High-Resolution Mapping of Japanese Microplastic and Macroplastic Emissions from the Land into the Sea

Round 1

Reviewer 1 Report

Review of the manuscript Water 740351

 

The manuscript by Nihei and colleagues presents an interesting dataset and an extensive study which combines research conducted in the field and water balance models, and basin analysis to assess the input of plastics from Japanese rivers into the sea according to the different type of land use and population density.

In my opinion, the study is very interesting and provides a great example for studies conducted in other parts of the world. The approach is novel, and the fact that mismanaged plastic waste is not considered makes the approach applicable to diverse regions worldwide and comparable among them. The authors certainly invested a lot of time in this research, and presented it with clarity of details. I particularly appreciate the fact that inland areas should also be considered to estimate plastic inputs into the sea.

I would suggest some minor revisions of the language style and grammar as the manuscript at times is hard to follow and there are minor mistakes.

I suggest that before publication, the current state of the manuscript needs implementation in some specific points, described specifically below.

Line 33: what do you mean by “water resistance”?

Line 44: physical fragmentation and biological fragmentation too shall be mentioned.

Line 47-48: “plastic pollution has progressed with plastic production” this, in my opinion, should not be directly a cause-effect relationship. I agree that is what we observe, but is it because of MMPW? I would make some reference to the fact that most of the plastic produced is single use plastic (in Europe alone almost 40%, PlasticsEurope 2019), and these polymers used in single-use items are the ones mostly found in the marine environment.

Line 60-61: please rephrase, this sentence seems contradictory.

Line 80-81: sentence unclear, I suggest to rephrase it

Line 91-93: it is unclear the approach. Particles or mass?

Line 98: Could you please add, later in the methods, how you estimated MacP from MicP? It makes it easier to understand the procedure.

Line 111: urban area ratio was considered per river basin or per the whole Japan?

Line 161-164: It is not totally clear why you excluded data from tidal area. Could you explain better?

Line 167: how did you estimate that there was “little influence of flood conditions”? How often did the areas of interest experience flooding,  and, for later in the results/discussion, how much would you increment your estimates of plastic inputs when considering the number of flooding events per year at your sampling sites?

Line 251-252: I think this is the only part where you explain how you related MicP to basin information. Is it based on a simple correlation of two parameters?

Line 253-254: could you explain better how the estimates of MacP was done? See comment for line 98.

Line 356: what do you mean by “rich data”?

Line 381: why do you think the data correlation showed some variation? How were the sites chosen for MicP analysis?

Line 517-520: this sentence is unclear, please rephrase

Line 526: Table A2: what does the orange color indicate?

Lines 534-545: Why do you think the study by Jambeck revealed higher values adnd less accurate? What are the reasons? Do you think that if you included the MMPW in your model, your estimates would have been higher?

Lines 581-591: based on your study and both modeling/field observations, which ones do you believe are the activities generating most of the MicP in order to tackle the problem at source?

I am somehow missing information on:

• The size range of MicP, with median and mean of the sizes, if available
• The most abundant polymer types
• The size range expected for MacP from your calculations: is it everything above 5 mm?
• What do you estimate being the minimum or mean residence time of MicP in rivers?
• Sediment retention: does infiltration underestimates microplastics export from rivers? Is it possible that MicP remain and are remobilized with the next flow? This also depends on the size of MicP.
• Did you consider the effect of waste water treatment plants? Or, were the sampling sites in rivers chosen after any treatment plant/possible urban input?
• How do your input estimates relate to actual measurements in seawater off Japan? This would be interesting to compare, I suggest adding this in the discussion.

 

 

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

L 35: Stating that plastics do not decompose naturally maybe a bit too general

L 41-44: Classification of plastic waste is not up to date: refer rather to the more recent GESAMP report No 99.

L 51: Suggest to delete the following part of the sentence: "..and when it does not sink in the water column,"

L 61: More recent publications could have been included

L 83: Delete "then"

L 91: Delete "among"

L 107: Revise sentence: "The basic structure proposed in this study on the assessment of plastic input from land and sea in two steps, is detailed in Figure 1."

L 121-123: Revise sentence: "In Step 2, a simple water balance analysis was performed in each grid, based on measured precipitation as input data, which was then distributed into evapotranspiration, surface runoff, and underground infiltration"

L 136: Replace "change" with "be different"

L 165: How is low-flow conditions defined?

L 176: Mesh size is given as 0.335 mm, whereas in L 219 it is given as 0.1mm.

L 184-186: Where each plankton net haul treated as replicates or pooled into one sample?

L 206: The consequence of incubating at this high temperature should be discussed. Common practice is not higher than 40 though it may be important to keep low temperatures for the detection of smaller particles than in this study.

L 212: Detection limit should be given

L 225: Only one blank test following all the samples? Please state clearly.

L 232-233: Delete part of sentence "and to calculate plastic input from Japan.

L 331-333: What was the size distribution of the particles?

L 391: How was functions (= models) tested for best fit?

L 535-536: Revise sentence "In previous studies by Jambeck (12) and Lebreton (41), the plastic inputs estimated for Japan ranged 21,000-57,000 t/yr and 190-1050 t/yr, respectively."

L 540: consider to replace the word "slightly higher than" with "more than twice"

 

 

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

I really like this study. Marine debris and plastic pollution are very complex substances and their quantification is tremendously difficult. The authors use very original ways to obtain general estimates without going into details of underlying sources and processes.

The only part that I would recommend to further expand is the discussion, in which the low numbers, estimated in this paper, are compared with previous results of Jambeck et al., Lebreton, and Schmidt. Understanding the total amounts of plastic, entering the ocean, is critical for making right decisions.

I would be great if the authors discuss what sources of marine plastic are possibly missing from their conceptual model, considering only transport by rivers. For example, such extreme events as tsunamis and hurricanes (not only rain but also wind) can significantly increase the input. Also, large sources may be located directly on the shoreline (such as public beaches, resort areas and industrial sites). In addition, large ports, commonly located in the mouths of rivers are not necessarily scaled by the size of associated cities and may be not adequately measured with river surveys.

If including these sources doesn't close the gap between different estimates, I would like to know the authors explanation -- is it because recycling and waste management systems are so advanced in Japan?

It would be also interesting to include estimates of the the seasonal cycle and interannual trend of the plastic input from Japan.

I also spotted numerous parts of the test that require some improvement of English. And sometimes grammatical roughness projects into misused mathematical terms or statements.

For example, in the title of section 3.2 the authors should use 'correspondence' not 'correlation' (which is just a number). In the caption to Fig 4 "numerical concentration" should be "abundance" or counts and "correlation" should be "regression coefficient", which is not non-dimensional in this case and should be described as 0.201 mg/particle. Figure 4 itself is not very helpful as a single observation in the upper right corner has a tremendous weight on determination of correlation and regression. Generally, this datum should be treated as an outlier and excluded from calculations. If regression is obtained as 'total mass' divided by 'total counts', this should be said clearly in the text.

Unfortunately, I don't have time to help with the editing but I recommend that the authors use help of an English-speaking scientist with expertise in the relevant area.

I recommend publication of this paper after minor revision.

Author Response

Please see the attachment.

Author Response File: Author Response.docx