Anthropogenic Microparticles in Sea-Surface Microlayer in Osaka Bay, Japan

Response to Reviewer 1 Comments

1. Summary

Thank you for providing such valuable advices and Issues of extraordinary significance on our paper. Your questions pointed out the shortcomings of the paper. Based on your corrections, I will provide the following answers and explanations.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Yes

Are all the cited references relevant to the research?

Yes

Is the research design appropriate?

Must be improved             

I have improved the content of research design, and the errors due to discrepancy of coordinate for one sampling site was pointed out. The reason of the selection of sampling site and rationality of sampling method was also illustrated.

Are the methods adequately described?

Yes

Are the results clearly presented?

Can be improved               

I have rephrased my explanation of the results.

Are the conclusions supported by the results?

Yes

3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: My major concern is the sampling design, with 9 samplings in the bay at specific dates and positions over two years (Tab. S1). Apparently some positions were visited more than once even though GPS coordinates differed slightly as for 15 Sept. 2021 site 5 34 35.639N, where site 5 is 34 35.553N on 5 Sept. 2022?

Response 1: Thank you for pointing this out. I agree with this comment. I think you raised a very professional issue, as your comment, every sampling sites had been visited more than once. Because the berths of experimental ship cannot be ensured exactly at the same coordinate of latitude and longitude every sampling time, the latitude and longitude for one site were different at every sampling time, so that there must be some errors for the one sampling site. For site 5, the GPS coordinates are 34°35.639’N 135°14.259’E for 15 Sept. 2021 and 34°35.602’N 135°14.030’E for 5 Sept. 2022, respectively. The distance between the two GPS coordinates is about 358m. The method of calculation is shown as the following figure. And the distance of 1’ at different latitude and longitude can be found at https://www.wingfield.gr.jp/archives/9721 .

And other sampling sites also had the same errors because the discrepancy of latitude and longitude at every sampling time. Thus, for one sampling site, even though the experimental ship was stopped at the same sampling site on by located by GPS, to the same sampling site, there was still an error about maximum 300 meters affected by sea current and tide.

This paragraph can now be seen at lines 229-237 of the amended manuscript.

Comments 2: What are the ranges (diameter) of a site?

Response 2: I have reconfirmed to emphasize this point. The experimental ship was located at the GPS coordinates. The electric cable and water collection tube was about 20 m, and the RDS was connected by a rope from the experimental ship with a distance about 12-15 m to collect the S-SML samples.

Comments 3: Why are data not shown as time-series when sites are visited more than once? Or give a description of the variation at sites.

Response 3: I have considered again to explain this point. Since there was no significant discrepancy of MPs abundance among time-series, the MPs abundance at one site was shown by the average data in order to facilitate intuitive comparison with data from other sites, and the discrepancy of MPs abundance among all samplings at one site was reflected by the error bar at Fig.S3.

Table S1 showed sampling date and GPS coordinates of every sampling, and Fig. S3 showed the average MPs abundance and the corresponding standard deviation at every sampling site. As the experimental design, we measured the average MPs abundance and the corresponding standard deviation at every sampling site, but the coefficient of variation of MPs abundance at every sampling site was large. The data shown as time-series is as the following figure. This figure contained the original data used for Figure 1. The X axis in this figure also displayed the date for each sampling.

Fig. 1 showed the average abundance of MPs among all different sampling times for every sampling site. However, notably, as shown in Fig. S3, fluctuation of MPs abundance among different samplings was extraordinarily large in S-SML both at S4 (ranging from 342 to 1070 items/kg with a CV of 105%) and S8 (ranging from 62.7 items/kg to 528 items/kg with a CV of 92%). This result may be due to the exchange of seawater through Akashi Strait in the western side of Osaka Bay. Freshwater inflow from all rivers on land to Seto Inland Sea has been reported to be approximately 14 km3 / year (38ⅹ106 m3 / day), [36], 91% of which corresponded to seawater flows through Akashi Strait from Harimanada [37]. As such, original MPs accumulated at S4 and S8 might be diluted by fresh seawater flows and then transported to the eastern part of Osaka Bay, which led to the relatively lower MPs abundances. In total, among all sampling sites, MPs abundance in S-SML always showed larger CV values and presented more intense fluctuation than that in bulk water. At the bay center where all seawater circulation can flow through, CV value was 110% in S-SML and 36% in bulk water respectively. Thus, we can formulate a hypothesis that in the highly mobile and easy diffused S-SML, MPs abundance constantly changes and a large fluctuation is noted in the process of long-distance transportation and migration.

This paragraph can now be seen at lines 286-302 of the amended manuscript.

Comments 4: It appears the water circulation is quite significant in OB with a tidal range of about 1 m, i.e. water is flowing in and out of OB, and is it reasonable to name st.2, 5, 6,7, and 8 as the navigational route in this context?

Response 4: Thank you for pointing this out. I agree with this comment. Site 2 is located at the coastal area and site 8 is located at center of Osaka Bay. Site 5, 6, and 7 were selected as navigation routes because a lot of ship activities take place there, as shown in the following figure according to the maritime route system on the Google Map. The blue lines showed navigation routes. And all the sampling sites at coastal area, navigation routes and center were decided subjectivity by the authors, to the same sampling site, there was still an error about maximum 300 meters affected by sea current and tide due to the deviation of longitude and latitude coordinates.

Comments 5: I would assume that micro particle concentrations would be higher at coastal stations, where the ships are located for a certain period of time for off and on-loading? This has also to be discussed more in detail.

Response 5: Thank you for pointing this out. I will explain it in detail. Although (meth) acrylic binders can be soluble in water by hydrolysis of the silyl-ester bond [47], for most self-polishing antifouling paints, their antifouling ability mainly depends on the shear of seawater flow and is therefore poor in static seawater [50]. Hence, once hydrolysis of acrylic binders is prevented by biofilm covered on surfaces, APPs from shipyards by wind and rain runoff may persist in marine environments with low water flow. In particular, during the navigating process, as hydrolysis occurs, more hydrophilic ester bonds (-COO-) are exposed on the paint surface, which makes the resin on the paint surface brittle and easy to peel off [70]. The friction and collision between the hull and seawater may exacerbate the exfoliation of these resins. Once detached from the hull bottom, these resin fragments can remain in a relatively static water environment without further hydrolysis for long time, then break into APPs due to UV weathering and biodegradation. By contrast, ships locating at coastal areas are always in a static water environment, and the antifouling paints cannot be hydrolyzed due to the shear of seawater flow, thus the resins on surface of paints may not delaminate from the hull bottom. This leads to lower APPs abundance at the coastal area compared to navigation routes. Actually, APPs detected at the coastal area mainly come from ship building and boat cleaning at shipyards.

The following paragraph has been incorporated in the revision. This paragraph can now be seen at lines 416-426 of the amended manuscript.

Comments 6: I would further recommend to keep Results and Discussion separated. Would make it more clear, and specifically the manuscript needs a strong revision of the English language.

Response 6: We thank you for this comment which is understandable in its way of presenting the data. We believe that since some hypotheses are also tested into action and some comparisons can directly be made, that discussion would be very difficult to be as stand alone. We have now tried to correct English language mistakes and make the text more friendly to the reader. Please see the amended discussion and results in the revised (non-track changes) version of the manuscript.

Comments 7: 49 Specify biological processes

Response 7: Biodegradation by bacteria, fungi and microalgae.

This can now be seen at lines 49 of the amended manuscript.

Comments 8: 54. All kinds? What kinds?

Response 8: Including invertebrates, planktonic organisms, seabirds, crustaceans, fish, aquatic animal and marine mammals.

This can now be seen at lines 55-56 of the amended manuscript.

Comments 9: 95-107. It's discussion.

Response 9: It was cited as the research background to show the recent research results that tiny MPs of 10-53 um occupy large amounts.

Comments 10: 108-115. There are no hypothesis!! 

Response 10: We thank you for this comment and we also think the hypothesis is necessary. This has now been amended. Based on the above theories, we make the hypothesis that more MPs may accumulate in S-SML and at navigation routes, and most of these MPs may be related to antifouling and marine paint.

This can now be seen at lines 107-109 of the amended manuscript.

Comments 11: 122. Tidal range?

Response 11: We thank you for this comment and the tidal range should also be considered for the experiment design. It should be noted that the water circulation is quite significant at Osaka Bay with a tidal range about 1m every day, as such the abundance of microplastics in the estuary was directly related to the tidal fluctuation [66], for this reason, the estuary was excluded from the sampling sites in this research.

An addition has been made. This can now be seen at lines 132-135 of the amended manuscript.

Response to Reviewer 2 Comments

1. Summary

Thank you for providing such valuable advices and Issues of extraordinary significance on our paper. Your questions pointed out the shortcomings of the paper. Based on your corrections, I will provide the following answers and explanations.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Yes

Are all the cited references relevant to the research?

Yes

Is the research design appropriate?

Yes             

Are the methods adequately described?

Can be revised as follows

Are the results clearly presented?

Can be revised as follows

Are the conclusions supported by the results?

Yes

3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: It should be noted that there are certain rules for processing experimental data and leaving the number of significant digits in calculations. As a rule, instrument errors and errors in arithmetic operations are taken into account. Experimental and calculated data should be given with due allowance for such errors and with the required number of significant digits. Not more, but also not less. I recommend that the authors revise the text, tables and figures and bring the number of significant digits to a uniform format.

Comments 2: And also indicate in the materials and methods section the error/accuracy of the methodology used.  

Comments 3: The «Materials and Methods» section is missing Fig. 1

Comments 4: Table S3 is unreadable because it is given in Japanese.

No.

Paint name

Manufacture

Polymer Type

Metal

Paint 1

Sea Blue Acepaint for outer side and superstructures

Dainippon Paint Co., Ltd

PMMA

Ti

Paint 2

CCP Cleaning

Kanae paint Co., Ltd.

Acrylic polymer

Cu Zn

Comments 5: In the reference list, source No,46 is in Japanese.