- Over the past decade, the Mediterranean Sea has experienced frequent, destructive marine heat waves that have impacted a diversity of marine life, including red gorgonians (Paramuricea clavata).
- In 2022, researchers launched “Noah’s Ark of the Deep,” an expedition to study the gorgonians in the western Mediterranean Basin. In April, the second mission of the expedition explored gorgonians below 50 meters (164 feet).
- While the gorgonians in shallow waters suffered as temperatures rose, corals in deeper waters appeared untouched by the impacts of thermal stress.
- Researchers are currently trying to understand if these deeper gorgonians can help repopulate shallow populations if climate conditions allow them to regenerate.
Scientists have discovered that soft, branching corals deep in the Mediterranean Sea have remained unharmed by heat stress. In contrast, the corals in shallow waters have noticeably suffered as temperatures have risen.
Over the past decade, the Mediterranean Sea has experienced frequent, destructive marine heat waves that have affected vast swaths of the Mediterranean’s surface. Sea surface temperatures have previously risen as high as 5° Celsius (9° Fahrenheit) above normal, killing animals from 50 taxonomic groups.
One affected species is the red gorgonian (Paramuricea clavata), also known as the violescent sea-whip, a soft coral species known to “engineer” the local ecosystem by creating three-dimensional habitats that attract a host of biodiversity.
In 2022, researchers working with UNESCO and Fondation 1 Ocean launched “Noah’s Ark of the Deep,” an expedition to study the gorgonians in the western Mediterranean Basin. After a period of abnormally high temperatures in the Mediterranean, they found that many gorgonians at 0 and 30 meters’ depth (up to 98 feet) died off as a result of the heat stress.
Yet a second mission of the expedition, launched in April, discovered that gorgonians in deeper waters — below 50 meters (164 feet) — appeared untouched by the impacts of thermal stress.
“From one side, it is good news because we know that there are some populations that are not affected,” Lorenzo Bramanti, a researcher at the LECOB-CNRS laboratory who led the expedition, told Mongabay. “On the other side, we know that this increase in the frequency and strength of the heat waves will also bring the warming of the water to that depth. I imagine that we have bought time … to understand how the population works and to do something to stop the destruction of the population.”
Many corals in the Mediterranean are highly vulnerable to heat stress due to their slow-growing nature. A 2021 study found that coral reef communities in the Mediterranean had yet to recover from a marine heat wave in 2003 due to subsequent heat events that have occurred on a near-annual basis.
Earlier this year, Bramanti and a team of divers, including photographer Alexis Rosenfeld from 1 Ocean, dived near the Saint-Raphaël region of the Var, France, going as deep as 70 meters (230 feet) to assess the health of the gorgonians. Rosenfeld said the coral reef communities at these depths were “full of life” and that he felt he was “opening the door to a new world.”
“This new discovery gives us real hope for the future of this scientific programme and, more broadly, for a whole area of marine biodiversity,” Rosenfeld said in a statement.
Bramanti said a vital component of the project will be understanding if the deep population of the gorgonians will act as a refuge for the species. This will depend on whether the deep populations of the gorgonians are genetically independent of the shallower populations, which will give them a greater chance of survival, he said. Additionally, Bamanti said he is trying to understand if these deeper populations can act as a “reservoir of larvae to repopulate the shallow population” if conditions allow the shallow corals to regrow.
“For the moment, we can’t do anything [but] try to regulate the climate … to stop these increasing heat waves,” Bramanti said. “But in the future, if we can reobtain acceptable environmental conditions for the shallow population, the population could be the vault from which we can repopulate the shallow ones.”
If climate change accelerates, however, experts believe marine heat will likely move deeper into the water.
The expedition team plans to take additional dives in the Mediterranean to continue collecting data on the health and survivability of the gorgonians. Another heat wave is predicted for this year due, in part, to a possible El Nino climate pattern that may begin in the near future.
“The temperature of the water now is quite warm already for the season,” Bramanti said, “but a strong tramontana, a [northern] wind phenomenon, could locally change the conditions, so it’s not so easy to forecast what will happen.”
‘There’s not much hope’: Mediterranean corals collapse under relentless heat
Banner image: An underwater forest of red gorgonians (Paramuricea clavata). Image by Alexis Rosenfeld.
Elizabeth Claire Alberts is a senior staff writer for Mongabay. Follow her on Twitter @ECAlberts.
Citation:
Gómez-Gras, D., Linares, C., López-Sanz, A., Amate, R., Ledoux, J. B., Bensoussan, N., … Garrabou, J. (2021). Population collapse of habitat-forming species in the Mediterranean: A long-term study of gorgonian populations affected by recurrent marine heatwaves. Proceedings of the Royal Society B: Biological Sciences, 288(1965). doi:10.1098/rspb.2021.2384
