Tidal variability of nutrients in a coastal coral reef system influenced by groundwater

Wang, Guizhi; Wang, Shuling; Wang, Zhangyong; Jing, Wenping; Xu, Yi; Zhang, Zhouling; Tan, Ehui; Dai, Minhan

doi:https://doi.org/10.5194/bg-15-997-2018

Articles | Volume 15, issue 4

https://doi.org/10.5194/bg-15-997-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-15-997-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 15, issue 4

Research article

|

20 Feb 2018

Research article |

| 20 Feb 2018

Tidal variability of nutrients in a coastal coral reef system influenced by groundwater

Guizhi Wang, Shuling Wang, Zhangyong Wang, Wenping Jing, Yi Xu, Zhouling Zhang, Ehui Tan, and Minhan Dai

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Final revised paper (published on 20 Feb 2018)
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Preprint (discussion started on 08 May 2017)
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Status: closed

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

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RC1: 'bg-2017-156 Submitted on 21 Apr 2017 Tidal variability of nutrients in a coastal coral reef system influenced by groundwater Guizhi Wang, Shuling Wang, Zhangyong Wang, Yi Xu, Zhouling Zhang, Ehui Tan, and Minhan Dai', Anonymous Referee #1, 01 Jun 2017
RC2: 'Review of the paper: Tidal variability of nutrients in a coastal coral reef system influenced by groundwater by Guizhi Wang and co-authors', Anonymous Referee #2, 26 Jul 2017
AC1: 'Response to comments', Guizhi Wang, 25 Aug 2017

Peer-review completion

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

ED: Reconsider after major revisions (27 Aug 2017) by S. Wajih A. Naqvi

AR by Guizhi Wang on behalf of the Authors (20 Sep 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (30 Sep 2017) by S. Wajih A. Naqvi

RR by Anonymous Referee #1 (17 Oct 2017)

RR by Anonymous Referee #2 (24 Oct 2017)

Suggestions for revision or reasons for rejection

Comments:
The authors conclude two key points: nutrient concentrations at station CT reflected the mixing of nitrite-deplete, nitrate and phosphate-rich less saline groundwater and nitrite-rich, nitrate and phosphate-deplete saline offshore seawater and quantified variations in oxidized nitrogen and phosphate contributed by biological processes. This manuscript addresses the small scale effects of groundwater discharge on nutrient loads in Sanya Bay and should be very important on understanding nutrient variability. However, I am not sure whether because I don’t read the other published and submitted papers closely related to this manuscript, I still have some questions for this revision.

In the authors’ response, they mentioned that “at Station CT, which is about 30 m away from the coast with water depth of 0.7-2.1 m, the groundwater discharge is from both the seabed and the coast”. If the groundwater discharge is non-point source, the groundwater discharge can be recognized in the bay or just at station CT? In addition, water masses mixing besides groundwater discharge exists in the bay?

Based on the relationship between nutrients and salinity, the authors tried to estimate the effects of biological processes. The most important is to confirm the two end members represent the groundwater discharge and offshore seawater.

Other comments:
P6, Discussion 4.1: The authors concluded that “the Sanya River plume affected the northeast of the bay with little impact on Station CT and the only source of freshwater at this site in February would be groundwater discharge”, I cannot understand how this conclusion comes out and what is the evidence, and why the minimum salinity scale for station CT (Figure 3) is lower than Sanya bay (Figure 1). The latter should cover station CT.

P8, Discussion 4.2: The authors contribute the correlation relationship between nutrients and salinity to “mixing between the groundwater discharge and the offshore seawater”. Why it is not mixing of different water masses? Thus, the two assumptions for estimate generation and consumption of NOx and phosphate need evidence! Even if we take these assumptions, the calculated NO3bio and Pbio contributed by biological processes should have similar ratios during both spring and neap tides. However, the fact is that “the relationship between NOxbio and Pbio during the spring tide differed from that during the neap tide”, which is unbelievable. Why primary producers assimilate NOx and phosphate with different ratios from the offshore seawater and groundwater discharge or the other sources. I guess the main problem is that both NO2 and NO3 concentrations were low and ammonium may be an important composition and influence the N/P ratio. But the authors don’t consider the contribution of ammonium.

P7, Lines 2-6, I totally lost. The authors mentioned “The daily maximum concentration of NOx, phosphate, and silicate appeared in the day time at relatively low tides……. During the neap tide, ……The daily maximum concentration of NOx and phosphate appeared around the mid-night, when a flood tide appeared”. Then the authors conclude that “This pattern reflected dominance of biological processes”. What “this pattern” means? and why?

P5, Please explain why nutrient variabilities were different, such as nitrite; nitrate and phosphate; silicate, which derived from different sources or others?

P7, line 2, compared with “offshore water”. You mean offshore seawater?

The authors should pay much attention to the use of significant digit. For example, “The NOx:P ratio varied from 4.78 to 12.94 in the spring-neap tide”.

Figure 1, the color scale is not clear. In addition, it is better to show water depth and current.

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ED: Reconsider after major revisions (02 Nov 2017) by S. Wajih A. Naqvi

AR by Guizhi Wang on behalf of the Authors (03 Dec 2017) Manuscript

ED: Referee Nomination & Report Request started (08 Dec 2017) by S. Wajih A. Naqvi

RR by Anonymous Referee #1 (03 Jan 2018)

ED: Publish subject to technical corrections (15 Jan 2018) by S. Wajih A. Naqvi

AR by Guizhi Wang on behalf of the Authors (16 Jan 2018) Author's response Manuscript

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Short summary

Time-series observations of nutrients and ²²⁸Ra, a groundwater discharge tracer, were carried out from spring to neap tide in the Luhuitou fringing reef at Sanya Bay in the South China Sea. Nitrate, phosphate, and silicate in the water column showed greater diurnal variation during the spring tide. Biological processes predominantly controlled the composition of nutrients, but there was less of an impact in the spring tide due to groundwater discharge in this reef system.