Depth-partitioned nitrogen uptake sustained by ammonium in a nitrate-rich reservoir

Thermal stratification in inland waters creates pronounced physicochemical gradients, yet a quantitative, depth-resolved understanding of how these gradients regulate nitrogen (N) dynamics remains lacking. Here, we conducted a two-year, depth-resolved study in a subtropical reservoir, quantifying uptake of ammonium (NH4+), nitrate (NO3-), and urea using the 15N tracer labeling technique. Although NO3- is the dominant component of the dissolved inorganic nitrogen (DIN) pool in the euphotic zone, NH4+ consistently sustained the majority of N assimilation, exceeding NO3- and urea uptake by approximately 7- and 11-fold, respectively, resulting in rapid NH4+ turnover (median τ≈4 days). Vertically, we observed a distinct three-layer structure of nitrogen assimilation along the light gradient: (1) phytoplankton preferentially utilized NH4+ and urea under high irradiance in the surface layer; (2) NO3- uptake peaked near the base of the euphotic zone, likely associated with low-light adapted diatoms; and (3) in the aphotic layer, substantial NH4+ assimilation persisted but decoupled from carbon fixation, indicating NH4+ consumption by heterotrophs. Our results provide a quantitative framework that refines our current view of nitrogen cycling in stratified inland waters and challenges concentration-based views of nutrient limitation. We demonstrate that effective management of eutrophication in stratified systems must account for N speciation, vertical stratification, and the distinct roles of autotrophic and heterotrophic communities.