Dynamics of ammonium biogeochemistry in an oligotrophic regime in the South China Sea

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 inventory upper 100 m showed considerable variation between two samplings, averaging 3.46 mmol m−2 almost doubling This could be attributed seasonality or more likely influence Kuroshio Current intrusion, prominent within El Niño years such as Kuroshio-influenced stations exhibited elevated decreased NO3− inventories. Higher at Kuroshio-SCS frontal zone consistent with prior observations enhanced microbial consumption Kuroshio-derived dissolved organic matter. Two patterns vertical distributions observed: profiles subsurface maximum (30.1–241 L−1), usually occurring depths 50–100 that closely coupled deep chlorophyll nitrite maximum, low generally uniform depth. depth appears controlled by euphotic depth, position nitracline, well difference affinity phytoplankton nitrifiers. We demonstrated precise measurements nanomolar levels facilitate improved understanding oceanic nitrogen biogeochemistry.