Concentrations, Fluxes And Influencing Factors Of Greenhouse Gases In Tidal Creeks In Coastal Wetlands Of The Yangtze Estuary

The aquatic ecosystem is an important source of greenhouse gases（GHGs）emissions,and their emission process and mechanism are crucial to understand and solve the global climate change crisis.At present,most of the research on GHG emission fluxes in aquatic ecosystems focuses on shelf seas,estuaries,rivers and lakes,etc.The understanding and estimation of GHG emission fluxes in coastal wetland aquatic ecosystems are still unclear.In addition,most of the existing studies on GHG fluxes in aquatic ecosystem are limited to the vertical diffusion fluxes between water-air interface.However,due to the highly dynamic water interaction between coastal wetlands and offsbinhaishidie water,the carbon and nitrogen cycling process in the lateral direction is complicated,so the lateral transport fluxes of GHGs are often ignored.Therefore,exploring the change process of dissolved GHGs concentrations in coastal wetland aquatic ecosystems and their responses to environmental factors,and accurately estimating the vertical diffusion and lateral transport fluxes of GHGs,are helpful for clarifying the carbon and nitrogen cycle process of coastal wetland or coastal aquatic ecosystems and accurately evaluating the carbon sequestration capacity at coastal wetland ecosystems.Based on this,we selected Jiuduansha tidal salt marsh（JDS）and Xisha tidal salt marsh（XS）in the Yangtze River Estuary as the research objects.We identified the variation characteristics of dissolved GHGs concentrations,water-air interface exchange fluxes and lateral exchange fluxes between inside and outside of wetlands,and investigated the responses of dissolved GHGs concentrations and exchange fluxes to key environmental factors.We also constructed multiple regression models of environmental parameters and different dissolved GHGs concentrations to estimate vertical diffusion and lateral transport fluxes of GHGs and analyze the source-sink transformation of GHGs in coastal wetlands aquatic ecosystems of the Yangtze Estuary by combining the traditional field interval sampling and in-situ continuous observations methods.The main conclusions of this study are as follows:（1）The seasonal changes of the dissolved CO₂,CH₄ and N₂O concentrations in JDS and XS aquatic ecosystems were significant（p<0.05）.The average dissolved CO₂concentration in JDS(83.86±15.88μmol·L^-1)was significantly higher than that in XS(75.63±9.00μmol·L^-1)（p<0.05）,and the average dissolved CH₄ concentration in JDS(0.18±0.17μmol·L^-1)was significantly lower than that in XS(0.56±0.91μmol·L^-1)（p<0.05）.The average dissolved N₂O concentration in JDS and XS were 0.093±0.118μmol·L^-1 and 0.035±0.026μmol·L^-1,respectively,with no significant difference（p>0.05）.Dissolved organic carbon is the main environmental factor for the spatio-temporal difference of dissolved CO₂ concentration in aquatic ecosystems;Water temperature,salinity,dissolved oxygen,p H,SO₄^2-and water level are the main factors for those of dissolved CH₄ concentration;Water temperature,salinity,dissolved oxygen,dissolved inorganic nitrogen and water level are the main factors for those of N₂O dissolved concentration.（2）The seasonal changes of water-air interface CO₂,CH₄ and N₂O exchange fluxes in JDS and XS aquatic ecosystems were significant（p<0.05）.The annual average water-air interface CO₂ and N₂O exchange fluxes in JDS(CO₂:200.07±115.55 nmol·m^-2·s^-1;N₂O:0.17±0.14 nmol·m^-2·s^-1)were significantly higher than those in XS(CO₂:112.84±76.50 nmol·m^-2·s^-1;N₂O:0.07±0.10 nmol·m^-2·s^-1)（p<0.05）,the annual average water-air interface CH₄ exchange flux in XS(1.23±3.15 nmol·m^-2·s^-1)was significantly higher than that in JDS(0.26±0.63 nmol·m^-2·s^-1)（p<0.05）.The lateral CO₂,CH₄ and N₂O exchange fluxes in JDS(CO₂:32.40±1668.32 nmol·m^-2·s^-1,CH₄:0.72±4.96nmol·m^-2·s^-1 and N₂O:-0.01±0.70 nmol·m^-2·s^-1)were slightly lower than that in XS(CO₂:74.53±784.88 nmol·m^-2·s^-1,CH₄:0.88±3.21 nmol·m^-2·s^-1 and N₂O:0.01±0.53nmol·m^-2·s^-1),but with no significant spatio-temporal difference（p>0.05）.In addition to the environmental factors affecting the dissolved GHGs concentration,wind speed is also the main environmental factor leading to the spatio-temporal differences in the water-air interface exchange fluxes of GHGs,water flow and velocity are also the main environmental factors affecting the spatial-temporal differences in the lateral exchange fluxes of GHGs.（3）In the lateral direction,the aquatic ecosystem of JDS is the sink for CO₂(-346.62 mg·m^-2·d^-1),the source for CH₄(1.32 mg·m^-2·d^-1)and the sink for N₂O(-0.66mg·m^-2·d^-1);the aquatic ecosystems of XS is the source of CO₂(262.71 mg·m^-2·d^-1),CH₄(0.34 mg·m^-2·d^-1)and N₂O(0.10 mg·m^-2·d^-1).In the vertical direction,the aquatic ecosystems of JDS and XS are both sources of atmospheric CO₂,CH₄ and N₂O.The annual average vertical diffusion fluxes of CO₂,CH₄ and N₂O in JDS were 684.47mg·m^-2·d^-1,0.33 mg·m^-2·d^-1 and 0.67 mg·m^-2·d^-1,respectively;those in XS were 236.28mg·m^-2·d^-1,0.95 mg·m^-2·d^-1 and 0.14 mg·m^-2·d^-1,respectively.（4）The GWP of GHGs in JDS aquatic ecosystem(876.84 mg·CO₂·m^-2·d^-1)is stronger than that in XS aquatic ecosystem(302.09 mg·CO₂·m^-2·d^-1).The GWP of CO₂and CH₄ in JDS aquatic ecosystem accounted for 7.95～12.96%of the absolute GWP of CO₂ and CH₄ in JDS wetland,which will offset carbon uptake of the JDS ecosystem.The GWP of CO₂ and CH₄ in XS aquatic ecosystem accounted for 3.66～9.87%of that in XS ecosystem.In summary,we used the self-developed,in situ continuous observation system to effectively increase the observation frequency of dissolved GHGs concentrations,accurately estimate GHGs fluxes in coastal wetland aquatic ecosystems of the Yangtze River estuary,which can perfect the coastal wetland blue carbon research about full carbon balance and achieve the strategy of"carbon neutral"to deal with global climate change.