The aim of the European program Life Integrated Project Environment (IPE) MarHa (2018-2025) is to restore and sustain the favorable conservation status of marine natural habitats in French Natura 2000 sites. In this context, Ecosystem Service Assessments (ESA) are carried out at various French sites including the Bay of Marseille (Provence, France). First, we applied the TRIAGE methodology: a strategic assessment of the issues with experts of the area (local MPA managers and scientists). TRIAGE raises two main concerns: (i) the intensification of recreational uses (by both residents and visitors), and (ii) the conservation of Posidonia oceanica (Linnaeus) Delile, 1813 seagrass meadows. In order to address both issues, we selected two adapted methodologies based on the strategic assessment: (i) a study oriented toward understanding the demand for ecosystem services (focused on recreational activities), and (ii) a study of the capacity of P. oceanica meadows to deliver ecosystem services using state-and-transition modeling. The objective of this work is to present the ESA process, from the strategic assessment to the analysis results. We focus on the study of the capacity of P. oceanica seagrass meadows to provide ecosystem services. State and transition models consist in defining alternative states of the habitat based on ecological indicators, identifying the bundle of services associated with each state and identifying transition vectors capable of explaining the shifts between each state. State-and-transition models can be very powerful frameworks for integrating multiple functions and services delivered by ecosystems while accounting for their temporal dynamics.