Program     Nitrogen cycling in the ocean: From genes to ecosystems and from the past to the future       Poster Nitrogen cycling in the Jiulong River Estuary evidenced from the stable isotopes ratios of dissolved inorganic nitrogen P-M5-10-S Dan Yu* , College of the Environment and Ecology, Xiamen University Julie Granger, Department of Marine Sciences, University of Connecticut, USA Craig R. Tobias, Department of Marine Sciences, University of Connecticut, USA Huasheng Hong, College of the Environment and Ecology; Fujian Provincial Key Laboratory for Coastal Ecology and Environmental Studies;State Key Laboratory of Marine Environmental Science, Xiamen University Nengwang Chen, College of the Environment and Ecology; Fujian Provincial Key Laboratory for Coastal Ecology and Environmental Studies;State Key Laboratory of Marine Environmental Science, Xiamen University Presenter Email: yudan417@163.com The Jiulong River is a eutrophied system that discharges into a macro-tidal estuary in the west of the Taiwan Strait. Inorganic nitrogen species in the river (nitrate, nitrite, and ammonium) occur at relatively elevated concentrations in the upper estuary especially in spring and mix non-conservatively with lower concentration marine end-members. To better understand the cycling and fate of dissolved inorganic nitrogen in the Jiulong River estuary, we sampled surface water and measured the naturally occurring nitrogen (and oxygen) isotope ratios of nitrate, nitrite, and ammonium with concentrations along the river-estuary-bay transect in April 2018. Net production of nitrate and nitrite in the estuary was suggested by non-conservative mixing. Relatively low δ15N values of the added nitrate and nitrite are consistent with production by nitrification. The net removal of ammonium in the estuary was apparent from non-conservative mixing, in tandem with a δ15N increase (0.08-4.8 ‰ deviated from the mixing curve), consistent with isotopic discrimination during ammonium oxidation. In all, N cycling in the estuary during our cruise was dominated by nitrification.

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