This article presents a model of plastic pollution in the Mediterranean area. A mass budget for the year 2015 was proposed, and a box-model was calibrated to evaluate different OECD policy scenarios up to 2060. All steps and assumptions of the method are well-justified, and the results and conclusions are thoroughly developed. The authors critically examine the limitations of the current model and provide suggestions for improvement, although some of these potential improvements strongly depend on further studies for determining microplastics in the various environmental compartments considered. In particular, the lack of information on the concentration of nanoplastics in these compartments leads to a portion of the plastic pollution being 'missed,' an issue that must be addressed in future research. This type of study is crucial for understanding the potential impact of plastic waste reduction policies and for emphasizing the lack of ambition in current proposals. It is key to fostering the development of more robust and effective policies. This article is recommended for publication after minor revisions. The comments are provided below.

Abstract

Line 4 : « based on recent observations. » be more specific, From what year is the literature used in this study?

Introduction :

Line 30 : Introduce the idea that plastic waste fragments in the environment, which in turn influences its mobility.

Line 51 : Please specify what is referred to as plastic litter in this context. Later in the text, it appears to include macroplastics, but does it also encompass microplastics?

Lines 50-55 : What can explain the variability between studies, and how does your study address these uncertainties?

Methods

Lines 90-91 : « small microplastics (SMP) defined as 1 μm ≤ L<0.3 mm ». "Why this category? It is not a standardized size classification in the literature."

Lines 96-97 : « Fragmentation is caused by (photo-)oxidation, structural fragilization and mechanical abrasion at a rate of approximately 3% per year (Chamas et al., 2020; Lebreton et al., 2019; Sonke et al., 2022, 2024) ». This information should already be presented in the introduction, see coment below

 Tables S1 and S2 : How do you explain that some of your sd are 0.0?

Lines 191-195 : SMP extrapolation from global data. Could you please explain in more detail how you performed this extrapolation? Why is this the best option? Could an extrapolation from P and LMP in the Mediterranean be considered as an alternative?

Lines 209-210 : It is more detailed, rather than being contradictory.

Results and Discussion

 -line 321 : It is important to note here that « The Ocean Cleanup » is a non-profit organization

This study explored the fate of plastic waste generated in the Mediterranean Sea catchment and authors proposed a box-model to evaluate different OECD policy scenarios toward 2060. This is a highly forward-looking study, essential for the development of public policy. The model is well presented and results are interesting. I recommend this paper for publication after major revision.

Major comments and other comments

I recommend authors stating right from the introduction that this is a prospective, exploratory exercise, that it tries to take into account a maximum of plastic pollution (large, small, etc.), but that it cannot consider all categories for all the compartments but this model tries to take as much as he can. I appreciated the limitation of the model section, which really help to understand what the model proposes or not.

My first and solely concern is the concept of runoff, which still unclear from my side. L70. of plastic runoff from land to sea. What do you mean by runoff form land-to-sea? Remobilization of all plastic litter undependably of the connection with river first and then Sea? Or direct runoff to the Sea? It’s also parametrized to consider the intensity of the rain? A major conclusion of this study is the important of runoff – what I would call river discharges during high flow periods or during flooded but not definitely runoff. From my point of view, this concept, or what runoff mean, should be better explained.

Implementation of the OECD Global Ambition policy scenario, that targets near-zero new plastics waste leakage, would not significantly lower this stock (25 Mt, median, IQR 12-44 Mt) by 2060. Totally agree with this conclusion. Very important as regard the OCED recommendation and the objectives. Behind this policy scenario, the main idea is to significantly reduce the plastic consumption and not necessary the plastic pollution in the environmental compartment. One other important idea is the “legacy stock” of plastic.

L15. his underlines the necessity to address upstream legacy plastic waste on land. How upstream? On land? Or for the consumption?

L24. Plastic items are also very mobile due to their relatively low density and buoyancy, and can travel long distances by rivers and ocean currents. Some studies suggest that significant amount of plastic litter can be also trapped along the river banks and flooded pain aera.

L69. This study investigates the fate of plastic waste in the Mediterranean Sea catchment across various environmental compartments 70 (terrestrial, sea surface and water column, shelf and deep sediments, beaches and atmosphere). Until here, we can not determine if the study will focus on plastic litter only or include microplastic. If atmosphere is included, probably microplastic is considered. If yes, why? It’s clearer L90-95. L280. Results on the atmospheric SMP cycle are not presented here, as they are not yet constrained enough in the Mediterranean. I suggest to remote this section to the core manuscript. It can be mentioned in model structure.

region to provide reasonable levels of uncertainty

Figure 2. Which data are used to build this figure?

L112. Plastic mass transport between boxes is approximated to be first order, where the plastic flux FA→B [Mt y-1] from box A to box B is proportional to the plastic mass MA [Mt] in box A. How can we justify this assumption? Which implication would have a different one? It’s here a question of the assumption sensibility.

Table 1. For each study, in addition to the stock considered, which compartments are considered? This is important since in your model you consider all compartements.

2.4.6 Runoff to sea surface. Here what is the concept of runoff? It’s the discharges by river during high flows/flooded?

1. The geographical distribution of plastic runoff was found to be dominated by S. Europe (87.9%), followed by N. Africa & M. East (12.0%) and the Nile basin (0.1%). Do this observation is linked to the crossed explanation, population density and high flows of river? In contradiction with other studies (as underlined by authors). This has a very strong implications for policy. I would suggest to explain more why you found these contradictory results and how our approach is relevant in regards to others studies.

L395. his means that most of MMPW is still in terrestrial areas, which are not detailed yet in the model. Totally agree.

Results section. Do the results/conclusions will be different by considering only the large fraction of plastic litter (> 5 mm)?

Minor comments

L4. for the Mediterranean region. Please clarify the aera.

L54. Simon-Sánchez et al. (2022) reviewed and reported concentrations in sediments (300 items kg-1) and beaches (60 item kg-1), insisting on the high uncertainties and 55 variability between studies. You discussed here about microplastics. What is the link with plastic litter mentioned?

L60. Sea litter by Cózar et al. (2024) highlighted the close relationship between marine litter occurrence and heavy rainfall events, pointing at Southern Europe as the largest macroplastic source to the Mediterranean Sea. What is their hypothesis?

L142. The year 2015 is chosen as reference for calibration. Why?

Table 2. What is the unit of the first reported concentration? 5.6 10-3

L289. we calculare. Calculate