Hope for the Great Barrier Reef | “Refuges” where corals thrive

Ocean warming is seriously affecting the Great Barrier Reef off the coast of Australia. But its vast expanse hides areas where corals resist climatic assaults, researchers have discovered.

Oasis of freshness

Since 2015, the Great Barrier Reef in northeastern Australia has experienced five of the six hottest temperatures in 400 years. And each time, it has lost hundreds of square kilometers of coral through “bleaching”.

Like the famous Gallic village of Asterix, two “refuges” of this Great Barrier are resisting global warming.

Geologist Benjamin Petrick analyzed samples from the ocean floor of the Coral Sea dating back 900,000 years. In the magazine Science Advancespublished at the beginning of December, it shows that the Great Barrier appeared 700,000 years ago when the temperature of this region of the Pacific rose from 26°C to 28°C.

“There appears to be a very narrow zone of maximum coral growth, between 28°C and 30°C,” says Mr Petrick.

This means that cooler refugia will experience faster coral growth in the coming decades.

The temperature of these Australian coasts has remained constant for 700,000 years, with variations of less than 4°C. How is this possible despite the glaciations? “Because there is a reservoir of warm water: the Indo-Pacific warm basin, which extends from eastern Australia to Southeast Asia,” explains the city geologist German from Kiel. These are the warmest waters on the entire planet. »

What is coral bleaching?

When it gets too hot, the algae that give corals their color, called zooxanthellae, are expelled. Instead of providing oxygen to their host, these algae begin to produce toxins. The coral immune system responds by putting an end to their symbiotic relationship.

Coral does not necessarily die after bleaching, but it is more fragile, in particular because it lacks nutrients provided by algae.

The latter feed on CO by photosynthesis.₂ generated by coral and present in the water, which produces oxygen.

Disease, pollution and increased UV rays from the sun can also cause coral bleaching, as can waters that are too cold, which limits the maximum depth of coral reefs.

The lifts

The cool refuges of the Great Barrier Reef owe their existence to “upwellings” of water from the depths. Another Australian researcher, Chaojiao Sun, from the Commonwealth Scientific and Industrial Research Organization, did the opposite exercise to Mr. Petrick: she simulated what will happen in the future to upwellings in these two refuges .

The climate models used by M^me Sun are so complex that it was only able to finance 20 years of simulations on supercomputers: between 2045 and 2055, and between 2075 and 2085. “In both cases, even with the worst climate scenarios, the refuges remain cooler than the rest of the Great Barrier,” notes the Perth oceanographer, who published his results in November in the journal Science Advances.

The depth

Less growth of corals in waters that are too warm could also affect their ability to keep up with rising sea levels. “Corals should not be too deep to have enough light,” emphasizes Mr. Petrick.

According to some researchers, deeper corals, species requiring less light, could also provide refuges because the deep water is cooler. “But they need to grow fast enough to keep up with rising sea levels, which remains to be determined because they are less well studied,” he says.

Other important questions to be resolved about the refuges in the Great Barrier Reef concern the one in the north, near Papua New Guinea. “We don’t know him well,” admits Mr. Petrick. It’s not clear why the waters are colder. »

A fundamental question is the impact of changes in global ocean circulation on the upwellings that create these refuges, says M.^me Sun. “There is still uncertainty about ocean circulation. » She also wants to understand why a bleaching event took place in 2024 in the southern refuge of the Great Barrier Reef.

The two refuges represent 15% to 20% of the surface area of ​​the Great Barrier Reef.

The transplants

These refuges allow corals to repopulate the central regions of the Great Barrier Reef, if they are decimated by heat stroke. They will also serve as a source of “grafts”, a strategy taken very seriously by the Australian government.

In November, Israeli researchers, who wanted to save the corals of the Mediterranean and Red Seas, demonstrated a more effective way of grafting corals: by using larger grafts.

“For the same quantity of biological material transplanted, the results are much better with entire communities,” underlines Natalie Levy, who is the lead author of the study published in November in the journal Nature Communications.

Often, the grafts are composed of coral larvae sprinkled into the seabed.

If the grafts are grown in aquariums before being introduced into damaged corals, a fairly large, natural-like structure must be provided. In the case of the Israeli researchers, these were textured ceramic tiles measuring 25 cm on a side.

Drones

Autonomous underwater technology could increase the efforts of coral protectors. “We can take samples much more easily,” says M.^me Levy.

For its part, the Queensland University of Technology has been testing, for half a dozen years, the “larvabot”, a drone that can self-sow hundreds of thousands of coral larvae directly above the Great Barrier.

Watch a video about the “larvabot” (in English)