Posidonia oceanica meadows represent one of the most important coastal habitats in the
Mediterranean Sea, providing multiple ecosystem services, among which carbon sequestration plays
a crucial role in climate regulation. Nevertheless, these ecosystems are still inadequately
represented within National Accounting Systems. This study proposes a spatially explicit approach
to quantify the extent, ecological condition, and the carbon sequestration service of Posidonia
oceanica meadows across the Italian coast, following the principles of the System of Environmental
Economic Accounting-Ecosystem Accounting (SEEA-EA) framework. Spatial information on
meadow density and rhizome primary productivity was used to quantify structural and functional
ecosystem properties and integrated into a composite indicator describing the ecosystem condition.
Carbon sequestration was estimated from rhizome production data, converted into carbon content
and subsequently expressed as CO₂ equivalents through stoichiometric conversion. Findings
indicate that Italian Posidonia meadows sequester nearly 630,000 t CO₂ yr⁻¹, with substantial
differences among regions. Economic assessment revealed a significant contribution, ranging from
approximately 42 M€ yr⁻¹ using market-based carbon values to about 93 M€ yr⁻¹ when applying
the social cost of carbon. Furthermore, cumulative risk analyses identified critical areas where
anthropogenic pressures could coincide with degraded ecosystem conditions, potentially reducing
service provision. The resulting losses were monetised through restoration-based replacement costs,
estimated as reforestation expenses required to compensate for the diminished service. Overall, the
proposed framework establishes a national baseline for integrating blue carbon ecosystems into
ecosystem accounting and supports marine spatial planning, environmental monitoring, and climate
mitigation strategies.