Ammonium (NH₄⁺) dynamics in the oligotrophic South China Sea (SCS) were examined using data collected during summer 2014 and spring 2016. A sensitive fluorometric technique was used to measure NH₄⁺ concentrations (detection limit of 0.7 nmol L⁻¹). The NH₄⁺ inventory of the upper 100 m showed considerable variation between two samplings, averaging 3.46 mmol m⁻² in 2014 and almost doubling in 2016. This could be attributed to seasonality or more likely to influence of Kuroshio Current intrusion, prominent within El Niño years such as 2016. Kuroshio-influenced stations exhibited elevated NH₄⁺ and decreased NO₃⁻ inventories. Higher NH₄⁺ concentrations at the Kuroshio-SCS frontal zone were consistent with prior observations of enhanced microbial consumption of Kuroshio-derived dissolved organic matter. Two patterns in vertical NH₄⁺ distributions were observed: profiles with subsurface NH₄⁺ maximum (30.1–241 nmol L⁻¹), usually occurring at depths of 50–100 m that were closely coupled with the deep chlorophyll maximum and nitrite maximum, and profiles with low and generally uniform NH₄⁺ concentrations with depth. The depth of the NH₄⁺ maximum appears to be controlled by the euphotic depth, position of the nitracline, as well as the difference in the affinity to NH₄⁺ between phytoplankton and nitrifiers. We demonstrated that precise measurements of NH₄⁺ concentrations at nanomolar levels facilitate improved understanding of oceanic nitrogen biogeochemistry.

使用 2014 年夏季和 2016 年春季收集的数据检查了贫营养南海 (SCS) 中的铵 (NH ₄ ⁺ ) 动态。使用灵敏的荧光技术测量 NH ₄ ⁺浓度（检测限为 0.7 nmol L ^-1） . 的NH ₄ ⁺的上百米库存表现出两个采样之间的相当大的变化，平均为3.46毫摩尔米^-2在2014年和2016年这几乎加倍可以归因于季节性或更可能内埃尔影响黑潮侵入，突出Niño 年，如 2016 年。受黑潮影响的台站表现出升高的 NH ₄ ⁺和降低的 NO₃ ^-库存。黑潮-SCS 锋区较高的 NH ₄ ⁺浓度与先前观察到的黑潮衍生的溶解有机物的微生物消耗增加一致。观察到垂直 NH ₄ ⁺分布的两种模式：具有地下 NH ₄ ⁺最大值（30.1-241 nmol L ^-1）的剖面，通常发生在 50-100 m 的深度，与深层叶绿素最大值和亚硝酸盐最大值密切相关，以及具有低且大体均匀的 NH ₄ ⁺浓度随深度分布的剖面。NH ₄ ⁺的深度最大似乎受透光深度、硝基胺的位置以及浮游植物和硝化生物之间对 NH ₄ ⁺的亲和力差异的控制。我们证明，精确测量纳摩尔级的 NH ₄ ⁺浓度有助于提高对海洋氮生物地球化学的理解。