Formation Mechanism Of Hypoxia In Pengxi Bay Of The Three Gorges Reservoir

Dissolved Oxygen(DO)is the most important indicator to maintain the health of water ecosystem.Under the comprehensive influence of global warming and the continuous increase of eutrophication in the watershed,hypoxia has become a serious global ecological and environmental problem.The previous monitoring found that Pengxi Bay(PXB)has a wide range of water hypoxia which located in the Three Gorges Reservoir(TGR).As a strategic water resource treasure in China,such a large-scale hypoxia in TGR poses a great safety risk to water quality.In view of the new environmental problem of TGR-hypoxia,the study of dissolved oxygen circulation process and hypoxia formation mechanism is crucial to improve the water ecological environment and implement sustainable lake and reservoir management.However,the physical and biogeochemical processes in TGR and its tributaries are much more complex than those in general water bodies,and dissolved oxygen is added,removed and transformed at different time scales and at different material interfaces.It is a very challenging proposition.In this study,the distribution and transformation of dissolved oxygen in a typical tributary reservoir,PXB,where large-scale hypoxia has occurred,were systematically investigated,including:(1)Spatial and temporal evolutionary characteristics of hypoxia/anoxic zone in PXBThe hypoxia/anoxic zone of PXB has a wide range of influence and evolves rapidly,with more than 70%of the reservoir being hypoxia/anoxic at the most serious stage.Field monitoring found that the anoxic zone(DO<2mg/L)had appeared in a wide range in PXB at the early stage of subsidence,and the distance of the anoxic zone extended from the river mouth to the upstream direction was about 20km,accounting for 64%of the backwater length of the survey area;by the end of subsidence,the extension length of the anoxic zone had increased to 23.75km,accounting for 76%of the backwater length of the survey area.During the period of water level rise,although there was no anoxic water body,an obvious process of dissolved oxygen decline was observed,and the extension length of the hypoxic zone(DO<4mg/L)increased by 23km in only one week,accounting for 74%of the backwater length.Such a large-scale decline in DO in PXB will pose a great security risk to the water quality health in TGR,a strategic water resource treasure in China.(2)Quantitative analysis of the relative contributions of different physical and biochemical processes to dissolved oxygen deficitApplying the end-member mixing model and carbon isotope mass conservation,the proportion of DIC generated by oxygen-depleted degradation of organic matter was resolved to be between 5.67 and 32.07%.Combined with the classical Redfield equation,the contribution of oxygen-depleted degradation of organic matter to dissolved oxygen deficit was further quantitatively estimated to be more than 70%and dominant.Combining the isotopic characteristic values of organic carbon from different sources,endogenous organic matter generated on-site by algae was screened as the main source of oxygen-depleted organic matter in the anoxic zone.By conducting sediment oxygen consumption simulation experiments,the sediment oxygen consumption rate variation range of 0.44～43.92 mg/(m~2·h)was determined in PXB.After analyzing the net loss of dissolved oxygen in the anoxic zone with the measured data,it was found that the contribution of sediment oxygen consumption process to the loss of dissolved oxygen did not exceed 28%.Using acoustic Doppler profiler and high-precision temperature chain,we calculated the vertical dissolved oxygen transport fluxes of 3.12±0.25g/(m~2·d)and 0.71±0.34 g/(m~2·d)in PXB during the water level subsidence and water level uplift periods,respectively,and obtained the dissolved oxygen transported downward across the leap layer in the water column for the two monitoring periods by integration,which were2.66±0.21mg/L and 0.42±0.20 mg/L,accounting for only 32.68±18.81%and 18.67±8.88%of the dissolved oxygen deficit,indicating that the stratified blockage of the water column prevents the surface oxygen-rich water column from replenishing the depleted oxygen downward in a timely manner.(3)Mechanisms and influencing factors for the formation of hypoxia/anoxia in PXBIt was screened that the oxygen-consuming degradation of large amounts of endogenous organic matter produced by phytoplankton on site under the stimulation of eutrophication was the key controlling factor leading to the formation of hypoxia/anoxia zone in the tributary.The mechanism of hypoxia/anoxia formation in the tributary is further summarized as follows:the backwater area formed after the completion of TGD impoundment has changed the hydrodynamic conditions of the tributary,which has transformed the tributary from a river-type water body to a lake-type water body.With the slowing down of flow velocity,the nutrient transport capacity slows down,and high nutrient stimulation leads to frequent occurrence of water blooms in the tributaries,and the dominant algal species evolves to lake-type algal species represented by cyanobacteria.In the process of decay and fragmentation,lacustrine algae represented by cyanobacteria rapidly release a large amount of endogenous organic matter,mainly aromatic protein unstable components with good biodegradation properties,which strengthen the metabolic process of aerobic microorganisms in the system and break the balance between physical process recharge and oxygen-consuming degradation of organic matter.When the dissolved oxygen recharge channel is blocked,it will rapidly consume the oxygen in the water in a short period of time and change the water body from an oxygen-rich state to hypoxia/anoxia state.On the other hand,the high concentration of organic matter released by algal decay also increases the content of reducing substances in the water,further maintaining the stability of the hypoxia/anoxia environment and eventually leading to the formation of a widespread hypoxia/anoxia event in the tributary.In summary,hypoxia/anoxia in TGR is the result of a combination of multiple driving factors that interact to create the basic pattern of the formation and evolution of the hypoxic zone in the tributaries of TGR.Among them,the large amount of endogenous organic matter released after the decay of algal blooms has a crucial contribution to hypoxia/anoxia.Further analysis revealed that there is a positive feedback between water eutrophication,algal bloom,hypoxia/anoxia and greenhouse effect.In view of this,future research on hypoxia/anoxia should further link hypoxia/anoxia with water eutrophication,water bloom outbreak and greenhouse effect as a whole system,and actively explore measures to cope with the adverse effects.