PONE-D-19-30825

Pump-underway ship intake: An unexploited opportunity for MSFD monitoring needs. First observations of microplastics on oceanic and coastal waters off the Canary Islands (Subtropical NE Atlantic)

PLOS ONE

Dear Mrs. Montoto-Martínez,

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I would particularly highlight the need to (i) make it clear that there is a potential bias in manual picking microplastics (ii) address the question of whether any confirmatory polymer analysis has been done or not (iii) provide a better explanation of the sampling design used and/or how its limitations may hamper result comparability, and (iv) add a description of the statistical analyses done as well as an indication of the estimate of variability used (SD, SE, other).

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Reviewers' comments:

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Reviewer #1: Partly

Reviewer #2: Yes

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: N/A

Reviewer #2: I Don't Know

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Reviewer #1: Yes

Reviewer #2: No

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Reviewer #1: No

Reviewer #2: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This study highlights the use of a ship’s underway pump system and size fractionated sieves to capture potential microplastics. Whilst this in itself is not new, as highlighted by the authors, the idea that it could be used for MSFD monitoring purposes is insightful. Comments regarding the manuscript are noted below;

Manually picking out suspected plastic particles from samples has many limitations, particularly in terms of bias. It is becoming more and more expected for environmental microplastics to be detected autonomously, for example via scanning or imaging mode using µFT-IR, especially for particles < 100 µm where the detection and analysis is highly skewed by manual selection (Lenz, R., Enders, K., Stedmon, C. A., Mackenzie, D. M., & Nielsen, T. G. (2015). A critical assessment of visual identification of marine microplastic using Raman spectroscopy for analysis improvement. Marine Pollution Bulletin, 100(1), 82-91.) This potential bias in picking should be made more transparent in this manuscript. In addition to this, there does not appear to have been any confirmatory polymer analysis (ie, via FT-IR or Raman) which is currently the minimum requirement for publication of environmental microplastic data. Without knowing whether an particle is actually synthetic or natural, microplastic concentrations cannot be reported and comparisons between other studies cannot be made.

Whilst the English in the manuscript is good, it is not quite up to the standard for publication in an international journal. I suggest that the authors enlist the help of someone who speaks English as their first language to edit the manuscript.

Some line specific comments;

Line 73 – I don’t understand why floating litter would play a role in the cycling of marine debris? Please calrify

Line 75 – micro-litter definition (particles < 5 mm) requires a reference as there are many different definitions. I would argue however, that a lower limit should also be specified so as not to include the realms of nano-particles.

Line 233 – Why was the number of foam particles observed not relevant? Please clarify.

Lines 239—240 - This should go into the Results section.

Reviewer #2: I carefully reviewed the manuscript “Pump-underway ship intake: An unexploited opportunity for MSFD monitoring needs. First observations of microplastics on oceanic and coastal waters off the Canary Islands (Subtropical NE Atlantic)” by Montoto-Martínez and colleagues. The paper presents an opportunistic sampling methodology for monitoring ocean microplastic using the pump-underway ship intake system of research vessels.

The introduction is appropriated, providing the reader a solid background on marine microplastic/litter research, while pointing out for its caveats and limitations. The objective is clearly stated. As mentioned by the authors, methods are not entirely new, and consist on a modification of similar sampling set ups by Lusher et al. (2014) Enders et al. (2015).

The method itself is clearly described (e.g. the pump system, filtering device apparatus and sample sorting) but the sampling scheme is somehow confusing preventing results to be properly analysed or discussed. There are different times of surveys, different “modes”, some samples come from stations, some come from replicates, other are transect derived. With such mix of variables, it is unclear how data is analysed. For instance, it seems that comparisons between the stationary and navigation approaches included samples from different campaign dates. In this case, any differences detected may have been due to temporal variability and not related to the approach itself. Data analyses need more clarification. Also, given the reduced number of samples, and/or the spatial variability nature of oceanic microplastics, means in Table 3 do not provide that much information e.g. almost all deviations are higher than means. With such a dispersion of data, there is likely any significant differences between sampling approaches and/or particle types. As I understand, and given the lack of a robust sampling scheme, the authors should put more emphasis on what the paper is really about: the usefulness of an opportunistic method to monitor ocean microplastic (which in fact is done in the first section of the discussion).

Other minor comments:

Title: I would avoid using abbreviations in the manuscript title

Ln43: avoid keywords already referred in the manuscript title

Ln55: sentence “...long list of even unsuspected places.” needs some references

Ln85: remove paragraph.

Ln98: replace WG by working group.

Ln105/106: there is no previous reference in text to MP abbreviation. Same in Table 2. Ln164: there is no need for the use of hyphens.

Ln175: If possible, I would complement (or replace) Fig. 2 with real pictures of the research vessel, pump system and filtering device apparatus.

Ln242-261: I would move this paragraph to discussion.

Ln:268: Identify which measure of dispersion is being used. In means, there is no indication whether the second figure is the standard deviation or the standard error (or indeed something else).

Ln291-305: Again, this comparison is obsolete if data is originated from distinct campaign times.

Ln375-445: The authors are the first to recognize that methods used for oceanic microplastic detection are diverse and that “no harmonization has been yet fully accomplished”. Moreover, they acknowledge that “The lack of a method standardization concerning sampling method, size fraction, and other variables reduce the comparability of

the results”. Yet, in this section they compare results with other studies, that use different methods, distinct approaches with no standardization between them whatsoever. I find this section very speculative and suggest its removal.

Overall, I find the study interesting and, provided that some minor changes are made, it should be considered for publication.

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Reviewer #1: No

Reviewer #2: No

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Pump-underway ship intake: An unexploited opportunity for Marine Strategy Framework Directive (MSFD) monitoring needs on coastal and oceanic waters.

PONE-D-19-30825R1

Dear Dr. Montoto-Martínez,

We are pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it complies with all outstanding technical requirements.

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With kind regards,

Gustavo M. Martins, PhD

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

I am pleased with the thorough revisions made by the authors and I find that the manuscript in now deemed suitable for publication in PLOS ONE as is.

Reviewers' comments: