| Since the initial filling of Three Gorges Reservoir(TGR),eutrophication had become a serious problem in its tributary bays and algal bloom had been observed frequently.According to the eutrophication of TGR,a large amount of research and exploration were carried out.As the material basis of eutrophication and algal bloom,nutrients are one of the key issues,especially the source of nutrients with some achievements.Xiangxi Bay(XXB)is the research object in this paper.Based on the monitoring data,this paper firstly analyzed the characteristics of the water environment in XXB,and discussed the spatial and temporal distribution characteristics of hydrology,hydrodynamics,nutrients,chlorophyll a and phytoplankton in XXB,and next calculable hydrodynamic and water quality prediction model of the XXB in the TGR was developed by using CE-QUAL-W2.In addition,the circulation characteristics and environmental effects of the water body are summarized on the strength of the simulation results.Finally analyzed the nutrients transport process and supply ways of sensitive area in different circulation patterns by using the particle tracck technology of the model,and verified which based on the measured hydrogen and oxygen isotopes.The main conclusions are as follows:(1)The vertical water temperature of XXB was dominated by the positive temperature and all the same temperature throughout the year,and the water temperature stratification was easy to occur in spring and summer.The main density current type in upstream of the XXB was down-slope density current.While the main density current type in downstream of the XXB was density current intrusion.The nitrogen,phosphorus and silicate of XXB showed a high level.The nitrogen concentrations of the upstream was generally lower than the downstream,the phosphorus concentrations of the upstream was higher than downstream,and the underlying phosphorus was higher than that of surface water,the nitrogen and phosphorus concentrations were higher than the threshold of eutrophication.There was no significant difference in space for silicon distribution.But the silicon concentration was higher after impoundment compared with other periods.The spatial and temporal differences of phytoplankton biomass and chlorophyll a concentrations in XXB were significant,and those were high in pre-flood season and flood season which were sensitive periods of algal bloom.Bacillariophyta and chlorophyta were relatively more,pyrrophyta was mainly distributed in the XX07 ~ XX09 of upstream,cyanophyta was mainly suitable for high temperature season,the relative biomass of xanthophyta and euglenophyta accounts for a small proportion in XXB.The dominant species were bacillariophyta and chlorophyta in spring and mainly cyanophyta and chlorophyta in summer.(2)In this paper,the CE-QUAL-W2 model could accurately simulate the vertical and longitudinal distribution characteristics of water temperature and velocity.What?s more,the two-dimensional water quality model could simulate nutrients distribution of XXB.Although there was some error in the model,the accuracy of that could meet the requirements,and could be used to simulate the nutrients transport of the XXB.(3)The main factors affecting density flow of the XXB were temperature and turbidity differences between the XXB and the Yangtze River,flow rates of the upstream of the XXB and wind current.The hydrodynamic process of XXB was mainly influenced by two kinds of driving forces,upstream inflow and downstream pouring.The downstream driving force was mainly the density difference caused by the warer temperature difference and turbidity difference between the XXB and the Yangtze River,the location and intensity of the infiltration point were closely related to the temperature difference.The approach and influence range of upstream inflow affected by flow rates of the upstream in different seasons.At the same time,the mixing of surface water caused by wind current affectd the warer temperature stratification and mixing depth,and then affected the vertical distribution structure and biomass of plankton of the surface water.(4)The typical circulation patterns of XXB were the downslope density current in the upstream,density current intrusion from surface layer,the middle layer and the bottom layer in the downstream.When it was the downslope density current in the upstream and density current intrusion from the surface layer in XXB,the water circulation process promoted the flow of water,reduced water remaining time,sped up the water exchange.When it was the downslope density current in the upstream and density current intrusion from the bottom layer in XXB,the driving forces of upstream and downstream would collide and cancel each other,forming a vortex of local water,which would increase the water remaining time in local water.When it was the downslope density current in the upstream and density current intrusion from the middle layer in XXB,the water remaining time was shorter and the flow velocity was greater than that of the upper water in XXB.(5)The algal bloom sensitive zone of XXB was divided into: XX08 and XX09 as the terminal of backwater were the strong sensitive area,that the middle reach from XX04 to XX07 were the middle sensitive area,that the downstream from XX00 to XX03 near the mouth of the river were weak sensitive areas.The nutrients supplies in strong sensitive area at the terminal of backwater mainly came from the upstream inflow by the way of level supply and mixing supply,and the contribution rate reached more than 85%,furthermore there was no supply for the weak sensitive area.In the weak sensitive area of the downstream,the nutrient was mainly from the main stream.With the movement of the density current intrusion from the surface to the bottom in the estuary,the upstream inflow had gradually enhanced the replenishment of the surface water,and the replenishment of the surface water had been gradually weakened by the density current intrusion in the downstream. |
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