Abstract
A cruise covering two transects in the Changjiang (Yangtze) estuary in July 2017 was conducted, aiming to explore the sources for riverine \(\rm{NO}_3^-\) and identify reactions involved in the \(\rm{NO}_3^-\) transformations along the transport of the Changjiang diluted water (CDW). In the river water, \(\rm{NO}_3^-\) was fundamentally contributed by chemical fertilizer leakage in the watershed according to isotope signals. Sewage discharge may also be significant on riverine \(\rm{NO}_3^-\) inventory, while the isotope signal was masked by nitrification. Together with the transport of the CDW, \(\rm{NO}_3^-\) production was observed in waters with low salinities (<20) and high turbidities. Nitrification resulted from the mineralization of riverine organic nitrogen; therefore, the high turbidity was linked to active production. In the outer plume, coupled with stratification, a significant decrease in \(\rm{NO}_3^-\) concentration was observed in the surface water. In parallel, enrichment in \(\rm{\delta}^{15}N-\rm{NO}_3^-\) and \(\rm{\delta}^{18}N-\rm{NO}_3^-\) was found, indicating biological consumption by phytoplankton. The difference in the stratification intensity between two transects led to variations in \(\rm{NO}_3^-\) concentrations and isotope compositions. In the benthic water, denitrification (sediment-water interface) and nitrification (bottom water) coexisted. Furthermore, accumulations of \(\rm{NH}_4^+\) and dissolved organic nitrogen in the bottom water were observed, indicating that nitrification was constrained by oxidant (mainly dissolved oxygen) supplies.
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Data Availability Statement
The data used in the present study are available from the corresponding author upon request.
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Acknowledgment
Technical support by Prof. GAO Yonghui, Prof. LI Bo, Ms. ZHENG Wei, Dr. ZHU Xunchi, Ms. QI Lijun, Mr. LIU Zhengbo, Ms. CAO Wanwan, Ms. JIANG Shuo, Mr. ZHU Kun, Mr. DAI Jinlong, and the R/V Zheyuke II crew during the cruise are acknowledged. The authors also appreciate the great assistance from Prof. YU Zhiming at the Institute of Oceanology, Chinese Academy of Sciences for the assistance in stable isotope analyses. Thanks are also due to the editor and anonymous reviewers, whose comments helped to improve the original manuscript.
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Supported by the National Natural Science Foundation of China (Nos. 41530960, 41706081) and the Scientific Research Foundation of SKLEC (No. 2017RCDW04)
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Jiang, S., Jin, J., Zhang, G. et al. Nitrate in the Changjiang diluted water: an isotopic evaluation on sources and reaction pathways. J. Ocean. Limnol. 39, 830–845 (2021). https://doi.org/10.1007/s00343-020-0149-8
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DOI: https://doi.org/10.1007/s00343-020-0149-8