Excessive Eutrophication as a Chemical Barrier for Fish Fauna Dispersion: A Case Study in the Emblematic Tietê River (São Paulo, Brazil)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

ABSTRACT

Provide more details about the sampling technique used, such as the specific parameters measured and the duration of sampling at each site

What is the significant statistical results or tests conducted to support the observed differences in water quality and fish composition since author use the PCA and nMDS.

DISCUSSION

Suggest to have subsection…

Provid a more detailed interpretation of the Principal Component Analysis (PCA) results. Explain the specific variables or factors that contributed most significantly to the distinctiveness of the Tietê River compared to the tributaries.

Discuss any observed temporal trends in environmental conditions and their implications for longitudinal impacts along the Tietê River.

Explore the concept of ecosystem resilience in response to pollution. Discuss how certain fish species, particularly detritivores like Prochilodus lineatus, exhibit resilience and adaptability to degraded environments.

Provide a comparative analysis with findings from other studies conducted in similar river systems or under comparable pollution conditions.

What are the implications of your findings for environmental management and conservation practices in the Tietê River basin.

CONCLUSION

Conclude outlining potential future research directions based on the gaps identified in your study.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this study, the authors addressed extreme pollution in the emblematic Tietê River in São Paulo, Brazil, which creates a chemical barrier to the spread of fish fauna and shed light on the impact of pollution on aquatic environments and the interconnectedness of river systems despite changing water quality conditions.

-Introduction

The study was introduced by emphasizing the importance of dispersal in maintaining the structure and functioning of populations and species and in ensuring continuous genetic flow through space and time. The study also focused on assessing the impact of water pollution restrictions on the distribution of fish fauna in the Tietê River basin, providing information in particular that pollution plays its role as a chemical barrier preventing the spread of sensitive organisms in the region. The introduction provided context for the ecological importance of dispersal, the challenges posed by pollution in aquatic ecosystems, and a specific focus on the Tietê River basin to address these issues, and effectively summarized the research focus, objectives, and justification of the study.

-Material and methods

This section presents the sampling method, data collection procedures, statistical analyses, data conversion techniques, software tools used, and environmental variables measured during the study. The authors have provided a clear and structured overview of the research, demonstrating a systematic approach to data collection and analysis. The containment of specific details such as using the gillnets to sample fish, measurement of limnological and water quality variables, and statistical methods reveals a comprehensive methodology.

-Results

The study presented results on how pollution affects fish distribution in the Tietê River basin. The main river was heavily polluted, leading to low oxygen levels and high nutrient concentrations. In contrast, tributaries had better water quality due to less urban impact. There were fewer fish species in the main river than in tributaries, and pollution hindered sensitive species. Correlation analysis revealed strong connections between fish communities and environmental factors, particularly dissolved oxygen. These differences were more pronounced during the rainy season. In general, pollution negatively affected fish distribution, reducing species richness and changing community structure. It is understood that urgent action is needed to improve water quality in the region and restore aquatic biodiversity.

-Discussion

The results of the study regarding the impact of pollution on the fish fauna of the Tietê River and environmental protection efforts are discussed. The authors emphasize the importance of biodiversity restoration, conservation of species diversity, implementation of integrated management approaches, and public awareness as the need for urgent action to combat pollution in the Tietê River. I am also of the opinion that these measures are necessary for the effective protection of the environment in polluted river systems and the sustainable management and protection of aquatic ecosystems and biodiversity.

Generally speaking, this study is of utmost importance as it highlights the role of pollution as a chemical barrier preventing the spread of sensitive organisms in the Tietê River basin and calls for urgent measures to improve water quality for the restoration of aquatic biodiversity and sustainable human-nature relationships. In my opinion, the article is suitable for publishing.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper presented for review is devoted to the impact of water quality on the ichthyofauna of a river and its tributaries. The authors analyse the role of water quality as a barrier for different fish species. However, when talking about pollutants they treat them collectively hence the analysis includes the overall water quality including phosphorus, nitrogen etc. It is evident that factors that cause severe eutrophication and reduce water quality negatively affect vertebrate life. The overall data is interesting, but the paper does not take into account the individual types of pollution, yet there is a complex network of interactions between them. Toxic effects are shown not only by water eutrophication agents but also by heavy metals and a wide range of organic pollutants, pesticide residues, etc. and each of these can significantly affect the ichthyofauna of rivers. Nevertheless, the paper contains interesting data, from a local point of view. The objectives of the work have been clearly defined and the results obtained respond to them. The statistical part of the paper should be particularly highlighted, as the data analysis model used seems to be interesting. It is only a pity that some of the accompanying figures are of poor quality, which should certainly be improved. This applies in particular to Figures 1 and 3, whose resolution is so poor that important details are lost. In my opinion, these should be corrected. I also recommend a detailed analysis of the text, as there are spelling errors. An example is the caption of Figure 9 where the word chthyofauna is used instead of the term ichthyofauna. In my opinion, the final conclusions of the paper, which do not flow directly from the research carried out, also need to be reworded. The authors concluded that the significant contamination of the waters of the studied river by numerous organic and inorganic pollutants acts as a chemical barrier, but after all they did not analyse the concentrations of individual compounds but the overall quality of the water on the basis of its basic physico-chemical parameters. I believe that the conclusion regarding the need for further research is also too general and rather wishful thinking. It should be replaced by elements resulting directly from the research conducted since it is clear that further research is necessary.

I believe that the paper is suitable for printing after a minor revision.

Comments on the Quality of English Language

The paper presented for review is devoted to the impact of water quality on the ichthyofauna of a river and its tributaries. The authors analyse the role of water quality as a barrier for different fish species. However, when talking about pollutants they treat them collectively hence the analysis includes the overall water quality including phosphorus, nitrogen etc. It is evident that factors that cause severe eutrophication and reduce water quality negatively affect vertebrate life. The overall data is interesting, but the paper does not take into account the individual types of pollution, yet there is a complex network of interactions between them. Toxic effects are shown not only by water eutrophication agents but also by heavy metals and a wide range of organic pollutants, pesticide residues, etc. and each of these can significantly affect the ichthyofauna of rivers. Nevertheless, the paper contains interesting data, from a local point of view. The objectives of the work have been clearly defined and the results obtained respond to them. The statistical part of the paper should be particularly highlighted, as the data analysis model used seems to be interesting. It is only a pity that some of the accompanying figures are of poor quality, which should certainly be improved. This applies in particular to Figures 1 and 3, whose resolution is so poor that important details are lost. In my opinion, these should be corrected. I also recommend a detailed analysis of the text, as there are spelling errors. An example is the caption of Figure 9 where the word chthyofauna is used instead of the term ichthyofauna. In my opinion, the final conclusions of the paper, which do not flow directly from the research carried out, also need to be reworded. The authors concluded that the significant contamination of the waters of the studied river by numerous organic and inorganic pollutants acts as a chemical barrier, but after all they did not analyse the concentrations of individual compounds but the overall quality of the water on the basis of its basic physico-chemical parameters. I believe that the conclusion regarding the need for further research is also too general and rather wishful thinking. It should be replaced by elements resulting directly from the research conducted since it is clear that further research is necessary.

I believe that the paper is suitable for printing after a minor revision.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Review for the paper “Excessive pollution as a chemical barrier for fish fauna dispersion: a case study in the emblematic Tietê River (São Paulo, Brazil)” by Bruna Quirici Urbanski, Marcos Gomes Nogueira submitted to “Water”.

The authors conducted a sampling campaign to study fish assemblages in the Tietê River at sites located far from a large city. The authors aimed to compare fish diversity and community structure in relation to pollution levels. They found a decrease in species richness at sites near the city due to lower oxygen levels and high concentrations of nutrients and solids. These results highlight the high pressure of human activities on local fish populations, including commercially important ones, and may be useful for monitoring and conservation purposes.

The paper needs to be revised as many of the authors' statements were not supported by statistical comparisons. The work concept also needs to be revised.

Recommendations.

The abstract is too long and should be shortened considerably to meet the 200 word limit suggested by the editor.

The authors should clarify their main hypothesis about the limits of faunal dispersal. Chemical pollution is already known to be an important factor influencing the distribution patterns of aquatic organisms. In the context of the current study, it is more reasonable to formulate the main hypothesis as "to assess the extent to which changes in environmental factors have affected the local fish fauna".

Figure 1. The authors should highlight the location of the State of São Paulo on the map.

The authors use the term "limnological", which is not quite correct, since they studied river systems. A broader term "aquatic" seems more appropriate.

It is unclear how many replicates were made at each site when measuring environmental variables and sampling fish. Sample sizes should be reported in the text.

Section 3.1.The authors should statistically compare the values presented in Table 3 using an appropriate approach and considering two factors - site and season.

Caption of Table 3. The authors stated that "Values in red are not in accordance with the limits established by CONAMA", but there is no red text in this table. If the authors meant the bold font, they should revise the caption and the font in the first column and in the legend.

Figure 3. All abbreviations should be defined in the caption.

Section 3.2. The authors should statistically compare the proportions shown in Figures 4 and 5. They should also explain what the black line on the green background in the last figures means.

Figures 6 and 7 and the text explaining them. The authors should perform statistical analysis to support the relevance of the patterns they describe in this text.

Figure 8. The authors should explain what the black line on the gray background is in the last figure.

P 16. The authors should supplement the NMDS analysis visualizing the site separation with an appropriate analysis, e.g. PERMANOVA, to support the statement about differences in fish composition between the main river and tributaries.

Table 9 Caption. Same concern as the caption for Table 3: the authors mention that "In red, the significant variables (p <0.05)." while there is no highlighted text in the table.

Figure 9. Abbreviations should be defined in the figure caption.

Section 3.3. It is recommended that the authors include the effect of each factor (positive or negative) on the structure of fish assemblages. It is also unclear what "the structure" means, i.e. what response variables were used in this analysis.

It would be useful to provide an analysis testing the effects of environmental variables on CPUE values.

In the discussion, the authors mention that a study conducted in the 1990s showed dominance of 1-2 species at most polluted sites, while in their study the most polluted sites supported 3-4 species. The authors should explain this discrepancy.

P. 20. The last paragraph of section 4.2 is a simple restatement of the results. The authors should discuss their results more thoroughly.

The authors should also reconsider the term "pollution" used throughout the text. They did not directly measure contaminant levels and did not relate them to fish fauna. The authors studied the consequences of pollution, and in this context the term "eutrophication" seems more relevant.

Comments on the Quality of English Language

Some revisions are required

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 4 Report

Comments and Suggestions for Authors

The paper has been revised.