Microplastic Contamination of Fine-Grained Sediments and Its Environmental Driving Factors along a Lowland River: Three-Year Monitoring of the Tisza River and Central Europe

Comment of the Reviewer

Response

Title: Change on to of. 

The title was corrected as:

“Microplastic contamination of fine-grained sediments and its environmental driving factors along a lowland river: Three-year monitoring of the Tisza River, Central Europe”

Introduction: What is a hotspot of microplastic?

Based on the literature every study defines the hot spots in a different way. In the present study we defined a hot spot as:

L. 263: “A site was defined as a hotspot if the MP content was ≥2000 items/kg”

Introduction: What is the significance of the current study? It might benefit from more direct connections between the problem statement and the specific aims of this study. 

The environmental driving factors are highlighted in the abstract. Unfortunately, there is a 200-word limit for the abstract, so we could not add everything. Thus, it was corrected as:

L. 19-24: “The MP pollution is influenced by highly variable environmental factors. (1) The geomorphological setting of a site is important, as most of the hot spots were on side bars. (2) The tributaries convey MP pollution to the Tisza River. (3) The bankfull or higher flood waves effectively rearrange the MP pollution. (4) The dams and their operation influence the downstream trend of MP pollution in the reservoir. (5) Downstream of a dam, the clear-water erosion increases the proportion of the pristine sediments; thus, the MP concentration decreases..”

Methods: Ensure consistent terminology throughout the section. Ensure clarity in technical descriptions related to laboratory techniques and equipment used in the analysis.

The Method chapter was checked to support consistent terminology. The wet sieving was explained.

L. 189-192: “Since the high clay content made it difficult to separate the MPs, the silt and clay particles were removed by wet sieving (mesh size: 90 µm) following Atwood et al. [26]. The samples were rinsed until the run-off became clear. The material from the sieve was washed into a glass jar. The remaining inorganic material was separated using a zinc chloride solution (1.8 g/cm³)…”

How were sediment samples collected and transported to the laboratory? Consider providing a brief rationale or justification for choosing specific sites. Explain why these were deemed important or representative of the overall study area.

Corrected as:

L. 164-167: “Preliminary sampling points were set in 2019 [21]. The sampling sites were evenly spaced in approx. every 30 km. [...] Every sampling was carried out on the middle of a sedimentary body at the water line.”

L. 180-181: “Samples were stored in glass jars.”

Results: If possible, provide concise explanations for unexpected or notable findings.

The explanations are in the Discussion section. But to make the more highlighted, we also made short explanations in the Conclusion. See below.

Line 258: what is fever? 

Corrected as:

L. 272 “They contained fewer MP particles (2260-2660 items/kg) than the hotspots in the previous year.”

5- Conclusion: provide a summary of the results, not the methods. 

It was a great suggestion! Thank you very much! The Conclusions chapter was corrected as:

L. 625-645: “The performed research in the Tisza River’s water system (Central Europe) is a good example of long-term (three years) monitoring along a ca. 750 km long reach. The study proved the high spatial and temporal variations of MP pollution and analyzed its various influencing factors. Flood waves can effectively rearrange the MP pollution along a river, and could support the emptying of former pollution hot spots and the development of new one. Floods with high specific stream power (e.g. bankfull floods) play very important role in this rearrangement [32]; but the event sequence is probably important too, as in case of several, subsequent flood waves the first one is the most effective. [7,31] Therefore, during the planning of the sampling it is advised to carefully study the hydrological characteristics of a river. Our sampling was performed on three different accumulational channel forms. It proved, that these forms trap the MPs with different efficiency, and collecting sediments from different forms could even influence the downstream trends in MP pollution. Thus, it is suggested to select a form which is common all along the river and perform the sampling from its freshly deposited, fine-grained sediments. Along the river any natural (e.g., tributaries, rapids and pools) or artificial structure (e.g., reservoirs, dams and grading structures) which influences the water and sediment discharge conditions of the main river must be considered during the sampling, as they could fundamentally influence the MP pollution of a site. It also must be emphasized that a fluvial system should be considered a whole unit because the spatiality of the elements (e.g., MP sources, tributaries, reservoirs, hotspots) influences the cause-effect relationships and reveals the processes at individual sites.”

Comment of the Reviewer

Response

The coverage of the river is good considering its great extension but, evidently, the great variability in microplastic contents at the same points is normal because the great variability of the environment…a river bed is something that changes in the order seconds/minutes…

Yes, we agree. But we think, that this kind of approach is very rare, so the fluvial (geographical) background of any MP study should get a highlight. Therefore we started the research.

Indeed, the observed variability in microplastics content is probably due to the limitations of the study, i.e. the number of sampled points, the surface surveyed at each point and the timing of surveys…but it is normal, it is impossible to cover a so large area at the same time…So, again, I liked and appreciated the manuscript but I strongly suggest Authors to explain the variability observed with the great variability of the environment….

Thanks, we agree again. To sample the entire length of the river lasted for 10 days; thus, not necessarily under the same hydrological conditions.

The great variability is emphasized in the Abstract and also in the Conclusion as:

L. 18-24: “The MP pollution is influenced by highly variable environmental factors. (1) The geomorphological setting of a site is important, as most of the hotspots were on side bars. (2) The tributaries convey MP pollution to the Tisza River. (3) The bankfull or higher flood waves effectively rearrange the MP pollution. (4) The dams and their operation influence the downstream trend of MP pollution in the reservoir. (5) Downstream of a dam, the clear-water erosion increases the proportion of the pristine sediments; thus, the MP concentration decreases.”

L. 626-642: “The study proved the high spatial and temporal variations of MP pollution and analyzed its various influencing factors. Flood waves can effectively rearrange the MP pollution along a river and could support the emptying of former pollution hotspots and the development of new one. Floods with high specific stream power (e.g. bankfull floods) play very important role in this rearrangement [32]; but the event sequence is probably important too, as in case of several, subsequent flood waves the first one is the most effective. [7,31]. Therefore, during the planning of the sampling it is advised to carefully study the hydrological characteristics of a river. Our sampling was performed on three different accumulational channel forms. It proved, that these forms trap the MPs with different efficiency, and collecting sediments from different forms could even influence the downstream trends in MP pollution. Thus, it is suggested to select a form which is common all along the river and perform the sampling from its freshly deposited, fine-grained sediments. Along the river any natural (e.g., tributaries, rapids and pools) or artificial structure (e.g., reservoirs, dams and grading structures) which influences the water and sediment discharge conditions of the main river must be considered during the sampling, as they could fundamentally influence the MP pollution of a site.”

I also agree with data observed at “key” points along the river, e.g. dams, joints, etc….such points have indeed a minor variability and a clearer trend….so differences are not so important from one year to another…I mean, upstream of a dam there is always accretion…see my comments in following lines..

Yes, the deposition is clear, but the one reservoir has declining MP concentration trend, while the other has increasing trend towards downstream.

Here the reason of the differences could be explained as:

L. 539-548: “The three dams on the Tisza River resulted in different reservoir types. The uppermost and the lowest dams (at Tiszalök and Novi Becej) impound the Tisza River to the bankfull level (in-channel reservoir). However, the reservoir behind the Kisköre Dam is different, as in its upstream half, the impounded water reaches the bankfull level, but in its downstream part, the impounded water inundates the floodplain, creating a shallow and wide lake. Therefore, the sediment dynamics of the reservoirs are different, especially because during flood waves, the floodgates are open so that the sediment can be mobilized from the in-channel reservoirs [40,41], but the deposited sediments in the floodplain lake cannot be redistributed. This hydrological difference explains the pollution patterns in the downstream part of the impounded water bodies.”

Another observation is about the analysis of microplastics….no results are presented at all about colors, typology and so on…..why? Maybe such characteristics can be useful to use microplastics as “natural” tracers as it has been done in coastal environments with heavy minerals or foraminifera….it is just an idea…..

Yes, you are right. Their detailed analysis was published previously in Balla et al. 2022. Spatial Variations in Microfiber Transport in a Transnational River Basin. But for clarity it was added:

L. 463-465: “As the dominance of fibres (≥92%) can be attributed to their waste-water origin [17], the high MP pollution of some tributaries suggests that some sub-catchments have better wastewater management or a smaller population.”

The title sound a “bit strange” to me but I am not English native speaking person….

Corrected as:

“Microplastic contamination of fine-grained sediments and its environmental driving factors along a lowland river:  Three-year monitoring of the Tisza River, Central Europe”

Line 11, I suggest to say: background IN microplastic…..

Corrected as:

L. 11: “background in microplastic “

Line 14, I suggest to say “Central Europe” after Tisza River

Corrected as:

L. 14 “Tisza River, Central Europe,”

Line 16, I suggest to add “content” or “amount” after this: The mean MP pollution ……

Corrected as:

L. 16 “The mean MP content…”

Keywords: I suggest to erase “Microplastic monitoring”since it is in the title and this “Hot spots, seems to me very generic…

Corrected as:

Keywords: Tributaries; Confluences; In-channel forms; Sediment samples; Downstream changes, Flood sequence “

Line 47, reference (15) is not well cited…should be Surname (15)……..the same at lines 75, 440,  

Corrected as:

L. 44 “For example, Ding et al. [15] found…”

L. 74 ”out by Hurley et al. [13] “

L. 462 “obtained by Wang et al. [17] and Horton et al. [32].“

Line 51, something is missing at the end of the sentence…..

Corrected as:

L. 47-48 “Furthermore, the MP spatial contamination is highly influenced by point sources [16,18] or inflowing, inadequately treated waste water [15].“

Line 70, I suggest to insert “the possible existence of” after “system due”….So it would be: river system due to possible existence of sources nearby.

Corrected as:

L. 68-69: “…pollution status of the entire river system due to the possible existence of pollution sources nearby..”

Line 85, I suggest to add “river” after Tisza

Corrected all over the text as:

L. 85 “Tisza River”

Lines 86-87, Since our previous results proved that MP content in sandy sediments  (≥0.1 mm) depends…….Please pay attention, sand sediments are > 2.0 mm

It depends on which grain size scale is applied.

We used the Wentworth scale for classification, where the limit of the fine sand is 0.1 mm.

Corrected as:

L. 90-93. “Since our previous results proved that MP content in very fine sandy, silty or clayey sediments (£0.1 mm) independent of grain size [14], in the present study, only freshly deposited clayey sediments were used to evaluate the long-term longitudinal changes. “

Figure 1, dam should be probably in red in legend.

Corrected, now on Fig. 1 all dams are indicated by red and it is also correct in the legend.

Line 197….was used to identify…WHAT? Something is missing.

Corrected as:

L. 207-208: “The Shimadzu Standard Library database (Shimadzu LabSolutions) was used to analyze the chemical composition of microparticles.”

Lines 203-205…this is repeated two times: Only glass and metal equipment

Corrected as:

L. 213-214: “Only glass and metal equipment were used to avoid contamination during sampling and laboratory work.”

Lines 212-213, I suggest: ..as a baseline and, therefore, are briefly introduced

Corrected as:

L. 220-222: “After the preliminary survey of 2019, 2020 could be considered as a baseline survey, as the MP measurements were complemented with more sites, grain size analysis and suspended sediment measurements [7,14,21,22,31].”

Line 221, I suggest Sajó rivers, that is add RIVERS

Corrected all over the text. Here:

L. 233-234: “were the Kraszna River (F2: 4220 items/kg), Hernád River (H1: 3440 items/kg), and Sajó River”

Line 267, very interesting, I guess it can be due to the great variability of the natural environment….

Thanks. Its explanation was given at:

L. 499-507: “The largest mobilization and emptying were detected between 2020 and 2021 when higher stages and higher and longer flood waves could successfully mobilize and transport the already deposited sediments and their MP content away from the fluvial system of the Tisza River. In the last year, smaller floods appeared, so they could only redistribute the MPs in the fluvial system: the MPs entered the tributaries but have not yet reached the main river. The deposition and remobilization of MP particles are highly influenced by flow velocity [5,13], and the role of bank-full flood waves is very important [7], as they are the most efficient mobilizers of the previously deposited sediments and MPs due to high specific stream power.”

Figures 6 and 7 have more sense because are places where changes take place too but general environmental conditions are more uniform in time, I mean if a trend it is observed probably it will be more “clear” and “durable” respect to other areas….and such two figures, seems to me, show lower variability respect to the previous one confirming my idea…

Mentioned and explained in the discussion:

L. 518-524: “For example, the slightly to intermediately contaminated Szamos River contributes to 36% of the discharge of the Tisza River. Nevertheless, it was able to triple or double the MP content of the Tisza River in just the first two years, and it had no effect in the last year. At the same time, the intermediately to heavily contaminated Kraszna River discharges only 1.5% of the Tisza River 's discharge; thus, despite its high pollution, it increased the Tisza River's MP content (by 125%) in 2022, while in the other two years, it had no measurable effect on the Tisza River.”

Figure 8, as above…these are “key” points in the river behavior that does not show great variability…..or, if any, it takes place within a clear trend….accretion upstream and erosion down-stream…

Mentioned in the discussion:

L. 539-547: “The three dams on the Tisza River resulted in different reservoir types. The uppermost and the lowest dams (at Tiszalök and Novi Becej) impound the Tisza River to the bankfull level (in-channel reservoir). However, the reservoir behind the Kisköre Dam is different, as in its upstream half, the impounded water reaches the bankfull level, but in its downstream part, the impounded water inundates the floodplain, creating a shallow and wide lake. Therefore, the sediment dynamics of the reservoirs are different, especially because during flood waves, the floodgates are open so that the sediment can be mobilized from the in-channel reservoirs [40,41], but the deposited sediments in the floodplain lake cannot be redistributed.”

Lines 458- 460, Ok but it can be just a limitation of the study...that is normal!!! It is impossible to cover the entire river bottom…..and sample it at the same time!

Corrected as:

L. 482-483: “This spatiotemporal redistribution could explain the contradictory results on longitudinal MP variations [21,35-37] and highlight the importance of repeated measurements.”

Lines 468-472, OK I agree with such statements…..considered points are places where changes are less relevant because are “key” points in the river

Thank you!

Lines 500-502, yes I agree…

Thank you!

Lines 570-571, I agree…

Thank you!

Lines 579-581, “In addition, the influence of tributaries and reservoirs on the spatial distribution of MPs should be investigated by dense spatial and temporal measurements since the MPs are probably mobilized and transported in pulses.”….I agree and this confirms my idea that the variability is normal and linked to the modality of the study….

Thanks!

Comment of the Reviewer

Response

This article is mainly research the environmental driving factors of MP deposition and mobilization. “The environmental driving” can be added.

The title was corrected as:

“Microplastic contamination of fine-grained sediments and its environmental driving factors along a lowland river: Three-year monitoring of the Tisza River, Central Europe”

Abstract: The entire summary summarizes the degree of microplastic pollution in sediments during the three-year period from 2020 to 2022, but it does not specify the reasons why the monitored data are affected by hydrological environmental conditions in each year, nor does it specify which hydrological factors may be related to the changes in the abundance of microplastics in the three years. What can be done to study the impact of hydrological environmental factors on microplastic pollution in the future.

The abstract should not exceed 200 words. So just limited info could be added:

“The MP pollution is influenced by highly variable environmental factors. (1) The geomorphological setting of a site is important, as most of the hotspots were on side bars. (2) The tributaries convey MP pollution to the Tisza River. (3) The bankfull or higher flood waves effectively rearrange the MP pollution. (4) The dams and their operation influence the downstream trend of MP pollution in the reservoir. (5) Downstream of a dam, the clear-water erosion increases the proportion of the pristine sediments; thus, the MP concentration decreases..”.

Line 50-51:Here it is said that microplastic space pollution is affected by point sources and inadequately treated waste pollution levels will be reduced, the specific reason is not given.

The sentence was not clear, so we deleted the “pollution levels will be reduced“ part: Corrected as:

L. 47-48 “Furthermore, the MP spatial contamination is highly influenced by point sources [16,18].“

Line57-80:The advantages and disadvantages of time and space measurement of sample points can be summarized and simplified in these two paragraphs.

We tried to shorten the Introduction. Each paragraph has a specific content:

1.    Lack of analysis on environmental factors influencing MP pollution

2.    Spatial monitoring

3.    Temporal monitoring

4.    Combined spatiotemporal monitoring

5.    Goals, aims

You asked to delete or shorten the 1-2 paragraphs. But they are as important as the 3 and 4 paragraphs you suggested to keep. So we decided to keep all of them.

Figure 2:The horizontal coordinate of the date should be clearly marked on the icon, and it is recommended to combine the data of 2019 and 2020 in one chart, so that the water level changes are more intuitive, and the same is true for 2021 and 2022.

The event sequence is more intuitive on single graphs than one figure with several overlapping graphs. So we kept the original setting. However, on the horizontal axis the names of the months were added for clarity.

Line 182:The concrete steps of wet screening and the reason of selecting 90μm as screen size are explained.

Corrected as:

L. 189-192: “Since the high clay content made it difficult to separate the MPs, the silt and clay particles were removed by wet sieving (mesh size: 90 µm) following Atwood et al. [26]. The samples were rinsed until the run-off became clear. The material from the sieve was washed into a glass jar.”

Line 202:Change the serial number to 2.4.

Corrected as:

L. 211: “2.4. Contamination control”

Line 212: Give reasons why the 2020 measurement results can be considered as a baseline.

Corrected as:

L 221-223: “After the preliminary survey of 2019, 2020 could be considered as a baseline survey, as the MP measurements were complemented with more sites, grain size analysis and suspended sediment measurements [7,14,21,22,31].”

Figure 3:The figure for microplastic pollution levels is not straightforward, giving a vague impression.

The blox plot is explained in the legend. The values are added in Table 1. We are sorry, but we could not figure out what did you mean on vague.

Line 386-390:The reason for the decrease in microplastic content in the viscous sediment downstream of the Kisköre dam should be to analyze the impact of clear water erosion rather than simply to say that microplastic pollution has decreased in every monitoring period in this range for three years.

As it was a Result chapter, the explanation was given I the Discussion chapter as:

L. 560-569: “Downstream of the Kisköre Dam, the impact of clear-water erosion on the MP content of sediments could be studied. As the transported sediments are deposited in the reservoir, it causes a sediment deficit below the dam, which, combined with the increased stream power of the river, leads to sediment production by vertical or horizontal erosion of the channel [38,41]. The effect of in-channel erosion is well reflected by the gradual decrease in MP content (‒50 to ‒75%) along a ca. 35 km long section downstream of the dam. It could be explained by the appearance of MP pristine material eroding from the river bed due to the clear-water erosion. As this material is thousands of years old, the pristine clayey material is mixed into the MP-polluted sediment. Thus, a decreasing spatial pattern in MP pollution evolves.”

Line 425-437:Here is a comparative discussion of the microplastic pollution points of the Disa River and other rivers, so if the pollution level is similar, it is necessary to analyze the differences in their hydrological environmental conditions.

We agree, that without hydrological data the comparison of the MP pollution of various rives is weak. However, most of these articles did not provide data on the hydrological background of their survey.

L. 458-459: “Unfortunately, most of these studies did not provide hydrological data of the analyzed rivers, so a hydrology-based comparison could not be performed.”

Based on the three years' continuous study of microplastics in the Disa River, the dynamic changes of microplastics in its section should be summarized in the conclusion. Possible causes of redistribution in microplastics hotspots; The impact of tributaries on microplastic pollution; And what other aspects can be explored in the future continuous research on the Disa River to explore the impact of its microplastic pollution.

The Conclusion was corrected as:

L. 625-645: “The performed research in the Tisza River’s water system (Central Europe) is a good example of long-term (three-year) monitoring along a ca. 750 km long reach. The study proved the high spatial and temporal variations of MP pollution and analyzed its various influencing factors. Flood waves can effectively rearrange the MP pollution along a river and could support the emptying of former pollution hotspots and the development of new one. Floods with high specific stream power (e.g. bankfull floods) play very important role in this rearrangement [32]; but the event sequence is probably important too, as in case of several, subsequent flood waves the first one is the most effective. [7,31]. Therefore, during the planning of the sampling it is advised to carefully study the hydrological characteristics of a river. Our sampling was performed on three different accumulational channel forms. It proved, that these forms trap the MPs with different efficiency, and collecting sediments from different forms could even influence the downstream trends in MP pollution. Thus, it is suggested to select a form which is common all along the river and perform the sampling from its freshly deposited, fine-grained sediments. Along the river any natural (e.g., tributaries, rapids and pools) or artificial structure (e.g., reservoirs, dams and grading structures) which influences the water and sediment discharge conditions of the main river must be considered during the sampling, as they could fundamentally influence the MP pollution of a site. It also must be emphasized that a fluvial system should be considered a whole unit because the spatiality of the elements (e.g., MP sources, tributaries, reservoirs, hotspots) influences the cause-effect relationships and reveals the processes at individual sites.“