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

CONTEXT: Seagrass meadows act as efficient natural carbon sinks by sequestering atmospheric CO₂ and through trapping of allochthonous organic material, thereby preserving organic carbon (Cₒᵣg) 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 Cₒᵣg in seagrass habitats. METHODS: Through predictive modelling, we assessed sedimentary Cₒᵣg 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 Cₒᵣg 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 Cₒᵣg levels in seagrass seascapes. There was higher Cₒᵣg content and a mangrove signal in seagrass surface sediments closer to the deforested mangrove area, possibly due to an escalated export of Cₒᵣg from deforested mangrove soils. Seascapes comprising large continuous seagrass meadows had higher sedimentary Cₒᵣg 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 Cₒᵣg 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.