Marine Plastic Drift from the Mekong River to Southeast Asia

Round 1

Reviewer 1 Report

The authors present new results on modeling floating plastic from the Mekong river. The study design is convincing and well described. The text is easy to follow and the results are clearly presented. The topic of the research is of high importance and will gain attendance of scientists working in this field. Therefore I recommend to publish the manuscript in J. Mar. Sci. Eng. with minor alterations.

Specific comments:

Line 15 ff Please add two sentences on outlook and overarching benefit of the study (compare my comment to section conclusion).

Line 25ff It remains unclear to the reader, if the present study focussed on floating microplastic only or if also floating plastic litter items are covered. Plaese explain.

Line 74: Concerning sinking plastic litter in the sea related to polymer densities, I suggest to consider https://doi.org/10.1016/j.marpolbul.2021.112876 for citation.

Line 177 Fig 4: legend and coordinates outside the map are hardly visible. Please consider an alternative layout.

Line 177 Fig 5: Text in the figure is too small and hardly readable. Please consider to use a bigger font or explain the countries in the figures legend.

Line 230 Fig 10: Blue dots are hardly visible on a blue background. Please consider to choose another colour ar any other way to enhance visibility.

Figs 6, 7,10, A7, A8: The subheadings inside the figure (not the caption), sometimes standing for (a) ob (b) part of the Figure should go to the figures caption.Otherwise fonts are mixed.

Line 331 ff Conclusion: The section "conclusion" is at the moment the summary of results only, as the authos state in line 338. What is missing here is a conclusion or outlook with a wider perspective. Please add a new paragraph here which may touch some of the following questions (or other higher level questions choosen by the authors): How does the findings help to improve future modelling of plastic debris? Do the findings help to describe e.g. global distribution or fate of floating plastic? How? Can the findings contribute to reduce plastic in the seas? How? Who could benefit from the findings? Are the results in line with overarching goals of international organisations protecting the environment?

Author Response

Reviewer #1:

The authors present new results on modeling floating plastic from the Mekong river. The study design is convincing and well described. The text is easy to follow and the results are clearly presented. The topic of the research is of high importance and will gain attendance of scientists working in this field. Therefore I recommend to publish the manuscript in J. Mar. Sci. Eng. with minor alterations.

Specific comments:

Line 15 ff Please add two sentences on outlook and overarching benefit of the study (compare my comment to section conclusion).

We have added a few sentences according to the reviewers recommendation.

Line 25ff It remains unclear to the reader, if the present study focussed on floating microplastic only or if also floating plastic litter items are covered. Please explain.

This study focused on microplastics only, and the plastics can both float and sink depending on their buoyancy. Buoyancy was expressed as terminal velocity, and is usually a function of particle density, diameter, and shape (line 148). Terminal velocity in this study by default is assumed to be +0.01 m/s, upward (line 150). However, when considering the influence of terminal velocity on plastic drift, the terminal velocity is changed to -2 and -5 m/d, downward (line 227).

This also may cover floating plastic litter items that have the same buoyancy. However, practically the buoyancy of plastic litter items greatly varies. 

Line 74: Concerning sinking plastic litter in the sea related to polymer densities, I suggest to consider https://doi.org/10.1016/j.marpolbul.2021.112876 for citation.

The article is now cited according to recommendation.

Line 177 Fig 4: legend and coordinates outside the map are hardly visible. Please consider an alternative layout. 

We have tried to improve this figure and hope it is satisfactory now.

Line 177 Fig 5: Text in the figure is too small and hardly readable. Please consider to use a bigger font or explain the countries in the figures legend. 

We have increased the font size. We hope it works.

Line 230 Fig 10: Blue dots are hardly visible on a blue background. Please consider to choose another colour ar any other way to enhance visibility. 

We have tried to improve the visibility of this figure.

Figs 6, 7,10, A7, A8: The subheadings inside the figure (not the caption), sometimes standing for (a) ob (b) part of the Figure should go to the figures caption.Otherwise fonts are mixed. 

We have corrected these issues according to recommendations. 

Line 331 ff Conclusion: The section "conclusion" is at the moment the summary of results only, as the authos state in line 338. What is missing here is a conclusion or outlook with a wider perspective. Please add a new paragraph here which may touch some of the following questions (or other higher level questions chosen by the authors): How does the findings help to improve future modelling of plastic debris? Do the findings help to describe e.g. global distribution or fate of floating plastic? How? Can the findings contribute to reduce plastic in the seas? How? Who could benefit from the findings? Are the results in line with overarching goals of international organisations protecting the environment?

We thank for these good suggestions. We have tried to put our results in a wider perspective and reformulated the conclusion.

Reviewer 2 Report

Dung et al. have modeled the dispersion of plastics carried out of the Mekong River.  The rationale for this work is compelling, and results detail the important roles of wind drift, vertical mixing, and sinking rate.  I have only a few minor comments. 

Line 42-45:  Move the questions addressed to the end of the Introduction

Line 51:  make clear these are wind speeds!!

Line 192 and throughout:  instead of “Fig 6 shows the seasonal drift and the influence of wind drift in the summer (a) and winter (b) with wind direction,” sometime like “Wind direction and season influence… (Fig. 6)”

Line 292:  “…more efficiently” than what?

Any observational studies that support the model predictions?

Author Response

Reviewer #2:

Dung et al. have modeled the dispersion of plastics carried out of the Mekong River.  The rationale for this work is compelling, and results detail the important roles of wind drift, vertical mixing, and sinking rate.  I have only a few minor comments. 

Line 42-45:  Move the questions addressed to the end of the Introduction

We have moved the questions according to the recommendation.

Line 51:  make clear these are wind speeds!! 

We have clarified according to recommendations. 

Line 192 and throughout:  instead of “Fig 6 shows the seasonal drift and the influence of wind drift in the summer (a) and winter (b) with wind direction,” sometime like “Wind direction and season influence… (Fig. 6)”

The sentence has been rewritten according to the suggestion.

Line 292:  “…more efficiently” than what?

The sentence has been reformulated to improve clarity.

Any observational studies that support the model predictions?

Unfortunately we have not found any large scale observations that can support our predictions.