Soutenance de thèse de ASTRUC Patrick

Thesis resume

Mediterranean marine and coastal ecosystems are increasingly affected by the impacts of Global Change (GC). Biodiversity and endemism hotspot, the Mediterranean Sea is particularly at risk, facing cumulative pressures from numerous human activities. The north-western Mediterranean, including the French coastline, is characterised by relatively cooler sea surface temperatures but also intense pressures from tourism, coastal development, wastewater inputs, and recurrent mass mortality events associated with marine heatwaves (MHWs). At international, national, and regional levels, several policy frameworks have been developed to regulate human activities and to achieve or maintain a good environmental status. These policies rely on the establishment of monitoring networks, the implementation of standardised protocols, and the use of ecological descriptors and indicators. Within this context, the ecosystem-based approach (EBA), promoted under the EU Marine Strategy Framework Directive and the Barcelona Convention, provides an integrated framework for the sustainable management and use of marine ecosystems. Unlike the traditional species-centred approach, the EBA emphasises ecosystem functioning and the services it supports. In this PhD, Chapter 1 illustrates multiple consequences of GC on coastal and marine ecosystems: the settlement of thermophilic teleost fish species along the French coast, temporal trajectories of species introductions in Berre Lagoon (Provence, France), the retreat of the deep limit of Posidonia oceanica meadows, and the unprecedented basin-wide mass flowering of P. oceanica following the record-breaking 2022 MHWs. These examples reveal an intensification of GC impacts, particularly those driven by ocean warming, raising critical management challenges at ecosystem level. Chapter 2 explores the operational implementation of the EBA through the development of original ecosystem-based quality indices (EBQIs) for emblematic ecosystems such as coralligenous reefs and coastal detrital bottoms. The reef-EBQI has been applied at the French Mediterranean scale to assess the ecological status of shallow rocky reefs. Training provided to managers of marine protected areas (MPAs) has demonstrated the reliability of EBQIs as management tools at both MPA and regional scales. Another case study concerns the saltmarshes of Hyères (Provence, France), where EBA principles were used to propose improvements in water circulation while safeguarding highly valuable ecosystem compartments. In a General Discussion, I describe how thermal trends, vulnerability to mass mortality events, rates of non-indigenous species introductions, and dynamics of P. oceanica flowering delineate cold- and warm-edge areas across Mediterranean ecoregions, while also identifying counterexamples with specific local trajectories. Applications of the EBA on diverse ecosystems are now increasing worldwide. This thesis demonstrates the relevance and operational value of EBQIs for monitoring key coastal marine ecosystems at both MPA and regional scales. To account for interactions among ecosystems, I propose an original multi-ecosystem model that identifies major functional linkages (e.g., trophic interactions, habitat use, litter and organogenic material flows). This model highlights how the alteration of one or several ecosystems can cascade across the entire coastal network. In the perspective of implementing the EBA under accelerating environmental change, the potential role of active restoration (focused here on the case study of P. oceanica meadow) is discussed as a complementary but not mandatory tool to support habitat conservation, in line with the EU Biodiversity Strategy and the Nature Restoration Regulation. Overall, this thesis underscores the urgent need for applied, large-scale, and multidisciplinary research to strengthen the implementation of ecosystem-based indices and to effectively monitor GC-driven changes across the Mediterranean basin.