Factors Affecting Water Quality and the Structure of Zooplankton Communities in Wastewater Reservoirs of the Right-Bank Sorbulak Canal System (South-Eastern Kazakhstan)

Round 1

Reviewer 1 Report

The manuscript describes in detail the physico-chemical parameters affecting water quality and zooplankton community composition in the wastewater reservoirs of the Right-Bank Sorbulak Canal system in the south-eastern part of Kazakhstan (Almaty region). The studies were conducted at three locations with a grid of 15 stations from April to September 2021 on a monthly basis. Spatial and temporal distribution was considered for the following environmental variables: depth, temperature, pH, transparency, total dissolved solids (TDS), total hardness, Ca²⁺, Mg²⁺, NA⁺, K⁺, Cl^-, SO₄^2-, CO₃^2-, HCO₃^-, NO₂^-, NO₃^-, NH₄⁺, PO₄^3-, Fe, Si, Mn, easily oxidizable organic matter, Cd, Pb, Zn, Cu, Cr, Co. Of the biological variables, the authors monitored the variability of abundance and biomass for the rotifers, cladocerans and copepod groups, including the number of species, Shannon index and average biomass of individuals. Using statistical correlation analyses and principal component analysis (PCA), the authors sought to define the relationships between abiotic and biotic parameters and determine environmental factors that influence zooplankton community structure.

General comments:

Although this manuscript is based on an extensive set of environmental variables and zooplankton groups/species, there is still a long way to go before publication. The authors obviously put a lot of effort into the analysis of physico-chemical parameters and taxonomic identification, resulting in a detailed listing of the species of rotifers, cladocerans and copepods in the studied wastewater reservoirs. Unfortunately, the same cannot be said about the presentation of the results, the analyses included, and the overall discussion and conclusions. The paper is not written in accordance with general scientific principles, and the results and discussion of the studied parameters show serious deficiencies in understanding the relationships between physical, chemical and biological interactions. The authors are therefore advised to reconsider their approach, re-examine their data, and focus on establishing a clear hypothesis. It is recommended that the authors consult the scientific literature on similar topics to perform appropriate statistical analyses and improve the presentation of the data, include various diversity indices to discuss community composition and abundance, and apply appropriate statistical methods to link environmental conditions to species distributions.

Unfortunately, for all the above reasons, I cannot recommend publication of this manuscript.

Specific comments:

The following are some specific comments on various aspects of the manuscript and some suggestions for improving the work.

• The entire manuscript is descriptive and too long with many repetitions of the same things in different ways.
• It would be interesting to see the similarity/dissimilarity between the stations in the Sorbulak watershed when you look at them together.
• In addition to the range values, the description of parameters is defined by an average and standard deviation rather than a standard error.
• The sampling and counting of zooplankton should be better explained. Considering the sampling method, it is not appropriate to use population density as the number of individuals per cubic metre. The volume sampled is much smaller than one cubic metre. You can concentrate samples into different volumes without affecting the volume of the sampled water column. The problem of clogging the plankton net in the highly eutrophic areas still exists.
• Principal component analysis is not an appropriate statistical method for analysing relationships between biotic and abiotic variables. In addition, the resolution of Figure 7 is so low that it is not possible to see what is written on it. Instead, consider canonical correspondence analysis (CCA) or redundancy analysis (RDA). A very useful analysis for detecting similarities between samples is multidimensional scaling (MDS).
• The justification for using the Shannon diversity index (H’), which uses biomass as a variable instead of abundance, is unclear.
• The focus of this research is on physico-chemical parameters and their influence on the zooplankton community. Except for some environmental variables (such as temperature, salinity, oxygen concentration), it is very difficult to discuss their influence on zooplankton, while completely ignoring the control of zooplankton by resources (as mentioned in the Discussion section). Therefore, it is better to focus on the differences in zooplankton community composition under different ecological conditions.
• The manuscript should be completely rewritten, without repeating the results in the Abstract and Discussion sections, but with clearly defined conclusions of the research.
• The authors should consider significantly reducing the number of references.

Author Response

Dear Reviewer, please see the attachment!

Author Response File: Author Response.docx

Reviewer 2 Report

water-1674206

Comments for Authors

This study aims to assess the main factors that determine the spatial and seasonal variability of environmental and biological parameters in Sorbulak (South-Eastern Kazakhstan), one of the largest wastewater reservoirs. Besides Sorbulak, the research was also conducted in two shallow ponds (RSC7 and RSC8) which were built in 1995 for discharging the part of the wastewater bypassing Sorbulak, since by the end of 1980, the filling of Sorbulak reached a critical level, and there was a threat of a dam breakthrough. Since the reservoir contains mixed pollution, in this study, as continuation of previous studies, the pH, permanganate index, total dissolved solids, N-NO2, N-NO3, N-NH4, PO4, Si, Mn, Fe, the content of some heavy metals such Cd, Co, Pb, Zn, Cr and Cu, as well as the structure of zooplankton communities were monitored. The sampling was conducted in period from April to September 2021, once a month using a grid of 15 stations.

The study is properly conducted and design, but there are parts in the manuscript that need improvement in order to increase the clearness of the manuscript and to highlight the main contribution of this research.

General comments

Authors should pay attention to the following:

• More information about the wastewater reservoirs in general is needed, about their purpose, to be understandable to readers who are not closely related to this field. Why are they being built at all, what is their main purpose, how are they being built, do they affect groundwater…?
• It is not clear whether such reservoirs are ultimately a positive or negative solution given the impact on the environment and human health?
• The purpose of research must be better explained. What is the main aim of this study? Only to monitor environmental and biological parameters in wastewater reservoirs? Will these results be helpful in wastewater management? Can this study in some way improve the wastewater management system, identify critical points of the system? Is there a solution to mitigate the negative effects of such reservoirs on the environment?

• What happens when the maximum capacity of such reservoirs/ponds is reached again? New ponds?
• What pre-treatment methods are used for the wastewater before inflowing in reservoirs? This is not mentioned in the manuscript and it is very important data. Can obtained results be helpful in changing approach of pre-treatment methods in order to reduce the impact on the environment?

• In some part of the manuscript the secondary pollution is mentioned in regards to desorption of heavy metals from the sediment in water column. Is there a chance that heavy metals are being constantly discharged into the reservoirs due to inadequate pre-treatment methods?

Detail comments:

Lines 1-2 - Title of the manuscript: it would be good to precise the wastewater reservoir, which one and where.

Abstract: as mentioned before, the data about reservoir is need, because in this way it seems generally, but is not. It is known which reservoir it is.

Line 14 – the permanganate index and its units, are they mgO/dm3 or mg O2/dm3 (molecule of oxygen)?

Line 39 – TDS – please give here firstly the full name and then the abbreviation in the brackets, which can be further used.

Lines 46-47 – Can the sentence “The critical nutrient level for lakes to become turbid is higher for smaller lakes” be incorporated in the sentence in lines 43-44? It seems better.

Line 50 – please give the name of region and state for lakes of the Kola Peninsula.

Line 54 - please give the name of region and state for the Calcasieu Estuary.

Line 66 – is the purpose of the Sorbulak reservoir only to store the wastewater? Can authors give the methods of wastewater pre-treatment? Also, what are the sources of wastewater flowing into Sorbulak and ponds? Can authors give these information? As I already commented in general comments, more information about the wastewater reservoirs in general is needed, about their purpose, to be understandable to readers who are not closely related to this field. Why are they being built at all, what is their main purpose, how are they being built, do they affect groundwater…?

Line 78 – What number is this 2.7709.8. Please check for this number, it is not clear.

Lines 78-81 – please equalize the units individual/m3 or ind./m3 here and elsewhere in the manuscript.

Lines 88-91 – The main purpose of assessment of these factors that determined the spatial and seasonal variability is not clear. As mentioned in general comments, is monitoring an end in itself or is there some higher goal, e.g. to maybe reduce negative environmental impact of this wastewater, or maybe to apply better pre-treatment methods or to suggest the remediation technique for these sites?

Line 91 – PCA – please firstly give the full name and then the abbreviation in the brackets, which can be further used.

Line 94 – name the state for Almaty.

Lines 128-129 – “Sorbulak and the entire system of wastewater reservoirs is a crucial ornithological area, and it is included in the list of wetlands of regional importance”. How come? Isn’t that wastewater with mixed pollution?

Line 133 – This grid of 15 stations, is it represents by black dots in Fig 1? If yes, this should be mentioned here.

Lines 142-149 – please equalize the units in litters here and elsewhere in the manuscript, both in text and figures, L or l, or better in dm3 since it is written in that way till now.

Lines 168-169 – “The detection limits for heavy metals are: Cd – 0.00006 mg/dm 3 , Co – 0.00007, Cr – 0.0004, Cu – 0.0005, Pb–0.00005, Zn – 0.001 mg/dm 3”. This sentence could be revised as for example: The detection limits for heavy metals in mg/dm3 are: for Cd - 0.00006, for Co - 0.00007, for Cr – 0.0004, for Cu – 0.0005, for Pb–0.00005, and for Zn – 0.001.

Line 171 – Please shortly explain of method followed by reference 45, not only give the reference number.

Lines 173 and 178 – be careful with mL, ml. The best solution is cm3 to be equal through entire manuscript.

Line 228 – the dot is at wrong place. It should be corrected.

Line 231 – It is not common to mention in the text the Figures 2d and 2h before Fig 2c. Maybe the figures should be reordered.

Line 235-236 – Is this water safe for irrigation purposes? Does rainfall influence the water level in these reservoirs and ponds?

Table 1 – units for PI (mgO2/dm3); MPCs are given as note at the bottom of the Table 2. Why not to add one more column in the table and give MPC values for all examined parameters? In that way it will be easier to compare the results.

Line 249 – are here ºC?

Line 251 – add also here the units (ºC).

Line 268 - units for PI (mgO2/dm3)

Lines 279-280 – what is the reason for elevated Cu concentration?

Lines 292-293 - units for PI (mgO2/dm3)

Figure 3 – the units in the title of y-axes mg/l should be mg/dm3

Lines 305-306 – for these parameters, somewhere in the text are given full names and somewhere are abbreviations. The abbreviation can be used after the first appearance of parameter with given full name followed by abbreviation in the brackets.

Table 2 – can the columns 5,6,7 and 8 be under the first 4 columns? In that way the table will be more clear.

Lines 318 and 320 – HCO3 appears in both cases, and seems best represented in both first and second principal component. Is this correct? The data in Figure 4 are hardly visible.

Figure 4 – very low resolution. It must be revised since some data are almost invisible.

Line 340 – one extra dot in subtitle. It should be deleted.

Table 3 – the empty spaces in table 3 mean that there is no particular species in that waterbody? In that case, it maybe will be better to put zero value (0) instead of empty space.

Table 4 – please equalize the units ind/m3 with those mentioned in lines 78-81.

Figure 5 - the unit in the title of y-axes should be corrected – number 3 in m3 should be as superscript.

Line 396 – D. (Daphnia) or Daphnia (Daphnia)? Please see line 398.

Figure 6 - the unit in the title of y-axes should be corrected – number 3 in m3 should be as superscript.

Line 420 – the dot at the end of the sentence instead of comma.

Lines 426 – 428 – is there any explanation for this? Especially for higher concentrations of Pb and Zn?

Figures 7 and 8 – very low resolution. The data are invisible. Authors must give better figures because in this way they have no purpose at all.

Line 488 – The sentence “Comparative characteristics of environmental variables in the wastewater canal and reservoirs.” should be checked. It seems unfinished. I assume that it refers to data in table 5.

Lines 489-490 – “In June 2021, the content of nitrogen compounds, phosphates, and iron in the wastewater canal was higher, and readily oxidizable organic substances, copper, and lead were lower than in ponds and Sorbulak (Table 5).” – Do authors know why is that so? With what this is related?

Table 5 – Again, if it is possible it would be good to move all variables in the first column, and to add one extra column with MPC values. In note of table 5 please check for PI units.

Line 506 – Can authors give more information about the sewage? Sewage from where? What was the source of Zn and Cd for example?

Line 508 – add units for other metals also. It is better to repeat than to give number without units. Or put units somewhere before listing.

Line 510 – “The results obtained allow us to conclude that, despite a significant decrease in the concentrations of heavy metals in wastewater in recent decades [36,38,39]…” how often this is monitored, regularly or periodically? It is important. If monitoring is not regularly, discharge of wastewater polluted with heavy metals will not be recorded.

Again I missing the pre-treatment methods. Are they performed in order to remove heavy metals from wastewaters?

Line 565 – instead of dash (−) put was as in line before.

Line 596 – “zinc binds with manganese and iron”…What is meant by this?

Lines 629-632 – Unfortunately, heavy metals are in the system and only circulates between different environmental compartments, depending on external conditions. It is well-known that once emitted/discharged, stays permanently in the environment since are not biodegradable. Authors should add some short explanation here in the text.

Table 6 – It is better to use the same abbreviation through the whole manuscript thus replace Ponds 7 and 8 with RSC 7 and RSC 8.

positive – positive

negative-negative

Conclusion – the main purpose of this study is missing, and what this research contributes to. This should be added also in the conclusion section.

Author Response

Dear Reviewer, please see the attachment! 

Author Response File: Author Response.docx

Reviewer 3 Report

Dear Editors and Authors,

I have carefully read your manuscript entitled "Factors affecting water quality and the structure of zooplankton communities in wastewater reservoirs". Environmental pollution is one of our biggest concerns right now. It also covers many water pollution problems, so manuscripts like this one are very important and interesting to readers. Please find below my comments:

• line 27 - zooplankton and wastewater are unnecessary in keywords because they are in the title;
• line 29 - add "total dissolved solids" and after that "(TDS)", because it is the first time with this abbreviation in the regular text;
• line 88 - which studies? those from the previous paragraph or yours previous studies? if yours, please add citation;
• in my version of the work (pdf) the quality of the figures is very poor; figures 4, 7 and 8 are completely illegible;
• are the maps in figure 1 the authors' property? look like graphics from other sources linked together, it's not very professional;
• for how many samples are each average value reported in table 1? for 84 samples each?
• line 249 - °C.

Best regards,

Reviewer

Author Response

Dear Reviewer, please see the attachment! 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Review – version 2

Page 1, lines 24-26: In the abstract, the explanation of the influence of environmental variables on zooplankton should be corrected. The authors comment the nonlinear relationship between planktonic invertebrates and environmental factors, and persistently use analyses based on linear relationships. An earlier comment on the influences of environmental factors on zooplankton is still valid. Except for some variables (temperature, salinity, oxygen), it is difficult to interpret the relationship between nutrients and zooplankton abundance/biomass. Certainly, the abstract should not emphasize that seasonal dynamics are influenced by something that has not been explored.

Page 2, lines 92-93: The abundance of zooplankton is still expressed as the number of individuals per cubic meter. Delete the word thousand and the unit is ind./dm³. Apply changes to the entire text.

Page 2, lines 89-101: This paragraph should only list previous zooplankton research with appropriate references, without numerical values, which should be commented on in the discussion section. Thus, emphasize only what studies were conducted (research area, year of investigation) and what those studies included (taxonomic composition, abundance and biomass of rotifers, cladocerans and copepods).

Page 2, lines 120-124: These sentences should be removed from the introduction section. (“Domestic animals (horses, cows, and sheep) from the same farmers use water for drinking (Figure 1c). The meat of these animals and fish are sold in large quantities in Almaty, while the content of heavy metals in the liver and muscle tissue of the fish reaches a high level [24]. The authors link pathological changes in the internal organs of fish with exposure to toxic substances.)

Page 3, lines 132-135: Point out what makes this manuscript special compared to similar papers you have already published (Krupa et al., 2020; Aubakirova et al., 2021) and why it would be worth publishing. Do not mention statistical analysis in this paragraph.

Page 3, lines 137-176: The description of the research area should be written as one unit. The description of the area refers to the two paragraphs in the introduction and then is repeated again in the materials and methods. Assemble the text into a meaningful whole and abbreviate it.

Page 4: Please add a scale bar (distance in km) in Figure 2. It would be useful to have the geographic coordinates of the stations.

Page 5, line 183: Give the diameter of the Secchi disc in centimetres.

Page 5, line 200: Also give the length of the plankton net.

Page 6, line 228: Zooplankton abundance is still given per cubic meter, although the sampled volume is much smaller.  In reality, sampled volumes ranged from 45 L to 283 L and estimated abundance was reported for 1000 L. It is not appropriate to base abundance on a volume larger than that sampled. In scientific papers, values in thousands are given with a number (x10³), and instead of a fraction bar, a negative exponent is written in the denominator (ind. L^-1). I have already expressed my concerns about the sampling area in the Sorbulak Lake, and the question arises whether it can be treated as a single water body at all?

Page 6, lines 245-246: The use of Shannon diversity index (H’) based on zooplankton biomass data is still unclear. Add a sentence to explain this.

Page 7, lines 281-286: For clarity, reduce the number of variables, taking into account their collinearity.

Page 7, line 290: delete “the water level… … to September,”

Page 8, Table 1: What is the difference between “-“ and “**there is no MPC”? Does “-“ mean not applicable?

Page 9, lines 342-343: To which area do these temperatures apply?

Page 9, line 351: Explain the meaning of “the most significant amount”. In relation to what is the value significant?

Page 9, lines 364-372: I wonder what the authors were trying to show with the correlation of the chemical parameters in the three wastewater reservoirs? I think it would be better if they tested whether there were differences between the reservoirs in terms of chemical parameters and focused on the variables where the differences were found to be significant (ANOVA).

Page 9 (and page 13) – It should be emphasised that the survey does not cover the whole year, but only the warm part of the year (6 months), which affects the interpretation of the results. Therefore, it is not appropriate to talk about the seasonal distribution, as only the spring-summer aspect of the studied parameters is presented. It would be good to briefly explain why this part of the year was chosen.

Page 10, lines 374-339: The explanation of PCA results needs further clarification. Define the number of principal components extracted by PCA and the environmental variables that determined each principal component with associated eigenvalue. The term biplot is reserved for simultaneous representations which respect the fact that the projection of observations onto the vectors must be representative of the input data for the same variables. Delete the number 2 in the labels: SO42, CO32, Mg2, Ca2 in Figure 5a.

Page 11, line 393: Which quadrant? It seems that depth, PI and transparency have a short vector length and are probably represented in a different PCA dimension. Please, check.

Page 11, line 403: Replace the word “waterbody” with the name “wastewater reservoir”. Zero values are missing in some places in Table 3. Align the numbers on the right side to make them easier to read.

Page 12, line 406: Define species richness. Do you mean the total number of species (S)?

Page 12, line 422: Copepods dominated the zooplankton in all wastewater reservoirs. – Is this true for RSC7? Do you mean abundance or biomass?

Page 13, line 427: The word “Table” is missing. Table 4. Abundance, biomass and diversity of the zooplankton community in the three wastewater reservoirs of the Right-Bank Sorbulak Canal system in 2021 (average value ± standard deviation). Also correct: Average individual mass, mg ind.^-1

Page 13, lines 439-446: This paragraph should have been rewritten by considering seasonal variations in abundance and biomass separately. If the differences between abundance and biomass fluctuations are not significant, the authors should focus on one of them.

Page 14: Figures 6 and 7: Instead of chronological order, the months are arranged alphabetically. The figures presented this way are counterintuitive. Be sure to correct this.

Page 15: 3.6. Impact of environmental variables on zooplankton: The comments written earlier about PCA and interpretation of results are still the same. PCA is not an appropriate statistical method for analysing relationships between biotic and abiotic variables. Except for some environmental variables (such as temperature, salinity, oxygen concentration), it is impossible to discuss their influence on zooplankton, while the control of zooplankton by resources is completely ignored. The authors must ask what is the point of comparing bicarbonate or magnesium concentrations with rotifers, or cladocerans with calcium, zinc or lead, … A correlation can be calculated between any two data sets, but that does not mean there are causal relationships. It makes no sense to include depth as an environmental variable because zooplankton are not sampled in layers and concentrations are expressed per unit volume, not per surface area. Therefore, I recommended that the authors test the gradient length of the selected data set prior to data analysis to define the relationship between the response and the explanatory variables. If you want to examine the effects of selected (meaningful) ecological variables (i.e., explanatory variables) on zooplankton (i.e., response variables) and consider the temporal and/or spatial distribution, you need to create a correlation triplot.

Page 17, lines 547-552: Table 5 is unnecessary. Why do you repeat the results again? Why do the water quality results for the incoming channel suddenly only appear in the discussion section? Did I misunderstand something? The issue is the reliability of the assessment, which was based on only one sampling.

Page 18, lines 567-570: Repetition of the results.

Page 19, line 654: “Distribution of heavy metals” instead of the word “migratory”?

Page 20, lines 704-706: The first sentence of this paragraph is redundant, omit it. What do you consider background species?

Page 21, line 717: prefer – tolerate?

Page 21, line 719: What are the thicket zooplankton species?

Page 21, lines 720-722: You are not looking at similarities/differences between stations, just calculating correlations between abiotic and biotic variables. Lake Sorbulak is certainly a deep-water body, even if it is deeper than ponds.

Page 21, lines 723-728: Does this mean that the winds blow only over Lake Sorbulak and not over the other reservoirs? It would be good if the authors would provide data to support this statement.

Page 21, lines 741-744: It would be useful to support this statement with relevant literature and avoid self-citations.

Page 21, line 766: Given all the previous comments on PCA, there is no reason to show the same data in the figure and in the table. Had you chosen the appropriate statistical analysis for what you wanted to show and interpreted the results accordingly, you would have drawn conclusions based on them in the discussion and commented on them according to the reference literature.

Page 21, line 768: You found that the relationship between biological variables and environmental factors is not linear. Why then did you use a PCA? As I mentioned earlier, it would be useful to substantiate your conclusions in the relevant scientific literature and refrain from self-citations.

Page 23, lines 787-790: The authors should carefully reconsider the conclusions of the manuscript and refrain from making general statements. As written, the main conclusions seem to be that most correlations between biological and environmental variables cannot be explained and that there is insufficient knowledge about the ecological preferences of zooplankton.

Page 23, lines 791-819: Although the manuscript shows that the authors have many years of experience in research on the ecological characteristics (biotic and abiotic) of wastewater reservoirs, their conclusions should be based primarily on this research. I propose to harmonize and shorten the conclusions on the basis of the above remarks. The main conclusions should be exhaustively stated.

In view of all these points, I do not recommend publishing the article in Water. It might become acceptable if rewritten under a different concept, because the basic environmental and zooplankton data are interesting. I have made detailed comments above and wish you good luck with a new attempt.

Author Response

Dear Reviewer,

Please see the attachment!

Authors

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript is significantly improved. The main purpose of the research is now more clear. There are still some parts that should be focused on:

1. In added methods of wastewater pre-treatment there are mostly physical and biological methods. What about the chemical treatment in order to remove heavy metals? Is there any chemical treatment of wastewater before entering the water ponds? If not, then, it is expected to find elevated concentrations of heavy metals which is not good if that water is used for irrigation purposes. The question is – the low concentration of most of the heavy metals that occurs in ponds is due to an adequate wastewater treatment or due to the decrease in industrial activity?
2. At the end of Table 3 there are still few empty spaces without zero value.
3. For future, it would be nice that authors in answers to the reviewer comments precise the lines in which the corrections were made to be easier to follow.

Author Response

Dear Reviewer,

Please see the attachment!

Authors

Author Response File: Author Response.docx