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Ocean Acidification and Coral Bleaching

Ocean Acidification and Coral Bleaching


Two major threats to the health of Earth’s oceans from the release of greenhouse gases are coral bleaching and ocean acidification.

Corals are sedentary animals that grow in colonies of many individual ‘polyps’ which live in a co-dependent relationship with single-celled algae that live within their tissues. Environmental stresses such as an increase in sea temperature force the coral polyps to eject the algae. As a result corals lose their color and their source of food and energy.

Additionally, bleaching can be particularly severe when during years when there is a strong El Niño event, which can lead to an even greater increase in sea temperatures (see Case Study). The overall warming of global sea water has led to bleaching at a large scale at coral reefs around the world.

Ocean acidification, caused by greenhouse gas emissions, is another major threat to the state of the world’s oceans.

Many human activities release carbon dioxide into the atmosphere. Oceans absorb some of this gas and when it dissolves in water if forms a mild acid, which makes a small but significant change to the overall acidity of the sea water.

Acidity is measured using the pH scale which runs from 0 (fully acid) through 7 (neutral) to 14 (fully alkaline). Sea water is normally slightly alkaline and the increase in carbon dioxide is making it less alkaline, but for the sake of convenience this is referred to as acidification.

So far, the change has been slight, but it has made the pH lower than it has been for tens of thousands of years, and even a small change in the pH of water can have a big effect.

Among the biggest concerns that scientists have about this trend is that it can interfere with the way many marine organisms produce shells or plates from calcium carbonate (CaCO3).

As the oceans become more acidic (less alkaline) there is less calcium carbonate available in the water and when this happens, the calcium carbonate in marine organisms can dissolve.

The UK science academy, the Royal Society, concluded in a 2005 report that the impacts of ocean acidification would vary between regions, species and ecosystems, and would be most severe for coral reefs and the Southern Ocean.

Both coral bleaching and ocean acidification pose threats to human food supplies. Many commercial fish species such as snappers and groupers live and breed among coral reefs, so they are threatened by bleaching.

Ocean acidification, meanwhile, threatens an even broader range of sea life including corals, shellfish (mollusks and crustaceans), some algae, sea urchins and starfish.

The smaller of these species form a large proportion of the microscopic plankton that is the basis of food chains across the oceans.

Scientists fear that if these microscopic organisms are no longer able to produce shells they will die out and trigger a cascade of extinction that affects larger and larger species up the food chain — and ultimately lead to a big reduction in food for human populations.

Greenhouse gas emissions are the main cause of ocean acidification and the increases in sea temperature that cause coral bleaching. Any efforts to reduce emissions will bring benefits both on land and at sea. However, global greenhouse gas emissions have risen steadily in recent years.

Without a drastic reduction in global emissions, there is so far little else that can be done to avert an increase in ocean acidity and temperature.

The US National Oceanic and Atmospheric Administration has developed an automated system that warns coral reef managers when the conditions are prime for coral bleaching.

This system uses satellites and monitoring equipment in the oceans and can alert managers so they can act to reduce other stresses on reefs.

Good sources for journalists include NOAA’s Coral Reef Watch, which warns of areas at risk of bleaching.

Nongovernmental sources include the Coral Reef Alliance has a list of experts and publications.

Reefbase is a good resource with access to country coral reef statistics along with relevant publications.

A note on pH: Journalists who cover ocean acidification should note that pH is measured on a logarithmic scale, meaning that a pH on 3 is ten times more acidic than a pH of 4, which is ten times more acidic that a pH of 5.

CASE STUDY – Coral bleaching in the Indian Ocean
In 1997-1998 between 50 and 90 percent of corals in shallow waters of the Indian Ocean were bleached and died. This extreme bleaching followed a very strong El Niño event that contributed to the high sea temperatures.

Research by scientists at the University of Bangor showed how this bleaching was unevenly across the region. In the granitic islands of the Seychelles more than 90 percent of the corals were bleached on most reefs.

In Mauritius, however, the bleaching affected under ten percent of the reefs. The researchers suggest that this is because Cyclone Anacelle produced wet and cloudy unsettled weather during February 1998.


BBC News – What is Ocean Acidification
Intergovernmental Panel on Climate Change – ocean acidification

Ocean Portal by the Smithsonian National Museum of Natural History

International Union for Conservation of Nature on Ocean Acidification
US National Oceanic and Atmospheric Administration (NOAA) – fact sheet on ocean acidification[pdf]

International Coral Reef Initiative
Royal Society – Ocean acidification due to increasing atmospheric carbon dioxide [pdf]