Seasonal Variations and Drivers of Total Nitrogen and Phosphorus in China’s Surface Waters

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

General comments

 

The paper is generally interesting and informative. The research was conducted on an extensive dataset at a large-scale spatial arrangement, using well-selected and advanced data analysis methods, which is undoubtedly a strength. The results and findings are not particularly novel, as they primarily confirm the well-known and documented relationships between TP and TN concentrations and various natural and anthropogenic factors. However, a significant contribution of the presented research lies in confirming that these relationships also hold true on a large spatial scale, which is rare in the literature that predominantly focuses on individual, often small catchments.

 

However, the paper requires significant refinement. The authors should carefully consider which aspects of their research provide valuable insights for an international audience and which may be superfluous—for instance, if they merely serve as a report of the conducted studies rather than a thorough scientific analysis.

 

In my opinion, the monthly analysis (subsection 4.4.2) is unnecessary, as it adds little new information compared to the seasonal analysis. Moreover, the spatial maps for individual months do not provide a clear picture of the situation. I suggest describing this part briefly at most, as a supplement to the seasonal analysis. Figures 11 and 12 (not referenced in the text) contain very interesting results and should be retained.

 

The "Discussion" section requires reorganization, as a significant portion of it is essentially a continuation of the "Results" rather than a genuine discussion. This particularly applies to the presentation of statistical analysis results discussed in the "Discussion."

The authors might consider combining the "Results" and "Discussion" into a single section, which is permissible according to the journal Water's author guidelines.

 

I believe that in the revised version of the paper, the authors could also attempt to explain the evident discrepancy between the studied elements, as the very high TN concentrations were accompanied by relatively low TP concentrations. This aspect intrigued me greatly from the beginning of the paper, and I expected the issue to be addressed. Unfortunately, the analysis lacks any reference to this topic.

 

The paper also contains numerous additional issues, including repeated content (the first paragraph of the introduction – lines 40-53 – requires reorganization), unclear and incompletely described figures, the absence of figure citations in the text, and several minor editorial errors.

The authors should adhere to the principle that figures/tables should be cited in the text before they appear, which is often not followed in this paper.

Additionally, some figures contribute little and could be omitted without compromising the scientific value of the paper. I will address this in more detail in the specific comments.

 

Overall, I selected the "Major revisions" option in the review form because the necessary changes are numerous, although they are primarily organizational in nature. Meanwhile, the general scientific value of the paper remains high.

 

Detailed comments

 

Lines 46-48: This is a repeat of the phrase from lines 44-46.

Lines 50-52: This is basically another repetition of the same statement.

Lines 64-64: Repeated same sentence from previous lines.

Line 80: Such use of the term "eutrophication" is incorrect, as eutrophication is a process rather than a state. I suggest presenting this information in terms of trophic state assessment categories or specifying the degree of advancement of the eutrophication process.

Line 107: Something should be added to: „land use and other”. “factors” seems to be the right word.

Line 120 (Fig. 1): The three small maps are virtually unreadable, particularly the soil map. Additionally, it has not been explained what the green points on the main map represent. The additional small map in the bottom right corner is unnecessary if it does not include any points where water quality studies referenced in the manuscript were conducted.

Lines 129-135, 170, 190, 219, 228, 240: Missing parentheses in reference links. Reference error at line 198.

Lines 200, 201: Tables 1 and 2 are not cited in the text.

Line 203 (Subsection 2.2.2): Presenting basic meteorological data would be highly advisable. At a minimum, the average annual temperatures and precipitation for the winter and summer halves of the year, for each of the analyzed catchments, would be very useful, especially since the authors identify precipitation as a key factor influencing the distribution of TN and TP concentrations.

Line 263: Values [-1, 1] are written twice.

Lines 264-265. Missing spaces.

Lines 302, 306: No superscript in R².

Line 306: no explanation of symbols in formulas (3) and (4). If they are the same as in formula (1), it is worth writing it down.

Lines 324-326: These are pieces of information related to the methods rather than the results; they are unnecessary in this section, as they have already been provided earlier.

Line 350: This is very interesting. Perhaps the authors could supplement the provided information with concentration values observed in atmospheric precipitation (which could be based on literature), as it is important to understand to what extent the increase in TP and TN concentrations results from direct delivery of these elements through precipitation, and to what extent it is due to runoff from the catchment area.

Line 362. Table 3 is not cited in the text. Is this table even necessary? You can add this data to tables 1 and 2.

Line 404, 405 (Fig. 5 and 6): The figure requires some explanation as to what the different sizes of the pie charts refer to.

Line 414 (Fig. 7): Wrong title of figure.

Lines 437-439 (Fig. 9 and 10): These figures add no value, as they are virtually indistinguishable. I believe they are unnecessary. A similar piece of information, but with better informational value, is provided by Figures 11 and 12. I suggest the authors reconsider whether the monthly analysis is even needed.

Lines 453, 455: Figures 11 and 12 have incorrect titles. These figures are not cited in the text.

Line 463: A subsection should not start with a figure, especially since it is not quoted in the text.

Line 465: No explanation of (a) and (b) in the figure. The figure is not cited in the text.

Lines 469-529: Are clusters and Types the same thing? If so, there is no need for duplicate naming.

Lines 547-551, 564, 574-581 and further: Missing parentheses on reference links.

Line 617 (Fig. 16): Missing punctuation in figure title explanations.

Lines 730, 732. Tables 5 and 6 are not cited in the text.

Line 757: Figure 19 is not cited.

Line 775. Figure 20 is not cited.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors of this manuscript conducted a study on the spatial-temporal variations and drivers of total nitrogen (TN) and total phosphorus (TP) concentrations in surface waters of China. Natural factors (elevation, rainfall, and soil type) and human factors (land use type, fertilizer application, and population density) were taken as potential factors influencing TN and TP concentrations in surface waters. Although the results showed some spatial and seasonal disciplines on TN and TP distributions, I have the following major concerns about this manuscript.  

First, the writing of this manuscript needs to be improved. This manuscript is long and has quite a lot of figures, which were more like a report. I suggest the authors summarize the most important findings into several major figures and put the secondary results into supporting information. The study period is only one year, which provides limited insights on the long-term water quality changes and the driving factors.  

Second, there were many descriptions that lacked evidence or in-depth discussion. For example, why was the elevation more important than rainfall in driving TP concentrations? Why should rainfall belong to “human activities” (line 822-823)? The authors mentioned “Nationwide, TP levels were generally negatively correlated with rainfall”, while “TP concentrations exhibit the opposite pattern, being higher in summer and lower in winter”. The reason for this phenomenon is not well discussed.

Third, some other important drivers on TN and TP concentrations are not considered or discussed. For example, the internal metabolism mediated by microbes were important drivers on seasonal changes of TN and TP concentrations. Therefore, the limitations of this study should be discussed.

Other minor comments:

 

Title: “A Short to Medium-Term Analysis” is unclear.

Line 62-65: There were two repeated sentences.

Line 69-77: These descriptions lack support from the literature. Descriptions like “the leading pollution indicators”, “mildly polluted”, and “moderately eutrophic” are unclear.

Line 107: It seems that this sentence was not finished.

Line 121: The boundaries of the nine river basins are not shown.

Line 195-196: Are all the “water quality monitoring stations” river stations? Please make it clear. 

Line 196-197: “the whole year of 2020-2021” is unclear.

Line 198-199: Please provide the citation of the figure/table/reference correctly.

Line 346-350: Please indicate the figure/table that supports the descriptions.

Line 414: Figure 7 shows seasonal changes. “monthly spatial distribution” is not accurate.

Line 391: Does the “TP/TN” mean “TP or TN”?

Line 427: Why did the authors conduct both seasonal analyses and monthly analyses?

Line 468: Figure 13 was not cited.

Line 591-595: The reason should be explained further.

Line 630-641: Please indicate the figure or table or evidence that support these descriptions.

Line 688-690: Should “rainfall” belong to “human activities”?

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper titled "Spatio-temporal variations and drivers of total nitrogen and phosphorus in China's surface waters: A short to medium-term analysis" presented for review concerns the analysis of spatio-temporal variability and factors influencing total nitrogen (TN) and total phosphorus (TP) concentrations in China's surface waters, using data from 1,387 monitoring stations collected in 2020-2021. The authors provide a detailed analysis of influence of both natural factors (rainfall, soil type, elevation) and anthropogenic factors (land use, fertilization, population density). The topic of the publication is significant in the context of water eutrophication and water resource protection. The study utilizes advanced analytical methods, such as K-means cluster analysis and geographically weighted regression (GWR), and holds substantial scientific value, but only in a local context. It provides data that can help in understanding local environmental issues and support informed decisions regarding the management and protection of aquatic ecosystems. In my opinion the paper deserves publication in Water Journal, but it needs to be improved, particularly in terms of providing more detailed and practical recommendations for stakeholders in specific regions. The current recommendations are too general and rather well-known in the scientific literature and water protection practices. This kind of recommendation undermines the validity of conducting extensive analyses using advanced tools, which should provide more precise conclusions tailored to specific regions. Below, I present my comments on the elements of the paper that, in my opinion, require improvement.

General comment:

The article is extensive and contains a large number of figures. Please consider whether some of them could be included in the “Supplementary Material”. It is also important to note the significant number of editorial errors that require correction.

 Specific comments:

Lines 60–62: The authors mention many sources for TP and TN, but provide only one reference. In my opinion, this is insufficient. Did all this detail really come from one reference?

Lines 70–72, 74–78: Please support this information with appropriate references.

Fig. 1: The figure is very unclear, especially the smaller versions on the right (i), (ii), (iii). Please explain the meaning of the Roman numerals and other abbreviations in the figure. The caption below the figure includes a, b, c, but these designations do not appear in the figure.

Lines 198–199: Please correct the error.

Tables 1 and 2: Please add the number of measurements, n.

Section 3.1.1.: What criterion was used to select the number of clusters?

Fig. 2: Is it possible to use more varied colors for the different trophic states? In my opinion, this would improve the readability of the figure, as well as the others. Is there a need to repeat concentration ranges for TP and TN when they have already been presented in the text? Currently, the figures are overloaded with information, reducing their readability. The above comment also applies to figures such as 5, 6, 9, 10.

Lines 339-354: Please provide the reasons for these results.

Lines 346-359: Where can I find these results in figures or charts?

Table 3 has no clear connection to the text. Is it necessary?

Fig. 7 and 8: Is "season^-1" on the Y-axis necessary?

Figs. 10 and 11 are not referenced in the text.

Section 4.2.3 begins with a figure, which is incorrect.

Figure 13: The figure is poorly described and cannot be interpreted without reading the text. Furthermore, there is no reference to it in the text, which makes it even more difficult to understand. Line 468 – Does it refer to Figure 13? Could there be an error in the numbering of the figures in the text? Check it carefully and correct it.

Figures 14 and 15: Cannot be interpreted without additional explanations.

Lines 547, 549, 551, 574, 575, 576, 581, 584, 595 and many others: editorial errors.

The “Discussion” section is basically a discussion of the results of the analyses carried out. References to the world scientific literature are very limited. The article would be more valuable if the authors compared their results with studies conducted in other countries, especially in the context of similar ecosystems.

The “Conclusions” section is very laconic. The authors suggest improving the management of the catchment, but there are no details. The current recommendations are rather obvious and not very innovative, considering the detail of the analysis. I propose to indicate areas where human activity had a particularly large impact on TN and TP concentrations, and taking into account their specificity, propose specific corrective actions that can be implemented in practice.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

General comments

 

I would like to thank the authors for their careful consideration of my comments and suggestions. The revised version meets my expectations, although the authors should correct a few more minor errors that appeared in the content.

The authors have moved the monthly analysis to the Supplement; however, the materials from this section should also be referenced in the main text of the article (they should be cited in the appropriate places). Otherwise, the Supplement becomes unnecessary. Figures in the Supplement should be numbered according to the following scheme: Figure S1, S2, etc., to distinguish them from Figures 1, 2, etc., in the main text of the paper.

 

Detailed comments

 

Line 191: Should be “Figure 1” not “Figure 2”

Line 194: Should be “Table 1”.

Line 195: Should be “Table 2”.

Line 207: Table 3 contains only seasonal data, and the title says annual. I suggest adding the column "Annual" with annual precipitation totals, and in the title "The seasonal and annual...". Please also add for which period these average values ​​are (from which years).

Line 221: This is Figure 2, not 3.

Line 245: The title of the figure cannot be so abbreviated, please expand it. The general rule is that the figure with its title should be understandable to the reader, even if the figure is taken out of the body of the article.

Line 631: The phrase „Take the Yellow River Basin as an example.” is redundant.

Line 638: Should be “Shi and Shao [18] conducted … “

Line 799: Statements like “The analysis of Table 5 and Table 6 revealed…” are unfortunate. You don’t analyze the table, you analyze the data that the table contains. I suggest changing it to “The analysis of the data contained in Table 5 and Table 6 revealed…”

Comments on the Quality of English Language

My proficiency in English is limited (I am not a native speaker); however, the article gives the impression that it would benefit from professional language editing.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

I still have the following concerns on the revised manuscript.

Overall, the writing of this manuscript still needs to be improved as there were many places like a report not a scientific paper (e.g., lines 116-183, 459-483, and 500-520). I strongly recommend the authors refer to papers published in this journal and write it more concisely and logically clearly (your data to support on the discussion of the potential sources of nutrients).

 

Line 51-52: The description of “direct indicators” is not appropriate.

Line 60-65: Again, descriptions like “leading pollution indicators”, “mildly eutrophic”, and “moderately polluted” are not appropriate. Regional differences described in this paragraph make the introduction too dispersive. Please focus on the background of the motivations of this study.    

Line 82: “on the plot scale” is hard to understand.

Line 115: “the measured sites” usually means that the data were measured by the authors.

Lines 116-183: Again, these paragraphs are quite like a report, and most of the information is lack of references to support. Please write like a scientific paper and give the most important information needed in this study, e.g., summarize it into one paragraph.

Line 186-193: How were the TN and TP measured?

Line 404: Please use “TP: TN” instead of “TP/TN”.

Lines 459-483 and 500-520: These writings are not like a scientific paper.

Line 536: “is shaped by” is not suitable.

Line 717-720: What are the implications of these correlations?

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The revision of the article was carried out without attention to detail, which significantly reduces the quality of the text. I believe the article can only be published if the authors thoroughly organize the numbering of tables and figures (e.g., there are currently two figures labeled as 'Figure 3,' Figure 2 is missing, and the order of tables in the text is chaotic – at the beginning, instead of Table 1, Table 3 appears, followed by Table 4, and then Table 3 again). It is also necessary to differentiate the numbering of figures in the paper (Fig. 1, Fig. 2, Fig. 3, etc.) from the numbering of figures included in the Supplementary Materials (Fig. S1, Fig. S2, Fig. S3, Fig. S4). All figures, including those in the Supplementary Materials, must be properly linked to the text. Additionally, captions under the figures should start with a capital letter. A detailed review of the consistency between the numbering of tables and figures and the content of the paper is also essential. Moreover, the text still contains numerous editorial errors, such as the frequent omission of spaces between words, which need to be corrected.

Author Response

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Author Response File: Author Response.pdf