## 2 Materials and Methods
### Flux Measurement of \(NO_{3}{}^{-}\), \(NH_{4}{}^{+}\), \(PO_{4}{}^{3-}\), \(DOC\)  
Flux experiments were run in September and November 2020 and March and June 2021 (Table S2). In the laboratory, to simulate the sediment-water interface, 20 mL of wet surface sediment and 150 mL of overlying water were mixed into a 500 mL chamber, which was scaled up by the nitrate flux experiment by [29]. At 2.5 min after mixing, the particle settlement was visualized. Experiments were run for 3 min and 10 min after mixing to measure the concentrations of NO\({}_{3}{}^{-}\), NH\({}_{4}{}^{+}\), PO\({}_{4}{}^{3-}\), and DOC to represent the process related to both physical disturbance and biological action [29]. We consider that water extraction at t = 3 min would centrally represent physical disturbances, while the experiment where water was extracted at t = 10 min represented a mix of processes. Allexperiments were run with 3 replicates. A DO logger (PME MiniDot oxygen logger) was also used to continuously measure the DO concentration in the overlying water during two exposures (0-3 min, 3-10 min).  
Nutrient fluxes were derived from Equation (1):  
\[f\ =\ \frac{(C_{2}-C_{1})\ V}{A\ (t_{2}-t_{1})} \tag{1}\]  
where \(C_{1}\) and \(C_{2}\) refer to the nutrient concentration at times t\({}_{1}\) and t\({}_{2}\), respectively; \(V\) is the volume of the overlying water (L); \(A\) is the surface area of the sediment surface (m\({}_{2}\)); \(t_{2}-t_{1}\) is the time (s) between the subsequent (\(t_{2}\)) and previous (\(t_{1}\)) water extraction. Fluxes are expressed as mg/(m\({}^{2}\cdot\)s). A positive flux indicates movement from sediments to the overlying water and a negative flux indicates movement from the overlying water to the sediment.  
During the summer flux experiment, we conducted an additional experiment to further verify the contribution of microbial action to the flux. During the 0-3 min exposure, experimental 3 replicates were run where we added 1 mL of 1 mg/L of zinc chloride with lethal effect on microorganisms [83]. Subsequently, flux was compared to experiments without zinc chloride (Table S3).  
Equilibrium Flux of NO\({}_{3}\)\({}^{-}\), NH\({}_{4}\)\({}^{+}\), PO\({}_{4}\)\({}^{3-}\), DOC  
The following nutrient standard solutions were configured: NO\({}_{3}\)\({}^{-}\) standard solution with concentration gradient of 10-50 mg/L with sodium nitrate; NH\({}_{4}\)\({}^{+}\) standard solution with concentration gradient of 0.5-2.0 mg/L with ammonium chloride; PO\({}_{4}\)\({}^{3-}\) standard solution with concentration gradient of 0.5-3.0 mg/L with monopotassium phosphate; DOC standard solution with concentration gradient of 5-20 mg/L with potassium biphthalate (Table S4). In each 500 mL chamber, 20 mL of wet surface sediment was weighed before adding to the chamber, and then 150 mL of standard solution with the above concentration gradient was added. Nutrient concentrations of the overlying solution were measured at t = 3 min. The amount of nutrients released/absorbed by sediments was derived from Equation (2):  
\[Q\ =\ \frac{(C_{0}-C_{1})\ V}{W} \tag{2}\]  
where \(Q\) is the amount of nutrient released/adsorbed by sediment per unit weight (mg/g); \(V\) is the volume of overlying standard solution; \(W\) is the weight of wet surface sediment. A positive \(Q\) represents the amount of nutrients absorbed by sediment; a negative \(Q\) represents the amount of nutrients released from sediment to the overlying standard solution.  
Taking the concentration \(C_{0}\) of the standard solution as the independent variable and \(Q\) as the dependent variable, the linear equation \(Q\)\(-\)\(C\) was established for each nutrient (Table S4). When \(Q\) = 0, it indicates that the uptake and regeneration flux of nutrients reach equilibrium, and the corresponding nutrient concentration is the equilibrium concentration at the sediment-water interface.