| Riverine carbon transport acts as a crucial biogeochemical role in the global cycling.An extensive investigation was conducted in the Longchuan River of the upper Yangtze basin and the Three Gorges Reservoir area.We focused on the CO2 area flux(FCO2)at the water-air interface and dissolved organic matter.Our results indicated that:Temporal and spatial characterization of partial pressure(pCO2)and CO2 areal flux at the water-air interface in the Longchuan River,to reveal the riverine pCO2 and their controls.Thin boundary layer model was adopted to estimate the CO2 flux via water-air interface.The results indicated the riverine pCO2 were 1740.5±1659.7,1731.3±1595.3 and 1552.4±1355.3 ?atm in spring,autumn and winter,respectively.Moreover,the median pCO2 in the main stem(1321.0±1243.2 ?atm)was lower than the tributary(1856.4±1633.5 ?atm).The FCO2 were 649.7±812.8?625.8±763.4 and 693.8±445.4 mmol/m2/d in spring,autumn and winter,respectively.In addition,the FCO2 averaged 709.1±803.1 mmol/m2/d in the main stem,2-fold higher than the average of 446.5±608 mmol/m2/d in the tributaries.In spring,the FCO2 had positive correlations with air temperature,Alk,DIC and ORP but negative correlations with the pH.In autumn,the FCO2 had positive correlations with Alk and DIC and ORP but negative correlations with the flow speed and pH.In winter,the-FCO2 had positive correlations with Alk and DIC but negative correlations with the flow speed and pH.Moreover,the main stem of FCO2 was negative correlations with the pH and TP,whereas the tributaries of FCO2 had positive correlations with Alk,DIC and TN but negative correlations with the flow speed and pH.We also found the biogeochemical process of within-river respiration of organic carbon rather than photosynthesis resulted in negative relationships between pCO2 and DO or pH because of the high velocities and short residence time in these mountainous rivers and streams.The differences in the resource and characteristics of DOM in Longchuan River were studied by fluorescence ultraviolet-visible(UV-vis)and excitation-emission matrix spectra(EEMs)combined with analysis.The results indicated that four fluorescent components were identified by PARAFAC model,including fulvic acid-like fluorescent components:C1,C2,humic acid-like fluorescent components:C2,C3 and protein-like fluorescent components:C4.The fluorescent identity and the relative proportions of CDOM showed obvious variations in seasons(P<0.01).We found the fluorescent components of C1,C2 and C3 in spring accounted for 36.1%,46.5%and 17.4%,respectively,and the fluorescent components of C1,C2,C3 and C4 in autumn made up 22.9%,34.9%,28.6%and 13.6%,respectively,and the fluorescent components of C1,C2 and C3 in winter accounted for 30.50,37.5%and 32.0%,respectively.The riverine CDOM exhibited a predominantly terrestrial origin in Longchuan River.Meanwhile,metabolism of the phytoplankton contributed little to the water DOM.During the whole time,the C2 fluorescent components could predict well the content of CDOM:CDOM=-987.137+3.12C2(R2=0.523,P<0.01).It was worthwhile that the CDOM had a strong correlation with the CO2 partial pressure.The S350-400 and/pCO2 had a significant positive correlation in spring.The A254,S350-400,FI,C1,C2,C3 and C4 were significantly positive related to pCO2 in autumn.In addition,the correlation between E2/E3 and pCO2 was significantly positive.This study demonstrated that the CDOM was the dissolved CO2 contribution.Diurnal and seasonal characterization of pCO2 and FCO2 in a tributary of the Jialing River in the TGR region was studied.The results indicated that pCO2 and FCO2 showed significant hourly,daily and seasonal variations(P<0.01).CO2 release peaked around 09:00 and then slightly decreased.On average,the riverine pCO2 was 2436.6±496.9?853.4±76.0 and 925.1±87.8 ?atm in spring,summer and autumn,respectively.Moreover,the FCO2 exhibited the similar seasonal trend:spring>autumn>summer.In summer,the FCO2 had negative correlations with pH?EC and temperature.In autumn,the FCO2 was positive correlations with but negative correlations with pH and temperature.In spring,the FCO2 showed significantly correlations with pH and EC.During the whole period,we also found that pH,Alk and water temperature were significantly related to pCO2 and FCO2,whereas pH and Alk were the best predictors of FCO2.This study aids understanding of the impacts of urbanization on CO2 emissions in the rivers and helps to re-evaluate local riverine CO2 budgets. |
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