Carbon Cycle Processes And Their Key Control Factors In Coastal Wetland Waters

As a transitional ecosystem linking inland waters to the sea,coastal wetlands are the first to be affected by sea level rise in the context of global warming.Coastal wetlands are highly productive and have high carbon sequestration capacity.The processes of material cycling in coastal wetlands are complex.The transport and transformation processes between different forms of carbon in coastal wetland waters are still relatively poorly studied.To investigate the processes and control factors of carbon transport and transformation in coastal wetland waters,this study sampled a total of 21 coastal wetlands in nine coastal provinces in China during the summer of 2019（July-September）.To further investigate the seasonal characteristics of the carbon cycle in coastal wetlands,in addition to the summer of 2019,additional sampling was conducted in October 2019 and March 2021 in the Beidagang Wetland in Tianjin.Basic water environmental factors,carbon concentrations of each form and relevant isotope data were measured.Gene sequencing of microorganisms in the wetland was also performed using gene chip technology to obtain data related to carbon fixation and carbon degradation genes in the wetland.The main findings of this study are as follows:（1）There is no significant north-south distribution pattern in the physical and chemical parameters,the concentration of carbon and the carbon isotopic composition in the coastal wetlands.The coefficients of variation of the parameters within the wetlands located in the Haihe,Yellow River and Yangtze River basins are larger,probably because the wetlands in these basins are generally located in areas of high human activity.The water bodies at the entrance to the coastal wetlands are mainly affected by rock weathering and evaporation while the internal water bodies are mainly affected by evaporation and seawater.（2）The water depth in the coastal wetland is shallow and there is no stratification of the water column.The physical and chemical parameters of the water and the carbon transport and transformation processes are different between the emergent vegetation zone and the open water zone in the wetland.The carbon content of the sediments in the emergent vegetation zone is higher than in the open water zone,and the influence of phytoplankton on the carbon cycling processes in the open water zone is stronger than in the emergent vegetation zone.（3）The dissolved inorganic carbon（DIC）in coastal wetlands is mainly influenced by a mineral weathering and phytoplankton within the wetland.The strong carbon fixation process by phytoplankton in the water column is the main reason for the positiveδ¹³C_DIC in the water.The particulate inorganic carbon（PIC）in the water column is controlled by a combination of exogenous inputs and endogenous precipitation.The most significant source of particulate organic carbon（POC）in the water column is the contribution of phytoplankton growth,followed by exogenous POC flowing into the wetland through the wetland inlet,with the least contribution from plant litter in the wetland.The most significant source of dissolved organic carbon（DOC）in the water column is DOC flowing into the wetland through the wetland inlet,followed by the precipitation and diffusion of soluble organic carbon from the sediment,with the least contribution from mineralised decomposition of POC in the wetland.（4）Dissolved carbon concentrations in the water column are lower in summer than in spring and autumn,and particulate carbon concentrations are relatively high.POC in the spring water column is mainly derived from phytoplankton.The dissolved carbon concentration in the spring water column is higher than in the summer and autumn due to the influence of the coastal water.（5）The carbon cycling process in coastal wetlands is mainly influenced by nutrients,external environmental factors,sediments and biological effects.The results of the quantitative analysis show that wind speed and water salinity in the wetland have an inhibitory effect on the carbon cycling process in the wetland.In the coastal wetlands,nutrients,dissolved oxygen,p H,phytoplankton and other microorganisms such as bacteria and archaea contribute to the process of carbon cycling in the water column;the concentration of organic carbon in the sediment also contributes to the process of carbon cycling in the water column;The biological effects of phytoplankton and other microorganisms such as bacteria and archaea in wetland water bodies are the strongest contributors to the carbon cycling process in wetlands.In summary,this study analysed the process of transport and transformation processes of carbon and their key controlling factors in coastal wetland waters in China.The study discusses the role of phytoplankton in the carbon cycle process,identifies the sources and cyclic processes of carbon in wetland waters,and identifies key control factors in the carbon cycle process.The results of the study provide a scientific basis for future research on the carbon cycle in coastal wetlands,and provide a theoretical reference for the ecological protection of coastal wetlands.