## 3 Results
### Water Quality Inversions  
To verify the inversion effect of the bagging ensemble model, the total phosphorus concentration of Taihu Lake was recorded on 19 December 2021, 24 December 2022, and 24 December 2023. Figures 8-10, respectively, represent the inversion of the total phosphorus in Taihu Lake on 19 December 2021, 24 December 2022, and 24 December 2023. From these figures, it can be seen that the overall water quality of Taihu Lake is good. The average total phosphorus concentrations of Taihu Lake obtained from the inversion at three time points in 2021, 2022, and 2023 were 0.0428 mg/L, 0.0403 mg/L, and 0.0388 mg/L, respectively. It indicates that the inversion of the model conforms to the actual situation, and the inversion effect is good. Regarding the temporal variation of total phosphorus in Taihu Lake, the average total phosphorus concentration in 2023 is the lowest, and it has decreased compared to the previous two years. The overall level of total phosphorus is relatively low. According to the relevant provisions of the Technological Regulations for Surface Water Resources Quality Assessment [49], when the total phosphorus concentration ranges from 0.01 mg/L to 0.05 mg/L, the trophic status of lake bays is classified as mesotrophic. The inversion results shown in Figures 8-10 indicate that the average total phosphorus concentrations in Taihu Lake are 0.0428 mg/L, 0.0403 mg/L, and 0.0388 mg/L, indicating that Taihu Lake as a whole is mesotrophic. Figures 8 and 9 show that the total phosphorus concentration in the peripheral areas of the lake bays in the northern and western parts of Taihu Lake is approximately 0.12 mg/L. Based on the standards, a total phosphorus concentration of 0.1 mg/L-0.2 mg/L corresponds to a mildly eutrophic level, indicating that these areas of Taihu Lake are mildly eutrophic. In contrast, the total phosphorus concentration in the central lake area of Taihu Lake is relatively low, all below 0.1 mg/L, with a correspondingly lower eutrophication level, classifying it as mesotrophic. And the water quality inversion results shown in Figure 10 indicate that the total phosphorus concentration in most areas is below 0.1 mg/L. Compared to the total phosphorus inversion results in Figures 8 and 9, the areas where the total phosphorus concentration exceeds 0.1 mg/L are noticeably fewer, suggesting that the overall nutrient status of Taihu Lake at this time is lower than the results in Figures 8 and 9. It also indicates that the overall eutrophication level of Taihu Lake has decreased, and the environmental quality has improved to some extent. From the spatial distribution of total phosphorus in Taihu Lake, the concentration of total phosphorus in the northern and western region is relatively higher compared to other areas. Therefore, in the subsequent environmental management processes, there should be a strengthened focus on pollution control and environmental protection in the northern and western region of the Taihu Lake basin. Additionally, compared to the total phosphorus concentration in the center of the lake, the concentration around the lake's periphery is relatively higher. The reason may be that the periphery of the lake is often closer to densely populated towns or agricultural areas. Human activities such as agricultural fertilization, industrial emissions, and domestic sewage discharge can lead to elevated total phosphorus concentrations around the lake. Therefore, it is essential to strengthen the control of industrial and agricultural wastewater and the treatment of domestic sewage in rural areas in the future. \begin{table}
\begin{tabular}{c c c c} \hline
**Models** & \(\mathbf{R^{2}}\) & **MSE** & **MAE** \\ \hline Linear & 0.42 & 0.0017 & 0.029 \\ Bagging & 0.9 & 0.00031 & 0.014 \\ Random Forest & 0.78 & 0.00065 & 0.019 \\ Desion Tree & 0.71 & 0.00086 & 0.023 \\ \hline \end{tabular}
\end{table} 
Table 4: Comparison of the performance of models trained on denoised data. Figure 8: Inverse map of total phosphorus in Taihu Lake on 19 December 2021.