthesis. Even if the concentration of carbon dioxide is double in the ocean, the photosynthesis rate will increase by roughly 12% to 14%. Hence photosynthesis rates show almost no change with the increase in carbon dioxide.   

The research for the effects of increased carbon dioxide in water on corals and other species is still going on. It is very hard to prove these assumptions and predict future damages. Earlier due to lack of advanced technology, makes misconceptions were created. But today researchers can predict the future effects up to a certain extent. 

The increase in carbon dioxide concentration might look like a very slow process, but its effects in near future are surely very harmful. Required steps have to be taken now to stop this.

The original article was published in nature.com