Study On Submarine Groundwater Discharge And Its Carried Flux Of Material To The Sea From Two Typical Chinese Mangrove Wetlands

Submarine Groundwater Discharge（SGD）is an important process in the global water cycle.During the interaction between land and sea in coastal areas,SGD can carry substances such as carbon,nutrients,and heavy metals into the ocean,causing significant impacts on the nearshore environment.Because SGD is different from traditional input pathways,it is often overlooked.Therefore,accurate evaluation of SGD and the material flux it carries is of great scientific significance for assessing the source and sink balance of the coastal zone’s biogenic elements.Mangrove wetland soil has extremely high carbon burial capacity,which is beneficial for reducing atmospheric CO₂ concentration and mitigating the adverse effects of global climate change.Its strong carbon sequestration potential is not only evident in sedimentary carbon burial,but also in the groundwater-surface water exchange process（i.e.,the SGD process）in the sedimentary environment.This lateral transport process is likely to be another important carbon sink besides sedimentary carbon burial.Due to the control of driving forces such as tidal pumping,hydraulic gradient,and atmospheric precipitation,the groundwater-surface water carbon exchange process in mangrove wetlands has significant uncertainty in blue carbon sink estimation in coastal zones.Therefore,quantitatively assessing SGD and its associated carbon flux in mangrove wetlands is highly challenging.Therefore,this study intends to select the typical representatives of Golded Bay mangroves in Guangxi and Zhangjiangkou mangroves in Fujian,and use the ²²²Rn isotope tracing technique to quantitatively evaluate the Submarine Groundwater Discharge（SGD）rate and the carbon flux it carries in the sediment of mangrove wetlands.The study aims to explore the impact mechanism of SGD and its effect on the carbon balance and cycling process of the ecosystem,with the goal of gaining a deeper understanding of the carbon sink function of mangrove wetlands as part of the coastal blue carbon system.The study also aims to provide a scientific basis for the source and sink balance of carbon in mangrove wetland ecosystems,thus deepening people’s understanding of the carbon cycling process in mangrove wetland systems.The study’s calculated results are as follows:（1）In August 2021 and March 2022（spring and neap tides）,by continuously observing and analyzing the tidal ditch water in Zhangjiangkou mangrove spatial and temporal distribution characteristics of ²²²Rn activity concentration,dissolved inorganic carbon（DIC）,and dissolved organic carbon（DOC）concentrations in different water bodies were obtained.Further,by constructing the mass balance system of 222Rn in the mangrove area,the SGD rate and dissolved carbon flux carried in different seasons and tides in Zhangjiangkou mangrove were calculated.The results show that the range of SGD rate in Zhangjiangkou mangrove in August 2021 was 0.78-1.31 cm/d,and the fluxes of DIC and DOC transported by SGD were（9.01-10.90）×10⁴ mol/d and（4.38-6.25）×10³mol/d,respectively.During the spring tides in March 2022,the range of SGD rate was0.94-1.42 cm/d,and the fluxes of DIC and DOC transported by SGD were（9.33-9.77）×10⁴ mol/d and（5.27-9.08）×10³ mol/d,respectively.During the neap tides in March 2022,the range of SGD rate was 1.15-1.37 cm/d,and the flux of DOC transported by SGD was（6.54-8.05）×10³ mol/d,with an average value of（7.30±0.76）×10³ mol/d.The dissolved carbon output carried by SGD in the Zhangjiangkou mangrove accounts for about11.71%-12.20%of the primary productivity of the mangrove ecosystem.（2）This paragraph describes the results of continuous online observations of the tidal channel water in the Golden Bay mangrove,Guangxi during the August 2021（spring-neap）period.The study found that during the spring tide period,the SGD rates in the Golden Bay mangrove area ranged from 1.12 to 1.51 cm/d,with DIC and DOC fluxes transported by SGD ranging from（1.37-1.43）×10⁵ mol/d and（0.72-1.27）×10⁴ mol/d,respectively.During the neap tide period,SGD rates ranged from 1.09 to 1.67 cm/d,with DIC and DOC fluxes transported by SGD ranging from（0.79-1.32）×10⁵ mol/d and（0.48-1.14）×10⁴ mol/d,respectively.Taking into account the relevant results reported in the literature,the dissolved carbon output carried by SGD in the Golden Bay mangrove accounts for about 20.00%-26.32%of the primary productivity of the mangrove ecosystem,（3）This passage summarizes the characteristics of changes in SGD rates and associated carbon fluxes in red mangrove wetland regions,based on the comparative analysis of the research results of Zhangjiangkou mangrove and Golden Bay mangrove wetlands in this study,as well as other related studies on global red mangrove regions reported in the literature.The following observations were made:1)In both mangrove,the SGD rates and associated carbon fluxes during the neap tide were higher than those during the spring tide.2)There were obvious seasonal differences in SGD rates and associated carbon fluxes in the tidal creek system of Zhangjiangkou mangrove,as observed in the results of the rainy and dry seasons.3)Significant spatial differences were observed in SGD rates between domestic and foreign research results,such as the SGD rates and associated carbon fluxes of Golden Bay mangrove being higher than those of Zhangjiangkou mangrove,but both were lower than those of other mangrove regions globally,demonstrating significant spatial differences.