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

Yifan Zhu, Old Dominion University, Ocean, Earth and Ocean Sciences, Norfolk, United States, Minhan Dai, Xiamen University, State Key Laboratory of Marine Environmental Science, Xiamen, China, Dennis Joseph McGillicuddy Jr, Woods Hole Oeanographic Institution, Woods Hole, MA, United States, Margaret R Mullholland, Old Dominion University, Ocean and Earth Sciences, Norfolk, VA, United States, Phoebe Dreux Chappell, Old Dominion University, Ocean, Earth and Atmospheric Sciences, Norfolk, United States and Sophie Clayton, National Oceanography Centre, Southampton, United Kingdom
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.