Distribution Patterns of Sediment Organic Carbon Stocks in Shallow Lakes and the Significance for Sustainable Lake Management: Chaohu Lake in Eastern China as a Case Study
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsDear Authors! I with interest read you manuscript entitled: “Distribution patterns of sediment organic carbon stocks in shallow lakes and the significance for sustainable lake management: Chaohu Lake as a case study”. The topic of article is relevant because carbon accumulation in lake sediments plays an important role in the carbon cycle. Organic material (both which delivered with slope runoff or/and produced inside water object), such as suspended sediments, plant debris and dead algae, settles to the bottom of the lake without completely decomposing due to the lack of oxygen. This creates conditions for carbon accumulation in the form of bottom sediments or peat. Sediments/peat contains huge reserves of carbon, and its accumulation helps sequester carbon from the atmosphere, which influences global climate and reduces atmospheric carbon dioxide levels. Studying carbon accumulation in lake sediments is important for understanding carbon dynamics in ecosystems, as well as for studying climate change and the impact of human activities on the environment.
The presented article is fits to Land journal, also it suitable for consideration in other MDPI journals like Water, Soil Systems, Sustainability, etc. At almost the article is “average” written, and I have some comments and suggestions:
1. In the title of article, suggested to add region of study (eastern China).
2. L. 25. In Keywords better to avoid repeating of “words” which were already mentioned in the Title of article.
3. L. 31. “increasingly public attention (Bastviken et al., 2011; Pekel et al., 2016)” The citation should be provided as “increasingly public attention [1, 2]”.
4. L. 60. Yes, sediment analysis can be useful in many studies, as well for example for reconstructing pollution dynamics in water objects (please see Reconstruction of time changes in radiocesium concentrations in the river of the Fukushima Dai-ichi NPP contaminated area based on its depth distribution in dam reservoir's bottom sediments).
5. In section 2.1. I suggest to provide an elevation map of lake bottom (or add data of depth in sampling points in Table 1). You know that siltation rates (sediments accumulation) depends of watershed area and its structure. I suggest to add maps of lake watershed with following data: elevation, land/use, and if possible erosion map of watershed, because the sediments accumulation rates is closely linked with erosion on watershed (for example, please see Siltation and radiocesium pollution of small lakes in different catchment types far from the Fukushima Daiichi nuclear power plant accident site).
6. L. 135. On some of map need to show the rivers what entering the lake.
7. L. 143. Which method or how you calculate the wet density? As well for section 3.2.2. Water content and particle size distribution pattern.
8. Figure 5. Particle size. Please check the unit (should be μm). Also the particle size distribution usually presented by combination of fractions of clay, silt and sand. According you data you have only 1 size.
9. “4.1. Mathematical fitting of SOC content” should be moved to methods section.
10. The reference list prepared not according to journal rules.
Author Response
[Dear Authors! I with interest read you manuscript entitled: “Distribution patterns of sediment organic carbon stocks in shallow lakes and the significance for sustainable lake management: Chaohu Lake as a case study”. The topic of article is relevant because carbon accumulation in lake sediments plays an important role in the carbon cycle. Organic material (both which delivered with slope runoff or/and produced inside water object), such as suspended sediments, plant debris and dead algae, settles to the bottom of the lake without completely decomposing due to the lack of oxygen. This creates conditions for carbon accumulation in the form of bottom sediments or peat. Sediments/peat contains huge reserves of carbon, and its accumulation helps sequester carbon from the atmosphere, which influences global climate and reduces atmospheric carbon dioxide levels. Studying carbon accumulation in lake sediments is important for understanding carbon dynamics in ecosystems, as well as for studying climate change and the impact of human activities on the environment.
The presented article is fits to Land journal, also it suitable for consideration in other MDPI journals like Water, Soil Systems, Sustainability, etc. At almost the article is “average” written]
Thank you very much for taking the time to review this manuscript. Your comments are of great help to improve the quality of our manuscript, and we have revised the paper carefully. Hope that it can be accepted by the journal.
Comments 1: [ In the title of article, suggested to add region of study (eastern China).]
Response 1: Thank you for pointing this out. We agree with this comment. Therefore, We have added “in eastern China” to the title.
Comments 2: [L. 25. In Keywords better to avoid repeating of “words” which were already mentioned in the Title of article]
Response 2: Thank you for pointing this out. We agree with this comment. Therefore, We have changed the keywords and changed them to “Shallow lakes; Bottom sediments; Carbon source; Carbon sink; Carbon emissions”.
Comments 3: [L. 31. “increasingly public attention (Bastviken et al., 2011; Pekel et al., 2016)” The citation should be provided as “increasingly public attention [1, 2]”]
Response 3: Thank you for pointing this out. We agree with this comment. Therefore, We have changed the format of all the references in the text.
Comments 4: [L. 60. Yes, sediment analysis can be useful in many studies, as well for example for reconstructing pollution dynamics in water objects (please see Reconstruction of time changes in radiocesium concentrations in the river of the Fukushima Dai-ichi NPP contaminated area based on its depth distribution in dam reservoir's bottom sediments)]
Response 4: Thank you for pointing this out. We agree with this comment. This literature is of great help to us, and we refer to it here.
Comments 5: [In section 2.1. I suggest to provide an elevation map of lake bottom (or add data of depth in sampling points in Table 1). You know that siltation rates (sediments accumulation) depends of watershed area and its structure. I suggest to add maps of lake watershed with following data: elevation, land/use, and if possible erosion map of watershed, because the sediments accumulation rates is closely linked with erosion on watershed (for example, please see Siltation and radiocesium pollution of small lakes in different catchment types far from the Fukushima Daiichi nuclear power plant accident site)]
Response 5: Thank you for pointing this out. We agree with this comment. Firstly, we have put the specific values of sediment thickness in Figure 2 into Table 1. In addition, we have added information on Chaohu water flow movement and land use (Lines 84-92). Here we would like to explain more about the elevation of the basin. In fact, in Figure 1, we have used cloud maps of different colors to express it. Although the difference in the scope of the legend is relatively large, the terrain and landform in the basin as a whole can still be seen. In addition, regarding the erosion map of the lake, this has not been studied at this stage, but we will go deeper into this part in the later stage.
Comments 6: [L. 135. On some of map need to show the rivers what entering the lake.]
Response 6: Thank you for pointing this out. We agree with this comment. Therefore, We have modified Figure 1 and added the rivers that entering the lake.
Comments 7: [L. 143. Which method or how you calculate the wet density? As well for section 3.2.2. Water content and particle size distribution pattern.]
Response 7: Thank you for pointing this out. In this study, we can maintain the sediment in its original state to the greatest extent through the sampling method of column core culture, without damaging its structure and physical and chemical properties. In the lab, we siphon off the water above the sediment, so we get a sample of the sediment in its original state. We then measure the wet density, water content and particle size in accordance with the methods required for laboratory.
Comments 8: [Figure 5. Particle size. Please check the unit (should be μm). Also the particle size distribution usually presented by combination of fractions of clay, silt and sand. According you data you have only 1 size]
Response 8: Thank you for pointing this out. We checked the unit of particle size in Figure 5 and determined that it was μm (The depth of the sediment is adopted in cm). In addition, we use a laser particle size analyzer to measure the particle size of the sediment in the laboratory, and in fact it is a wide range of interval values. It is recommended to use particle size indicators that occupy 10%, 50% and 90% of the particle size. What we have chosen here is an eigenvalue of 50% median particle size as a representative.
Comments 9: [“4.1. Mathematical fitting of SOC content” should be moved to methods section]
Response 9: Thank you for pointing this out. We agree with this comment. Therefore, We've moved this section to methods section. In addition, combined with the opinions of the second reviewer, the comparison with the existing research is reserved in the discussion section.
Comments 10: [The reference list prepared not according to journal rules.]
Response 10: Thank you for pointing this out. We agree with this comment. Therefore, We have revised the list of references.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for AuthorsLuo et al has carried out a study about the patterns of lake sediment OC and report the differences of the OC content among the sub-regions in the Chaohu Lake. Indeed, OC buried in the lake sediment is the key issues that concerned by scientific community. The results highlight the specific OC stocks in the sediment in the Chaohu Lake and inferred the impact of human activities on OC stocks. However, there are several issues which are of concern to me and I would like to draw the attention of the authors to this sectors. I recommend Reconsider after major revision. My specific comments are enumerated below:
1. The introduction part has been written in a very loose manner. The importance of the study should be elucidated in deep. As far as I am concerned, sediment OC in Chaohu Lake has been widely studied. The authors should tell us the novelty of the present study.
2. In the Study area and methodology part, some important information is missed. For example, why the authors divided the Chaohu Lake into three subzones rather than two subzones such as western and eastern? When introducing the sample site, the authors did not comply with caitation standards. “It has an average water level of 8.37 m, a lake basin length of 61.7 km, a width of 12.47 km, a water area of 769.55 km2 and an average depth of 2.89 m. The average flow velocity in the lake ranges from 0.02 to 0.07 m/s, and the maximum flow velocity is around 0.62 m/s.” If these values were not determined by the authors, they should cite the relative references. In Figure 1, the sampling sites were not evenly distributed in three subzones. In addition, the authors did not use the standard map of China.
3. In the “Sample collection and analysis” part, the analysis and calculation methods of the relative parameters in sediments were not mentioned in the manuscript. We did not know how were the parameters calculated? The authors said “three layers of detected sediment samples were selected between the 5-10 cm layer and the bottom in accordance with the principle of equidistant equal division.”, but it was not evident from Figure 2.
4. In “3.1. Sediment depth distribution” part, there were several channels for freight transport in the Chaohu Lake, so did the deeper sites avoid the channels?
5. L197-202, the authors should explain whether this formular is applicable to Chaohu Lake or similar lakes.
6. In Figure 7 and 9, the authors should checked the table, R2=adjusted R2?
7. The Table 1 is redundant in the text.
8. L252-255, lack the robust evidence.
9. L317-321, in eutrophic regions, phytoplankton biomass and aquatic plant biomass were high, which may cause the organic carbon buried.
10. In the discussion part, the authors should discuss the difference between the present results and the existing studies.
11. L344-345, how could the authors draw the conclusion that LUCC affect SOC stocks.
Author Response
（Luo et al has carried out a study about the patterns of lake sediment OC and report the differences of the OC content among the sub-regions in the Chaohu Lake. Indeed, OC buried in the lake sediment is the key issues that concerned by scientific community. The results highlight the specific OC stocks in the sediment in the Chaohu Lake and inferred the impact of human activities on OC stocks. However, there are several issues which are of concern to me and I would like to draw the attention of the authors to this sectors. I recommend Reconsider after major revision.）
Thank you very much for taking the time to review this manuscript. Your comments are of great help to improve the quality of our manuscript, and we have revised the paper carefully. Hope that it can be accepted by the journal.
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Comments 1: [The introduction part has been written in a very loose manner. The importance of the study should be elucidated in deep. As far as I am concerned, sediment OC in Chaohu Lake has been widely studied. The authors should tell us the novelty of the present study.] |
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Response 1: Thank you for pointing this out. We agree with this comment. Therefore, We have increased the necessity of studying the distribution of sediment organic carbon in Chaohu Lake in the introduction section.
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Comments 2: [In the Study area and methodology part, some important information is missed. For example, why the authors divided the Chaohu Lake into three subzones rather than two subzones such as western and eastern? When introducing the sample site, the authors did not comply with caitation standards. “It has an average water level of 8.37 m, a lake basin length of 61.7 km, a width of 12.47 km, a water area of 769.55 km2 and an average depth of 2.89 m. The average flow velocity in the lake ranges from 0.02 to 0.07 m/s, and the maximum flow velocity is around 0.62 m/s.” If these values were not determined by the authors, they should cite the relative references. In Figure 1, the sampling sites were not evenly distributed in three subzones. In addition, the authors did not use the standard map of China.] |
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Response 2: Thank you for pointing this out. First, we have added reasons for dividing Chaohu Lake into three subzones. Secondly, we have also added references to the basic information of Chaohu Lake in the text. Finally, we have removed the "average" and replaced the standard map of China in Figure 1.
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Comments 3: [In the “Sample collection and analysis” part, the analysis and calculation methods of the relative parameters in sediments were not mentioned in the manuscript. We did not know how were the parameters calculated? The authors said “three layers of detected sediment samples were selected between the 5-10 cm layer and the bottom in accordance with the principle of equidistant equal division.”, but it was not evident from Figure 2.] |
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Response 3: Thank you for pointing this out. We agree with this comment. First of all, according to the comments of the first reviewer, we have adjusted the part about parameter calculation in 4.1 to the method in the second part, so that this parameter calculation will be clearer. Second, we have corrected the errors in the representation in Figure 2.
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Comments 4: [In “3.1. Sediment depth distribution” part, there were several channels for freight transport in the Chaohu Lake, so did the deeper sites avoid the channels?] |
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Response 4: Thank you for pointing this out. In fact, our 12 sampling points are arranged according to the direction of the current mainstream and the course of the air. In addition, the results of the previous investigation on water and sediment pollution were combined, so the points with larger sediment depth after sampling did not deliberately avoid the channels.
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Comments 5: [L197-202, the authors should explain whether this formular is applicable to Chaohu Lake or similar lakes.] Response 5: Thank you for pointing this out. We agree with this comment. Based on the comments of the first reviewer, we adjusted the previous part of the mathematical relationship to the second part of the body, and added a description of the applicability of this formula to Chaohu Lake (Lines135-139).
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Comments 6: [In Figure 7 and 9, the authors should checked the table, R2=adjusted R2?] Response 6: Thank you for pointing this out. This may be a misunderstanding caused by our expression. Adjusted R2 is a value automatically generated in origin's graph. When the curve relationship in the graph is not adjusted, R2=adjusted R2.
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Comments 7: [The Table 1 is redundant in the text.] Response 7: Thank you for pointing this out. Based on the comments of the first reviewer, we adjusted Table 1 to increase the sediment depth.
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Comments 8: [L252-255, lack the robust evidence.] Response 8: Thank you for pointing this out. In fact, the depth at which the mud-water interface is affected has been studied extensively. Different studies have drawn different conclusions, the smallest is only millimeter level, there are about 1cm, there are about 10cm, but the maximum is not more than 20cm. Therefore, we chose a relatively average of 10cm as credible in the study. This can be supported in the reference of Fan (2019). We have also added this reference to the text.
Comments 9: [L317-321, in eutrophic regions, phytoplankton biomass and aquatic plant biomass were high, which may cause the organic carbon buried.] Response 9: Thank you for pointing this out. In eutrophication regions, the natural state does lead to carbon burial. However, if human activities are frequent, too much human interference will lead to carbon emissions. In the text we emphasize the impact of human activities.
Comments 10: [In the discussion part, the authors should discuss the difference between the present results and the existing studies.] Response 10: Thank you for pointing this out. In fact, we have compared the existing studies in Section 4.1, and verified that some formulas we optimized have better accuracy with the measured data.
Comments 11: [L344-345, how could the authors draw the conclusion that LUCC affect SOC stocks.] Response 11: Thank you for pointing this out. In fact, what we have here is the intensity of the impact of human activity. In 2.1 above, we also added the explanation that human activities around Chaohu Lake mainly affect the use of cultivated land, so the impact of land use on sediment carbon sources must be dominant. But it does seem a bit obtrusive here, so we've removed "land use" from the text. |
Author Response File: 
Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for Authors1. In response 4, the authors said "sampling did not deliberately avoid the channels", so the effects of channels on sediment OC should discuss in the ms.
2. in response 5, I could not find the corrections. The authors should point out the corrections in the response letter.
3. in response 6, why you put the tables in Fig. 7 and Fig. 9. To avoid misunderstanding， you can present it in equation.
4. in response 10, I could not find the comparisons, could you list them in the response letter?
5. in response 11, the corrections should highlight in the revised manuscript or list in the response letter.
Author Response
Comments 1: [ In response 4, the authors said "sampling did not deliberately avoid the channels", so the effects of channels on sediment OC should discuss in the ms. ]
Response 1: Thank you for pointing this out. We agree with this comment. In Section 2.3 of the manuscript (Lines131-133), we have added a discussion on the influence of channels on sediment sampling, although the arranged sampling points will inevitably intersect with channels. However, in the process of sediment sampling, our sampling process will conduct a test sample of the surrounding area to ensure that the sediment obtained can represent the average sediment depth of the area. Therefore, the influence of channels on sediment samples can be ignored in this study.
Comments 2: [In response 5, I could not find the corrections. The authors should point out the corrections in the response letter.]
Response 2: Thank you for pointing this out. In fact, we have revised the explanation in section 2.4 of the manuscript according to the comments of the reviewers. We have added to the manuscript (Lines162-167) a revision of this opinion:” The verification results of the above formula with the measured data are good. The similar response of aquatic sediments in ponds, rivers and lakes, in different climatic and edaphic regions, suggests that conclusions drawn from this formula have a prac-tical value in a wide range of aquatic systems. Therefore, in this study, we applied this formula to the estimation of SOC in Chaohu Lake, and used the measured data to fur-ther optimize the formula.”
Comments 3: [In response 6, why you put the tables in Fig. 7 and Fig. 9. To avoid misunderstanding， you can present it in equation.]
Response 3: Thank you for pointing this out. We used the now more recognized mapping software "Origin 2023b" to produce figures 7-9. In this mapping software, the information in this table is automatically generated for the fitted curve to ensure the reliability of the data and the drawing. Therefore, we recommend that the data reliability of the figures in the manuscript be preserved in this original format.
Comments 4: [In response 10, I could not find the comparisons, could you list them in the response letter?]
Response 4: Thank you for pointing this out. In order to more clearly respond to the comments of reviewers, we have added a discussion analysis different from the existing research in Section 4.1 of the main body (Lines291-299), the specific content is" Compared with previous research methods, our method improves the interaction mechanism between sediment organic carbon distribution and sediment physical properties. The quantitative method of sediment carbon sink is further proposed with reference to the sediment organic matter balance theory. Although this method still needs further scientific definition and different academic viewpoints in the analysis of stable organic carbon content and carbon equivalent calculation, its progressive sig-nificance lies in the fact that it puts forward an operable quantitative accounting method. In the future investigation of lake sediment organic carbon emissions, it has some progressive value or enlightenment to provide a quantitative accounting tool.”
Comments 5: [In response 11, the corrections should highlight in the revised manuscript or list in the response letter.]
Response 5: Thank you for pointing this out. In view of the discussion and analysis of the impact of land use on sediment organic carbon, we have added a relevant paragraph in the body of section 4.3 (Lines368-373), specifically modified as " This is mainly because with the social and economic development, frequent human activities lead to changes in the nature of land use in Chaohu Lake Basin (the most obvious one is the reduction of cultivated land and the increase of urban land), resulting in the increase of impervious underlying surface, which inputs a large amount of nutrients originally left in cultivated land into the lake, thus affecting the nutrient status of the lake.".
And for the conclusion part (Line399), we highlight the impact of human activities, and take land use and land cover as its representative content, and specifically revise it as " human activities (land use and land cover)".
Author Response File: Author Response.pdf
