The Mediterranean is warming rapidly and transforming in alarming ways. Divers are witnessing fish die-offs and a surge of exotic species. Reporterre met them after a dive.
The gray boat of the International Diving Center (CIP) returns to the port of Nice. As soon as it is moored, heavy tanks and fins are unloaded. Only the wetsuits feel light. Beginner divers entered the water in sleeveless neoprene suits. Aurélie descended to 6 meters and “wasn’t cold.” The water temperature reached 26°C. As across the Mediterranean, a marine heatwave is affecting Villefranche-sur-Mer, a popular dive spot. “Water temperatures could easily rise to 30°C,” reports La Chaîne Météo—a phenomenon not seen since August 2003.
Aurélie surfaced with lasting impressions. She saw “octopuses, starfish, sargo, salpa, sea urchins, a grouper,” listing them as if describing an aquarium. For her first dive, she was also accompanied by peacock wrasse, small iridescent fish native to the southern Mediterranean. “When I was little, we saw one or two per dive,” notes instructor Laura Bottau, wearing a nurse shark pendant and a turtle tattoo on her forearm. “Today, there are plenty.”
She often swims among barracudas, parrotfish, and species migrating from the Red Sea. Warmer waters are shifting marine species’ distribution. This is known as the “tropicalization of the Mediterranean.” “The climate favors the expansion of invasive exotic species,” says Alexandre Iaschine, Director of the Marine Foundation. “They move north via the Suez Canal, such as the voracious herbivore rabbitfish.”
Underwater Fire
Richard Vial has been diving since the 1980s—even marrying at 12 meters deep. His latest dive, on Monday morning, confirmed what he had noticed: exotic species are appearing where they weren’t before. Previously, he could observe “gorgonian colonies, coral, and sponges above 15 meters,” which have now disappeared from the surface.
“There is massive mortality of all species attached to the seafloor,” notes Alexandre Iaschine. Thermal stress will kill them, as they cannot move. These heatwaves are “earlier, more frequent, and more intense.” They are defined when temperatures exceed normal levels—25-26°C—for more than five consecutive days. “It’s like an underwater fire,” he adds.
In 2022, a team of about sixty international researchers studied Mediterranean heatwaves between 2015 and 2019. Published in Global Change Biology, their findings show that marine heatwaves can cause massive die-offs of around fifty species of fish, sponges, algae, and mollusks, down to 40 meters depth.
Seafloor Disruption
When Richard Vial puts on his wetsuit, mask, and tanks and descends, he feels a welcome coolness. At 20 meters, the water drops to 20°C. “It’s the first year like this,” he says. “A cooling layer is saving us.”
This is due to the thermocline, the separation of water layers. “During particularly intense heatwaves, surface and deep water layers no longer mix, like oil and water,” explains Alexandre Iaschine.
In July, upwelling (cold water rising) also cooled the Mediterranean. The mistral wind pushed warm surface water offshore and brought cold water up, giving the struggling sea some relief.
At the boat’s descent, diver Alexia watches her ears. Experienced, she isn’t worried about pressure after a 30-meter round trip. She fears bacteria thriving in warm water, causing ear infections. The marine heatwave is expected to last ten days.
“Then, each winter, the water drops to 13°C, and everything regenerates,” hopes Richard Vial, who continues diving year-round in the bay. Alexandre Iaschine is more cautious: “Using the fire analogy, once it passes, the damage is done. There is a fundamental disruption affecting the Mediterranean’s balance.”
