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Nitrogen biogeochemistry during the beginning of a spring phytoplankton bloom in the Yellow Sea was investigated based on nutrient concentrations, benthic fluxes of nutrients, the nitrogen and oxygen isotope composition of NO3−, and the hydrological conditions. The δ15N and δ18O values for NO3− in the Yellow Sea were more variable in surface waters than in near‐bottom waters and were generally 6.3–8.2‰ for δ15N and 6.2–9.7‰ for δ18O, except in the Yellow Sea Coastal Current Water (YSCCW), where the maximum δ15N and δ18O values found were 13.2‰ and 18.8‰, respectively. Both the δ15N and δ18O values varied among the different water masses with higher values in the YSCCW than in the Yellow Sea Mixed Water and the Yellow Sea Warm Current Water. Higher δ18O relative to δ15N in NO3− in the Yellow Sea reflects contributions from multiple NO3− sources, including nitrification and atmospheric deposition with the later accounting for 71% of total external NO3− input. A simple nitrogen budget indicated the importance of nutrient regeneration from sediments as a source for water column NO3− representing 74% of total NO3− input in the YS.

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