Spatial-Temporal Distribution And Source Identification Of Dissolved Carbon In The Dagu River,Shandong Peninsula

Carbon is not only the most basic element of organisms but also the main component of many minerals in the natural environment,playing an important role in maintaining the stability of ecosystem.Rivers play an important role in transporting terrestrial carbon to the ocean and atmosphere.Understanding the dynamic distribution and biogeochemical processes of carbon in river ecosystems is very important for knowing the global carbon cycles.Carbon in rivers mainly dominated by dissolved carbon and particulate carbon.Among them,dissolved carbon concluding dissolved organic carbon（DOC）and dissolved inorganic carbon（DIC）is an important part of river carbon flux,and also the key to anyalze the land-sea carbon cyles.Affected by different environmental and human factors,the distribution and transport processes of dissolved carbon in rivers exhibit obvious tempo-spatial differences.The Dagu river,as the largest river in shandong peninsula which dischargeing into the Jiaozhou bay,has obvious regional characteristics.In recent years,due to strong influcene of human activities such as land use,sewage discharge and river damming,the ecological environment of the Dagru River basin has been damaged,producing a number of problems such as no flow in most of the year,water quality degradation,river estuary wetland shrink,seawater intrusion,decrease or loss of land-sea connectivity,which significantly changed the distribution,cycle and flux of river carbon.In this study,the field investigation and water samplings were conduced in the Dagu River from 2018 to 2019.Based on the in-depth data anlyasis,the seasonal and spatial characteristics of dissolved carbon（DIC and DOC）concentrations were revealed.Then,the influencing factors of dissolved carbon in rivers were discussed using water chemistry and isotope analysis.The C/N ratio and the dual terminal mixed method were used identify the sources of dissolved carbon quantitatively.The results show that:（1）Mainly affected by geological environment,climate and hydrological conditions and other natural factors,the dissolved carbon in the Dagu River showed seasonal differences.The DIC concentration is largest in autumn,then followed by winter,summer,spring,with the average of 10.97 mg/L.The DOC concentration is largest in autumn,followed by spring,summer,winter with the mean of 12.85 mg/L.Different from most of global rivers,DOC ranther than DIC is the main form of dissolved carbon in the Dagu River and it is also higher than the average value of global rivers(5.75 mg·L^-1).（2）Spatially,river dissolved carbon shows obvious spatial differences due to the influences of geological characteristics,land use pattern,sewage discharge,river damming in the Dagu River basin,and generally,it indicates an increase from the upstream to the downstream.The spatial difference of DIC concentration is mainly attributed to the difference of geological environment and river damming.The spatial distribution of DOC is mainly affected by land use pattern,sewage discharge,river damming and other factors,having obvious characteristics of exogenous input.The DOC concentration of rivers flowing through rural settlements and cities is the highest,then the cultivated land,and the lowest for the forest land.Moreover,it is significantly higher before the dam than that after the dam.（3）The tempo-spatial distribution of dissolved carbon in rivers is also controlled by the CO₂ exchange at the water-air interface.Therefore,the distribution characteristics of CO₂partial pressure（pCO₂）in the Dagu River water are also discussed.The pCO₂ in the Dagu River ranges from 14.16 to 1772.47μatm,with an average of 313.79μatm,generally expressed as the atmospheric CO₂ sink.The pCO₂ is significantly higher in the upper reaches than that in the middle and lower reaches,and higher in summer and autumn than that in spring and winter.pCO₂ is significantly positively correlated with DIC,but negatively correlated with dissolved oxygen,pH and Chla,which reflectets the strong biological photosynthesis in water.（4）The results of the mixed model show that the contribution of weathering of carbonate weathering to DIC in spring,summer,autumn and winter are 78%,53%,59%and 68%,respectively,and the contribution of soil organic matter are 22%,47%,41%and 32%,respectively.Therefore,carbonate weathering is the main source of DIC in the Dagu river for any season,and its contribution is relatively higher in spring and winter.The sources of DOC in the Dagu River are identified using the C/N ratio.In summer and autumn,the C/N ratio of most sampling points ranges between 4 and 12,implying that DOC mainly comes from aquatic plants for these two seasons.In contrast,it for most of the samplings in winter is less than 4,suggesting that DOC may mainly sourced from sewage discharge for this season.