Ecological connectivity between mangroves and seagrasses increases sediment blue carbon storage

Mangrove forests and seagrass meadows are important coastal blue carbon ecosystems for mitigating climate change and often co-occur as coastal continuums in tropical and subtropical regions. While mangrove contributions to the enhancement of carbon storage in seagrass sediments is well-known, the potential impact of seagrass meadows—whose biomass is an order of magnitude smaller than that of mangroves—on mangrove ecosystems remains unclear. In this study, we investigated differences in shallow surface (0–30 cm) sediment carbon storage across two mangrove-seagrass continuums and two mangrove-only sites in a semi-closed bay in Zhanjiang, Guangdong, China. Higher sediment carbon densities (by 80.7%–124.3%) in continuum sites indicate that seagrass presence enhanced the carbon stocks of both mangrove and unvegetated tidal flats. Stable isotope analysis showed that the enhancement of sediment carbon was due to increased deposition of both autochthonous and allochthonous organic carbon (in a lesser degree) from adjacent ecosystems. Therefore, ecosystem connectivity between mangrove forests and seagrass meadows can substantially increase sediment carbon densities due to dense seagrass vegetation reducing water velocity and turbulence, highlighting the importance of protecting interconnected ecosystems and coastal landscape-scale management for climate change mitigation.