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[eng] Climate change has emerged as one of the greatest and most pervasive threats that our natural heritage will have to face in the coming decades. Together with other anthropogenic pressures such as pollution, overfishing or habitat degradation, climate change is causing enormous impacts on oceans, affecting all kind of marine communities and driving major losses to biodiversity. In this framework of global change, the Mediterranean Sea, which has been identified as one of the major hotspots of marine biodiversity, has also become a hotspot of climate change. Consequently, some of its most emblematic and ecologically important communities are now under threat. This is the case of the coralligenous assemblages, which are temperate benthic communities which stand out for their great structural complexity and exceptional biodiversity (they harbour approximately 10% of Mediterranean species). Most of the constituent species of these communities exhibit high longevity and slow population dynamics. Consequently, they are especially vulnerable to disturbances that increase adults mortality. In fact, thermal anomalies linked to ocean warming have impacted the coralligenous assemblages in several ways during last decades, triggering changes that go from the observed mass mortalities of benthic organisms to potential changes at the community and ecosystem levels. However, despite the increasing interest of the scientific community to conserve the coralligenous, how these benthic communities are responding to climate change at the community level is poorly understood. In this thesis, we have taken this leap by integrating responses from species to the ecosystem level, through populations and communities. In particular, we have combined field observations of ecological and environmental changes, aquaria experiments, ecological theory, and cutting-edge quantitative research to improve our understanding of the bio-ecological consequences of ocean warming for Mediterranean coralligenous assemblages. In the first three chapters, we gathered field and experimental data on sensitivity of several coralligenous species and populations to marine heatwaves (MHWs) by conducting a literature review on previous mass mortality events occurred in the Mediterranean and by carrying out scaled-up thermotolerance experiments. The results of Chapter 1 revealed contrasting patterns of sensitivity to MHWs among different coralligenous species and populations. The posterior multi-specific thermotolerance experiment (including corals, sponges and tunicates) conducted in Chapter 2 confirmed the high response diversity to warming among co-occurring species observed in the field. These results demonstrate that co-occurring species that are taxonomically and morpho- functionally similar can have contrasting thermotolerances. Moreover, the observed patterns suggest that coralligenous assemblages could have certain degree of insurance against MHWs, since the decline of sensitive species could potentially be compensated by other functionally similar species that are more resistant. The results of the multi- population thermal experiment conducted in Chapter 3 with 12 populations of the red gorgonian Paramuricea clavata (Risso, 1826) revealed that this species, which has a key habitat-forming role in coralligenous assemblages, is highly sensitive to thermal stress across its distributional range. Moreover, we found little local adaptation to average thermal regime and little acclimatization to local above-average warm temperatures prior to the experiment. These findings, which were obtained at a spatial scale never addressed before (> 2000 km) for P. clavata, question the adaptive potential of this species to climate change. In Chapter 4, we further explored the vulnerability of Mediterranean habitat- forming octocorals to climate change by exploring the long-term recovery capacity of Corallium rubrum (Linnaeus, 1758) and P. clavata from recurrent MHWs. We explored the long-term trajectories (15 years) of different populations that were firstly impacted by the 2003-MHW in terms of changes in biomass, density and size structure. We found that all impacted populations have followed collapse trajectories. These results further highlight the high vulnerability of these species to the ongoing climate change. Finally, we took the leap from species and populations to the community and ecosystem levels in Chapter 5, to explore if MHWs have driven changes in the functional community structure of coralligenous assemblages, which could imply changes for ecosystem functioning. Specifically, we conducted a multi-taxa, trait-based analysis and found that MHW are shifting the functional identity of coralligenous assemblages (i.e., dominant functional traits). Interestingly, the observed changes were driven primarily by the decline of a single functional group (habitat-forming octocorals), whose species are functionally unique. Consequently, by severely affecting habitat-forming octocorals, MHWs are leading to assemblages that are deficient in key functional traits, likely causing changes in ecosystem functioning. Overall, this thesis provides an integrated, long-term view of the interconnected responses to ocean warming found in coralligenous assemblages. The combined results of this thesis suggest that despite structural changes induced by climate change, most ecosystem functions count on certain degree of insurance against MHWs. However, some essential functions (e.g., 3D-habitat provision) could be highly compromised or even totally disappear, threatening the overall assemblage stability. Given the current climate change trends, identifying and preserving the mechanisms that maintain essential ecosystem functions and the overall ecosystem stability is critical. In this thesis, we have identified the general loss of arborescent octocorals as the greatest disruptive process for coralligenous assemblages. Therefore, the future trajectories of these emblematic Mediterranean communities will greatly depend on the design and implementation of measures that favour the conservation and adaptive management of these key habitat-forming species in the face of climate change.

The temperate coralligenous habitats are considered hotspots of Mediterranean marine biodiversity. These coralligenous bioconcretions are built primarily by the accumulation of encrusting coralline algae growing at low irradiance levels that host numerous long-lived macroinvertebrates with prominent ecological roles. Several pressures, such as fishing activities, invasive species, and recurrent warming-induced mass mortalities, affect coralligenous assemblages and lead to dramatic loss of habitat complexity and biodiversity. One aim of the EU-funded project MERCES, dedicated to marine ecosystems restoration, was to develop innovative active restoration protocols with a focus on coralligenous habitat-forming macroinvertebrates from three key taxonomic groups: Cnidaria/Anthozoa, Porifera/Demospongiae and Bryozoa. These protocols combined transplants from donor organisms and recruitment-enhancing devices, taking into account the life-history traits, population dynamics, and genetics of targeted habitat formers. Our results show that transplantations require low initial effort due to high survival of transplants, but decades will be needed to fully recover habitat complexity because targeted species are slow- growing. In order to speed up recovery, we explored the potential influence of facilitation processes, i.e. positive species interactions. In order to guide the choice of transplant donors, we also aimed to identify thermo-resistant populations and specimens. We thus combined a transregional common garden and whole-genome sequencing to unravel the eco- evolutionary processes driving the differential responses to thermal stress. The outcomes of these works will implement the EU directives and contribute to the strategic planning of restoration initiatives for temperate marine ecosystems in the context of global change.

Este artículo contiene 18 páginas, 4 figuras., Climate change is causing an increase in the frequency and intensity of marine heatwaves (MHWs) and mass mortality events (MMEs) of marine organisms are one of their main ecological impacts. Here, we show that during the 2015–2019 period, the Mediterranean Sea has experienced exceptional thermal conditions resulting in the onset of five consecutive years of widespread MMEs across the basin. These MMEs affected thousands of kilometers of coastline from the surface to 45 m, across a range of marine habitats and taxa (50 taxa across 8 phyla). Significant relationships were found between the incidence of MMEs and the heat exposure associated with MHWs observed both at the surface and across depths. Our findings reveal that the Mediterranean Sea is experiencing an acceleration of the ecological impacts of MHWs which poses an unprecedented threat to its ecosystems' health and functioning. Overall, we show that increasing the resolution of empirical observation is critical to enhancing our ability to more effectively understand and manage the consequences of climate change., This paper was supported by Euromarine. Joaquim Garrabou acknowledges the funding by the “Severo Ochoa Centre of Excellence” (CEX2019-000928-S), the MCIU/AEI/FEDER [HEATMED; RTI2018-095346-B-I00], Interreg-Med Programme MPA-Engage (1MED15_3.2_M2_337), the European Union Horizon 2020 research and innovation programme (Futuremares SEP-210597628). Nuria Teixido acknowledges the French National Research Agency (4Oceans-MOPGA grant, ANR-17-MPGA-0001) and internal funds from the Stazione Zoologica Anton Dohrn. Gil Rilov was supported by the Israel Ministry of Environmental Protection. Hocein Bazairi, Free Spinosa, and Vasilis Gerovasileiou acknowledge the funding by the MAVA Fondation (MedKeyHabitats I Project) and the European Commission (Ecap-MED II Project; projects implemented by UNEP/MAP-RAC/SPA). Alfonso Ramos was supported the CIESM “Tropical Signals,” Stelios Katsanevakis and Maria Sini were supported by the Project “Coastal Environment Observatory and Risk Management in Island Regions AEGIS+” (MIS 5047038), implemented within the Operational Programme “Competitiveness, Entrepreneurship and Innovation” (NSRF 2014–2020), co-financed by the Hellenic Government (Ministry of Development and Investments) and the European Union (European Regional Development Fund, Cohesion Fund), Stelios Katsanevakis, Maria Sini and Vasilis Gerovasileiou acknowledge the support of the MARISCA Project, co-funded by 85% by the EEA GRANTS, 2009–2014, and 15% by the Public Investments Programme (PIP) of the Hellenic Republic. Ivan Guala and Daniele Grech thanks thank the support of the project “Pinna nobilis—ricerca per la sopravvivenza: un'iniziativa di Citizen Science per tracciare la mortalità di massa di Pinna nobilis in Sardegna” project and the L/7 grant (CUP 87G17000070002) funded by the Regione Autonoma Sardegna. Jean-Baptiste Ledoux was funded by an assistant researcher contract framework of the RD Unit—UID/Multi/04423/2019—Interdisciplinary Centre of Marine and Environmental Research—financed by the European Regional Development Fund (ERDF) through COMPETE2020—Operational Program for Competitiveness and Internationalization (POCI) and national funds through FCT/MCTES (PIDDAC). This research was supported by the Strategic Funding UIDB/04423/2020 and UIDP/04423/2020 through national funds provided by the FCT—Foundation for Science and Technology and European Regional Development Fund (ERDF), in the framework of the program PT2020. Bernat Hereu and Cristina Linares acknowledge the support of the long-term monitoring programme of the catalan Natural Parks, funded by the Departament de Territori i Sostenibilitat of the Generalitat de Catalunya. Cristina Linares acknowledges the support of the ICREA Academia programme. David Díaz acknowledges the support the research grant CTM2016-77027-R of the Programa Estatal de Investigación, Desarrollo e Innovación Orientada a los Retos de la Sociedad and Program of Marines Strategies of Spain funded by MITERD. Jamila Ben Soussi was partially funded by the Fondation Albert 2 Monaco (MIMOSA Project) and the Tropical Signals Program of CIESM. Giovanni Chimienti was supported by the Italian Ministry of Education, University and Research (PON 2014–2020, AIM 1807508–1, Linea 1), by the Ente Parco Nazionale del Gargano (Research agreement with CoNISMa N. 21/2018), and by the National Geographic Society (Grant EC-176R-18). Nathaniel Bensoussan acknowledges financial support from the European Commission through the programme “Caroline Herschell” in the context of the action “Developing Downstream applications and services on BIO-PHYsical characterization of the seascape for COASTal management” (BIOPHYCOAST). Monica Montefalcone and Annalissa Azzola collected some of their data on MMEs in the frame of the project “Mare Caldo” funded by Greenpeace Italy. Núria Marbà acknowledges financial support from the Spanish Ministries of Economy and Competitiveness (CTM2012-32603, CGL2015-71809-P) and Ministerio de Ciencia, Innovación y Universidades (RTI2018-095441-B-C21). Diego K. Kersting acknowledges support by the postdoctoral fellowship programme Beatriu de Pinós funded by the Secretary of Universities and Research (Government of Catalonia) and the Horizon 2020 programme of research and innovation of the European Union under the Marie Sklodowska-Curie grant agreement No 801370.

Bazairi, Hocein/0000-0002-1788-0580; Gattuso, Jean-Pierre/0000-0002-4533-4114; Gerovasileiou, Vasilis/0000-0002-9143-7480; Cerrano, Carlo/0000-0001-9580-5546; Turicchia, Eva/0000-0002-8952-9028; Topcu, Nur Eda/0000-0003-2734-2695; Ponti, Massimo/0000-0002-6521-1330; deudero, salud/0000-0001-5584-2801; Ledoux, Jean-Baptiste/0000-0001-8796-6163; Fraschetti, Simonetta/0000-0002-1780-1651; BERTOLINO, Marco/0000-0003-3233-303X; Duarte, Carlos M./0000-0002-1213-1361, WOS: 000499719400001, [No abstract available], postdoctoral contract Juan de la Cierva-Incorporacion [IJCI-2016-29329]; Ministerio de Ciencia, Innovacion y Universidades [PTA2015-10829-I]; Interreg Med Programme [MPA-Adapt 1MED15_3.2_M2_337]; European Regional Development Fund - Foundation Prince Albert II Monaco; European UnionEuropean Union (EU) [689518, FPU15/05457]; Spanish Ministry of EducationSpanish Government [UID/Multi/04423/2013]; FCT - Foundation for Science and TechnologyPortuguese Foundation for Science and Technology; European Regional Development Fund (ERDF)European Union (EU), MV-L was supported by a postdoctoral contract Juan de la Cierva-Incorporacion (IJCI-2016-29329) of Ministerio de Ciencia, Innovacion y Universidades. AI was supported by a Technical staff contract (PTA2015-10829-I) Ayudas Personal Tecnico de Apoyo of Ministerio de Economia y Competitividad (2015). Interreg Med Programme (grant number Project MPA-Adapt 1MED15_3.2_M2_337) 85% cofunded by the European Regional Development Fund, the MIMOSA project funded by the Foundation Prince Albert II Monaco and the European Union's Horizon 2020 research and innovation programme under grant agreement no 689518 (MERCES). DG-G was supported by an FPU grant (FPU15/05457) from the Spanish Ministry of Education. J-BL was partially supported by the Strategic Funding UID/Multi/04423/2013 through national funds provided by FCT - Foundation for Science and Technology and European Regional Development Fund (ERDF), in the framework of the programme PT2020.

9 pages, 7 figures, supplementary information https://doi.org/10.1038/s41598-018-36094-9, Climate change threatens the structure and function of marine ecosystems, highlighting the importance of understanding the response of species to changing environmental conditions. However, thermal tolerance determining the vulnerability to warming of many abundant marine species is still poorly understood. In this study, we quantified in the field the effects of a temperature anomaly recorded in the Mediterranean Sea during the summer of 2015 on populations of two common sympatric bryozoans, Myriapora truncata and Pentapora fascialis. Then, we experimentally assessed their thermal tolerances in aquaria as well as different sublethal responses to warming. Differences between species were found in survival patterns in natural populations, P. fascialis showing significantly lower survival rates than M. truncata. The thermotolerance experiments supported field observations: P. fascialis started to show signs of necrosis when the temperature was raised to 25–26 °C and completely died between 28–29 °C, coinciding with the temperature when we observed first signs of necrosis in M. truncata. The results from this study reflect different responses to warming between these two co-occurring species, highlighting the importance of combining multiple approaches to assess the vulnerability of benthic species in a changing climate world, This study was partially funded by the “Parc Natural del Montgrí, Illes Medes i Baix Ter” and the “Departament de Territori i Sostenibilitat” of the Catalan Government project, the SMART project (CGL2012-32194) and the European Union’s Horizon 2020 research and innovation program under grant agreement No 689518 (MERCES). This output reflects only the author’s view and the European Union cannot be held responsible for any use that may be made of the information contained M.P.E was supported by an APIF grant (APIF2016), I.M.S by an FPI grant (BES-2013-066150), D.G.G by an FPU grant (FPU15/05457) and B.F by a SENACYT & STRI Postdoctoral Fellow (47-2017-4-FID16-239)