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We constraint the nitrogen budget and in particular the internal sources and sinks of nitrate in the Bohai Sea by using a mass-based and dual stable-isotope approach based on δ15N and δ18O of nitrate. Based on the available mass fluxes and isotope data an updated nitrogen budget is proposed. Compared to previous estimates, it is more complete and includes the impact of interior cycling (nitrification) on the nitrate pool. The main external nitrogen sources are rivers contributing 17.5 %–20.6 %.
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https://doi.org/10.5194/bg-2020-471
https://doi.org/10.5194/bg-2020-471

  21 Dec 2020

21 Dec 2020

Review status: this preprint is currently under review for the journal BG.

A reactive nitrogen budget of the Bohai Sea based on an isotope mass balance model

Shichao Tian1, Birgit Gaye1, Jianhui Tang2, Yongming Luo2,3, Wenguo Li4, Niko Lahajnar1, Kirstin Dähnke5, Tina Sanders5, Tianqi Xiong6, Weidong Zhai6, and Kay-Christian Emeis1,5 Shichao Tian et al.
  • 1Institute for Geology, Universität Hamburg, Hamburg, 20146, Germany
  • 2Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai, China
  • 3Institute of Soil Science, Chinese Academy of Siences, Nanjing, China
  • 4Institute of Oceanography, Universität Hamburg, Hamburg, 20146, Germany
  • 5Helmholtz-Zentrum Geesthacht (HZG), Institute for Coastal Research, Geesthacht, 21502, Germany
  • 6Institute of Marine Science and Technology, Shandong University, Qingdao, China

Abstract. The Bohai Sea is a semi-closed marginal sea impacted by one of the most populated areas of China. The supply of nutrients, markedly that of reactive nitrogen, via fluvial and atmospheric transport has strongly increased in parallel with the growing population. It is therefore crucial to quantify the reactive nitrogen input to the BHS and to understand the processes and determine the quantities of nitrogen eliminated in and exported from the BHS. The nitrogen budget and in particular the internal sources and sinks of nitrate were constrained by using a mass-based and dual stable-isotope approach based on δ15N and δ18O of nitrate. Samples of water, suspended matter and sediments were taken in the BHS in spring (March and April) and summer (July and August) 2018. The Yellow River was sampled in May, July to November and Daliao River, Hai River, Luan River and Xiaoqing River were sampled in November of 2018. In addition to nutrient, particulate organic carbon and nitrogen concentrations, the dual isotopes of nitrate (δ15N and δ18O), δ15N of suspended matters and sediments were determined. Based on the available mass fluxes and isotope data an updated nitrogen budget is proposed. Compared to previous estimates, it is more complete and includes the impact of interior cycling (nitrification) on the nitrate pool. The main nitrogen sources are rivers contributing 17.5 %–20.6 % and the combined terrestrial runoff (including submarine discharge of nitrate with fresh ground water) accounting for 22.6 %–26.5 % of the nitrate input to the BHS while atmospheric input contributes only 6.3 %–7.4 % to total nitrate. An unusually active interior nitrogen cycling contributes 59.1 %–71.2 % to total nitrate via nitrification. Nitrogen is mainly trapped in the BHS and mainly removed by sedimentation (96.4 %–96.9 %) and only very little is exported to the YS (only 1.7 %–2.0 %). At present denitrification is only active in the sediments and removes 1.4 %–1.7 % of nitrate from the pool. A further eutrophication of the BHS could, however, induce water column hypoxia and denitrification as already observed – often seasonally off river mouths – in other marginal seas.

Shichao Tian et al.

 
Status: open (until 11 Feb 2021)
Status: open (until 11 Feb 2021)
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Shichao Tian et al.

Shichao Tian et al.

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Short summary
We constraint the nitrogen budget and in particular the internal sources and sinks of nitrate in the Bohai Sea by using a mass-based and dual stable-isotope approach based on δ15N and δ18O of nitrate. Based on the available mass fluxes and isotope data an updated nitrogen budget is proposed. Compared to previous estimates, it is more complete and includes the impact of interior cycling (nitrification) on the nitrate pool. The main external nitrogen sources are rivers contributing 17.5 %–20.6 %.
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