Sediment properties influence seagrass sedimentary carbon stocks, challenging uniform blue carbon accounting

Seagrass meadows are increasingly recognised as important blue carbon ecosystems, yet large uncertainties in regional sedimentary organic carbon (Corg) stocks limit their inclusion in greenhouse gas accounting. Northern Ireland supports extensive seagrass meadows across diverse hydrogeomorphic settings but lacks empirically derived carbon stock estimates. Here, we present the first regionally representative assessment of sedimentary Corg storage in Northern Ireland seagrass meadows. Sediment cores were collected from nine intertidal and subtidal meadows and analysed for carbon content, sediment properties, and stable carbon isotopes, with adjacent unvegetated sediments used as references where available. Sediment Corg stocks to 1 m depth averaged 109.7 ± 18.9 Mg C ha-1 but varied by more than an order of magnitude among sites (18.6 - 280.8 Mg C ha-1). Seagrass sediments retained higher carbon at depth than unvegetated sediments, indicating enhanced burial and carbon permanence, although surface carbon enhancement was highly site dependent. Multivariate analyses identified sediment properties, used as proxies for hydrodynamic setting, as dominant factors influencing carbon storage, with fine-grained, low-energy systems supporting substantially higher stocks per hectare than more exposed, coarse-grained environments. Stable isotope mixing models showed that sediment carbon commonly comprised a mixture of seagrass-derived and allochthonous material, indicating that high carbon stocks are not restricted to autochthonous production alone. These results show that seagrass carbon storage in Northern Ireland is highly heterogeneous, influenced by physical setting rather than seagrass presence alone, and must be represented using stratified, regionally resolved approaches in blue carbon accounting.