Water eutrophication in Poyang Lake, the largest freshwater lake in China, has been considered to be an obstacle to aquatic environment protection and regional sustainable development. Chlorophyll-a concentration is one of the most important indices of water eutrophication. This paper builds seasonal chlorophyll-a concentration retrieval models using a semi-analytical model. Quarterly distributions of chlorophyll-a concentration from 2009 to 2012 are explored using multi-spectra data from a moderate-resolution imaging spectroradiometer (MODIS). The correlation coefficient of the retrieval models primarily ranged from 0.6 to 0.9. The results show that the chlorophyll-a concentration in Poyang Lake has significant seasonality characteristics that present low values in the winter and spring, and present relatively high values in the summer and autumn; this report also presents an obvious, increasing trend of inter-annual variability from 2009 to 2012. The spatial distribution of the chlorophyll-a concentration has regional differences that give relatively high values adjacent to the shore in the north area of Poyang Lake, in the flow in river entries, and in the main channel area in the central and south areas of Poyang Lake. The natural hydrology features have a close relationship with the variation in the chlorophyll-a concentration. Intensive human activities are the main driving forces for the increasing chlorophyll-a concentration.

Dongting Lake is the largest lake in the middle reaches of the Yangtze River in China. For centuries, people inhabiting the Dongting Lake area have been reclaiming land and constructing dams for flood resistance, agricultural production, and rural settlement, forming geographical entities known as polders. In this study, the regional spatial distributions of polders in the Dongting Lake area in 1949, 1998, and 2013 were obtained using historical maps and modern remotely sensed data, revealing changes since the establishment of the People's Republic of China. Nanxian County was then selected to demonstrate polder changes at the county level, because it has undergone the most dramatic changes in the area. Different polder change models for the Datonghu, Yule, and Renhe polders were analyzed for eight periods: 1644 (the early Qing Dynasty), 1911 (the late Qing Dynasty), 1930 (the Republic of China), 1949 (the People's Republic of China), 1963, 1970, 1998, and 2013. Three resulting polder evolution models are: 1) reclaiming polders from lakes, 2) integrating polders by stream merging, and 3) abandoning polders for flood release. The polder evolution models demonstrate the wisdom of local people in using land resources according to the specific regional conditions. Throughout their long-term historical evolution, the spatial distribution of polders in the Dongting Lake area tended to be homogeneous, and the degree of human disturbance tended to be stable. However, a shift occurred, from pure polder area growth or removal to more comprehensive management and protection of the regional environment.

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