A Multidisciplinary Approach for Restoration Ecology of Shallow Coastal Lagoons, a Case Study in South France

Abstract

International audience By the end of the 20th century, many of the coastal lagoons along the FrenchMediterranean coast showed insufficient water quality and degraded ecosystem statesdue to anthropogenic impacts. Among these, nutrient over-enrichment, resulting ineutrophication, has been a major concern. The EU Water Framework Directive (WFD)has initiated public action to improve their water quality and ecosystem state using anapproach rooted in restoration ecology. Here we analyze how this has been appliedfor the coastal lagoons in South France, considering eutrophication as an exampleof ecosystem degradation and oligotrophication as the corresponding trajectory forecological restoration of the eutrophied coastal lagoons. Oligotrophication trajectories,initiated by the reduction of external nutrient loading, have resulted in a quick recovery(i.e., within 3 years) of integrative water column variables (Chlorophyll a, total N and P)and phytoplankton. Starting from hypertrophic systems, the oligotrophication trajectoryis described by a sequence of three ecosystem states dominated respectively by (i)phytoplankton with bare non-vegetated sediments, (ii) opportunistic macroalgae, (iii)angiosperm and perennial macroalgae, punctuated by regime shifts between theseecosystem states. Nevertheless, the latter regime shift has not been observed for themost degraded ecosystems after 10-years oligotrophication. The N and P accumulatedin sediments during eutrophication may also retard the ecological restoration. Inshallow freshwater lakes, the phytoplankton-dominated and the angiosperm-dominatedstates are also characteristic for highly degraded and fully restored ecosystemsstates, respectively. In contrast, opportunistic macroalgae do not bloom in thesesystems. Hence, the alternative stable state model, used successfully for these lakes,cannot be applied straightforwardly for coastal lagoons. To be successful, ecologicalrestoration should consider the legislative and societal questions as according theDPSIR framework it ...