Accumulation of Methylmercury in the High-Altitude Lake Uru Uru (3686 m a.s.l, Bolivia) Controlled by Sediment Efflux and Photodegradation

Round 1

Reviewer 1 Report

The article is devoted to the important problem of the transformation of mercury pollution of lake waters in high mountain conditions. It is known that mercury is one of the most toxic elements in nature. This alpine lake at an altitude of more than 3,600 meters was a victim of mine water discharge, as it is located below the mine and in lower relief. The shallow lake, in addition, is subject to significant seasonal fluctuations in the water level, which triggers the mechanism for the release of elements from the pore solution of bottom sediments. The authors studied both the daily and seasonal dynamics of mercury concentrations in surface waters, as well as in the pore solution of bottom sediments in connection with changes in other environmental, including meteorological, indicators. The authors drew attention not only to concentration fluctuations but also to environmental factors such as ultraviolet intensity and eutrophication processes. The authors' conclusions complement and make more detailed the conclusions of their earlier studies of the ecosystem of this lake. In addition, a monitoring plan is being drawn up with an emphasis on some of the most common indicators of mercury pollution. The article can be published with minor changes, including:

1). The aims of the work can be added at the end of the Introduction part.

2). Forms of mercury can be unified in lines 103, 142, 317.

3). Line 293. Is = lake uru-Uru, must be Lake Uru-Uru.

4). In the references can be checked the authors name on lines 410 and 524.

Author Response

Dear Reviewer 1,

We sincerely thank you for reviewing our paper and for your report.

• We added a line at the end of the Introduction to describe the aims of the work
• Forms of mercury have been unified in lines 103. For the result section, we chose to keep the entire name for the first time of presentation to ease the reading. We also kept the reference to "DGM (i.e. Hg(0))", as DGM can be composed of other species than Hg(0), but in our case it is dominantly Hg(0).
• Line 293. Is = lake uru-Uru, has been corrected to Lake Uru-Uru
• We checked and corrected the author’s name in the references on lines 410 and 524.

Reviewer 2 Report

The manuscript entitled " Accumulation of methylmercury in the high altitude Lake Uru-Uru (3686 m a.s.l, Bolivia) controlled by sediment efflux and photodegradation" by Guédron et al., presented a seasonal and diel variation of mercury (Hg) and methylmercury (MMHg) sediment-water column exchanges in Lake Uru-Uru, Bolivia. A good mercury speciation study was done in the water column and in surface sediment porewaters. Additionally, biogeochemical and physicochemical factors influencing these exchanges were discussed. This study is a good contribution for the Hg biogeochemical cycle in this particular wetland and will certainly have a good impact in the scientific community mostly the one that dedicate his work studying Hg behaviour in aquatic ecosystems and its contribution to Hg global cycle.

The manuscript is very well written, with a short but clear introduction. The material and methods section is well presented with all the necessary details indicated. The results have a good presentation, although little details should be changed in some Figures so that the results obtained are better understood. The authors made a brilliant relation between Hg and MMHg with all the biogeochemical and physicochemical factors studied. The discussion are in agreement with the results presented and was written strictly. Everything is explained in a very clear way and some previous hypotheses have been correctly proved. The conclusions strikingly indicated the main results of this study.

With minor revision, I recommend, without a doubt, this manuscript for publication in the Applied Sciences journal.

General comments:

In the abstract the last sentence is exactly the same that the authors have in the conclusions (also as the last sentence). I suggest a minor change without losing the important information that this sentence gives, just to be slightly different from the sentence in the conclusions.

Specific comments:

Figure 1: Where is a) and b) indicated in the figure? Because you have this in the figure caption but not in the figure himself. Please correct accordingly.

Line 124: I suggest to change to: “Sediment/water fluxes measurements and calculations”

Figure 2: in c) I suggest to use full black symbol for THg_UNF instead of crossed symbols; in d) use green empty symbol for MMHg_UNF instead of crossed full symbol; Do the same to e) for the MMHg/THg unfiltered symbol. Also, the lines that indicated the average values should be more clear. The night period is not perfectly indicated in the figures and the legends of “late wet season” and “late dry season” should appear above the figures and not below. The information about the period of sampling in each season should be indicated in the caption of the figure.

Line 163: What do you mean with “aquatic ecotopes' respiration”?

Lines 163-167: This paragraph is quite confuse. I do not see these values that you presented here. Please check accordingly.

Figure 3: in b) the y axis from the left panel should be green. Correct accordingly. In the caption of the figure you should also indicate what are the dashed lines.

Figure 4: In the caption of the figure you have a) sentence repeated. Please delete. Correct the rest of the caption to: “b) filtered Hg(II)(=THg-MMHg) and MMHg diffusive fluxes in sediment/water interface and c) partitioning of THg and MMHg between filtered and particulate fractions in surface water, during the wet (left panels) and the dry (right panels) seasons.

Line 218: Write as: “supporting the sulfate reduction…”

Line 294: write as: “ 80% higher than those measured…”

Line 312: Change “radiation” to “ radiation effect”

Lines 337-339: Attention! This sentence is exactly the same that the one that you used in the end of the abstract, as I mentioned in the beginning. In here or in the abstract you should try to change a bit in order to not be exactly the same and without losing the important information and a correct conclusion.

Figure S3:Change the caption to: “Scatter plot for ; (upper panels) PW MMHg concentrations vs H₂S and vanadium (V), and (bottom panels) MMHg diffusive fluxes (Jsed MMHg) vs H₂S and Redox potential (measured in epibenthic water above the SWI with a multiparameter probe).”

Author Response

Dear Reviewer 2,

We sincerely and greatly thank you for reviewing our paper and for your report.

• We modified the last sentence of the conclusion accordingly
• Line 124: we changed to: “Sediment/water fluxes measurements and calculations”
• Figure 2 was modified accordingly
• Figure 3 was modified accordingly
• Line 163: The “aquatic ecotopes' respiration” :

Aquatic ecotope is the common terminology for aquatic algae and benthic macrophytes. There night activity is mainly respiration; they switch from photosynthesis during the day to respiration during the night.

• True; the paragraph was confusing. We revised it accordingly (Lines 163-167).
• Figure 3 was modified accordingly
• Figure 4 was modified accordingly
• Ok done
• Ok done
• Ok done
• Ok we modified the conclusion sentence.
• Figure S3 was modified accordingly