This is the main conclusion of a study co-led by the ICM and the UB, which warns of the lack of substitutes for these species responsible for structuring one of the most emblematic habitats in the Mediterranean.
Marine heatwaves are severely affecting marine ecosystems around the world and the Mediterranean is not an exception. Here, these extreme weather events and the resulting mass mortality events are becoming increasingly intense and frequent. To date, most studies have studied their effects at the level of individual species and populations, but how these disturbances affect the functioning of the affected ecosystems is still unknown.
A new study co-led by the Institut de Ciències del Mar (ICM) and the Universitat of Barcelona (UB) has found that marine heatwaves are having a strong impact on the functioning of coralligenous assemblages -one of the most emblematic habitats in the Mediterranean given the high diversity of species they host and the large number of ecosystem services they provide-, mainly due to the loss of the few unique species that provide structural complexity to these communities.
We are talking about the architects of these habitats, such as gorgonians, which act like trees in forests and provide shelter to a large number of marine organisms. According to the study, recently published in the journal Ecology Letters, the gorgonians have been severely affected in the Mediterranean in recent years by different mass mortality events caused by marine heatwaves.
"Gorgonians are very sensitive to climate change. In fact, their abundance has been reduced by as much as 93% in some of the sampled sites where heatwaves have impacted. This is worrying because gorgonians play a critical role in coralligenous assemblages. They are the organisms that provide the three-dimensional structure that allows more than 1600 species to thrive in the coralligenous", explains the ICM and UB pre-doctoral researcher Daniel Gómez-Gras.
"Thanks to this study we have been able to verify that the coralligenous reefs are losing the dominance of key functions, which are precisely those provided by the gorgonians. This implies the loss of essential ecosystem services such as the potential decline of species with great commercial value such as scorpionfish and lobsters, which typically prefer habitats with high structural complexity" stresses the ICREA professor Cristina Linares, who carries out her research in the Faculty of Biology of the UB and is a member of the UB's Institute for Research on Biodiversity (IRBio).
Increased ecosystem vulnerability
The authors warn that, unlike in tropical reefs, where many species act as architects of this type of habitat, in the Mediterranean there are very few and, therefore, their function could be lost. The best known are the red coral (Corallium rubrum) and the red gorgonian (Paramuricea clavata), which are precisely the species considered in this study.
"The loss of structural complexity makes coralligenous assemblages more vulnerable to disturbances, as the disappearance of some species makes it difficult for others to survive, leading to an acceleration of the degradation process of the community. It's like if the roof of a building collapses and the windows break. If they are not fixed, the building will eventually collapse", explains the ICM-CSIC researcher Joaquim Garrabou, who also participated in this work.
Effects also inside marine protected areas
The study analysed the results obtained in monitoring programmes carried out by the MedRecover research group for more than a decade in the Port-Cros National Park and the Scandola Nature Reserve, both located in France. These are two of the oldest and most effective marine protected areas in terms of protection and conservation of their natural resources in the Mediterranean.
According to the researchers, the effects of marine heatwaves on the functional structure of coralligenous assemblages from these protected areas have been investigated by exploring temporal changes in demographic and functional traits of coralligenous species. To this end, the research team has collaborated with experts from the University of Hawaii (United States) and the University of Saint Andrews, Scotland (United Kingdom).
The results of the study highlight the devastating effects of climate change on habitats as essential for marine life in the Mediterranean as coralligenous assemblages, which, according to the authors, "will be severely compromised in the near future due to marine heat waves".
This study has been funded by different projects in which the ICM and the UB participate: the Heatmed project, from the Spanish Ministry of Science, Innovation and Universities, and the European projects MPA-Engage, from the Interreg MED programme, and FutureMARES, from the H2020 programme. Their main objective is to analyse the effects of ocean warming on coastal marine ecosystems, as well as to promote the efficiency of nature-based solutions as one of the pillars of the climate change adaptation and mitigation strategy.