Seasonal Water Column Stratification and Manganese and Iron Distribution in a Water Reservoir: The Case of Pinios Dam (Western Greece)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

in the last part of abstract some quantity results can be presented.

Author Response

1. In the last part of abstract some quantity results can be presented.

Answer: Some quantity results were added in the abstract concerning Mn and Fe concentrations as well as the anoxic layer and duration of anoxia.

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors,

research on seasonal changes in the water column is extremely important for the study of changes in water quality in reservoirs. Your research is, in my opinion, monitoring research that has been presented in an interesting way. Below are a few comments that will improve the quality of the article:

1. In the title, you refer to the distribution of heavy metals. Are iron and manganese actually heavy metals?
2. Line 14: Do climate changes really affect water column stratification?
3. Throughout the article: since you are not using abbreviations (such as T, DO) for water quality indicators, remove them from the text.
4. Keywords: Prove to me that the ‘temperate lake’ term can be applied to dams.
5. Introduction: Add similar studies that have been conducted on this or other reservoirs in Greece or Europe.
6. Line 53: You are certainly not investigating the mechanism in your research. If I’m wrong, prove it. I suggest change mechanism to variability: “…to describe the water stratification variability…”
7. Results: mark the epilimnion, metalimnion, and hypolimnion on the graphs (Fig. 3–6).
8. Lines 45, 142, 186, 204, 212, 263: What statistical tests did you use? What is the p-value level? If you didn’t use statistical tests, replace the word significant with substantial.
9. Discussion: The discussion needs to be expanded. Check what results other researchers obtained and compare them with yours.
10. Line 223: The reference [12] in this place is unclear. Are the authors of [12] referring to the Pinios reservoir, or are you rather describing your own results?
11. Conclusions: The conclusions are too long. In my opinion, the conclusions should arise from the study, and it seems that in this section you are repeating the results and discussion.

Author Response

Comment 1: In the title, you refer to the distribution of heavy metals. Are iron and manganese actually heavy metals?

Response 1: The reviewer is right. We have changed the term “heavy metals” in the title to “Manganese and Iron”. We have replaced the term accordingly in the text by replacing the term “heavy metal” with the term “elements”, “metal(s)” or “Manganese and iron”.

 

Comment 2: Line 14: Do climate changes really affect water column stratification?

Response 2: A short paragraph has been added in the introduction (Lines 92 – 100) on the effect of climate change on stratification.

 

Comment 3: Throughout the article: since you are not using abbreviations (such as T, DO) for water quality indicators, remove them from the text.

Response 3: The abbreviations for temperature and dissolved oxygen have been removed from the text.

 

Comment 4: Keywords: Prove to me that the ‘temperate lake’ term can be applied to dams.

Response 4: According to our opinion, the term “temperate lake” could be applied to Pinios reservoir as it is in the temperate zone, it exhibits seasonal temperature stratification, and it presents a complete mixing during winter. It is also deep enough and with the exception of the larger variation in water level, it behaves like a natural temperate lake.

Yet, under stricter definition, there are some important differences with a natural lake, such as water residence time and habitat stability, all linked to the usage of water from the dam.

We have decided to remove the term from the keywords as it does not affect in any way the ms, and in the strict sense of the term, the reviewer is right.

 

Comment 5: Introduction: Add similar studies that have been conducted on this or other reservoirs in Greece or Europe.

Response 5: To the best of our knowledge there are no other studies in Greece that we have not included in the ms. In the revised version of the ms we have added 10 more studies dealing with the topics of stratification, anoxia and metals (i.e. in between references 10 – 25).

 

Comment 6: Line 53: You are certainly not investigating the mechanism in your research. If I’m wrong, prove it. I suggest change mechanism to variability: “…to describe the water stratification variability…”

Response 6: According to the reviewer’s suggestion, the term “mechanism” has been replaced by the term “variability” in the sentence (Line 113 in the revised version of the ms).

 

Comment 7: Results: mark the epilimnion, metalimnion, and hypolimnion on the graphs (Fig. 3–6).

Response 7: The epilimnion and hypolimnion have been marked in Figures 4, 5 and 6. In Figure 3, only the hypolimnion has been marked. The hypolimnion was defined at 25m (the depth below which temperature drop was negligible). The depth of the epilimnion was not constant among seasons, so its presentation in Figure 3 would be rather confusing.  

 

Comment 8: Lines 45, 142, 186, 204, 212, 263: What statistical tests did you use? What is the p-value level? If you didn’t use statistical tests, replace the word significant with substantial.

Response 8: According to the reviewer’s suggestion, the term “significant” has been replaced by the following terms:

Lines 43, 48, 77 & 139: The term has remained the same as the sentence is referring to literature and not our results.

Line 207: No substantial temperature differences were observed among the sampling stations.

Line 240: Manganese and iron concentrations exhibited obvious depth-dependent trends.

Line 258: The duration and intensity of thermal stratification are among the most important factors affecting the dissolved oxygen dynamics in lakes

Line 266: ….without a substantial increase in temperature which could affect the solubility of oxygen.

Line 333: The onset of hypoxia and anoxia in the hypolimnion was closely linked to a substantial rise in Mn and Fe concentrations….

 

Comment 9: Discussion: The discussion needs to be expanded. Check what results other researchers obtained and compare them with yours.

Response 9: The discussion has been expanded also according to comments 7 & 8 of Reviewer #2. Some references have been added, and a more thorough discussion has been written concerning the water treatment and possible management solutions.

 

Comment 10: Line 223: The reference [12] in this place is unclear. Are the authors of [12] referring to the Pinios reservoir, or are you rather describing your own results?

Response 10: The sentence has been rephrased to be clearer (Lines 276-277 in the revised version of the ms):

“Anoxia in the hypolimnion of Pinios reservoir has also been reported in a previous study [12], a fact showing that the problem is relatively constant.”

 

Comment 11: Conclusions: The conclusions are too long. In my opinion, the conclusions should arise from the study, and it seems that in this section you are repeating the results and discussion.

Response 11: The conclusions section has been drastically reduced, keeping only the most important concepts of the work.

 

Reviewer 3 Report

Comments and Suggestions for Authors

Reviewer comments:

The scope of the present study is to describe the water stratification mechanism of the Pinios reservoir Dam, as well as the seasonal variations in physicochemical parameters and the heavy metals manganese (Mn) and iron (Fe). For this study a detailed bathymetry survey was conducted, four monitoring stations were defined and seasonal water column profiles as well as physicochemical parameters such as pH, temperature, conductivity, dissolved oxygen and heavy metals of Mn and Fe were measured.

The article is written at a good professional level, it is understandable and well designed in terms of the experiments performed. On the other hand, it is a common procedure in bathymetric measurements, bathymetry is mainly used to measure the bottom of a water reservoir, measure the height of sediments, water depth. This is not clearly presented from the results. The reader of the article will probably not be interested in the ship, its parameters, but the measured values ​​of the monitored parameters in individual horizons. It is difficult to determine this from the images used.

I have a few questions and comments about the article:

1. Please explain the concept of elevation, at what height is the surface and bottom of the reservoir, it does not suit me, I would rotate the y-axis in all the images. After all, at 80 m there will be less oxygen and a lower temperature than at 60 m. Fig. 6 indicates that these are temporal changes in Mn and Fe with water depth.
2. This is an artificial reservoir or water reservoir, it must probably be supplemented by some inflow, is there an inflow? It would be appropriate to indicate it there. Thus, the direction of flow. This can affect the quality of water in individual horizons.
3. page 2, line 62: I quote from the text: The Pinios Dam is a heterogeneous, earth filled structure standing meters high with a length of 2,175 meters. What it means “structure meters high”?
4. Four sampling locations were selected (ST1, ST2, ST3, and ST4), can you write me the water depth for these locations?
5. If the reservoir is used as a source of drinking water, I am missing some data from what depth (horizon) water is taken for drinking purposes, perhaps it would be enough to change the horizon, considering the concentration of Fe and Mn in the months of August to October.
6. The article often mentions the terms epilimnion, metalimnion and hypolimnion, could this be shown in the pictures?
7. Please explain how the higher concentration of Fe and Mn is solved from the perspective of drinking water, the article does not mention water treatment?
8. I was struck by the sentence that the lake is oxygenated hypolimnion as a way to reduce the concentration of Fe and Mn, I can't imagine this, water treatment is simpler. Can you explain it?
9. The article mentions the word eutrophication several times. It would be necessary to know the water quality, not only temperature, pH, oxygen and conductivity. Please provide data on water quality, e.g. COD, total phosphorus, total nitrogen, or individual forms of nitrogen, insoluble substances, turbidity.... at least in the period August-October. If there are anoxic conditions, the occurrence of NO2, and NH4 is also possible.
10. Figure. A1, Table A1 are missing
11. spectrophotometer (HACH-LANGE), DR??? What method was used to determine Mn and Fe.

Author Response

Comment 1: On the other hand, it is a common procedure in bathymetric measurements, bathymetry is mainly used to measure the bottom of a water reservoir, measure the height of sediments, water depth. This is not clearly presented from the results. The reader of the article will probably not be interested in the ship, its parameters, but the measured values ​​of the monitored parameters in individual horizons. It is difficult to determine this from the images used.

Response 1: We agree with the comment. Many details concerning the depth-data acquisition have been removed from the M&M section in the revised version of the ms. We also consider that the detailed presentation of the bathymetry would not provide any significant value in the results and all that is needed about bathymetry has been already presented in Figures 4 & 5 and in the text.

 

Comment 2: Please explain the concept of elevation, at what height is the surface and bottom of the reservoir, it does not suit me, I would rotate the y-axis in all the images. After all, at 80 m there will be less oxygen and a lower temperature than at 60 m. Fig. 6 indicates that these are temporal changes in Mn and Fe with water depth.

Response 2: The elevation is the altitude above sea level, so the smallest values represent the bottom of the lake. It is common to present profiles in reservoirs with elevation in the y-axis, as the water level is variable through time.

In order to make it clearer, we have added this information to the M & M section (Lines 106 -111) as well as in Figure legends (Figures 4, 5 & 6). 

 

Comment 3: This is an artificial reservoir or water reservoir, it must probably be supplemented by some inflow, is there an inflow? It would be appropriate to indicate it there. Thus, the direction of flow. This can affect the quality of water in individual horizons.

Response 3: There are 3 main tributaries in the SE, E and NE part of the reservoir. We revised Figure 2 in order to show them. Moreover, the tributaries have been added in the M&M section (Lines 133-134):

“The inflow of the reservoir is mainly located in the Southeastern, Eastern and Northeastern parts of the reservoir (Figure 2).”

 

Comment 4: page 2, line 62: I quote from the text: The Pinios Dam is a heterogeneous, earth filled structure standing meters high with a length of 2,175 meters. What it means “structure meters high”?

Response 4: The sentence has been improved:

“The Pinios Dam is a heterogeneous, earth-filled structure with a height of 50 meters and a length of 2,175 meters.”

 

Comment 5: Four sampling locations were selected (ST1, ST2, ST3, and ST4), can you write me the water depth for these locations?

Response 5: In the M&M section the water depth of each station has been added, along with the variation of water level in the reservoir:

Lines 168 – 173: “These stations were located along the reservoir’s main axis, with water depths ranging from 12 m to 36 m (ST1: 36m, ST2: 28m, ST3: 20m, ST4: 12m) when the reservoir was at its maximum water capacity. The maximum capacity of the reservoir is reached at +93m (elevation above sea level) and minimum water level is at +51m. During the sampling period, water level in the reservoir varied from +79.8m to +88.0m presenting the lowest level in October 2023. Sampling occurred seasonally, between February 2023 and October 2023.”

 

Comment 6: If the reservoir is used as a source of drinking water, I am missing some data from what depth (horizon) water is taken for drinking purposes, perhaps it would be enough to change the horizon, considering the concentration of Fe and Mn in the months of August to October.

Response 6: The reviewer is right. This important information was missing from the description of the study area. Detailed information on this subject has been included in the revised version of the ms (Discussion, Lines 300 – 311) 

 

Comment 7: The article often mentions the terms epilimnion, metalimnion and hypolimnion, could this be shown in the pictures?

Response 7 (same as in comment 7 of Reviewer #2): The epilimnion and hypolimnion have been marked in Figures 4, 5 and 6. In Figure 3, only the hypolimnion has been marked. The hypolimnion was defined at 25m (the depth below which temperature drop was negligible). The depth of the epilimnion was not constant among seasons, so its presentation in figure 3 would be rather confusing. 

 

Comment 8: Please explain how the higher concentration of Fe and Mn is solved from the perspective of drinking water, the article does not mention water treatment?

Response 8: In the revised version of the ms the existing water treatment along with its limitations is described in the discussion section (Lines 299 – 318)

 

Comment 9: I was struck by the sentence that the lake is oxygenated hypolimnion as a way to reduce the concentration of Fe and Mn, I can't imagine this, water treatment is simpler. Can you explain it?

Response 9: Water column aeration has been applied quite extensively during the last decades in water reservoirs with different characteristics and/or purposes. The results of these practices depend on the specifics of each system, but we considered that a detailed analysis of these results are out of scope of the ms. In the revised version of the ms we added a few references in order to support the application of aeration systems and we also added details on the existing water treatment plant as well as suggestions for improvement (Lines 312 – 319).

 

Comment 10: The article mentions the word eutrophication several times. It would be necessary to know the water quality, not only temperature, pH, oxygen and conductivity. Please provide data on water quality, e.g. COD, total phosphorus, total nitrogen, or individual forms of nitrogen, insoluble substances, turbidity.... at least in the period August-October. If there are anoxic conditions, the occurrence of NO2, and NH4 is also possible.

Response 10: Unfortunately, there is no available data on nutrients for the study period. Moreover, there is no past nutrient data adequate for evaluating the trophic status of the reservoir. The topic concerning the trophic status of the reservoir is mentioned in the discussion and a new sentence has been added (Lines 276 – 278) stating the lack of data on nutrients:

“Anoxia in the hypolimnion of Pinios reservoir has also been reported in a previous study [22], a fact showing that the problem is relatively constant but there are very few available data concerning nutrient concentration and distribution [22] to evaluate the trophic status of the reservoir.”

 

Comment 11: Figure. A1, Table A1 are missing

Response 11: We are not sure what the reviewer means. Figure A1 and Table A1 are shown in page 11 of the ms.

 

Comment 12: spectrophotometer (HACH-LANGE), DR??? What method was used to determine Mn and Fe.

Response 12: The reviewer is right. The methodology has been added in the revised version of the ms (Lines 184 – 186):

“Water samples were analyzed in the laboratory for manganese (Mn), and iron (Fe) using a DR3800 HACH-LANGE spectrophotometer according to LCW532 and LCK321 water quality analysis procedures”

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,
thank you for sending the revised manuscript. I appreciate all the responses to my comments.