1 Introduction
Water, as the source of life, is a fundamental component of all living organisms and sustains all vital activities in the human body (Sajeev et al., 2020; Westall and Brack, 2018). Nevertheless, the overuse and pollution of water resources by human activities (continuous population growth, rapid progress in agriculture and industry, and high rates of urbanization) have rendered water resources one of the most threatened resources in the world (Varol, 2019). As the important tool for humans to utilize water resources, reservoirs play an extremely prominent role in the agricultural irrigation of rural acres, the living supply of urban residents, the storage project during the drought season, and the flood control project during the rainstorm season (Mamun et al., 2020). However, increased environmental degradation has substantially reduced the role of reservoirs. And under the combined influence of natural and anthropogenic factors (Liu et al., 2022; Paerl and Otten, 2013), causing severe environmental problems of water eutrophication worldwide (Le Moal et al., 2019). When eutrophication occurs in reservoirs, surplus nutrients stimulate the excessive growth of algae, phytoplankton, and other aquatic plants in the water, decreasing DO and increasing Chl-a levels (Li et al., 2021). Currently, the eutrophication of reservoirs has resulted in multiple impacts on ecosystems and social stability (Chen et al., 2020; Mäler, 2000), such as water hypoxia, water bloom, aquatic animal mortality, simplification of food webs, and reduction of biodiversity. According to the studies, eutrophication has occurred in up to 54% of reservoirs in Asia, and its incidence continues to increase (Fink et al., 2018). In conclusion, reservoir eutrophication has been recognized as the most frequent and severe environmental hazard in aquatic ecosystems (Wu et al., 2018).
Proactive gathering of reliable information on changes in water quality for eutrophication management in reservoirs, which has been demonstrated in many countries and regions (Astel et al., 2006; Behmel et al., 2016; Romero et al., 2016). At present, various multivariate statistical analysis techniques are applied in the academic community for multiple and complex data on water quality, such as Cluster Analysis (CA) (Hajigholizadeh and Melesse, 2017), Discriminant Analysis (DA) (Li et al., 2018), Factor Analysis (FA) (Mamun et al., 2021), Principal Component Analysis (PCA) (Zeinalzadeh and Rezaei, 2017), Correlation Analysis (Liu et al., 2010), and Analysis of Variance (ANOVA) (Chen and Lu, 2014), not only do they provide an effective tool for interpreting monitoring data sets, but they also provide significant assistance in identifying the major factors affecting water quality. In addition, the academic community has likewise developed multiple methods for evaluating water quality, examples include the fuzzy mathematical method (Peche and Rodríguez, 2012), gray clustering method (Wong and Hu, 2014), neural network method (Zhou et al., 2020), matter element analysis method (Chen et al., 2012), Water Quality Index (WQI) (Wang et al., 2019), and Trophic level Index (TLI) (Li et al., 2021), they provide an excellent foundation for comprehensively characterizing the degree of eutrophication in reservoirs and for scientifically improving reservoir water quality.
Chuzhou City is located in the eastern part of China’s Anhui Province, with a land area of 13,300 km2, and belongs to the Yangtze and Huaihe River Basin, which is a typical area of the Jianghuai Watershed. The average annual precipitation is 1,035.5mm, and the total water resource is 33.5×108m3, but the per capita water resource is only 908m3, which is far lower than the national average value of 2,304m3, and is also lower than the water scarcity warning line of 1,000m3, which is a water scarcity area. Therefore, sufficient and healthy water resource is particularly essential for the city. The three reservoirs in Chuzhou City (Shahe Reservoir, Huanglishu Reservoir, and Chengxi Reservoir) have been tasked with alleviating water scarcity since their construction. However, as the central lifeblood of the city, the reservoirs have been suffering from eutrophication of water bodies in recent years, which directly or indirectly affects the normal life of the people in the area and the sustainable economic development of the city (Dokulil et al., 2000). Consequently, the evaluation of eutrophication in reservoirs has great significance in providing the scientific basis and control strategies for local water quality management and ecological protection.
Given the limitations of multivariate statistical analysis techniques for water quality assessment when used alone (Chen and Lu, 2014; Güler et al., 2002), in this study, multiple statistical analysis techniques were jointly applied to minimize their limitations and preserve their respective strengths (Chen et al., 2016; Zhou et al., 2007). At the same time, selected a higher scientific basis of the TLI method to analyze and evaluate the trophic state of three reservoirs in Chuzhou City (Liu et al., 2021). Aims: (1) To research the drivers of water quality parameter changes over time in three reservoirs between 2019 and 2021; (2) To evaluate the trophic levels and causes of occurrence in reservoirs using the TLI method; (3) To analyze the differences in nutrient levels in the water bodies of the three reservoirs.