Bleaching kills off 'third of Great Barrier Reef coral'

• Bleaching is destroying great coral reefs in Australia. It is estimated that about one third of corals in the northern and central parts of Australia’s Great Barrier Reef got bleached.
• The Great Barrier Reef stretches for 1,400 miles along Australia's northeastern coast. It consists of 3,000 individual reefs and is home to 1,500 species of fish.
• The southern part has long sustained heavy damage from tourism, pollution, and invasive species. But the northern part has always been more pristine, located far from (most) human activity. It's usually a bright feeding ground for dugongs, sea turtles, and other marine life.
• Ever since December 2015, as the Australian summer got underway, ocean temperatures around the reef have surged to record highs, aided by global warming and a powerful El Niño currently raging in the Pacific
• That's helped trigger severe coral bleaching in the Great Barrier Reef .
• However, bleached corals that haven’t died can recover if the water temperature drops, older corals take longer to bounce back and likely won’t have a chance to recover before the next bleaching event occurs.
• This is the third and most extreme mass bleaching event in 18 years to strike the Great Barrier Reef.

About coral reefs:
Coral reefs are often dubbed the rain forests of the ocean. Anchored by millions of coral polyps — tiny, soft-bodied animals that create elaborate calcium carbonate skeletons that shelter fish — these reefs cover just 0.1 percent of the sea floor but are home to 25 percent of marine fish species. They're popular spots for divers and tourists. They protect coasts from storms. They sustain food for half a billion people.
What is coral bleaching

• Many types of coral have a special symbiotic relationship with a tiny marine algae (zooxanthellae) that live inside corals' tissue and are very efficient food producers that provide up to 90 per cent of the energy corals require to grow and reproduce.
• Coral bleaching occurs when the relationship between the coral host and zooxanthellae, which give coral much of their colour, breaks down. Without the zooxanthellae, the tissue of the coral animal appears transparent and the coral's bright white skeleton is revealed.
• Corals begin to starve once they bleach. While some corals are able to feed themselves, most corals struggle to survive without their zooxanthellae.
• If conditions return to normal, corals can regain their zooxanthellae, return to their normal colour and survive. However, this stress is likely to cause decreased coral growth and reproduction, and increased susceptibility to disease.
• Bleached corals often die if the stress persists. Coral reefs that have high rates of coral death following bleaching can take many years or decades to recover.

What causes coral bleaching

• The main cause of coral bleaching is heat stress resulting from high sea temperatures. Temperature increases of only one degree celsius for only four weeks can trigger bleaching events.
• If these temperatures persist for longer periods (eight weeks or more) corals begin to die. High water temperatures can affect reefs at regional and global scale.
• Other stressors can also cause bleaching, including freshwater inundation (low salinity) and poor water quality from sediment or pollutant run-off.

Coral bleaching and the Great Barrier Reef

• The Great Barrier Reef has experienced mass coral bleaching events in the past.
• In 1998, there was a global mass bleaching event where 50 per cent of the reefs on the Great Barrier Reef suffered bleaching. During this time, sea temperatures on the Great Barrier Reef were the highest ever recorded.
• Mass bleaching also occurred in 2002, with 60 per cent of reefs were affected. This was the largest coral bleaching event on record. Two periods of hot weather resulted in sea surface temperatures a few degrees centigrade higher than long-term summer maxima.In both events, about five per cent of the Great Barrier Reef's coral reefs were severely damaged

By: Vishal