Organic Carbon Accumulation And Its Lateral Transportation In A Typical Tidal Creek System Of The Yangtze River Estuary

Plants,sediments and water are the three major carbon pools in coastal salt marsh system.Quantitative estimation of its carbon sink capacity is critical to accurately assess the value of blue carbon ecosystems.As one of the Ramsar Sites,the Chongming Dongtan Wetland is located at the northern part of the intersection of the Yangtze River and the East China Sea.It is densely covered with tidal creeks and is periodically submerged by rising tides,while siltation and erosion are dynamically balanced in this area with continuous vegetation succession.The rapidly changing environment made it difficult to study the carbon budget in the tidal creek system.In this study,a typical tidal creek system in the southern Chongming Dongtan with zonal vegetation was selected and its boundary was delineated.Based on the measured data in 2013 and 2017,remotely-sensed image interpretation,model retrieval and statistical analysis,the carbon sink capacity of the 42 hm² tidal creek system and the lateral transportation of organic carbon in water were evaluated,and the sources of organic carbon in the three major carbon pools were estimated.The main research results were as follows:(1)Affected by low halophytes and low coverage,the carbon sequestration capacity of salt marsh vegetation at the southern part of Chongming Dongtan was still lower than that of the global average level of coastal wetlands.With the positive succession of salt marsh plants,the carbon sequestration capacity of vegetation gradually increases,and the carbon sequestration potential is great.The net primary productivity of halophytes showed significant temporal and spatial heterogeneity.The temporal differences were mainly reflected by seasonal variations.In 2013 and 2017 the net primary productivity of halophytes in summer(>97.70±69.31 g C/m²·month)was significantly higher than that in other seasons.The spatial difference showed that the net primary productivity of Phragmites australis cohorts(>1079.64±168.34 g C/m²·yr)was significantly higher than that of other cover types.Until 2017?2018,the carbon sequestration capacity of Chongming Dongtan salt marshes were further improved.Under the background of sea level rise,the positive succession of salt marsh vegetation compensates for the organic carbon loss caused by the erosion of salt marsh vegetation.The annual total carbon sequestration potential increased by 310.1 g C/m²?595.5 g C/m².In 2017,the total organic carbon storage of vegetation in the study area increased compared with that in 2013,reaching 294.2 t.The continuous rise of global temperature and dynamic changes of estuarine environment,such as sea level rise,shoreline erosion,vegetation succession and human disturbance,increases the uncertainty of salt marsh carbon sequestration ability.(2)The content of organic carbon in the salt marsh sediments is closely related to the species and biomass of halophytes,and carbon storage of sediments showed obvious difference among different cover types.As the sediment in the study area continued to accumulate,the carbon storage continued to increase.The organic carbon content(0?30 cm)of the study area varied within the range of 0.7?10.93 g/kg horizontally,which was positively correlated with the biomass of halophytes,and decreased with increasing depth vertically(0?100 cm).The order of carbon density of different halophyte communities was Phragmites australis cohorts(57.7 t/hm²)>mixed cohorts(49.2 t/hm²)>sedge cohorts(25.5 t/hm²)>mudflats(21.5t/hm²).Combined with remotely sensed image interpretation,the total organic carbon storage in the aboveground and belowground biomass,and 100 cm deep sediments of the selected tidal creek system was estimated as 1493 t in 2013.Due to the low biomass and fast organic carbon turnover rate,organic carbon increased by 0.34 t/hm²(Sedge cohorts)to 2.06 t/hm²(Mixed cohorts)for different cover types in the study area from 2013 to 2017.The newly added sediments provided new storage space for salt marsh organic carbon.(3)Influenced by tidal action,the organic carbon concentration and fluxes of the tidal creek system in the study area showed obvious seasonal variations,with the highest in summer and lowest in spring.From 2017 to 2018,during the tidal cycles,the dissolved organic carbon concentration and particulate organic carbon concentration in the salt marsh waters of the southern Chongming Dongtan both reached the highest in autumn,3.54 mg/L and5.48 mg/L,respectively,and the lowest concentration in winter and spring was 1.87mg/L and 1.51 mg/L,respectively.The two fluxes showed significant positive correlation in different seasons affected by the water flux,but the two concentrations only showed a positive correlation in autumn and winter.In this study,the tidal creek system in different seasons showed organic carbon export,and the organic carbon fluxes during tidal cycles ranged from-12.65 g C/m² to4.04 g C/m².The intensity showed significant seasonal differences,with the highest in summer,the second in autumn,and the lowest in spring.In different seasons,organic carbon flux during spring tides were significantly higher than that during neap tides.Due to the unique hydrological conditions of the Yangtze River estuary and the relatively young stage,the salt marshes in the study area showed a strong lateral carbon source function.As the salt marsh matures,the output of organic carbon will gradually decrease.(4)In the tidal creek system investigated,terrestrial organic components accounted for a relatively high proportion in the surface sediments organic carbon and the dissolved organic matter,while the marine authigenic components accounted for a relatively high proportion in deep sediments organic carbon.Sediment and phytoplankton were the main sources of particulate organic matter.The organic carbon in the sediments of plant cohorts was a mixture of marine and terrestrial sources and showed differences among vegetation coverage types.The contribution rate of terrestrial-source organic carbon in the Phragmites australis and mixed cohorts exceeded 60%,and with the increase of sediment depth,the proportion of terrestrial-sourced carbon gradually decreased.The proportion of terrestrial sources in the surface sediments of the sedge cohorts was not significantly different from that of the other two vegetation types,but it did not decrease significantly with depth.The results showed that the dissolved organic matter in salt marsh water was a mixture of terrigenous humus,marine-derived humus and protein-like substances under the combined influence of resuspension,pore water release and tidal action.The terrigenous components of dissolved organic matter from higher plants or soil were dominant,and the autogenetic capacity of water was weak.However,phytoplankton and sediment re-suspension are the main sources of particulate organic matter.With terrestrial succession of vegetation and continuous sediment deposition,the carbon sequestration capacity of vegetation and the carbon storage of sediment in the study tidal creek system increased,and the organic carbon export decreased gradually.The carbon sink capacity of study tidal creek system will gradually increase.In the context of future environmental changes,the protection of the salt marsh ecosystem will feedback positively to the mitigation of CO₂ emissions.