Review Reports

Round 1

Reviewer 1 Report

The paper focus on the effect on three types of lights (blue, red and white) application on the growth of three submerged macrophytes. In purpose to study this effect, experiment was established for 100 days. The study was carried out during winter time.  In 12 tanks (of 800 l) response to different light treatments of three common aquatic plants: Vallisneria natans, Myriophyllum spicatum and Ceratophyllum demersum has been studied.

Paper is interesting and well written, however I found a few issues which I expect to be explain by authors. 

I am not confident with the title of the paper. Effect on light application is very important because we observe strong and significant influence of big cities on the water quality in the lakes located in the big agglomerations. However, these effect is negative due to intensive growth of phytoplankton and negative influence of prolonged daily light availability on fauna.

Base also on my experiences with aquatic macrophytes I had big problem with very intensive growth of phytoplankton in my tanks before plants development. I guess during these experiments temperature was rather low to avoid intensive growth of phytoplankton. Could you add information about temperature in the water during the experiment? Water in the tanks was stagnant or did you forced the water movement in tanks?

I am not agree with the title of the paper: “implications for lake restoration”. I think if we not reduced nutrient concentrations in water than you not improve conditions for plants growth (reduction of phytoplankton and visibility). Actually, Authors indicated this issues in the introduction. I suggest to remove it. Anyway this methods are not sufficient for natural lakes. I recommend to correct the application of the achieved results.

Author Response

Response to Reviewer 1 Comments

 

Thank you very much for your comments. We have revised the manuscript carefully and incorporated many points raised by you and another reviewer. Our responses are given as follows:

 

Point 1: I am not confident with the title of the paper. Effect on light application is very important because we observe strong and significant influence of big cities on the water quality in the lakes located in the big agglomerations. However, these effect is negative due to intensive growth of phytoplankton and negative influence of prolonged daily light availability on fauna.

 

Response 1: Thanks for the comments. We agree that light application might, to some extent, stimulate the growth of phytoplankton. However, the idea is that the macrophytes by enhanced growth may help shifting the lake to a clear state with less algae when external nutrient loading is reduced sufficiently for such a shift to have the potential to occur. We have now made this clear in the end of the discussion (please see answers to point 3). Our intention with the title was not to express any opinions of positive or negative effects of light application. We have modified the text somehow to account for your concern. “Effects of artificial LED light on the growth of three submerged macrophyte species in the low-light, low-growth winter season: implications for lake restoration”

 

Point 2: Base also on my experiences with aquatic macrophytes I had big problem with very intensive growth of phytoplankton in my tanks before plants development. I guess during these experiments temperature was rather low to avoid intensive growth of phytoplankton. Could you add information about temperature in the water during the experiment? Water in the tanks was stagnant or did you forced the water movement in tanks?

 

Response 2: Data on temperature have been included in Table 1. Water in the tanks was stagnant during the experiment.

 

 

Point 3: I am not agree with the title of the paper: “implications for lake restoration”. I think if we not reduced nutrient concentrations in water than you not improve conditions for plants growth (reduction of phytoplankton and visibility). Actually, Authors indicated this issues in the introduction. I suggest to remove it. Anyway this methods are not sufficient for natural lakes. I recommend to correct the application of the achieved results.

 

Response 3: Thanks for the comments. We did not intend to suggest that application of artificial light should be done without simultaneously reducing external nutrient loading. Nutrient reduction is always the fundamental way to mitigate eutrophication and restore lake ecosystem. In this study, our purpose was to see the effects of artificial light on macrophytes and the potential application of artificial light to facilitate lake restoration combined with efforts of nutrient reduction. Artificial might not be sufficient for large lakes, but still possible in small lakes. Further studies are needed in the future. We have now added a sentence to account for your concern “whether artificial light can be used as a tool in lake restoration when plants are delayed after the external loading is reduced sufficiently to create a shift to a clear state needs further studies. If not reduced sufficiently to help creating a shift to a macrophyte state the light may just enhance eutrophication by stimulating phytoplankton growth in winter.” (please see Line 235 - 238 in the revised manuscript). We also highlight this in the final conclusion “Side effects of artificial light on aquatic systems and studies at in situ conditions should be considered before its application as a restoration tool when the external loading has been sufficiently reduced to be able to push the system from a turbid to a clear state.” (please see Line 272 - 274 in the revised manuscript).

 

I hope that the manuscript has been satisfactorily improved. If you have any further questions, please do not hesitate to contact me.

 

Best regards,

 

Haijun Wang

State Key Laboratory of Freshwater Ecology and Biotechnology,

Institute of Hydrobiology, Chinese Academy of Sciences,

Wuhan 430072, China

Tel: +86 27 68780225/Fax: +86 27 68780064

E-mail: wanghj@ihb.ac.cn

Author Response File: Author Response.docx

Reviewer 2 Report

This is  a controlled experimental, feasibility analysis for use of artificial illumination to improve recovery of macrophytes in turbid water where light limitations impair plant growth, especially during the winter months in geographic areas at higher latitudes where day length may be limiting. Three species of submerged macrophytes were used: Vallisneria natans, Myriophyllum spicatum and Ceratophyllum demersum. It might be helpful if the authors briefly explain why they chose these three species for their study.

Overall, the study used a standard experimental design of three experimental treatments (three different LED sources of light: red, blue, and white) and one control group at an ambient intensity of approximately 1% of natural incident radiation in a  shaded and breathable shed. In this respect, it would be helpful if the authors more clearly label the control condition. The word ‘dark’ is not quite clear, and perhaps ‘shaded’ would be a better term simply to make the Results more interpretable. This is a minor terminological matter.

While the experiments seem to provide some useful approximations to possible applications in natural environments, I believe it would be helpful if the authors could comment on the turbidity of the water collected from Lake Bao’an in relation to their rationale for, and justification of, their study. That is, to what extent is the Lake Bao’an water of approximately equivalent turbidity to lakes that are experiencing the most stress for macrophyte survival due to turbidity and limited natural day length. Most readers may not be informed enough to judge the NTU values relative to troubled, turbid lakes; and it would help if the authors simply clarify this, perhaps in their Discussion. Also, it would be helpful in the Methods to clearly define NTU. For example, add a sentence such as the following that defines the abbreviation NTU following the definition of other parameters as shown below.

 

The water used in the experiment 100 was taken from Lake Bao’an. According to our survey from 2015 through 2016, annual mean water 101 depth was 2.29 m, Secchi depth (ZSD) was 0.66 m, pH 8.54, dissolved oxygen (DO) 9.77 mg L−1, total 102 nitrogen (TN) 0.83 mg L−1, total phosphorus (TP) 0.08 mg L−1 and chlorophyll a (Chl a) 36.69 μg L−1. Turbidity was measured in nephelometric turbidity units (NTU).

 

The authors explain the experimental set up, and particularly the orientation and adjustments of the LED light sources as follows: LED lights were hung 20 cm above the plants and regulated relative to plant growth (Figure. 1).

Does this mean that the lights were adjusted during plant growth to ensure they were 20 cm above the plants as the plants grew ?

 

While this study provides useful benchmark data for future field-based applications, I believe that the authors should be somewhat more clear about the issues involved in scaling up their proposed methods to the natural environment. They make some estimates of the likely energy costs to maintain the illumination, but they offer little insight on how the infrastructure to suspend the lights over a large area of lakes might be designed, etc. Again, this goes beyond the experimental aspects of their study, but I think some advice in the Discussion would be helpful.

 

There are some small typographical corrections.

Lines 114-115, the species names of the plants should be in italics.

Lines 150 – 151  Correct the superscript notations: nitrogen (mg L-1); TP, total phosphorus (mg L-1); Turb, turbidity (NTU); Chl a, chlorophyll a (mg m-150 3); PPFD, photosynthetic photon flux density (μmol m-2 s-1).  They are not properly superscripted.

The study is interesting from a scientific perspective, in addition to possible practical applications.

Author Response

Response to Reviewer 2 Comments

 

Thank you very much for your comments. We have revised the manuscript carefully and incorporated many points raised by you and another reviewer. Our responses are given as follows:

 

Point 1: Overall, the study used a standard experimental design of three experimental treatments (three different LED sources of light: red, blue, and white) and one control group at an ambient intensity of approximately 1% of natural incident radiation in a shaded and breathable shed. In this respect, it would be helpful if the authors more clearly label the control condition. The word ‘dark’ is not quite clear, and perhaps ‘shaded’ would be a better term simply to make the Results more interpretable. This is a minor terminological matter.

 

Response 1: Revised as suggested (please see Line 22 and Line 114 in the revised manuscript).

 

 

Point 2: While the experiments seem to provide some useful approximations to possible applications in natural environments, I believe it would be helpful if the authors could comment on the turbidity of the water collected from Lake Bao’an in relation to their rationale for, and justification of, their study. That is, to what extent is the Lake Bao’an water of approximately equivalent turbidity to lakes that are experiencing the most stress for macrophyte survival due to turbidity and limited natural day length. Most readers may not be informed enough to judge the NTU values relative to troubled, turbid lakes; and it would help if the authors simply clarify this, perhaps in their Discussion. Also, it would be helpful in the Methods to clearly define NTU. For example, add a sentence such as the following that defines the abbreviation NTU following the definition of other parameters as shown below.

The water used in the experiment 100 was taken from Lake Bao’an. According to our survey from 2015 through 2016, annual mean water 101 depth was 2.29 m, Secchi depth (ZSD) was 0.66 m, pH 8.54, dissolved oxygen (DO) 9.77 mg L−1, total 102 nitrogen (TN) 0.83 mg L−1, total phosphorus (TP) 0.08 mg L−1 and chlorophyll a (Chl a) 36.69 μg L−1. Turbidity was measured in nephelometric turbidity units (NTU).

 

Response 2: Data of NTU has been included in the revised paper (please see Line 110 in the revised manuscript).

 

Point 3: The authors explain the experimental set up, and particularly the orientation and adjustments of the LED light sources as follows: LED lights were hung 20 cm above the plants and regulated relative to plant growth (Figure. 1).

Does this mean that the lights were adjusted during plant growth to ensure they were 20 cm above the plants as the plants grew?

 

Response 3: Yes. This is now explained better in the revised manuscript. “LED lights were regulated relative to plant growth to ensure that they were 20 cm above the plants.” (please see Line 117-119 in the revised manuscript).

 

Point 4: While this study provides useful benchmark data for future field-based applications, I believe that the authors should be somewhat more clear about the issues involved in scaling up their proposed methods to the natural environment. They make some estimates of the likely energy costs to maintain the illumination, but they offer little insight on how the infrastructure to suspend the lights over a large area of lakes might be designed, etc. Again, this goes beyond the experimental aspects of their study, but I think some advice in the Discussion would be helpful.

 

Response 4: Many thanks for this comment. Actually, more studies are needed in the future to elucidate the effects of application of artificial light, for example, as to the issue of the dependence of light effects on light strength, season and macrophytes species. It is in our opinion premature to discuss infrastructure at this stage.

 

Point 5: Lines 114-115, the species names of the plants should be in italics.

Lines 150 – 151 Correct the superscript notations: nitrogen (mg L-1); TP, total phosphorus (mg L-1); Turb, turbidity (NTU); Chl a, chlorophyll a (mg m-150 3); PPFD, photosynthetic photon flux density (μmol m-2 s-1).  They are not properly superscripted.

 

Response 5: Done as suggested (please see Line 122-123 and Line 160-162 in the revised manuscript).

 

I hope that the manuscript has been satisfactorily improved. If you have any further questions, please do not hesitate to contact me.

 

Best regards,

 

Haijun Wang

State Key Laboratory of Freshwater Ecology and Biotechnology,

Institute of Hydrobiology, Chinese Academy of Sciences,

Wuhan 430072, China

Tel: +86 27 68780225/Fax: +86 27 68780064

E-mail: wanghj@ihb.ac.cn

Author Response File: Author Response.docx

Reviewer 3 Report

Overall a good study. Some points of concern need clarification. Referenced with line number

Line 17: Need to mention turbidity up front, since that's the real concern

Line 99: Soil was taken from a different lake than plants? Why? If a different lake, please explain. If the same lake as plant source, make more clear

Line 111: Why only 6hr and not a normal photo-period? Explain and cite

Line 114: Format scientific names in caption

Line 117: Sampling every 20 days seems like a very long interval. Why? Explain and cite

Line 125: Is chl-a coming from phytoplankton or plants?

Line 134: What was Excel used for? 

Line 148: Table 1 - Is this the final day of the experiment? Some compilation of those 20-day intervals? Also, the total PPFD mean&SD...is this a useful metric to report given that all the different light sources have distinctly differed outputs and hopefully there was no light pollution between mesocosms

Line 163: What is meant by "initial survival"? Did some plants die off? Did this affect total n? Explain this

Line 168: Figure 2 - I would remove the CBRW from the columns. The key is sufficient, and the individual letters are not explicitly defined

Line 172: Figure 2 caption - This is the first I'm seeing of an "initial value". How was this calculated? Nowhere in the methods.

Line 234-241: This whole paragraph seems like an afterthought. Needs to be fleshed out more, with citations, perhaps integrated with preceding paragraphs

Line 243-254: Each sentence does not need to be indented. Make one uniform paragraph

Figures generally: Put the unit in the Y-axis. It's confusing to go hunting for it in the caption.

Author Response

Response to Reviewer 3 Comments

 

Thank you very much for your comments. We have revised the manuscript carefully and incorporated many points raised by you and another reviewer. Our responses are given as follows:

 

Point 1: Line 17: Need to mention turbidity up front, since that's the real concern

 

Response 1: Thanks for the comments. “increased turbidity” was included before “loss of submerged macrophytes” in the revision (please see Line 18 in the revised manuscript)..

 

Point 2: Line 99: Soil was taken from a different lake than plants? Why? If a different lake, please explain. If the same lake as plant source, make more clear

 

Response 2: Sediment and plants were introduced from different lakes. We performed the test at the Lake Bao’an field station.  We therefore introduced sediment and water from Lake Bao’an. No plant was found in Lake Bao’an. The plants were taken from Lake Xiliang, where there are abundant submerged macrophytes.

                                               

 

Point 3: Line 111: Why only 6hr and not a normal photo-period? Explain and cite.

 

Response 3:Our ultimate target is to provide theoretical basis for applying artificial light in lake restoration. At present, application of artificial light is a costly approach. A normal photo-period of 12h light: 12h dark was not possible due to funding limitation. We rewrote the sentence as “The light was restricted to 6 h per day (9:00 ~ 15:00) due to funding limitation” (please see Line 120 - 121 in the revised manuscript).

 

Point 4: Line 114: Format scientific names in caption

 

Response 4: Revised as suggested. (please see Line 123-124 in the revised manuscript).

 

Point 5: Line 117: Sampling every 20 days seems like a very long interval. Why? Explain and cite

 

Response 5: We disagree – we a searching for longer term responses.

 

Point 6: Line 125: Is chl-a coming from phytoplankton or plants?

 

Response 6: Yes, Chl a was from phytoplankton. It is now explained in the revised paper. (please see Line 134).

 

Point 7: Line 134: What was Excel used for?

 

Response 7: Excel 2016 was used for data preprocessing. It is now explained in the revised paper (please see Line 144-145).

 

Point 8: Line 148: Table 1 - Is this the final day of the experiment? Some compilation of those 20-day intervals? Also, the total PPFD mean&SD...is this a useful metric to report given that all the different light sources have distinctly differed outputs and hopefully there was no light pollution between mesocosms.

 

Response 8: The table compiled the results averaged from the 5 samplings across the whole experiment. In terms of PPFD, we believe it is useful to report. The data showed quantitative differences in light strength among treatments and replicates.  They are also very useful in quantitatively explaining the variations in plant growth among treatments.

 

Point 9: Line 163: What is meant by "initial survival"? Did some plants die off? Did this affect total n? Explain this

 

Response 9: "initial survival" means the shoot number of plants at the beginning of the experiment. Ms and Cd died off in some treatments during the experiment. For the treatments in which plants died off, comparison was not considered anymore.

 

Point 10: Line 168: Figure 2 - I would remove the CBRW from the columns. The key is sufficient, and the individual letters are not explicitly defined.

 

Response 10: Done as suggested.

 

Point 11: Line 172: Figure 2 caption - This is the first I'm seeing of an "initial value". How was this calculated? Nowhere in the methods.

 

Response 11: Thanks for the comments. "initial value" was the growth value of plants in the beginning of the experiment. we add a sentence “We selected 20 plants from each of the three submerged macrophytes species to measure their growth value as the initial value at the beginning of experiment.” (please see Line 103 – 105 in revised manuscript.)

 

Point 12: Line 234-241: This whole paragraph seems like an afterthought. Needs to be fleshed out more, with citations, perhaps integrated with preceding paragraphs

 

Response 12: Thanks for the comments. We have now added a sentence to account for your concern “whether artificial light can be used as a tool in lake restoration when plants are delayed after the external loading is reduced sufficiently to create a shift to a clear state needs further studies. If not reduced sufficiently to help creating a shift to a macrophyte state the light may just enhance eutrophication by stimulating phytoplankton growth in winter.” (please see Line 236 - 239 in the revised manuscript).

 

Point 13: Line 243-254: Each sentence does not need to be indented. Make one uniform paragraph

 

Response 13: Done as suggested. 

 

Point 14: Figures generally: Put the unit in the Y-axis. It's confusing to go hunting for it in the caption.

 

Response 14: Done as suggested.

 

I hope that the manuscript has been satisfactorily improved. If you have any further questions, please do not hesitate to contact me.

 

Best regards,

 

Haijun Wang

State Key Laboratory of Freshwater Ecology and Biotechnology,

Institute of Hydrobiology, Chinese Academy of Sciences,

Wuhan 430072, China

Tel: +86 27 68780225/Fax: +86 27 68780064

E-mail: wanghj@ihb.ac.cn

 

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Abstract: WHY is there increased turbidity

Methods: I see no clarification on my issue of the "experimental pond". Why use sediment from here and not the plant-parent or water-parent lake? Why are there so many different lakes being used? Please clarify this issue, otherwise it speaks to potentially confounding variables

My question about the sparse measurements (every 20 days) was also not addressed. Please explain why.

Results: The time frame of table 1 is still not described, either in the text or in the legend. Additionally, the meaning or rationale for the overall mean PPFD (110.39 +/- 70.09) is not given. As my understanding goes, this is an average of the photon flux for plants growing in a variety of light wavelengths. I do not see a purpose or meaning for this value other than to not have a blank space on the table. I'd rather that than a number with no meaning.

In Section 3.2, the question of what "initial survival" means is unanswered. Did a number of plants die off? Were the replaced? Did you end up with replicates of n=1 due to die off? What does "initial survival" mean?

Author Response

Response to Reviewer 3 Comments

Thank you very much for your comments. We have revised the manuscript carefully and incorporated many points raised by you and other reviewers. Our responses are given as follows:

 

Point 1: Abstract: WHY is there increased turbidity

Response 1: With an increase of eutrophication, turbidity also increases along with the growth of phytoplankton. We added a sentence “increased growth of phytoplankton leading to increased turbidity,” in the revised manuscript (please see Line 18 in the revised manuscript).

 

Point 2: Methods: I see no clarification on my issue of the "experimental pond". Why use sediment from here and not the plant-parent or water-parent lake? Why are there so many different lakes being used? Please clarify this issue, otherwise it speaks to potentially confounding variables

 

Response 2: Feel sorry not to make a clear explanation on this issue. Our purpose was to see the possibility of using artificial light to restore submerged macrophytes in eutrophic lake. Therefore, it doesn’t matter to take water and sediment from different places. For convenience, we introduced water from Lake Bao’an and sediment from an adjacent experimental pond. The plants were taken from Lake Xiliang, as there are abundant submerged macrophytes there. The water quality, underwater light climate and sediment quality there provide good conditions for submersed macrophytes. That why we decided to introduce plants from Lake Xiliang, while introduce water from Lake Bao’an and introduce sediment from an adjacent experimental pond. Hope our explanation is clear and reasonable now. We now added a sentence in the material and method section to explain this (Please see Line 105-106 in the revised manuscript).

 

Point 3: My question about the sparse measurements (every 20 days) was also not addressed. Please explain why.

 

Response 3: Our purpose was to elucidate the general responses of plant growth in a longer term as we wrote in the last response letter.

 

Point 4: Results: The time frame of table 1 is still not described, either in the text or in the legend. Additionally, the meaning or rationale for the overall mean PPFD (110.39 +/- 70.09) is not given. As my understanding goes, this is an average of the photon flux for plants growing in a variety of light wavelengths. I do not see a purpose or meaning for this value other than to not have a blank space on the table. I'd rather that than a number with no meaning.

 

Response 4: Thanks for the comments. We rewrote the sentence as “The variables of water quality including TN, TP, WT and Turb were generally comparable among the treatments during the 100-day experiment” in the revised manuscript (please see Line 154 in the revised manuscript). And we delete the mean±SD of PPFD (110.39 +/- 70.09) in Table 1.

 

Point 5: In Section 3.2, the question of what "initial survival" means is unanswered. Did a number of plants die off? Were the replaced? Did you end up with replicates of n=1 due to die off? What does "initial survival" mean?

 

Response 5: "Initial survival" means the number of plants at the beginning of the experiment. Some of the Ms and Cd died off in a few treatments during the experiment and were not replaced. For the treatments in which plants died off, comparison was not considered anymore. To avoid confusion, we changed “initial survival” with “compared to the initial levels.” (Please see Line 175 in the revised manuscript), and deleted the second “initial survival” in the revision manuscript. (Please see Line 178 in the revised manuscript).

 

Please note that, in the revision, we also add some colleagues who offered helps to this study in the acknowledgement.

I hope that the manuscript has been satisfactorily improved. If you have any further questions, please do not hesitate to contact me.

 

Best regards,

 

Haijun Wang

State Key Laboratory of Freshwater Ecology and Biotechnology,

Institute of Hydrobiology, Chinese Academy of Sciences,

Wuhan 430072, China

Tel: +86 27 68780225/Fax: +86 27 68780064

E-mail: wanghj@ihb.ac.cn

Author Response File: Author Response.docx