Abstract
Following the unprecedented marine heatwaves of summer 2022, extensive flowering of the endemic seagrass Posidonia oceanica was witnessed across the western Mediterranean. To unravel the causes of this event, we conducted a pan-Mediterranean analysis across 442 sites spanning all Mediterranean ecoregions, with 76% exhibiting flowering. Flowering differed regionally, with both highest flowering prevalence (over 90%) and flowering intensity (up to 0.75) recorded in the Liguro-Provençal and Balearic Seas. We demonstrate that regions experiencing large summer sea surface temperature anomaly with high marine heatwave cumulative intensity displayed the highest flowering intensity. The probability of flowering was very likely when marine heatwave cumulative intensity exceeded ⁓120°C days. These findings suggest that marine heatwaves trigger flowering in P. oceanica and, given the high energetic cost of sexual reproduction, continued ocean warming may shift energy allocation toward flowering. This shift could profoundly affect the species’ reproductive strategies, posing major challenges and uncertainties for its long-term evolution.
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Acknowledgements
This work is dedicated to the memory of Monica Montefalcone, our co-author and colleague, who passed away a few days before the publication of this article. We warmly thank the management teams of Marine Protected Areas which helped in providing flowering records (Marine Natural Reserve of Cerbère-Banyuls, Nature Marine Park of the Gulf of Lions, Port-Cros National Park, Côte Bleue Marine Park, Côte Agathoise Marine Protected Area, Association Monégasque pour la Protection de la Nature), diving centres (Costa Verde Loisir, Albatros Diving, EPIR Plongée Ile Rousse, Scuba Ibiza, Blaumar Bivalvia, Federación Catalana de Actividades Subacuáticas), citizens and volunteers, in particular in the frame of the ‘Observadores del mar’ network https://www.seawatchers.net/ for the huge dataset made available for this work .
Funding
Fiona Tomas would like to acknowledge funding from the Government of the Balearic Islands (CAIB), under the research program “Ajudes a Projectes Biodiversitat Emmarcats dins els Plans Complementaris” through the project “Incorporación de nuevas tecnologías en la caracterización del estado de conservación de los bosques submarinos de Baleares y su contribución a servicios ecosistémicos (BIO022 PRTR BIODIV ConBoSer-Bio)”. IMEDEA is an accredited “Maria de Maeztu Excellence Unit” (Grant CEX2021-001198, funded byMICIU/AEI/10.13039/501100011033). Núria Teixidó was supported by the French government through the National Research Agency Investments for the Future (“4Oceans-Make Our Planet Great Again” grant, ANR-17-MOPGA-0001) and the European Union Next-Generation EU (PIANO NAZIONALE DI RIPRESA E RESILIENZA (PNRR) – MISSIONE 4 COMPONENTE 2, “Dalla ricerca all’impresa” INVESTIMENTO 1.4 – D.D. 1034 17/06/2022, CN00000033). Jérémy Carlot was supported by a Beatriu de Pinós postdoctoral fellowship (2024 BP 00106), funded by the Government of Catalonia and co-funded by the European Union under the Marie Skłodowska-Curie COFUND programme. Jordi Boada acknowledges funding received by the H2020 program (MSCA-IF SHIFT2SOLVEXXX), and by the I + D + i project RYC2021-033650-I and PID2022-140652NA-I00 MITnPOINT funded by the MCIN/AIEI/10.13039/501100011033 and FEDER. Jorge Terrados would like to acknowledge the research contracts “Restoration of Posidonia oceanica meadow, REE Marine Forest” funded by Redeia (Spain) and “High-quality sequencing of Posidonia oceanica genome” funded by Iberostar Foundation (Spain). Andres Izquierdo study forms part of the ThinkInAzul programme and was supported by MCIN with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Generalitat Valenciana (GVA-THINKINAZUL/2021/016; F. Gimenez-Casalduero, UA). Daniele Grech was supported by the LIFE PINNA (LIFE20 NAT/IT/001122) “Conservation and re-stocking of the Pinna nobilis in the western Mediterranean and Adriatic Sea” and Sea Urchin monitoring program (Paracentrotus lividus) funded by the Autonomous Region of Sardinia (RAS). He would like also to thank the entire citizen science underwater group “Subacquei per la Scienza” for flowering sightings. In Andalusia (Spain), the authors are grateful to Eugenio Montes and Soledad Vivas, responsible in the Junta de Andalucía for the Sustainable Marine Environment Management Program, and to the rest of the team members, as well as to Alejandra Pérez and Fernando Orri from Aquatours for coordinating the work of the volunteers at the Aguadulce and Cerrillos stations (Almería). Arturo Zenone was funded under the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.4 – Call for tender No. 3138 of 16 December 2021, rectified by Decree n.3175 of 18 December 2021 of Italian Ministry of University and Research funded by the European Union – NextGenerationEU. Project code CN_00000033, Concession Decree No. 1034 of 17 June 2022 adopted by the Italian Ministry of University and Research, CUP B83C22002930006 (CNR). Arnaud Boulenger was supported by the University of Liege (Belgium) and the Fonds National de la Recherche Scientifique – FNRS (grants ASP 40006932 and CDR J.0076.23). Periklis Kleitou, George Constantinou and Demetris Kletou (MER) are grateful to Christina Michail (MER) and Melina Marcou (DFMR) for documenting and providing information about flowering events in Cyprus. Eugenia T. Apostolaki and Catalina A. Garcia-Escudero acknowledge supported by DRESSAGE (No MIS5045792 through the Operational Program ‘Competitiveness, Entrepreneurship and Innovation’ (EPAnEK 2014-2020, NSRF), CHRISI (‘Implementation of measures for the Action Plan of Chrisi island, Cretan Sea’ funded by Region of Crete) and AGOGOS (No 81/10.4.23 HCMR own funding) projects. Vesna Mačić would like to thank to RAC SPA for support in implementation of national monitoring program in line with the IMAP, MoU N°22/2024-SPA/RAC We are deeply thankful to T-Med Network for providing temperature analyses at the Mediterranean scale.
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Astruch, P., Bensoussan, N., André, S. et al. Mass flowering of the seagrass Posidonia oceanica after 2022 record-breaking marine heatwaves, a Pan-Mediterranean study.
Commun Earth Environ (2026). https://doi.org/10.1038/s43247-026-03631-8
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DOI: https://doi.org/10.1038/s43247-026-03631-8
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