The Impact Of POC On Methane In Upper Yangtze’s River Reservoir Systems

Reducing carbon emissions from fossil consumption plays the key role of slowing down global warming.Hydropower is different from other clean energy,needed building dam and storing water,destroying the continuity of the original water body,causing parts of organic matter stay in reservoirs instead of transporting to the ocean and translated into hotspots of production and emission of greenhouse gases（GHGs）.In order to make full use of hydropower resources,cascade dam systems are popular in many large basins,forming fluvial reservoirs ecosystem having a significant discrepancy from natural rivers.However,the destruction of river connectivity by dams will not only reduce the amount of sediment transported to the ocean,but also increase the primary productivity of the river,to change carbon cycle process of aquatic ecosystems.Meanwhile,the increasing of human activities a certain degree changes the land use and make a disturbance of carbon cycle.For in-depth understanding of the influence from different origin of POC to methane production in fluvial reservoir systems,this study based on two campaigns（upper reaches of Yangtze river from 2015 to 2016 and downstream of Jinsha river from2018 to 2019）,combined with dam operation,hydrology and human activities,analyzing the source of particulate organic matter（POM）by isotope mixing model after end-member calibration and relationship from allochthonous and autochthonous POM to methane.The paper has the following conclusions:（1）In the upper reaches of the Yangtze River,POC has a large range(0.07 mg·L^-1-3.31 mg·L^-1,0.49±0.61 mg·L^-1).In flood season,allochthonous（50.3±23.7%）and autochthonous（46.8±23.5%）POC has same contribution,but the concentration of allochthonous(0.38±0.48 mg·L^-1)and autochthonous(0.28±0.27 mg·L^-1)POC.In dry season,the contribution of autochthonous（56.5±23.3%）is greater than that of allochthonous（41.2±23.4%）,but the concentration difference between autochthonous(0.14±0.14 mg·L^-1)and allochthonous(0.15±0.28 mg·L^-1),indicating that allochthonous POC is the dominate factor in the increase of POC.In the lower reaches of the Jinsha River,the concentration of POC(0.35±0.39 mg·L^-1)is slightly lower than that of the upper reaches of the Yangtze River as a whole.Allochthonous POC has a greater contribution to the increasing of POC and the area of its input and the CH₄ hotspot are highly overlapped,indicating that allochthonous POC is the main driving factor to vary the CH₄ concentration in reservoirs.（2）In the upper reaches of the Yangtze River,the cumulative effect of dams on POC reduction is only evident in the upper and middle reaches of the Jinsha River,indicating that the POC in the middle and upper reaches of the Jinsha River has a very limited impact on the POC of downstream.In the other words,the input POM characteristics in local environmental are one of the dominant factors to determine the source and distribution of POC in downstream reservoir.In addition to the dam,the impact of human activities on POC influencing CH₄ concentration mainly occurs in the Three Gorges Reservoir and its upstream.The CH₄ concentration of the Jinsha River Basin is mainly affected by dam interception,natural POM input and regional water temperature rather than other human activities.In the lower reaches of the Jinsha River,the impact of dam operation on POC is mainly reflected in the fluctuation of the reservoir water level.The fall of the water level during the dry and flood transition period will increase the possibility of allochthonous POC input into fluvial reservoir by landslides or mudslides.The increase of water level during the flood season may cause the POM imported by the tributary to settle in its backwater area and the main stream to decrease the concentration of terrestrial POC.（3）In the lower reaches of the Jinsha River,allochthonous POC and CH₄ are positively correlated with the flow.Spatially,hydraulic retention time（HRT）has a significant positive correlation with autochthonous POC,while allochthonous POC has a unimodal distribution with maximum values located in the transition zone of the reservoir,indicating that the transition zone is critical area where CH₄ is generated and released into the atmosphere in the fluvial reservoir ecosystem.（4）In the lower reaches of the Jinsha River,CH₄ oxidation process appears obviously in the reservoir,and this process presents a longitudinal trend after the CH₄ hot spot.In the flood season,the low input and interception of allochthonous POC and the CH₄oxidation cause the appearance of low CH₄ concentration in the reservoir.Future research can further use different land use and pollutant by human as end member to explore how human activities specifically affect the POC.At the same time,the reason why CH₄ oxidation in the reservoir occurs in water bodies with almost no hypoxia requires more research and discussion.Finally,tributary backwater area may be a methane hot spot escaped monitoring in the form of bubbles due to the shallow water depth of the reservoir.