Program     Nitrogen cycling in the ocean: From genes to ecosystems and from the past to the future       Poster The transformation processes of ammonium at the Pearl River Estuary in summer P-M5-04-S Ling Chen* , State Key Laboratory of Marine Environmental Science, Xiamen University Jing Liu, State Key Laboratory of Marine Environmental Science Shuh-ji Kao, State Key Laboratory of Marine Environmental Science Presenter Email: chenling522@stu.xmu.edu.cn Nitrification is a two-step process whereby ammonium (NH4+) is oxidized to nitrite (NO2-) and then to nitrate (NO3-). It is an essential process in nitrogen cycle for it helps to regulate the overall distributions of different nitrogen species. Besides, nitrification would contribute to hypoxia development in coastal oceans. NH4+ is the substrate for nitrification, meanwhile, phytoplankton also use NH4+ for assimilation even in completely dark condition. NH4+ is the highest concentration of nutrients in the Pearl River Estuary (PRE), in order to understand the transformation processes in dark condition,we determined the bulk nitrification rate (NRb) and NH4+ uptake rate (UR) along the PRE, especially for hypoxia zone in July 2017 by nitrogen isotope tracer technique. The NRb of section A ranged from undetectable up to 2.3 umol L-1 d-1, peaking at a salinity of 0.1. As for section 01, the maximum value (0.8 umol L-1 d-1) of NRb occurred at 1F701, which owns the highest concentration of NH4+. UR was almost higher than NRb in the surface, while UR was lower than NRb in the bottom, indicating that NH4+ is mainly absorbed by phytoplankton for assimilation in the surface even in completely dark condition, however, as for bottom water, nitrifier can compete with phytoplankton and make use of NH4+ for nitrification. Through principal component analysis (PCA), we found both NRb and UR presents positively correlation with NH4+ concentration, suggesting the importance of NH4+ in the transformation processes of nitrogen in the Pearl River Estuary.

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