Program     Ocean Circulation, Ecosystem and Hypoxia around Hong Kong Waters       Poster Dynamics of ammonium in the Pearl River Estuary under summer hypoxic conditions P-SPS4-02-S Jing Liu* , State Key Laboratory of Marine Science Yangyang Zhao, State Key Laboratory of Marine Science Tao Huang, State Key Laboratory of Marine Science Lifang Wang, State Key Laboratory of Marine Science Zhongwei Yuan, Minhan Dai, State Key Laboratory of Marine Science Presenter Email: LiuJing@st.,xmu.edu.cn We examine ammonium dynamics in the Pearl River Estuary (PRE), a highly eutrophic system where extensive hypoxia is emerging in its lower estuary in the summer time. A total of 312 water samples for ammonium were collected from the PRE in July 2017 when an extensive area in the lower PRE was under hypoxic conditions. Ammonium was measured on board by using the indophenol blue colorimetric method at mM level concentrations or by using a more sensitive fluorescence method with preconcentration was at nM concentration levels. Ammonium concentration ranged from 37 mM in the upper estuary down to 100nM~2 mM in the lower estuary, and to 35 nM~300 nM in the offshore area. Our data showed that ammonium was overall non-conservative during the estuarine mixing showing rapid removal in the upper estuary due probably to nitrification but significant additions in the lower estuary around Shenzhen, Hong Kong and Macau due likely to the sewage discharge. Such ammonium additions were also obvious in the subsurface and bottom waters where we found consistently high concentrations of total suspend matter suggesting that degradation of organic particles is a major source of ammonium therein. We adopted a multiple end-members model to further characterize the ammonium dynamics in different regimes of the lower estuary. Ammonium was slightly removed in the surface layer (0.8±0.3 mmol/kg) where nitrate was much more profoundly removed (28±5 mmol/kg), indicating that nitrate was the major nitrogen species fueled the phytoplankton bloom. Ammonium was slightly added in the hypoxic bottom water (<0.5 mmol/kg) where nitrate was much more significantly regenerated (15±5 mmol/kg). This inferred that nitrification contributed to the oxygen consumption in the hypoxic zone. However, the ratio between oxygen consumption vs inorganic nitrogen production amounted to 25±5.1, which was much higher than the Redfield stoichiometry, indicating nitrogen losses relative to the oxygen consumption, the exact reason of which remains however unknown. We contend that ammonia dynamics in the hypoxic zone is much more complex than what was previously thought.