Seasonal variability of nitrous oxide concentrations and  emissions in a temperate estuary

Schulz, Gesa; Sanders, Tina; Voynova, Yoana G.; Bange, Hermann W.; Dähnke, Kirstin

doi:https://doi.org/10.5194/bg-20-3229-2023

Articles | Volume 20, issue 15

https://doi.org/10.5194/bg-20-3229-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-20-3229-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 20, issue 15

Research article

|

07 Aug 2023

Research article |

| 07 Aug 2023

Seasonal variability of nitrous oxide concentrations and emissions in a temperate estuary

Gesa Schulz, Tina Sanders, Yoana G. Voynova, Hermann W. Bange, and Kirstin Dähnke

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Final revised paper (published on 07 Aug 2023)
Supplement to the final revised paper
Preprint (discussion started on 17 Feb 2023)
Supplement to the preprint

Interactive discussion

Status: closed

RC1:
'Comment on bg-2023-35', Anonymous Referee #1, 08 Mar 2023

The comment was uploaded in the form of a supplement: https://bg.copernicus.org/preprints/bg-2023-35/bg-2023-35-RC1-supplement.pdf

Citation: https://doi.org/10.5194/bg-2023-35-RC1
- AC1: 'Reply on RC1', Gesa Schulz, 13 Apr 2023
  
  We thank the reviewer for their constructive and helpful comments and suggestions about our manuscript. For a detailed response to the individual issues, please see the attached pdf (for formatting reasons).
  
  Citation: https://doi.org/10.5194/bg-2023-35-AC1
RC2:
'Comment on bg-2023-35', Anonymous Referee #2, 04 Apr 2023

Please see attached pdf

Citation: https://doi.org/10.5194/bg-2023-35-RC2
- AC2: 'Reply on RC2', Gesa Schulz, 13 Apr 2023
  
  We thank the reviewer for their constructive and helpful comments and suggestions about our manuscript. For a detailed response to the individual issues, please see the attached pdf (for formatting reasons).
  
  Citation: https://doi.org/10.5194/bg-2023-35-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (20 Apr 2023) by Jack Middelburg

AR by Gesa Schulz on behalf of the Authors (11 May 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (12 May 2023) by Jack Middelburg

RR by Anonymous Referee #1 (26 May 2023)

Suggestions for revision or reasons for rejection

I want to thank the authors for addressing my last round of comments. For example, the N2O flux and emission have been comprehensively estimated using different parameterizations and wind speeds. However, some of authors’ arguments need to be supported by evidence or the authors need to discuss the caveats in the conclusion instead of making assumptions, e.g., the effect of temperature on N2O production, and N2O discharge from wastewater treatment plants. Here are my additional comments (line number in the modified manuscript).

23: It is not clear how N2O production was enhanced by warmer temperature based on the data presented in this study. Is it due to positive correlation between temperature and N2O saturation shown in Table 4? But not all the datasets had such pattern.

25-26: “N2O saturation did not decrease alongside the decrease in DIN concentration…”

26-27: What does it mean for phytoplankton to reestablish? Were phytoplankton limited before 1990 when the water quality was bad?

61-62: The emission factor (N2O:DIN) derived from the Elbe River could be compared to different types of riverine systems, not only the rivers with high agricultural nutrient inputs. Such comparison would broaden the results.

Figure 1: What does the blue color mean in the figure? How is the section separated? Are they the same as shown in Figure 3 for AOU and excess N2O or for emission calculation?

125: “N2O saturation was calculated…”

155: N2Ocw has already been defined.

158: “an indicator for N2O production from nitrification”

206-209: It may be useful to have a figure to show the spatial distribution of average N2O flux density as shown in Figure 2 for N2O saturation and nutrients.

Section 3.5: It’s clear to have a table to show the correlations between N2O and environmental factors. However, no results were described in this section. How about listing a few key results here such as the positive correlation with nitrite and negative relation to oxygen?

278-279: If the N loading decreased, but N2O concentration or flux kept constant, it may suggest the yield of N2O production increased or the emission factor increased.

Figure 4: Colormap of the relative change of SPM concentration is not shown for (a) and (b).

304: (d) and (f)

305: What does “This” represent?

319-320: Is there any evidence to support this argument (e.g., correlation between PN and N2O)?

334-338: Does the 5% means the fraction of DIN loading from WWTP to the total N load into the Elbe River? Are there any measurements of N2O associated with WWTP discharge or in the WWTP? Previous studies have shown that discharge of WWTPs may be important source of N2O to the aquatic system (e.g., Beaulieu et al., 2010; Chun et al., 2020). The authors may need to acknowledge the possibility of N2O discharge from WWTP instead of rejecting such possibility.

357-358: Why is nitrite favorable for N2O production by nitrification? Low oxygen does not facilitate nitrification but facilitates N2O production from nitrification.

361-364: Not sure how sedimentary denitrification is proved to be an important source of N2O. For instance, do you have depth profiles of N2O to show high N2O concentration near the bottom water?

407: This is a hypothesis but not an assumption: elevated ammonium concentration led to higher N2O production.

408-411: Why is the case (your hypothesis) if temperature is an important driver of N2O production (warmer water likely facilitates N2O production)?

434: Any ideas about the drivers? temperature and oxygen?

456-454: I am trying to understand the decoupling of historical trend of N2O and DIN. Is there any historical change in the concentration of organic matter in the river?

Reference:
Beaulieu, J. J., Shuster, W. D., & Rebholz, J. A. (2010). Nitrous oxide emissions from a large, impounded river: The Ohio River. Environmental Science & Technology, 44(19), 7527-7533.

Chun, Y., Kim, D., Hattori, S., Toyoda, S., Yoshida, N., Huh, J., ... & Park, J. H. (2020). Temperature control on wastewater and downstream nitrous oxide emissions in an urbanized river system. Water Research, 187, 116417.

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RR by Perran Cook (13 Jun 2023)

ED: Publish subject to minor revisions (review by editor) (21 Jun 2023) by Jack Middelburg

AR by Gesa Schulz on behalf of the Authors (30 Jun 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (05 Jul 2023) by Jack Middelburg

AR by Gesa Schulz on behalf of the Authors (05 Jul 2023) Author's response Manuscript

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Short summary

Nitrous oxide (N₂O) is an important greenhouse gas. However, N₂O emissions from estuaries underlie significant uncertainties due to limited data availability and high spatiotemporal variability. We found the Elbe Estuary (Germany) to be a year-round source of N₂O, with the highest emissions in winter along with high nitrogen loads. However, in spring and summer, N₂O emissions did not decrease alongside lower nitrogen loads because organic matter fueled in situ N₂O production along the estuary.