Microplastics in Inland Saline Lakes of the Central Ebro Basin, NE Spain

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study investigates microplastics in inland saline lakes, a topic of growing environmental concern. However, several issues should be addressed to enhance the manuscript:

Recent Literature on Airborne Microplastic Fibers (Line 118):
The authors should incorporate recent research on airborne microplastic fibers. For example, Xiao et al. (2023) demonstrated that the long-distance atmospheric transport of microplastic fibers is strongly influenced by their shapes (Xiao, S., Cui, Y., Brahney, J., Mahowald, N. M., & Li, Q., 2023, Nature Geoscience, 16(10), 863–870). Including this reference would provide a more comprehensive literature context and strengthen the discussion.

Discussion on the Predominance of Microplastics (Line 223):
The manuscript should reference the above study and discuss why microplastics are found in such high abundance. A detailed discussion on the mechanisms, such as atmospheric transport processes and particle shape influences, would help clarify the observed predominance of microplastics.

Illustrative Figure for Sample Morphology:
It would improve the manuscript if a figure were added to display representative samples of the microplastic particles. Such a figure would visually support the text and offer insights into the morphological characteristics of the particles analyzed.

Clarification of Particle Origin in Figure 3:
In Figure 3, many particles appear black, which may indicate contamination from vehicle tire wear. Given that tire wear particles are typically not fiber-like, the authors should discuss this possibility and clarify the criteria used to distinguish microplastic fibers from other black particles.

Source Attribution of Atmospheric Deposition (Line 399):
The claim that atmospheric deposition is the most significant source of microplastics requires further substantiation. The manuscript should provide a detailed explanation of the methodology used to identify and quantify atmospheric deposition and justify why it is considered the dominant source.

Addressing these points will substantially improve the clarity and scientific rigor of the manuscript.

Author Response

Comments provided by Reviewer #1:

This study investigates microplastics in inland saline lakes, a topic of growing environmental concern. However, several issues should be addressed to enhance the manuscript:

 

POINT #1: Recent Literature on Airborne Microplastic Fibers (Line 118):

The authors should incorporate recent research on airborne microplastic fibers. For example, Xiao et al. (2023) demonstrated that the long-distance atmospheric transport of microplastic fibers is strongly influenced by their shapes (Xiao, S., Cui, Y., Brahney, J., Mahowald, N. M., & Li, Q., 2023, Nature Geoscience, 16(10), 863–870). Including this reference would provide a more comprehensive literature context and strengthen the discussion.

 

RESPONSE #1: Following reviewer’s suggestion, we have included the above study in the manuscript as we think it is relevant to the discussion of our results. This reference has been included in section “4. Discussion” as number [56] along with the following sentence:

 

“According to Xiao et al. [56], MPs with a fiber-like morphology present a lower density, high resistance, and slower sedimentation rate, which contributes to remain suspended and be air transported over long distances”.

 

POINT #2: Discussion on the Predominance of Microplastics (Line 223):

The manuscript should reference the above study and discuss why microplastics are found in such high abundance. A detailed discussion on the mechanisms, such as atmospheric transport processes and particle shape influences, would help clarify the observed predominance of microplastics.

 

RESPONSE #2: Explanation of the high abundance of MPs in these saline lakes, compared to previous reported results in different lakes around the world, is given in section “4. Discussion”. But, following reviewer's comment, we have included the following paragraph in section “3.1. Presence of MPs”, in order to clarify the observed predominance of MPs:

 

“The studied saline lakes are endorheic and thus terminal basins and suffer from irregular and long drought cycles, so pollutants can accumulate during the filling and evaporation cycles of the lakes and never outflow the basin. Given the ephemeral nature of the lakes, the aeolian dispersal of MPs retained on the surface of sediments may also be a contributing factor. This effect can be especially observed in lakes C and D, which are significantly smaller than lakes A and B and show a higher presence of MPs. Besides this, these lakes are surrounded by farmland and industrial activities, and lakes C and D are also located in closed depressions delimited by escarpments. Also, the size of the filter, smaller than filters used elsewhere, contributes to retaining more MPs”.

 

 

 

 

POINT #3: Illustrative Figure for Sample Morphology:

It would improve the manuscript if a figure were added to display representative samples of the microplastic particles. Such a figure would visually support the text and offer insights into the morphological characteristics of the particles analyzed.

 

RESPONSE #3: We thank the reviewer for this suggestion. An illustrative figure with different color MPs was already included in Supplementary Material, but we have added it to the manuscript as Figure 2 in section “2.4. Analytical procedure”.

 

POINT #4: Clarification of Particle Origin in Figure 3:

In Figure 3, many particles appear black, which may indicate contamination from vehicle tire wear. Given that tire wear particles are typically not fiber-like, the authors should discuss this possibility and clarify the criteria used to distinguish microplastic fibers from other black particles.

 

RESPONSE #4: We agree with reviewer’s opinion as tire wear particles are a common source of MP pollution, although these lakes are located in remote areas with very low road and traffic density. But our results show that the morphology of the black microparticles are fiber-like. This morphology can be appreciated in Figure 2 (a, d, and f) in the manuscript. The studied saline lakes are surrounded by farmland, industrial and farm activities, animal feed factories, and plastic greenhouses. Black MPs can be derived from the plastic fabrics used in these activities, which would explain the fiber-like morphology of the MPs.

 

POINT #5: Source Attribution of Atmospheric Deposition (Line 399):

The claim that atmospheric deposition is the most significant source of microplastics requires further substantiation. The manuscript should provide a detailed explanation of the methodology used to identify and quantify atmospheric deposition and justify why it is considered the dominant source.

 

RESPONSE #5: We also consider that a detailed explanation to identify and quantify atmospheric deposition is very interesting and important to complete our study. Unfortunately, the scope of our manuscript does not include this study. Despite this, we have included several reasons to justify why atmospheric deposition is considered a dominant source in section “4. Discussion”:

• Lines 338-340: “Particularly, Gallocanta Lake is a large lake also with outstanding wind activity affecting the water and sediments.”
• Lines 341-343: “Salineta and Pito are found in closed depressions delimited by escarpments and located close to industrial and intensive farm activities, thereby facilitating the accumulation of plastic debris and MP fallout.”
• Lines 359-361: “The presence of MPs in saline wetlands in a region (Gallocanta Lake) that features one of the lowest population densities in Europe, further hints at its atmospheric origin.”
• Lines 369-373: “The shape of the bottom of the basins may in turn influence the flow of water, which in combination with the wind, may facilitate accumulation in certain parts of the basin. This is especially relevant in lakes with windward barriers, longshore currents and coastal sedimentary forms favoring the accumulation and sedimentation processes as it happens in Gallocanta Lake.”

 

In general, there is no human activity in the lakes, and therefore MPs may reach them either by runoff from nearby fields or from aeolian dispersal. Plastics from runoff tend to be more irregularly shaped, and not fiber-like, as they come from bags or tires.

Reviewer 2 Report

Comments and Suggestions for Authors

General comments: This study provides a valuable contribution to the emerging field of microplastic (MP) pollution in fragile saline lake ecosystems. The authors present compelling evidence of exceptionally high MP concentrations in the Central Ebro Basin (CEB) lakes, highlighting their endorheic nature and anthropogenic pressures as key drivers. The methodological rigor, including micro-FTIR analysis and consideration of atmospheric deposition, strengthens the findings. However, the limited sample size (four lakes, single sampling campaign) and lack of sediment or biota data constrain broader ecological implications. The discussion effectively contextualizes results within global MP research but could better address uncertainties (e.g., cellulose fiber origins, aging effects). Overall, this work underscores the urgency of protecting saline wetlands and lays a foundation for future studies on MP dynamics in arid regions.

Specific comments:
Sampling Depth: The paper states samples were taken at 20 cm depth but does not justify why this depth was chosen. How representative is this of the entire water column, especially in deeper lakes like Gallocanta?
Fiber Dominance: While fibers dominate (>98%), no discussion addresses potential contamination from lab materials (e.g., cotton coats) despite cellulose fibers being detected. Clarify mitigation steps.
Polymer Variability: The high PA/PP percentages in lakes C/D are attributed to nearby industries, but no direct evidence (e.g., polymer matching to local sources) is provided.
Hydrological Data: Water inflow/outflow percentages (e.g., 50% atmospheric precipitation) lack citations. Provide sources or clarify if these are original estimates.
Temporal Limitations: A single sampling in December 2022 may not capture seasonal variability (e.g., agricultural runoff peaks). Address this limitation.
MP Concentration Units: Table 2 reports MPs/L, but the text later compares MPs/m³ (e.g., Alfonso et al., 2020). Standardize units for clarity.
Cellulose Exclusion: The decision to exclude cellulose fibers is reasonable but requires validation (e.g., FTIR spectra in Supplementary Material).

Author Response

Comments provided by Reviewer #2

 

This study provides a valuable contribution to the emerging field of microplastic (MP) pollution in fragile saline lake ecosystems. The authors present compelling evidence of exceptionally high MP concentrations in the Central Ebro Basin (CEB) lakes, highlighting their endorheic nature and anthropogenic pressures as key drivers. The methodological rigor, including micro-FTIR analysis and consideration of atmospheric deposition, strengthens the findings. However, the limited sample size (four lakes, single sampling campaign) and lack of sediment or biota data constrain broader ecological implications. The discussion effectively contextualizes results within global MP research but could better address uncertainties (e.g., cellulose fiber origins, aging effects). Overall, this work underscores the urgency of protecting saline wetlands and lays a foundation for future studies on MP dynamics in arid regions. 

Specific comments: 

POINT #1: Sampling Depth: The paper states samples were taken at 20 cm depth but does not justify why this depth was chosen. How representative is this of the entire water column, especially in deeper lakes like Gallocanta? 

 

RESPONSE #1: Indeed, studying the biota and the sediment would provide invaluable data for the ecological processes occurring in the lake and this is a line of research that should be explored in the future. Given the fact that these are terminal lakes, the age of the MPs would also be a good indicator of their origin and dispersal. Hence, another line of research that opens is studying the age of MPs, a line the authors are currently exploring.

 

The 20 cm depth was to avoid capturing floating debris due to surface tension. Most basins are not much deeper than that, and we wanted to avoid stirring the sediment. It is also practical from the point of view of fieldwork, as it does not need extra equipment (e.g., ropes) that risk contaminating the samples.

POINT #2: Fiber Dominance: While fibers dominate (>98%), no discussion addresses potential contamination from lab materials (e.g., cotton coats) despite cellulose fibers being detected. Clarify mitigation steps. 

 

RESPONSE #2: Potential contamination from lab materials was controlled with three blank samples. As explained in the manuscript in lines 192-194 in section “2.3. Materials and sample preparation”: “Three 1L glass bottles were used as a blank, with 10 mL of H₂O₂ 30% v/v and 50 mL of ultrapure water, to control possible contamination during laboratory procedures.” The same procedure used to prepare and analyze the saline water samples was followed with only ultrapure water. All samples were manipulated in the hood, to avoid contamination from coats and background cellulose. After that, MP presence in the three blank experiments was subtracted from MP presence in saline water samples. Blank samples helped assess this background contamination and subtract it. Therefore, results shown in the manuscript do not include any kind of lab contamination.

 

POINT #3: Polymer Variability: The high PA/PP percentages in lakes C/D are attributed to nearby industries, but no direct evidence (e.g., polymer matching to local sources) is provided. 

 

RESPONSE #3: The main human activities in the areas sampled were irrigated and rainfed agriculture. In some areas (e.g. Sariñena) there is a presence of plastic greenhouses, thereby increasing the presence of PP MPs.

POINT #4: Hydrological Data: Water inflow/outflow percentages (e.g., 50% atmospheric precipitation) lack citations. Provide sources or clarify if these are original estimates. 

 

RESPONSE #4: We apologize for this mistake. Reference for water inflow/outflow percentages is:

 

[45] García, M. A., & Castañeda, C. Las saladas de Bujaraloz-Sástago. Naturaleza Aragonesa 2005, 14, 52-58.  

 

This source has been included in line 160 of the manuscript.

POINT #5: Temporal Limitations: A single sampling in December 2022 may not capture seasonal variability (e.g., agricultural runoff peaks). Address this limitation. 

 

RESPONSE #5: While we understand this is a limitation, we are confident that a years’ worth of MPs is captured, given the terminal nature of the lakes. In fact, most MPs may be there for a long time and studying their age may help assess their distribution and dispersal.

POINT #6: MP Concentration Units: Table 2 reports MPs/L, but the text later compares MPs/m³ (e.g., Alfonso et al., 2020). Standardize units for clarity. 

 

RESPONSE #6: We have standardized MP concentration units to MPs/L in the manuscript in section “4. Discussion”. The reason for having shown Alfonso et al.’s results as MPs/m³ was that these authors used this concentration unit in their paper.

 

POINT #7: Cellulose Exclusion: The decision to exclude cellulose fibers is reasonable but requires validation (e.g., FTIR spectra in Supplementary Material). 

 

RESPONSE #7: The cellulose FTIR spectrum is already included in Figure S1 Supplementary Material, along with linen, cotton, and the artificial polymers found in our study (polyester, polypropylene (PP), polyethylene terephthalate (PET), polyacrylonitrile (PA), nylon, styrene/butadiene copolymer (STY/BUT), and polyvinyl chloride (PVC)).

 

 

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript presents a well-grounded study within a theoretical framework that is both solid and easy to understand. The importance of the sample and the sampling site is clearly established. The results are thoroughly systematized, described, and analyzed. The information regarding the quantity, size distribution, morphology, and color of the microplastics (MPs) reveals that highly relevant and novel environmental data were obtained for the sampled sites. The interpretation of the results is strongly supported by theory and the existing literature. Comparisons with findings from other studies in lakes are excellent and demonstrate that the level of contamination is very high, which adds further value to the publication of these results.

The discussion provides deep insights, especially concerning the importance of pursuing future research on other aspects—such as assessing the age of MPs deposited at these sites—to better understand their distribution mechanisms. The final paragraphs of the discussion are particularly inspiring in this regard. Although the sample size of this study is limited, it was sufficient to draw meaningful environmental conclusions and to guide future research in this field.

The methodology used to identify MPs includes enough detail to allow reproducibility. However, some aspects regarding its underlying rationale are not entirely clear. For instance, the manuscript states that water samples were treated with H₂O₂, which is a reasonable approach for removing organic matter. Nevertheless, it is not specified whether this procedure has been validated in the literature in terms of both its efficiency in removing organic material and its ability to preserve MPs. I recommend including scientific references to support this procedure.

In conclusion, I suggest that this manuscript be accepted for publication, with only minor revisions to strengthen the methodological justification for the sample treatment process.

Author Response

Comments provided by Reviewer #3

This manuscript presents a well-grounded study within a theoretical framework that is both solid and easy to understand. The importance of the sample and the sampling site is clearly established. The results are thoroughly systematized, described, and analyzed. The information regarding the quantity, size distribution, morphology, and color of the microplastics (MPs) reveals that highly relevant and novel environmental data were obtained for the sampled sites. The interpretation of the results is strongly supported by theory and the existing literature. Comparisons with findings from other studies in lakes are excellent and demonstrate that the level of contamination is very high, which adds further value to the publication of these results. 

The discussion provides deep insights, especially concerning the importance of pursuing future research on other aspects—such as assessing the age of MPs deposited at these sites—to better understand their distribution mechanisms. The final paragraphs of the discussion are particularly inspiring in this regard. Although the sample size of this study is limited, it was sufficient to draw meaningful environmental conclusions and to guide future research in this field. 

The methodology used to identify MPs includes enough detail to allow reproducibility. However, some aspects regarding its underlying rationale are not entirely clear. For instance, the manuscript states that water samples were treated with H₂O₂, which is a reasonable approach for removing organic matter. Nevertheless, it is not specified whether this procedure has been validated in the literature in terms of both its efficiency in removing organic material and its ability to preserve MPs. I recommend including scientific references to support this procedure. 

In conclusion, I suggest that this manuscript be accepted for publication, with only minor revisions to strengthen the methodological justification for the sample treatment process. 

 

RESPONSE:

We truly appreciate the comments provided by the reviewer.

The use of H₂O₂ to remove organic matter from the sample has been previously validated in the literature. A solution of H₂O₂ 30% v/v can degrade organic residues but not MPs and allows targeted MPs to be freely from this residue and to be observed by microscopy. Some references using the same methodology are the following ones:

• Dwiyitno, D., Sturm, M.T., Januar, H.I., Schuhen, K. Influence of various production methods on the microplastic contamination of sea salt produced in Java, Indonesia. Environmental Science and Pollution Research, 2021, 28(23), 30409-30413.
• Yaranal, N.A., Subbiah, S., Mohanty, K. Identification, extraction of microplastics from edible salts and its removal from contaminated seawater. Environmental Technology & Innovation, 2021, 21, 101253.
• Putri, E.B.P, Syafiuddin, A., Aini, S.A., Iswahyudi, I., Garfansa, M.P. Identification and quantification of microplastics in seawater and sea salt collected from sea salt ponds. Desalination and Water Treatment, 2023, 300, 130-135.