Ammonium Dynamics in the South China Sea-Kuroshio and the Mid-Atlantic Shelf Break Frontal Zones

Studies have shown that frontal zones can facilitate new production by upwelling nutrients and through enhancement of dinitrogen fixation. This study compared ammonium (NH₄⁺) concentrations from two frontal zones, one in the South China Sea (SCS) where the Kuroshio intrudes and the other at the mid-Atlantic shelf break front, and the end-member water masses. Concentrations of NH₄⁺ in the oligotrophic South China Sea (SCS) were measured onboard ship in summer 2014 and spring 2016, using a sensitive fluorometric technique capable of high precision measurements to concentrations as low as 0.7 nmol L^-1. Concentrations of NH₄⁺ in mid-Atlantic shelf break frontal zone were measured onboard ship in July 2019, using a fluorometric method with detection limit of 10.0 nmol L^-1. Elevated NH₄⁺ inventories were observed at both SCS-Kuroshio and mid-Atlantic Shelf break frontal zones. The higher NH₄⁺ inventories in the SCS were coincident with a previous observation of enhanced microbial consumption of Kuroshio-derived DON. A similar process may be responsible for the high NH₄⁺ inventories in the mid-Atlantic shelf break frontal zone. In addition to elevated surface concentrations of NH₄⁺ in frontal zones, two patterns in vertical NH₄⁺ distributions were observed; subsurface NH₄⁺ maximum (AM), ranging from 30.1-241 nmol L^-1 in the SCS , and usually occurring between 50 and 130 m, closely coupled with the deep chlorophyll maximum (DCM) and the primary nitrite maximum (PNM); and another in which NH₄⁺concentrations were uniformly low throughout the water column. AM varied from 33.7-1108.7 nmol L^-1 in the mid-Atlantic shelf break front in a depth range of 30-60 m, overlapped with the chlorophyll maximum. Depths of the euphotic zone and nitracline as well as the discrepancy between phytoplankton and nitrifiers in their affinities for NH₄⁺ may be the dominant factors in creating niches for the AM and PNM in the SCS.