Published online before print November 8, 2010

vol. 107 no. 47 20400-20404

doi:10.1073/pnas.1007273107

This scientific report, like many others, speaks of the effects of increased dissolved CO₂ (pCO₂) in ocean water. This study focuses expressly on how varied pCO₂ levels affect the recruitment and reproduction success of a specific Caribbean coral species Acropora palmate. This study used current pCO₂ conditions as a basis and then continued on to test the recruitment process of this coral species in progressively higher pCO₂ levels, all of which are expected to be reached by the end of the century. The impact of the elevated pCO₂ levels tested showed a 52% and 73% reduction in the number of larval settlers on the reef under pCO₂ conditions projected for the middle and the end of this century.

This study shows that pCO₂, a recognized component of ocean acidification, without a doubt affects the success and longevity of a coral ecosystem. Not only does pCO₂ do all this, but it also alters fundamentals of ocean chemistry such as the pH and aragonite saturation, furthering the detrimental effect on the Caribbean coral ecosystem.

The ocean has absorbed carbon dioxide (CO2) from the atmosphere since the beginning of time. The ocean is known as a carbon dioxide sink because of its absorption capabilities. This has been viewed as a positive in the past few decades since global warming has become a pressing issue. The ocean absorbs CO2 from the atmosphere, which helps lessen the threat of global warming to the earth. Carbon dioxide levels have risen since the Industrial Revolution due to automobile emissions, cement production, industrial power plants and other contributing factors. Now, with the oceans absorbing absorbing about 1/3 of the earth’s carbon dioxide, the sea chemistry is being affected. The ocean cannot handle such high levels of CO2 as a result, its pH is becoming more acidic. This effect is known as ocean acidification.

Before the Industrial Revolution, the ocean’s pH levels were stable. Since then, the pH has dropped 0.1 units. Though this may seem like a relatively small change, the world’s ocean has a very high buffering, or acid neutralizing, ability so the fact that the pH has declined to this extent is very startling. PH levels are predicted to drop even more in the next century.

Like all ecosystems, marine ecosystems are comprised of a vast array of interactions between different species and different types of organisms (both alive and dead) and the physical environment. Therefore, a change in any of these aspects will lead to a plethora of changes throughout the rest of the ecosystem. Because of the sheer quantity of life present and the significant interactions between species, coral reefs are prime candidates to be affected greatly by ocean acidification. These reefs provide food and shelter to hundreds of thousands of marine organisms so when the reef is harmed by acidification, so are all of the organisms that interact with it. When global warming, one of the world’s most alarming environmental threats directly affects the coral reef, one of the ocean’s most important ecosystems, the ramifications are dire.

Coral reefs are created by large calcium carbonate colonies known as coral. These reef structures are the home and feeding grounds to a wide array of organisms. Coral reef ecosystems have been called “cradles of evolution” because more marine organisms evolve from coral reefs than from any other ecosystem.

Ocean acidification may actually alter the physical structure of coral reefs. Acidification affects the organisms that build the reef because it lowers calcification rates and pH, inhibiting the creature’s skeletal growth. Without these reef-building organisms, coral reefs cannot exist.

Aside from hindering the organisms that physically build the reefs, ocean acidification also increases the probability that existing reef structures may dissolve. Reef erosion is likely, given the vulnerability inevitable with increased acidification.

Acidification raises the possibility of coral mortality. It can cause coral bleaching, which can cause the coral to die. As the coral tries to survive and is in a weakened state, they become vulnerable to encroachment by other marine organisms. Some species can benefit from higher water acidity, like macroalgae. As these algae thrive, they block sunlight from getting to the coral and they may be abrasive to coral structures as they move through the water in the current.  Both low light and abrasive contact can weaken the coral, or even kill the reef structure.

Dissolving and eroding coral reefs, as well as coral that is lost because of displacement by other organisms that can survive better in the high acidity all lead to what is known as “reef flattening”. This is a phenomenon that creates a loss in the “architectural complexity” of the reef.  This affects all of the organisms that live within and rely on the reef as a key part of their survival methods. Reef flattening diminishes reef structure and habitats, and reduces organism populations and biodiversity.

Coral reefs are home to over 25% of all known species of fish and exhibit the highest biodiversity of any ecosystem in the entire ocean. Threats to coral reefs are a threat to thousands of other organisms, so as we see ocean acidification harming our world’s coral reefs, we should be very concerned.  Ocean acidification does not mean that the oceans will die, but the survivors may be algae and jellyfish.  For the ocean to be sustainable in its present form, with coral reefs the prominent sanctuaries for marine life, the pH of the ocean has to maintain acidity within relatively narrow boundaries.  With the alarming increase in CO2 being absorbed into our great carbon dioxide sink known as the ocean, the coral reef is in jeopardy.

Sources:

“Chapter 4.” Ocean Acidification: a National Strategy to Meet the Challenges of a Changing Ocean. Washington, D.C.: National Academies, 2010. Print.

Eilperin, Juliet. “Growing Acidity of Oceans May Kill Corals.” The Washington Post: National, World & D.C. Area News and Headlines – The Washington Post. 5 July 2006. Web. 04 Sept. 2011. <http://www.washingtonpost.com/wp-dyn/content/article/2006/07/04/AR2006070400772.html>