Mediterranean coral gardens may inevitably be compromised

Based on vast experience at sea and in the laboratory over the last 10 years, the latest study by the Interdisciplinary Center for Marine and Environmental Research at the University of Porto (CIIMAR-UP), Portugal, in collaboration with the Institute of Marine Sciences (ICM-CSIC) in Barcelona, Spain, predicts that Mediterranean corals and the communities that depend on them may be permanently compromised.

Extreme climatic events, such as marine heat waves, pose a strong threat to the biodiversity of marine ecosystems, including Mediterranean coral gardens, which are home to a huge number of organisms. However, little is known about how corals respond to the recurrence of these events, which threatens the efficiency of conservation and restoration actions.

Most existing studies focus on single heat wave events and only allow us to understand the momentary effects and reactions of the species in question. For this reason, they are unable to capture patterns of adaptability, whether due to genetics or phenotypic plasticity, of populations to the scenario of recurring events, risking predictions about their real and cumulative effect on biodiversity.

However, understanding this adaptability is a fundamental issue, given that marine heat wave events have become increasingly frequent in the Mediterranean, with surface temperatures of around 30ºC in 2024.

A multi-event study in the laboratory and in situ

The study, “Recurrent Extreme Climatic Events Are Driving Gorgonian Populations to Local Extinction: Low Adaptive Potential to Marine Heatwaves,” was published in Global Change Biology. The work involved researcher Jean-Baptiste Ledoux from CIIMAR’s Evolutionary genomics and Bioinformatics group, who monitored and reproduced the effects of marine heat wave events in the laboratory to assess the response of colonies of the red gorgonian species Paramuricea clavata (P. clavata).

The aim was to try to understand how their response to repetitive thermal stress varies, as well as the factors that most influence this variability. According to the two authors, Ledoux and Joaquim Garrabou, from the Institut de Ciències del Mar, “Our study combined aquarium experiments repeated over three years and carried out with the same colonies to replicate the effect of the recurrence of extreme weather events in the sea.”

Heartbreaking results

The percentage of necrosis in the samples was measured as an indicator of this coral species’ responses to thermal stress. While in 2015 and 2016 the percentages of necrosis were less than 60%, in 2017 there was a significant increase in mortality, with almost all colonies disappearing.

“As the same individuals from three populations and exactly the same experimental stress conditions were used during the three trials, we can conclude that the environmental component, i.e., sea temperatures during the summer, was the main factor underlying the responses of these colonies,” explains Sandra Ramirez-Calero, a Ph.D. student at the University of Barcelona and first author of the study.

The results show that ecological responses, i.e., resistance to heat stress, are not related to the genetic make-up of the population and that all individuals have a high environmental sensitivity with little ability to adapt. These experimental results agreed with the monitoring of this species at sea after the two marine heat waves in 2018 and 2022.

This leads to the conclusion that P. clavata populations have very limited adaptive capacities for extreme and recurrent climatic events, particularly marine heat wave events. “Both the experiments and the field surveys carried out here support a limited adaptability potential, both based on the genetic component and the plastic component,” says Ledoux.

This low or non-existent adaptive response, combined with high environmental sensitivity and a potential intensification of marine heat wave events, leads us to believe that this habitat-creating species is likely to face an inevitable population collapse in the shallow waters of the Mediterranean.

“Given the recurrence of extreme weather events, our results point to a potential collapse of many of the shallow populations of P. clavata,” explains Garrabou, co-author of the article. “Unfortunately, this study is yet another example of the poor adaptation potential of Mediterranean corals to marine heat waves linked to anthropogenic climate change,” emphasizes Ledoux.

What happens to ecosystems?

Corals, like sponges, are habitat-forming species. Like trees in a forest, when they exist, they provide shelter for a considerable number of other species and are the basis of these habitats.

As a result of the marine heat waves, there will be a change in the distribution limits of these species, which will be unable to occupy environments with temperatures higher than they can bear. The collapse of this species will consequently bring about a profound change in the ecosystems where it will cease to exist, with a drastic decrease in biodiversity and failures in ecosystem functions and services.

However, the researchers are not giving up the fight to support conservation. “We need to work on the causes of anthropogenic climate change, which means immediate action on greenhouse gas emissions, if we want to protect these species and their associated communities,” explains Ledoux.