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‘There’s not much hope’: Mediterranean corals collapse under relentless heat

A red gorgonian coral (Paramuricea clavata) partially dead due to a marine heatwave. The lefthand side is still alive, while the righthand side is dead and the skeleton is exposed. Image by Eneko Aspillaga.

  • In 2003, a marine heat wave devastated coral reef communities in the Mediterranean Sea, including the reefs in the Scandola Marine Reserve, a protected region off the coast of Corsica.
  • More than 15 years later, the coral reef communities in Scandola still have not recovered.
  • Researchers determined that persistent marine heat waves, which are now happening every year in the Mediterranean, are preventing Scandola’s slow-growing coral reefs from recuperating.
  • Human-induced climate change is the culprit; persistent rising temperatures in the ocean have normalized marine heat waves, not only in the Mediterranean, but in the global oceans.

For years, Joaquim Garrabou donned scuba gear and dove into the waters of the Scandola Marine Reserve in Corsica to find a paradise. Twenty meters (66 feet) beneath the surface, there were reef walls draped with soft red coral (Corallium rubrum) and red gorgonian sea-whips (Paramuricea clavata), all swarming with fish and other sea creatures. But in 2003, a marine heat wave hit Scandola, leading to the death of many coral reefs. More than 15 years later, the reefs have still not recovered.

Now when Garrabou dives at Scandola, he’s greeted by the skeletons of once-thriving corals.

“It’s like seeing someone who is ill, who has a disease that you cannot find the solution for,” Garrabou told Mongabay in a video interview. “You hope that someday there will be a [solution] but you see that there’s not much hope.”

After the 2003 marine heat wave, Garrabou and colleagues began monitoring Scandola’s coral reefs to track their recovery. But after accumulating reef survey data and temperature data over many years, they eventually realized they were actually tracking the reefs’ collapse. The results of their long-term study were recently published in Proceedings of the Royal Society B.

“We knew something bad was happening to the corals around the world, but we weren’t expecting a collapse in all of the populations that we studied,” study lead author Daniel Gómez-Gras, a marine ecologist at the Institut de Ciències del Mar in Barcelona, told Mongabay in a video interview. “The point of tracking these populations for such a long time was to show recovery in the long term because we expected that the populations — maybe not in five years, but in 15, 20 years — [would be] able to recover. However, we saw a collapse.”

A researcher monitoring a red coral population in the Mediterranean. Image by Medrecover.

‘We don’t call it bleaching’

The data showed that marine heat waves were happening every year in different parts of the Mediterranean between 2003 and 2018. For 12 of those years, the water temperature at a depth of 20 m reached more than 23° Celsius (73.4° Fahrenheit), which is considered a sublethal threshold for corals. And for four of those years — 2009, 2016, 2017 and 2018 — temperatures at that depth breached the lethal threshold for corals at 25°C (77°F).

The researchers found that the ceaseless heat wasn’t allowing these slow-growing coral reefs to recover.

​​“Frankly, I never thought that I would be seeing it,” Garrabou said. “And it’s happening really fast.”

Soft coral species in the Mediterranean don’t “bleach” the way that tropical corals do, Gómez-Gras said. That’s because Mediterranean corals don’t have a symbiotic relationship with zooxanthellae, the algae that tropical corals expel when they experience heat stress.

“We don’t call it bleaching here in the Mediterranean for these coral species, since they don’t bleach,” he said. “They directly die with a loss of tissue and skeletons being exposed.”

While the results of the study are relevant to many coral communities across the Mediterranean, the researchers chose to focus their study on Scandola because the area had been established as a marine protected area (MPA) in 1975, and had been relatively free from other human pressures such as fishing and pollution. This helped them eliminate other possibilities for the coral reef population collapses and to pinpoint marine heat waves as the reason for their demise.

Researchers used to think that deeper reef communities might shelter coral species from heat stress. But it’s becoming increasingly clear that this isn’t the case, not only in the Mediterranean, but in other parts of the world, including coral reef sites in the Pacific.

“We are witnessing that if you go deeper, [there is still] impact,” Garrabou said.

Maps showing the location of Scandola within the Mediterranean, as well as the location, species and depth of the five monitored populations within Scandola MPA. The graph shows the number of marine heat waves that occurred between 1982 and 2018, and severity of each event. Image by Gómez-Gras et al (2022).

‘The new normal’

Human-induced climate change is responsible for the heating of the oceans — and it’s becoming hotter and hotter in the water. According to another study, the global oceans have broken a heat record for the sixth year in a row. As the oceans warm, heat penetrates downward — and this heating trend will continue even if emissions stop tomorrow, Kevin Trenberth, co-author of this separate study, told Mongabay in January.

A related study also found that extreme heat have become the new normal for the global oceans as climate change rapidly transforms our world.

The Mediterranean may be feeling the impacts of climate change even more intensely than other parts of the world. A report published last year by WWF found that the Mediterranean was warming 20% faster than the rest of the world’s oceans.

Gómez-Gras said the accelerated warming in the Mediterranean has partly to do with its semi-enclosed shape. While this is unique to the region, he added that the Mediterranean shows what will happen in other parts of the ocean due to climate change.

“Marine heat waves are becoming the new normal in the Mediterranean Sea,” Gómez-Gras said. “So you can guess that in the future, it can become the new normal [elsewhere] in the world.”

Georgios Tsounis, a marine biologist at California State University, whose work was based in the Mediterranean for 11 years, but who was not involved in this research, praised the new study in Proceedings of the Royal Society B for its “valuable approach.”

“We need more long-term demographic studies such as this one to better understand where our environment is heading in the future,” Tsounis told Mongabay in an email.

While the study is focused on the soft coral communities of the Mediterranean, Tsounis said the research can help us understand how other coral communities “may or may not recover from repeated stress over a period of 15 years.”

“We are seeing coral mortality in other parts of the world as well,” he said. “The tropical coral reefs make sad headlines every year. But in the tropics we are mainly concerned with reef-building hard corals (as opposed to the soft corals in this Mediterranean study). The temperature range and entire cause-effect mechanism differ between these two examples. What is common to most of these scenarios is that the corals have adapted to a narrow set of environmental conditions, such as temperature, over a long period of time, and are sensitive to changing climate.”

A red coral population with dead colonies after a marine heat wave in 2016. Image by Olivier Bianchimani.

‘It has to happen fast’

The researchers said they are searching the Mediterranean for “refugia,” places that offer coral reefs protection from thermal stress. One possible place could be the waters off the coast of the Calanques near Marseille, France, which seems to get enough cold water to protect its corals, Garrabou said. That said, the coral reef communities here experienced mass die-offs during marine heat waves in both 1999 and 2003. But since then, the region hasn’t had any major warming, and the corals have been able to slowly recover, he said.

While there are currently many places of refugia for coral communities across the world, a new study found that most of these places will disappear once the world reaches 1.5°C (2.7°F) of warming above pre-industrial levels, which is likely to happen within the next decade.

But it’s not just climate change placing pressure on the Mediterranean — fishing and pollution are additional stressors to the region. Because of this, Garrabou said it’s important to establish MPAs with strict protective measures to enhance the resilience of coral reef communities.

Currently, there are more than 1,200 MPAs in the Mediterranean, but only about 0.02% of the area they cover is closed to fishing year-round.

While the future looks grim for coral reefs, Garrabou said he feels hopeful about the momentum that’s building for the establishment of MPAs, especially with global efforts to protect 30% of the world’s oceans by 2030.

“When we provide the right conditions and the right tools, nature can be really generous and nature has demonstrated that it can bounce back,” he said.

But he said that MPAs need to be urgently established for the oceans to reap their benefits. “It has to happen,” he said, “and it has to happen fast.”

Citations:

Bongaerts, P., Ridgway, T., Sampayo, E. M., & Hoegh-Guldberg, O. (2010). Assessing the ‘deep reef refugia’ hypothesis: Focus on Caribbean reefs. Coral Reefs, 29(2), 309-327. doi:10.1007/s00338-009-0581-x

Cheng, L., Abraham, J., Trenberth, K. E., Fasullo, J., Boyer, T., Mann, M. E., … Reagan, J. (2022). Another record: Ocean warming continues through 2021 despite La Niña conditions. Advances in Atmospheric Sciences. doi:10.1007/s00376-022-1461-3

Dixon, A. M., Forster, P. M., Heron, S. F., Stoner, A. M., & Beger, M. (2022). Future loss of local-scale thermal refugia in coral reef ecosystems. PLOS Climate, 1(2), e0000004. doi:10.1371/journal.pclm.0000004

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

Jeffries, E., & Campogianni, S. (2021). The climate change effect in the Mediterranean. Six stories from an overheating sea. Retrieved from WWF website: https://www.wwf.de/fileadmin/fm-wwf/Publikationen-PDF/Meere/WWF-Report-The-Climate-Change-Effect-in-the-Mediterranean-2021.pdf

Tanaka, K. R., & Van Houtan, K. S. (2022). The recent normalization of historical marine heat extremes. PLOS CLIMATE. doi:10.1371/journal.pclm.0000007

Banner image caption: A red gorgonian coral (Paramuricea clavata) partially dead due to a marine heatwave. The lefthand side is still alive, while the righthand side is dead and the skeleton is exposed. Image by Eneko Aspillaga.

Correction (02/16/2022): An earlier version of this article incorrectly stated that a study by Tanaka and Van Houton (2022) suggested that marine heat waves were the new normal for the oceans. However, this study found that extreme heat was the new normal for the marine environment.

Elizabeth Claire Alberts is a staff writer for Mongabay. Follow her on Twitter @ECAlberts.

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