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Coral 101.

The Great Barrier Reef is made up of 3000+ reefs, and each of those reefs consists of millions of individual coral colonies. It’s a diverse and complex ecosystem that supports an array of fish and sea creatures but it’s under stress. 

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What we’re starting to see is fast-growing corals are continuing to recover, but the slow-growing corals which may take hundreds of years to replace are not getting that opportunity.

What is coral?

Corals are colonies of teeny tiny animals called coral polyps (which are closely related to jellyfish!). The polyps have a symbiotic relationship with microscopic algae called zooxanthellae that live in their tissues. These algae are the polyp’s primary food source and give coral their colour.

What are the biggest threats to the reef?

Climate change and warming oceans are the biggest threat to the Great Barrier Reef because coral is notoriously fussy and likes the water to be a nice and consistent 23°–29°C. Other threats include ocean acidification, poor water quality flowing off the land, coastal development and over-fishing.

Why does coral bleach?

Coral is easily stressed by changes in conditions, which cause it to expel the zooxanthellae and lose its colour (hence the bleaching). Bleached coral is not dead but if it goes too long without its zooxanthellae, it will starve and die.

Although most corals can cope with higher temperatures for short periods of time, they don’t like it if temperatures stay high for too long. For example, a coral that lives in 27°C water may cope with the water being 29°C for a day or two, but may start to bleach if the temperature rises to 31°C for a week.

Does coral recover from bleaching?

Under moderate levels of temperature stress, individual colonies often bleach and recover as temperatures return to normal. However, if exposed to higher temperatures, bleached coral may die, and take 10-15 years to replace under stable conditions. The problem now is that with climate change, bleaching events are happening more often so there’s less recovery time between the events. Meanwhile, natural disturbances like cyclones also knock them around and impact recovery.

What does coral need to stay healthy?

Coral health is really about the colony as a whole, which as we’ve seen above, relies on a balance between disturbances and recovery. Having favourable conditions like clear water so that light can penetrate and enable the zooxanthellae to photosynthesise, as well as clean water with minimal pollutants, are also part of keeping the overall resilience of the reef up.

How is the reef changing?

Angus Thompson is a coral scientist with the Australian Institute of Marine Science. He’s been monitoring coral and fish communities since the late ‘80s.

“What we’re starting to see is fast-growing corals are continuing to recover, but the slow-growing corals which may take hundreds of years to replace are not getting that opportunity. So over time the composition of the reef is changing.”

Why is it so important to protect the reef?

Coral reefs are often called the ‘rainforests of the sea’ because of the diversity of life that shelters, feeds and reproduces in coral habitat. But there’s even more to the Great Barrier Reef. It underpins our commercial fishing and tourism industries, which support over 60,000 jobs, and it provides important ‘ecosystem services’ including carbon sequestration and coastal protection from cyclones.

How can we protect coral?

First and foremost, coral reefs need strong climate action. Locally there’s also a lot we can do to minimise stressors on the reef by improving water quality, addressing overfishing and rolling out restoration and resilience technologies so the reef has more chance of bouncing back from disturbances like cyclones and bleaching events.


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