The supply of nitrogen (N) from various external and internal sources, e.g., atmospheric deposition, upwelling, lateral intrusion and remineralization, into the euphotic layer modulates the biogeochemical and climatic role of the oligotrophic ocean. In marginal seas, multiple sources and processes may effectively contribute to complicate the dynamics of N in the two-regime structure, namely nitrate (NO₃^–)-depleted layer and NO₃^–-replete layer, of euphotic zone. We investigated the spatial distributions in concentrations and isotopic compositions of NO₃^– (δ¹⁵N_NO3 and δ¹⁸O_NO3) in the upper 200 m of northern South China Sea (SCS). High vertical resolution sampling (10-32 layers for each station within 200 m) showed that NO₃^– concentrations decreased significantly upward to the surface layer while its δ¹⁵N_NO3 and δ¹⁸O_NO3 covary becoming higher due to phytoplankton assimilation. The isotope effect during NO₃^– uptake was estimated to be 2.6±1.3‰ (n=17) for N and 4.1±1.9‰ (n=18) for O based on Rayleigh model, with a ratio of 2.0±0.9 (O/N, n=16). Such high O/N ratio was attributable to heterotrophic bacterial assimilation of NO₃^– and/or in situ nitrification (remineralization). The regenerated NO₃^– occupied 18±21% (n=54) the total NO₃^– pool in the lower euphotic zone at 100-200 m, suggesting the importance of nitrification on modulating the size of NO₃^– pool in the NO₃^–-replete layer. At subsurface of some stations, we found negative shift in δ¹⁵N_NO3 significantly deviating from the vertical pattern of δ¹⁸O_NO3, suggesting an addition of isotopically light N. Positive N* and reduction of Δ(15-18) was supportive of the external inputs from atmospheric deposition and/or N₂ fixation, contributing 18±23% (n=11) to NO₃^– uptake in the NO₃^–-depleted layer. These findings demonstrate that NO₃^– dynamics in the euphotic zone of marginal seas is co-influenced by various external N sources and internal N processes.