Mediterranean coral pops under pressure from climate crisis

A new study by the Faculty of Biology, the Biodiversity Institute (IRBio), and the Institute of Marine Sciences(ICM-CSIC), has shown that the Mediterranean’s coral populations are being reduced by heatwaves. In some cases, their biomass has fallen by 80 to 90%.

According to the study, published by the journal Proceedings of the Royal Society B, coral populations of the Mediterranean —essential for the functioning of the coral reefs, one of the most emblematic habitats in this sea— could be unable to recover from the recurrent impact of these extreme episodes, with water temperatures reaching high degrees for days and even weeks.

This is the first study to evaluate the long-term recovery potential of populations of two emblematic species in Mediterranean coral: the red gargonian ().Paramuricea clavata) and the red coral (Corallium rubrumComplex habitats are vital for a wide range of fauna. It is important to understand how they cope with heatwaves that are more frequent and intense.

Mass mortalities

The global climate crisis is affecting marine ecosystems severely, and the Mediterranean is no different. The climate crisis is causing mass deaths in all coastal ecosystems. In particular, the heatwaves that affect the marine heatwaves are affecting the Mediterranean corals.

Knowledge on the coral’s long-term resilience is still scarce, despite the studies that analysed the immediate impact of marine heatwaves on these organisms. These coral species are long-lived (more than 100 years in some cases) with slow populational dynamics (organisms with low growth or recruitment rates). Therefore, researchers need to perform long temporary series (decades). To assess their recovery ability.

The study involved the analysis of long-term monitoring data on coral populations that were affected by a large mass death in 2003 by a heatwave in the protected area of Scandola, France. They analysed data from the MedRecover research group that included experts from the UB and ICM–CSIC on the state of these populations (density and size structure, biomass, and other factors) over the fifteen years following the heatwave.

The results show that all of the population analyses showed signs of collapse after the 2003 heatwave. These populations are now considered extinct from a functional perspective 15 years after the 2003 heatwave.

“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, researcher at ICM-CSIC.

“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.

Recurrent heatwave exposure

Cristina Linares, professor at the Department of Evolutionary Biology, Ecology and Environmental Sciences of the Faculty of Biology and member of IRBio, says that “we believe one of the main reasons why we observed these collapse trajectories is the potential recurrent exposure to heatwaves, incompatible with the slow populational dynamics of these species”. They experienced significant heatwaves in at most four years during the study period (2003-2018), including 2009, 2016, 2017, and 2018.

“During these heatwaves,” continues Linares, “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.”

The climate crisis is expected to increase the intensity and frequency of heatwaves in marine waters over the next decades, which could lead to coral populations being seriously endangered.

“However, there will probably be some areas in the Mediterranean in which, due to several factors, the recurrence of such climate impacts may be lower. This makes it especially relevant to keep —regarding other potential impacts— these climate refuges where the trajectories of coral populations could be more positive than those observed in this study,” notes the research team.

“Nevertheless, there is an urgent need for stronger measures to be implemented against the climate crisis before the loss of biodiversity becomes irreplaceable,” conclude the experts.

This study has been carried out thanks to the support of the Regional Natural Park of Corsica, the HEATMED National Plan project (RTI2018-095346-B-485 I00), the European projects H2020, MERCES, H2020 Futurmares, Interreg MED MPA-Engage and Interreg MED MPA-Adapt.