Salt Marsh Plant Community Response to Freshwater Inflow Management: Implications for Sustainable Coastal Lagoon Restorations in the Northern Venice Lagoon, Italy

Coastal lagoons are increasingly affected by altered salinity regimes due to river diversion and hydrological regulation, with major impacts on ecosystem structure and functioning. The Venice Lagoon is a paradigmatic case, where centuries of river diversion have reduced freshwater inputs, causing widespread marinization and the decline of brackish habitats such as reedbeds (Phragmites australis). Within the LIFE Lagoon Refresh project, controlled freshwater inputs from the Sile River (300–1000 L s−1 since 2020) were reintroduced into the Ca’ Zane Valley to restore salinity gradients. Vegetation responses were assessed by comparing pre-diversion (2018) and post-diversion (2024) conditions across 28 salt marsh platforms (9.82 ha) using field surveys, UAV imagery, satellite data and GIS analysis. Both freshwater inflow, which reduced salinity from values > 30 psu to mean values of 0.22 and 5.6 psu near the canal inlet and within a few hundred meters, respectively, and reed transplants triggered rapid changes in plant communities. Hypersaline species such as Salicornia procumbens subsp. veneta, Limonium narbonense, and Sarcocornia fruticosa declined, while brackish species such as Galatella tripolium and Phragmites australis expanded, reaching up to 75% cover in areas with salinity < 10 psu. These findings demonstrate that controlled freshwater inflows and transplants of suitable species can represent an effective nature-based solution for the sustainable restoration of coastal lagoons, habitat diversity, ecosystem functionality, biodiversity conservation, and long-term resilience to environmental change.