Let’s talk about coral bleaching.

Did you know that corals are actually part-animal, part-plant? Corals are invertebrates made up of tiny, individual organisms that come together and form colonies, and, eventually, a reef. So coral reefs are large, living structures. Each coral polyp has a mouth at one end, surrounded by tentacles which gather food (around 10% of a coral’s food intake) and have stingers (nematocysts) which help protect the coral.

Corals also have tiny algae called zooxanthellae living inside them, which gives them their colourful exterior. A coral and its zooxanthellae have a mutually beneficial relationship. The coral benefits from the zooxanthellae producing food through photosynthesis (this accounts for about 90% of a coral’s food), which leads to further coral growth, and the zooxanthellae also remove waste from the coral. In return, the coral provides a safe structure for the zooxanthellae to live and thrive. So the coral and the algae rely on one another for survival. 

When corals become stressed through a change in their environment, whether that’s a change in temperature, a change in salinity, or even a change in light or pollution levels, they expel their zooxanthellae that live inside them, causing the coral to turn completely white. This is known as bleaching. Most bleaching is caused by increased temperatures, and a temperature increase of just one degree (Celsius) over a period of four weeks can be enough to induce bleaching in many corals.

So when the coral turns completely white, does this mean that the coral is dead? Thankfully, no, not straight away BUT if the environmental stressor i.e. temperature, doesn’t return to within a normal range quickly then the coral is likely to die as it can’t survive without its supportive algae for long. As mentioned above, the zooxanthellae provide the coral with around 90% of its food which is (obviously) essential for survival. It is difficult to pinpoint exactly how long a coral is likely to survive without its zooxanthellae, but most corals are expected to be able to survive for a few weeks, so won’t die if their zooxanthellae return within that timeframe.

Mass bleaching events happen when corals and whole reef areas cannot tolerate prolonged, changed, environmental conditions. This can lead to widespread devastation on our reefs, so big that they can be detected from aerial surveys. Reefs can take years, if not decades, to recover as corals are slow to grow, and are particular about the environment in which they can thrive. Research in 2019 which studied the recovery of a coral reef in Palau following a mass bleaching event in 1998 found that the reefs needed at least 9 - 12 years to recover, and that’s without any further bleaching or other disturbance to the reef in the meantime.

The science is clear that there have been more mass bleaching events in recent years than ever before, and at an alarming rate given the time required for recovery. The Great Barrier Reef has experienced five mass bleaching events since 2015, and just last year, the world’s 4th global coral bleaching event was confirmed by NOAA, with significant bleaching documented in both the Northern and Southern Hemisphere.

There’s not much good to say about coral bleaching. Some level of dead coral can be helpful to a reef as it turns into rocky substrate where new corals can eventually take hold and start to grow. However, the frequency and intensity of bleaching (and mass bleaching) events is far surpassing any level which could be considered ‘natural’, and this is a direct result of climate change and warming oceans. So anything we do individually, or as a collective, to slow, or prevent, climate change from getting worse, will help to protect our precious coral reefs, as well as this rest of this wonderful world that we call home. If you’re based in Australia, consider signing the Australian Marine Conservation Society’s campaign to push for the government to commit to a roadmap to help save the iconic Great Barrier Reef. 

To see some coral bleaching in action, check out this video on our Instagram

Written by Charlotte Hansford

Photo: Dan Nicholson