Turbulence and hypoxia contribute to dense biological scattering layers in a Patagonian fjord system

Pérez-Santos, Iván; Castro, Leonardo; Ross, Lauren; Niklitschek, Edwin; Mayorga, Nicolás; Cubillos, Luis; Gutierrez, Mariano; Escalona, Eduardo; Castillo, Manuel; Alegría, Nicolás; Daneri, Giovanni

doi:https://doi.org/10.5194/os-14-1185-2018

Articles | Volume 14, issue 5

https://doi.org/10.5194/os-14-1185-2018

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

https://doi.org/10.5194/os-14-1185-2018

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

Articles | Volume 14, issue 5

Research article

|

09 Oct 2018

Research article |

| 09 Oct 2018

Turbulence and hypoxia contribute to dense biological scattering layers in a Patagonian fjord system

Iván Pérez-Santos, Leonardo Castro, Lauren Ross, Edwin Niklitschek, Nicolás Mayorga, Luis Cubillos, Mariano Gutierrez, Eduardo Escalona, Manuel Castillo, Nicolás Alegría, and Giovanni Daneri

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Final revised paper (published on 09 Oct 2018)
Preprint (discussion started on 24 Oct 2017)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review of “Turbulence and hypoxia contribute to dense zooplankton scattering layers in Patagonian Fjord System”', Anonymous Referee #1, 29 Nov 2017
- AC1: 'author's response to Referee #1', Iván Pérez-Santos, 15 Mar 2018
RC2: 'Review', Anonymous Referee #2, 18 Dec 2017
- AC2: 'author's response to Referee #2', Iván Pérez-Santos, 15 Mar 2018
- AC5: 'New manuscript', Iván Pérez-Santos, 15 Mar 2018
RC3: 'Review of "Turbulence and hypoxia contribute to dense zooplankton scattering layers in Patagonian Fjord System"', Anonymous Referee #3, 19 Dec 2017
- AC3: 'author's response to Referee #3', Iván Pérez-Santos, 15 Mar 2018
- AC6: 'New manuscript', Iván Pérez-Santos, 15 Mar 2018
RC4: 'review of os-2017-89', Anonymous Referee #4, 28 Dec 2017
- AC4: 'author's response to Referee #4', Iván Pérez-Santos, 15 Mar 2018
- AC7: 'New manuscript', Iván Pérez-Santos, 15 Mar 2018

Peer-review completion

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

AR by Iván Pérez-Santos on behalf of the Authors (15 Mar 2018) Author's response

ED: Referee Nomination & Report Request started (08 Apr 2018) by Markus Meier

RR by Anonymous Referee #2 (23 Apr 2018)

Suggestions for revision or reasons for rejection

In their revised version of the manuscript, the authors have reorganised their results and rewritten large parts of the introduction and discussion. Despite these efforts, which one has to acknowledge, the paper remains weak in several respects:

The introduction still does not identify relevant research questions. Three topics are introduced that are related by physical-biological interactions on different spatial scales (turbulence, vertical migration, hypoxic zones), but open scientific questions regarding these or the context of addressing these together in a common study are insufficiently specified. Relevant literature is largely ignored. There is quite some published work related to the distribution of zooplankton in oxygen minimum zones including work using acoustics (to name a few: Maas et al. 2014, Hauss et al. 2016, Seibel et al. 2016). The same applies for the discussion, which has a strong focus on Patagonian fjords, but does not address for instance the recent efforts to understand the effect of expanding minimum zones in the world ocean. Doing so, the study remains of local interest.

The presentation of the data is still confusing. Although the objectives are primarily addressing the biotic-physical interactions as stated in the introduction, the results do not. Physical data from years in which no zooplankton sampling has been conducted is presented without a relationship to the objectives. The zooplankton data still remains poorly explored, selected data is presented (e.g. a minor proportion of the available vertical profiles) and the presentation does not fit the purpose (although the vertical distribution is highlighted as a major aspect, integrated stocks are presented). Correlations and the data on which these are based are not shown or described in the methods.

Finally, the conclusion that zooplankton distribution is affected by the vertical structure in turbulence is supported by little data. Correlations are not shown and also not properly explained in the methods.

Summing up, I find the manuscript not acceptable for publication in Ocean Science. It is repetitive and primarily of local interest

Minor comments

Title:
The title is misleading as primarily macrozooplankton was analysed from acoustic data.

Introduction:
1st paragraph: 7 out of 12 citation in the paragraph are missing in the references. I haven’t checked this in the remainder of the manuscript, because the first one in the second paragraph is missing as well… Many citations appear to be related to fjords around Southern America, but more general literature should be cited.

Line 75: The authors imply that the all the effects on zooplankton listed here are referenced by Basedow et al. 2004.I have strong doubts on this. Again the citation is missing in the references. In addition, it is not correct that effects on larvae imply also general effects on zooplankton community composition, biomass, productivity and the rest.

Line 96: Why however? How are small scale, short term processes (feeding interactions) related to larger scale processes acting on longer time scales (productivity)? I do not understand what the authors actual mean.

Line 98: Change wording. ‘inform our understanding’ is not appropriate.

Line 101: How are hypoxia related to turbulence?

Line 107: ‘the sensitivity varies among organisms in different environment’: here, ‘different
environment’ needs to be explained.

Line 118: This chapter is related to Patagonian fjords. No general questions and hypotheses are identified that would make the study relevant beyond local interests.

Line 164 following: Delete this introduction sentences and provide clear scientific hypothesis related to the effects of a) micro-turbulence, b) hypoxia) and c) small scale vertical distribution of zooplankton in fjords in general.

Study area:

Line 172 following: A very detailed description of the fjord system that can be shortened. Instead, one expects a short justification of the location of the stations chosen for acoustic measurements and net catches and a description why their features are representative.

Data and methodology

Line 199 following: Why do the authors refer to results already without having presented the methods? When were CTD measurements conducted? On a seasonal basis, during one cruise? Similarly, when were microstructure measurements taken? With each CTD cast? From 1995-2016? This is still very confusing. Particularly, because ADCP measurements and net catches were temporarily restricted to a few observation periods. This contrasts strongly with the goals of the study which relate primarily to the interaction between physics and biology.

Line 237: 300 kHz are given in Table 1. The provision of the table is good for an overview, but reveals that there was no clear strategy in this study to resolve the effects of the interaction of physical structures with the distribution of organisms. For instance, during the investigation in 2016 in Puyuhapi Fjord and in 2012-2013 in Jacaf Channel no zooplankton samples were taken. This is not consistent with a study that primarily aims to study physical-biological interactions. Communities change with time/area, and a few time points for ‘validation’ are not sufficient.

Line 288: Table 1 contains information about additional measurements in 2012/13 that are not described here.

Line 276-287: The authors are inconsistent here: macrozooplankton is by definition > 2 mm. Why do the authors refer to small copepods here, which contrast with the applicability of the algorithm?

Line 304: Brierley et al. 2006 is missing in the references.

Line 322: How long did the sampling last? Making 5 separate net catches over 150 m is time consuming. What was the hauling speed for both nets?

Line 327: Please correct: < 1 mm doesn’t represent a 5 mm threshold.
Results:

Line 346: The study aims to investigate the interactions between physics and biology. Why do the authors present physical data for a period (2016) in which no biological data was collected. I suggest to delete this. In the next chapter backscatter data for 2013-2014 is presented. This does not make sense to me.

Line 356: In the methods it is stated that the algorithm integrates particles sizes >0.5 mm. Here data for only > 5mm is presented. The text states with reference to Fig. 3 that copepods < 5 mm were most abundant. This is not shown. Why are selected data shown and not all, especially when the text refers to ‘data not shown’?

Line 368: This also contrasts with Fig. 4 in which data < 5mm is shown out of the sudden. Analysis of the backscatter requires that the vertical resolution is shown and not depth integrated data. The methods state 5 depths for sampling, only 3 are shown. Why?

Line 377: This description implies that euphausiids migate from 50m to the surface which is certainly not the case.

Line 384: Fig 3 shows results for the top 50m only, so results cannot match. Furthermore, Fig 4 shows that the surface 80 m were devoid of large zooplankton during daytime. This looks different in Fig 5. Is the NASC for day and night-time significantly different?

Line 404/428: the correlations should be shown? On how many observations are these based?

Line 474: This is unclear to me: was this a positive correlation? ‘also’ in the next sentence implies this.

Line 476: The correlation analyses are not described in the methods. Since little data on zooplankton vertical distribution is shown, one wonders on how much data this correlations are based.

Discussion:

Line 494: The ‘clear’ increment of zooplankton related to increased turbulence is not clearly sown. This refers to correlations for which little information on procedures (statistics, analysis) and data is presented (vertical distribution of zooplankton) and needs to be improved.

The discussion still suffers from a very narrow representation of results obtained in other regions of the world (particularly DVM and zooplankton relationship to hypoxia).

Line 498: The study actually shows very little data related to DVM-patterns, several vertical profiles with nets have been obtained. Little of this data is shown.

Line 532: This is incorrect. No zooplankton sampling has been conducted in hypoxic conditions, which could give some ideas which organisms actually migrated into hypoxic water (see Fig 9).

Line 536: What about major studies at sites of oxygen minimum zones in the ocean? There are more studies to mention here in the relationship of organisms to oxygen than Ekau et al. (2010). I also think, to make statements for zooplankton in general are not appropriate considering that macrozooplankton data is reported.

Line 594: Actually, very little data is provided (vertical distribution patterns of zooplankton) to support the statement of ‘evident’ aggregation and to support the conceptual model concluding that vertical mixing influences the vertical distribution of zooplankton and fish. I suggest to omit lines 594-627, because very little own data is presented on this.

Line 625: Theses statements certainly need some references.

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RR by Anonymous Referee #3 (28 Apr 2018)

RR by Anonymous Referee #1 (28 Apr 2018)

Suggestions for revision or reasons for rejection

Summary
Perez-Santos et al. revised the manuscript significantly by including more details on data analysis in the Methods and improving figure presentations. However, the manuscript still lacks significance of the study. The authors provided intensive literature reviews in the Introduction, but did not identify a research question in larger context. Various data sets having both spatial and temporal mismatches are presented, which present challenges in comparing interdisciplinary data and interpreting the results. By identifying a clear objective in the Introduction, I believe that the authors can identify the appropriate data sets to answer a specific research question. In addition, I still have a major concern on the acoustic data analysis provided. Even though the 38 kHz is not relevant to examine zooplankton (as stated in the first review), some of the results are still based on 38-kHz data sets alone. This manuscript is not yet ready for publication.

General comments
Introduction heavily focuses on literature reviews on physical processes and hypoxia effects on zooplankton, but fails to identify the significance of this study. It is important to identify the goal of the study, which will help identifying data sets needed to answer the question.

Too many data sets collected at different sites, time, equipment, and sampling strategies were analyzed in this manuscript. These temporal and spatial mismatches combined with different sampling strategies make it difficult to compare between data sets and fail to identify seasonal and inter-annual variability. To compare the effects of environmental properties on zooplankton distributions, it is critical to analyze the biological and physical data which were collected simultaneously.

I still have major concerns on the analysis of echosounder data to extract zooplankton backscatter. As I stated in the first review, zooplankton species found in their net samples (e.g., copepods, euphausiids) cannot be detected at 38 kHz, because they are too small to be significant backscatterers compared to the wavelength of 38 kHz. Yet, Fig. 5 uses only 38 kHz data to isolate zooplankton which is impossible: high backscatter shown as zooplankton (Fig. 5a, d) are likely fish. There is no description in the Methods about how the authors separated zooplankton vs. fish when only 38 kHz data were collected (e.g., January 2014). Low correlations between Sv and zooplankton taxa can be explained by the use of 38 kHz to detect them (lines 403-407).

Regions outside of the usable acoustic data range (i.e., deeper than 250 m) should be identified in Figs. 6 and 7.

What animals fall into “blue noise” organisms in your study site?

How does “Tidal regime” (Section 4.5) related to the goal of this study?

No need to explain data that are not analyzed/presented in the manuscript. Remove lines 199-200, and lines 256-257.

The manuscript needs to be carefully reviewed for typos and grammatical errors. For example;
Line 91: Replace “preys” by “prey”.
Line 215: Insert space between “Using the values” and “epsilon”.
Lines 316, 318: Replace “flow meter” by “flowmeter”.
Line 350: Remove comma before “(Fig. 2c)”.
Line 365: When describing times, it should be “17:00 (local time)”, instead of “17 h”. Please correct the description of times throughout the text.
Line 378: Insert “m” after “20-50”.
Line 403: Insert a space between “mL” and “L^-1”.
Line 425: Insert a space between “the” and “elevated”.
Line 482: Insert space between a period and “Maximum”.
Line 1031: Insert space between “35 km” and “on”.
Line 1043: What do you mean by “contined”?
Figs. 6 and 7: Clarify which frequencies are used to present (b) and (c).

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ED: Reconsider after major revisions (04 May 2018) by Markus Meier

AR by Iván Pérez-Santos on behalf of the Authors (15 Jun 2018) Manuscript

ED: Referee Nomination & Report Request started (22 Jun 2018) by Markus Meier

RR by Anonymous Referee #5 (13 Sep 2018)

ED: Publish subject to minor revisions (review by editor) (13 Sep 2018) by Markus Meier

AR by Iván Pérez-Santos on behalf of the Authors (14 Sep 2018) Author's response Manuscript

ED: Publish as is (19 Sep 2018) by Markus Meier

AR by Iván Pérez-Santos on behalf of the Authors (19 Sep 2018)

Short summary

Fjord systems play an important role in primary production and carbon export. Acoustic, hydrographic and in situ abundance measurements were used to study macrozooplankton assemblages at 44.7° S. Diel vertical migration of zooplankton stopped at the hypoxic boundary layer and apparently did not tolerate the hypoxic conditions. Turbulence appears to be the oceanographic process that contributes to vertical mixing around the sill, helping the interchange of nutrients, feeding and carbon export.