Millennial Posidonia oceanica reefs reveal contrasting growth trajectories across temporal scales in the Central Mediterranean

The Mediterranean seagrass Posidonia oceanica forms millennia-old biogenic barrier reefs through the vertical growth of rhizomes, roots, and sediments, collectively known as matte. It was hypothesised that P. oceanica reefs subjected to contrasting hydrodynamic and sedimentary regimes would exhibit divergent long-term accretion trajectories and contemporary growth dynamics, and that millennial-scale matte development may reflect present-day plant performance. A multi-proxy analysis on three barrier reefs along the Sicilian coast (Solanto, Maragani and Marsala) was therefore conducted to investigate their long-term development patterns and recent growth dynamics, and to shed light on their vulnerability to climate change. Radiocarbon dating, δ13C analysis, and lepidochronology revealed contrasting age, growth and sedimentation patterns among the three barrier reefs. The oldest reefs (>1000 years old), Solanto and Maragani, showed contrasting matte accretion and rhizome productivity patterns presumably linked to differing hydrodynamics and habitat conditions. The younger and exposed Marsala reef (∼600 years old), exhibited the highest matte accretion rate, as well as the greatest variability in rhizome speed of growth and primary production likely associated to the highest hydrodynamic regimes coupled with its unique location. Notably, the Solanto reef represents the oldest P. oceanica barrier reef dated so far in Italy and ranks among the oldest documented in the Mediterranean Sea. δ13C values indicated site-specific organic matter preservation and carbon dynamics, with more sheltered reefs showing depleted but variable values over time, indicative of susceptibility to carbon loss. These findings highlight the complex interplay between long-term reef formation and present-day dynamics, providing key insights into the resilience, carbon storage capacity and conservation of shallow P. oceanica barrier reefs in the Mediterranean Sea.