## 3 Results  
### Nutrient Dynamics in Urban River Ecosystems  
#### 3.1.1 Water Quality of Study Rivers  
Nitrate: During the study period, there were significant differences in NO3- concentrations among the three rivers (\(F\) = 12.737, \(p\) < 0.001) and a negative correlation between NO3- and NH4+ concentrations was observed (\(r^{2}\) = \(-\)0.557, \(p\) < 0.001). Specifically, the concentrations at relative downstream sites were significantly higher than at upstream sites across rivers (\(p\) < 0.001, Figure 3a). The maximum and minimum mean NO3- concentrations appeared at the sites T2 and W1, respectively. Among these rivers, the NO3- concentration in winter was moderately higher than in autumn (\(p\) = 0.310) and then decreased continuously in spring (\(p\) < 0.001) and summer (\(p\) < 0.001). Overall values ranged from 13.8 to 51.9 mg/L (Figure 3b). Ammonium: During the study period, there were significant differences in NH4+ concentrations among the three rivers (\(F\) = 4.760, \(p\) = 0.010). The maximum and minimum mean NH4+ concentrations appeared at the sites L1 and W1, respectively. However, no significant difference in the concentration between the two sites of each river was observed (Figure 3a). A positive correlation between NH4+ concentrations and water temperatures was observed (\(r^{2}\) = 0.707, \(p\) < 0.001, Table 1), resulting in the lowest concentration in winter and the highest concentration in summer, with a range of 0.3-3.4 mg/L (Figure 3b). There were significant differences in the NH4+ concentration (\(p\) < 0.001) and water temperature (\(p\) < 0.001) between seasons. Figure 3: (**a**) Concentrations of nutrients and DO and water temperature across sampling sites. Each box contains all data points from sampling events during the study period. The concentrations of NO3–, NH4+ and PO4– across the three rivers were significantly different. (**b**) Concentrations of nutrients and DO and water temperature in four seasons during the study period. Each box contains data points from all sampling sites in the corresponding season. The concentrations of all nutrients changed significantly seasonally. Phosphate: During the study period, there were significant differences in the PO4\({}^{3-}\) concentrations among the three rivers with a range of 0.2-1.5 mg/L (\(F\) = 32.916, \(p\) < 0.001), and significant differences between the two sites of each river (River Liangshui: \(p\) = 0.001; River Tonghui: \(p\) = 0.027; River Wenyu: \(p\) = 0.038, Figure 3a). The maximum and minimum mean PO4\({}^{3-}\) concentrations appeared at W2 and T1 sites, respectively. A negative correlation between PO4\({}^{3-}\) and DO concentrations were found (\(r^{2}\) = \(-\)0.321, \(p\) < 0.001), resulting in the highest concentration in winter with the lowest DO. However, the lowest PO4\({}^{3-}\) concentrations were recorded in spring (Figure 3b). There were significant differences in the PO4\({}^{3-}\) concentrations (\(p\) < 0.001 from autumn to winter; \(p\) < 0.001 from winter to spring; \(p\) = 0.023 from spring to summer) and the DO concentrations (\(p\) < 0.001 from autumn to winter; \(p\) = 0.001 from winter to spring; \(p\) < 0.001 from spring to summer) between seasons. Dissolved organic carbon: During the study period, there was no difference in the DOC concentration among the three rivers with a range of 4.3-16.7 mg/L, and no significant difference in the concentration between the two sites of each river was observed (Figure 3a). The maximum and minimum mean DOC concentrations appeared at L1 and W1, respectively. Among these rivers, the DOC concentration in winter was significantly higher than in autumn (\(p\) = 0.014), and then decreased significantly in spring (\(p\) < 0.001) but slightly in summer (Figure 3b). Across all sites, DOC was positively correlated with NO3\({}^{-}\) (\(r^{2}\) = 0.669, \(p\) < 0.001) and PO4\({}^{3-}\) (\(r^{2}\) = 0.518, \(p\) < 0.001) concentrations and negatively correlated with NH4\({}^{+}\) concentrations (\(r^{2}\) = \(-\)0.757, \(p\) < 0.001). the highest in summer.