## 4 Discussion
Notably, Canal and Pond are relatively less subjected to exogenous interference, but the analysis of the \(\delta^{15}\)N value of the sedimentary organic matter shows  
Figure 4: PCA results for water quality and chemical indicators from canals, lakes, ponds, and rivers. that terrigenous organic matter is the main source of sedimentary organic matter in Canal and Pond, including soil organic matter and terrestrial plant debris. In addition, large values of \(\delta^{13}\)C (\(-\)22.47%) and \(\delta^{15}\)N (5.76%) of sedimentary organic matter were found in the urban ponds, which indicates that some ponds in urban areas may be subject to sewage input. This analysis shows that part of the source contribution of sediment organic matter in urban ponds is mainly sewage source organic matter. The \(\delta^{13}\)C and \(\delta^{15}\)N ranges of Lake and River sediments in urban areas are similar, and the \(\delta^{15}\)N ranges are widely distributed. This shows that the sources of organic matter in Lake and River sediments in urban areas are relatively complex. The relatively high \(\delta^{13}\)C value, medium C/N value, and low \(\delta^{15}\)N value of sedimentary organic matter in Lake and River indicate that sedimentary organic matter is primarily influenced by submerged water and phytoplankton [19]. Shaoxing City has a subtropical monsoon climate characterized by four distinct seasons and abundant rainfall. Soil organic matter and terrestrial plants around water bodies are easily affected by rainfall and enter water bodies through leaching or runoff, thus affecting changes in sedimentary organic matter in water bodies. Indeed, carbon and nitrogen isotope analysis methods (\(\delta^{13}\)C and \(\delta^{15}\)N) can underrepresent or misidentify sources due to overlapping signatures. Especially, in urban environments, multiple organic matter sources, such as sewage, terrestrial runoff, and phytoplankton, often exhibit similar isotopic compositions, complicating accurate differentiation. Moreover, environmental factors like temperature and pH can influence isotope fractionation, further obscuring source signals. Therefore, to enhance the precision of carbon and nitrogen source apportionment in future studies, it is recommended that stable isotope analysis be complemented with additional tracers such as heavy metals or polycyclic aromatic hydrocarbons (PAHs) [42]. This integrated approach can provide more robust source identification. Furthermore, the application of Bayesian mixing models can help quantify the relative contributions of different sources, even when isotopic signatures overlap, thereby reducing uncertainties in the source apportionment process [43; 44; 45]. In summary, the aforementioned comparative analysis highlights the unique characteristics of each water body type based on their quality and chemical properties. Understanding these variations is crucial for effective environmental management and conservation efforts tailored for specific aquatic ecosystems. Furthermore, using C/N ratio, \(\delta^{13}\)C and \(\delta^{15}\)N values to identify the sources of organic matter in urban water sediments has certain practical value and importance. According to different sources of water pollution, this study suggests that water bodies with numerous phytoplankton and aquatic plants should be regularly recovered, water and soil conservation should be conducted around the water bodies, and the input of terrestrial organic matter (soil organic matter and terrestrial plant litter) into the water bodies should be controlled as much as possible in Shaoxing City. Sewage treatment should strictly adhere to local standards, ensuring discharge only after compliance with these regulations. This study highlights phytoplankton and sewage as key sources of organic carbon and nitrogen in urban water bodies. To mitigate eutrophication, the recommend policies about Shaoxing City for monitoring nutrient inputs, stricter effluent regulations, and best practices for fertilizer use.