Assessing the contribution of carbon export from Seychelles’ seagrass meadows to blue carbon sequestration

Details

Supervisors

Hanert, Emmanuel

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en sciences et technologies de l'environnement, à finalité spécialisée

Abstract

enMarine ecosystems are increasingly recognised as a complementary lever to emission cuts in climate change mitigation. Seagrass meadows sequester carbon locally through burial beneath the canopy and remotely through the lateral export of detached fragments to the deep ocean and sedimentation. National inventories credit only the buried share. This study aims to quantify the missing lateral pathway for Thalassodendron ciliatum meadows in the Seychelles, coupling the multiscale hydrodynamic model SLIM (Second-generation Louvain-la-Neuve Ice-ocean Model) with an offline Lagrangian particle tracking module developed for this thesis that releases 253 million T. ciliatum propagules between 2016 and 2020. Long-term sequestration captures 71 % of the exported carbon under the model's assumptions, equivalent to 12 % of meadow net primary production. Within this pool, 36.0 ± 2.2 % settles inside the Exclusive Economic Zone (EEZ), 35.0 ± 6.8 % settles beyond national borders with comparable residence times, and 29.0 ± 5.2 % is lost to coastal stranding and in-transit decomposition. Meadow location shapes the within-EEZ share more than the monsoon does, ranging from near zero to 44.4 % across the archipelago. Estimated through a Monte Carlo approach on meadow production, the median within-EEZ flux reaches 53 kt C per year (193 kt CO₂e, full range 59–447 across production parameters). This flux is not yet recognised in carbon accounting frameworks, yet it is estimated to be substantial: it would raise the buried-only national baseline of 510 kt CO₂e by 38 %, approach the national transport sector, and reach 40 % of the energy sector. Under emerging voluntary blue-carbon markets, such a flux could also be valorised. More broadly, this thesis underlines why carbon export from seagrass meadows warrants closer investigation as a component of blue-carbon accounting.