Relationship of the “Dequada” Phenomenon with Mercury Methylation in Pantanal, Brazil

Round 1

Reviewer 1 Report

This is a very interesting paper looking at the effects of the dequada on mercury concentration in water and sediment and on methylation. This can have implications for bioaccumulation and potential negative effects of Hg in organisms exposed to this water and sediments, especially given gold mining in the regions studied.

The work is well thought out and the experiments are sound. Here are a few comments to improve the manuscript and clarify some of the analysis and discussion.

1.      The dequada phenomenon is fascinating and I had not heard about it. The authors did a good job describing it, but a little more content explaining it would be useful.  Are the effects of the dequada mostly in the river itself or the flood plain (flooded area surrounding the river) or both? Are the effects mainly seen downstream (for example, in the bays) or are they seen along the rivers as well?

2.      The authors collected water and sediment samples. It seems that they may have been used in separate analyses or combined, as for example if the microcosm experiments. It would be useful to stat this in the methods so the reader can understand what each part of the research involves and represents.

3.      Three samples were combined for each microcosm experiment, instead of running three separate experiments. Therefore, there is no experiment replication to look at variation. I understand there may be reasons for that that related to logistics and the difficulty of conducting this work, but it would be useful to state that, and also to compare with similar established work in the published literature.

4.      Were there any statistical (Table 2) analyses conducted?

5.      In Figure 2 the graph for BC1 is missing.

6.      All the measurements in figure 3 are mentioned in the text but there are no statistics on whether the values are different.

7.      It would be good to have a reference point as to what values may be of concern for methylation, for example when 6.4% of the added Hg is methylated is that significant.

8.      I think the EH measurements are especially interesting because the temporal patterns are so different the two microcosms.

9.      Can you explain or re-write the paragraph in page 9, lines 310-315? I cannot understand the point being made.

10.   Figure 4 is counter intuitive and seems to go against the point the authors are making about higher methylation in the dequada vs without the dequada; it shows lower methylation in dequada. Why?

11.   Can you explain the point you are making in page 11, lines 372-374?

12.   Why do you think the methylation differences were not consistent within both experiments, and in the Saracura Bay microcosms there was not high levels of methylation?

Thank you for the opportunity to review this paper, I enjoyed reading it.

Quality of English is sufficient.

Author Response

Response to Reviewer 1

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

1. The dequada phenomenon is fascinating and I had not heard about it. The authors did a good job describing it, but a little more content explaining it would be useful.  Are the effects of the dequada mostly in the river itself or the flood plain (flooded area surrounding the river) or both? Are the effects mainly seen downstream (for example, in the bays) or are they seen along the rivers as well?

R: It is a natural phenomenon in the flooded plains. However, its effects are observed in the bays where the phenomenon occurs and consequently in the river, albeit with less intensity.

Text modification made on page 2, lines 49-56.

2. The authors collected water and sediment samples. It seems that they may have been used in separate analyses or combined, as for example if the microcosm experiments. It would be useful to stat this in the methods so the reader can understand what each part of the research involves and represents.

R: Water and sediment analyses were carried out separately (detailed in sections 2.3, 2.5, 2.6, and 2.7) and combined, but only combined in the microcosm experiments, as mentioned in the article (section 2.4).

3. Three samples were combined for each microcosm experiment, instead of running three separate experiments. Therefore, there is no experiment replication to look at variation. I understand there may be reasons for that that related to logistics and the difficulty of conducting this work, but it would be useful to state that, and also to compare with similar established work in the published literature.

R: Quotations written in the text, page 4 line 138.

4. Were there any statistical (Table 2) analyses conducted?

R: No, as per the studies by Gomes et al. (2019) and Rosas (2016), it is not common to conduct statistical analyses for the physicochemical parameters of water.

5. In Figure 2 the graph for BC1 is missing.

R: Figure 2, graph BC1 inserted in the text on page 7.

6. All the measurements in figure 3 are mentioned in the text but there are no statistics on whether the values are different.

R: No, as per the studies by Gomes et al. (2019) and Rosas (2016), it is not common to conduct statistical analyses for the physicochemical parameters of water.

7. It would be good to have a reference point as to what values may be of concern for methylation, for example when 6.4% of the added Hg is methylated is that significant.

R: Given that the environmental conditions are favorable for the methylation of Hg, any value can be considered significant due to the process of biomagnification.

9. Can you explain or re-write the paragraph in page 9, lines 310-315? I cannot understand the point being made.

R: Text modification made on page 9, lines 310-315.

10. Figure 4 is counter intuitive and seems to go against the point the authors are making about higher methylation in the dequada vs without the dequada; it shows lower methylation in dequada. Why?

R: As mentioned in the text, the ‘dequada’ phenomenon occurs in the plain with a secondary effect on the bays. In this case, the size of the bay matters, since the Castelo bay is approximately 3 times larger than the Saracura bay, thus receiving a larger load of organic matter. These results can be explained by the organic matter content presented in Figures 3 and 5.

11. Can you explain the point you are making in page 11, lines 372-374?

R: Text modification made on page 11, lines 371-373.

12. Why do you think the methylation differences were not consistent within both experiments, and in the Saracura Bay microcosms there was not high levels of methylation?

R: As mentioned in the text, the ‘dequada’ phenomenon occurs in the plain with a secondary effect on the bays. In this case, the size of the bay matters, since the Castelo bay is approximately 3 times larger than the Saracura bay, thus receiving a larger load of organic matter. These results can be explained by the organic matter content presented in Figures 3 and 5.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors have thoroughly analyzed the effects of physical and chemical parameters such as pH, redox potential, and dissolved oxygen using a multi-parameter probe on mercury methylation. They strongly reported that during "dequada" in Castelo Bay, the methylation rate was several times higher than without "dequada". Finally, they concluded that gold mining activity in Pantanal can cause significant damage to the ecosystem because this environment can favor mercury methylation, which is the most toxic form active in the aquatic ecosystem of Pantanal. The presented data here are robust. From a reviewer's standpoint, the manuscript holds promise for publication without requiring any revisions.

Author Response

Thank you very much for taking the time to review this manuscript.

Reviewer 3 Report

The research paper titled “Relationship of the “Dequada” phenomenon with mercury methylation in Pantanal, Brazil” is a great accomplishment because of the relatively unexplored nature of research in this area. The research work is original and will contribute significantly to the existing literature.  Due consideration has been given to the theoretical part. Experiments have been performed with precision and the data obtained has been presented systematically and satisfactorily. The article has a good format and is supported by updated references; however, there are some typographical/grammatical/technical mistakes in it.  Furthermore, the scale on the Y-axis of Figure 2 is vast range due to which all the data points seem to be zero. The range of this scale should be set up to 2000. Similarly, a sub-figure of Figure 3, Figure 4, and two sub-figure of Figure 5 should be revised for the Y-axis.

As given above.

Author Response

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

R: Since Figure 2, graph BC1, which shows values close to 7000, was missing from the work and has now been added, it may have confused Reviewer 3 regarding the scales. The scales have been standardized based on this value.

Author Response File: Author Response.pdf