Thanks to its 12 overseas territories, France has 60,000 km2 of coral reefs, or 10% of the world's surface area. If in Reunion this reef only measures 18km2 and 25 km as a coral reef, its role nonetheless remains essential in the ecosystem. This marine oasis is home to more than 3,500 animal and plant species, including around 200 species of coral. But behind the postcard of lagoons of white sand and turquoise water, hides a coral reef that is degraded and threatened by many factors.
The role of global warming… but not only!
Globally, 20% of coral reefs have permanently disappeared over the past 30 years. And Reunion Island is no exception: in the 1980s, corals occupied more than 65% of the reef. Today, they are estimated between 20 and 25%. The threats are global but also local. “Since 1998, we have regularly observed episodes of coral bleaching, which particularly concerns branching corals. Every 4 or 5 years, we observe major episodes of coral bleaching” explains Bruce Cauvin, head of the education center within the Réunion National Marine Reserve. To schematize the phenomenon, we must remember that the coral is an animal, a polyp, which creates a limestone skeleton. This small polyp lives in symbiosis with an algae: the zooxanthella. However, when the temperature becomes too high, the zooxanthella flees the polyp and causes it to weaken or even die if the temperature does not drop quickly.In Réunion, following its episodes, a small part will die, around 10%. It is therefore one of the strong threats weighing on our coral reefs in Reunion Island.“, details Bruce Cauvin.
But on the Indian Ocean island, the threat also lies in the watershed. The steep slopes of the territory lead to potential mudslides due to increased runoff of surface water, including inputs present in agricultural soils or hydrocarbons. Materials dangerous for the coral reefs into which they flow. The cause is deforestation that has been taking place since the colonization of the island, accompanied by increasing concreteization which makes the soil less permeable.
Act on anthropogenic causes
If the impact of users should not be neglected (trampling of corals, use of toxic sunscreen, recovery of corals on beaches), anthropogenic causes appear much more important in the degradation of reefs. Léo Broudic is a doctoral student in socio-ecology (University of Rennes and Sciences Po Paris). Also an employee of the ARBRE association (research association for biodiversity in Reunion), he is developing a thesis entitled 'From landscape ecology to territorial planning: towards integrated management of the coral reefs of Reunion'. We meet him in Saint-Pierre, near the lagoon and an evacuation nicknamed the mermaid buzzard. “These are wastewater and rainwater that mix and come out directly into the reef. A plume that will impact all the surrounding coral communities. With our results, we see that the corals here are dead.“explains the researcher.”Each of the threats has its perpetrators. The idea of the thesis is to see: who is responsible for what, and who has the power to change these trajectories” argues Léo Broudic. His objective is to bring political decision-makers face to face with realities. To do this, he wants to develop fun and concrete tools for public authorities in connection with regional planning in order to take into account the risks linked to reef. This involves in particular the land use planning plan which must soon be revised.There would remain around 3,200 hectares to be urbanized in Reunion. Now we need to ask ourselves, with all the stakeholders, not only economic ones but also the population, and say: what do we want?“. He also advocates the development of participatory democracy tools so that the population can be listened to and their voice taken into account.
Know to better protect
Among the awareness-raising actions, we also find the practice of coral cuttings. The principle is simple, recover fragments of dead corals and re-establish them on stainless steel structures submerged in the lagoon, called frames. In Reunion, the Co-récif association installed two in the dugout basin of Étang Salé two years ago. “We have very good results and colonies which are starting to reach a significant size.“, says Caroline Massac, representative of the association. Indeed, a few dozen meters from the shore, we discover underwater two structures loaded with a hundred colorful corals, arranged like a work of art Co-récif supports Reunion students and volunteers to discover this practice, but more broadly to learn about the reef.We take cuttings to raise awareness, we want to show what coral is, that it is a living, fragile animal. And show the pressures he is under. Cuttings in terms of restoration are the ultimate solution. We can't do this by saying 'it's okay, we did our part, we saved the reef'. What will work is: observe why he has difficulty developing, what these pressures are and how to fight against them. If we always have poor quality water arriving in the lagoon, the corals will not develop and that will be of no use..” concludes the specialist in tropical marine biology.
What if raising awareness was no longer enough? Indeed, there is an emergency, confirms Tévamie Rungassamy, head of the Natural Heritage division at the Marine Reserve: “We have been in an emergency situation for several years, the particularity is that we are in an ecosystem which will try to maintain itself but at one point we will reach a point of collapse. This last phase is often linked to a runaway, that is to say an acceleration of degradation phenomena. For the moment, it is holding up but less and less well.” explains the scientist. “Today, coral reefs and associated ecosystems still respond as best they can to the ecosystem services they provide us. Thus, the coral reef protects us against the aggression of waves, the coral skeletons allow, among other things, the production of white sand for our beaches, the reefs attract tourists and constitute a food resource, for example. But all this may collapse further in the years to come.”
However, combined, the various nuisance factors continue to increase. As the Marine Reserve points out, globally, 75% of reefs could reach a critical stage of threat and disappear by 2050.
