Enhanced Ammonia Oxidation Caused by Lateral Kuroshio Intrusion in the Boundary Zone of the Northern South China Sea

Abstract

Lateral input of dissolved organics may play a significant role to support productivity in oligotrophic ocean although associated biogeochemical evidences are lacking in the field. Ammonia oxidation (AO), the first step of nitrification that bridges organic remineralization and nitrate, is potentially an immediate responder. By using 15N‐NH4+, the spatial distribution of AO was investigated in the northern South China Sea, where Kuroshio Current intrudes frequently. AO ranged widely (0.001 to 134 nmol · L−1 · day−1) in space and the depth‐integrated (200 m) AO peaked where the Kuroshio influence is moderate suggesting that enhanced AO had occurred due to lateral mixing. Since oligotrophic Kuroshio is characterized by high dissolved organic nitrogen (DON), such lateral mixing not only introduces external DON into the northern South China Sea but also enhances NH4+ regeneration and subsequent oxidation to complicate the conventional new production in the boundary zone with DON gradient.

Stimulation of Heterotrophic and Autotrophic Metabolism in the Mixing Zone of the Kuroshio Current and Northern South China Sea: Implications for Export Production

“…A full description of the Kuroshio intrusion and nutrient concentrations during the sampling period has been reported in Xu et al (). As seen from the T‐S diagram ( θ ‐ S ) in Figure a, the hydrologic characterization of incubation stations indicated a mixture of NSCS and Kuroshio water, and all the data points fell within limits defined by the two representative end‐members: SCS water (SS) and Kuroshio water (P4).…”

“…(a) Integrated NO 3 − concentration and (b) integrated NH 4 + versus the fraction of Kuroshio intrusion in the upper 100 m ( R K ). The NH 4 + and NO 3 − data were compiled from Xu et al (). The data on the ordinate are plotted on a logarithmic scale.…”

“…During the summer, the NSCS basin is typically characterized by oligotrophic conditions, with low biomass and nutrient concentrations due to the extensive stratification that occurs in the upper water column (Gong et al, ; Lee Chen & Chen, ). Recent studies have demonstrated that the intrusion of the Kuroshio Current into the NSCS significantly impacts the nutrient inventory, DOC pool, and nitrogen cycling in the upper 100 m of the water column (Du et al, ; Wu et al, ; Xu et al, ).…”

“…edu.cn) if any questions. And the nutrients concentration data could be acquired from the supporting information of Xu et al (, https://doi.org/10.1029/2018GL077896). We also acknowledged the crew in R/V Dongfanghong II at the University of Ocean, China.…”

“…Ammonium concentrations were determined by the fluorometric o-phthaldialdehyde method with a detection limit of 0.7 nmol/L (Zhu et al, 2013). The chemical data from the same cruise have been published by Xu et al (2018).…”