Nitrogen Source Preferences and Ecological Implications of Phytoplankton Primary Production in the Yellow Sea, South Sea of Korea, and East/Japan Sea, 2018

Abstract Understanding the partitioning of primary production into new and regenerated productions, based on nitrate or ammonium utilization, is crucial to understanding biogeochemical processes and marine ecosystems. This study addresses the scarcity of information on new and regenerated productions in the Yellow Sea (YS), South Sea (SS), and East/Japan Sea (EJS). Employing the 13 C– 15 N tracer method, we quantified carbon and nitrogen uptake rates of phytoplankton across four seasons in 2018. Seasonal nitrate and ammonium uptake rates exhibited distinct ranges in the YS (0.2–6.1 and 0.4–22.3 mg N m −2 hr −1 ), SS (1.0–15.6 and 4.5–15.4 mg N m −2 hr −1 ), and EJS (1.5–7.3 and 4.5–15.4 mg N m −2 hr −1 ). Notably, nitrate uptake rates in the YS (except spring), SS, and EJS were generally lower than ammonium uptake rates, attributed to the dominance of pico‐sized (<2 μm) phytoplankton with a high affinity for ammonium. Carbon uptake rates in all seas displayed significant positive correlations with nitrate uptakes rather than ammonium uptakes, suggesting that the prevailing ammonium assimilations by dominant pico‐sized phytoplankton contributed to the lower primary productions in 2018. Estimated annual new productions (30, 24, and 43 g C m −2 y −1 ) in the YS, SS, and EJS fall within the reported ranges for other regions. However, the EJS exhibited lower productions than previous reports (146 g C m −2 y −1 ), potentially impacting fishery yields and export production to the deep ocean in 2018. These findings, in the context of rapid environmental changes, provide crucial baseline information for monitoring future marine ecosystems in Korean seas.