INT-02 Marine Nitrogen Cycle
Diel change in inorganic nitrogenous nutrient dynamics and associated oxygen stoichiometry along the Pearl River Estuary
Min Xu* , State Key Laboratory of Marine Resources Utilization in South China Sea, Hainan University, Haikou 570228, China
Yanhua Wu, Shenzhen Marine Environment Monitoring Center Station, State Oceanic Administration, Shenzhen 518067, China
Xiao Zhang, State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, 361102, China
Jin-Ming Tang, State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, 361102, China
Ehui Tan, State Key Laboratory of Marine Resources Utilization in South China Sea, Hainan University, Haikou 570228, China
Zhen-Zhen Zheng, State Key Laboratory of Marine Resources Utilization in South China Sea, Hainan University, Haikou 570228, China
Moge Du, State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, 361102, China
Xiuli Yan, Marine Science Institute and Guangdong Provincial Key Laboratory of Marine Biotechnology College of Science, Shantou University, Shantou, China, 515063
Shuh-Ji Kao, State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen, 361102, China

The reactive nitrogen (N) emitted from continents significantly perturbs the pristine N cycle around the land-ocean boundary resulting in eutrophication and hypoxia. As nutrients are transported downstream through an estuary, various types of biological processes co-occur to modulate nitrogen speciation to influence the biogeochemical habitats for downstream microorganisms. We surveyed the Pearl River Estuary to examine the N transfer dynamics among nitrogen species with considering process-specific oxygen production and consumption. By using 15N pulse-tracing techniques, we measured ammonia oxidation and uptakes of ammonium, nitrite, and nitrate simultaneously under dark and light conditions in parallel. Light strongly inhibited nitrification but enhanced N uptake, and such light effect was further considered in the calculation for nitrogen transformation rates over a diel cycle. We found both oxidation and uptake of ammonium decreased seaward as substrate decreased. The nitrifier and phytoplankton work in the antiphase to draw down incoming ammonium rapidly. Contrary to ammonium uptake, the uptake of nitrite and nitrate showed a seaward increasing pattern. Such an inverse spatial pattern implies a shift in N preference for phytoplankton. Such high ammonium preference inhibits nitrate/nitrite uptake allowing them to behave conservatively in the estuary and to travel farther to the outer estuary. By integrating oxygen consumption and production induced by N transformation processes over the diel cycle, oxygen was produced although allochthonous ammonium input is high (~250 μM). For most stations, ammonium was completely consumed within 2 days, some stations even less than 0.5 days, implying that although the water residence time is short (2-15 d), tremendous input of ammonium N from upstream was transformed into particulate organic or nitrate forms during traveling to modulate the biogeochemical niche, including the substrate, organics, and oxygen, of coastal microbes in water column and sediments.