Drivers of Particle Sinking Velocities in the Peruvian Upwelling System

Baumann, Moritz; Paul, Allanah Joy; Taucher, Jan; Bach, Lennart Thomas; Goldenberg, Silvan; Stange, Paul; Minutolo, Fabrizio; Riebesell, Ulf

doi:https://doi.org/10.5194/egusphere-2022-814

Preprints

https://doi.org/10.5194/egusphere-2022-814

Preprints

23 Aug 2022

| 23 Aug 2022

Drivers of Particle Sinking Velocities in the Peruvian Upwelling System

Moritz Baumann, Allanah Joy Paul, Jan Taucher, Lennart Thomas Bach, Silvan Goldenberg, Paul Stange, Fabrizio Minutolo, and Ulf Riebesell

Abstract. As one of Earth’s most productive marine ecosystems, the Peruvian Upwelling System transports large amounts of biogenic matter from the surface to the deep ocean. Whilst particle sinking velocity is a key factor controlling the biological pump, thereby affecting carbon sequestration and O₂-depletion, it has not yet been measured in this system. During a 50-day mesocosm experiment in the surface waters off the coast of Peru, we measured particle sinking velocities and their biogeochemical and physical drivers. We further characterized the general properties of exported particles under different phytoplankton communities and nutritional states. Average sinking velocities varied between size classes and ranged from 12.8 ± 0.7 m d^-1 (particles 40–100 µm), to 19.4 ± 0.7 m d^-1 (particles 100–250 µm), and 34.2 ± 1.5 m d^-1 (particles 250–1000 µm) (±95% CI). Surprisingly, no relationship between opal ballast and sinking velocity could be identified, despite the presence of diatoms, questioning the importance of opal ballast in freshly produced material sinking from the surface. In contrast, we found higher sinking velocities with increasing particle size, compactness and roundness. Size had by far the strongest influence among these physical particle properties. Our study provides a detailed analysis of the drivers of particle sinking velocity in the Peruvian Upwelling System, which allows modelers to optimize local particle flux parameterization. This will help to better project oxygen concentrations and carbon sequestration in a region that is subject to substantial climate-driven changes.

Received: 20 Aug 2022 – Discussion started: 23 Aug 2022

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Journal article(s) based on this preprint

05 Jul 2023

Drivers of particle sinking velocities in the Peruvian upwelling system

Moritz Baumann, Allanah Joy Paul, Jan Taucher, Lennart Thomas Bach, Silvan Goldenberg, Paul Stange, Fabrizio Minutolo, and Ulf Riebesell

Biogeosciences, 20, 2595–2612, https://doi.org/10.5194/bg-20-2595-2023,https://doi.org/10.5194/bg-20-2595-2023, 2023

Short summary

Moritz Baumann, Allanah Joy Paul, Jan Taucher, Lennart Thomas Bach, Silvan Goldenberg, Paul Stange, Fabrizio Minutolo, and Ulf Riebesell

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-814', Emmanuel C. Laurenceau-Cornec, 27 Sep 2022

Review of "Drivers of Sinking Velocities in the Peruvian Upwelling System" by Baumann et al.

Summary

This manuscript explores the relative influences of biogeochemical and physical properties of marine particles on their sinking velocity (SV) in the Peruvian Upwelling System . An experimental setting consisting of eight mesocosms deployed off the coast of Peru allowed the production and sampling of sinking particles needed for the analyses. The effect of an upwelling event ("OMZ treatment") on particle SVs was tested via "deep water additions" consisting of the addition of nitrate-depleted water (2 levels) inside the mesocosms. This mesocosm setup has been validated in previous works and the protocols appear to have been followed thoroughly and rigorously. The sampling of the particles was done via a long 10mm tube connected to the bottom of the mesocosms and using a manual vacuum pump to suck the material all the way up to the surface lab inside a 5L bottle. Measurements of particle sinking velocities were conducted following a method developped by Bach et al. (2012) and using the FlowCAM as a settling column and video recording apparatus. Over the particle size range 40-1000µm, the SV recorded varied between 19.4±0.7 and 34.2±1.5 m d^-1. According to their results the authors suggest size as the main driver of particle sinking velocities, while porosity and shape (aspect ratio) exert measurable but weaker controls. The authors note the unexpected absence of relationship between the opal content of the particle and their SV.

General comments and recommendations

This manuscript is well structured, and easy to follow. The figures are clear and most of them follow a very similar model to those used in several previous publications (Bach et al., 2019; Bach et al., 2020). The statistical analyses appear very robust and are used to support most of the conclusions drawn. I acknowledge the value of the data presented here and strongly support works conducted in regions where there is a lack of observation.

However, while reading this manuscript was very satisfaying, the final impression I had was a little disappointment that some interesting aspects were not explored more deeply. The take-home messages advertised here — i.e. the absence of relationship between opal content and SV or that size is the main controlling factor over porosity and shape — remain very poor in comparison to the abundant literature that precedes this study. The main interest thus lies on the regional / local aspect of the SV data, and their possible interaction with upwelling events and the OMZ persistence. The range of SV reported here is very narrow and the conclusion could be that there is no significant SV variation despite a change in plankton community. This could be a very interesting result. I was disappointed to find in the Discussion a significant part allocated to a list comparing the SV obtained here with others. Unfortunately, either a match or mismatch of the SV observed here and elsewhere would not lead to any useful conclusion. I find such listing unecessary and pointless if all other particle properties are not considered and proved to be unvariant across studies (which is of course never the case). This brings me to my main concern. I may have missed it but I don't think there is any report on the nature of the particles sampled. Was it dominated by micro-aggregates? Fecal pellets? Large individual diatom or dinoflagellate cells? Did the category of the dominant particle pool change over time? The absence of data on the nature of the particle is an enormous obstacle to the understanding of the processes ongoing during this experiment. I assume that at least a part if not all of the images taken by the FlowCAM have been stored. Why not using these images to try and understand for example why the diatom opal content was not correlated to the SV? Images showing for example when and where fecal pellets were produced would inform on the role that zooplankton might have played here. Bretagnon et al. (2018) mentioned the abundance of fecal pellets in this region (their study is less than 50 km away). This is an important aspect because some assumptions made here on the fractal structure of the particles (see detailed comments) cannot apply to the biologically-aggregated particle such as fecal pellets. Finally, I find the extrapolation of the present results out of this particular experiment setting being a bit too confident. Results from one isolated study should be taken with caution and used to draw conclusions inside this context. Unfortunately, the relative influences of particle size, porosity, composition, structure, etc. on their SV will remain an ongoing debate for probably many years, but every little contribution is welcome, even if it does not give any useful relationship to global numerical modelers. I have no doubt that this overall good article will find its place among this large litterature and will bring a small but still important contribution.

The attached pdf includes all my detailed comments, which I hope will help the authors improve further their manuscript.

E.C. Laurenceau-Cornec

Citation: https://doi.org/10.5194/egusphere-2022-814-RC1
- AC1:
  'Reply on RC1 and RC2', Moritz Baumann, 12 Nov 2022
  Dear Mrs. Cavan and Mr. Laurenceau-Cornec,
  
  first of all, we would like to express our thankfulness for your friendly and constructive feedback. You have obviously spent a lot of time going through the manuscript and have given lots of thought as to how to improve it.
  The comments from both of you are quite compatible and we have identified among them three main concerns:
  More complete information about the methodology should be provided. Additionally, a discussion about the limitations of the deployed methods is needed.
  
  Information on the nature of sampled particles is of great importance in order to better interpret a variety of the shown data and understand the involved processes.
  
  The conclusions drawn from our analyses need to factor in the uncertainties of this highly regional and specific data set and need to be tuned down accordingly.
  
  We fully agree with each of these points and believe that they will greatly enhance the value of our submitted manuscript. Furthermore, we are very confident that we can address most of your concerns to your full satisfaction.
  However, we have relatively limited options at our disposal regarding the second point. Doing an in-depth analysis on the particle pictures, in order to provide information on the particle types, will unfortunately not be possible. The reason for this is that we experienced a grave loss of video data on the FlowCAM device. In consequence, we ended up with particle picture collages that are by no means representative and thus unfit to perform (semi-) quantitative analyses on. We can neither link particle pictures to sinking velocities, nor perform a random sampling of representative particle pictures without bias. We will hence not be able to link particle classes to sinking velocities, different experimental time points or the prevalent phytoplankton community composition.
  One important point regarding the nature of sinking particles was the contribution of fecal pellets to the downward flux. To this end, we can incorporate biomass data from zooplankton net sampling in the manuscript, which could give some evidence on the possible contribution of fecal pellets to the downward flux. This would at least partly address the issue of missing knowledge about particle types.
  
  Typically, we would have sent you the answers to your comments and entailed revisions in the manuscript today. However, since I, the main author, defended my PhD thesis just last week, there was insufficient time to do so. As I am now not under a working contract in the scientific field anymore, taking the time to work on your comments will be scarce. Therefore, before working in the revisions, we would like to kindly ask for your opinion. Do you think the changes to the manuscript, that we deem possible, will be sufficient to pursue the publication of the paper?
  We thank you very much for your time and consideration.
  
  With kind regards,
  Moritz Baumann
  
  Citation: https://doi.org/10.5194/egusphere-2022-814-AC1
- EC1: 'Reply on RC1', Hans-Peter Grossart, 15 Nov 2022
  
  Dear Emmanuel,
  
  could you please tell me whether the revisions suggested by the authors would be sufficient to warrant the publication of the manuscript?
  as hort response would be very useful. Many thanks and best wishes, yours Hans-Peter
  
  Citation: https://doi.org/10.5194/egusphere-2022-814-EC1
RC2:
'Comment on egusphere-2022-814', Emma Cavan, 29 Sep 2022

The authors present results from a mesocosm study off Peru investigating the drivers of particle sinking rates in the ocean. Understanding what controls sinking rates of carbon-rich particles is important so we can calculate the magnitude of sinking carbon by ocean life. Sinking rate parameterisations are often over simplified in models compared to reality due to this knowledge gap. In particular upwelling systems are of special interest due to their often high productivity compared to locations of the same latitude elsehwere. The methods followed in this study seem robust as are the statistics applied. This article is a good fit for the journal and special issue and should be published after corrections.

The manuscript is exceptionally well written. In addition I applaud the team's efforts to run these mesocosm experiments to get this data, which must be no simple feat. My main comments to improve the manuscript are around representing more methods as figures/tables in the manuscript, with less reference to other manuscripts, particularly Bach 2020 which explains the set-up. Althought this is part of a special issue the reader needs to understand the methods used without referring constantly to another manuscript. I also have other comments about acknowledging limitations of the approaches used and drawing out more from the data you have, although the latter is really up to the authors and is more a suggestion.

My comments are as follows:

- Please include a figure showing a map of mesocosms and a diagram of a mesocosm with the set up.

- A table with initial nox conditions in the mesocsms and how these changed after addition of OMZ waters. I can see these exist in Bach et al 2020 but without these the methods are not fully understandable if only reading this manuscript.

- There must be images of the particles, can you show these to show as example 1) particle type, and 2) a porous and non-porous particle. I was surprised there was not any mention of particle type and wonder if perhaps if this data is being saved for another manuscript, if not please include the data here. If it is, please summarise what the particles were and if composition changed over time.

- in line 234 you state seabirds stimulate new production, it's a bit pedantic but isnt this regeneration of nutrients and so they are stimulating production, but by definition (F-ratio) its regenerated nutrients (ammonium) not nitrate.

- consider putting some of Fig.1 plots on same scale so can compare Sv.

- Did you compare the means of your variables with time for the different OMZ water treatments? I can see from your statistics and figures there is no difference in magnitude, but there might be interesting (or not) subtle differences in the trends of some of your variables. For instance instead of just the solid black line you could have a blue and red one too. If there isnt any thing interesting to be seen with time then this can just be stated.

- in your discussion around size vs Sv you note that a low sample number (n) could be the reason. We also looked into this using the Cavan et al 2017 data from Guatemala, but published the size vs Sv in a seperate paper (Cavan et al 2018, JGR), https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JG004392. You can see in Fig.1 below 1250um theres a signal of size vs Sv, but above this there is not, which we hypothesised was due to a low sample size. This could be useful to further back up your discussion (~ line 387) and is from a similar location.

- As the main motivation for this study is to improve models it would be helpful to expand discussion on your coefficients for the size vs Sv relationship, ~ line 395. what was the range reported by Cael? Were your coefficients on the lower or higher end, how do they compare with global values, does this location have a lower or higher slope than others? What does this infer about biology? What else needs to be done to get this paramterisation in models and how would a modeller scale the coefficients with location/chl/temperature etc. to represent this location differently?

- A large proportion of the discussion is given to the opal ballast, which the authors find is not a key driver here (would we expect that in the tropics?) and the wider community has already shown ballast is often not a driver. I would encourage the authors to bring out more novel aspects, particularly around porosity which has only more recently been quantified and could be really important in driving fluxes. For instance, pellets that are more porous (e.g. salps) are less efficient vectors of carbon to the deep. I think some interesting discussion could be had here at the authors discretion.

Some discussion around particular limitations is needed, especially:

- The use of mesocosms, what are the limitations of artificially manipulating the ecosystem in this way? are there wall effects for example?

- What is the mixed layer depth? and as the mesocsms are only 19 m deep, do you think the particles you sample would actually be exported out of the mixed layer? many would be recycled before reaching the mixed layer if its much deeper than 19 m.

- I am really glad to see the seabird discussion, but did you count seabirds per mesocosm and compare to N or P? Can you absolutely prove that seabirds increased N and P in the mesocsoms? I think its still fine to mention the birds in the manuscript but need to acknowledge you hypothesise the affect the birds had rather than can prove it. This sets up a nice future experiment to be had.

- Are there limitations to your methods of measuring Sv? might the material have aggregated once settled and before you sample it? there are also limitations to the flow cam method which need to be ackowledge. Around line 391 would be a good space to introduce discussion on this.

Citation: https://doi.org/10.5194/egusphere-2022-814-RC2
- AC2:
  'Reply on RC1 and RC2', Moritz Baumann, 12 Nov 2022
  Dear Mrs. Cavan and Mr. Laurenceau-Cornec,
  
  first of all, we would like to express our thankfulness for your friendly and constructive feedback. You have obviously spent a lot of time going through the manuscript and have given lots of thought as to how to improve it.
  The comments from both of you are quite compatible and we have identified among them three main concerns:
  More complete information about the methodology should be provided. Additionally, a discussion about the limitations of the deployed methods is needed.
  
  Information on the nature of sampled particles is of great importance in order to better interpret a variety of the shown data and understand the involved processes.
  
  The conclusions drawn from our analyses need to factor in the uncertainties of this highly regional and specific data set and need to be tuned down accordingly.
  
  We fully agree with each of these points and believe that they will greatly enhance the value of our submitted manuscript. Furthermore, we are very confident that we can address most of your concerns to your full satisfaction.
  However, we have relatively limited options at our disposal regarding the second point. Doing an in-depth analysis on the particle pictures, in order to provide information on the particle types, will unfortunately not be possible. The reason for this is that we experienced a grave loss of video data on the FlowCAM device. In consequence, we ended up with particle picture collages that are by no means representative and thus unfit to perform (semi-) quantitative analyses on. We can neither link particle pictures to sinking velocities, nor perform a random sampling of representative particle pictures without bias. We will hence not be able to link particle classes to sinking velocities, different experimental time points or the prevalent phytoplankton community composition.
  One important point regarding the nature of sinking particles was the contribution of fecal pellets to the downward flux. To this end, we can incorporate biomass data from zooplankton net sampling in the manuscript, which could give some evidence on the possible contribution of fecal pellets to the downward flux. This would at least partly address the issue of missing knowledge about particle types.
  
  Typically, we would have sent you the answers to your comments and entailed revisions in the manuscript today. However, since I, the main author, defended my PhD thesis just last week, there was insufficient time to do so. As I am now not under a working contract in the scientific field anymore, taking the time to work on your comments will be scarce. Therefore, before working in the revisions, we would like to kindly ask for your opinion. Do you think the changes to the manuscript, that we deem possible, will be sufficient to pursue the publication of the paper?
  We thank you very much for your time and consideration.
  
  With kind regards,
  Moritz Baumann
  
  Citation: https://doi.org/10.5194/egusphere-2022-814-AC2
- EC2: 'Reply on RC2', Hans-Peter Grossart, 15 Nov 2022
  
  Dear Emma,
  could you please tell me whether the revisions suggested by the authors would be sufficient to warrant the publication of the manuscript?
  A short response would be very useful. Many thanks and best wishes, yours Hans-Peter
  
  Citation: https://doi.org/10.5194/egusphere-2022-814-EC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-814', Emmanuel C. Laurenceau-Cornec, 27 Sep 2022

Review of "Drivers of Sinking Velocities in the Peruvian Upwelling System" by Baumann et al.

Summary

This manuscript explores the relative influences of biogeochemical and physical properties of marine particles on their sinking velocity (SV) in the Peruvian Upwelling System . An experimental setting consisting of eight mesocosms deployed off the coast of Peru allowed the production and sampling of sinking particles needed for the analyses. The effect of an upwelling event ("OMZ treatment") on particle SVs was tested via "deep water additions" consisting of the addition of nitrate-depleted water (2 levels) inside the mesocosms. This mesocosm setup has been validated in previous works and the protocols appear to have been followed thoroughly and rigorously. The sampling of the particles was done via a long 10mm tube connected to the bottom of the mesocosms and using a manual vacuum pump to suck the material all the way up to the surface lab inside a 5L bottle. Measurements of particle sinking velocities were conducted following a method developped by Bach et al. (2012) and using the FlowCAM as a settling column and video recording apparatus. Over the particle size range 40-1000µm, the SV recorded varied between 19.4±0.7 and 34.2±1.5 m d^-1. According to their results the authors suggest size as the main driver of particle sinking velocities, while porosity and shape (aspect ratio) exert measurable but weaker controls. The authors note the unexpected absence of relationship between the opal content of the particle and their SV.

General comments and recommendations

This manuscript is well structured, and easy to follow. The figures are clear and most of them follow a very similar model to those used in several previous publications (Bach et al., 2019; Bach et al., 2020). The statistical analyses appear very robust and are used to support most of the conclusions drawn. I acknowledge the value of the data presented here and strongly support works conducted in regions where there is a lack of observation.

However, while reading this manuscript was very satisfaying, the final impression I had was a little disappointment that some interesting aspects were not explored more deeply. The take-home messages advertised here — i.e. the absence of relationship between opal content and SV or that size is the main controlling factor over porosity and shape — remain very poor in comparison to the abundant literature that precedes this study. The main interest thus lies on the regional / local aspect of the SV data, and their possible interaction with upwelling events and the OMZ persistence. The range of SV reported here is very narrow and the conclusion could be that there is no significant SV variation despite a change in plankton community. This could be a very interesting result. I was disappointed to find in the Discussion a significant part allocated to a list comparing the SV obtained here with others. Unfortunately, either a match or mismatch of the SV observed here and elsewhere would not lead to any useful conclusion. I find such listing unecessary and pointless if all other particle properties are not considered and proved to be unvariant across studies (which is of course never the case). This brings me to my main concern. I may have missed it but I don't think there is any report on the nature of the particles sampled. Was it dominated by micro-aggregates? Fecal pellets? Large individual diatom or dinoflagellate cells? Did the category of the dominant particle pool change over time? The absence of data on the nature of the particle is an enormous obstacle to the understanding of the processes ongoing during this experiment. I assume that at least a part if not all of the images taken by the FlowCAM have been stored. Why not using these images to try and understand for example why the diatom opal content was not correlated to the SV? Images showing for example when and where fecal pellets were produced would inform on the role that zooplankton might have played here. Bretagnon et al. (2018) mentioned the abundance of fecal pellets in this region (their study is less than 50 km away). This is an important aspect because some assumptions made here on the fractal structure of the particles (see detailed comments) cannot apply to the biologically-aggregated particle such as fecal pellets. Finally, I find the extrapolation of the present results out of this particular experiment setting being a bit too confident. Results from one isolated study should be taken with caution and used to draw conclusions inside this context. Unfortunately, the relative influences of particle size, porosity, composition, structure, etc. on their SV will remain an ongoing debate for probably many years, but every little contribution is welcome, even if it does not give any useful relationship to global numerical modelers. I have no doubt that this overall good article will find its place among this large litterature and will bring a small but still important contribution.

The attached pdf includes all my detailed comments, which I hope will help the authors improve further their manuscript.

E.C. Laurenceau-Cornec

Citation: https://doi.org/10.5194/egusphere-2022-814-RC1
- AC1:
  'Reply on RC1 and RC2', Moritz Baumann, 12 Nov 2022
  Dear Mrs. Cavan and Mr. Laurenceau-Cornec,
  
  first of all, we would like to express our thankfulness for your friendly and constructive feedback. You have obviously spent a lot of time going through the manuscript and have given lots of thought as to how to improve it.
  The comments from both of you are quite compatible and we have identified among them three main concerns:
  More complete information about the methodology should be provided. Additionally, a discussion about the limitations of the deployed methods is needed.
  
  Information on the nature of sampled particles is of great importance in order to better interpret a variety of the shown data and understand the involved processes.
  
  The conclusions drawn from our analyses need to factor in the uncertainties of this highly regional and specific data set and need to be tuned down accordingly.
  
  We fully agree with each of these points and believe that they will greatly enhance the value of our submitted manuscript. Furthermore, we are very confident that we can address most of your concerns to your full satisfaction.
  However, we have relatively limited options at our disposal regarding the second point. Doing an in-depth analysis on the particle pictures, in order to provide information on the particle types, will unfortunately not be possible. The reason for this is that we experienced a grave loss of video data on the FlowCAM device. In consequence, we ended up with particle picture collages that are by no means representative and thus unfit to perform (semi-) quantitative analyses on. We can neither link particle pictures to sinking velocities, nor perform a random sampling of representative particle pictures without bias. We will hence not be able to link particle classes to sinking velocities, different experimental time points or the prevalent phytoplankton community composition.
  One important point regarding the nature of sinking particles was the contribution of fecal pellets to the downward flux. To this end, we can incorporate biomass data from zooplankton net sampling in the manuscript, which could give some evidence on the possible contribution of fecal pellets to the downward flux. This would at least partly address the issue of missing knowledge about particle types.
  
  Typically, we would have sent you the answers to your comments and entailed revisions in the manuscript today. However, since I, the main author, defended my PhD thesis just last week, there was insufficient time to do so. As I am now not under a working contract in the scientific field anymore, taking the time to work on your comments will be scarce. Therefore, before working in the revisions, we would like to kindly ask for your opinion. Do you think the changes to the manuscript, that we deem possible, will be sufficient to pursue the publication of the paper?
  We thank you very much for your time and consideration.
  
  With kind regards,
  Moritz Baumann
  
  Citation: https://doi.org/10.5194/egusphere-2022-814-AC1
- EC1: 'Reply on RC1', Hans-Peter Grossart, 15 Nov 2022
  
  Dear Emmanuel,
  
  could you please tell me whether the revisions suggested by the authors would be sufficient to warrant the publication of the manuscript?
  as hort response would be very useful. Many thanks and best wishes, yours Hans-Peter
  
  Citation: https://doi.org/10.5194/egusphere-2022-814-EC1
RC2:
'Comment on egusphere-2022-814', Emma Cavan, 29 Sep 2022

The authors present results from a mesocosm study off Peru investigating the drivers of particle sinking rates in the ocean. Understanding what controls sinking rates of carbon-rich particles is important so we can calculate the magnitude of sinking carbon by ocean life. Sinking rate parameterisations are often over simplified in models compared to reality due to this knowledge gap. In particular upwelling systems are of special interest due to their often high productivity compared to locations of the same latitude elsehwere. The methods followed in this study seem robust as are the statistics applied. This article is a good fit for the journal and special issue and should be published after corrections.

The manuscript is exceptionally well written. In addition I applaud the team's efforts to run these mesocosm experiments to get this data, which must be no simple feat. My main comments to improve the manuscript are around representing more methods as figures/tables in the manuscript, with less reference to other manuscripts, particularly Bach 2020 which explains the set-up. Althought this is part of a special issue the reader needs to understand the methods used without referring constantly to another manuscript. I also have other comments about acknowledging limitations of the approaches used and drawing out more from the data you have, although the latter is really up to the authors and is more a suggestion.

My comments are as follows:

- Please include a figure showing a map of mesocosms and a diagram of a mesocosm with the set up.

- A table with initial nox conditions in the mesocsms and how these changed after addition of OMZ waters. I can see these exist in Bach et al 2020 but without these the methods are not fully understandable if only reading this manuscript.

- There must be images of the particles, can you show these to show as example 1) particle type, and 2) a porous and non-porous particle. I was surprised there was not any mention of particle type and wonder if perhaps if this data is being saved for another manuscript, if not please include the data here. If it is, please summarise what the particles were and if composition changed over time.

- in line 234 you state seabirds stimulate new production, it's a bit pedantic but isnt this regeneration of nutrients and so they are stimulating production, but by definition (F-ratio) its regenerated nutrients (ammonium) not nitrate.

- consider putting some of Fig.1 plots on same scale so can compare Sv.

- Did you compare the means of your variables with time for the different OMZ water treatments? I can see from your statistics and figures there is no difference in magnitude, but there might be interesting (or not) subtle differences in the trends of some of your variables. For instance instead of just the solid black line you could have a blue and red one too. If there isnt any thing interesting to be seen with time then this can just be stated.

- in your discussion around size vs Sv you note that a low sample number (n) could be the reason. We also looked into this using the Cavan et al 2017 data from Guatemala, but published the size vs Sv in a seperate paper (Cavan et al 2018, JGR), https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JG004392. You can see in Fig.1 below 1250um theres a signal of size vs Sv, but above this there is not, which we hypothesised was due to a low sample size. This could be useful to further back up your discussion (~ line 387) and is from a similar location.

- As the main motivation for this study is to improve models it would be helpful to expand discussion on your coefficients for the size vs Sv relationship, ~ line 395. what was the range reported by Cael? Were your coefficients on the lower or higher end, how do they compare with global values, does this location have a lower or higher slope than others? What does this infer about biology? What else needs to be done to get this paramterisation in models and how would a modeller scale the coefficients with location/chl/temperature etc. to represent this location differently?

- A large proportion of the discussion is given to the opal ballast, which the authors find is not a key driver here (would we expect that in the tropics?) and the wider community has already shown ballast is often not a driver. I would encourage the authors to bring out more novel aspects, particularly around porosity which has only more recently been quantified and could be really important in driving fluxes. For instance, pellets that are more porous (e.g. salps) are less efficient vectors of carbon to the deep. I think some interesting discussion could be had here at the authors discretion.

Some discussion around particular limitations is needed, especially:

- The use of mesocosms, what are the limitations of artificially manipulating the ecosystem in this way? are there wall effects for example?

- What is the mixed layer depth? and as the mesocsms are only 19 m deep, do you think the particles you sample would actually be exported out of the mixed layer? many would be recycled before reaching the mixed layer if its much deeper than 19 m.

- I am really glad to see the seabird discussion, but did you count seabirds per mesocosm and compare to N or P? Can you absolutely prove that seabirds increased N and P in the mesocsoms? I think its still fine to mention the birds in the manuscript but need to acknowledge you hypothesise the affect the birds had rather than can prove it. This sets up a nice future experiment to be had.

- Are there limitations to your methods of measuring Sv? might the material have aggregated once settled and before you sample it? there are also limitations to the flow cam method which need to be ackowledge. Around line 391 would be a good space to introduce discussion on this.

Citation: https://doi.org/10.5194/egusphere-2022-814-RC2
- AC2:
  'Reply on RC1 and RC2', Moritz Baumann, 12 Nov 2022
  Dear Mrs. Cavan and Mr. Laurenceau-Cornec,
  
  first of all, we would like to express our thankfulness for your friendly and constructive feedback. You have obviously spent a lot of time going through the manuscript and have given lots of thought as to how to improve it.
  The comments from both of you are quite compatible and we have identified among them three main concerns:
  More complete information about the methodology should be provided. Additionally, a discussion about the limitations of the deployed methods is needed.
  
  Information on the nature of sampled particles is of great importance in order to better interpret a variety of the shown data and understand the involved processes.
  
  The conclusions drawn from our analyses need to factor in the uncertainties of this highly regional and specific data set and need to be tuned down accordingly.
  
  We fully agree with each of these points and believe that they will greatly enhance the value of our submitted manuscript. Furthermore, we are very confident that we can address most of your concerns to your full satisfaction.
  However, we have relatively limited options at our disposal regarding the second point. Doing an in-depth analysis on the particle pictures, in order to provide information on the particle types, will unfortunately not be possible. The reason for this is that we experienced a grave loss of video data on the FlowCAM device. In consequence, we ended up with particle picture collages that are by no means representative and thus unfit to perform (semi-) quantitative analyses on. We can neither link particle pictures to sinking velocities, nor perform a random sampling of representative particle pictures without bias. We will hence not be able to link particle classes to sinking velocities, different experimental time points or the prevalent phytoplankton community composition.
  One important point regarding the nature of sinking particles was the contribution of fecal pellets to the downward flux. To this end, we can incorporate biomass data from zooplankton net sampling in the manuscript, which could give some evidence on the possible contribution of fecal pellets to the downward flux. This would at least partly address the issue of missing knowledge about particle types.
  
  Typically, we would have sent you the answers to your comments and entailed revisions in the manuscript today. However, since I, the main author, defended my PhD thesis just last week, there was insufficient time to do so. As I am now not under a working contract in the scientific field anymore, taking the time to work on your comments will be scarce. Therefore, before working in the revisions, we would like to kindly ask for your opinion. Do you think the changes to the manuscript, that we deem possible, will be sufficient to pursue the publication of the paper?
  We thank you very much for your time and consideration.
  
  With kind regards,
  Moritz Baumann
  
  Citation: https://doi.org/10.5194/egusphere-2022-814-AC2
- EC2: 'Reply on RC2', Hans-Peter Grossart, 15 Nov 2022
  
  Dear Emma,
  could you please tell me whether the revisions suggested by the authors would be sufficient to warrant the publication of the manuscript?
  A short response would be very useful. Many thanks and best wishes, yours Hans-Peter
  
  Citation: https://doi.org/10.5194/egusphere-2022-814-EC2

Peer review completion

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

ED: Reconsider after major revisions (30 Nov 2022) by Hans-Peter Grossart

AR by Moritz Baumann on behalf of the Authors (14 Apr 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (17 Apr 2023) by Hans-Peter Grossart

RR by Sven Tobias-Hunefeld (25 Apr 2023)

Suggestions for revision or reasons for rejection

Summary:
Overall, the introduction summarised the literature and established context well. Methods were relevant and fitting to the study objectives. The discussion was detailed, well considered, and was realistic in the scope of their findings and any limitations.

The authors carried out a mesocosm experiment off the coat of Peru, under the context of OMZ upwelling events. They set out to identify the major drivers of particle sinking velocity. Linking this both to particle properties such as porosity and carbon concentrations, and environmental factors such as the phytoplankton community. Establishing these values and relationships is important due to the importance of carbon transport to sediments. The applied methods seem robust and have been previously validated. Statistical methods were well chosen and applied.

The major findings include the importance of particle size to sinking velocity, validated with over 100.000 individual particles samples and analysed (however, sample type could not be identified due to catastrophic raw image data loss, this is a significant limitation and is acknowledged as such). Meanwhile porosity and shape are only weak drivers of sinking velocity. Minimum differences could be identified between two different OMZ (low and very low NOx-) treatments, therefore this factor was excluded from further analyses.

Comments:
Previous Referee comments have been well considered and their integration have improved the text to a publishable degree.

Minor comments:
L81 - many times repeated statements, remove superflous sentence: Our OMZ treatment thus had two levels: low NOx- and very low NOx-.

L320-329 - You are presenting results here for the first time, this should be in the results section.

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ED: Publish subject to minor revisions (review by editor) (17 May 2023) by Hans-Peter Grossart

AR by Moritz Baumann on behalf of the Authors (18 May 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (23 May 2023) by Hans-Peter Grossart

AR by Moritz Baumann on behalf of the Authors (29 May 2023)

Journal article(s) based on this preprint

05 Jul 2023

Drivers of particle sinking velocities in the Peruvian upwelling system

Moritz Baumann, Allanah Joy Paul, Jan Taucher, Lennart Thomas Bach, Silvan Goldenberg, Paul Stange, Fabrizio Minutolo, and Ulf Riebesell

Biogeosciences, 20, 2595–2612, https://doi.org/10.5194/bg-20-2595-2023,https://doi.org/10.5194/bg-20-2595-2023, 2023

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Moritz Baumann, Allanah Joy Paul, Jan Taucher, Lennart Thomas Bach, Silvan Goldenberg, Paul Stange, Fabrizio Minutolo, and Ulf Riebesell

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Aug 2022	115	43	3	161
Sep 2022	74	30	4	108
Oct 2022	34	15	2	51
Nov 2022	84	25	8	117
Dec 2022	16	8	0	24
Jan 2023	20	8	0	28
Feb 2023	30	16	1	47
Mar 2023	17	7	0	24
Apr 2023	20	15	0	35
May 2023	19	11	0	30
Jun 2023	18	12	1	31
Jul 2023	5	4	0	9
Aug 2023	0
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

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Short summary

The sinking velocity of marine particles affects how much atmospheric CO₂ is stored inside our oceans. We measured particle sinking velocities in the Peruvian Upwelling System and assessed their physical and biochemical drivers. We found that sinking velocity was mainly influenced by particle size and compactness, while ballasting minerals played only a minor role. Our findings help to better understand the particle sinking dynamics in this highly productive marine system.


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Drivers of Particle Sinking Velocities in the Peruvian Upwelling System

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