## 3 Results and Discussion
### The Relationship between N, P, and Water Quality  
Water quality parameters can be devoted to expressing the degree of water environmental quality. The migration of N and P in water is inextricably related to water quality parameters, and the distribution of N and P can also affect the water quality parameters. The correlations between N, P, and water quality parameters are shown in Table 2.  
There was a certain correlation between the environmental quality parameters (pH, COD, DO, and Eh) and the N and P contents of overlying water in Table 2. The positive correlation between pH and TP in the overlying water could be attributed to the increase in P release in sediment due to the strong alkalinity of the water body [20]. There was a significant positive correlation between COD and NH\({}_{3}\)-N, TN, and TP in overlying water, and DO was negatively correlated with TN and TP in water. The increase in nutrient (N, P, and NH\({}_{3}\)-N) contents might lead to the eutrophication of the water system; tiny plankton, protozoa, or bacteria could proliferate, consume dissolved oxygen, and accumulate organic matter. Thus, COD would increase and DO would decrease, which was consistent with the results of their correlation analysis. Eh, reflecting the redox ability of water, was only negatively correlated with TP in the sediment. Since the Eh values measured in this experiment were all positive (as shown in Table S4), the regional water bodies were considered to have a reduction reaction. With the increase in Eh, the oxidation of the water body was strengthened, and the metal ions complexing with P in the water increased, leading to the enhanced release of P and the corresponding reduction in TP content in the sediment [21]. Therefore, the water quality parameters had the strongest correlation with P content in the overlying water and sediment, while in sediment, the P absorption depended on precipitation and biological adsorption, and the P release was affected by the chemical change of the solution, biomass release, and hydrolysis of organic P [22]. This manifested in the P concentration being mutually restricted with the water environment, especially in the overlying water. Moreover, the migration and transformation of N in the water and sediment also interacted with the water environment quality [23]. Therefore, the interaction between N, P, and water quality parameters is complex, and no single parameter works alone. COD is an important quality parameter of a water environment. It measures the amount of reducing substances that need to be oxidized in samples, and reflects the pollution degree of reducing organic matter in water. Based on the high correlation between COD and NH\({}_{3}\)-N, TN, and TP in the overlying water in Table 2, the partial least squares (PLS) method was used to explore the establishment of their regression equations, with the aim of simply estimating the COD value in water bodies from the contents of TP, TN, and NH\({}_{3}\)-N in the absence of actual COD measurements. The basic principle of this method was to extract the principal components (several comprehensive variables with the best explanatory power for both the independent variables of TP, TN, and NH\({}_{3}\)-N and the dependent variable of COD) from the independent variable system consisting of TP, TN, and NH\({}_{3}\)-N; establish the regression equation of COD to the principal components; and then restore the above equation to the PLS regression equation of COD to TP, TN, and