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

Große, Fabian; Fennel, Katja; Zhang, Haiyan; Laurent, Arnaud

doi:https://doi.org/10.5194/bg-17-2701-2020

Articles | Volume 17, issue 10

https://doi.org/10.5194/bg-17-2701-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Ocean deoxygenation: drivers and consequences – past, present...

https://doi.org/10.5194/bg-17-2701-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 10

Research article

|

19 May 2020

Research article |

| 19 May 2020

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

Fabian Große, Katja Fennel, Haiyan Zhang, and Arnaud Laurent

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Final revised paper (published on 19 May 2020)
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Preprint (discussion started on 02 Sep 2019)
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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Major revision', Hagen Radtke, 08 Oct 2019
- AC1: 'Response to review', Fabian Große, 30 Oct 2019
- AC2: 'Response to review - corrected version', Fabian Große, 30 Oct 2019
RC2: 'comments', Anonymous Referee #2, 24 Oct 2019
- AC1: 'Response to review', Fabian Große, 30 Oct 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (15 Nov 2019) by Marilaure Grégoire

AR by Fabian Große on behalf of the Authors (08 Dec 2019) Author's response

ED: Referee Nomination & Report Request started (15 Jan 2020) by Marilaure Grégoire

RR by Hagen Radtke (14 Feb 2020)

RR by Anonymous Referee #2 (27 Feb 2020)

Suggestions for revision or reasons for rejection

Thank the authors for their effort on improving this manuscript. I carefully read the response notes and revised manuscript and agree that the authors clarified some points in this revision. However, there are still some unclear and even wrong points. Following are my notes as I read revised manuscript (line number is from the manuscript with modification marked).

1. (line 20): should be m^3 yr^-1.
2. (lines 29-32): The authors cited Bian et al. (2013) to relate seasonal variation of Kuroshio intrusion and monsoon winds. However, this relation is wrong. Please read carefully the papers by Yang et al. (2011, 2012) and Guo et al. (2006). The authors cited these papers but failed to understand their conclusion on seasonal variation in Kuroshio intrusion: change in density field is important while that in local wind is not as important as the authors described here (actually, not only here. The authors cited this relation many times in this manuscript). Following papers are also helpful for the authors to understand Kuroshio water intrusion into the East China Sea. Please note the Kuroshio intrusion is strong from autumn to winter but weak in summer.
Yang, et al. (2018). Topographic beta spiral and onshore intrusion of the Kuroshio Current. Geophysical Research Letters,45, 287–296.
Oey, L. Y., Hsin, Y. C., & Wu, C. R. (2010). Why does the Kuroshio northeast of Taiwan shift shelf ward in winter? Ocean Dynamics, 60(2), 413–426.
3. (line 61): Does the model include oceanic dissolved organic matter? If not, why? If the influence from rivers includes DON’s effect but that from open ocean does not, the compassion for role of river and oceanic water is unfair.
4. (line 66): what is evidence for 75%?
5. (line 67): what is evidence for 115:4?
6. (line 76-77): what is physical background for 7 and 10 days? Why should they be different?
7. (line 80): not 11 rivers.
8. (line 81-83): I cannot understand the relation between each state variable here and TN given in Fig. 2.
9. (line 85): what are evidences for these values?
10. (line 90-91): what is background for reduction in only N load. Since you also include phosphate as state variable, reduction in only N load will change N/P ratio in the region affected by the Changjiang River in the model calculation, which is however not a realistic situation because phosphate load also changes if N load changes.
11. (line 95): the same question is also for 20% O2 reduction case. What is reason for O2 reduction in a warming situation? If it is the biogeochemical process, reduction in O2 concentration and increasing in nutrient concentration occur together. Therefore, it is not reasonable to reduce only O2 concentration but keep the same nutrient concentration in oceanic water.
12. (Line 118): what is the exact way to diagnose the N tracer? Does this means that you did not solve the equation for each source of nitrate (Equation 2))?
13. (line 118): I am wondering the separation here will cause some artificial problem in the area where the phosphate, not nitrate, is a limiting element for phytoplankton growth. Can the authors add some words to address this concern? As we know, the offshore area of Changjiang River has a very high N/P ratio.
14. (Figure 5): As I comment before, the anomaly of winds speed is not very large. The authors emphasize the positive anomaly of wind speed on August 2013 when there is no contribution of Changjiang River. However, this positive anomaly plus the climatology on August 2013 is not larger than wind speed on July 2013. However, we can find contribution of Changjiang River on July 2013 in this figure. Therefore, disappear of contribution from Changjiang River is not necessarily caused by local winds. Another question for Fig. 5 is what is the area for averaging the meridional wind speed.
15. (line 189-190): Again, I have to say that the relation between wind and movement of Kuroshio and Taiwan Strait water given here is wrong.
16. (line 221): I would like to suggest the authors to present the same Fig. 5 for this case (50% reduction of N load). The comparison between two cases is very helpful to understand the role of Changjiang source of nitrate.
17. (line 250): resuspension in this area is largely attributed to strong tidal currents. The authors can easily confirm this from satellite images for suspended matter (e.g., Shi et al., 2011). There is always high concentration of suspended matter in this area although the wind wave is not always high. In addition, Shi et al. (2011) also demonstrated a strong neap and spring tidal cycle in suspended matter in this area.

Shi, W., M. Wang, and L. Jiang (2011), Spring-neap tidal effects on satellite ocean color observations in the Bohai Sea, Yellow Sea, and East China Sea, J. Geophys. Res.,116, C12032, doi:10.1029/2011JC007234.

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ED: Reconsider after major revisions (10 Mar 2020) by Marilaure Grégoire

ED: Reconsider after major revisions (11 Mar 2020) by Marilaure Grégoire (Co-editor-in-chief)

AR by Fabian Große on behalf of the Authors (31 Mar 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 Apr 2020) by Marilaure Grégoire

RR by Anonymous Referee #2 (17 Apr 2020)

ED: Publish as is (22 Apr 2020) by Marilaure Grégoire

ED: Publish as is (22 Apr 2020) by Marilaure Grégoire (Co-editor-in-chief)

AR by Fabian Große on behalf of the Authors (23 Apr 2020) Author's response Manuscript

Short summary

In the East China Sea, hypoxia occurs frequently from spring to fall due to high primary production and subsequent decomposition of organic matter. Nitrogen inputs from the Changjiang and the open ocean have been suggested to contribute to hypoxia formation. We used a numerical modelling approach to quantify the relative contributions of these nitrogen sources. We found that the Changjiang dominates, which suggests that nitrogen management in the watershed would improve oxygen conditions.