Monitoring water clarity of lakes in the Middle-Lower Yangtze Plain using Landsat observations (1984-2023)

Using the improved Quasi-Analytical Algorithm (QAA) and Landsat data, we documented the long-term changes in water clarity of the 17 largest lakes in the Middle-Lower Yangtze Plain from 1984 to 2023. A comprehensive dataset with over 4600 water clarity maps, reconstructed from 2511 Landsat series images, was compiled. The water clarity changes of these 17 lakes have a clear seasonal variation pattern, with the highest in summer and the lowest in winter. Over the past 40 years, water clarity of 59 % lakes shows a downward trend, with Junshan Lake showing the highest decrease rate of -0.0231 m/yr. 41 % lakes water clarity are showing an upward trend. From 1984 to 1990, the highest average ZSD (Secchi disk depth) values were recorded in Changhu Lake, Junshan Lake, Honghu Lake, Futou Lake, and Gehu Lake. These lakes, however, experienced the most significant declines compared to period from 2021 to 2023. Chaohu Lake, Taihu Lake, Caizi Lake, and Nanyi Lake, which had the lowest average ZSD during 1984-1990, showed the most substantial increases. Analyzing the monthly distribution of water level and ZSD data reveals that water clarity exhibits different seasonal variations in relation to water levels. The comprehensive dataset and analysis provide a crucial scientific basis for informed water resource management and policy-making in the region.