Abstract

L33: “Some of the highest plastic-polluted rivers end up in the IO with all this…” – this sentence reads a little awkward to me. Suggest changing to something along the lines of “Some of the most plastic-polluted rivers empty into the IO suggesting the IO…”

L48: slight issue with the phrasing, for example discuss vs discussed and identify vs identified. Pick one and be consistent throughout.

Introduction

L33-46: The first paragraph is fascinating; however, it feels inappropriate for this paper (and journal - sorry) given these statements are focused on a historical account of the evolution of plastics in the late 1850s. Even if this were condensed significantly (which I would argue it needs to be, at 14 lines of text it feels too long and detailed – for example, “billiard balls” are mentioned five times), I’m still not convinced it’s the best fit. Instead, could you provide historical context for plastic usage in the SE Asia/IO region? I’ve not seen this information compiled/synthesised in other papers, so that would be a useful contribution.

L49 (and some of the sentences in the paragraph above; also line 57 “35% of all plastic materials”): references are somewhat minimal and/or missing in a few places. For example, this sentence “Since a large percentage of all plastics are single use, “throwaway" packaging items, plastic waste has increased at a similar rate.

Section 2 Sources

L97-100 – no reference(s) provided

L102 – “the total amount of plastic waste produced in 2010 by the USA and China” Here and elsewhere, how do these values such as these compare to more recent (2015) estimates? http://advances.sciencemag.org/content/advances/3/7/e1700782.full.pdf

L110-114 – suggest providing more information on how and when this error was identified, and more importantly, how it has been rectified (this could potentially be included as Supp Info). The level of detail provided here is a little lacking. For example, what exactly is the error originating from the World Bank Data?

L123-124 “The estimates of the amount of plastic waste entering the oceans through rivers by Lebreton et al. (2017) and Schmidt et al. (2017) agree relatively well with each other. In contrast, the estimates by Jambeck et al. (2015) of the amount of plastic waste entering the oceans through coasts are an order of magnitude higher” – it’s useful to synthesise these 3 studies like this, but I’m left wondering what the take home message is beyond what you’ve stated here. Could the authors make some sort of recommendation on how the plastics community should move forward, in light of this? Do we need another of these modelling papers to try and figure out who is “most right” or is the more useful path forward to fill an obvious data gap that would help refine one of the existing models? As the authors are aiming to synthesise information and “recommend future research strategies” it would be useful to answer the “now what” question.

L135 – I find the wording of this sentence to be in an odd order, sorry “Although the International Convention prohibited the dumping of waste from vessels in 1988 for the Prevention of Pollution from Ships (MARPOL)”

L136 – what about ghost nets in Carpentaria, do you have any information on whether some could make their way across to the IO? https://www.sciencedirect.com/science/article/pii/S0016718516302603

L149 – “Commonly used type categories are plastic fibres, fragments, films, and pellets” this is a brief and somewhat uninformative statement as it lacks references and other information. For example, why are these the commonly used categories (why does this matter to the reader)? Consider this paper, or others like it: Serra-Gonçalves, C., Lavers, J.L., Bond, A.L., 2019. Global review of beach debris monitoring and future recommendations. Environ. Sci. Technol. 53, 12158-12167.

L151 – “Size categories as defined by…” this is actually a very complex and actively debated issue that is often over-simplified. GESAMP may have been (one of) the original groups to define these categories, but there’s been much development and learning in the 12 years since the report was published. For example, see Gigault et al. 2018. Current opinion: What is a nanoplastic? Environ. Pollut. 235, 1030-1034.

L207 – definition of Convergent flows isn’t provided until line 210, after it’s first mention. This is a little confusing for readers not familiar with this concept

Sections 4.1 to 4.3 – these are well-written and referenced sections. The level of detail is high, explanations are clear, and I found this useful and enjoyable to read. However, it stands out against other sections which, in comparison, are brief and sometimes feel incomplete (or a little unnecessary). I’m not suggesting you write more elsewhere as your article is already 18 pages – instead, is it possible to focus the paper more on these sections where the author’s clearly have a wealth of knowledge and experience? (and less on the tangential topics, many of which have already been covered in other papers).

Section 5 (fate)

This 1^st paragraph is redundant with earlier sections which also talk about buoyant plastics (e.g., line 181-186) and sinks (e.g., line 59, 80-84, and 174).

L391-395 – an example of one of the brief sections that seems “thrown in” at the last minute (sorry). While this is interesting and does indeed occur, you either need to provide more information on the mechanism of how this actually occurs, or disregard this entirely and focus on other fates. Two refs that you may want to consider:

Cartraud etal. 2019. Plastic ingestion in seabirds of the western Indian Ocean. Mar. Pollut. Bull. 140, 308-314.

Fujieda etal. 2008. Ingestion case of plastics by black marlin and lancetfish caught in the east Indian Ocean. Memoirs of Faculty of Fisheries 57, 47-48.

Section 5.1 – well-written, however I’m not entirely convinced this section adds anything new as it essentially summarises the findings of one paper written by the authors (van der Mheen et al. 2019).

L490 - Abandoned, lost, and discarded fishing gear (ALDFG) already defined on line 136

section 6.1 (ghost nests) is 2 paragraphs, but only one sentence (line 489) contain information or direction refers to the Indian Ocean – can you replace some of this with information more specific to the region?

Acknowledgements - Australia nPostgraduate Award

Figure 7 – the brown arrows and red boundaries are a little difficult to distinguish (perhaps even more so when the image is reduced in size during printing). Can you select another colour, being mindful of folks with colour blindness https://www.ascb.org/science-news/how-to-make-scientific-figures-accessible-to-readers-with-color-blindness/

Table 1 – some entries seem incomplete, plastic size and type data is vailable  at least for Cocos, yes?

Please check attached file.

The manuscript entitled “Plastics in the Indian Ocean – sources, fate, distribution and impacts” written by Charitha Pattiaratchi et al. is a review of plastic pollution in the Indian Ocean. In general, the manuscript has an excellent proposal to show for the scientific community an actual scenario of plastic pollution in the Indian Ocean, mainly when it has scarce information related to other oceans. The manuscript was organized in the following topics: sources (section 2), observations (section 3), transport (section 4), fate (section 5), impact (section 6), prevention and mitigation (section 6) of plastic debris in the Indian Ocean as well as highlight some of the emerging policies and initiatives, knowledge gaps and recommend future research strategies (section 7) (lines 92-95).

However, the manuscript does not have a section for methodology. Then, it does not possible to know how the authors found the papers for this study. 

The authors should be clear in:

1. what platform of science (Scopus, Scholar Google, Web of Science, Science Direct, and other) these papers were downloaded;
2. what keywords were used to find the articles;
3. in what period (time limit) they were downloaded (perhaps from 1980 to 2020 - lines 145-147/Table 01);
4. What criteria were used for inclusion or exclusion of papers?

These questions must be answered because a review article should provide a comprehensive foundation on a topic, explain the current state of knowledge, identify gaps in existing studies for potential future research, and/or highlight the main methodologies research techniques. The authors tried to do it during the manuscript, but I do not have not access their methodology so I can not able to understand the database of the article to build this study.

I reinforce again that in a systematic review with a focused question, the research methods must be clearly described. Besides, a review article does not have an input of new data/results. Therefore, the illustration made by the authors should be excluded from the manuscript (lines 298-315). Also, I think the name of the program is wrong. The correct would not be ichthyop instead of  ICHYTOPOP? (line 300).

2.1 Land-based sources

This topic needs to have an increment of articles, reports, or data from NGOs local, regional about the situation of waste management or plastic pollution in the land.

On Scholar Google, I searched these references:

Vidanaarachchi, C. K., Yuen, S. T., & Pilapitiya, S. (2006). Municipal solid waste management in the Southern Province of Sri Lanka: Problems, issues, and challenges. Waste Management, 26(8), 920-930.

Talyan, V., Dahiya, R. P., & Sreekrishnan, T. R. (2008). State of municipal solid waste management in Delhi, the capital of India. Waste Management, 28(7), 1276-1287.

Patti, T. B., Fobert, E. K., Reeves, S. E., & da Silva, K. B. (2020). Spatial distribution of microplastics around an inhabited coral island in the Maldives, Indian Ocean. Science of The Total Environment, 748, 141263.

2.2 Ocean-based sources

Oceanic islands act as a source and/or a sink of plastic waste. Different studies in both the Atlantic and the Pacific Ocean have been discussing it and I think it should be discussed or at least presented on this topic. Oceanic Islands could be a temporary reservoir when plastics items fragmenting on beaches, for example, and physical forcing takes out them to water surrounding. On the other hand, plastic items could stay there for a long time on the supratidal zone fragmenting itself (final reservoir).

I looked for some articles in the Indian Ocean, but I can find nothing. Therefore, I suggest looking for some articles that could bring this discussion.

On Scholar Google, I find these references for example:

Pham, C. K., Pereira, J. M., Frias, J. P., Ríos, N., Carriço, R., Juliano, M., & Rodríguez, Y. (2020). Beaches of the Azores archipelago as transitory repositories for small plastic fragments floating in the North-East Atlantic. Environmental Pollution, 263, 114494.

Monteiro, R. C., do Sul, J. A. I., & Costa, M. F. (2018). Plastic pollution in islands of the Atlantic Ocean. Environmental Pollution, 238, 103-110.

2.2 Ocean-based sources

Oceanic islands act as a source and/or a sink of plastic waste. Different studies in both the Atlantic and the Pacific Ocean have been discussing it and I think it should be discussed or at least presented on this topic. Oceanic Islands could be a temporary reservoir when plastics items fragmenting on beaches, for example, and physical forcing takes out them to water surrounding. On the other hand, plastic items could stay there for a long time on the supratidal zone fragmenting itself (final reservoir) infinitely.

I looked for some articles in the Indian Ocean, but I can find nothing. Therefore, I suggest looking for some articles that could bring this discussion.

On Scholar Google, I searched these references

Pham, C. K., Pereira, J. M., Frias, J. P., Ríos, N., Carriço, R., Juliano, M., & Rodríguez, Y. (2020). Beaches of the Azores archipelago as transitory repositories for small plastic fragments floating in the North-East Atlantic. Environmental Pollution, 263, 114494.

Monteiro, R. C., do Sul, J. A. I., & Costa, M. F. (2018). Plastic pollution in islands of the Atlantic Ocean. Environmental Pollution, 238, 103-110.

3 Observations

Perhaps this topic is the most approximated to a methodological topic. Thus, this topic should be worked on to improve the mechanism to search articles in this manuscript. Here, it could be defined the kind of reservoirs (biota -  seabirds, invertebrates, mammals, reptiles; sediment – sand mud, water; deep sea) among many other variables. 

On the Scopus base, I used the following keywords "Indian Ocean" and plastic or microplastic and, I found 227 documents (1972-2021). After It was limited to review papers and, I found only seven articles and no one of them was about the purpose of this manuscript. Therefore, the authors have a great an fantastic study proposal. However, it needs to be improved, mainly in the methodology. After that, this article could be reference in plastic pollution in Indian Ocean.

About the whole physical section (section 4)

The proposition of information among these sections with the other is too much different. The topics need an equilibrium because a review is constructed by a global vision of the theme. As a researcher, I know we ended up talking more about what we understand, but we have to control it. 

5. Fate

This topic is good writing in this manuscript because the authors bring a diversity of articles cited. But, because it is the Indian Ocean I think the manuscript should have more information about biological sinks since the literature has some articles about them.

Some suggestions of references

Cliff, G., Dudley, S. F., Ryan, P. G., & Singleton, N. (2002). Large sharks and plastic debris in KwaZulu-Natal, South Africa. Marine and Freshwater Research, 53(2), 575-581.

Carey, M. J. (2011). Intergenerational transfer of plastic debris by Short-tailed Shearwaters (Ardenna tenuirostris). Emu-Austral Ornithology, 111(3), 229-234.

Roman, L., Paterson, H., Townsend, K. A., Wilcox, C., Hardesty, B. D., & Hindell, M. A. (2019). Size of marine debris items ingested and retained by petrels. Marine pollution bulletin, 142, 569-575.

Ryan, P. G. (2008). Seabirds indicate decreases in plastic pellet litter in the Atlantic and south-western Indian Ocean. Mar. Pollut. Bull, 56, 1406-1409.

Sparks, C., & Immelman, S. (2020). Microplastics in offshore fish from the Agulhas Bank, South Africa. Marine pollution bulletin, 156, 111216.

Cartraud, A. E., Le Corre, M., Turquet, J., & Tourmetz, J. (2019). Plastic ingestion in seabirds of the western Indian Ocean. Marine pollution bulletin, 140, 308-314.

Crutchett, T., Paterson, H., Ford, B. M., & Speldewinde, P. (2020). Plastic Ingestion in Sardines (Sardinops sagax) From Frenchman Bay, Western Australia, Highlights a Problem in a Ubiquitous Fish. Frontiers in Marine Science, 7, 526.

McGregor, S., & Strydom, N. A. (2020). Feeding ecology and microplastic ingestion in Chelon richardsonii (Mugilidae) associated with surf diatom Anaulus australis accumulations in a warm temperate South African surf zone. Marine Pollution Bulletin, 158, 111430.

Hoarau, L., Ainley, L., Jean, C., & Ciccione, S. (2014). Ingestion and defecation of marine debris by loggerhead sea turtles, Caretta caretta, from by-catches in the South-West Indian Ocean. Marine Pollution Bulletin, 84(1-2), 90-96.

Pfeiffer, M. B., Venter, J. A., & Downs, C. T. (2017). Observations of microtrash ingestion in Cape vultures in the Eastern Cape, South Africa. African Zoology, 52(1), 65-67.

Lavers, J. L., & Bond, A. L. (2016). Selectivity of flesh-footed shearwaters for plastic colour: evidence for differential provisioning in adults and fledglings. Marine environmental research, 113, 1-6.

Cherel, Y., Xavier, J. C., de Grissac, S., Trouvé, C., & Weimerskirch, H. (2017). Feeding ecology, isotopic niche, and ingestion of fishery-related items of the wandering albatross Diomedea exulans at Kerguelen and Crozet Islands. Marine Ecology Progress Series, 565, 197-215.

Please consider the information and suggestions given for improving the article. I think it has a lot of potentials, but we need to improve some points. The work done was a lot and I'm sure it can get even better.