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 NO3-, and the hydrological conditions. The N-15 and O-18 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 N-15 and 6.2-9.7 for O-18, except in the Yellow Sea Coastal Current Water (YSCCW), where the maximum N-15 and O-18 values found were 13.2 and 18.8, respectively. Both the N-15 and O-18 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 O-18 relative to N-15 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.