Distribution Characteristics of Carbon, Nitrogen, and Phosphorus and Pollution Load Estimation of Sediments in Danjiangkou Reservoir, China

Round 1

Reviewer 1 Report

I' wondering the authors have submitted this manuscript as a "Research Article" or a "Case Study". My suggestion is acceptance as a "Case Study" with considering the comments given in the attached file.

Good luck!

Comments for author File: Comments.pdf

Author Response

Reviewer #1:

General comments: I' wondering the authors have submitted this manuscript as a "Research Article" or a "Case Study". My suggestion is acceptance as a "Case Study" with considering the comments given in the attached file.

Response: Thanks very much for your encouragement and suggestions to our manuscript. We have submitted the manuscript as a "Research Article". However, I appreciate your suggestion to consider acceptance as a "Case Study" and will take into account the comments provided in the attached file. We have marked the sections of manuscript revision using red color. We hope this paper would have the pleasure of considering for publication in Water.

The responses to other comments can be seen the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Review of the manuscript titled "Distribution characteristics of carbon, nitrogen, and phosphorus and pollution load estimation of sediments in Danjiangkou Reservoir, China". The paper presents the distribution of carbon, nitrogen, and phosphorus in reservoir sediments and their pollution assessments at different water periods. Considering the new version supplied, the following comments are expressed. Recommend to revise considering the comments provided.

1. Without providing any abbreviation list, the authors directly use abbreviations in the abstract. What are TN, TP, and TOM?

2. It is advised to split the Introduction section into four parts: introduction, related literature review, motivation, and research objectives. The current contents are insufficient and try to review more recent related literature (For example: https://doi.org/10.1016/j.envpol.2022.120471; https://doi.org/10.1016/j.gexplo.2023.107181; https://doi.org/10.1016/j.chemosphere.2022.134589; https://doi.org/10.1016/j.jenvman.2022.116712)

Research motivation or a problem statement must be clearly emphasized.

3. From Figure 2, it can be seen that only D1-D14 samples were analyzed. For such a huge area, the number of sampling points or samples considered to arrive at any conclusions becomes very small. Provide your comments on this issue. Why did you restrict to less number of sampling locations?

4. The authors need to expand from the local pollution data to general interpretations, which is a main challenge as the information provided is insufficient, necessitating assumptions that are not easily validated. 

5. Limitations and future scope of work must be presented in the Conclusions section.

Author Response

Reviewer #2:

General comments: Review of the manuscript titled "Distribution characteristics of carbon, nitrogen, and phosphorus and pollution load estimation of sediments in Danjiangkou Reservoir, China". The paper presents the distribution of carbon, nitrogen, and phosphorus in reservoir sediments and their pollution assessments at different water periods. Considering the new version supplied, the following comments are expressed. Recommend to revise considering the comments provided.
Response: Thanks very much for your efforts and suggestions to our manuscript. We have newly made a carefully revision on our manuscript according to the Reviewer's comments. A detailed point-by point list of the responses to the Reviewer's comments is attached. We have also marked the sections of manuscript revision using red color.

1. Without providing any abbreviation list, the authors directly use abbreviations in the abstract. What are TN, TP, and TOM?
Response:Thanks very much for your reminding. We have added the full name of abbreviation appeared in Abstract. Revised as suggested (see page 2 lines 22-23 ).

2. It is advised to split the Introduction section into four parts: introduction, related literature review, motivation, and research objectives. The current contents are insufficient and try to review more recent related literature (For example: https://doi.org/10.1016/j.envpol.2022.120471; https://doi.org/10.1016/j.gexplo.2023.107181; https://doi.org/10.1016/j.chemosphere.2022.134589; https://doi.org/10.1016/j.jenvman.2022.116712)
Research motivation or a problem statement must be clearly emphasized.

Response: Thanks for your suggestions. We have re-written the introduction part in this paper to make them clearer and more concise (see page 4 lines 65-68, 82-89 and page 5 lines 93-97). The source apportionment and probabilistic risk of internal nutrition release in Dangjiangkou Reservoir are the main problems that this study hopes to solve. The added references are as following.
References:

(1) Chen, M.J., Janssen, A.B.G., de Klein, J.J.M., Du, X.Z., Lei, Q.L., Li, Y., Zhang, T.P., Pei, W., Kroeze, C., Liu, H.B. Comparing critical source areas for the sediment and nutrients of calibrated and uncalibrated models in a plateau watershed in southwest China[J]. J. Environ. Manage, 2023, 326(B): 116712.

(2) Fu, Z.B., He, N.J., Ma, M., Bao, Z.Y., Xie, S.Y., Gu, Y.S. Source apportionment and probabilistic risk assessment of heavy metals in selenium-rich soils in Hainan Province, China[J]. J. Geochem. Explor, 2023, 251: 107241.

(3) Pan, X., Chen, Z.Y., Zhai, W.L., Dong, L., Lin, L., Li, Y., Yang, Y.Y. Distribution of antibiotic resistance genes in the sediments of Erhai Lake, Yunnan-Kweichow Plateau, China: Their linear relations with nonpoint source pollution discharges from 26 tributaries[J]. Environ. Pollut, 2023, 316: 120471.

(4) Mohanavelu, A., Shrivastava, S., Naganna, S.R. Streambed pollution: A comprehensive review of its sources, eco-hydro-geo-chemical impacts, assessment, and mitigation strategies[J]. Chemosphere, 2022, 300:134589.

3. From Figure 2, it can be seen that only D1-D14 samples were analyzed. For such a huge area, the number of sampling points or samples considered to arrive at any conclusions becomes very small. Provide your comments on this issue. Why did you restrict to less number of sampling locations?
Response:Thanks for your questions. Obviously, the more cross-section we have investigated, the more accurate and reliable our findings will be. We agree with the reviewer’s comment that more sampling points are needed to investigate. Danjiangkou, a vast reservoir, has been routinely monitored every month for more than a decade as a water resource of the middle route of the South-to-North Water Diversion project by us. We have a sufficient understanding of the hydrology, hydrodynamic conditions, and the spatial and temporal distribution characteristics of water quality of the reservoir. On this basis, it has formed a fixed national water quality monitoring point of the reservoir, and the monitoring results can represent the water quality of the reservoir. The sampling points of sediments in this study are in coincide with these water quality monitoring points.

4. The authors need to expand from the local pollution data to general interpretations, which is a main challenge as the information provided is insufficient, necessitating assumptions that are not easily validated. 
Response:Thank you for your query. Revised as suggested. We have added some discussion to the resultsin the “Results and discussion” part. Such as “Zhai et al. (2022) statistically divided the monthly TN and TP inflow fluxes and runoff in the main tributaries of Danjiangkou Reservoir, and the results showed that the two changes were consistent and showed a very significant positive correlation. The most non-point source pollutants were imported in flood season. ”

5. Limitations and future scope of work must be presented in the Conclusions section.

Response: Revised as suggested. Danjiangkou Reservoir is a multi-year regulating reservoir. Unlike the common lakes and reservoirs, the nutrient contents in this reservoir sediments fluctuate greatly in different water periods. The reasons for this phenomenon need to be further studied. In addition, there are some limitations in the laboratory simulated experiment of nitrogen and phosphorus nutrient release in  sediments. And the in-situ observation of nutrient release in reservoir sediments can be carried out in the future.

Author Response File: Author Response.docx

Reviewer 3 Report

The paper studied the distribution characteristics of carbon, nitrogen, and phosphorus in Danjiangkou Reservoir sediments, China, and estimated the pollution load of those nutrients. They have done a detailed work about this topic. It is a necessary and significant work for the protection and management of water ecological environment in this super large water sources, Danjiangkou Reservoir. And it was suitable for publication in Water. But there are still some problems with the draft. Several revisions are needed.

1. Abstract When the abbreviation appears for the first time, the full name is required.

2. Grammar and tenses should be checked throughout, e.g., line 23-24.

3. The previous studies also give the information about the characteristics and distribution of the nitrogen and phosphorus forms in the sediments of Danjiangkou Reservoir, author should give the newer ideas in introduction part.

4. The quality control during the analysis of the sediment samples need to be supplemented.

5. The discussion part should be developed more. In order to make the discussion visible for wider international readers, the discussion should also be made by comparing the results of nutrients in sediments with what reported previously by other researchers across the world.

6. Why do nitrogen and phosphorus contents in sediments change differently in different water periods?

7. Compared with previous studies, is the pollution risk of nitrogen and phosphorus release from Danjiangkou Reservoir sediment increasing or decreasing in recent years?

Author Response

Reviewer #3:

General comments: The paper studied the distribution characteristics of carbon, nitrogen, and phosphorus in Danjiangkou Reservoir sediments, China, and estimated the pollution load of those nutrients. They have done a detailed work about this topic. It is a necessary and significant work for the protection and management of water ecological environment in this super large water sources, Danjiangkou Reservoir. And it was suitable for publication in Water. But there are still some problems with the draft. Several revisions are needed.

Response: We appreciate the positive comments of the reviewer. And we have newly made a carefully revision on our manuscript according to the Reviewer's comments. The revised sections of manuscript have been marked by red color.

1. Abstract When the abbreviation appears for the first time, the full name is required.

Response: Thanks very much for your reminding. We have added the full name of abbreviation appeared in Abstract. Revised as suggested (see page 2 lines 22-23).

2. Grammar and tenses should be checked throughout, e.g., line 23-24.

Response: This is our negligence. Thanks very much for your suggestion. We have checked throughout the paper carefully. Revised as suggested can be seen in red.

3. The previous studies also give the information about the characteristics and distribution of the nitrogen and phosphorus forms in the sediments of Danjiangkou Reservoir, author should give the newer ideas in introduction part.

Response: Thanks for your suggestions. We have re-written the introduction part in this paper to make them clearer. Previous studies on nitrogen and phosphorus nutrients in Danjiangkou Reservoir sediments are rare, and there is no report on pollution load estimation. So, we would like to pay more attentions to the source apportionment and probabilistic risk of internal nutrition release in this deep-water reservoir.

4. The quality control during the analysis of the sediment samples need to be supplemented.

Response: Thank you. Revised as suggested. “All the instruments used sediment sample analysis had passed national measurement examination. And the reference materials (GSD-9) and standard reagents were used for quality control. ” has been add in lines 153-155.

5. The discussion part should be developed more. In order to make the discussion visible for wider international readers, the discussion should also be made by comparing the results of nutrients in sediments with what reported previously by other researchers across the world.

Response: Thanks for your suggestions. We have added some discussion to the results in this part to make them clearer and more concise. And we have also made the discussion and comparison with the results in previous studies (see page 10 lines 217-219, lines 227-230; page 11 lines 251-254; page 18 lines 389-395).

6. Why do nitrogen and phosphorus contents in sediments change differently in different water periods?

Response: By comparing the sediment TN content in different water periods, we found that TN content in most sampling points, especially the core reservoir area, increased significantly during the flood season. The reason might be the increase of rainfall in flood season had leaded to the significant increase of exogenous N pollution and sediment input in the inflow river and surrounding reservoir area. In flood season, the phosphorus entering the reservoir with rainfall is significantly less than nitrogen, and the phosphorus in sediments is more easily released under the influence of floods than nitrogen, which results in the decrease of TP content in sediments in flood season.

7. Compared with previous studies, is the pollution risk of nitrogen and phosphorus release from Danjiangkou Reservoir sediment increasing or decreasing in recent years?

Response: Compared with these monitoring results from 2011 to 2016 reported by Li et al. (2018), of which the concentrations ranged from 720 to 1910 mg/kg, TN content in Dan River Reservoir sediments decreased slightly. The maximum reported TP content in Danjiangkou Reservoir sediments was 821.4 mg/kg before 2016 (Tang et al., 2014; Li et al., 2018). It indicated the enrichment of phosphorus in reservoir sediments.

References:

1. Li, B.;  Wang, Y., Zheng, Z.; Xu, X.; Xin, Y.D.; Huang, J.; Bi, S.G.; Hu, L.Q.; Chen, Z.J.; Li, Y.Y. Temporal and spatial changes in sediment nutrients and heavy metals of the danjiangkou reservoir before and after water division of the mid-route project. Environ. Sci, 2018, 39(08), 3591-3600.(in Chinese)
2. Tang, X.Q.; Wu, M.; Dai, X.C.; Chai, P.H. Phosphorus storage dynamics and adsorption characteristics for sediment from a drinking water source reservoir and its relation with sediment compositions. Ecol. Eng, 2014, 64(3), 276-284.

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear Authors;

Thank you for addressing my comments. My suggestion is acceptance.

Good luck!

Reviewer 2 Report

The authors have sufficiently revised the manuscript as per suggestion.