Accumulation and Dispersion of Microplastics near A Submerged Structure: Basic Study Using A Numerical Wave Tank

Round 1

Reviewer 1 Report

This study simulated the surface wave and microplastic movements over a submerged structure, in order to investigate the physical behaviour of microplastics near man-made coastal structures, such as artificial reefs.

The manuscript was generally well written. My only major concern is the implication of the findings, since man-made coastal structures, particularly artificial reefs, come in all different sizes and shapes. I would suggest the authors to further explain how the results reported here could be useful in the real situations.

And please report the shape and test ranges of particle size and density of microplastics in Abstract.

Author Response

Please see the attachment. 

Author Response File: Author Response.pdf

Reviewer 2 Report

This MS discusses the physical movements (dispersion and accumulation) of microplastics near a submerged structure using computational fluid dynamics (CDF). The study is a basic study at the initial stage. There are limitations of the study to derive strong useful conclusions. However still the content is interesting for the readers. There are some suggestions to improve. 

1. All the abbreviations should spell in full in the first place (i.e. CFD in Abstract). 

2. Lines 30-32: "Most plastic waste 30 generated by river runoff and ocean farms or [1-3] can be broken down into microplastics of size smaller than 5 mm." This can be improved to represent present-day knowledge more accurately.  Refer to the below references. 

A. "Ocean farms" shall be replaced with a term to represent marine plastic inputs 

B. "broken down" better replacing with something like "physical, chemical, and biological degradation". Pelase check the references. 

Rapid sampling of suspended and floating microplastics in challenging riverine and coastal water environments in Japan. Water, 12(7), p.1903.

https://doi.org/10.3390/w12071903

"The environmental degradation of plastic is governed by a synergic effect of photo- and thermo-oxidative degradation, abrasion, and biological activities"

 

3. Microplastic Size, " size smaller than 5 mm". Better mention as a range indicating the lower range also. please see these references. 

Chapter 11—Microplastics in wastewater treatment plants. Cur. Dev. Biotechnol. Bioeng. 2022, 1, 311–337. https://doi.org/10.1016/B978-0-323-99874-1.00010-5 

Unraveling Macroplastic Pollution in Rural and Urban Beaches in Sarangani Bay Protected Seascape, Mindanao, Philippines. Journal of Marine Science and Engineering, 10(10), p.1532.

https://doi.org/10.3390/jmse10101532

Rapid sampling of suspended and floating microplastics in challenging riverine and coastal water environments in Japan. Water, 12(7), p.1903.

https://doi.org/10.3390/w12071903

"Based on size, plastics are categorized into macro- (>25 mm), meso- (25–5 mm), micro- (5 mm–0.1 μm), and nano-plastics (<0.1 μm)."

4. Line 118-120: What are the reasons for selecting the two densities 1.225 and 998.2 kg/m3? please explain it in the methods. 

5. Line 146-148: "0.2, 1, and 5 mm as small, medium, and large, respectively" In microplastics studies, 1 mm is within the large-sized particle range.  the 0.2 mm is not small.  Since the range goes from 0.001 mm to 5 mm. the small size range is less than 0.05 or 0.1 mm.  Authors may describe in the MS that the sides considered in the study are larger microplastic ranges only. Within that considered range then authors can describe the large particles and small particles. Then little bit extend the discussion into a small range of microplastics considering extrapolation with a certain level of uncertainty. 

 

 6. Figure 5: provide axes titles (consider the readers with less knowledge of the simulation modeling. provide information to help to understand). 

7. Figures 6 and 7 are too small. enlarge those. and axes descriptions (also legends). 

8. Did the authors discuss the effect of the assumptions made in this study and the impact on the results? (i.e. shape of the particles. Authors assumed spherical particles). Pelase improves the discussion by considering those assumptions and provides recommendations for future studies to improve. 

 

9. Conclusions: Pelase limit only to the supported outcome of the study. No need of writing the general background. (first few sentences are redundant). Line 364 "the results ...." can be the starting sentence.

 

Author Response

Please see the attachment. 

Author Response File: Author Response.pdf

Reviewer 3 Report

General Comments

Le Duc Quyen and Jun Myoung Choi performed a Computational fluid dynamics (CFD) simulations study to simulate the accumulation and dispersion of microplastics in marine environments­­ near a submerged structure. The paper has a good rationale and focuses interesting aspects of the topic.

It is my opinion that the paper has a good structure, but it could be improved. Please consider highlighting in the whole manuscript the fact that the results have limitation in their application to “natural” conditions, and to submerged structures that soon have polygonal and/ or fragmented shape.

 

Specific comments

 

Title:

The title well frames the performed study. Please avoid the use of acronyms.

 

Abstract:

Lines 16-20 are not clear, please rephrase. The conclusion of the abstract is speculative, please consider more caution in defining such an elucidation of all these phenomena on a basis of computational-obtained data.

 

Keywords:

Keywords should not contain terms already used in the title. Please provide different keywords

 

 

Introduction:

The authors well introduce the focused topic. Despite this, it is necessary to make the following corrections:

Line 40: microplastics are not “absorbed by marine organisms”, it is better to say that these particles can enter in contact with marine organisms trough ingestion and/or superficial adhesion. Please have a look to numerous papers about this topic; e.g “Ingestion of plastic and non-plastic microfibers by farmed gilthead sea bream (Sparus aurata) and common carp (Cyprinus carpio) at different life stages”, Savoca et al., 2021 STOTEN.

Line 41: “ingested by humans” also in this case it is better too better define what the authors are writing; in fact, only in the case of whole-body marine products there is the microparticles ingestion risks by humans; e.g.  Plastics occurrence in juveniles of Engraulis encrasicolus and Sardina pilchardus in the Southern Tyrrhenian Sea, Savoca et al., 2020 STOTEN.

Line 43: not all the coastal marine ecosystems are hotspots of MPs abundance, think for example at uninhabited sites. Please correct it specifying which coastal marine ecosystems are more affected by plastic pollution.

Author Response

Please see the attachment. 

Reviewer 4 Report

The article is well written and the quality of the figure and of the manuscript seems to be high. The only comment i have is that many results are actually presented in the discussion section, so i would suggest the authors to first present their results in the appropriate result section, and then discuss them in light of the most recent recent and relevant literature, together with their implications, in the discussion section. 

Author Response

Please see the attachment. 

Round 2

Reviewer 2 Report

The authors have addressed most of the comments in the revised MS. 

 

Just one comment: 

4. Line 118-120: What are the reasons for selecting the two densities 1.225 and 998.2 kg/m3? please explain it in the methods. We took the standard lab environment into account. We added this in lines 121-122.

Line 121-122: We considered the standard laboratory environment such that two domains of air and water phases were created with ambient densities of 1.225 and 998.2 kg/m3 , respectively.

This is done for the coastal environments. But the density used is water. What are the effects of this? please state this as a limitation of the study and consider addressing this in your future studies (actual coastal water density which is higher than 998.2 kg/m3. 

 

Figures 2, 3, 4, 5, 6, etc. Please name the axes with specific titles (do not just provide y (m) and x(m). the reader needs to understand the figure content by looking at the figures.