4.3 The reasons for the differences in the eutrophication level of the three reservoirs in Chuzhou City
According to the CA method, the sampling time groups of all three reservoirs were divided into three statistically significant clusters, primarily because the three reservoirs belong to the same city of Chuzhou, have the same geographic characteristics, and natural background, and are influenced by similar natural environments and human activities. However, in this case, three reservoirs did not show a consistent degree of eutrophication. The eutrophication degree of Shahe Reservoir and Huanglishu Reservoir was significantly higher than that of Chengxi Reservoir, both in the course of the annual change (Fig. S4) and the monthly change (Fig. S5). The reason for this is mostly due to the biased utilization of reservoir functions by the region. Shahe Reservoir is a large reservoir with comprehensive benefits in Chuzhou City, which not only serves as an important local aquaculture site but also undertakes irrigation and water supply for neighboring agriculture. Among that, aquaculture often requires abundant imported food and feed, which is highly susceptible to imbalances in N concentrations in the water body (Zhou et al., 2011). The average TN concentration in the reservoir reached 0.98 mg/L in November, which already belongs to Class III water (≤1.00 mg/L) in China’s environmental quality standard for surface water (GB3838-2002). Not only that, the secretions and excretions produced by fish and shrimp contain a large number of nutrients, which are dispersed from the nets or suspended or deposited at the bottom of the reservoir, greatly increasing the P concentration in the water body. TP concentrations in Shahe Reservoir reached 0.05 mg/L in multiple months and were notably higher than those in Huanglishu Reservoir and Chengxi Reservoir. It was pointed out that the discharge from farmed fish accounted for 10 − 20% of the total phosphorus load in some reservoirs and had a much larger actual impact on eutrophication than other sources of phosphorus (Zhou et al., 2011). Besides, the agricultural development around the Shahe Reservoir has been using chemical fertilizers for a long time. After being irrigated by the Reservoir, the chemical fertilizers will be completely dissolved, and the nutrients in its composition will slowly flow into the Reservoir, which will ultimately cause the N and P content of the water body to exceed the standard, and aggravate the eutrophication of the Reservoir. It is worrying that the chemical fertilizer application rate has been increasing to date (Kim et al., 2001). Huanglishu Reservoir also serves as a comprehensive reservoir that not only has the two functions of the Shahe Reservoir mentioned above, but also has the title of a 4A-level scenic area. After being renamed Biyun Lake, it became a famous tourist resort in Quanjiao County, Anhui Province, attracting countless tourists, which has become an important cause of the serious eutrophication of the water body. It is well known that human activities contribute significantly to the occurrence of eutrophication in reservoir waters (Huang et al., 2014). With the rise in the number of tourists, the amount of domestic sewage discharged from scenic tourist areas has increased dramatically, and the water bodies are prone to enrich a large number of highly concentrated nutrients, which in turn causes eutrophication in the reservoirs (Le et al., 2010). CODMn can indicate the extent of organic pollution in water bodies (Li et al., 2018). As expected, Huanglishu Reservoir exceeded the remaining two reservoirs in average CODMnconcentrations each year. Although Chengxi Reservoir has multiple functions, its main function is to undertake the water supply needs of the residents of Chuzhou City, and other functions only play a role in a small number of areas. This is the important reason why the eutrophication level of Chengxi Reservoir is the lowest. According to the TLI method, the concentrations of all five indicators in Chengxi Reservoir are more reasonable and stable than the other two reservoirs. The degree of eutrophication in Huanglishu Reservoir is slightly higher than that in Shahe Reservoir (Fig. 3), due in large part to the fact that the Huanglishu Reservoir is much older and more severely aged, resulting in slower water movement and decreased self − purification capacity (Holz et al., 1997), which affects the degree of eutrophication in the reservoir.