Abstract
The Thau lagoon is one of the largest Mediterranean coastal lagoons and it supports many uses such as shellfish farming and fishing, priority activities of the territory, and more recently recreational activities. Since the 1960s, the increase in anthropogenic inputs, linked to the sudden growth of the population, led to microbiological contaminations of shellfish and to the eutrophication of the lagoon, with negative impacts at sanitary, ecological and socio-economic levels. Since the 1970s, the considerable work carried out on the waste water system on the watershed has made it possible to restore the lagoon, which then began an oligotrophication trajectory, a process that has still few been studied in coastal environments.Subject to a variety of pressures and disturbances, Thau can be studied as a complex system, within which the ecosystem interacts with society, constituting a socio-ecosystem (SES). The general objective of the thesis is to propose benchmarks able to assess the state and to analyze the trajectory of the Thau SES from 1970 to 2018, in a dynamic and integrated way, based on the approaches and tools proposed by: i) restoration ecology and ii) ecosystem services (ES) approach. From the point of view of the state and functioning of the ecosystem, statistical analyzes of long-term data series on autotrophic compartments have highlighted shifts in the composition of communities, as well as phenomena of resilience or inertia during oligotrophication. By applying the interdisciplinary tool of timelines, we distinguished three contrasting periods in the trajectory of the SES: P1) 1970-1989: degraded state, predominant traditional uses and sectoral management; P2) 1990-2004, transition period: start of improvement in water quality, development of recreational uses and structuring of management; P3) 2005-2018: oligotrophication, diversification of uses and integrated management. Through the dynamic analysis of: i) bundles of ecosystem services (matches or gaps between ES’ potential and consumption); ii) and of forms of ES demands potentially driving change (impacting, unsatisfied, conservation), we identified the main interactions between ecological and social processes and the main determinants of the phases of stability or change in the SES.