Climate change is affecting corals everywhere — including in the Mediterranean, according to a new study.

Coral populations in the Mediterranean are experiencing immense damage due to climate-change-induced heatwaves. Two emblematic species, the red gorgonian (Paramuricea clavata) and the red coral (Corallium rubrum), have lost 80 to 90% of their total biomass since 2003, according to new research.

The findings are very concerning. Coral populations are a linchpin of the marine ecosystems they belong to, providing food and shelter for a multitude of other species. The incredible decline seen in this paper is likely indicative of the broader coral communities in the Mediterranean. If so, wildlife in the sea could be in a much more dire situation than anyone believed.

A sea of troubles

“We observed an average biomass loss regarding the initial biomass of 80% in populations of red gorgonian, and up to a 93% regarding the studied population of red coral,” notes Daniel Gómez, a researcher at the Institute of Marine Sciences of Barcelona (ICM-CSIC) and lead author of the study.

“These data are worrying for the conservation of these emblematic species, and it indicates that the effects of the climate crisis are speeding up with obvious consequences for the submarine landscapes, where the loss of coral equals the loss of trees in forests,” notes Joaquim Garrabou, also member of ICM-CSIC.

The authors explain that populations of the two studied coral species could be unable to recover under current conditions. Their plight comes down to rising temperatures, but especially to significant heatwaves that have impacted the region repeatedly, with the first one occurring in 2003.

Water temperatures are reaching levels that are completely unbearable for these corals and maintaining those temperatures for days, even weeks at a time, the authors explain. While corals all over the world are affected, this is the first study to quantify the effects of climate change and heatwaves on Mediterranean corals in particular. Here, as in other areas, climate change is causing mass mortality in the sea’s coastal ecosystems.

Both species are emblematic of the Mediterranean, underpinning the area’s complex ecosystems. They also have a large role to play in shaping the sea’s distinctive landscapes and look.

Researchers currently have information on the short-term response of corals to marine heatwaves. That being said, corals are long-lived creatures with very slow population dynamics — they are slow to grow and slow to spawn new generations) — so accurately understanding their response to climate change required decades of study. And that’s what the team did.

They used data from a long-term project by the MedRecover research group, which monitored different populations of coral in the protected marine area of Scandola (in Corsega, France) which saw mass mortality after the 2003 heatwave. Of particular interest were population density, size structure, and total biomass, which were used as proxies to estimate the overall health of these coral communities. Data was collected for fifteen years following the heatwave (up to 2018).

The data showed that all populations monitored in the study hadn’t recovered following the heatwave. In fact, they tended to collapse. Today, they are functionally extinct, the team explains.

“We believe one of the main reasons why we observed these collapse trajectories is the potential recurrent exposure to heatwaves [in 2009, 2016, 2017, 2018], incompatible with the slow populational dynamics of these species,” says Cristina Linares, professor at the Department of Evolutionary Biology, Ecology and Environmental Sciences of the Faculty of Biology and member of IRBio, co-author of the paper.

“During these heatwaves, the temperature conditions in the studied area reached extreme levels which are incompatible with the life of these corals, which probably caused new mortality events to the decimated populations and made the recovery impossible.”

These populations are at serious risk of actual extinction, especially since the number and intensity of marine heatwaves is set to increase in the future as the climate crisis deepens. However, the team adds that there are likely some areas in the Mediterranean where the impacts of climate change may be lower due to local factors. These should act as ‘climate refuges’ to help preserve the corals, they conclude.

The team “Population collapse of habitat-forming species in the Mediterranean: a long-term study of gorgonian populations affected by recurrent marine heatwaves” has been published in the journal Proceedings of the Royal Society B: Biological Sciences.