## 4 Discussion
### Metabolic Patterns of Urban River Ecosystems
#### 4.2.2 Factors Driving GPP and ER  
In this study, the WWTP effluents and the tributary inflow were used as natural experiments to investigate the effects of elevated nutrient concentrations and water temperature on urban river metabolism. We found similar GPP across our study sites but higher ER in the effluent/tribulatory-affected reaches (Table 1). [109] proposed criteria to assess the functioning impact on river ecosystems according to GPP and ER values (Table 3). Based on this, the ecological function of the WWTP effluent reservoir where T1 is located was mildly to severely impaired; the ecological function at other sites changed from no functional effect in summer or autumn to severe impairment in winter. Although GPP and ER were measured at specific locations, they can represent the metabolic patterns of the studied reaches and can be used to assess ecological functions, because of the natural movement and mixing of water in a river [109]. Overall, this suggests that both the WWTP effluents and the tributary rich in nutrients have negative impacts on river ecological function and health for urbanized catchments. Our SEM model revealed that NH4+ and PO43- were key nutrients stimulating aquatic bioactivity (Figure 9), which was directly associated with the promotion of growth and reproduction of aquatic organisms (e.g., macrophytes) and increased both GPP and ER [1,2]. For example, the mean GPP and ER at L1 with effluent outfall were higher than at L2 in the four seasons (Table 1). [74] referred to the increase in GPP and ER in heterotrophic rivers as the \"subsidy\" effect of WWTP effluents. As a result of this, a positive correlation between GPP and ER was found (\(r^{2}=0.262,p<0.001\)), which has also been widely reported by many studies (e.g., [110; 111; 112]). However, fine particulate organic matter (FPOM) and DOC associated with WWTP effluents or tributary inflows can be an important energy input for supporting in-situ ER [74; 113]. Additionally, GPP is directly controlled by light availability [66; 114]. Therefore, the increase for ER and GPP in reaches with elevated nutrients is not always consistent [55], which explains the inconsistency between the increased significance of GPP and ER across the study sites. Furthermore, the change of light availability led to the diurnal and seasonal variations of GPP as we observed that the mean GPP was the highest in summer across all sites with the peak between 1-3 pm, followed by autumn and spring and was the lowest in winter with the peak at noon (Figure 6). Regarding the River Wenyu, GPP at W2 with higher nutrient concentrations was not always higher than at W1 (i.e., in autumn and spring) because in addition to nutrient supply, water temperature, turbidity and light availability also likely led to GPP differences [55; 115]. \begin{table}
\begin{tabular}{c} Mild to severe \\ effect \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mean and seasonal range of GPP and ER (both were multiplied by water depth) across the study sites during the study period. The minimum and maximum GPP appeared in winter and summer, respectively. The minimum and maximum ER appeared in winter and autumn (or summer at W1), respectively. Impacts relate to the degree of impaired ecosystem functioning. We suspect that this is especially the case for light availability as the water at W2 was deeper than at W1. This difference in depth resulted in more attenuation of light at the riverbed depth at W2 [116]. These conditions reduced photosynthesis at the riverbed and caused lower observed DO concentrations. There was a similar situation in the River Tonghui, where at T1 (e.g., a site where a pool is formed by the WWTP effluent) GPP was not always higher than at T2 (i.e., in spring and summer). A potential explanation for this observation is related to algal senescence at T1 [117]. ER was positively correlated with water temperature (\(r^{2}\) = 0.567, \(p\) < 0.001) due to temperature-related enzyme activity involved in respiration [63; 64].