Spatiotemporal dynamics of submerged macrophyte status and watershed exploitation in a Mediterranean coastal lagoon: Understanding critical factors in ecosystem degradation and restoration

Increases in the intensity of disturbances in coastal lagoons can lead to shifts in vegetation from aquatic angiosperms to macroalgal or phytoplankton communities. Such abrupt and discontinuous responses are facilitated by instability in the equilibrium controlling the trajectory of the community response. We hypothesized that the shift in macrophyte populations is reversible, and that this reversibility is dependent on changes in the pressures exerted on the watershed and lagoon functioning. Biguglia lagoon (Mediterranean Sea, Corsica) is an interesting case study for the evaluation of long-term coastal lagoon ecosystem functioning and the trajectory of submerged macrophyte responses to disturbances, to facilitate the appropriate restoration of ecosystems. We used historical data for a two hundred-year period to assess changes in human activities on the watershed of the Biguglia lagoon. Macrophyte mapping (from 1970) and monitoring data for dynamics (from 1999) were used to investigate the trajectory of the community response. The changes observed in this watershed included a large number of hydrological developments affecting salinity and resulting in changes in macrophyte distribution. Nutrient inputs over the last 40 years have led to a shift in the aquatic vegetation from predominantly aquatic angiosperm community to macroalgae and phytoplankton in 2007 (dystrophic crisis). Changes in hydrological management and improvements in sewage treatment after 2007 led to a significant increase of aquatic angiosperms over a relatively short period of time (4-5 years), particularly for Ruppia cirrhosa and Stuckenia pectinata. There has been a significant resurgence of Najas marina, due to changes in salinity. The observed community shift suggests that Biguglia lagoon is resilient and that the transition may be reversible. The restored communities closely resemble those present before disturbance. These findings demonstrate the need to understand watershed exploitation and ecosystem variability in lagoon restoration.