A GIS-Based Methodology for Mapping and Inventorying Posidonia oceanica Beach-Casts: A Framework for Coastal Management

Posidonia oceanica (Delile) is a Mediterranean seagrass species whose fallen leaves accumulate along shorelines, forming compact structures known as banquettes. Despite their ecological and protective functions, these deposits remain insufficiently documented across the Mediterranean and are frequently removed before the summer season due to tourism and aesthetic considerations. This study develops and tests a Geographic Information System (GIS)-based methodology for mapping and inventorying P. oceanica beach-casts across five representative pilot areas along the Italian coastline (Western Sardinia; Liguria; Northwestern Sicily; Elba and Giglio Islands, Tuscany; Gargano coast, Apulia). The approach integrates historical Google Earth imagery with spatial datasets to characterize beach-cast occurrence and persistence and to describe their distribution in relation to selected geomorphological, environmental, and anthropogenic variables. Based on historical Google Earth imagery covering approximately three decades (~1998–present), beach-casts were identified at least once on 94% of the 393 surveyed beaches, indicating that the phenomenon is widespread across the investigated areas, with permanent banquettes representing a substantial component (63%). The spatial distribution of beach-casts was also examined in relation to shoreline dynamics, beach surface type, coastal vulnerability, and beach use, providing a descriptive characterization of their environmental and anthropogenic context, while the relative influence of the considered factors requires further investigation. The resulting standardized and reproducible framework provides a basis for future monitoring and susceptibility assessments across broader coastal sectors and can support evidence-based management of P. oceanica beach-casts.