Tracing Nitrogen Biogeochemistry During the Beginning of a Spring Phytoplankton Bloom in the Yellow Sea Using Coupled Nitrate Nitrogen and Oxygen Isotope Ratios

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 NO 3 − , and the hydrological conditions. The δ 15 N and δ 18 O values for NO 3 − in the Yellow Sea were more variable in surface waters than in near‐bottom waters and were generally 6.3–8.2‰ for δ 15 N and 6.2–9.7‰ for δ 18 O, except in the Yellow Sea Coastal Current Water (YSCCW), where the maximum δ 15 N and δ 18 O values found were 13.2‰ and 18.8‰, respectively. Both the δ 15 N and δ 18 O 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 δ 18 O relative to δ 15 N in NO 3 − in the Yellow Sea reflects contributions from multiple NO 3 − sources, including nitrification and atmospheric deposition with the later accounting for 71% of total external NO 3 − input. A simple nitrogen budget indicated the importance of nutrient regeneration from sediments as a source for water column NO 3 − representing 74% of total NO 3 − input in the YS.