Carbon Fluxes Via Submarine Groundwater Discharge In A Typical Coastal Saltmarsh Wetland-nearshore-estuary Continuum

As one of the blue carbon ecosystems with the highest carbon burial efficiency,it is not only an important part of the blue carbon sink in coastal zones,but also seriously affects the carbon budget in coastal and estuarine areas which play a vital role in tackling global climate change.However,the buried carbon in the saltmarsh wetlands would still be exported to the adjacent sea through submarine groundwater discharge（SGD）,a process that has often been overlooked in previous studies.Therefore,in the present study,we take the coastal saltmarsh wetland-nearshore-estuary continuum as the research object,based on the geochemical tracing method of natural radioactive radium isotope to assess the SGD fluxes and the associated carbon fluxes,aiming to discuss the influence and contribution of SGD on the carbon budgets of the coastal saltmarsh wetlands and nearshore and estuarine areas.In a typical tidal creek of the Chongming Dongtan saltmarsh wetland,based on the measured data of ²²⁴Ra and ²²³Ra and the hydrological data such as water flow in the tidal creek water,the SGD rate in the tidal creek was estimated to be 20.8±18.3cm d^-1.Combined with the carbon concentrations in the porewater,the fluxes of dissolved inorganic carbon（DIC）,dissolved organic carbon（DOC）,CH₄ and CO₂exported from SGD to the tidal creek can be derived as（2.54±0.72）×10⁴,（2.77±1.68）×10³,34±33 and（6.76±4.67）×10³ mol d^-1,respectively.And through the investigation of different forms of carbon in the creek water,the fluxes of DIC,DOC,CH₄,and CO₂exported laterally from the tidal creek to the nearshore water can be derived as（2.43±0.60）×10⁴,（3.18±0.96）×10³,0.48±0.26 and 639±288 mol d^-1,respectively,which showed that the SGD-derived carbon fluxes accounted for a large portion of the lateral carbon output,and even much higher than the lateral output.Meanwhile,this study further investigated the effect of SGD on the carbon budgets of the tidal creek system,and found that carbon exported from SGD to the tidal creek accounted for 50%of the carbon burial,of which DIC accounted for 73%of the total carbon flux transported by SGD.These indicated that carbon flux imported into tidal creek water through SGD have caused a overestimation in the carbon sequestration capacity of saltmarsh wetlands,which also reveals the importance of SGD for the carbon cycle of tidal creek system of saltmarsh wetland.The carbon in saltmarsh wetland enters the nearshore water through tidal creek and SGD,which will further affect the carbon cycle in nearshore water.In order to explore the influence of this process,we conducted continuous observations in the nearshore water of Chongming Dongtan.Based on the measured ²²⁴Ra,²²³Ra,²²⁶Ra,and ²²⁸Ra,two methods were applied to estimate SGD rates,including the three-endmember model and mass balance model.All methods revealed that the SGD rate in Chongming Dongtan saltmarsh ranged from 1.81 to 5.39 cm d^-1(mean value:3.89±1.20 cm d^-1).The SGD-derived DIC and DOC fluxes were then estimated to be（1.74±0.65）×10⁷ and（1.15±0.73）×10⁶ mol d^-1,respectively.The established carbon budgets in the Dongtan saltmarsh wetland showed that the fluxes of DIC and DOC transported by SGD were at least twice as much as the riverine input,and SGD correspondingly made up 66%and 64%,of the total inputs into the Chongming Dongtan saltmarsh nearshore water by comparing the carbon sources of lateral output of tidal creek,river,sediment diffusion and SGD,it can be seen that SGD is the most important carbon source in the nearshore water of Chongming Dongtan saltmarsh.In addition,comparing with carbon storage in Chongming Dongtan saltmarsh wetland,carbon export through the SGD(4.46±1.58 g C m^-2 d^-1)was 1.5-fold greater than the carbon burial rate,indicating that SGD plays an important role in evaluating carbon sequestration capacity of saltmarsh,and suggests that SGD should not be overlooked in blue carbon assessments of saltmarshes.The carbon transported by coastal saltmarsh wetland and its SGD has an important effect on the carbon budget of coastal and estuarine water.Therefore,in the Changjiang River Estuary which affect by saltmarsh wetland,through our collected surface seawater,coastal groundwater water and river water samples,we estimated SGD rate based on the ²²⁴Ra mass balance model was 12.8±19.5 cm d^-1 and its-derived DIC and DOC fluxes were then estimated to be（4.09±6.21）×10⁹ and（2.91±4.42）×10⁸mol d^-1.By establishing the carbon budgets in our study region of the Changjiang River Estuary,it can be seen in the SGD,river,sediment diffusion and lateral output of DIC and DOC flux,F_SGD（72.6%）>F_River（23.3%）>F_Diff（4.1%）>F_Lateral（0.1%）for DIC and F_SGD（45.0%）>F_River（21.4%）>F_Diff（20.3%）>F_Pro（13.3%）>F_Lateral（0.1%）for DOC.Meanwhile,it is found that SGD plays an important role in the coastal saltmarsh wetland-nearshore-estuary continuum,which can not only carry a large amount of carbon into tidal creek and then transport it to nearshore water,but also plays a dominant role in the carbon budget of estuarine areas.In summary,this study elucidated the carbon pattern of the coastal saltmarsh wetland-nearshore-estuarine continuum of the Chongming Dongtan tidal creek system,Chongming Dongtan nearshore water,and the Changjiang River Estuary under the influence of submarine groundwater discharge,and highlighted the carbon transport of SGD and lateral export by saltmarsh wetland in this system,which revealed the significance of SGD in the carbon sink function of the saltmarsh wetland ecosystem.The achievement of the study can not only enrich the theoretical system of the regional carbon cycle,but also provide scientific data support for the assessment of blue carbon of saltmarsh ecosystems in global carbon cycles,and serves the national strategy of our government to actively respond to and commit to the ambitious goal of"achieving carbon neutrality by 2060".