Fixed-Station Carbon Density Observations in a Zostera marina Meadow at Caofeidian

Site-level carbon density data remain limited for many seagrass meadows, especially where the mapped meadow extent has not been paired with repeated carbon-pool measurements. This study quantified biomass carbon density, 0–1 m sediment organic carbon density, total carbon density, and sediment contribution at 10 fixed stations in a meadow dominated by Zostera marina Linnaeus in Caofeidian, Bohai Bay, China, in 2021, 2024, and 2025. Sediment grain-size composition and five water-quality variables were also summarized for 2025 as environmental context. Sediment organic carbon dominated the station-level carbon pool. In 2025, biomass carbon density was 0.269–0.900 Mg C ha−1, sediment organic carbon density was 8.108–39.730 Mg C ha−1, and total carbon density was 8.377–40.566 Mg C ha−1. The median total carbon density was 23.391 Mg C ha−1 in 2021, 18.827 Mg C ha−1 in 2024, and 20.040 Mg C ha−1 in 2025. The rank association between fine sediment fraction and carbon density was positive in 2025 (Spearman’s ρ = 0.491, p = 0.150). These fixed-station records support seagrass monitoring, restoration planning, and sustainable coastal management. They also help link the mapped meadow extent with field carbon data for blue carbon assessment under climate and marine sustainability goals.