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 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.