Résumé
Increased nitrogen and phosphorus inputs from watersheds, especially since the 1960s with the increased anthropogenic activities is recognized as a major cause of environmental degradation of coastal ecosystems. On the Mediterranean coast, lagoons are particularly affected by the eutrophication leading to significant changes in ecosystem structure, functioning and services. Since 2000, the “Région Languedoc-Roussillon”, the “Agence de l’Eau Rhône Mediterranée & Corse” and Ifremer developed a lagoon monitoring network (RSL) focusses upon eutrophication to assess and monitor status of 24 lagoons of Languedoc-Roussillon and Corsica. Many actions on watershed led to a significant reduction of nutrient inputs. However, the time required to restore these degraded environments to achieve good ecological status under the Water Framework Directive (WFD) is unknown. Coastal restoration is characterized by a hysteresis, i.e. a partial or complete restoration involving different processes and generally longer than the rapid degradation mechanisms. This hysteresis is due to a complex ecosystem functioning involving all physical, chemical and biological processes that play a role in the matter cycles. In a lagoon, the recovery time will depend on: - the structure and functioning of the communities - the release of nitrogen and phosphorus implicated as the internal load - the ability to export the excess nitrogen and phosphorus to the open areas. Aims of the study : - to characterize and quantify the main nutrient flows participating in the reduction of internal nitrogen and phosphorus content in Mediterranean lagoons - to improve the parametrization of mathematical models - to define dynamic restoration scenarios of eutrophised Mediterranean lagoons