The alert was given at the end of August by divers “sentinels“, connoisseurs of these multicolored coral forests off the coast of the second city of France.
“It hurts the heart, the deterioration is super fast, only two months ago we were diving here to explore this magnificent fully colored forest“, deplores Tristan Estaque, in charge of scientific missions at Septentrion environnement, aboard the boat of this association studying Mediterranean marine ecosystems.
Meanwhile, a major marine heat wave hit the western Mediterranean this summer with water temperatures “4 to 5 degrees“Normal, according to Mercator Ocean International, the organization leading the European ocean monitoring service. The water temperature rose up to 30 degrees in places.
– “ghost forest“-
Coming back from an exploratory dive on this September day, Tristan Estaque described to AFP an apocalyptic landscape under water, a “ghost forest“: “You have to imagine a tree where there are no more leaves, no more bark“.
In his hands, a piece of dead gorgonian, a small beige tree with bare branches: “Normally on this species there is a purple tissue full of polyps“, he laments. Under the pressure of continuous and intense heat, the gorgonian dies and its tissue turns gray and crumbles to dust.
According to surveys from Septentrion environnement, near Marseilles “70 to 90% of the population of red gorgonians” in the area of 10 to 20 meters are dead.
The few living tissues removed (about 20% of each “tree“) are collected and tagged on the boat, for genetic analysis.
Gorgonian mortality has also been observed on the Spanish coast, in the Toulon region or even around the Italian island of Sardinia, according to Stéphane Sartoretto, who participates in species monitoring for the French Research Institute for Exploitation. from the sea (Ifremer).
In the Calanques National Park, these spectacular creeks scattered on the coast near Marseille, it was especially intense due to the shallow implantation of the gorgonians, only six meters from the surface in certain areas. In the Balearics, they live deeper, at 40 meters, and have therefore been less impacted, according to Mr Sartoretto.
– Risk of disappearance –
In addition to the red gorgonian, sponges or even bivalves were well affected, he notes, as well as a fish, the mostelle.
The first well-documented episode of gorgonian mortality was recorded in 1999, notes Mr. Sartoretto, “but the heat stroke was in the fall, whereas there it happened very early, so we are not immune to new heat strokes in October…”
The low temperatures seen in recent days can perhaps “preserve those that were not affected“, according to Solène Bastard-Gogain, without being sure that it stops the phenomenon because”if heat has favored a pathogen, it is likely“that he is still present.
For the destroyed gorgonians, scientists are pessimistic: “As for a forest fire on land, the resilience is very low for the gorgonians, they will take decades to regenerate“, deplores Mr. Sartoretto, also emphasizing the low reproduction rate of these marine animals.
“We can ask ourselves the question of their disappearance if the heat strokes are repeated too often, and in this case, what will happen?“, wonders the scientist. Because the gorgonians are “engineer species, which structure the seabed“.
They constitute “real +animal forests+ which are home to 15 to 20% of known species in the Mediterranean“, according to the Calanques National Park.
If this sea covers less than 1% of the planet’s ocean surface, it is home to “18% of all known marine species“, according to a report by the network of Mediterranean experts on climate change (Medecc), and already presents “the highest proportion of threatened marine habitats in Europe“.
There “marine heat wave“has also affected other species, especially mussels: in Spain, 150 tonnes of commercial mussels and 1,000 tonnes of farmed mussels were lost this summer.