What causes coral bleaching?

Say you got really bad food poisoning. Then you had to work outside in searing sun. This is what our coral reefs are being subjected to.

Say you went out for dinner, ate some dodgy food and got really bad food poisoning.

Now imagine you were forced to go outside and work under a searing sun, developing a case of heat-stroke on top of it.

This is essentially what many of our coral reefs are being subjected to, says Dr Jennie Mallela of the ANU Research School of Biology and the Research School of Earth Sciences.

Dr Jennie Malella
Dr Jennie Malella

“If you think of any animal, when fed a very poor diet of polluted food—like contaminated water for coral—they’re going to get very sick.

“And then if you were to subject that animal which is already suffering from food poisoning to heat stress—global warming in this instance—the system is just not going to cope.”

Coral is part animal (polyp) and part plant (an algae called Zooxanthellae). Their skeleton is made up of the same stuff as our bones, calcium carbonate (fun fact: this means that coral can get osteoporosis!).

They have what’s called a ‘symbiotic’ or interdependent relationship with Zooxanthellae that lives in their ‘skin’ follicles. The algae needs the coral and the coral needs the algae to survive.

When the coral gets stressed, it physically expels the algae which is providing the coral both with food and colour, causing it to go white.

What is commonly known as coral bleaching, therefore, is the coral without its algal residents. It doesn’t actually die straight away, but if it doesn’t get those algal symbionts back, it will slowly starve.

Coral bleaching has seen several epidemics worldwide over the past decade and the impact goes beyond ruining your scuba diving trips.

“Coral reefs are really important for human societies not only for food (fishing) but also shoreline protection. They take out up to 90 per cent of the wave energy during extreme weather events in the tropics, and protect us from cyclones, hurricanes and land erosion.” Dr Mallela herself has been involved in investigations of the Caribbean bleaching incident of 2005, and more recently the Christmas Islands and Great Barrier Reef from 2016 to 2017.

She and her research team showed that contaminated water contributed to these bleaching epidemics.

To do so, they studied annual coral growth bands, which are like the rings found inside tree trunks. Corals lay down a growth band every year, the exception can be when they bleach, causing stress bands in the skeleton.

Coral growth
Studying coral growth

Dr Mallela and her team examined many decades of growth history of the same coral species, but from sites with historically different water qualities.

“What was really interesting was that at sites that were not polluted, the corals didn’t bleach. But at the sites that were polluted, at the same time with the same heat stress, they bleached.”

This means that some of the bleaching we’re seeing is a natural consequence of copping the worst in terms of local pollution, and climate change globally.

The good news?

We have the power to change at least one half of that equation, explains Dr Mallela:

“We can actually improve the resilience of coral reefs by ensuring that their water and their eco-systems they live in are clean and healthy. This will make them more robust and resilient to periods of extreme heat stress.”

So if you want colour back in the sea, the first step is to take the pollution out.

Coral bleaching on the Great Barrier Reef