Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
doi:10.5194/bg-2017-43
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
22 Feb 2017
Review status
This discussion paper is under review for the journal Biogeosciences (BG).
Tracing the origin of the oxygen-consuming organic matter in the hypoxic zone in a large eutrophic estuary: the lower reach of the Pearl River Estuary, China
Jianzhong Su1, Minhan Dai1, Biyan He1,2, Lifang Wang1, Jianping Gan3, Xianghui Guo1, Huade Zhao1, and Fengling Yu1 1State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
2College of Food and Biological Engineering, Jimei University, Xiamen, China
3Department of Mathematics and Division of Environment, Hong Kong University of Science and Technology, Kowloon, Hong Kong, China
Abstract. We assess the relative contributions of differently sourced organic matter, marine vs. terrestrial, to oxygen consumption in an emerging hypoxic zone in the lower Pearl River Estuary (PRE), a large eutrophic estuary located in Southern China. Our cruise, conducted in July 2014, contained two legs before and after a passing through of Typhoon Rammasun, which destroyed the water stratification. The stratification recovered rather quickly within one day after the typhoon. We observed, in both legs, algal blooms in the surface water with hypoxias underneath in the bottom water. The repeated samplings at the initial hypoxic station showed severe oxygen depletion down to 30.3 μmol kg−1 before the typhoon and a clear drawdown of dissolved oxygen after the typhoon. Based on a three end-member mixing model and the mass balance of dissolved inorganic carbon and its isotopic composition, we derived the δ13C of the remineralized organic carbon as −21.8±0.6 ‰ in the hypoxia. We estimated that 73±10 % of the oxygen-consuming organic matter was marine sourced and, the rest 27±10 % was terrestrially sourced. Contrasting to the hypoxic site in the East China Sea off the Changjiang Estuary where eutrophication-stimulated marine sourced organic matter prevailed the oxygen consumption, the terrestrially sourced organic matter significantly contributed to the formation and maintenance of the hypoxia in the present study site. The difference in oxygen sinks in different systems has important implications in better understanding to the controls of hypoxias and their mitigations.

Citation: Su, J., Dai, M., He, B., Wang, L., Gan, J., Guo, X., Zhao, H., and Yu, F.: Tracing the origin of the oxygen-consuming organic matter in the hypoxic zone in a large eutrophic estuary: the lower reach of the Pearl River Estuary, China, Biogeosciences Discuss., doi:10.5194/bg-2017-43, in review, 2017.
Jianzhong Su et al.
Jianzhong Su et al.
Jianzhong Su et al.

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Short summary
We provide direct and quantitative assessments showing the marine organic matter from eutrophication-induced primary production dominated oxygen consumption in the hypoxic zone while the terrestrially sourced organic matter also significantly contributed to the formation and maintenance of hypoxia in the lower Pearl River Estuary (PRE) and the adjacent coastal water.
We provide direct and quantitative assessments showing the marine organic matter from...
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