Assessment of the Impact of Small Hydropower Plants on the Ecological Status Indicators of Water Bodies: A Case Study in Lithuania

Reviewer’s comment

Our response

Reviewer #1:

1.       

This paper describes a potentially interesting study of the effects of small hydropower plants on water quality and biological systems. However, I have significant concerns about the clarity of presentation (including English language usage), statistical analyses, comparison of the findings with existing literature, and conclusions.

We are very grateful to the anonymous reviewer for investing their own time reviewing our manuscript and the valuable comments.

We agree with almost all comments, and we have revised our manuscript accordingly.

The manuscript was edited before it was submitted to the journal.

The English Editing of the manuscript:

Date of Invoice: 11 December 2020

English Editing Invoice ID: english-25048

Basel, 14 December 2020

MDPI St. Alban-Anlage 66

4052 Basel

Switzerland

2.       

 As detailed in specific comments presented below, I have a number of concerns about the manuscript. An important concern relates to the statistical analyses used to test the significance of results. The authors rely on multiple t-tests to evaluate upstream vs downstream conditions without adjustment of the α-level despite the large number of tests performed. In addition, the linear regressions could have been conducted in a more rigorous fashion to determine the best regression possible eliminating independent variables that do not contribute meaningfully to the regression.

Finally, I am perplexed that the effects of hydropeaking on water quality and biotic indices were not evaluated statistically in spite of the fact that the authors were considering the need to impose restrictions on them. Without a supporting analysis, I believe the hydropeaking portions of the paper should be omitted. PP

We have used the statistical analysis, which in our opinion, was appropriate for data processing.

When assessing the impact of SPP in a complex way, hydropeaking (hydrograph ramping) cannot be ignored. In this paper, it was examined from a hydraulic or hydrodynamic point of view. Quantitative linking hydropeaking to water quality or biotic indices was beyond the scope of our study. This is the task of another study.

We believe that it is not good to highlight negative effects alone, without considering mitigation measures. The latter is only discussed here and is not mandatory. The article clearly identifies SHPs where hydrograph ramping exceeds imposed thresholds. This is just one SHP out of ten.

3.       

Title, line 2. Delete the abbreviation for small hydropower plants in the title.

Corrected in the manuscript.

4.       

Abstract, line 17. The phrase “have been sufficiently studied” implies that further study is not needed. Change “sufficiently” to “extensively” or some other term that indicates many studies have been conducted but does not imply further study is not needed. Change similar wording on line 52.

Corrected in the manuscript.

5.       

Abstract, line 21. It is stated that “available information is not sufficient.” Not sufficient for what? Please clarify

Clarified in the manuscript.

6.       

Abstract, line 27. Change “This study proved” to “This studied demonstrated” or some other term that is less suggestive of a final, definitive result.

Corrected in the manuscript.

7.       

Abstract, line 30. I am not sure what “(droplets, early, and edging) taxa” means

Corrected in the manuscript.

8.       

Abstract, line 32-33. I am not sure what is meant by “may yield somewhat significant hydrograph ramping but will not have a significant impact on the biological indices.” I assume ramping is referring to the rates of increase or decrease in flows during peaking. Are the authors saying that ramping or ramping rates do not affect any of the biological indices they examined? Unfortunately, it doesn’t appear that that relationship was examined in this study.

Here, “hydrograph ramping” was considered only from a hydraulic/ hydrodynamic point of view. Previous studies have identified its threshold values, which do not yet significantly affect biological indices (see Section 4.2, lines 608-627). The down ramping values found in our study were compared with those of other studies, and therefore the following conclusion was reached.

9.       

Section 1, line 51. Change “biota diversity” to “biodiversity.”

Corrected in the manuscript.

10.     

Section 1, line 56. Delete “dams from these” from the phrase “how dams from these power plants.” I assume the authors are referring to impacts from all components of infrastructure as well as operations.

Corrected in the manuscript.

11.     

Section 1, line 57. Is this sentence referring to differences between segments of river upstream and downstream of hydropower plants or differences between higher and lower elevation rivers? I presume the former rather than the latter. Please clarify with revised wording.

Corrected in the manuscript.

12.     

Section 1, line 72. In the phrase “impact of small dams decreases to eligible ecological levels,” I assume the word “eligible” should be “negligible.” I would revise to read “impacts of SHPs on ecological systems decrease to negligible levels.”

Corrected in the manuscript.

13.     

Section 1, line 75-76. Provide some additional information to justify the statement “The biophysical impact of SHPs may outweigh the impact of large hydropower plants in some respects [29].” Everything else in the paragraph indicated otherwise.

For clarification, additional information was provided in the article.

14.     

Section 1, line 77. In the phrase “impact of hydroelectric dams on river ecosystems,” change “hydroelectric dams” to “hydropower plants” to include the impacts from all infrastructure and operations. In general, throughout the ms, change “dams” to “plants” or “SHPs.”

Changed in the manuscript.

15.     

Section 1, line 89. Provide specificity to “habitat abundance.” Are the authors referring to the abundance of specific types of habitats (e.g., riffles, runs, pools, low-velocity channel-margin habitats, floodplain habitats) or the overall availability of aquatic habitat (sometimes referred to as “wetted area”)?

Section 1, line 91. I am not sure what is being referred to by “fish life characteristics.”

Corrected in the manuscript.

Corrected in the manuscript.

16.     

Section 1, line 104. I assume “water abstraction” should be “water extraction,” but “drawdown” would be the preferred word in this context.

Corrected in the manuscript.

17.     

Section 1, line 122. Change “ichthyological examinations” to “fish surveys.”

Changed in the manuscript.

18.     

Section 1, line 125. Delete first “trout” from phrase “trout brown trout.”

Corrected in the manuscript.

19.     

Section 1, line 128. Change “man cause” to “main cause.”

Corrected in the manuscript.

20.     

Section 1, line 133. Change “the quality of water bodies” to “water quality.”

Corrected in the manuscript.

21.     

Section 1, line 135. Change “total bundance” to “total abundance.”

Corrected in the manuscript.

22.     

Section 2.2, line 190. Define “SKN.”

Sorry for the mistake -SCN (standard climatic norm) Corrected in the manuscript.

23.     

Section 2.2, line 195. Define “PDO.” September was similar to what?

Sorry for the mistake -SCN (standard climatic norm). September was similar to August. Corrected in the manuscript.

24.     

Section 2.2, line 196. I am not sure what is meant by “perennial average.” Revise to “During the study period, the August temperature was 3 ºC higher than the average August temperature.”

Corrected in the manuscript.

25.     

Section 2.3, line 204. In the phrase “hydroelectric power plants (SHPs)” delete the first three words and take SHPs out of the parentheses.

Corrected in the manuscript.

26.     

Section 2.3, line 209. Replace “macrozoobenthos” with the more typical “benthic macroinvertebrates” here and elsewhere in the ms.

Corrected in the manuscript.

27.     

Section 2.4, line 248-249. I do not understand this sentence. Were the authors intending to compare the impact of downramp rates to upramp rates?

Accepted and corrected. This paragraph was clarified (Doubtful sentence removed). 

28.     

Section 2.4, line 251-252. What does the phrase “downstream hydrograph ramping parameters were not directly related to the macrozoobenthos or fish samples.” Do the authors mean there was no detectable effect of ramping parameters or that they were not accounted for in the sampling?

Here we wanted to clarify a bit this phrase.  We did not account for the instantaneous samplings in dewatered channel areas related to fish and macrozoobenthos' strandings during our hydrograph ramping experiments. This was outside of our tasks.

29.     

Section 2.4, line 253-260. The results of this portion of the study are not presented in this ms.

Accepted (doubtful statements were removed). Fig. 8 to 11 are specific outcomes of the methodology undertaken in this study.

30.     

Section 2.5, line 265-267. Please address the following issues:

Specify if the t-tests were 1- or 2-tailed.

Significance level should be presented as P < 0.05, not t < 0.05 (note that t < 0.05 is used throughout the ms).

Did the authors use paired t-tests or unpaired tests.

Were multiple t-tests performed (i.e., one per SHP as implied in line 311)? If so, a Bonferroni correction should be applied to adjust the α-value to maintain an across-tests level of 0.05.

Rather than performing t-tests to determine significance between upstream and downstream locations, the authors should perform repeated measures analyses of variance for each dependent variable accounting for the variability within and among SHPs.

Used t-test independent by groups.

Due to the small amount of data, a t-test was applied only between upstream and downstream values.

t changed to P in the manuscript.

31.     

Section 2.5, line 269. Change “quality elements of the ecological status of water bodies” to “water quality.”

Corrected in the manuscript.

32.     

Section 2.5, line 272. Change “percentage of the bottom of the vegetation river” to “percentage of the river bottom occupied by vegetation.”

Corrected in the manuscript.

33.     

Section 2.5, equation 2. Define the terms in the regression equation.

The terms of the regression equation are defined

34.     

Section 2.5, line 275-282. It is unclear how the statistical tests described in this paragraph relate to the t-tests describes in the first paragraph of the section. From what I can tell from the description, the authors are using both tests to make upstream and downstream comparisons.

Student's t test was used to assess differences of the water quality indicators upstream and downstream the hydroelectric power plant.

The impact of the SHPs on water quality was calculated using multiple linear regression. A significant factor when p <0.05.

35.     

Section 2.5, line 281. Define DSFI and FAI at first use.

Defined in the manuscript.

Fauna Autochthonous Index

36.     

Section 3.1, Figure 3. X-axis should be labelled “Upstream of SHP    Downstream of SHP”. The existing caption implies that the SHPs could be divided into upstream and downstream SHPs. Instead, there are upstream and downstream samples for each SHP.

Corrected in the manuscript.

37.     

Section 3.1, line 290. Higher values of these indicators are generally considered to indicate poorer conditions, so the differences, although not significant, suggest conditions downstream of SHPs are better than upstream of the SHP reservoir. This should be acknowledged.

Corrected in the manuscript.

38.     

Section 3.2, line 299. For all figures listed here, provided a callout when the data on the figures are described later in this section.

Corrected in the manuscript.

39.     

Section 3.2, line 303-306. I have several comments on this paragraph:

It is stated that for 7 of 10 SHPs, LFI values suggest conditions are worse upstream than downstream. This appears to be a significant finding that is not discussed here or in Section 4. Are these results statistically significant? What are the underlying factors resulting in this observed difference?

In Figure 4, upstream values of LFI are higher at upstream sites than downstream sites for 7 SHPs. I would assume LFI ranges from 0 to 1 with 0 being worst and 1 being best condition. If so, the figure indicates that conditions are better upstream than downstream.

The LFI and how to interpret LFI values should be clearly described in the methods section.

One of the 7 SHPs (Kavarskas) is not listed with the rest in 304-305.

Section 3.2, line 305-306. Revise the sentence “The mean LFI values were also worse that the dams downstream from the SHPs.” I suggest revising to “Mean LFI values also were higher upstream than downstream of SHPs.”

The results are statistically significant; the reasons are discussed in section3.3. Influence of hydroelectric and chemical water quality indicators on hydrobiological indicators (DSFI, FAI, and LFI)

1.000–0.940 – very good, 0.109–0.000 very bad - in Procedure for Assessing the Ecological Status of Surface Water Bodies [73].

The ecological status of water bodies at risk was assessed in accordance with the Procedure for Assessing the Ecological Status of Surface Water Bodies [73]. The status of the water body is divided into five ecological classes according to all quality indicators. Putting them all together in the methodology, I would greatly expand the methodological part. The methodological part provides a reference to the main document.

40.     

Section 3.2, Figure 5 and line 307-308. The units for fish abundance (N) are presented in three different ways (unit/100 m2, qty./ 100 m2, and ind./100 m2). Pick one and be consistent, “ind.” (presumed to mean individuals) is the best of the three but would need to be defined at first use.

Corrected in the manuscript

41.     

Section 3.2, line 308-314. Explain the relationship between LFI and the two other fish-related indices, abundance and biomass. Both of these are higher upstream than downstream of SHPs, which may reflect better conditions upstream than downstream. Yet the higher LFI values are interpreted to indicate poorer conditions upstream than downstream.

For calculating the Lithuanian fish index (LFI) evaluates the species composition, abundance, biomass, age structure of fish.

Revised - all indicators are worse in the downstream reach.

42.     

Section 3.2, line 317-318. The authors state that 7 of the 10 SHPs have downstream DSFI values that are “worse” than those upstream, but Figure 6 shows 6 with lower values and the authors list 6. This should be corrected. In addition, and in general, when describing index values, it would be preferable to first state the comparative magnitude of the values (e.g., lower, higher, or the same) then describe whether the values suggest conditions are better or worse upstream or downstream.

According to Procedure for Assessing the Ecological Status of Surface Water Bodies [73] very good class – 1, very bad – 0.

The status of the water body is divided into five ecological classes according to all quality indicators. Putting them all together in the methodology, I would greatly expand the methodological part. The methodological part provides a reference to the main document.

43.     

Section 3.2, line 321. Revise the last sentence of the paragraph to be consistent with the suggested revision for the sentence in lines 305-306.

Section 3.2, line 323. Spell out “FAI” in figure caption.

According to Procedure for Assessing the Ecological Status of Surface Water Bodies [73] very good class – 1, very bad – 0.

The status of the water body is divided into five ecological classes according to all quality indicators. Putting them all together in the methodology, I would greatly expand the methodological part. The methodological part provides a reference to the main document.

44.     

Section 3.2, line 324. The authors state that 4 of the 10 SHPs have downstream FAI values that are “worse” than those upstream, but Figure 7 shows 2 with lower values. As stated in comments on previous index values, provide a description of the index (i.e., what are its components, how it is calculated, and how it is interpreted) in the methods section.

According to Procedure for Assessing the Ecological Status of Surface Water Bodies [73] very good class – 1, very bad – 0.

45.     

Section 3.2, line 330-331. What is the statistical analysis being described in this section? Since an R2 is presented, I assume a linear regression was performed, but such an analysis implies one of the variables may be dependent on the other. This is clearly not the case for these two indices and a parametric (Pearson) or non-parametric (Spearman) correlation analysis would be the more appropriate analysis.

This sentence has been removed from the manuscript.

46.     

Section 3.2, line 333-346. The presentation of the Wilcoxon test is very difficult to follow. I am not sure what variables are being tested or what the yes/no comparison refers to.

The description also seems to imply that lower index values indicate greater effect, which is the opposite of the way these results are described earlier. See my comments above on interpretation of LFI, DSFI, and FAI. 

Accepted and corrected.

Only in two or three lower sections of the SHPs, the LFI, DSFI and FAI values were higher than those behind the respective dams.  In general, this does not contradict the previous results.

47.     

Section 3.2, Table 2. The table should be simplified. The calculated W-value, critical W-value, hypothesis, and conclusion could be replaced with a single column showing the calculated P-value for the test.

The comment partially accepted. 

48.     

Section 3.3. There are no callouts for the tables in this section.

Corrected in the manuscript

49.     

Section 3.3, Table 3, 4, 5, 6, and 7. I suggest several changes to these tables:

Provide an overall F-statistic, associated P-value, and R2 for the full model

Change the final column heading to P > |t|, which is what the values presented in the column represent

Change the footnote to read “Significant at α = 0.05”

Rearrange the rows into two groups representing water quality variables and hydropower variables

Consider redoing the regression using an approach that determines the best multiple regression from among those possible with the available independent variables using Akaike’s Information Criteria (AIC) and forward, backward, and stepwise procedures.

Section 3.3, line 358-362. The wording in this sentence is awkward. The independent variables listed do not “mean” higher or lower values of TS, but rather that higher values for independent variables were correlated with decreases or increases in TS. The wording for the EPT analysis results is much better.

Thank you very much for the improvement tips in statistical analysis.

We used SPSS - 10 - Analyze- Regression- Linear.

The program presents the results in a fixed table. Therefore, the indicators cannot be changed.

We will use your advice to analyse other data in the future.

50.     

Section 3.3, line 378, 391, and 396. What does the parenthetical “Y” mean after LFI, fish abundance, and fish biomass?

The multiple linear regression model is

Y = a + b_1x1 + b_2x2 +….+ b_{kxk. (2)}

51.     

Section 3.3, line 385 and 399. There is no need for the footnotes in Tables 6 and 7 because none of the variable effects are significant.

Corrected in the manuscript

52.     

Section 3.4. Why were ramping rates not considered as an independent variable in the multiple regressions presented in Section 3.3? 

Please see answers # 28 and #59.

53.     

Section 3.4, line 404. It is stated that “fluctuations in the water levels of several SHPs” are shown in Figure 8, but only fluctuations downstream of the Kavarskas SHP are shown.

Sorry for this typo. Accepted and corrected.

54.     

Section 3.4, line 407 and 415. I assume “P” is referring to “power,” but I would avoid that use because you already use “P” for phosphorous. You do not have an abbreviation for turbine power in the regression tables.

Accepted. This ambiguity was corrected (“P” - power removed)

55.     

Section 3.4, line 410. Does stage range refer to the difference between the maximum daily stage and the minimum daily stage or the difference between maximum and minimum stage of individual waves within the day ? If the former, there appears to be about a 25 cm stage change shown in Figure 8, not a 10 cm change as stated in the text.

The latter statement is correct. This is the difference between the maximum and the minimum stage of individual waves within the day.

Regarding a 25 cm range of the stage. This was an unusual situation on 1^st July. There was an increase in the inflow to the reservoir and consequently an increased release by turbines. The successive days (from 2^nd to 10^th Jul) demonstrates clearly a 10 cm change in the stages.

56.     

Section 3.4, Figure 10. Are the values presented in this graph mean daily values? Note that first letter of the name of one of the SHPs is cut off from view.

Yes, they are.

The full name of this SHP can be seen by zooming a bit this graph. This is not a raster – jpeg, but xlsx embedded chart.

57.     

Section 4.1, line 435-452. I have several concerns with this section:

In general, the text is overly simplistic and without supporting references. Many of the relationships between biotic and physical conditions are presented as if a strictly linear relationship exists, but in most instances, there are intermediate optimum values below and above which conditions are less favorable.

Some of the terms are seldom used by other researchers in the context of aquatic systems:

“hydrobiont” instead of “aquatic organism”

“soil” instead of “sediment”

“edgings”

Section 4.1, line 454-455. “SPH” should be changed to “SHP” for consistency with the rest of the ms.

We are very grateful for this comment, continuing this study, we will try to make a deeper analysis of the data. We collect additional data.

Corrected in the manuscript.

Sorry, it is corrected in the manuscript.

58.     

Section 4.1, line 462 and after. There should be more direct comparison between your results and the results presented here for other studies. There is a huge amount of information presented in this section with little attempt to integrate the results of the present study with others.

The text has been edited trying to integrate the results of the present study with others. The part of the material of other authors has been reduced.

59.     

Section 4.2, line 556 and after. This is an interesting discussion, but it’s unclear why the authors did not evaluate the effects of hydropeaking on water quality and biota. This seems like a missed opportunity. Without this evaluation, the authors are basing recommendations on the literature rather than their own work.

We completely share your point of view. Unfortunately, the lack of detailed sampling biota data during hydrograph ramping (hydropeaking) events prevented us from this undertaking. There were only spot samplings not linked simultaneously to the changes in the river flow. Please see also answer # 28.

60.     

Section 5. There are no conclusions related to the various biotic indices evaluated.

Accepted. The new conclusion (No.2) related to the biotic indices evaluated is presented in the manuscript.

Reviewer’s comment

Our response

Reviewer #2:

1

The Ms addresses an important topic as the effects of SHP in several aspects of freshwater/river ecology. Overall, it reads well and is well organised along the several sections. The originality of the approach is not demonstrated as this study replicates similar approaches carried out in rivers elsewhere.

Many thanks for the comments and advice on our manuscript.

In our opinion, the originality of our manuscript is that the majority such kind of studies were carried out in mountainous or steep topography regions, whereas the region considered in this manuscript is a typical lowland.

2

The title adequately describes the overall contents of the Ms. The Abstract includes some general information (lines 14-219) that might/should be rephrased/reduced/deleted and this is not relevant for a general (condensed) description of the study. Furthermore, most of this information is repeated in the first part of the Introduction. The Abstract provided no information on the fish assemblages that are a part of the biological communities addressed in this investigation

The comment accepted. The abstract was corrected: duplicate and redundant information was removed, and supplemented by data on fish.

3

The Introduction might be a bit condensed as some concepts (as, for example, the results from previous studied) might be more properly addressed as supporting inform

ation for the Discussion section.

The introduction was abbreviated, but according to other reviewer’s comments, necessary explanations were made in some places.

4

The Discussion section insufficiently addressed the results of this study regarding the fish assemblages and only provided information on the results obtained in other studies. It is showed that there are significant differences in the fish communities upstream and downstream and this is not appropriately discussed. I consider that this part has to be deeply revised before accepting this Ms.

For calculating the Lithuanian Fish Index evaluates the communities, abundance, biomass, age structure of fish.

A multiple regression analysis of the influence of hydroelectric and chemical water quality indicators on the Lithuanian Fish Index showed that the value of the LFI is influenced by the average depth and area of the river. LFI values are worse in the downstream reach.

5

The Discussion is too much biased towards the hydrographic approach of the study and thus disregarding or miss-considering the biological compartments of the ecosystem.

We cannot entirely agree with this comment. The hydrographic approach can be attributed only to the analysis of the water level regime downstream SHP and a short description of these power plants. And this part makes up to 5 pages of the text out of 22 pages in the Ms (without references). On the other hand, biological compartments of the ecosystem cannot be considered a part – a holistic approach is needed; first of all the changes in the river flow regime under operations of power plants.

Reviewer’s comment

Our response

Reviewer #3:

1

The objectives are not explicitly stated. Here, I mean you need to list 1 to 3 objectives instead of mentioning the work's general goal.

The authors need to put a bit more emphasis on the e-flows implementation; please highlight the importance of e-flows implementation in riverine ecosystems conservation and also the possibilities in reaching some trade-offs among ecosystem conservation and hydropower production. The following literature might be useful in this regard << Flow regime aspects in determining environmental flows and maximising energy production at run-of-river hydropower plants. >>, <<Water-energy-ecosystem nexus: Balancing competing interests at a run-of-river hydropower plant coupling a hydrologic–ecohydraulic approach>> and << Influence of hydrologically based environmental flow methods on flow alteration and energy production in a run-of-river hydropower plant>> you may review other additional relevant references as well.

What is the novelty of this work? This needs to be clearly stated.

We appreciate the comments and advice, which gave us a chance to improve our manuscript.

We have read many articles published in Water (MDPI) and have found that most of them do not specify the objectives. Therefore, we have chosen only to provide the goal and structure of the text's performed activities.

Thank you very much for your suggestions. 94 references were used in preparing this manuscript. Therefore, we will use the sources you suggest in our future study.

As stated in our manuscript, the effects of large HPPs on the ecological conditions of surface water bodies have been extensively studied, but less attention has been paid to environmental impact studies of small hydropower plants. The impact of hydropeaking on both the river flow regime and ecosystems has been well-studied for peaking mode plants, mainly medium to large-sized ones. However, for SHPs, especially those in lowland rivers (in Lithuania and other Baltic region countries), the available information on water quality, benthic macroinvertebrates communities and fish abundance, and biomass is not sufficient due to little research carried out.

2

Methods

I would suggest showing the methodological approach through a flowchart; it will be more understandable for readers.

Methodology limitations should be mentioned.

All variables should be explained.

What was the data resolution used in the computations (daily, monthly …)?

A citation is needed in the following sentence:

‘’Note that nonparametric tests do not involve precise 278 parameters or assumptions about the underlying distribution and are most suitable for 279 short data series.’’ (lines 278-279)

The methodology is set out in the five sub-sections: Study area, Climate conditions, Sample preparation (collection), Water level regime downstream from the SHPs, Statistical analyses.

The quality standards were used in the methodology.

Accepted. All variables were explained in the methodology.

Accepted. Citation is provided.

3

Results

This section is well written.

How do you explain that LFI was worse upstream than downstream?

According to Procedure for Assessing the Ecological Status of Surface Water Bodies [73] very good class – 1, very bad – 0.

A multiple regression analysis of the influence of hydroelectric and chemical water quality indicators on the Lithuanian Fish Index showed that the value of the LFI is influenced by the average depth and area of the river. LFI values are worse in the downstream reach.

4

Discussion

The discussion should summarize the main finding(s) of the manuscript in the context of the broader scientific literature and address any limitations of the study or results that conflict with other published work—the connectivity impacts which one of the main impacts of SHPs are not discussed at all.

Some corrections in the discussion part were made.

5

Conclusions

This section needs to be elaborated a bit more; it is too short.

Accepted. The new conclusion (No.2) related to the biotic indices evaluated is presented in the manuscript.

Reviewer’s comment

Our response

Reviewer 1:

1

Review of Assessment of the Impact of Small Hydropower Plants on the Ecological Status Indicators of Water Bodies: A Case Study in Lithuania by L. Česonienė, Midona Dapkienė, and Petras Punys

            The authors made many of my suggested revisions, and the ms is much improved. However, I continue to have several of the concerns raised in my initial review. (1) The authors should provide a better description of the types of t-tests performed (one- or two-tailed, paired or unpaired). (2) The extensive discussion of hydropeaking is not warranted given the fact that the effect of ramping on water quality and biota was not examined in the study.

Specific Comments

Abstract, line 13. “HPP” is introduced here as an abbreviation, but then not used consistently in the ms.

.

Many thanks for the comments and advice on our manuscript.

In our opinion, the originality of our manuscript is that the majority such kind of studies were carried out in mountainous or steep topography regions, whereas the region considered in this manuscript is a typical lowland.

Thank you for your feedback. The abbreviation has been deleted because SHP is used everywhere later in the text

2

Abstract, line 14. “Hydrobionts” should be changed to “aquatic organisms throughout.

Abstract, line 14. “Hydrobionts”  changed to “aquatic organisms”.

3

Abstract, line 32-33. Wording was not clarified as requested.

Sorry for this oversight.  Now, this wording is as follows:

“Some SHPs operating in lowland areas may yield somewhat significant hydrograph ramping but more detailed investigation is needed to support the significance of this impact on the biological indices”.

4

Section 1, line 89. The revision is satisfactory, but delete the word “abundance.”

Section 1, line 89. The word “abundance” has been deleted

5

Section 2.4, line 251-252. Although a change was made, the authors have not addressed my question in their revision. A statement should be added that the study did not include measurements of the biotic effect of ramping.

Was added that in the study did not include measurements of the biotic effect of ramping.

6

Section 2.5, line 265-267. Some of the parts of this comment were not addressed. The authors should specify if the t-tests were one- or two-tailed, paired or unpaired, Bonferroni corrections should be applied since it appears the authors performed a test for each of the SHPs. As mentioned in the comment, repeated measures analyses of variance would be preferred, but the other parts of the comment need to be addressed

Used t-test independent by groups (upstream and downstream) The Statistica 9 soft-ware was used for this purpose, with the significance level set to p<0.05.

7

Section 2.5, line 275-282. In their response, the authors do not specify what the Wilcoxon tests were used for

If we understand the sense of this question well,  in the original Ms lines 280 -282 reply to this comment.

“This test was applied to reveal the homogeneity of the two samples consisting of the DSFI index, the FAI index, and the Lithuanian fish index (LFI) values determined upstream and downstream from the SHP dams”.

8

Section 3.2, line 299. The authors did not make the requested change.

Your note was: “ Section 3.2, line 299. For all figures listed here, provided a callout when the data on the figures are described later in this section”.  Corrected in the manuscript. In line  313.

9

Section 3.2, line 303-306: (1) In their response, the authors provided information that confirmed my suspicion that higher values of LFI indicate better conditions upstream in 7 of 10 SHPs, but they continue to say that conditions were worse upstream than downstream. (2) The authors should state that LFI values can range from 0 to 1 with 0 being worst and 1 being best condition.

Thank you very much. Surely we made a mistake in describing. Now corrected.

10

Section 3.2, Figure 5 and line 307-308. Suggested changes were made, but I should mention that “Q” is typically used to represent stream flow. I would use a different variable abbreviation for biomass to avoid confusion.

Section 3.2, Figure 5 and line 307-308. Q changed to Bm.

11

Section 3.2, line 308-314. The authors did not provide the requested description of the relationship between LFI and other indices

For the calculating the Lithuanian fish index (LFI) evaluates the species composition, abundance (N), biomass (Bm), age structure of fish.

12

Section 3.2, line 317-318. The authors did not correct the identified mistake in their description of the graph, nor did they indicate that values of the indices range from 1 (best) to 0 (worst). These are simple fixes, not the complete descriptions they seem to think I am requesting.

Corrected in the manuscript

13

Section 3.2, line 321. The authors did not make the requested revision.

DSFI values can range from 0 to 1 with 0 being worst and 1 being best condition. The results of the research showed that out of the 10 SHPs, 7  featured DSFI values downstream from the SHPs that were worse than those upstream.

The mean DSFI values were also found to be worse downstream from the SHPs.

14

Section 3.2, line 324. Simple descriptions of the FAI and other biotic indices should be provided. Simple statements are all that is needed.

The significance of the effect was determined according to the determined values of Lithuanian fish index (LFI), Danish Stream Fauna Index (DSFI), and Fauna Autoch-thonous index (FAI) upstream and downstream from the SHP

15

Section 3.2, line 333-346. The presentation is still confusing. The Wilcoxon test does not quantify the impact as stated, but rather provides a test of the statistical significance of that impact. I would drop the parenthetical phrase regarding the binary expression. It is confusing and not necessary

Thank you, this comment accepted and corrected accordingly.

A Wilcoxon test was used to quantify the  statistical significance  of the impact  of SHP water level fluctuations in the lower reaches on aquatic ecosystems, as expressed by the biological indices (DSFI, FAI and LFI).

16

Section 3.3, Table 3, 4, 5, 6, and 7. Suggested changes were not made. The requested changes would improve the manuscript, but are not essential.

Thank you very much for the improvement tips in statistical analysis.

We used SPSS - 10 - Analyze- Regression- Linear.

The program presents the results in a fixed table. Therefore, the indicators cannot be changed.

We will use your advice to analyse other data in the future.

17

Section 3.3, line 358-362. Requested changes were not made.

Your previous remark was:  Section 3.3, line 385 and 399. There is no need for the footnotes in Tables 6 and 7 because none of the variable effects are significant.

Footnotes under the tables 6 and 7 have been deflected.

18

Section 3.3, line 378, 391, and 396. I suggest deleting the parenthetical “Y.”

The parentheticals "Y" was deleted

19

Section 3.4, Figure 10. The authors should indicate in the y-axis label that the values are mean daily values.

Accepted.   A small correction – maximum daily values. Wordings added to the  title of Figure 10.

Figure 10. Ramping range (amplitude) (dH) and ramping rate (V) in the tailwater of SHP plants (maximum daily values).

20

Section 4.2, line 556 and after. The extensive discussion of hydropeaking is not warranted given the fact that the effect of ramping on water quality and biota was not examined in the study.

We cannot accept this comment.

In previous responses, we recognized that this discussion is of hydraulic, hydropower nature. The properties of hydrograph ramping (hydropeaking) were considered in RoR schemes and lowland rivers. The types of turbines on which depend hydrograph ramping were discussed. This information definitely will support future detailed studies on the ramping effects on biodiversity and water quality.

Referring to the threshold values of hydropeaking determined in other regions,   we compared them with those identified in our SHPs. Based on this, a  general conclusion was made regarding likely ramping effects of the studied SHPs on biodiversity.  Possible measures to make the river flow regime more natural, which would undoubtedly have less impact on biodiversity, were briefly discussed.

And this section goes hand in hand with the title of this Ms.

21

Section 5. The authors added a conclusion regarding LFI, and linked these two effects on fish biomass and abundance, but the relationship of these variables to LFI was not described in the results or discussion portions of the ms.

For the calculating the Lithuanian fish index (LFI) evaluates the species composi-tion, abundance (N), biomass (Bm), age structure of fish.

Reviewer’s comment

Our response

Reviewer #2:

1.       

The authors improved this revised Ms by considering most of the comments provided for the earlier version.

I still consider that the originality of the investigation is unconvincingly demonstrated. Anyway, I think that the revised Ms might be accepted if this is ok for the editorial board.

We are very grateful to the anonymous reviewer for investing their own time reviewing our manuscript and the valuable comments.