Research On Spatial-temporal Distribution Characteristics Of Dissolved Oxygen And Its Formation Mechanism In A Convection-dominated Reservoir

Dissolved oxygen(DO)is an important indicator to measure the self-purification ability of water bodies and characterize water quality.The spatial-temporal distribution of DO in reservoirs and lakes is essential to understanding the distribution and behavior of aquatic organisms.Therefore,understanding the temporal and spatial characteristics of DO in lakes and reservoirs can provide theoretical basis for water quality management.To investigate characteristics of the DO distribution,field observations including temperature and DO profiles(from March 2018 to December 2018)were conducted in Fusshanqian Reservoir.A one-dimensional temperature-DO model was developed in AQUASIM program,revealing the cause of rapid decrease of DO in small reservoirs after a storm.The oxygen-consuming processes in the hypolimnia of lakes leading to deep-water anoxia in summer are still not well understood.In this study,R language(ReacTran)was used to develop the model of DO diffusion-reaction to explore the mechanism of anoxic formation.Many researchers directly ignore the DO consumption in a diffusive boundary layer(DBL)when studying DO transfer across the sediment-water interface.However,abundant organic matter and microorganisms often exist in DBL,resulting in a very dynamic chemical environment.The oxygen consumption in a DBL will affect the DO flux at the sediment-water interface.In this study,the critical value of Da is determined by numerical simulation as a criterion for judging DO consumption in a DBL that can be neglected.The research results are as follows:(1)DO profiles in the reservoir are mainly affected by stratification and mixing of the water column and are of obvious seasonal variation characteristics.During the heat-stratification-formation period(March to April),DO gradually forms stratification in the vertical direction and a supersaturated DO "peak" exists in the thermocline;DO in the deep water is gradually consumed and forms an hypoxic environment.During the stable stratification period(April to August),the "peak" of DO in the thermocline still exists,and the anoxic layer gradually thickened.During the stratification extinction period(October to December),the water is completely mixed vertically and the DO is nearly saturated.(2)DO profiles showed that the mixed layer has a obvious diurnal variation.During the day,the transport of DO is hindered and forms DO gradients under the influence of water stratification.At night,DO in surface water was evenly distributed due to convectionmixing.The diurnal variation of DO content and the main influencing factors are also slightly different in periods of stratification and "turning pond".In the stratification period,the diurnal variation of DO is affected slightly by convection,and mainly affected by the photosynthesis and respiration of algae.During "turning pond" period,the DO content was significantly affected by convection mixing.(3)The profiles of DO and phytoplankton in reservoirs can be changed directly by a storm,and the DO profile can be further affected by the photosynthesis and respiration of phytoplankton.The decreased transparency,on the one hand,limited the light penetration weakening the overall photosynthetic process of the water column.On the other hand,the original chlorophyll peak was not disappeared after the storm,and it located below the euphotic layer.Therefore,the respiration of the phytoplankton below the euphotic layer accelerated the DO consumption process.The amount of DO consumption is greater than that of the input in the reservoir,resulting in a rapid decrease in DO concentration after the storm.(4)The numerical simulation results show that the oxygen consumption in the bottom water accelerates the formation of hypoxia,which is an important reason for the decrease in DO concentration and vertical DO concentration gradient.In the initial stage of hypoxia formation,the DO consumption of sediments accounts for a large proportion.Meanwhile,the DO consumption in water gradually decreases the DO gradient at the sediment-water interface.Until the formation of hypoxia,the vertical profile of DO is nearly vertical,and the DO consumption in water column becomes the main reason for the thickening of the hypoxia layer.(5)The oxygen-consuming process within a DBL decreases the diffusive flux of DO into the sediment by changing the DO profile at the sediment-water interface.Moreover,the DO flux at the sediment-water interface decreases with thicker DBL and higher microbial activity in a DBL.Based on the condition that DO consumption in the DBL decreases the DO flux at the sediment-water interface by less than 10%,the critical Da value in a diffusion-reaction system is determined to be 0.6.Therefore,When Da<0.6,the oxygen consumption in the DBL can be neglected in the numerical simulation study.