An internationally renowned scientist, researcher and professor, Bernard Salvat has dedicated his life to studying coral reefs and their conservation. In 1971 in Moorea, he created Criobe, centre de recherches insulaires et observatoire de l’environnement (Center for island research and environmental observatory). He is our guide as we discovery this fascinating ecosystem that is an essential part of life and the environment in French Polynesia. A vital perspective at a time when corals are threatened worldwide.
Can you tell us how long coral reefs have existed on Earth?
Bernard Salvat : Present-day reef-building corals have extremely old ancestors and have existed for hundreds of millions of years. Coral reefs in the time of dinosaurs looked much like they do today, even if they were made up of different species.
How are corals able to build these imposing reefs and build islands?
Their secret lies in their association with microscopic algae that we call zooxanthellae; they are just single cells no larger than several microns (editor’s note: unit of measurement, one micron is a thousandth of a millimeter.The living part of the coral are the polyps, tiny sorts of sea anemone growing side by side, there are millions in a cubic centimeter. It is through this association between an animal and a plant, which can also be called a symbiosis, that corals are able to build their limestone exoskeletons. Corals covering a surface of one square meter can, on average, produce around 10kg of skeleton every year. When corals die, the limestone skeleton remains and that is how the reef grows. Today there are around 800 species of corals that produce coral reefs.
Are there different « types » of coral reefs ?
Yes, there are several different types of coral reefs, at least we try to classify them neatly, even if nature is not easily categorized. The most common in French Polynesia are fringing reefs and barrier reefs that surround high volcanic islands, forming a highly developed reef complex, starting at the coastline and stretching out to the reef face that is battered by the ocean waves. And then beyond that there are other coral populations that grown on the external reef face, to depths of around 80 meters if the water clarity is good. And then there are atolls, where there is only coral, the debris of limestone skeletons are what make up the motu that rise out of the water, the atoll’s crown, with living coral populating the steep reef face and form coral heads in the lagoon. The difference between the reefs of a high volcanic island, like Moorea, and an atoll, are that in the atoll the volcanic edifice has sunk down under its own weight, back into the oceanic plate while the corals continue to keep growing upwards piled one atop the other over time, countering the sinking. The volcanic portion of the atoll is found deep underwater ; for an atoll like Mataiva or Takapoto it is several hundred meters down, or even further.
You mentioned – 80 m. Why can’t corals live deeper than this?
Quite simply because their symbiotic algae need light for photosynthesis. Below that there is not enough light. However, at greater depths, right down as far as the ocean abysses, at depths of several kilometers, other kinds of corals that do not possess zooxanthellae can be found. Such corals are called « ahermatypic » as opposed to the reef constructors at the surface (hermatypic). They manage to build what are incorrectly called « deep-sea reefs » that are « bioconstructed ». The term “reef” is inappropriate because the word’s meaning is an obstacle on the ocean’s surface.
Have our islands’ reefs been well studied and are they well understood ?
The first descriptions of coral reefs date from the 19th century and early 20th century, but in reality it is only after WWII that in-depth research began. It was not until 1969 that the first coral reef researchers – around fifty of them – gathered for the first international conference and the creation of an international association. The most recent « reef » congress was attended by more than 2,500 research scientists. For our islands the first descriptions date from the start of the 20th century with L-G Seurat and the zoology laboratory on Rikitea in the Gambier Islands, who visited and published work on many atolls in the Tuamotu, like Marutea. Since 1965, research on our islands reefs has multiplied. There must be more than 3,000 published scientific articles. The Tiahura reef in Moorea has been a focus of study for over 300 scientists and is known worldwide thanks to the work of Criobe, that you mentioned at the start of the interview. It can be said that French Polynesia’s reefs are relatively well known, even if there are still atolls that have not been studied.
But what is the broader significance of this research ?
I was coming to that, researchers work within a cultural, economic and social context. Understanding helps us to better manage and the researchers participate and are committed to establishing marine protected areas, combating pollution, and creating laws… But they can only provide their opinion, the final decisions are political .
The Society Islands, Tuamotu and part of the Australs have flourishing reefs, why is this not the case in the Marquesas and Rapa the southernmost of the Austral Islands ?
As you know, during the last ice age sea level was around 140m lower than it is currently. The polar ice caps locked up a large part of the world’s water as ice. When the ice melted, around 14,000 years ago, the sea-level rose. We have been in an interglacial period these last 14-millennia, with sea-level rising to its current level. In the Society and Tuamotu Islands the corals continued to grow and, as their skeletons accumulated, the reefs kept pace with the rising water level. About 40m of layered corals have accumulated over the last 14,000 years below Tahiti’s barrier reef. This rate of growth has been possible because water conditions, particularly temperature, have allowed the corals to thrive. However, in the Marquesas the local conditions have not been favorable to coral growth, undoubtedly because the water temperature has been too cool, due to the upwelling of deep ocean currents, and so the corals have not been able to keep up. The oldest reefs are now 110m below the surface. In Rapa, in the Australs it is the ambient temperature that is just slightly too cool, and so coral reef formations not been able to establish.
Are there any fundamental differences between the coral reefs from the different archipelagos possess them ?
There are morphological differences that we refer to when distinguishing high island reefs and those of atolls. We can also note that poorly developed reefs on the volcanic island Mehetia, to the East of Tahiti, where the last volcanic eruptions occurred within the last thousand years and where reef formation has just started. But where reefs have formed, there are no real fundamental differences between the archipelagos except in terms of the coral species and their diversity. It is the Society Islands reefs that contain the greatest number of coral species, mollusks and fish, but that does translate into a measure of their abundance . The different archipelagos have seen species evolve that are specific to them, as in the Marquesas where levels of endemism, in several groups, exceeds 10% of all species.
Other than providing emblematic landscapes for French Polynesian tourism, what are the other advantages of having coral reefs and lagoons ?
Of course, as tourism based on these emblematic landscapes is only just a very recent aspect of our Polynesian islands’ economy. Foremost, without corals and reefs the 85 atolls and 118 islands that make up French Polynesia simply would not exist. In fact, the corals attach themselves to the underwater slopes of volcanic islands when they first form and continue to grow as the volcanic edifice subsides; the basalt disappears under the corals that now form an atoll. A second advantage of having a reef is that it protects the island from rough seas and cyclones. The third and quite important advantage of having a reef is that it provides a food source for the inhabitants, an aspect that has deeply influenced the Polynesian culture. And let’s not forget about pearl farming.
What is the current state of the coral reefs in our territory and what major damage have they suffered ?
The state of coral reefs in the world is not very encouraging because of damage and pollution caused by human activity and demographic pressure. Researchers started to sound the alarm in the early 80s, and at a time before climate change was a concern. French Polynesia’s reefs are in relatively good condition on the scale of the 118 islands, which includes around thirty uninhabited atolls. Of course, reefs in urban zones have suffered, but this is only within the lagoons, not the outside reef slopes, which are the living part of the ecosystem. Aside from human impacts, reefs are also devastated by cyclones, or by abnormally warm water, which triggers bleaching (the corals lose their algae and their color) which can result in coral die-off, or the invasion of taramea, the Crown of thorns starfish, that feeds on coral polyps. All these phenomena have always existed, it is the increased frequency and intensity of these events that are raising questions about the future of coral reefs.
Are you referring to climate change ? What is going to happen to our reefs ?
Obviously, there is great concern about the future of coral reefs and certain islands. The rise in ocean water temperature is the greatest worry. Corals cannot tolerate an increase in summer water temperatures of more than 1°C above the norm. If the water stays warmer than 29-30 °C for several days, they will bleach and die. It is expected that by around 2040 or 2050, according to estimates of greenhouse gas emissions, with a temperature increase of 1.5 to 2.5 °C, bleaching events risk becoming annual phenomena, and under this scenario the reefs will lose their “resiliency”: and will have difficulty to recover. An associated acidification of the water, causing a reduction in the pH, will also reduce the calcification potential of corals, creating a serious long-term threat. In the short-term, however, it is the temperature that is the most worrying. As for cyclones, the predictions are not in agreement: there may be slightly more of them, but undoubtedly they will be more powerful.
But you didn’t talk about sea-level that is going to rise ? What about our islands ?
The previous question focused on coral reefs. Sea-level increases are a good thing if anything for them, as they will have more space to develop. But for the inhabitability of an island that’s another story. Researchers are not in agreement about what will happen to atolls, who have an altitude of no more than 4m. Staying within French Polynesia, where sea-level has increased by some twenty centimeters in 70 years (since 1950), observations made on atolls in the northwest Tuamotu (Mataiva, Rangiroa, Takapoto…) show that most motu have stayed the same size (77 %) or increased in size (15 %) with just a minority (8 %) seeing their surface area diminish.
What should be done to protect our reefs ?
Aside from the global responsibility to reduce greenhouse gas emissions, there are public policy measures and citizen-based initiatives . For the political leaders, it is important to minimize pollution and physical destruction of reefs by humans, processes that have been damaging reefs for years now, because a healthy reef will be better able to withstand the effects of climate change. Coral reefs must therefore be better managed to ensure their resilience. On an individual level, it is necessary to respect the environment, particularly to bear in mind that « everything ends up in the ocean » and to support associations that protect nature against destructive development that is all too often focused on short-term profit. It’s our survival that’s at stake here !
Interviewed by Ludovic Lardière