## 4 Discussion
Canals also displayed elevated CODMn levels (mean = 3.41 mg/L), attributable to urban and industrial discharges [36]. The PCA analysis elucidates distinct water quality patterns across Canal, Lake, Pond, and River ecosystems (Figure 4). Specifically, PC1 and PC2 together captured more than 85% of the data variations, respectively highlighting the influence of external inputs (such as nutrients and pollutants) and internal dynamics (such as biological activity and natural circulation) on the water quality characteristics of different types of water bodies. In canals, ORP, EC, TDS, NH4, pH, DO, COD, and Chl-a are positively associated with PC1, indicating a strong oxidative environment with high conductivity, and a potential for nutrient enrichment and algal growth. Lakes exhibit a similar pattern, with ORP, EC, TDS, NH4, and Chl-a being positively correlated with PC1, suggesting oxidative conditions and nutrient-rich environments, while DO and pH negatively correlate with PC2, indicating lower oxygen levels and acidity. Ponds show a unique profile with ORP and TDN positively linked to PC1, reflecting oxidative conditions and high dissolved nitrogen, while NH4, EC, TDS, DO and pH negatively correlate with PC2, suggesting nutrient-rich conditions with lower oxygen and higher acidity. Rivers display a distinct pattern with EC and TDS positively correlated with PC1, indicating high conductivity and dissolved solids, while ORP, TN, TDN, NO3, NH4, Chl-a, DO, and pH show varying correlations across PCs, highlighting diverse environmental conditions and potential nutrient dynamics. These insights further underscore the complexity and heterogeneity of water quality across different aquatic ecosystems. The composition and sources of organic matter in aquatic sediments are critical for understanding nutrient cycling, ecosystem productivity, and the impact of anthropogenic activities on water bodies [37]. Canal sediments have the lowest mean TON and TOC values, suggesting a low input of organic matter or efficient decomposition processes. Meanwhile, lake sediments have the highest mean TON and TOC values, indicating a high input of organic matter, potentially from both internal and external sources. The high TOC in lakes may also reflect the high input of terrestrial organic matter due to high connectivity with terrestrial ecosystems [38]. The C/N ratio is also a valuable indicator of the sources and diagenetic history of organic matter. Canal sediments have the lowest mean C/N ratio, suggesting a high proportion of nitrogen-rich organic matter, possibly from sewage or agricultural runoff sources. Pond sediments have the highest mean C/N ratio, indicating a high proportion of carbon-rich organic matter, attributable to inputs from vascular plants or a lower degree of decomposition. Furthermore, the sources of sedimentary organic matter in four urban water bodies were evaluated using the C/N ratios and \(\delta^{13}\)C and \(\delta^{15}\)N values. According to previous studies [24,39], phytoplankton use airborne CO\({}_{2}\) containing \({}^{12}\)C for photosynthesis, and their \(\delta^{13}\)C values are generally more negative than those of other organic matter sources such as aquatic plants. In addition, when C/N \(\leq\) 10, phytoplankton can be considered a source of deposited organic matter [40]. In this study, the \(\delta^{13}\)C value of the sedimentary organic matter of the four urban water bodies was relatively negative, among which the \(\delta^{13}\)C value of Canal was the most negative (\(-\)28.70%), and the C/N value of the sedimentary organic matter was <10. Therefore, C/N-integrated \(\delta^{13}\)C values can explain why phytoplankton is one of the three sources of sedimentary organic matter in urban water bodies. The growth of phytoplankton (algae) in water is caused by sewage with a high nutrient load [41].