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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-342
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-342
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 02 Sep 2019

Submitted as: research article | 02 Sep 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Quantifying the contributions of riverine vs. oceanic nitrogen to hypoxia in the East China Sea

Fabian Große1,2, Katja Fennel1, Haiyan Zhang1,3, and Arnaud Laurent1 Fabian Große et al.
  • 1Department of Oceanography, Dalhousie University, Halifax, NS, Canada
  • 2Department of Mathematics and Statistics, University of Strathclyde, Glasgow, United Kingdom
  • 3School of Marine Science and Technology, Tianjin University, Tianjin, China

Abstract. In the East China Sea, hypoxia (oxygen ≤ 62.5 mmol m−3) is frequently observed off the Changjiang (or Yangtze) River estuary covering up to about 15,000 km2. The Changjiang River is a major contributor to hypoxia formation because it discharges large amounts of freshwater and nutrients into the region. However, modelling and observational studies have suggested that intrusions of nutrient-rich oceanic water from the Kuroshio also contribute to hypoxia formation. The relative contributions of riverine versus oceanic nutrient sources to hypoxia have not been estimated before. Here, we combine a three-dimensional, physical-biogeochemical model with an element tracing method to quantify the relative contributions of nitrogen from different riverine and oceanic sources to hypoxia formation during 2008–2013. Our results suggest that the hypoxic region north of 30° N is dominated by Changjiang River inputs, with its nitrogen loads supporting 74 % of oxygen consumption. South of 30° N, oceanic nitrogen sources become more important supporting 39 % of oxygen consumption during the hypoxic season, but the Changjiang River remains the main control of hypoxia formation also in this region. Model scenarios with reduced Changjiang River nitrogen loads and reduced open-ocean oxygen levels suggest that nitrogen load reductions can significantly reduce hypoxia in the East China Sea and counteract a potential future decline in oxygen supply from the open ocean into the region.

Fabian Große et al.
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Status: open (until 14 Oct 2019)
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Fabian Große et al.
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Short summary
Low oxygen conditions negatively affect marine life and can occur due to strong growth of algae, which die and sink while being degraded consuming oxygen. In the East China Sea, both the Changjiang River and oceanic nutrient sources are considered relevant. We use a computer-based model to quantify the contributions of the different nutrient sources to the formation of low oxygen conditions. We find the Changjiang is the most important and river load reductions can improve oxygen conditions.
Low oxygen conditions negatively affect marine life and can occur due to strong growth of algae,...
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