How Brown The Corals – That Were Pink Last Year

Written by:
13 October 2023
How Brown The Corals – That Were Pink Last Year - Featured image
Originally Appeared In

In this article, Dr Jennifer Marohasy contextualises and disseminates the findings of the IPA’s research into the Great Barrier Reef, conducted as part of the IPA’s research into Climate Change.


I was back at John Brewer Reef last week and many of the corals are now dark brown. John Brewer reef has lost its pink, for the moment.

The corals in this section of John Brewer Reef are now very brown, or green, because they are replete with zooxanthellae.
The corals in this section of John Brewer Reef are now very brown, or green, because they are replete with zooxanthellae.

There is a Coral Watch program that was developed at Heron Island by the University of Queensland. The ‘Coral Watch Coral Health Chart’ quantifies the health of a coral according to the intensity of its colour that is considered a proxy for the concentration of zooxanthellae. A report on the status of corals at Heron Island indicates that the corals tend to score between 3 and 4, which is considered healthy.

Dive Master Paul Crocombie holds a Coral Health Chart against a branching Acropora at John Brewer Reef last Tuesday, 3rd October 2023.
Dive Master Paul Crocombie holds a Coral Health Chart against a branching Acropora at John Brewer Reef last Tuesday, 3rd October 2023.

According to The Coral Watch website, to score a reef:
1. Choose a random coral and select the lightest area. [Don’t include the growing tip that is usually white.]
2. Rotate the chart to find the closest colour match.

Photographed at John Brewer Reef last Tuesday by Leonard Lim, colour chart held by Paul Crocombe. The growing tips of the coral are not bleached, the growing tips are naturally white.
Photographed at John Brewer Reef last Tuesday by Leonard Lim, colour chart held by Paul Crocombe. The growing tips of the coral are not bleached, the growing tips are naturally white.

3. Record the colour code on a data slate.
4. Select the darkest area of the coral and record the matching colour code.
5. Record the coral type.
6. Continue your survey with other corals. Record at least 20 corals.
7. Submit your data online at www.coralwatch.org

Ideally this is done with the chart beside the coral at the reef. But, given the extraordinary quality of the underwater photographs taken by Leonard Lim last Tuesday, and that some include the colour chart held by Paul Crocombe, a Dive Master, Skipper and Owner of Adrenalin Snorkel and Dive, I am attempting some interpretation here, so we might be able to quantify how brown the corals were last week for that patch of reef.

All four colours of the symbiotic algae/zooxanthellae in the Coral Watch Coral Health Chart are shown in this photograph. I calculate the average colour to be 3.7, I would have guessed that it was even higher.
All four colours of the symbiotic algae/zooxanthellae in the Coral Watch Coral Health Chart are shown in this photograph. I calculate the average colour to be 3.7, I would have guessed that it was even higher.
Photograph taken at John Brewer Reef on Tuesday 3rd October 2023 by Leonard Lim.
Photograph taken at John Brewer Reef on Tuesday 3rd October 2023 by Leonard Lim.
I can’t bring myself to mark the corals in this photograph that is so beautiful. Another from under-the-water at John Brewer Reef taken by Leonard Lim on 3rd October 2023.
I can’t bring myself to mark the corals in this photograph that is so beautiful. Another from under-the-water at John Brewer Reef taken by Leonard Lim on 3rd October 2023.
The photographs by Leonard Lim are all from a section of reef has particularly healthy corals and exceptionally high coral cover. There are other sections of John Brewer reef that consist of coral rubble. Coral cover and coral colour always varies with the particular habitat at a reef and its position relative to the prevailing wind, the last cyclone, geological history and the current season. The drone photograph was taken by Stuart Ireland from the back of the Adrenalin Snorkel and Dive boat.
The photographs by Leonard Lim are all from a section of reef has particularly healthy corals and exceptionally high coral cover. There are other sections of John Brewer reef that consist of coral rubble. Coral cover and coral colour always varies with the particular habitat at a reef and its position relative to the prevailing wind, the last cyclone, geological history and the current season. The drone photograph was taken by Stuart Ireland from the back of the Adrenalin Snorkel and Dive boat.

I first visited John Brewer reef on 10th April last year, with photographer Leonard Lim and cinematographer Stuart Ireland, specifically to photograph and film the bleaching. At the time this reef, John Brewer Reef, was reportedly the centre of a sixth mass coral bleaching – and I wanted it all recorded, for that moment in time.

We did find some corals that had bleached white, but mostly I was surprised at how pink the reef was back then. Leonard took some exquisite photographs and Stuart filmed sections of coral that can be viewed in part 1 of a documentary entitled ‘Bleached Colourful’.

The same section of reef, but quite a different colour in April 2022.
The same section of reef, but quite a different colour in April 2022.

Back in April 2022, many of the corals had ‘kicked out’ their symbiotic algae and so it was possible to see more of their natural pink colour. The pink is from a florescent protein that can ‘upregulated’ when corals are stressed.

It is somewhat counter intuitive that the more symbiotic algae, also known as zooxanthellae, the healthier a coral and the less colourful!

According to the Coral Watch Website:

The Coral Health Charts are based on the actual colours of bleached and healthy corals. Each colour square corresponds to a concentration of symbionts contained in the coral tissue. The concentration of symbionts is directly linked to the health of the coral. All you have to do is match the colour of the coral with one of the colours on the chart. You then record the lightest and darkest colour codes, along with coral type, on a waterproof data slate.

The hues on the chart represent the most common colours of corals, and help our eyes to make an accurate match. The brightness of the colours ranging from 1 to 6 are the same on every side of the chart, so you can mix and match sides.

How brown are these two corals, photographed at John Brewer Reef last Tuesday. This reef is known in the mainstream political media, including in The Guardian, as stark white. In reality it is rather brown.
How brown are these two corals, photographed at John Brewer Reef last Tuesday. This reef is known in the mainstream political media, including in The Guardian, as stark white. In reality it is rather brown.
Another Leonard Lim photograph from last Tuesday. There is a dead plate/tabulate coral clearly visible to the left in this photograph.
Another Leonard Lim photograph from last Tuesday. There is a dead plate/tabulate coral clearly visible to the left in this photograph.
What a special coral reef: John Brewer Reef with Paul Crocombe, photographed by Leonard Lim on 3rd October 2023
What a special coral reef: John Brewer Reef with Paul Crocombe, photographed by Leonard Lim on 3rd October 2023
Every photograph shows something different. The purple is from a florescent protein, not zooxanthellae/symbiotic algae.
Every photograph shows something different. The purple is from a florescent protein, not zooxanthellae/symbiotic algae.
Coral reefs are the most extraordinary places, and they are essentially layer upon layer of dead coral topped with a thin veneer of living coral.
Coral reefs are the most extraordinary places, and they are essentially layer upon layer of dead coral topped with a thin veneer of living coral.

Stuart Ireland, Saxon Davidson (Research Fellow at the IPA), Jennifer Marohasy and Leonard Lim, at the Breakwater Marina, Townsville, after a great day out at John Brewer Reef on 3rd October, 2023.
Stuart Ireland, Saxon Davidson (Research Fellow at the IPA), Jennifer Marohasy and Leonard Lim, at the Breakwater Marina, Townsville, after a great day out at John Brewer Reef on 3rd October, 2023.

So much thanks to Adrenalin Snorkel and Dive for a great day at the reef.

You can read more about the structure and history of coral reefs in this central region of the Great Barrier Reef at a previous blog post, about Britomart Reef by clicking here.

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