Evaluation of the seasonal formation of subsurface negative preformed nitrate anomalies in the subtropical North Pacific and North Atlantic

Letscher, Robert T.; Villareal, Tracy A.

doi:https://doi.org/10.5194/bg-15-6461-2018

Articles | Volume 15, issue 21

https://doi.org/10.5194/bg-15-6461-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-15-6461-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 21

Research article

|

02 Nov 2018

Research article |

| 02 Nov 2018

Evaluation of the seasonal formation of subsurface negative preformed nitrate anomalies in the subtropical North Pacific and North Atlantic

Robert T. Letscher and Tracy A. Villareal

Download

Final revised paper (published on 02 Nov 2018)
Supplement to the final revised paper
Preprint (discussion started on 09 Apr 2018)
Supplement to the preprint

Interactive discussion

Status: closed

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

- Printer-friendly version

- Supplement

RC1: 'nutrient biogeochemistry', Anonymous Referee #1, 07 May 2018
- AC1: 'Response to Anonymous Referee #1', Robert Letscher, 12 Jul 2018
RC2: 'Negative preformed nitrate and the speculative role of migrating phytoplankton', Anonymous Referee #3, 19 Jun 2018
- AC2: 'Response to Anonymous Referee #3', Robert Letscher, 12 Jul 2018

Peer-review completion

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

ED: Reconsider after major revisions (24 Jul 2018) by Manmohan Sarin

AR by Robert Letscher on behalf of the Authors (25 Jul 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (31 Jul 2018) by Manmohan Sarin

RR by Anonymous Referee #4 (17 Sep 2018)

Suggestions for revision or reasons for rejection

Review- Letscher and Villareal: Evaluation of the seasonal subsurface negative preformed nitrate anomalies in the subtropical North Pacific and North Atlantic

Letscher and Villareal have analyzed pre-formed NO3 signals from the HOT and BATS site to try to explain subsurface negative anomalies at both sites, as well as positive anomalies in the euphotic zone. Additionally, they have a goal of trying to explain the summertime seasonal DIC drawdown that is detected at these sites that happens when little nutrients are present in the system. In their analysis they decompose the preformed NO3 signal into that which comes from non-Redfield remineralization of dissolved and particulate organic matter, this calculation is named the residual pre-NO3. Thus, residual pre-NO3 is pre-NO3 not attributable to non-Redfield-O2:N DOM remineralization dynamics. The non-Redfield dynamics that could be responsible for contributing to the residual pre-NO3 signal are listed as 1) bacterial uptake of NO3 to mineralize N-poor organic matter, 2) biological production, export, and remineralization of N-deficient transparent exopolymer particles (TEP) and 3) vertical migration of phytoplankton between the euphotic zone and the nutricline and the accompanying transport of NO3 between the latter and former parts of the water column. The authors argue the bacterial uptake of NO3 is a minor component, TEP remineralization can account for up to 50% of the residual pre-NO3 signal and vertically migrating phytoplankton account for 50 – 90% of the signal. They also conclude that vertically migrating phytoplankton can transport enough NO3 from the nutricline to the euphotic zone to account for observed DIC drawdown.

Overall, I find this paper very difficult to read. It is overly cumbersome and difficult to follow. I understand the topic of pre-formed NO3 is not intuitive but creating new terms such as residual pre-NO3 and negative and positive residual pre-NO3 make the topic even more difficult to understand. As such the authors need to be extremely clear as to what each term represents and how each term is used. Perhaps some conceptual diagram is needed. I spent a lot of time going back through the paper trying to re-learn what the residuals represent. Below are listed concerns I have with the manuscript in its present form:

1) It is unclear what the motivation of the study is. I thought I understood that the seasonal drawdown of DIC in the absence nutrients would be the motivation, as the intro started with this. However, this was a small part of the paper in the end. Instead the majority of the paper was about explaining negative and positive residual pre-NO3 signals, but I am unsure why this is important. It is not explained clearly and should be.

2) I am also having difficulty in understanding the term residual pre-NO3. To me this is a new calculation of pre-NO3 that specifically accounts for remineralization DOM and POM, as well as non-Redfield stoichiometry. So this is the NO3 present in the system not accounted for by non-Redfield remineralization of organic matter, where as standard pre-NO3 assumes Redfield stoichiometry and does not decompose remineralization into DOM and POM. I do not understand where the term residual comes in. This is just pre-formed NO3 calculated in a new way isn’t it? Residual pre-NO3 implies there is some component of pre-formed NO3 that is left over, yet the new calculation is defining all the preformed NO3 as residual. What am I missing?

3) Another major concern I have with this paper is that the contribution of vertically migrating phytoplankton to the positive and negative residual pre-NO3 signals is determined via difference. Essentially, the residual pre-NO3 is calculated, the contribution of both TEP and bacteria is accounted for and then the remainder is stated to be from vertically migrating phytoplankton. I am not sure why a calculation has not been made that shows phytoplankton can account for these residual pre-NO3 signals. Some type of mass balance estimate is required that shows phytoplankton can move this much NO3 between the upper mesopelagic and the euphotic zone.

4) Shouldn’t recycled production be accounted for in the positive residual pre-NO3 (rPPN) calculation? The rPPN is O2 production without the expected (non-Redfield) drawdown of NO3. In the euphotic zone of both environments analyzed here there is little NO3 and productivity is primarily recycled (or NH4 or organic N based). Thus, O2 can be produced without any drawdown of NO3, shouldn’t this be included somehow?

5) Can significant N2 fixation in these regions affect the residual pre-NO3 signals? Wouldn’t this be similar to cells introducing N to the euphotic zone through vertical migration? N2 fixation has been used to explain DIC drawdown in oligotrophic regions as well, how does it compare to vertical migration.

6) Lastly, I do not find the pre-PO4 part of the paper adding much. Likewise, the DIC drawdown discussion is small and I find this to be potentially the most significant part of the paper. I suggest nixing both parts unless they are expanded and made more central to the paper’s objectives (which I state above need clarification).

Hide

ED: Reconsider after major revisions (26 Sep 2018) by Manmohan Sarin

AR by Robert Letscher on behalf of the Authors (02 Oct 2018) Author's response Manuscript

ED: Publish as is (09 Oct 2018) by Manmohan Sarin

AR by Robert Letscher on behalf of the Authors (17 Oct 2018)

Download

Article (4487 KB)
Full-text XML

Short summary

The formation rates of oxygen to nitrogen anomalies in the subtropical North Pacific and North Atlantic were estimated from time series data. We find that vertically migrating phytoplankton, which traverse ~ 100–150 m in the upper ocean over days to acquire nutrients from waters at depth and return to the surface for photosynthesis, likely explain the observed anomalies and help sustain surface ocean productivity and the biological pump throughout the annual cycle in the subtropical ocean.