## 4 Discussion
### Nutrient Dynamics across the Sediment-Water Interface
#### 4.1.1 Mechanisms Driving NO\({}_{3}\)- and NH\({}_{4}\)+ Concentrations and Fluxes  
Among the study rivers, NO\({}_{3}\)- concentrations were highest in winter and the lowest in summer (Figure 3b). On the contrary, NH\({}_{4}\)+ concentrations were highest in summer and lowest in winter. After water and sediment samples were mixed, disturbances across the sediment-water interface for the 0-3 min exposure caused NO\({}_{3}\)- and NH\({}_{4}\)+ to migrate along the concentration gradients. In winter, the lowest NO\({}_{3}\)- equilibrium concentration (26.4 mg/L) and the highest water concentration (43.3 mg/L) were found at site L2 and resulted in the maximum NO\({}_{3}\)- uptake flux rate (0.31 mg/(m\({}^{2}\cdot\)s)) observed in the 0-3 min exposure (Figure 4). For site L1 (i.e., highest NO\({}_{3}\)- equilibrium concentration among all sites in winter (45.5 mg/L) but lower water concentration than L2), we observed NO\({}_{3}\)- regeneration. For NH\({}_{4}\)+ in winter, the water concentration at L2 was lower than at L1, while the equilibrium concentration at L2 was the highest among all sites (3.9 mg/L). This likely indicated higher sediment NH\({}_{4}\)+ concentrations here which resulted in a greater concentration gradient and an overall higher regeneration rate (0.04 mg/(m\({}^{2}\cdot\)s); Figure 4). In summer the equilibrium NO\({}_{3}\)- concentration at L1 dropped to the lowest in the four seasons (20.9 mg/L) and was lower than the mean water concentration (26.2 mg/L). These conditions indicated lower sediment concentrations than in the overlying water and contributed to NO\({}_{3}\)- uptake in the 0-3 min exposure. The NO\({}_{3}\)- concentration change at L2 was similar to that at L1, hence the flux still presented uptake in this exposure. The likely decreased concentration gradient also led to a decreased flux rate from 0.09 mg/(m\({}^{2}\cdot\)s) in spring to 0.05 mg/(m\({}^{2}\cdot\)s) in summer. For NH\({}_{4}\)+ in summer, water concentrations at L1 were lower than at L2, while the equilibrium concentration at L1 was higher. This suggests that higher sediment NH\({}_{4}\)+ concentrations at L1 resulted in a greater concentration gradient of NH\({}_{4}\)+ and this was observed as regeneration (0.04 mg/(m\({}^{2}\cdot\)s)). For spring and autumn, as well as other study rivers, seasonal variations in the water concentrations and equilibrium concentrations of NO\({}_{3}\)- and NH\({}_{4}\)+ caused flux transformation to vary between uptake and regeneration (Figure 4). However, the highest water concentration or equilibrium concentration did not correspond to the maximum flux rates. In terms of biological processes (i.e., 3-10 min exposure), the movement of water transferred NO\({}_{3}\)- into the sediment and likely made it bioavailable to microbes associated with sediment surfaces. The DO consumption during both exposures (Figure S1) favored the occurrence of the dissimilatory nitrate reduction to ammonium (DNRA), so that NO\({}_{3}\)- was transformed into NH\({}_{4}\)+ and returned to the overlying water along these concentration gradients [92]. This was consistent with both NO\({}_{3}\)- uptake and NH\({}_{4}\)+ regeneration fluxes shown at some sites (e.g., L1 and L2 in winter; T1 and T2 in summer; Figure 4). However, when the overlying water could not provide sufficient NO\({}_{3}\)- DNRA were likely inhibited resulting in decreased NH\({}_{4}\)+ regeneration fluxes and overall sediment-associated uptake. Seasonal variations of NO3\({}^{-}\) and NH4\({}^{+}\) fluxes were determined by a combination of physical and biological factors. There was no significant seasonal difference in the NO3\({}^{-}\) equilibrium concentration among the three rivers which resulted in a lack of seasonal difference in the NO3\({}^{-}\) flux in the 0-3 min exposure. On the contrary, significant seasonal differences in the NH4\({}^{+}\) equilibrium concentration (\(F=30.272\), \(p<0.001\)) led to the observed seasonal difference in NH4\({}^{+}\) flux (\(F=34.857\), \(p<0.001\)).