## 3 Results
### Nutrient Dynamics in Urban River Ecosystems
#### 3.1.3 Nutrient Fluxes at the Sediment-Water Interface  
Nitrate and ammonium: Across all sites, NO3- fluxes in both exposure periods ranged from \(-\)0.31 to +0.30 mg/(m2-s) and NH4+ ranged from \(-\)0.01 to +0.05 mg/(m2-s) (Figure 4). There were no significant variations in the NO3- flux between the three rivers, but there were significant differences in flux between the sites on the River Wenyu (\(p\) = 0.002). Significant seasonal variations were observed in the River Wenyu (\(F\) = 4.129, \(p\) = 0.032) but again not for other rivers. No significant spatial differences in the NH4+ flux between the three rivers or between sites on each river were found. NH4+ flux generally presented as regeneration, but no significant seasonal variations were observed among the study rivers. Both NO3- and NH4+ flux rates were slower in the 3-10 min exposure compared with the 0-3 min exposure. Figure 4: NO3– and NH4+ fluxes of sampling events during the study period for each study river. Downwards flux represents uptake/removal of nutrients from overlying water to sediment and upwards flux represents regeneration/release of nutrients from sediment to water. The flux experiment at each site has three replicates, if the results are different, the error line is marked on the bar. Phosphate: PO4\({}^{3-}\) fluxes in both exposure periods ranged from \(-\)0.01 to +0.01 mg/(m\({}^{2}\)-s) (Figure 5). For the River Liangshui, L2 showed the maximum regeneration flux in autumn and L1 showed the maximum uptake flux in winter, but no overall significant seasonal variations were recorded. The two sites on the River Tonghui only presented regeneration fluxes in the 0-3 min exposure in autumn. Similarly, seasonal variations did not vary significantly. The two sites on the River Wenyu presented uptake fluxes in the 0-3 min exposure in all seasons, but again no significant seasonal variations were observed. Spatially, no significant differences among rivers or between sites on each river were observed. Dissolved organic carbon: DOC fluxes in both exposure periods ranged from \(-\)0.04 to +0.13 mg/(m\({}^{2}\)-s) (Figure 5). In the 0-3 min exposure, DOC at the sites on the River Liangshui and the River Tonghui was released from sediments except for L1 in autumn. In the 3-10 min exposure, DOC at the sites on the River Wenyu showed uptake flux except for W1 in autumn. The maximum uptake and regeneration fluxes occurred at W2 in winter and at T1 in summer, respectively. There were no overall significant variations in the DOC flux  
Figure 5: PO4\({}^{3-}\) and DOC fluxes of sampling events during the study period for each study river. Downwards flux represents uptake/removal of nutrients from overlying water to sediment and upwards flux represents regeneration/release of nutrients from sediment to water. The flux experiment at each site has three replicates, if the results are different, the error line is marked on the bar. across the three rivers or between the sites on each river. Further, no significant seasonal variations were observed in each river. DO flux: In both exposures, DO was consumed across all sites with the consumption flux ranging from 0.01 to 0.05 mg/(m\({}^{2}\)-s) (Figure S1). The DO consumption flux was positively correlated with regeneration fluxes of NH4\({}^{+}\) (\(p<0.001\)) and DOC (\(p<0.001\)), but no correlation was observed with NO3\({}^{-}\) or PO4\({}^{3-}\) flux; only the initial PO4\({}^{3-}\) concentration was positively correlated with PO4\({}^{3-}\) regeneration flux (\(p=0.002\)), indicating that in the overlying water DO and initial nutrient concentrations did not independently affect nutrient fluxes. When the water-specific nutrient concentration increased/decreased seasonally, an increase in the equilibrium concentration strongly indicated an increase in the sediment-specific nutrient concentration, while a decrease in the equilibrium concentration indicated a decrease in the sediment-specific nutrient concentration (Table S10).