Removal of phosphorus and nitrogen in sediments of the eutrophic Stockholm archipelago, Baltic Sea

van Helmond, Niels A. G. M.; Robertson, Elizabeth K.; Conley, Daniel J.; Hermans, Martijn; Humborg, Christoph; Kubeneck, L. Joëlle; Lenstra, Wytze K.; Slomp, Caroline P.

doi:https://doi.org/10.5194/bg-17-2745-2020

Articles | Volume 17, issue 10

https://doi.org/10.5194/bg-17-2745-2020

© 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-17-2745-2020

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

Articles | Volume 17, issue 10

Research article

|

24 May 2020

Research article |

| 24 May 2020

Removal of phosphorus and nitrogen in sediments of the eutrophic Stockholm archipelago, Baltic Sea

Niels A. G. M. van Helmond, Elizabeth K. Robertson, Daniel J. Conley, Martijn Hermans, Christoph Humborg, L. Joëlle Kubeneck, Wytze K. Lenstra, and Caroline P. Slomp

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Final revised paper (published on 24 May 2020)
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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: 'review', Emil Rydin, 02 Dec 2019
- AC1: 'Reply to Reviewer 1', Niels van Helmond, 29 Jan 2020
RC2: 'Reviewer's comments', Anonymous Referee #2, 30 Dec 2019
- AC2: 'Reply to Reviewer 2', Niels van Helmond, 29 Jan 2020
RC3: 'Interactive comment on “Efficient removal of phosphorus and nitrogen in sediments of the eutrophic Stockholm Archipelago, Baltic Sea” by Niels A.G.M. van Helmond et al.', Anders Stigebrandt, 09 Jan 2020
- AC3: 'Reply to Reviewer 3', Niels van Helmond, 29 Jan 2020

Peer-review completion

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

ED: Reconsider after major revisions (31 Jan 2020) by Hermann Bange

AR by Niels van Helmond on behalf of the Authors (17 Feb 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (18 Feb 2020) by Hermann Bange

RR by Anders Stigebrandt (06 Mar 2020)

Suggestions for revision or reasons for rejection

Review of the revised version of “Removal of phosphorus and nitrogen in sediments of the eutrophic Stockholm Archipelago, Baltic Sea” by Niels A.G.M. van Helmond et al.,

By Anders Stigebrandt

This revised version of the MS contains, unfortunately, some important conclusions at the system level that are not correct due to ignorance regarding oceanographic conditions at the study sites. Additional revision of the MS is needed as detailed below.

Line 21-23. This statement is refuted by your own data. Ingaröfjärden has a higher upward sulphide flux towards the sediment surface than Baggensfjärden (Table 3). Despite this, Ingaröfjärden has five times larger Fe-oxide-bound P pool. The explanation for this is the much better oxygen penetration of the sediment due to higher oxygen concentration in the bottom water in Ingaröfjärden than in Baggensfjärden (Fig. 2a), see also comments for Line 133-143.

Line 32-34. This should not be in the Abstract because it is not a conclusion based on results achieved in this MS.

Line 85-86. In your reply to my comments for Line 85 and Line 475 (in the original MS) you say that Hansen et al. (2019) does not contain new data. This is wrong, these authors included new data showing that the amount of anoxic water and the area of anoxic bottoms were record high in 2018. This is not a sign of recovery in the region which should be mentioned to give a balanced view.

Line 99-101. I cannot see that the objective “to determine the time scales that govern removal and the implications for management strategies” is met.

Line 133-143. (Study sites)
The much greater temperature range in the bottom water in Ingaröfjärden (Fig. 2) than in Baggensfjärden proves that the water exchange in summer and autumn is much more frequent in Ingaröfjärden than in Baggensfjärden where winter temperatures prevail all the year around. This explains the much smaller amplitudes of the recorded annual cycles of P and O2 in Ingaröfjärden than in Baggensfjärden (Fig. 3a).

The solid black line in Fig. 3a is not explained. I assume that it is the theoretical relationship between O2 and P in a closed water volume where organic matter of Redfield composition is decomposed. Data from Ingaröfjärden have a much smaller slope than this line, which demonstrates the vivid exchange of bottom water in Ingaröfjärden. Data from Slussen are clearly displaced above the solid line which may be explained by large contributions from the submerged outlets, interleaved in the deepwater, from sewage treatment plants. The oceanographic conditions at the study sites are of great importance for the interpretation of the surface layer sediment data and should thus be more fully described in the MS.

Line 341. Is this true also for Slussen? Please explain the effect of the contribution from the submerged outlets from the sewage treatment plants.

Line 389-391. The conclusion here is wrong. Ingaröfjärden has higher H2S flux to the surface sediment than both Baggensfjärden and Erstaviken (Table 3) but this does not hinder the formation of a thick Fe-bound P pool in the surface sediment in Ingaröfjärden. This shows that high oxygen concentration in the bottom water (Fig. 2a) is the key factor for the formation of a thick Fe-bound P pool is the surface sediment.

Line 400. Sink-switching should be defined at its first occurrence in the text.

Line 467-469. It should be discussed if the high oxygen concentration in the bottom water in Ingaröfjärden may be an important factor.

Line 514 – 523 The conclusion here is not in accordance with your own data. As pointed out above, the high upward H2S flux (the “legacy” effect) does not hinder the build-up of a large Fe-P pool if the oxygen concentration is high in the bottom water like it is in Ingaröfjärden. This shows that long-term artificial reoxygenation (e.g. Stigebrandt and Gustafsson, 2007) would be an effective measure towards improving the water quality in the Stockholm archipelago. As serious scientists, you should of course report and not withhold this important conclusion.

References

Hansson, M., Viktorsson, L., Andersson, L., 2019. Oxygen Survey in the Baltic Sea 2018-Extent of Anoxia and Hypoxia, 1960-2018. SMHI Swedish Meteorological and Hydrological Institute. Report Oceanography No, 65, 11 pp + 2 Appendices.

Stigebrandt, A., Gustafsson, B.G., 2007. Improvement of Baltic proper water quality using large-scale ecological engineering. Ambio 36, 280-286.

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RR by Anonymous Referee #2 (28 Mar 2020)

ED: Reconsider after major revisions (29 Mar 2020) by Hermann Bange

ED: Reconsider after major revisions (30 Mar 2020) by Katja Fennel (Co-editor-in-chief)

AR by Niels van Helmond on behalf of the Authors (06 Apr 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (06 Apr 2020) by Hermann Bange

RR by Anders Stigebrandt (18 Apr 2020)

Suggestions for revision or reasons for rejection

The MS is now fine with many useful observations and analyses. However, there is still a serious inconsistency, between two different cases, in the evaluation of the effects of recovery from hypoxia upon P burial. I have asked questions about these evaluations in my previous reviews. The inconsistency should be solved, and the text revised accordingly before publication is recommended.

Case I; L 94 – 95 and L 571-572. In the Stockholm Archipelago recovery of hypoxia may be associated with increased P burial (Norkko et al., 2010).

Case II; L 550 – 555. Increases in the bottom water O2 concentration would allow thicker Fe-oxide bearing layers and a larger Fe-bound P pool in the surface sediments, hence a larger surface sedimentary P pool. This process will however be delayed due to prior deposition of organic rich sediments which results in a high upward flux of H2S hindering the formation of Fe-oxides that can bind P. Because of this we expect that artificial reoxygenation of bottom waters (e.g. Stigebrandt and Gustafsson, 2007), if applied in the Stockholm Archipelago, is unlikely to be a long-term effective measure towards improving the water quality since it does not stimulate permanent burial in these sediments and a large impact on the Fe-P pool is hindered by the high upward H2S flux.

Please explain how you define increased P burial in Case I. How is it related to permanent P burial?

Please explain why recovery from hypoxia in Case I, but not recovery from hypoxia by artificial reoxygenation in Case II, will lead to increased P burial.

L 35 – 38. It is postulated that nutrient load reductions support the capacity of the sediments to remove part of the P load. Is this true? The mechanism(s) behind this effect should be explained in the MS.

Finally, the theoretical paper by Stigebrandt and Gustafsson (2007) is not particularly relevant for oxygenation of the Stockholm Archipelago. The authors should instead refer to the relevant inshore oxygenation experiment in the Byfjord described in Stigebrandt et al. (Ambio, 44, 42-54 2015).

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ED: Publish subject to minor revisions (review by editor) (19 Apr 2020) by Hermann Bange

ED: Publish subject to minor revisions (review by editor) (20 Apr 2020) by Katja Fennel (Co-editor-in-chief)

AR by Niels van Helmond on behalf of the Authors (20 Apr 2020) Author's response Manuscript

ED: Publish as is (21 Apr 2020) by Hermann Bange

ED: Publish as is (21 Apr 2020) by Katja Fennel (Co-editor-in-chief)

AR by Niels van Helmond on behalf of the Authors (22 Apr 2020) Manuscript

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Short summary

We studied the removal of phosphorus (P) and nitrogen (N) in the eutrophic Stockholm archipelago (SA). High sedimentation rates and sediment P contents lead to high P burial. Benthic denitrification is the primary nitrate-reducing pathway. Together, these mechanisms limit P and N transport to the open Baltic Sea. We expect that further nutrient load reduction will contribute to recovery of the SA from low-oxygen conditions and that the sediments will continue to remove part of the P and N loads.