Abstract:Seagrass meadows are one of the most abundant systems in the coastal area. They absorb and sequester a large amount of atmospheric carbon dioxide (CO2) forming an important blue carbon system. The strong ability of seagrass beds to absorb CO2 and store organic carbon (Corg) could be attributed to their ability to convert CO2 into plant biomass and to reduce water flow and sediment re-suspension, capturing organic particles from outside the ecosystem. Thus, the source and the stock of carbon have attracted much attention. In certain areas, this value exhibits a large range. In some meadows, endogenous organic matter (OM) such as seagrass litter (leaves and roots) and epiphytes account for a larger contribution to carbon burial, while in most other grass beds, exogenous OM such as phytoplankton, suspended particulate matter, and mangroves in adjacent systems dominate. This is primarily because all marine ecosystems exchange energy and matter with the surrounding systems through water flow, and carbon is no exception. Sanggou Bay is one of the main seagrass distribution areas in northern China. It is also a typical mariculture bay in China. In this bay, the long-term large-scale bivalve culture activities have given a special ecological scene to the adjacent culture ecosystem and seagrass beds. Based on stable isotopes, this study analyzed the sources of organic carbon in the sediments of the two eelgrass meadows in Sanggou Bay and evaluated the organic carbon stock. The results showed that the isotope δ13C of the eelgrass bed in Sanggou Bay was in the range of (20.31~ –21.99)‰, compared to 12.30‰ of the eelgrass itself. The difference (8.2‰) shows the typical characteristics of allochthonous organic carbon. The estimation from the Isosource 1.3 isotope mixing model software showed that the surface organic carbon in the two eelgrass beds mainly originated from the phytoplankton (34.0%~41.4%). Bio-deposit from cultured bivalve also contributed 23.9%~25.3%, while eelgrass itself only contributed about 8.3%~17.1%. The contribution of shellfish bio-deposit was about 23.9%~25.3%, while that of macroalgae was about 25.0%. Around the Chudao eelgrass bed, the carbon output from the eelgrass contributed about 5.2% to 10.7% to the organic carbon deposited on the surface site within 2 km. The carbon stock estimation showed that the organic carbon storage at the depth of 0~30 cm in the two grass beds was 2.01 and 3.75 Mg C/hm2, with an average of 2.88 Mg C/hm2, and about 0.71 Mg C/hm2 was from bivalve deposition. In addition, eelgrass also contributed (average 5.2%~10.7%) to sediment carbon in the surrounding system. The contribution of eelgrass to surface sediment organic carbon in the study site was lower than that of the average contribution of eelgrass in the north temperate eelgrass beds (20%~40%). This result could be linked to the fact that both seagrass beds are in an environment with strong nearshore hydrodynamics. Under the strong hydrodynamic action, the exfoliated materials such as leaves are carried out of the grass bed and get accumulated in the surrounding environment, while the bio-deposit of the surrounding cultured shellfish are carried into the meadows by the resuspension. Strong water flow presents a weaker rate of carbon accumulation due to the high microbial decomposition. Compared to the average carbon stock in the temperate eelgrass beds (27.2 Mg C/hm2) in the world, the carbon stock in Sanggou Bay eelgrass meadows is low. The primary reason is the substrate characteristics of the eelgrass bed. The average grain size is relatively large, with relatively low mud content, which results in strong microbial decomposition, making it difficult for organic carbon from various sources to accumulate. The decline of seagrass will lead to a large loss of sedimentary organic carbon and eelgrass bed restoration could be an effective means to curb this trend. Strengthening the protection of seagrass in Sanggou Bay and carrying out effective transplantation and restoration could be an important measure to increase the carbon storage and would help to provide more ecological services such as increasing biodiversity and maintaining environmental health. This study shows that the carbon storage of the seagrass system in Sanggou Bay is relatively low. Bio-deposit from farmed shellfish is an important source of organic carbon in eelgrass beds. The results provide an in-depth analysis of the source of sedimentary carbon sink in the eelgrass bed in Sanggou Bay and the contribution of large-scale mariculture activities to the seagrass blue carbon.
