Seagrass meadows, salt marshes, mangroves and marine sediments sequester carbon dioxide (CO₂), mitigating climate change while increasing local marine biodiversity and providing coastal protection. These ecosystems act as carbon sinks due to their high productivity and capacity to trap and preserve sedimentary carbon. While carbon
uptake in coastal vegetated zones has been extensively studied, accounting, monitoring and reporting for carbon sequestration enhancement in open coastal and ocean environments remain difficult, as the natural carbon sink is influenced by dynamic lateral transport between various ecosystems and areas and additionally modified by human activities. Furthermore, the durability of carbon sinks in these blue carbon ecosystems is uncertain, particularly in the face of climate change and increasing anthropogenic stressors. 
A key question for blue carbon enhancement is to properly specify the baseline, distinguishing autochthonous and allochthonous carbon in each blue carbon system. Accounting for carbon imported into the blue carbon ecosystems from external sources, e.g. through terrestrial carbon input from land and/or lateral transport among different ecosystems and habitats, would overestimate the CO2 removal from atmosphere. Therefore, an understanding and quantification of lateral carbon fluxes is essential for determining the role of allochthonous carbon and, thereby, for accurate carbon budgeting in Blue Carbon ecosystems.
In this talk, I will present our recent studies addressing lateral carbon connectivity among different blue carbon ecosystems and unvegetated sediments in the North Sea and the Baltic Sea, highlighting the potential of numerical modeling in helping define the baseline for carbon crediting in blue carbon ecosystems.