Formation Mechanism And Regulation Of Metalimnetic Oxygen Minimum In Typical Reservoir In Lancang River

In weak hydrodynamic situation,water temperature stratification was a common natural phenomenon.At the premise of severe stratification of water body,the stratification effect led to the loss of dissolved oxygen in the metalimnion of deep reservoir,and further formed the Metalimnetic Oxygen Minimum(MOM).Generally,the appearance of MOM indicated the obvious loss of dissolved oxygen in metalimnion,which affected the ecological environment of reservoir.In order to study the relationship between water temperature stratification and vertical distribution of dissolved oxygen in different water depth reservoirs in Lancang River and the law of MOM occurrence,the group monitored the related environmental factors in Xiaowan Reservoir(supplemented by Manwan Reservoir,Dachaoshan Reservoir and Nuozhadu Reservoir)in June 2017(high water period)and January 2018(dry water period).This study summarized the water temperature stratification and the vertical distribution of dissolved oxygen.A comparative analysis of MOM phenomena in deep water reservoirs was presented.Meanwhile,the model of Xiaowan Reservoir was modeled using CE-QUAL-W2 hydrodynamic-water quality model.Analyze the stratified strength of the metalimnion(set 0~0.9℃/m temperature gradient)and get the mechanism(the process of organic matter diffusion and reoxygenation)of MOM phenomenon formation.After that,the reservoir surface water intake(0.5m),middle water intake(86m),bottom water intake(172m),upper metalimnion water intake(6m),middle metalimnion water intake(9.5m)and lower metalimnion water intake(13m)were simulated to explore the regulation of MOM phenomenon by stratified water intake(flow rate 1000 m~3/s,duration 5 d,total 120 h).The results showed that:1.In actual sampling monitoring,Manwan Reservoir and Dachaoshan Reservoir were shallow(46 m、53m)whose retention time of water body were short(8.66days,8.18 days).It led to obvious mixing of water body and no stratification.The Xiaowan Reservoir and Nuozhadu Reservoir were deep(172 m、105m)and the retention time of water body were long(142.66 days,158.58 days),which led to uneven distribution of surface energy input and slow renewal of water body.It caused the stratification of water body(In june,the metalimnion was in 3~15m and 5~12 m,respectively.In January,the metalimnion was in 43~55m and 35~52 m,respectively.).2.The vertical difference of dissolved oxygen was small in Manwan Reservoir and Dachaoshan Reservoir(7.03~9.24mg/L、7.49~9.05mg/L).However,the dissolved oxygen in the Xiaowan Reservoir and Nuozhadu Reservoir varied greatly in the vertical direction(3~8.05mg/L、1.28~7.71mg/L).The dissolved oxygen in the metalimnion showed a"inverted V"distribution pattern which decreased first and then rose.The MOM presence in the reservoir was associated with a higher buoyancy frequency,it indicated the density stratification in the water body.Retention of organic matter and obstruction of dissolved oxygen diffusion were both the important causes of MOM phenomena.3.Owing to the low salinity of the Xiaowan Reservoir,the temperature stratification was related to the density stratification.When the temperature gradient of the metalimnion was large(≥0.7℃/m),the diffusions of the surface dissolved organic matter and the particulate oxygen-consuming organic matter were obviously blocked,and the reoxygenation process was also affected.4.After simulating the MOM phenomenon,stratified water intake of reservoirs at different depths under set different conditions.As a result,the MOM disappeared rapidly when the water temperature stratification was obviously weakened by the stratified water intake(surface water intake,upper metalimnion water intake).If there was still obvious stratification of water temperature after the stratified water intake(middle water intake,bottom water intake),the MOM persisted.MOM usually move with the metalimnion,and when the metalimnion was close to the surface water body,the MOM disappeared.