Temporal And Spatial Pattern Of Dissolved Carbon Dioxide And Methane Concentrations In Xialaoxi Stream, Yichang, Hubei Province

Since the Industrial Revolution,anthropogenic emission factors have led to an increase in the concentration of greenhouse gases in the atmosphere,leading to global warming.Freshwater ecosystems are an important part of the global carbon cycle.Although the rivers surface area accounts for only 20%of inland water,the total amount of CO₂ escaping by rivers（1.8 Pg/year）accounts for about 85%of annual emissions from inland water.CH₄accounts for a small part of river carbon emissions,but its centennial-scale global warming potential is 28-34 times that of CO₂,and its impact on climate change cannot be ignored.Affected by factors such as fast flow velocity,strong disturbance,complex sources,and small area that difficult to identify,the dissolved CO₂ and CH₄ concentrations showed significant spatial and temporal variability,resulting in large errors in the estimation of global river carbon emissions.Existing studies on the characteristics of CO₂ and CH₄ concentrations in rivers mostly focus on large rivers,and relatively few studies on small and medium-sized rivers.This study focused on Xialaoxi,a mountain stream in Yichang,Hubei Province,China.In order to enrich the understanding of greenhouse gases in mountain streams,and provide a scientific basis for accurate estimation of carbon emissions in river ecosystems.We conducted field surveys and indoor analyses at different temporal and spatial resolutions and obtained the diurnal and intra-annual variation characteristics and spatial distribution of dissolved CO₂and CH₄ concentrations in the Xialaoxi stream.We combined it with hydrometeorological elements,landform characteristics and water environment factors and analyed the main influencing factors of CO₂and CH₄ concentrations.The main understandings of this paper are as follows:（1）Through the simultaneous monitoring of water quality,water quantity and dissolved greenhouse gas concentrations at 12 sampling sites in Xialaoxi stream for one year,combined with environmental factors,the characteristics and main influencing factors of CO₂and CH₄concentration changes during the year were analyzed.The results show that the concentration of dissolved CO₂ in Xialaoxi stream in 2021 will vary from 10.851 to 112.676μmol/L,with an annual average concentration of 30.665±13.966μmol/L,indicating a net source of CO₂ to the atmosphere.The average concentrations in spring,summer,autumn and winter were29.676,32.786,31.772 and 27.305μmol/L,respectively.CO₂ showed a significant negative correlation with NH₄⁺-N（P<0.05）,a very significant positive correlation with conductivity and TP（P<0.01）,and a very significant negative correlation with p H and DO（P<0.01）.Water temperature and precipitation both have great influence on dissolved CO₂ concentration in stream.The annual average concentration of dissolved CH₄ was 0.078±0.211μmol/L,and the variation range was 0.002～1.991μmol/L.Except for a few months in winter at site 9,the dissolved CH₄ concentration in the Xialaoxi stream reached supersaturation,which indicates a net source of CH₄ to the atmosphere.The mean concentrations of CH₄ in the four seasons were:0.038,0.093,0.132 and 0.036μmol/L,respectively.The CH₄ concentration in summer and autumn was significantly higher than that in spring and winter（P<0.05）,and the effect of rainfall and runoff was obvious.CH₄ showed a significant positive correlation with TP（P<0.05）,a very significant positive correlation with water temperature and DO（P<0.01）,and a very significant negative correlation with NO₃^--N,DTN,TN and p H（P<0.01）.Temperature was the main controlling factor of CH₄ concentrations in Xialaoxi stream during 2021,which was consistent with the overall trend of CH₄.Precipitation has a greater impact on low-velocity reach but has little effect on mountain streams that are in a state of strong disturbance.Maybe the main factor that reduces the dissolved CH₄ concentrations in low-velocity reach is not"dilution",but caused by raindrops hitting the water surface and increasing the emissions.（2）Through 72h high-frequency continuous monitoring of dissolved greenhouse gas concentrations and water physicochemical factors in the trunk and tributaries of Xialaoxi stream under different climatic conditions,the diurnal variation of CO₂ and CH₄concentrations in the stream was discussed.The west（W）site dissolved CO₂ concentration in spring ranges from 11.493 to 38.782（25.453±9.390）μmol/L,the E site ranges from 12.861 to43.427（26.693±9.423）μmol/L;The W site dissolved CO₂ concentration in summer ranges from 19.755 to 46.314（34.970±8.910）μmol/L,the E site ranges from 25.740 to 53.082μmol/L（41.416±8.259）;The W site dissolved CO₂ concentration in autumn ranges from11.578 to 35.901（26.403±8.505）μmol/L,the E site ranges from 7.235 to 38.706（24.338±10.721）μmol/L;The W site and E site dissolved CO₂ concentration in winter ranges from 12.274 to 33.438（24.872±6.533）μmol/L,7.756～29.147（19.023±6.934）μmol/L,respectively.The results showed that under the interaction of respiration and photosynthesis,the CO₂ concentration in the Xialaoxi stream showed an obvious diurnal variation pattern,with the day decreasing and the night rising,and the maximum value within 24 hours could reach 5 times the minimum value.There were extremely significant differences（P<0.01）.The24h average concentration of Xialaoxi stream can be accurately obtained by sampling at equal time intervals at a frequency of 6 times/day（error less than 5%）.CH₄ showed complex diurnal variation characteristics in different tributaries and different seasons,and there was no significant difference overall.The variation range of dissolved CH₄ concentration in the western site in spring is 0.008～0.028（0.012±0.003）μmol/L,and the variation range of the east site is 0.037～0.024（0.030±0.003）μmol/L.In summer,the concentration range of the western site was 0.022～0.115（0.043±0.017）μmol/L,and the variation range of the east site was 0.018～0.034（0.027±0.004）μmol/L.The concentration range of the western site in autumn was 0.009～0.073（0.012±0.005）μmol/L,and the concentration range of the east site was 0.039～0.116（0.062±0.010）μmol/L.The concentration range of the western site in winter was 0.006～0.011（0.007±0.001）μmol/L,and the concentration range of the east site was 0.024～0.052（0.033±0.004）μmol/L.The wavelet analysis results show that the dissolved CH₄ in Xialaoxi stream has no continuous and stable periodic variation.The specific performance is that the west site rises at daytime and decreases at night in summer,the east site decreases at daytime and rises at night in spring and autumn,and there is no significant change in the two tributaries during the rest of the time.（3）Through intensive monitoring（46 sampling points）in January,April,July and October,the spatial distribution characteristics and influencing factors of dissolved CO₂ and CH₄ concentrations in Xialaoxi stream were discussed in combination with landuse types and geological and geomorphological characteristics.The results showed that the CO₂concentration in 1^st order stream was 34.387μmol/L and that in 2^nd order stream was 30.729μmol/L.There was no significant difference between rivers of different orders.With the increase of the proportion of cultivated land,the CO₂ concentration in 2^nd order stream showed an upward trend.However,when the proportion of forest increases,there is no obvious regularity in CO₂ changes.The degassing effect of small dams on CO₂ is not obvious,and the biogeochemical processes under the dams have a greater impact on the dissolved CO₂concentration than upstream flows.The spatial heterogeneity of CH₄ in Xialaoxi stream is significant,the maximum value in the same season can reach 219 times the minimum value,the median CH₄ concentration in the1^st order stream is 0.021μmol/L,and the median in the 2^nd order stream is 0.033μmol/L,which is 53%higher than the 1^st order stream.When the proportion of cultivated land increased,the CH₄ concentrations in 1^st order stream did not change significantly,and the CH₄concentrations in 2^nd order stream showed an upward trend.The CH₄ concentrations before the small dam was significantly higher than that in channel without human influence（P<0.01）.Due to the water fall,the concentration below the dam is about 45%lower than above it.Agricultural activities,residents’life,leisure and building dams all increased the CH₄concentration level at corresponding sites.Human activities are an important factor affecting the spatial distribution pattern of CH₄ concentration in small watershed.The removal of small dams can reduce stream greenhouse gas emissions.