Dynamics and fate of blue carbon in a mangrove–seagrass seascape: influence of landscape configuration and land-use change

International audienceAbstract Context Seagrass meadows act as efficient natural carbon sinks by sequestering atmospheric CO 2 and through trapping of allochthonous organic material, thereby preserving organic carbon (C org ) in their sediments. Less understood is the influence of landscape configuration and transformation (land-use change) on carbon sequestration dynamics in coastal seascapes across the land–sea interface. Objectives We explored the influence of landscape configuration and degradation of adjacent mangroves on the dynamics and fate of C org in seagrass habitats. Methods Through predictive modelling, we assessed sedimentary C org content, stocks and source composition in multiple seascapes (km-wide buffer zones) dominated by different seagrass communities in northwest Madagascar. The study area encompassed seagrass meadows adjacent to intact and deforested mangroves. Results The sedimentary C org content was influenced by a combination of landscape metrics and inherent habitat plant- and sediment-properties. We found a strong land-to-sea gradient, likely driven by hydrodynamic forces, generating distinct patterns in sedimentary C org levels in seagrass seascapes. There was higher C org content and a mangrove signal in seagrass surface sediments closer to the deforested mangrove area, possibly due to an escalated export of C org from deforested mangrove soils. Seascapes comprising large continuous seagrass meadows had higher sedimentary C org levels in comparison to more diverse and patchy seascapes. Conclusion Our results emphasize the benefit to consider the influence of seascape configuration and connectivity to accurately assess C org content in coastal habitats. Understanding spatial patterns of variability and what is driving the observed patterns is useful for identifying carbon sink hotspots and develop management prioritizations