Global Ocean Warming: ‘We Are Observing Fourth Mass Coral Bleaching’

Jérémie Vidal-Dupiol is a researcher in molecular ecology at the French Research Institute for the Use of the Sea Ifremer. Originally from the Arcachon basin, it is attached to the IHPE (Interactions host pathogen environments) laboratory on the university’s website. from Montpellier. A coral specialist, he works in the Pacific region, particularly on the reefs of New Caledonia, French Polynesia and Fiji. It analyzes the consequences of the massive bleaching that affects the colonies of these marine animals.

Jérémie Vidal-Dupiol, researcher in molecular ecology at Ifremer.


Jérémie Vidal-Dupiol, researcher in molecular ecology at Ifremer.

JVD

What is coral reef bleaching?

It shows the separation between…

Jérémie Vidal-Dupiol is a researcher in molecular ecology at the French Research Institute for the Use of the Sea Ifremer. Originally from the Arcachon basin, it is attached to the IHPE (Interactions host pathogen environments) laboratory on the university website. from Montpellier. A coral specialist, he works in the Pacific region, particularly on the reefs of New Caledonia, French Polynesia and Fiji. It analyzes the consequences of the massive bleaching that affects the colonies of these marine animals.

Jérémie Vidal-Dupiol, researcher in molecular ecology at Ifremer.


Jérémie Vidal-Dupiol, molecular ecology researcher at Ifremer.

JVD

What is coral reef bleaching?

It demonstrates the separation of two organisms that live together: corals and the microalgae with which they are in symbiosis. This separation is caused by abnormally high water temperatures over a long period of time. A 24-hour temperature jump does not cause bleaching. On the other hand, it can lead to higher-than-usual temperatures – even slightly higher – that persist for several weeks.

Why is the separation of these two organisms likely to lead to coral death?

Microalgae transfer up to 90% of the nutrients they produce through photosynthesis to the corals. Without microalgae, the coral loses a vital source of food. It acclimatises by releasing large numbers of “hunting cells” to find its own food: for example, planktonic microorganisms and small crustaceans. It can thus survive for a period of time until the symbiosis with the microalgae is restored when the episode of heat stress ends. But the longer this symbiosis is interrupted, the greater the risk of coral mortality.

For a year there have been warnings about the extent of heat waves at sea. Is it a consequence of coral bleaching?

Quite. We are experiencing an El Nino episode that started last year (warming of the surface waters of the eastern Pacific, near the coast of South America, editor’s note). Its effects are exacerbated by global warming caused by greenhouse gas emissions related to human activity. As a result, we have bubbles of warm water that slowly cross the oceans and cause the bleaching of almost all of the planet’s coral reefs. We are thus observing the fourth mass bleaching of corals recorded in the world after the episodes of 1998, 2010 and 2016-2017.

Are some corals more resilient than others?

Yes, for various reasons. There is ample evidence to suggest that corals develop the ability to learn from past stressful events. Around the world, corals, which are often exposed to heat stress, have a better ability to tolerate new heat waves. A comparison can be made with sports practice. Someone who has never run will suffer a lot when they try running for the first time. The more he practices after that, the better he will be. In the same way, corals are increasingly used to tolerate high temperatures.

Red Sea corals are known to be more resistant to the heat of the water. why?

This is an exception. This is partly explained by the colonization of the Red Sea by corals that come from the Persian Gulf, the place in the world where coral reefs experience the highest temperatures. These corals have been adapted to warm water for hundreds, even thousands of years. Because the Red Sea is colder, their genetic pre-adaptation would protect them.

On April 4, a completely bleached coral atoll near Lizard Island north of Australia's Great Barrier Reef.


On April 4, a completely bleached coral atoll near Lizard Island north of Australia’s Great Barrier Reef.

DAVID GRAY/AFP

Will coral tolerance to heat waves increase fast enough to keep up with global warming?

It’s too early to answer for sure. But I think corals have much more adaptive potential than was thought about fifteen years ago. There are signs that allow us to be optimistic. But we are only talking about the effects of climate change. What is certain, however, is that the accumulation of heat stress, overfishing, water pollution, tourist pressure and other impacts associated with human activity on the reef results in almost zero chance of adaptation and survival.

Are there places in the world where this accumulation has already wreaked havoc?

Jamaica’s reefs were among the most beautiful in the Caribbean. They suffered a chain of stresses: bleaching, overfishing, pollution. Today, there is hardly a single living coral in Jamaica.

Isn’t the rapid succession of sea heats threatening to weaken corals instead of “boosting” them?

Australia’s Great Barrier Reef has been affected by two consecutive bleaching episodes, in 2016 and 2017, with a comparable intensity of stress, or even greater in 2017. And yet the corals bleached less in 2017 than in 2016. We hope that if we bearing in mind that it is essential to reduce greenhouse gas emissions and act on all other pressures to maximize the adaptive capacities of reefs.

Can a dying reef regenerate?

A reef experiencing no stress other than bleaching will be able to recover in about ten years and return to its original state. If not, the reef will change state. It will no longer be dominated by corals, but by algae, which are their main competitors. Going back will be very complicated.

What consequences does the death of a coral reef have on the ecosystem?

We are witnessing very significant losses of biodiversity. Both the diversity and abundance of fish are decreasing. It is a loss of “ecosystem services” for the marine environment and for the human societies that depend on it. Fewer fish means a less abundant food source. The impacts on tourism are also significant.

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