Carbon, Nutrients and Methylmercury in Water from Small Catchments Affected by Various Forest Management Operations

Round 1

Reviewer 1 Report

- "Appendix Table A2: Check unit for parameter ORP".

The topic of the paper focuses on the effects of forestry activities on water catchments and provides data in an area not previously investigated.

The authors present the topic in a clear and exhaustive way.
The methods used are indicated with international reference standards that ensure good data quality. The data is clearly presented in the graphs and tables both in the text and in the supplementary material.

Authors compared the obtained data with other published material and their conclusions agree for the correlation between MeHg and SO42--S.

In conclusion, the paper is clear and well written. 

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

This is a novel study that is the first of its kind in the region, and fills a gap concerning the impacts of particular forest management practices on water chemistry. I think it is quite comprehensive and well supported by references and figures. I have only minor comments and edits, which are marked in the attached PDF.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

General comments

The manuscript is well written and does a great job of interesting several data sets into an interesting story. There are a lot of good data in this study and the concept is novel. The tables and figures are of good quality and the use of existing literature is great. The study is clearly intended to be a short, first pass about nutrient, DOC and MeHg transport in Latvian catchments which is needed data. For this reason, I have aimed to be less demanding on a complete picture being provided of these catchments but it is a limitation of this study.

My main concerns are the experimental design lacks a clear set of hypotheses and there are several disconnects between the findings and takeaway points from the description of the problems. The authors should consider including seasonality into the introduction and objectives. Furthermore, if disturbances are expected to be an important variable controlling nutrient and MeHg export, it should be more clearly described as a goal and include mechanisms. For example, we expected rivers with higher disturbance to have higher concentrations or export rates of nutrient and MeHg because of sediments or DOC released to the rivers. Or we expected higher soil concentrations to be negatively related to river water MeHg because of greater sequestration of pollutants. Right now, there is no clear cause-effect relationship in the study and that is major detraction for being accepted as the manuscript is currently.

Specific comments:

Abstract

Line 18: What classifies as a disturbed catchment is not clear.

Line 21-23: Please rephrase the sentence as it is unclear.

Line 24-26: The information needed to compare forest management activities and disturbances is not articulated clear enough for readers to draw the same conclusion.

Introduction:

Lines 64-93: While I thought these paragraphs were written very well and did a great job of using existing literature, it was a bit long and redundant in some aspects of soil sequestration of Hg, methylation and bioaccumulation. Please shorten, and if possible, combine into one paragraph.

Line 95: Ditch network maintenance is not introduced well. I recommend separating the beginning of the final paragraph and adding a few lines to describe what it is and how commonly it is practiced.

The final paragraph is missing a set of clear objectives and hypotheses for this study. I recommend re-writing lines 107 – 118: to describe what is being tested here? Is this simply monitoring or were the catchments selected purposely to investigate a specific cause-effect relationship?

Materials and Methods

Lines 138: I think it is very important to articulate clearly what is meant by disturbance. Does construction or storm events count as disturbances?

Figure 1: What are the blue lines? It is not clear if they are only surface waters or include storm drainage systems below ground.

Table 1: I like the idea of the table overall, but the characteristics of the sediments are qualitative and not quantitative enough for readers to use. What does “rich” and “poor” mean in terms of concentrations?

Lines 183: The use of standard reference materials and recovery rates of standards solutions should be presented here for MeHg since it occurred at low concentrations.

Line 211: Can you add which packages were used? I liked how the data stacked in your boxplots and will have to use that package!

Results

The first few paragraphs of the results would be more effective if framed to focus on the differences of interest in a consistent way. First descriptive statistics, then if there are differences among major, minor, and no disturbances and then seasonality. The way nutrients, DOC, and MeHg are currently presented is not consistent and makes it hard to figure out differences.

Line 267: Either use ‘catchment’ or ‘flow paths’ to be more specific instead of ‘watercourses’.

Discussion:

I think the discussion about water table and flow rates. The connectivity of the soils to the rivers is unclear in the discussion. Further, the flow rates of the river and potential dilution effects are not clearly covered. Maybe the authors can provide some numbers of qualitative information about output of water across these rivers studied here?

Discussion of the course of the rivers and if wetlands or heavily vegetated areas with slow water movement where methylation occurs is not clearly described here. Maybe the authors can provide some numbers of qualitative information about wetlands in the catchments?

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

It’s a good paper but requires a bit clarification. Some of the comments/suggestions are:

Since rainfall in this area is 636 mm, it is hard to visualize the temporal pattern of rainfall without a graph. The seasonal effect on the results (Figure 2) can be substantiated with the monthly average rainfall pattern derived from a long-term climate data of the area.

A little description on the ditch cleaning practice would be helpful. Was it carried out once a year? For how many days? How vigorous? The ditch cleaning activities may have affected the water quality as reported in this study. So, a bit information on cleaning activities would help a reader to visualize the physical impact on the results.

Figure 1 should show the location of beaver dam which was removed. When was it removed? A bit history of the dam will be necessary. The dam withdrawal action may have impacted results in some area. Which site will represent potential impact of dam removal (based on runoff)?

Line 128: A bit more information on the peat layer is needed. For example, is it predominantly peat or clayey loam, or silty loam type? Peat soil tend to be very acidic. The pH values in water (Figure 2) don’t show so. A map under each sampling site will help ascertain possible source of nutrients and Hg in the sample. Did the peat soil flood with high rainfall event during the study period or anytime prior?

Per Table 1, all the undisturbed sites have high organic content, but DOC, TN and TP values in Figure 2 indicate they have the lowest.  

Line 394-396: From Figure 2, it seems that MeHg/DOC for minor disturbed area could be different from the undisturbed area. The DOC value ranged highly for the minor disturbed area (Figure 2). Is there any potential reason?

I was expecting Site 8 will give results a bit different than other sites as it is outside of the cluster of Sites 1 through 7. Site 8 has the smallest forest area (68%) of total catchment area among all sites and the least drained peatland (2.4%) except Site 3 (Table A1). The effect of peat would be the least for Site 8. Some comparative discussion among the sites with geophysical characteristics would be interesting. Discussion on the effect of forest canopy on the THg and MeHg values among sites will help understand the results.

Table A2: Please include mean value for ORP

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 4 Report

Th authors have addressed the review comments and sufficiently improved the manuscript.