Passive restoration of a Mediterranean Posidonia oceanica seagrass meadow: A 40-year survey

The seagrass Posidonia oceanica, endemic to the Mediterranean Sea, forms extensive meadows that provide a wide range of ecosystem services. However, P. oceanica meadows have declined in many parts of the Mediterranean due to human activities, particularly near large cities and ports, such as the Marseilles area (France, northwestern Mediterranean Sea). This decline has been especially pronounced in Prado Bay, located at the core of a 2-million-inhabitants metropolis, and has been linked to organic pollution from a river and an untreated sewage outfall, as well as to the construction of a marina and artificial beaches. In 1986, two permanent 36-m² quadrats (PQs) were established and monitored until 2025 to assess potential natural recovery of the meadow following the implementation of a wastewater treatment plant and in the absence of further coastal development, in accordance with national and European Union (EU) policies for the protection of P. oceanica and marine habitats. Recolonization occurred at an unexpectedly high rate, reaching up to 8.6%/year, although with marked interannual variability. By 2025, P. oceanica cover (94% and 81% in the two PQs) indicated near-complete restoration, as small unvegetated patches can be a natural feature in healthy meadows. These results, from one of the longest datasets for seagrass monitoring, demonstrate that, once the drivers of P. oceanica decline are removed, passive restoration is a viable approach that can be achieved within a realistic timeframe. Our findings emphasise the need for coordinated anthropogenic stressor mitigation to support effective ecological restoration under the EU Nature Restoration Regulation.