Marine heat waves devastate the red gorgonians of the Medes Islands

The increase in the frequency and intensity of marine heat waves over recent decades is one of the effects of global climate change. A new study shows that the extreme heat wave that occurred in 2022 caused an unprecedented increase in the mortality of the red gorgonian, the octocoral Paramuricea clavata, which affected 70% of the colonies located in the waters of the Montgrí natural park, the Medes Islands and the Baix Ter, in the province of Gerona, Catalonia. According to the researchers, these results are quite alarming and threaten the viability of this species of great value for the biodiversity of benthic ecosystems, since it is considered a habitat-former.

Graciel·la Rovira, researcher at the University of Barcelona (UB), is the first author of this study, which has been coordinated by professor Cristina Linares, professor at the Biodiversity Research Institute of the UB (IRBio). Researchers from the Mediterranean Institute for Advanced Studies (IMEDEA, dependent on the University of the Balearic Islands (UIB) and the Higher Council for Scientific Research (CSIC) in Spain) and the University of Toulon in France have also participated.

The work has evaluated the impact of marine heat waves, produced between 2016 and 2022, on seven populations of P. clavata, located in this marine protected area. The analysis of the results has shown that the mortality rates—obtained from the calculation of the cumulative proportion of affected colonies and the dead area—increased significantly in the years in which these episodes were warmer and longer: 2017, 2018 , 2019 and 2022.

In these four years, temperatures were higher than 24.3 degrees Celsius, and 2022 is the year with the highest mortality. «In 2022, the total proportion of colonies affected by the fifty days of heat wave was close to 70%, with a percentage of damaged surface of almost 40%, values ​​that had never been observed in this area since the began monitoring these populations,” detail the authors of the study.

This negative impact is also the result of the events of previous years, since the recovery of these organisms is, according to researchers, very slow. “The mortality of a specific year is also observed that of the previous year and, therefore, 2022 presents an accumulated mortality of all past years,” emphasizes Graciel·la Rovira, who is a member of the department of Evolutionary Biology, Ecology and Sciences Environmental Studies of the UB and IRBio.

Graciel·la Rovira. (Photo: University of Barcelona. CC BY)

Point of no return

Until now, P. clavata populations in this small marine reserve had withstood the heat waves of previous years better than other Mediterranean populations. This resilience had made researchers think that these colonies could be considered a climate refuge. “The impacts documented in this study show a worrying future for P. clavata populations throughout the Mediterranean, and that suggests that the resilience of this species may not be sufficient to maintain its populations in the predicted warming scenario,” the researchers highlight.

Furthermore, these results have important implications beyond the survival of the species itself. “As P. clavata is a habitat-forming species – it provides habitat to other species, that is, they use it, for example, as a refuge – its disappearance could have a strong impact on the biodiversity and the functioning of these ecosystems,” he highlights. Graciel·la Rovira.

This critical situation means that, even when applying measures to reduce “as much impact as possible” in the protected area, researchers are pessimistic about the possibility of recovery of the species. “The unprecedentedly high mortality rate, together with the predicted climate change scenarios, mean that these populations are probably at a point of no return,” concludes Rovira.

The study is titled “When resilience is not enough: 2022 extreme marine heatwave threatens climatic refuge for a habitat-forming Mediterranean octocoral.” And it has been published in the academic journal Journal of Animal Ecology. (Source: University of Barcelona)