The Lateral Flux And Its Influencing Factors Of Dissolved Carbon In Tidal Creeks Of Coastal Wetlands Around The Yangtze Estuary

Coastal blue carbon ecosystem,the ecological buffer zone between the land and ocean,is a large soil carbon stock with high carbon sequestration capacity per unit area.Exploring the processes and the driving mechanism of coastal blue carbon is essential to accurately estimate the blue carbon storage and strengthen the understanding of carbon cycle in coastal wetlands in the context of climate change.The coastal plants can absorb atmospheric CO₂via high photosynthetic capacity,then the CO₂ achieve long-term carbon storage in sediments.However,recent reports showed that lateral carbon exports from coastal wetlands may challenge the assumed efficiency of the carbon sequestration,and comprehensive studies on the lateral carbon loss are still lacking in the coastal wetlands.In this study,we investigated the transportation processes and influencing factors on the lateral flux of the dissolved carbon in coastal wetlands around the Yangtze Estuary.The conventional discrete sampling strategies produce substantial uncertainties in estimating the lateral carbon flux due to the extreme intertidal variability.Therefore,we developed a set of in-situ observation system for dissolved carbon(including dissolved organic carbon(DOC)and dissolved inorganic carbon(DIC))lateral flux in coastal wetlands and realized the high-frequency continuous observation of dissolved carbon in coastal wetlands of the Yangtze Estuary for the first time.DOC and DIC concentration multi-parameter regression models were developed by using discrete sample measurements and sensor measurements to derive high-frequency DOC and DIC concentrations.The main results in this study are as follows:(1)Based on the high-frequency in-situ observations,there were significant spatio-temporal variations of the concentrations and lateral fluxes of DOC and DIC in coastal wetlands around the Yangtze Estuary,correlating to the tidal cycle and intensity.In summer and fall,the DOC concentration increased significantly(p<0.001)result of the high productivity and decomposition rates in wetlands,and the mean DOC concentration was 50%higher than that in spring and winter.The lateral flux of dissolved carbon also raised rapidly,which coincided with the variation of water flux in the astronomical tide period in summer and fall.The lateral flux of the dissolved carbon increased significantly during the spring tide along with the great accumulations of the net flux.However,in the neap tide period,the lateral flux of dissolved carbon was less than 10%of that during the spring tide.The DOC and DIC concentration increased during the ebb tide,and tidal variations of the DOC and DIC lateral flux were similar to that of the water flux.As for the spatial differences,the mean concentrations of DOC and DIC were 2.31 mg L^-1 and 27.9 mg L^-1 in Jiuduansha saltmarsh(JDS),respectively,and in Xisha tidal freshwater wetland(XS)the mean concentrations of DOC and DIC were 2.63 mg L^-1 and 24.5 mg L^-1,respectively.During the spring tide,the DOC concentration increased and the DIC concentration decreased significantly(p<0.001)in JDS,whereas the concentrations did not change with the variation of the tide intensity in XS.(2)Combined with the high-frequency in-situ observations of dissolved carbon flux and the water qualities parameters,we can confirm that the hydrodynamics and sources of dissolved carbon were the critical influencing factors in regulating the variation of the lateral carbon flux of the dissolved carbon across different time scales.On short-time scales(hour),hydrodynamics was the dominant factor that promoted the variations of the dissolved carbon flux.On medium-time scales(tidal to week),the dissolved carbon concentration was the main impact factor,and the variation of dissolved carbon flux lagged due to the limitation of transportation processes in the wetland.On a long-time scale(over weekly),both hydrodynamics and temperature can control the change of the lateral flux.(3)This study evaluated the organic composition,degradation process,and influencing factors of the dissolved organic matter(DOM)in coastal wetlands around the Yangtze Estuary based on the optical properties of dissolved organic matter.In the tidal cycle,ebbing waters draining the marshes were enriched with highly absorbing and aromatic humic-like organic matter compared to the flood tidewaters,while protein-like organic matter did not change significantly.In winter,the protein-like component raised to 50-60%in JDS due to the seawater incursion,whereas the composition of DOM did not change significantly in XS,under the control of the Yangtze River runoff.The productivity and the river runoff were the important factors for the DOM composition,as well as the resuspension of the sediment,hydrological regime and landscape age.Onsite water incubation experiments with photochemical and microbial alterations revealed that photochemistry was primarily responsible for the removal of optically active components in the marsh DOM aquatic pools,whereas the contribution of microbial degradations was minor.(4)The lateral flux of dissolved carbon was one of the carbon losses of the coastal wetlands around the Yangtze Estuary.JDS and XS exported DOC flux about 60.7±2.4and 64.6±1.4 g C m^-2 yr^-1,and DIC fluxes about 619±8 and 497±15 g C m^-2 yr^-1,respectively.The DIC flux also increased the buffering properties of coastal waters.The lateral flux of dissolved carbon accounted for 75.4%and 53.4%of the net ecosystem exchange in JDS and XS,respectively.Based on the results of the high-frequency in-situ observations,we characterized the flux and variation of dissolved carbon in the coastal wetlands around the Yangtze Estuary on tidal and seasonal scale,and explore the environmental impacts of the lateral flux of dissolved carbon across different time scales.Combined with the net ecosystem exchange,we found that the lateral flux of dissolved carbon was the critical carbon loss of coastal wetlands.This study will make a difference to recent carbon cycle models of the coastal wetlands around the Yangtze Estuary.