Hydrodynamic controls on nitrogen distribution and removal in aquatic ecosystems

The impact of nitrogen (N) on water eutrophication is well-known, but the specific influence of hydrodynamic factors on N occurrence in aquatic systems has remained unclear. This lack of understanding has hindered our ability to assess the self-purification function of aquatic ecosystems and address water pollution problem. Here, we collected overlying water and sediment samples from different aquatic ecosystems (ditch, pond, river, and reservoir) in the Danjiangkou Reservoir area and compared the variation characteristics of various N components, and further conducted an incubation experiment to investigate the rate of N removal. We found that the concentration of total N and its N components decreased from ditches and ponds to rivers and reservoirs, indicating that N removal occurred during water flow, with up to 43% of total N concentration reduction rate. Additionally, we observed higher heterogeneity in eco-stoichiometric characteristics of N components in ditches and ponds compared to rivers and reservoirs. Interestingly, the ditches and ponds exhibited stronger interactions between overlying water and sediment, with higher rates of denitrification and anaerobic ammonium oxidation (anammox). Our findings highlight the need to focus on the upper reaches of agricultural catchments, such as ditches and ponds, for N removal and emphasize the importance of developing region-specific conservation strategies to mitigate N pollution and protect water resources.