Remote Sensing-based Spatial-temporal Dynamics Of Sediment Concentrations And Its Relationship With Wetland Vegetation Coverage In Large Lakes And Reservoirs Of The Middle And Lower Reaches Of The Yangtze River

As an important water quality parameter,total suspended sediments(TSS)can not only affect the color of the water,but also influence the light transmission efficiency of water,and finally influence the survival of underwater phytoplankton and wetland vegetation.Lake wetland vegetation plays an important role in slowing down sediment resuspension and improving lake water quality.Carrying out research on lake TSS concentration and wetland vegetation and exploring the relationship between TSS concentration and wetland vegetation,can provide scientific basis for lake water quality protection and water environment restoration.The middle and lower reaches of the Yangtze River(MLY)basin is the region with the most abundant freshwater resources in China.The changes in the ecological environment of freshwater bodies will directly affect human survival and social and economic development.However,limited by remote sensing technology and the in situ measured data,the long-term spatial-temporal distribution of TSS concentration and wetland vegetation of lakes in the MLY basin have never been comprehensively evaluated,not to mention to study the effects of watershed soil erosion on the changes of TSS concentration and the relationship between TSS concentration change and the distribution of wetland vegetation in lakes.In this study,it's the first time to obtain TSS concentration of 102 large lakes and reserviors in the MLY basin using long-term MODIS data.The causes of the spatialtemporal dynamic changes of TSS are analyzed in combination with meteorological data,vegetation cover data,and watershed soil erosion data.It's also the first time to reveal the spatial and temporal distribution of wetland vegetation in 25 large lakes in MLY basin using MODIS NDVI time series data,to investigate the causes of vegetation change and to analyze the relationship between TSS concentration and the growth of wetland vegetation in lake,details are as follows:Considering that the difference in optical characteristics of different water bodies will affect the inversion accuracy of TSS concentration,a universal quantitative remote sensing inversion model that is suitable for lake groups in the MLY basin is constructed using MODIS surface reflectance data and the in situ measured data of 58 lakes in the MLY basin.Verified by the measured data,the root mean square error of the inversion model is 34.2%.On the basis of considering and solving the influence of land adjacency effect,the 15-year spatial-temporal distribution of TSS concentration in 102 large lakes and reserviors in the MLY basin is obtained using MODIS data from 2000 to 2014.The TSS of all the lakes show obvious inter-annual change,in the past 15 years,the TSS concentration of 49 no river-connected lakes show a decreasing trend,and 29 riverconnected lakes exhibite a significant(p<0.05)decreasing trend.The TSS concentration of the river-connected lakes such as Dongting Lake also decrease significantly.Using MODIS surface reflectance data,MODIS NDVI,MODIS Land Cover data,digital elevation model data,Chinese soil type data,and precipitation data from 122 meteorological stations in the MLY basin,based on the soil erosion model(RUSLE),The long-term spatial-temporal distribution of soil erosion in the MLY basin is obtain.Results show that from 2001 to 2014,the soil erosion intensity of over 80% region in the MLY basin shows a decreasing trend,indicating that the soil and water conservation in the MLY basin is improved.Correlation analysis with meteorological factors shows that the intra-annual variations of TSS appear to be significantly correlated with the precipitation,with a time lag of two months for TSS,while the relationship between wind and TSS concentration is not significant.Analysis of changes in vegetation coverage in the MLY basin show that the vegetation coverage in all sub-basins except Taihu basin have shown a significant increase trend,and the increased vegetation could improve soil and water conservation,which might cause the reduced TSS concentration of lakes and reservoirs in the MLY basin.Combined with the amount of soil erosion derived from remote sensing data,the effect of soil erosion on the change of TSS concentration in lakes and reservoirs is further analyzed.Result shows that the reduction of TSS concentration in about 45% of lakes and reserviors may be related to the reduction of soil erosion,and the increase of TSS concentration in around 43% of lakes and reserviors may be affected by the increase of soil erosion.Considering that the differences of wetland vegetation communities in lakes with different hydrological conditions,based on the unique phenological characteristics of different features,a suitable accurate lake wetland vegetation classification algorithm is established using the NDVI time series constructed by MODIS data.The spatialtemporal dynamics of wetland vegetation in 25 large lakes in the MLY basin from 2000to 2014 are obtained.It's found that from 2000 to 2014,68% of the lakes show a decrease trend in their wetland vegetation coverage.Correlation analysis between TSS concentration,precipitation,temperature and the amount of fertilizer used around the lake and the vegetation coverage of the lakes show that the vegetation coverage of 48%of lakes are negatively correlated with the TSS concentration,and the vegetation coverage of 92% of lakes are negatively correlated with local precipitation,the vegetation coverage of 68% of lakes are positively correlated with temperature,while for the relationship between the vegetation coverage and the amount of fertilizer,half of lakes show positive correlations and half of lakes demonstrate negative correlations.Based on the obtained TSS concentration and wetland vegetation coverage data,the 15-year variations of TSS concentration in the high and low vegetation coverage area are analyzed,results show that the TSS concentration is lower and more stable in high vegetation coverage area than that in the area with less vegetation coverage.Analysis of the 15-year mean TSS concentration in the high aquatic vegetation coverage area of lakes indicates that the TSS concentration should be less than 20 mg/L to help and protect the growth of the aquatic vegetation in the lakes in MLY basin.These research results can provide an important imformation for the improvement of lake water quality in the MLY basin and the protection of wetland vegetation.