###### Abstract  
This study evaluated nutrient flux (nitrate (NO\({}_{3}\)\({}^{-}\)), ammonium (NH\({}_{4}\)\({}^{+}\)), phosphate (PO\({}_{4}\)[3]\({}^{-}\)), and dissolved organic carbon (DOC) at the sediment-water interface and river ecosystem metabolism (REM) to investigate how these ecological functions vary in Beijing's urban waterways. Three tributaries of the River Beijun were selected. Water quality varied across the study sites as each receives a mixture of wastewater treatment plant (WWTP) effluents and tributary inflows. A chamber technique was applied where water-specific nutrient concentrations were measured at two exposure times (3 and 10 min). Under the actions of physical and biological processes, NO\({}_{3}\)\({}^{-}\) and NH\({}_{4}\)\({}^{+}\) flux was primarily controlled by equilibrium concentration and the N-cycle. However, bioabsorption appeared to regulate DOC flux. Specifically, NO\({}_{3}\)\({}^{-}\) flux ranged from \(-\)0.31 to +0.30 mg/(m\({}^{2}\)-s), NH\({}_{4}\)\({}^{+}\) was \(-\)0.01 to +0.05 mg/(m\({}^{2}\)-s), PO\({}_{4}\)\({}^{-}\) was \(-\)0.01 to +0.01 mg/(m\({}^{2}\)-s), DOC was \(-\)0.04 to +0.13 mg/(m\({}^{2}\)-s). We applied the nighttime slope regression to estimate gross primary production (GPP) and ecosystem respiration (ER). Except in summer, net ecosystem production (GPP+ER) less than 0 indicated heterotrophic study reaches. Structural equation modelling revealed that nutrient dynamics and water temperature were the primary factors driving REM. Our study provides the needed systems-based understanding of vital ecological processes to improve in-stream management. nutrient dynamics; river metabolism; ecosystem function; urban rivers 2022142963103390/14/19/29/11/03/11/  
matter discharged by human activities, WWTP effluents, and biological residues produced as a result of photosynthetic processes of aquatic vegetation [26; 27; 28]. As an important part of river ecosystems, river sediments play a key role in nutrient dynamics and water quality and can serve as both a source and sink of nutrients [16; 29; 30]. The uptake and/or regeneration of nutrients by sediments depend on a variety of influencing factors, making river ecological protection more challenging. Nitrate (NO\({}_{3}\)\({}^{-}\)) and ammonium (NH\({}_{4}\)\({}^{+}\)) are the main forms of nitrogen in sediments [31; 32], and their fluxes are primarily controlled by equilibrium concentration at the sediment-water interface and nitrogen form transformation within the N-cycle [29; 33; 34], including nitrification, denitrification, dissimulatory nitrate reduction to ammonium (DNRA), mineralization and anammox. These processes are mainly performed by microorganisms such as nitrifying bacteria and denitrifying bacteria, which are affected by a range of factors including water temperature, dissolved oxygen (DO) and associated microbial activity [35; 36]. Phosphate (PO\({}_{4}\)\({}^{3-}\)) is another macronutrient required by aquatic organisms [37]. PO\({}_{4}\)\({}^{3-}\) flux is related to the adsorption of sediment minerals, for example, iron insoluble compounds can adsorb a large amount of PO\({}_{4}\)\({}^{3-}\) under oxidation conditions [38; 39; 40; 41], resulting in the uptake flux of PO\({}_{4}\)\({}^{3-}\). In the river restoration practice, alum is often added to water to promote PO\({}_{4}\)\({}^{3-}\) adsorption [42; 43; 44; 45]. Additionally, it has been demonstrated that sediment particle size plays an important role in PO\({}_{4}\)\({}^{3-}\) flux, such as PO\({}_{4}\)\({}^{3-}\) release caused by resuspension of fine sediment particles [46; 47; 48; 49; 50]. Dissolved organic carbon (DOC) can be an important carbon source in rivers to support biological activities which can regulate fluxes. Furthermore, some studies have found that anions in DOC and PO\({}_{4}\)\({}^{3-}\) compete for adsorption sites, which can affect the rates of both fluxes [51; 52]. Nutrient supply affects aquatic bioactivity, which in turn affects gross primary production (GPP) and ecosystem respiration (ER).