Are marine protected areas securing blue carbon sequestration in Posidonia oceanica meadows ?

Seagrass meadows are among the most efficient ecosystems for long-term carbon sequestration, alongside mangroves and salt marshes. Posidonia oceanica is of particular interest, as this seagrass forms a root and rhizome system, which stores carbon in the sediment for millennia as the meadow continues to grow. Over the last 50 years, a 34% decline of Posidonia meadows has been documented due to both direct and indirect anthropic activities, one of them being boat anchoring. This study aims to assess if anchorage in Posidonia meadows has effects on the sedimentary carbon stock. Three sites around Cap Martin, south of France, were assessed. The sites follow a gradient of anchoring pressure, the first one is a marine protected area where anchoring has been prohibited for decades. Large boats used to anchor in the second site until 2020, while the third site is a patchy, unhealthy meadow, exhibiting anchoring scars. Three sediment cores were collected from each site and analyzed for ²¹⁰Pb and inorganic and organic carbon content to assess the sediment and carbon accumulation rates. Sediment accumulation rates at these sites were relatively low, varying between 0.6 and 0.9 mm·yr^-1. The concentrations and inventories of excess ²¹⁰Pb at the sites where anchoring occurred were significantly lower than those at the protected site, suggesting that boating activities led to losses of sediment, further confirmed by the lower content of fine sediment. This allows estimating the magnitude of the effects on the carbon storage at the anchoring sites and thus evaluating the relevance of regulation on anchoring to ensure blue carbon ecosystems efficiency.