Study On The Mechanism And Its Influencing Factors Of Sediment-water Interface Oxygen Flux

Dissolved oxygen is the most basic element in aquatic ecosystems.Compared with other parameters,dissolved oxygen can better reflect the metabolism of aquatic ecosystems.Sediment is an important site for the metabolism of matter and energy in water bodies.It contains numerous microbial populations and various compounds.The oxygen environment determines the occurrence and final fate of substances in sediment.Dissolved oxygen in sediment mainly comes from the transmission of overlying water bodies,and sediment-water interface（SWI）is an important area where oxygen transmission occurs.In this dissertation,eddy covariance method is used to test the SWI oxygen flux under different hydrodynamic conditions,and the influence of hydrodynamic conditions on SWI oxygen flux is explored.Based on the measured flux results,combined with oxygen profile analysis and oxygen uptake rate（OUR）of sediment,as well as changes in Fe²⁺,Mn²⁺,and organic matter mass fractions before and after the experiment,the biological oxygen consumption and chemical oxygen consumption of sediment are calculated,the mechanism of oxygen consumption in sediment is investigated,the response relationship between hydrodynamic conditions and SWI oxygen flux as well as sediment oxygen consumption is established,and the mechanism of SWI oxygen flux is obtained.At the same time,the high-throughput sequencing technology is used to test the sediment inside the device.From the perspective of microbial molecular biology,the information on the functional bacteria that may consume dissolved oxygen in sediment is analyzed.The conclusions of this dissertation are summarized as follows:（1）The different hydrodynamic conditions have influence on SWI dissolved oxygen concentration.When the average flow velocity is low（the corresponding average flow velocity is 0.00m/s,0.03m/s）,the SWI dissolved oxygen concentration can not only fluctuate around the average,but also increase slowly,or both.When the average flow velocity is high（the corresponding average flow velocity is 0.07m/s,0.12 m/s,0.20 m/s）,the SWI dissolved oxygen concentration increases with the increase of the average flow velocity.（2）The SWI oxygen flux and coefficient of vertical eddy diffusion（K_V）are significantly affected by the hydrodynamic conditions and increase with the increase of the average flow velocity.The minimum SWI oxygen flux and K_V are obtained at an average flow velocity of 0.00 m/s,and the average values are 1.197±0.121 mmol·m^-2·h^-1and 1.859×10^-9±8.716×10^-11 m²·s^-1 respectively.The maximum of SWI oxygen flux and K_V are obtained at an average flow velocity of 0.20 m/s,and the average values are43.981±1.326 mmol·m^-2·h^-1 and 1.137×10^-6±7.299×10^-8 m²·s^-1 respectively.（3）The SWI oxygen flux and K_V are consistent with the changing trend of hydrodynamic conditions,and can be divided into three stages.In the first stage,the oxygen transfer in SWI is mainly determined by molecular diffusion.The water body starts to flow slowly from a stationary state,and the oxygen flux is small.In the second stage,the oxygen transfer in SWI is mainly determined by the eddy diffusion.The flow velocity is accelerated,and the clear and stable SWI disappears.The sediments begin to suspend,and the oxygen flux grows faster.In the third stage,the oxygen transfer in SWI is mainly determined by the eddy diffusion.The flow velocity is further faster.The surface sediments of SWI are completely suspended,and the overlying water becomes turbid,as well as the oxygen flux reaches the highest value.（4）The dissolved oxygen in sediment mainly comes from atmospheric reoxygenation.Using the law of conservation of mass,the equations between the SWI oxygen flux and sediment oxygen consumption during the calculation period of per unit sediment area can be established.The total amount of dissolved oxygen transport into the sediment through the oxygen flux is 0.662 mmol on average.The average biological oxygen consumption of sediments is 0.175 mmol,accounting for 26.4%.The average chemical oxygen consumption is 0.045 mmol,accounting for 6.8%.The average increase in oxygen residue is 0.082 mmol,accounting for 12.4%.And the other oxygen consumption is 0.360 mmol,accounting for 54.4%.（5）The hydrodynamic conditions affect the oxygen environment of the sediment profoundly by determining the rates of atmospheric reoxygenation and oxygen diffusion to the bottom.When the average flow velocity is low and the sediment is in an anoxic state,the oxygen consumption of the sediment is mainly based on the biological oxygen consumption.When the average flow velocity is high and the sediment changes from an anoxic state to an aerobic state,the biological oxygen consumption decreases,and the proportion of chemical oxygen consumption as well as other oxygen consumption caused by chemical process increase gradually,and the sediment resuspension occurred at this time has a great influence on the composition of the sediment oxygen consumption.（6）Compared with the Greengene database,there are 18 species with species abundance greater than 0.5%in both L and R samples.Between these two samples,there are 7 phylums with average abundance greater than 3%,of which Proteobacteria accounted for the largest proportion,52.54%,followed by Bacteroidetes（8.60%）,Euryarchaeota（7.56%）,Verrucomicrobia（5.39%）,Chloroflexi（4.94%）,Nitrospirae（4.32%）,Acidobacteria（3.66%）.（7）The Proteobacteria is the most abundant bacteria in the sediment.It has complex functions and diverse nutrient conditions.It is the key strain that controls the sediment environment and causes the oxygen consumption of the biological process and chemical process in the sediment.In the experiments,the sulfate-reducing bacteria（SRB）attribute to the Proteobacteria are detected in the L and R samples.They are mainly Desulfarculales,Desulfobacterales,Desulfovibrionales,Desulfuromonadales,and Syntrophobacterale.The detected ammonia-oxidizing bacteria（AOB）mainly include Alcaligenaceae and Nitrosomonadaceae.No Proteobacteria with nitrite-oxidizing ability are detected except Alcaligenaceae and Nitrosomonadaceae,and Nitrospira dominate the nitrification.