Distribution And Mechanism Of Dissolved Methane Concentration In The Channel-type Reservoir

Freshwater ecosystems are considered as an important source of greenhouse gas emissions.Due to land use changes caused by anthropogenic activities,contribution of hydropower reservoirs to the atmospheric greenhouse gases cannot be ignored under the circumstance of its increasing proportion in freshwater ecosystems.Methane（CH₄）,as the second most abundant greenhouse gas in the atmosphere,has a much higher warming potential than the first abundant carbon dioxide（CO₂）.Due to the unique physical geography characteristics and water environment characteristics of reservoirs（especially channel-type reservoirs）,the dissolved CH₄ concentration（DMC）in the water has a strong spatiotemporal heterogeneity,which leads to greater uncertainty in the accurate estimation of its CH₄ emissions.Therefore,systematic research on the distribution and spatiotemporal variation mechanism of CH₄ in channel-type reservoirs is of great significance for assessing carbon emissions in reservoirs.This study take the channel-type reservoir-Xibeikou Reservoir as an example.From November 2021 to October 2022,on-site investigations were conducted to monitor the DMC and environmental physicochemical factors within the reservoir water body.Samples of water and sediments were collected spatiotemporally to determine the CH₄ oxidation rate in the water column and the sediment CH₄ production rate by indoor simulation cultivation experiments,and furthermore,the spatiotemporal distribution and mechanism of DMC in channel-type reservoir were determined based on the spatiotemporal changes in DMC,oxidation,and production.The main results and understanding of this research were as follows:（1）Temporal and spatial distribution of CH₄ oxidation in the water column and CH₄production in sedimentThe oxidation rate of dissolved CH₄ in spring,summer,autumn,and winter water were0.0019±0.0009μmol·L^-1·h^-1,0.0029±0.0024μmol·L^-1·h^-1,0.0044±0.0015μmol·L^-1·h^-1,0.0017±0.0012μmol·L^-1·h^-1.In comparison,the oxidation rate of dissolved CH₄ in autumn was higher,and the oxidation rate of dissolved CH₄ in end of the reservoir was higher.The CH₄rate of surface sediment production in spring,summer,autumn,and winter were 0.695±0.386μmol·L^-1·h^-1,3.518±2.720μmol·L^-1·h^-1,3.301±2.359μmol·L^-1·h^-1,0.474±0.276μmol·L^-1·h^-1.The CH₄ production rate of sediment in summer was highest among four seasons,and the sediment CH₄ production rate in end of the reservoir was much higher than other sections.The sediment CH₄ production rate in typical river sections was greatly regulated by seasonal temperature,a higher temperature could promote the sediment CH₄ production rate;The quality and quantity of organic carbon in sediment may be the main reason for the spatial differences in CH₄ production rates among different river sections.During the monitoring period,the sediment CH₄ production was dominated by the pathway of acetic acid fermentation.The CH₄oxidation rate of surface water was seasonally strongly influenced by water temperature and reaches the maximum rate under approximate 25℃;and its spatial differences could be attributed to the total phosphorus content in the water.（2）Temporal and spatial distribution of dissolved CH₄ concentration in the reservoirDuring the monitoring period,DMC in the water of the Xibeikou Reservoir ranges from0.01 to 1.19μmol·L^-1,with an average of 0.235±0.123μmol·L^-1.Spatially,the the end of the reservoir characterizes as the hot zone of higher CH₄ concentration,and DMC showed a decreasing trend from the end to the head of the reservoir;Temporally,DMC in the reservoir water was the highest in summer and the lowest in winter.In general,DMC in the reservoir showed a strong spatiotemporal heterogeneity and was closely related to physical and chemical factors such as water temperature,dissolved oxygen,water depth,conductivity,chlorophyll,etc.The spatial heterogeneity of DMC was regulated by the CH₄production versus oxidation in the reservoir which were affected by all physical and chemical factors.By fitting the annual DMC in surface water and the distance from the dam（backwater end）,the physical and chemical factors（water temperature,dissolved oxygen,chlorophyll）of surface water could be used to jointly predict the spatiotemporal variation of DMC in the surface water.（3）Distribution patterns of CH₄ diffusive flux at the water-air interface of reservoirBased on the thin boundary layer model,the annual variation range of the CH₄ diffusive flux at the water-air interface is 0.262～1.982（1.020±0.611）μmol·m^-2·h^-1 and the Xibeikou Reservoir turned out to be the atmospheric CH₄ source characterized by high CH₄ diffusive flux in summer and low CH₄ diffusive flux in winter.The diffusive CH₄ flux at the water-air interface was jointly regulated by the DMC,water environmental factors,and meteorological factors.The annual average CH₄ diffusive flux at the water-air interface of the Xibeikou Reservoir was slightly lower than that of the lakes in the same latitude region,1-2 orders of magnitude smaller than that of the lakes in the low latitude region,and greater than that of the lakes in the high latitude region.