| Damming is one of the most important approaches to solve the shortage and uneven distribution of water resources in time and space,and the water quality safety of reservoir is the key to the sustainable utilization of water resources.Reservoir bays are the initial and most sensitive areas in the evolution process of reservoir water quality.The water quality of the reservoir bay is affected by both the water environment and local watershed landscape(i.e.topography,soil,land use composition,and landscape pattern).Understanding the response mechanism of the water quality in reservoir bay to the complex landscape background,clarifying the complex relationship between water environment characteristics and landscape background and eutrophication,and evaluating the eutrophication status of the water body of the reservoir bays are critical for developing water quality models,revealing the evolution mechanism of water quality,and providing reliable scientific and technological support for the prevention and control of eutrophication,water quality assurance,and sustainable development in reservoirs.We selected 66 reservoir bays with different landscape backgrounds in the Danjiangkou Reservoir,the water source for the Middle Route of the South-to-North Water Diversion Project,as our study area.Combined with the monitoring data of the reservoir bays from 2015 to 2018,our study used space-time interaction analysis,K-means clustering analysis,partial least-squares regression,partial least squares-structural equation modeling,and other methods to reveal the impact mechanism of local watershed landscape background and water environment on eutrophication in reservoir bays.We systematically studied the Spatiotemporal variation of the water quality in reservoir bays,quantified the impact of local watershed landscape on the concentration of nitrogen and phosphorus in the water body of the reservoir bays,clarified the response mechanism of chlorophyll a concentration in the water body to the water environment and local watershed landscape,evaluated the eutrophication status of the reservoir bays and proposed corresponding prevention measures.The main conclusions of the study are as follows:(1)The water quality of reservoir bays showed significant temporal and spatial variability.After the dam heightened,the water quality of the reservoir bay gradually improved over time.The mean values of water quality indicators,such as ammonia nitrogen,nitrate nitrogen,total nitrogen(TN),total phosphorus(TP),turbidity,and chlorophyll a(Chl a),during the storage period are higher than that during the discharge period.The total nitrogen and total phosphorus concentrations in the storage and discharge period were all greater than the eutrophication threshold(TN:0.2 mg·L-1,and TP:0.02 mg·L-1),and chlorophyll a concentration in some reservoir bays were significantly higher than ecologically acceptable boundaries for Chl a concentration(5?g/L).Reservoir bays with higher risks of eutrophication and water bloom decreased year by year during the study period and were more spatially concentrated near towns.(2)The responses of TN and TP concentration of reservoir bays to the local watershed landscape background were different during the storage and discharge period.Soil erodibility had significant effects on the TN and TP concentrations.The key factors affecting the TN concentration were the slope gradient,basin relief,topographic wetness index,forest,and agricultural land use,whereas the factors controlling the TP concentration were the landscape shape index,edge density,Shannon's diversity index and,grass land use,although the TP concentration was also controlled by the patch density and contagion during the storage period,and by mean patch size and largest patch index during the discharge period.(3)Water chemistry and nutrients have decisive effects on Chl a concentration in reservoir bays.Both water chemistry and nutrients had a major positive impact on Chl a concentration,as their total effects account for more than 40%of two PLS-SEMs,especially during the discharge period.The land use latent variable affected the pollution load from the source and transportation process,the positive effect of land use was more obvious during the storage period,accounting for 30%of the relative contribution of the Chl a concentration.Complex landscape patch shape helps to reduce the transport of nutrients to the reservoir bays,a fragmented landscape has a significant negative effect on the Chl a concentration,especially during the discharge period,accounting for 19%of the relative contribution of Chl a concentration.(4)The water body of the reservoir bays was generally mesotropher,part of tributary reservoir bays,closed reservoir bays,and bays with towns and farmland distribution in local watershed were light eutropher,middle eutropher,even hyper eutropher,facing greater risks of eutrophication.On this basis,we have proposed corresponding countermeasures to prevent and control eutrophication in reservoir bays such as the construction of ecologically clean watersheds,the establishment of ecological barriers in the buffer zone of the reservoir,water circulation systems and pollution retardation systems,strengthening the pollution prevention and control around the reservoir,continuous monitoring and in-depth study of eutrophication and prevention of algal blooms in reservoir bays.Focus on the key issue that the reservoir bay is the sensitive area for water quality pollution in the reservoir,we selected typical reservoir bays with different landscape backgrounds.Combined with the hydrological rhythm,we assessed the temporal and spatial variation of the reservoir bay water quality,explored the contribution of different landscape background to the water quality dynamic of reservoir bays,and revealed the response mechanism of the temporal and spatial variation of water quality to the landscape background and hydrological rhythm.This study has important practical value for the management of the water quality of the reservoir. |
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