Physicochemical Differentiation of the Muskau Arch Pit Lakes in the Light of Long-Term Changes
Round 1
Reviewer 1 Report
The authors describe two datasets of physico-chemical data in mine pit lakes 3 decades apart. This is an exciting opportunity to speculate on the evolution of mine pit lakes, relevant for many other regions in the world where mining comes to an end and management options for newly created lakes need to take into account physico-chemical processes over longer time-scales. I state "speculate" as there are only two points in time and not a series of measurements over this 30 year period to be able to follow the evolution over time.
The authors claim that lake depths have increased, infering this from the maximum measuring depths of an instrument lowered from a boat somewhere on a lake. This is totally inacceptable; if there are satellite images of those lakes, which the authors refer to in section 2, this is the way to infer a change in lake level - or observed inundated trees etc.
My main objection, is that the description and discussion of physical/hydrodynamic processes is insufficient and would need a major overhaul of the interpretation and the paper itself.
General:
- Table 1 not referenced in text
- Citation numbers are not corresponding to references
Introduction:
- physical/hydrodynamic processes not well explained, needs a better explanation.
- L47: "thermal diversity" not clear what this refers to.
- L55/56: "...(monimolimnion) do not mix at all." please correct:
the monimolimnion by itself can well mix by internal processes albeit very slowly, e.g. groundwater inflow, molecular diffusion etc.; ther is no or very limited exchange between mixolomnion and monimolimnion. - L58: "mostly retains" -> "is persistent"
- L58/59: rewrite sentences: "... plays an important part" + "in stratification. However, in shallower or less strongly stratified lakes external forces (e.g., wind, heat exchange, inflows) can lead to complete mixing and thus meromixis is not a common characteristic of pit lakes."
- L62: "are significant for the landscape" - explain in what respect?
- L66: convection mixing -> convective mixing
- L67: this is wrong, mixing cannot be reversed, potentially permanent stratification can build up again but this needs specific conditions.
- L72: add something like "... agent to reduce acidity."
- L79 no new paragraph
- L82: poor knowledge of these systems in the Mukau Arch are probably not so much the result of inaccessibilty but willingness (costs?) to do regular measurements. This sentence should be reformulated stating the fact that there is no consistent monitoring plan.
Study area and methods
- L98: not of interest to specify the instrument type in determining water depth.
- L98 ff: surface area was determined, but not discussed in the remainder. Although, this should give a hint whether the lake surfaces have changed shape and increased in area and thus an increase in depth is a fact (see later remarks: authors use maximum depth of lowering an instrument to argue for depth changes)
- cite Table 1
- Table 1: insert number of references; specify in text what "age" means in this context.
- EC is mostly called electrical conductivity
- L121: the numbering scheme used here is that of Pukacz not Jedrczak
- "additional measurements were performed": please define and discuss these measurements
Results
- L165: correct units superscript and use of l or L for litre (also Table 2)
- Figure 4: need to include date of profiles to be able to interprete them (seasonal variations)
Use same temperature scales for all plots - L206 "A clear increase in the maximum depth was found in all lakes"
This statement is hillarious. One cannot measure the depth of a lake from the lowering of an instrument alone, as it is unlikely to do a measurment at exactly the same point (assuming that the instrument is not inclined but); more so after 30 years. - L208: "in 2016, higher temperatures were observed". Off course this needs to be related to the seasonal cycle of water temperature. From a single measurements nothing can be inferred. This needs to be reflected with observed air temperatures (or modelled water temperatures).
- Figure 4 and interpretation in the text: Need to plot conductivity profiles, physical processes in chemical stratified systems are not primarily reflected in the temperature profiles but in density. Either need to show both, conductivity and temperature, or show a calculated density profile.
Discussion
- L268ff: the morphologic effect or increase in depth cannot be infered from the lowering of a temperture profiler and reached maximum depth. As long as this is not based on clear evidence from other observations, this paragraph has no value.
- L283: "speed and depth of water mixing" - not sure what the authors refer to (there is nothing like speed of mixing). Mixing depth is mainly determined by heat exchange, wind stress, and light absorption, to a lesser degree by morphometry; Shading by trees or topographic effects might havean influence. Most of those lakes are in forested areas and thus wind and solar shading play their roles.
- L285: it is not possible to state that the mictic type of the lakes has not changes from 1986 to 2016 having only two points of measurements; there could have been a change and reverse of change in these 30 years. It is not even possible to clearly define the mictic type from a single profile (more so when no conductivity profiles are present). This could be a seasonal effect, with mixing in autumnn, or it could be a permanent stratification.
What can be stated here is that the stratification structure is similar and further investigation of possible mixing scenarios using modelling would be needed to infer possible transitions over these 30 years.
Conclusion
- needs reevaluating depending on the points raised above.
- Missing a discussion/conclusion to broaden the context what this research means for other similar systems in past development and for future mine pit lakes.
Author Response
We sincerely thank you for the criticism and valuable time spent for performing the review. We thoroughly considered all remarks and the manuscript was modified according to the suggestions.
Please find below the detailed answers to the comments and description of corrections made in the revised manuscript.
We believe that the the improvement is sufficient and meets your expectations and we hope our manuscript can now be accepted for publication in the Water journal.
On behalf of all co-authors,
Andrzej Pukacz
Comment: The authors describe two datasets of physico-chemical data in mine pit lakes 3 decades apart. This is an exciting opportunity to speculate on the evolution of mine pit lakes, relevant for many other regions in the world where mining comes to an end and management options for newly created lakes need to take into account physico-chemical processes over longer time-scales. I state "speculate" as there are only two points in time and not a series of measurements over this 30 year period to be able to follow the evolution over time.
Response 1. I agree with the opinion that some of our conclusions are bit speculative. Unfortunately, the results available for these exceptional lakes are very poor (mainly due to a very difficult terrain to access). We have included this information in the introduction:
“The hydrochemical and hydrobiological properties of lakes located within the Muskau Arch are still not well known. Most of the available literature data from this area concern either single objects […] or, if more lakes were analysed, they are only regional-wide reports […]. It mainly results from the fact that they are located in poorly accessible places (with hills and forests). Therefore, many of them have never been transformed by human activity, maintaining their original character.”
This was also one of the main arguments why we took up this difficult topic. Nevertheless, we tried to be as careful in our conclusions as possible and to analyse the available data in as much detail as possible.
We believe that our results, when corrected, will provide a good starting point for further research.
Comment: The authors claim that lake depths have increased, infering this from the maximum measuring depths of an instrument lowered from a boat somewhere on a lake. This is totally inacceptable; if there are satellite images of those lakes, which the authors refer to in section 2, this is the way to infer a change in lake level - or observed inundated trees etc.
Response: It is not possible to determine the depth of these objects on the basis of satellite image analysis - there is no relevant data. In our research we used an echo sounder:
“Before the survey, Lowrance Mark 5x Pro sonic depth finder was used in each lake to determine its maximum depth and any potential bottom anomalies. The measurement was performed by a transect method.”
This is a standard method for measuring depth in lakes. In the case of the 1986 data, the measurement was used by a transect method with the use of portable steel roller water level meter with weighter, which at that time was a standard method. Since these lakes are very small and the bottom is mostly mineral, the probability of a measurement error (more than 0.2 m) was very low. Additionally, on the basis of observations of trees, fishing platforms and shoreline we have not found any clear changes in water level over the last 30 years. We also added this information in the discussion.
Comment: My main objection, is that the description and discussion of physical/hydrodynamic processes is insufficient and would need a major overhaul of the interpretation and the paper itself.
Response: We have done our best to improve the description and interpretation of physical and chemical processes, both in the introduction and discussion chapter.
General:
- Table 1 not referenced in text
- Citation numbers are not corresponding to references
Response: Both has been corrected
Introduction:
- physical/hydrodynamic processes not well explained, needs a better explanation.
Response: The physical/hydrodynamic processes have been supplemented with some additional information relevant to the topic.
- L47: "thermal diversity" not clear what this refers to.
Response: We have corrected that information as follows: “…stratification results mostly from variability temperature, according to the seasons…”
- L55/56: "...(monimolimnion) do not mix at all." please correct:
the monimolimnion by itself can well mix by internal processes albeit very slowly, e.g. groundwater inflow, molecular diffusion etc.; ther is no or very limited exchange between mixolomnion and monimolimnion.
Response: We have added this information.
- L58: "mostly retains" -> "is persistent"
Response: We have corrected this phrase.
- L58/59: rewrite sentences: "... plays an important part" + "in stratification. However, in shallower or less strongly stratified lakes external forces (e.g., wind, heat exchange, inflows) can lead to complete mixing and thus meromixis is not a common characteristic of pit lakes."
Response: We have corrected this information according to your suggestion.
- L62: "are significant for the landscape" - explain in what respect?
Response: We have corrected that information as follows: “…currently constitute an integral and unique element of the landscape.”
- L66: convection mixing -> convective mixing
Response: We have corrected this phrase.
- L67: this is wrong, mixing cannot be reversed, potentially permanent stratification can build up again but this needs specific conditions.
Response: We have corrected this information.
- L72: add something like "... agent to reduce acidity."
Response: We have added this information.
- L79 no new paragraph
- Response: We have corrected this section.
- L82: poor knowledge of these systems in the Mukau Arch are probably not so much the result of inaccessibilty but willingness (costs?) to do regular measurements. This sentence should be reformulated stating the fact that there is no consistent monitoring plan.
Response: We have corrected that information as follows: “Although the area is under protection as the Muskau Arch Landscape Park since 2001 and its most valuable area is a part of the first in Poland Geopark (Muskau Arch Geopark), there is still no consistent monitoring plan for the lakes.”
I would like to add that, although I grew up in that area, some of the lakes I discovered for the first time during this research. Finding some of the lakes in dense and undulating terrain is very difficult, and providing research equipment there (without access by car) - even more difficult.
Study area and methods
- L98: not of interest to specify the instrument type in determining water depth.
Response: We disagree with that remark. This is just as necessary as giving other equipment and research methods
- L98 ff: surface area was determined, but not discussed in the remainder. Although, this should give a hint whether the lake surfaces have changed shape and increased in area and thus an increase in depth is a fact (see later remarks: authors use maximum depth of lowering an instrument to argue for depth changes)
Response: We have added the data od max depth and surface in Table 1 described it separately.
- cite Table 1
Response: We have added this information.
- Table 1: insert number of references; specify in text what "age" means in this context.
Response: We have added this information.
- EC is mostly called electrical conductivity
Response: We have corrected this information.
- L121: the numbering scheme used here is that of Pukacz not Jedrczak
Response: We have corrected this information.
- "additional measurements were performed": please define and discuss these measurements
Response: We have corrected this information.
Results
- L165: correct units superscript and use of l or L for litre (also Table 2)
Response: We have corrected the units throughout the MS.
- Figure 4: need to include date of profiles to be able to interprete them (seasonal variations)
Use same temperature scales for all plots
Response: We have added this information in the study area and methods section: “The measurements and samplings were performed at the beginning of August, in the deepest point of each lake (the same was in 1986)”
- L206 "A clear increase in the maximum depth was found in all lakes"
This statement is hillarious. One cannot measure the depth of a lake from the lowering of an instrument alone, as it is unlikely to do a measurment at exactly the same point (assuming that the instrument is not inclined but); more so after 30 years.
Response: We have corrected this information, according to our response given above.
- L208: "in 2016, higher temperatures were observed". Off course this needs to be related to the seasonal cycle of water temperature. From a single measurements nothing can be inferred. This needs to be reflected with observed air temperatures (or modelled water temperatures).
Response: It is not possible to correlate our data with modelled temperatures. However, based on literature data concerning lakes, as well as our own measurements in natural lakes we believe that this argumentation is justified. Nevertheless, we have tried to choose our words carefully:
“An important factor affecting water chemistry of the studied lakes is the changing climatic conditions, which affect the inflow of water and the biogeochemical changes, e.g. the redox potential and microbiota activity [16]. The analysis of temperature in the lakes showed that in 2016, temperature in all lakes was higher at the surface than in 1986. Although our measurements do not cover intermediate years and were made in particular months this difference may result from the climatic changes occurring over the years that have been confirmed for natural lakes in many regions of the world [Woolway, and Merchant 2019].”
- Figure 4 and interpretation in the text: Need to plot conductivity profiles, physical processes in chemical stratified systems are not primarily reflected in the temperature profiles but in density. Either need to show both, conductivity and temperature, or show a calculated density profile.
Response: It is well known that both temperature and conductivity have an important impact into density. However, we decided do not calculate the density because we do not have sufficient data to do this (no ion specification in the whole vertical profile). Moreover, there is too many factors decided on density (also strongly connected with ion balance and ions e.g.: HCO3-, SO42-, Cl-, Ca2+, Mg2+, Na+, K+ calculated based on gram equivalent unit) all these ions are very variable in these lakes, thus the potential error is very high. This is why we decided to extend the interpretation to the values of conductivity at opposite depths.
Discussion
- L268ff: the morphologic effect or increase in depth cannot be infered from the lowering of a temperture profiler and reached maximum depth. As long as this is not based on clear evidence from other observations, this paragraph has no value.
Response: We have changed that information both in the results and discussion section. The interpretation of morphometric changes was described separately:
“In our opinion, changes in the morphometric structure also contributed significantly to the diversity of water chemistry. Our measurements, compared with the data by Jędrczak [1992], showed a considerable increase in the depth (even up to 6 m) of most the lakes. Based on observations of trees, fishing platforms and shoreline we have also concluded there were no significant changes in water level over the last 30 years. Still, it was found that all the lakes have reduced their surface area.”
- L283: "speed and depth of water mixing" - not sure what the authors refer to (there is nothing like speed of mixing). Mixing depth is mainly determined by heat exchange, wind stress, and light absorption, to a lesser degree by morphometry; Shading by trees or topographic effects might havean influence. Most of those lakes are in forested areas and thus wind and solar shading play their roles.
Response: We have changed this information as follows: “Changes in the depth of lakes have a significant impact on the occurring hydrochemical processes, including the ratio of the epilimniom and hypolimnion [4]. However, mixing depth is also determined by heat exchange, wind stress, and light absorption [10]. Thus, shading by trees or topographic effects might have an influence. Most of those lakes are in forested areas and thus wind and solar shading play their roles.”
- L285: it is not possible to state that the mictic type of the lakes has not changes from 1986 to 2016 having only two points of measurements; there could have been a change and reverse of change in these 30 years. It is not even possible to clearly define the mictic type from a single profile (more so when no conductivity profiles are present). This could be a seasonal effect, with mixing in autumnn, or it could be a permanent stratification.
What can be stated here is that the stratification structure is similar and further investigation of possible mixing scenarios using modelling would be needed to infer possible transitions over these 30 years.
Response: We have changed this information according to this suggestion.
Conclusion
- needs reevaluating depending on the points raised above.
- Missing a discussion/conclusion to broaden the context what this research means for other similar systems in past development and for future mine pit lakes.
Response: We have corrected the conclusions according to this suggestion
Reviewer 2 Report
Dear author, congratulation, you are studying an important topic. However, I would suggest adding results to the abstract. You did not show any results in the abstract. I have also recommendations, how many samples did you analyze from each lake, how many samples did you analyze from historical water collection. You should show the variability of chemical composition from each lake. Is there seasonality in studied analyses? Such data will improve the research paper. I suggest improving the paper.
Author Response
We sincerely thank you for the criticism and valuable time spent for performing the review. We thoroughly considered all remarks and the manuscript was modified according to the suggestions.
The abstract has been slightly corrected. Due to the wide range of presented results and word number limitations of the abstract we presented the most important information about our research including the highlights of results and conclusions. In our opinion, the presented information provides a comprehensive overview of the content of the article.
The variability of chemical composition from each lake has been presented both in Figure 2 and Table 2. Our research concerned the differentiation of physicochemical parameters and their comparison with previous studies. Thus, it is based on characteristics in the summer season (as was studied before) and it was not possible to analyse seasonal variability in particular years of research.
In order to improve the comprehensibility of our research, we have corrected and detailed both the description of the results and their interpretation.
We believe that the the improvement is sufficient and meets your expectations and we hope our manuscript can now be accepted for publication in the Water journal.
On behalf of all co-authors,
Andrzej Pukacz
Reviewer 3 Report
The present paper presents the result of a physical and chemical characterization of the Muskau Arch pit Lakes in the period between 1986 - 2016. In my opinion the paper is poor in novelty and data analysis conducted is not enough to support the discussion. Therefore, I would like to see more data analysis in order to put in evidence the interrelation between the physico - chemical characterization and natural processes, human activities and climate.
Figure 1 the resolution must be improved. Please insert North direction and scale of representation.
Author Response
We sincerely thank you for the criticism and valuable time spent for performing the review. We thoroughly considered all remarks and the manuscript was modified according to the suggestions.
We have not performed additional analyses related to climate and natural processes because there is not sufficient data (especially for natural processes). This area is still one of the least recognised in terms of nature, especially in terms of hydrochemical water research.
In order to improve the comprehensibility of our research, we have corrected and detailed both the description of the results and their interpretation.
We believe that the the improvement is sufficient and meets your expectations and we hope our manuscript can now be accepted for publication in the Water journal.
On behalf of all co-authors,
Andrzej Pukacz
Reviewer 4 Report
The overall paper is generally well written and clear. The laboratory and statistical methods are clearly described. The data is well organized and clearly presented. The study design is very simple and clearly laid out.
There are some problems with the English but they are I believed easily fixed. I do not believe that the authors are using some of their words correct. I cannot see how their use of the word "mictic" correct. Also "fertilizing" does not seem right.
The statistical section could use some strengthening. First it is critical to assess the normality of the data. They authors need to determine whether the data is normally distributed. They need to assess the using the Kolmogorov-Smirnov test or Shapiro-Wilk test or any number of others. The authors compared data between lakes using ANOVA with the Kruskal-Wallis (KW) test. That should be a One-Way ANOVA on Ranks. K-W is a non-parametric test used for non-normally distributed data. The authors need to describe the normality of the data and explain how the K-W test is appropriate. The results for the K-W test are shown on Table 2 with the heading "H-Test". This should be H Value, not H test. The authors need to explain the relationship between the K-W and the H value results. They then only list the p values, which are not the actual H values. The authors wrote: " Different lake groups were compared using the ANOVA..." but in Table 2, one lake is not compared to another lake but rather three different time periods are compared for the same lake.
The authors wrote: "Covariation was tested using non-parametric Spearman Rank Correlation. Significant levels were set to α = 0.05." As far as I can tell, they never used the Spearman Rank Sum test. They authors need to either remove that sentence or identify where in the paper the SRS is used.
The authors use the Principal Component Analysis but do not explain how this test works or how to interpret the results. What are these r values, how are they calculated, what do they mean? Is there a critical r value which indicates significance? The number are just hanging there without any context. Should there p values as well?
"PCA was performed in order to determine the the temporal diversity of the lakes against the background of the analyzed physico-chemical parameters. The outcome of the analysis (Figure 3) showed that the first and the second component explain approx. 60% of the general variability. Out of the tested properties, the strongest relationship with the first principal component (r > 0.75) occurred in the case of pH, EC, and the concentrations of Ca, Mg and Fetot. The first component was also highly correlated with K and NH4-N (r = 0.54 for both). The second principal component was the most strongly correlated with Na, K, NO3-N and NO2-N (r = 0.70; 0.66, 0.63 and 0.56, respectively). The outcome of the analysis showed clear temporal variation of the cases with consideration of the second principal component. The biggest differences were found between the years 1986 and 1992."
The authors already compared the "temporal diversity" of the lake using the KW test, why do it again with PCA? What is PCA telling us that K-W is not? It is just just laundry list of numbers without any context.
The main problem with the paper and why I am believe that it needs a major re-write is that the significance of the study is not really clear. It reads more like an engineering report rather than a scientific paper. It is largely just a collection of data, describing how different physio-chemicals test results at different locations and times. There is little explanation of what it is all supposed to mean, why the reader cares about these numbers. Is the water in the lakes being used for agricultural irrigation, fish farming, recreation, wildlife conservation, or something else? The authors need to explain to reader what the importance of this data is.
Author Response
We sincerely thank you for the criticism and valuable time spent for performing the review. We thoroughly considered all remarks and the manuscript was modified according to the suggestions.
The English language and style has been corrected by a native English speaker. We have checked and corrected the use of the indicated terms.
We have supplemented the information on the statistical methods used. The normality of distributions of the analyzed variables and the homoscedasticity of the samples were tested with the Shapiro-Wilk and Levene tests, respectively. Since non-normal distribution or unequal variances were stated for most of the variables the multiple comparisons were conducted using the ANOVA with the Kruskal-Wallis test.
We have removed the information on Spearman Rank Correlation – it was our mistake.
In the case of applying the significance of differences test, the aim was to compare the differences in parameters over different study periods. Due to the lack of relevant historical data, we could not apply the classical time-variation models.
The data from individual study periods were independent groups of variables, so we used a simple K-W test. Due to the heterogeneous distribution of variables, we did not apply other tests (e.g. one-way ANOVA), thus more detailed interpretations in the text. The results of this test, in particular those given in the table of values, complement the results of the PCA analysis and allow for a more detailed interpretation. The r values given in the description of the PCA results are the standard correlation coefficients calculated from metrical variables that are given in the description of the results in Statistica.
In order to make the results more detailed, graphs showing the differences between the epilimnion and hypolimnion were made. In this case, it was not possible to use statistical analyses as single samples were taken from each of these layers.
We believe that the the improvement is sufficient and meets your expectations and we hope our manuscript can now be accepted for publication in the Water journal.
On behalf of all co-authors,
Andrzej Pukacz
Round 2
Reviewer 1 Report
After this intense revision the paper is now acceptable for publication in Water with some minor changes (no further review necessary).
The authors have clarified the raised points. The added information removed my objection against the "depth issue". This was obviously a misunderstanding based on a piece of missing information and awkward formulation in the text. Please provide this info on depth measurements in 1986 also in the paper (see remark below).
The description of hydrodynamic processes has also improved as far as is possible with the restricted data set.
Specific comments:
In Figure 4 you now use the same axis range for temperature. You could do this also for Oxygen (the discussed features should still be visual)
Some minor textual changes I have marked in the pdf (you might switch on Comments in your pdf viewer to see them).
Just want to ensure that at
L124ff (section 2, end of 3rd par): Please add the sentence you stated in your reply to my question (this rules out any confusion, a trap I fell in when raising this issue in the first review):
"In the case of the 1986 data, depth measurement were done by a transect method with the use of portable steel roller water level meter with weighter, which at that time was a standard method. "
Comments for author File: 
Comments.pdf
Author Response
Thank you for the positive opinion and additional comments. We fully agree with all the remarks and have corrected the manuscript as suggested both in the specific comments and those marked in pdf.
We believe that these improvements will meets your expectations and we hope our manuscript can now be accepted for publication in the Water journal.
On behalf of all co-authors,
Andrzej Pukacz
Reviewer 2 Report
Thank you, you have improved Manuscript substantially.
Author Response
Thank you for your positive opinion.
We believe Your time and effort invested in the revision are reflected now in the improved manuscript that we hope can now be accepted for publication in Water journal.
On behalf of all co-authors,
Andrzej Pukacz
Reviewer 3 Report
The paper has been improved in several aspect. I suggest publishing.
Author Response
Thank you for your positive opinion.
We believe Your time and effort invested in the revision are reflected now in the improved manuscript that we hope can now be accepted for publication in Water journal.
On behalf of all co-authors,
Andrzej Pukacz
Reviewer 4 Report
Part 1.
In my original review I wrote:
"The main problem with the paper and why I am believe that it needs a major re-write is that the significance of the study is not really clear. It reads more like an engineering report rather than a scientific paper. It is largely just a collection of data, describing how different physio-chemicals test results at different locations and times. There is little explanation of what it is all supposed to mean, why the reader cares about these numbers. Is the water in the lakes being used for agricultural irrigation, fish farming, recreation, wildlife conservation, or something else? The authors need to explain to reader what the importance of this data is."
The authors completely ignored this, the main point I was trying to make, and made no attempt to respond. The conclusion of the paper is that the water chemistry changed over the course of the study period. That is clear enough but there is still no explanation of why the readers of this journal should care about these results.
At the beginning of the paper the authors write: "The aim of the study was to determine temporal changes in the physico-chemical conditions of
the Muskau Arch pit lakes within the years 1986-2016 and the main factors influencing their evolution." This is not scientific hypothesis, there is nothing to test. In the conclusion they write: "The performed analyses showed that the pit lakes of the Muskau Arch are very diverse in terms of the physico-chemical water conditions. This particularly refers to mineralization, including the concentration of the analyzed metal ions."
The authors need a hypothesis, such as "Because of the dynamic hydrology of the pit lakes, the concentrations of analytes x, y, and z should increase with time". They can test that and conclude "our hypothesis was correct because there was an increase in concentration" or they could conclude "our hypothesis was incorrect because the concentrations remained steady or decreased". Right now they are saying that they wanted to see if the concentrations changed and they did.
The authors need to establish what the readership of this journal will learn from this paper that they do not already know that is worth learning. That these particular pit lakes changed over time in terms of mineral content is of minimal interest.
Part 2.
In my original review I wrote: "The results for the K-W test are shown on Table 2 with the heading "H-Test". This should be H Value, not H test. The authors need to explain the relationship between the K-W and the H value results. They then only list the p values, which are not the actual H values."
This was not corrected. They need to explain what the H value is and why it matters, they need to show the actual H values, and then they need to explain what the p values are and their relationship to the H values.
Part 3.
In my original review I wrote: "The authors use the Principal Component Analysis but do not explain how this test works or how to interpret the results."
In the paper the authors use the term PCA three times.
"The Principal Components Analysis (PCA) was performed for the statistical analysis of the physico chemical parameters data set."
"PCA was performed in order to determine the the temporal diversity of the lakes against the background of the analyzed physico-chemical parameters."
"Although PCA showed an increase in the values of some parameters for five lakes, the mean values of all the analyzed parameters decreased between the years 1986 and 2016."
The authors never explain what PCA or why they are applying it. The K-W test has already demonstrated that the concentrations of some analytes have changed. What is the PCA test showing that the K-W did not?
PCA is used to find which variables, analytes in this case, are the main source of change and which are not.
Part 4.
I also wrote in regard to PCA.
"What are these r values, how are they calculated, what do they mean? Is there a critical r value which indicates significance? The number are just hanging there without any context. Should there p values as well?"
The authors need to explain what these r values are, what they have to do with PCA, and how what do they mean.
Author Response
Once again we sincerely thank you for the criticism and time spent for performing the review.
We thoroughly considered all your remarks and the manuscript was modified according to the suggestions.
We have tried to make improvements both with your suggestions and with examples known from literature.
Please find below the detailed answers to the comments and description of corrections made in the revised manuscript.
We believe that the improvement is sufficient and meets your expectations.
On behalf of all co-authors,
Andrzej Pukacz
Part 1.
It was not our intention to ignore any of your suggestions. The fact that we have not made sufficient improvement is only due to a misunderstanding of your comments.
We agree with the suggestion that we should formulate our research assumptions clearly.
As suggested, we have put the hypothesis in the introduction:
“The aim of the study was to determine temporal changes in the physico-chemical conditions of the Muskau Arch pit lakes within the years 1986-2016 and the main factors influencing their evolution. We hypothesized that the lakes undergo temporal variability of physical-chemical water conditions both through natural evolution and dynamic hydrology induced by external factors.”
and have reformulated the final conclusions accordingly:
“The overall analysis of the obtained results seems to confirm our hypothesis that the lakes undergo temporal variability of physical-chemical water conditions resulting not only from the natural evolution (e.g. hydrochemical processes, secondary succession) but also from dynamic hydrology induced by human activity (neutralization). Taking into consideration the unique character of these lakes and their environment and landscape formation role, it is necessary to learn the exact processes occurring therein and to protect them.”
Part 2.
We have completed the results and the description in the table two, presenting the results of the Kruskal-Wallis test: we have added both H and P values. Therefore, in the header of this column we left the term "Kruskal-Wallis test" (according presentation of such results in literature).
Additionally, we have improved and completed the description of this method in the “Study area and methods” section:
“To determine which of the parameters differed significantly over the years a non-parametric ANOVA was applied via the Kruskal–Wallis H-test. With these test three independent groups (water parameters in 1986, 1992 and 2016) were tested. The H values larger than 5.9915 (df = 2), allowed to reject the null hypothesis there is no year-to-year differentiation. P < 0.05 was accepted as statistically sound.
We also changed the description of the results accordingly:
“The detailed analysis of the mean values (Table 2) confirmed significant temporal variability among the studied lakes with consideration of most the parameters studied. Although PCA showed an increase in the values of some parameters for five lakes (11, 12, 14, 22, 26), the mean values of all the analyzed parameters decreased between the years 1986 and 2016.
The results of the Kruskal-Wallis H test (H(2) > 5.9915) showed statistically significant differences (P < 0.05,) for six of the studied variables: O2, Na, K, Mg, NO3-N and NO2-N. The greatest differences (P < 0.000) were found for K, NO3-N and NO2-N. These parameters were also among the most important for the year-to-year lake differentiation in the PCA output. Interestingly, no significant variation was found for parameters most strongly correlated with the first principal component, i.e., pH, EC, Ca and Fetot.”
Since our goal with this test was only to determine which parameters and to what degree differ signifficantly between the particular study periods (as explained in the “Study area and methods” section), we did not undertake a more detailed description of the results for this test.
Both the description and interpretation of the results for this method are consistent with our previous publications and literature on similar environmental research.
Part 3.
According to this suggestion, we have completed the description to the PCA method in the “Study area and methods” section:
“The Principal Components Analysis (PCA) was performed to determine physico-chemical variability among the studied lakes. Prior to this analysis, the data (except pH values) was subject to logarithmic transformation (log (1+x)) to minimize the discrepancies between the empirical distribution of the variables and their theoretical normal variation. Additionally, to reduce the number of variables, simple factor analysis was applied.”
We also clearly distinguished between the assumptions of the PCA method and the Kruskal-Wallis test.
Similarly to the K-W test, we also changed the description and interpretation of the results.
“PCA was performed in order to determine the physico-chemical variability among the studied lakes. The outcome of the analysis (Figure 3) showed that the first and the second component explain 65.23 % of the general variance. The PCA output proved that pH, EC, Ca, Mg and Fetot. (correlated with the first axis, r > 0.75) and Na, K, NO3-N (correlated with the second axis, r > 0.6) to be primarily responsible for the variance observed. These parameters were the most important parameters for the lake-to-lake differentiation (the distance between particular cases). The weaker correlations (lesser importance) were found for K and NH4-N (correlated with the first axis, r = 0.54 for both) and NO2-N (correlated with the second axis, r = 0.56). In addition to lake-to lake differentiation distinct clusters of cases (distinguished groups of lakes) indicate temporal variation with consideration of the second principal component. The greatest differences were found between the years 1986 and 1992.
Despite year-to-year differentiation in each group, a similar pattern of distribution of cases (lakes) within the distinguished groups (years of the study) was observed. The greatest diversity between the lakes occurred in 1992. The outstanding lakes were lakes no. 11, 12, 14, 22 and 26. Those lakes had the lowest pH values and the highest values of mineralization and NO2-N, NH4-N and Fetot. ion concentrations.
The highest pH values and relatively lowest values of conductivity occurred in lakes no. 8, 10, 18, 20 and 25. In those lakes, a clear decrease in Na, K, NO3-N, Ca and Mg occurred between the years 1986 and 2016. This makes them different from the other lakes.”
Part 4.
We have changed the description of the results so that we believe it makes clear what the correlations mean (please see the paragraph quoted above).
The r values given to PCA results provide a detailed interpretation - in what degree the analysed variables influence the observed variability of cases. The higher the correlation, the greater the importance within a particular principal component. We provide and explain these values as standard in all our publications. However, we don't explain how to interpret it, because it is based on the assumptions of the method.
Round 3
Reviewer 4 Report
I have made this comment twice before and the authors have not responded to it. It is the most important comment that I made. The authors do not explain the significance of their study. It is simply a collection of laboratory results. In their conclusion they wrote: "The performed analyses showed that the pit lakes of the Muskau Arch are very diverse in terms of the physico-chemical water conditions. This particularly refers to mineralization, including the concentration of the analyzed metal ions." I am quite certain that this is correct but why would the reader of this journal care about that? What does the reader learn that he or she did not know before? Pit lakes mineralize and different pit lakes mineralize in different ways, I think everyone knows that already.
They also wrote: "Distinct temporal variability of the analyzed physico-chemical parameters within the last 30 years has been found. An important factor for temporal and spatial variability alike is the secondary deepening of the lakes as a result of bottom subsiding, which we found in our research."
Again, I do not doubt that that is valid but what is the significance? Are these results surprizing or unexpected? If so how and why are the results unexpected? There needs to be some sort of background. There are plenty of papers that have been published on pit lakes, including coal mine pit lakes, including mineralization and change over time. How is this paper different from all of those other papers? What does the reader learn from this paper that he or she does not learn from those papers?
Author Response
Thank you very much for your additional comments. We appreciate your efforts to improve our manuscript. I apologize for not referring to that comment. Unfortunately, I didn't realize until after I sent the last revised version. I sent a corrigendum to Editors-in-Chief and I hope you also received it.
We fully agree with your opinion - such information should have been included in the manuscript. As suggested, we added additional information highlighting the importance of our results and scientific novelties, both in the introduction:
"Most of the available literature data from this area concern either single objects [e.g. 21; 22] or, if more lakes were analysed, there are only regional-wide reports [e.g., 23, 24]. It mainly results from the fact that they are located in poorly accessible places (with hills and forests), thus for years they were known only to the local people. Therefore, many of them have never been transformed by human activity, maintaining their original character. However, although the area is under protection as the Muskau Arch Landscape Park since 2001 and its most valuable area is a part of the first in Poland Geopark (Muskau Arch Geopark), there is still no consistent monitoring and management plan for the lakes. As a result, degradation processes and accelerated decline of lakes occur in many places. Liming and fertilization (e.g., with domestic wastewater) in order to stock the lakes with fishes (mostly with carps), despite being legally forbidden, are quite common in these lakes. The results from other regions shows, these are one of the main processes that lead to irreversible negative changes in such ecosystems [25, 26]. In order to protect the unique character and diversity of these lakes, it seems necessary to recognize the trends of water chemistry changes and their causes."
as well as in the discussion:
"The overall analysis of the obtained results seems to confirm our hypothesis that the lakes undergo temporal variability of physical-chemical water conditions resulting not only from the natural evolution (e.g. hydrochemical processes, secondary succession) but also from dynamic hydrology induced by human activity (neutralization). Unexpectedly, our analyses also showed an increase in the depth of most lakes, which may also affect the dynamics of their transformation. As a result, it may also be important for landscape diversification in the future. Similar processes are probably taking place in other post-mining regions, but there is no literature data concerning this issue. Taking into consideration the unique character of these lakes and their environment and landscape formation role, it is necessary to learn the exact processes occurring therein and to protect them."
…and the conclusions:
"Distinct temporal variability of the analyzed physico-chemical parameters within the last 30 years has been found. An important and unexpected factor for temporal and spatial variability alike is the secondary deepening of the lakes as a result of bottom subsiding, which we found in our research. This phenomenon requires detailed research, since there is no literature data to compare with. In addition, temporal variability of water chemistry does not only result from natural processes, but also from human activity (neutralization and water fertilization). An important process that may contribute to further variability is the increasing temperature and the related lowering of the water table.
The presented results and numerous data gaps over the years indicate the need to extend the research on the Muskau Arch lakes. A consistent monitoring plan seems necessary to understand the complex processes taking place and effectively protect these valuable objects."
We hope that we have understood your suggestions well and the changes made are sufficient.
Thank to your comments, we plan to expand our future research, focusing on a thorough analysis of the lake variability in the context of changes in the morphometry. This will also enable more detailed statistical testing.
