# Centennial Lake Environmental Evolution Reflected by Diatoms in Yilong Lake, Yunnan Province, China
## 1 Introduction  
Lakes are important freshwater resources on the Earth's surface, with various functions such as regulating river runoff, improving the ecological environment, providing drinking water sources, and breeding aquatic organisms. They play an important role in maintaining regional ecological and environmental security and achieving sustainable human development [1]. However, under the influence of natural changes and human activities, the ecosystem of lakes has undergone changes in the area, shape, water quality, etc., resulting in a series of problems such as lake drying up [2, 3], water quality deterioration [4, 5], and ecological degradation [6, 7]. The main factors affecting the lakes include rainfall and evaporation [8], acid deposition and climate change [9], human activities during historical periods [10, 11], as well as the intensity of development and extreme climate events [12, 13, 14]. Usually, the evolution of the lakes is extremely slow and takes thousands of years. However, it can be accelerated under the influence of human activities [15, 16], which leads to numbers of environmental problems. Among them, lake eutrophication is a major water environment problem that many countries are facing [17, 18, 19], especially China [20]. A large number of solutions have been adopted for the restoration and treatment of lake eutrophication, such as planting aquatic plants to absorb nutrients in water [21, 22], using sediment microbial fuel cells (SMFCs) to inhibit or reduce nitrogen and phosphorus [23, 24, 25, 26], and introducing microorganisms to purify water [27, 28], etc. However, it is not only an ecological issue but also a reflection of social issues. Therefore, social management strategies need to be carried out simultaneously, such as strengthening government management [29],planning human resources [30], improving lake management, guiding citizens to participate in lake protection, and promoting social innovation [31], etc. Until then, understanding the evolution process and the driving mechanism is a great foundation for the restoration and management of the lakes, which leads to more scientific and targeted solutions to eutrophication lakes. The Yilong Lake is a natural freshwater lake that belongs to the Pearl River water system. In addition, it is one of the nine plateau lakes in Yunnan Province that is located at 10\({}^{\circ}\)29\({}^{\prime}\)-10\({}^{\circ}\)37\({}^{\prime}\) E and 23\({}^{\circ}\)38\({}^{\prime}\)-23\({}^{\circ}\)42\({}^{\prime}\) N, 3 km southeast of Shiping County of Yunnan Province [32]. With an elevation of 1414 m, the lake is located on the plateau of southern China. It is a rift lake in an east-west strip shape. The shoreline is 62.9 km, and the drainage area is 360.4 km\({}^{2}\). The maximum water storage is 1.05 \(\times\) 10\({}^{8}\) m\({}^{3}\), with a maximum water depth of 6.55 m and an average water depth of about 2.75 m [32]. In total, 20 rivers are entering the lake, and most of them are seasonal. Yilong Lake belongs to the subtropical plateau monsoon climate, and the rainy season is mainly concentrated from May to October [32]. The average annual water temperature is 18.0 \({}^{\circ}\)C; the precipitation is 919.9 mm, and the evaporation is 1034.5 mm [32]. The water is mainly supplied by precipitation. Yilong Lake is a heavily eutrophic lake [33], with serious pollution in the basin. Urban and rural domestic sewage, industrial and agricultural wastewater, rainfall scouring, and surface runoff are the main pollution sources [34]. Diatom is a kind of unicellular algae with a siliceous shell outside. Its habitat is extensive and can be found almost everywhere where water is retained [34]. The siliceous shell can be preserved in the sediment for a long time after the diatom is died [35]. Therefore, sediment diatoms can accurately record the past's environmental information, making it possible to study environmental changes.