It’s not just coral: The unseen toll of record-breaking heat on the Great Barrier Reef

During past Great Barrier Reef bleaching events, southern areas have sometimes not suffered the worst of the bleaching. Not this time. This year’s intense underwater temperatures triggered some of the worst heat stress ever recorded. Of the southern reefs surveyed, only 3% showed no bleaching at all. It would be the worst and most widespread bleaching event on the southern reef, with other parts of the reef also experiencing mass bleaching, the fifth in eight years.

Shocked and saddened by the image of a pure white coral skeleton. But the damage caused by high temperatures underwater is more severe. A living coral reef is a complex ecosystem with many more species than coral. Not only that, but 95% of the habitat on coral reefs is not coral, but sediment and sand, making it a hidden biodiversity hotspot. So what happens to this cornucopia of life when subjected to extreme temperature stress?

We are currently on One Tree Island in the southern reef. It is home to a research station and one of the most protected in the entire reef.

What did we find? So far, the signs are not good. As we dive underwater, we can taste the changes in stress and chemicals the dying corals release into the water. When we brought the sample back, the smell was distinctly sulfuric and sour. Not only did we see bleached hard corals, we also saw bleached anemones and soft corals. There are few starfish or sea urchins, but the algae growing on the dead coral attracts more herbivorous fish.

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Read more: The Great Barrier Reef’s latest bleaching is the fifth in eight summers and corals now have little respite

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Coral reefs are made up of interlocking sections

Coral reefs are complex, interlocking systems built on interrelationships. Organisms need other organisms to survive, whether as food, homes, symbiotic partners, or substrate for growth.

Consider the famous relationship between anemone fish and their anemones. These fish drive away predators from the anemone host, and their droppings provide nutrients to the anemone. In turn, the anemone’s stinging cells protect fish from predators.

We are working to uncover the relationships between different parts of the coral reef ecosystem, such as corals, fish, starfish, worms and microorganisms. We wanted to know the effect of higher water temperatures on these relationships.

To remove this complexity, we need large amounts of data. We conduct live underwater surveys, photogrammetry, convert photos into 3D models of coral reefs and sediment collection and classification. We also record temperatures, deploy underwater coral hatcheries to study respiration, and analyze environmental DNA and nutrients in sediments.

Smell of sick coral

From the first day we arrive at a tree, we can see the consequences of high temperatures. There were bleached, fluorescent and dead corals on every reef we surveyed in the lagoon. The newly dead coral becomes colonized by wavy, dense algae called filamentous turf algae.

Near each reef, we could see more biofilm than usual on the top of the sediment section, which looked like brown sand when viewed from above. The sediment has a sticky feel to the touch, indicating that the slime shed by bleached corals has settled here.

So far, fish communities appear to be relatively untouched. However, as the coming months and years change, the knock-on effects may be delayed.

All of the large anemones we saw had expelled their symbiotic algae and bleached, suggesting that some anemone fish may soon lose their homes.

Damselfish and gobies that normally hide in live coral heads now have to hide in dead, algae-covered coral. There are large numbers of common parrotfish there, suggesting they benefit from eating algae on dead coral.

Starfish, shellfish and sea urchins are rare. This is as we feared as these mobile macroinvertebrates are in general decline. If they disappear, coral reefs will lose the ecological services provided by these animals, as these organisms recycle nutrients and eat detritus, while some also hunt prey.

Viewed from the air, the barrier reef appears as sheet-like and linear reefs scattered with large amounts of sand. These sediment and sand communities are not barren. They are hotspots for crustaceans and worms, feeding grounds for many species of fish, and where massive nutrient cycling occurs. They are likely to be hit hard by the heat.

What future do coral reefs have?

It’s easy to tell when corals are bleaching. It looks very different. But things become more difficult when the wider coral reef ecosystem shrinks. Much of the damage done this summer will take months or even years to show.

What will it look like? Research on other coral reefs can serve as a guide. First, we expect populations of coral-eating animals such as butterflyfish to decline. We then expect a decline in the number of coral residents such as damselfish and gobies that live on the coral heads.

For herbivores, such as algae-eating fish, we would expect to first see an increase in abundance as their populations expand to eat algae, followed by potentially significant declines as the overall health, diversity, and structure of the reef decline .

The chain reaction caused by bleaching is not always one-way. Some of the immediate damage can be reversed or even recovered, but we won’t know that for some time.

As we continue to heat the planet, the frequency and intensity of extreme underwater heat waves are expected to increase. We are entering uncharted territory for these extremely diverse and highly valued ecosystems, the flow-on effects of which we are only beginning to understand.

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Read more: Global coral bleaching due to global warming requires global response

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Acknowledgments: This work would not have been possible without the support of One Tree Island Research Station Managers Ruby Holmes and Heinrich Breuer