## 3 Results
### Metabolic Patterns of Urban River Ecosystems
#### 3.2.2 GPP, ER and NEP Estimates  
The minimum mean GPP appeared in winter and decreased from noon; the maximum mean GPP appeared in summer and peaked between 1 and 3 pm (Figure 6). The mean GPP in autumn was higher than in spring except for T2, while the maximum GPP at T2 in autumn was higher than spring values. Compared with autumn, GPP at L2 and T2 decreased significantly in winter (L2: \(p=0.011\); T2: \(p=0.028\)) then increased significantly in spring (L2: \(p=0.048\); T2: \(p=0.020\)); GPP at W2 increased significantly from spring to summer (\(p=0.044\)). However, no significant differences were observed in GPP for all rivers spatially.  
ER among the three rivers was the lowest in winter and the highest in autumn except for W1, where the highest ER was observed in summer (Figure 7). Significant seasonal variations at all sites were recorded from autumn to spring (\(p<0.001\), except \(p=0.006\) for W2 from winter to spring); in summer, ER at L1, T2 and W1 still increased significantly (\(p<0.001\)). Different from GPP, significant differences in ER were observed between the two sites of each river (\(p<0.001\), except River Tonghui and River Wenyu in spring). Further, significant spatial differences in ER were also observed across the three rivers except River Liangshui and River Tonghui in winter (\(p=0.002\) between River Tonghui and River Wenyu in summer; \(p<0.001\) among the three rivers in other seasons). There was a positive correlation between GPP and ER across the three rivers during the study period (\(r^{2}=0.462\), \(p<0.001\)).  
Figure 6: Diurnal variations of GPP in the four seasons across study sites. GPP dropped from the peak to 0 at night and increased from sunrise. The minimum mean GPP appeared in winter when GPP decreased from noon; The maximum mean GPP appeared in summer when GPP peaked between 1 and 3 pm.  
In each season, NEP decreased to the lowest value within one hour after sunset and then gradually increased (Figure 8). The maximum mean NEP across all sites appeared in summer and reached the peak between 10 am-2 pm. The minimum NEP at L1 was observed in spring, while the minimum values of other sites appeared in autumn when diurnal NEP peaked between 10 am-12 pm. The summertime positive mean NEP indicated that these river ecosystems were overall sinks of atmospheric CO\({}_{2}\). For these periods mean absorption rates ranged from 1.6 to 12.7 gCO\({}_{2}\)/(m\({}^{3}\cdot\)d). At other times, the mean NEP values were negative indicating these sites were sources of CO\({}_{2}\) with the mean release rate ranging from 2.2 to 16.0 gCO\({}_{2}\)/(m\({}^{3}\cdot\)d). The springtime NEP in the River Tonghui was significantly different from the River Liangshui (\(p\) = 0.006) and the River Wenyu (\(p\) < 0.001). Also, there was a significant difference between the two sites of the River Liangshui in spring (\(p\) = 0.005).  
Figure 7: Diurnal variations of ER in the four seasons across study sites. ER was the lowest in winter and the highest in autumn across study sites except W1. Significant seasonal variations of ER at all sites were recorded from autumn to spring.