An Operational Conservation Prioritization Model for Seagrass Ecosystems in the Eastern Mediterranean: National Posidonia oceanica Priority Index (NPOI)

Posidonia oceanica meadows represent one of the most valuable coastal ecosystems in the Mediterranean Sea due to their biodiversity support, sediment stabilization, and long-term carbon sequestration capacity. However, conservation planning often relies on present-day ecological status indicators without incorporating temporal persistence and carbon lock-in potential, which are fundamental to long-term ecosystem resilience. This study introduces the National Posidonia oceanica Priority Index (NPOI), a standardized and operational decision-support model developed for Eastern Mediterranean coastal management. The index integrates four equally weighted components: (A) spatial continuity and area, (Y) temporal persistence derived from lepidochronological age structure, (C) carbon lock-in potential based on matte thickness, and (P) anthropogenic pressure. Anthropogenic pressure is quantified through a six-component weighted model reflecting mechanistic impact pathways. Each component is scored on a 1–5 scale using transparent and repeatable criteria. The index does not assess legal protection eligibility, as all Posidonia oceanica meadows are protected habitats; rather, it quantifies conservation priority and restoration urgency. The methodological framework, scoring protocols, and management interpretation are presented in detail to ensure reproducibility and policy applicability. Application to three Eastern Mediterranean meadows yielded NPOI values ranging from 2.05 to 2.46 (mean ± SD: 2.19 ± 0.23), consistently classifying all sites as restoration-priority systems.