Coral bleaching
The largest structure made completely by living organisms anywhere in the world, the Great Barrier Reef is under threat from global warming.
Coral bleaching
Global warming has the potential to cause large areas of the Great Barrier Reef to become bleached and lifeless, as the warmer water disrupts the delicate patterns of life in this sensitive marine environment.
Coral reefs are very sensitive to changes in sea temperatures outside their normal seasonal range. Just 1°C above the long-term monthly summer average is enough to cause coral bleaching, and a 2°C increase in average water temperature will be enough to lose a World Heritage treasure, and jeopardise a $5.6 billion tourism industry.
Coral bleaching
Higher ocean temperatures will change the Great Barrier Reef, the world’s largest and most diverse marine park. So, what do we stand to lose?
- The largest natural feature on earth, stretching more than 2,300km and covering an area of 345,000 square km
- The largest structure made completely by living organisms anywhere in the world
- More diverse species of animals and plants in a cubic metre of the reef than in any other environment in the world – including tropical rainforests (some reefs in the Great Barrier Reef have more different fish types than in the entire Caribbean Ocean; our reef has no less than 1500 species, while Caribbean coral has fewer than 900)
- Six of the world’s seven species of marine turtles
Coral bleaching occurs when the trillions of zooxanthellae, the microscopic organisms that build the reef, are subjected to habitat stress. The most common cause of stress is elevated sea temperature, but additional stresses such as high light intensity, low salinity and pollutants can contribute. If the stress is too great, or persists too long, corals can die and the reef-building process stops.
Reef corals are very sensitive to sea temperatures outside their normal range. Just 1°C above the long-term monthly summer average is enough to cause coral bleaching in many dominant coral species.
What causes the colour to disappear?
When temperatures exceed threshold levels for prolonged periods, the relationship between the zooxanthellae and the corals breaks down. Because the zooxanthellae are responsible for the colours that characterise most corals, the loss of zooxanthellae can leave the coral tissue colourless, rendering the bright white of the coral skeleton starkly visible.
If stressful conditions prevail long enough, bleached corals will die. However, if temperature return to their normal range then the bleached corals can return to their normal, healthy colour. The severity of bleaching can vary substantially according to water depth, location and species of corals.
