Fifteen-year monitoring of the turbidity dynamics in large lakes and reservoirs in the middle and lower basin of the Yangtze River, China

The Middle and Lower Yangtze River (MLY) basin holds the most freshwater in East Asia; however, the conditions of basin-scale water turbidity remain unknown. In this work, a remote sensing algorithm was developed to estimate the concentrations of the total suspended sediments (TSS) in large lakes and reservoirs over the MLY basin and was based on a band ratio between 555 nm and 645 nm of the atmospherically corrected surface reflectance of the Moderate Resolution Imaging Spectroradiometer (MODIS). In situ samples used to calibrate the algorithm were collected from 58 lakes and reservoirs with a TSS range of 1 to 300 mg L− 1, and the uncertainty of this algorithm was 30-40%. The algorithm was subsequently applied to a total of 102 lakes and reservoirs located in the MLY basin to derive TSS maps from 2000 to 2014 at a 250 m spatial resolution, and the first comprehensive document of the TSS distributions and dynamics of large inland waters of the MLY basin was created. The seasonal patterns among the selected water bodies were similar, with the largest TSS concentrations occurring in the first and fourth quarters in a year and the lowest values occurring in the third quarter. In contrast, spatial heterogeneities were revealed by the 15-year long-term mean TSS climatology information. Although most lakes downstream of Poyang Lake were turbid with 15-year TSS climatology values of 45-100 mg L− 1, waters between Poyang and Doting Lake were relatively clearer with TSS climatology values of 15-45 mg L− 1, and the clearest waters (< 15 mg L− 1) were found in reservoirs. The turbidity of 64.5% (e.g., 49/76) for lakes in Class II exhibited a decreasing trend over the 15-year period, and the Three Gorges Reservoir (TGR) and Dongting Lake in Class I also showed significant TSS declines. Analysis with meteorological data shows that the intra-annual variations appear to be significantly correlated with local precipitation, with a time lag of two months for TSS. The prominent TSS decreasing trend of the lakes in Class II was probably linked to the significant NDVI increase in the MLY basin, whereas the TSS decrease in the TGR and Dongting Lake is likely to be attributed to the impoundment of the Three Gorges Dam. The TSS environmental data record (EDR) of large inland waters presented in this study serves as an important reference for future water quality monitoring and evaluation in the MLY and in China.