Fine-Scale Assessment of Greenhouse Gases Fluxes from a Boreal Peatland Pond

Round 1

Reviewer 1 Report

This is an interesting paper and I commend the authors for giving insight into GHGs emission of aquatic systems which have been understudied over the years. 

I would suggest the following to help improve the paper:

Abstract:

- Use the known names of the chemical formula i.e. carbon dioxide (CO2), etc, initially so readers have an understanding of what the formula represents.

- Identify what Carex orthostachys is e.g. ....dominated by the herb Carex 

Introduction:

It would be helpful to give some more background on the type of peatland in the surrounding landscapes of your study site. Several authors e.g. Woodman et al (2021) and Tanentzap have shown that the accumulation of GHG in lakes/ponds are often associated with their terrestrial connectivity. The introduction would benefit from a sentence or two on this aquatic-terrestrial connectivity. 

- Line 32: Readers may not be familiar with chemical formulas so would suggest initially identifying them e.g. methane (CH4), etc. and than you can use CH4 throughout the paper. 

- Line 53-54: Suggested rephrasing of sentence e.g., Firstly, through diffusive transport..

Results:

- Would suggest not placing the meterological update as your first results as it seem to be lost on why this info was necessary. You need to connect it directly to your results especially if it influenced the outcome of a measurement/variable. 

- It would be interesting to see what the different measurements looked it vs the average of these measurements across the growing season for each variable. What are the patterns/trends that they are showing?

- Table 2 is showing the TOC for the different zones but you need to identify e.g. what the different measurements produce vs the average, and what the terrestrial connectivity is e.g. how close to terrestrial and aquatic vegetation was your sampling point? Did this influence TN or TOC? If you're assuming that each zone has x amount of TOC base on 4 sampling sites, then you need to make that assumption clear. 

Sections 3.2 to 3.4 are well written. The figures are a bit difficult to read due to the size. 

Discussion:

This section is well written and connected to other known studies within similar ecozones. There seem to be a missed opportunity on connecting the data gathered from this pond to the larger issue of (a) importance of peatlands, (b) aquatic-terrestrial connectivity and (c) policy recommendations. I would suggest a paragraph which could address the above and conclude the paper. 

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

The authors present a study focused on the GHG emissions of a pond in an ombrotrophic peatland. The subject is in principle interesting, as ponds have importance to global GHG budgets larger than their size.

Unfortunately, the manuscript suffers from serious flaws in the way the methods and data are presented. Also the novelty and scientific significance of the results is not stated out anywhere in the manuscript. In the discussion most of the observations made by the authors are stated as finding support from earlier research, and there is nothing which is stated as being something new and unique to this study. More specific comments below.

To begin with the site description, there is no information about how many flux measurement plots there were on the three different sub-sites (bog, littoral zone, pond), and on what kind of surface were the non-aquatic plots placed (hummock, lawn, hollow). Also there is no information on whether there was a collar in the soil for the measurement chamber, and whether there were boardwalks to prevent disturbance of the measurement sites by the chamber operator. Walking around the measurement plot can induce ebullition when the peat matrix is pumped by the weight of the operator. This can lead either to overestimation of the real flux (if the ebullition occurs during measurement) or underestimation, if the peat column under the chamber is temporarily emptied of CH4 if the walking-induced ebullition happens before the measurement. There is no information on how the plant biomass was estimated, what the relative abundance of the species were, what the significance of plant height to the story is, how the height was calculated, or how the active layer depth was estimated.

In the gas flux measurement section, there is no information on how many (or what %) flux measurements were filtered out by the r2 filter (do note that using r2 as the filtering criterion for bidirectional fluxes such as CH4 and N2O in dark chambers filters out measurements with fluxes close to zero, and thus it's better to use for example the r2 of a simultaneously measured unidirectional flux, such as that of CO2 under a dark chamber, as the filtering criterion for a closure). The description of the gradient flux estimation method is really confusing. If you look at Eq. 2, the product of the equation is surely not mmol m-2 day-1. In Eq. 3, the decision to use 0.66 as the exponent is not explained. In Eq. 4, the term t is not described. Also, using dark chambers, the CO2 flux should be called ecosystem respiration. I would also like to know from which depth the soil temperature was measured.

In the results section, I can't see the relevance of the wind rose and the precipitation time series plot, as they are not employed in any of the calculations. In Figure 4., why are the y-axis scales different between the panels? The panels presumably contain the same CH4 measurements..

In figures 3 and 4, why is there no S.E. for the CH4 fluxes from the pelagic zone? If the fluxes were calculated with 3 parallel samples, as described in the Methods, the variation in the results should be presented. Also, if, as it appears, the flux was measured on only one spot on the pelagic zone per measurement, then the estimation of ebullition is very uncertain, as the process is highly localized and there could be large spatial variation in the flux at any given time, and the temporal variation seen in Fig. 5a could be just random variation.

The CO2 fluxes, as I stated above, are ecosystem respiration rates if they were measured with dark chambers as presented in the methods, and comparison between the sub-sites is not meaningful. The real CO2 balance of the littoral zone and the bog depend on photosynthesis as well as respiration, while the apparently non-vegetated pond only has the respiration component.

For the N2O results, there are mixed references to three and four sites, which is confusing. On lines 261-264, there is first a statement that there were factors affecting the N2O flux, and then a statement that there were no factors affecting the N2O flux. Please decide which it is.

In figures 3, 6, and 8, there is no explanation of the panels a and b. A is the temporal variation, where the bars and lines represent mean and SE, b is the spatial variation, where the box-and-whiskers-plot has different elements entirely. This should be presented in the captions.

There is a lot of focus in the introduction and abstract on the littoral zone, but then there is no attempt at upscaling the fluxes to the areas and over the growing season. This would enable the reader to estimate the actual significance of the littoral zone to the GHG fluxes of the pond and its surrounding areas.

In the discussion, 4.1.1, the differences between the ponds and lakes presented is surely something else than the country they're in? If some factors affecting the fluxes are different between the studies, that could be considered here.

4.1.2, has one very long paragraph which descends into a mess of conflicting explanations to the observed results. The language is sub-par, and a thorough revision is necessary to make the subsection sensible. There is unnecessary and confusing repetition of the effect of Carex on CH4 fluxes on rows 297-303. On row 306, what is apomictic material? Trees and shrubs create debris that is slower to decompose than herbaceous species, but it is not at all unsuitable for CH4 production. On lines 307-308 it looks like the authors state that aerenchymatic tissues are not conductive for CH4 transport. Sphagnum doesn't oxidise CH4, microbes on and within Sphagnum do. Anaerobic degradation of any plant material produces CH4, not just of Sphagnum (l. 310-311). L. 316-317, the CH4 emissions from any ecosystem depend on the sum of CH4 production and consumption. L. 318, "very low" should be called "negative" if there is consumption of CH4. L. 319-322, I don't understand what is attempted to say with this sentence.

4.2, on rows 325-334, it is first stated that the driver of CO2 fluxes is the sediment temperature, then that it is the water temperature, and then that it is DOC. This makes no sense. Then on row 343-344 the major driver is hydrological events. So which one is the major driver?

Also in 4.2, on lines 362-373 there is again comparison of fluxes between ponds and lakes in different countries, but no consideration on what could be behind the differences between the observed fluxes.

4.3, line 375, there surely is a lot of temporal and spatial variation in N2O fluxes, if you look at Fig. 8? LInes 389-393, where is this data? On line 403-404, is the pond in Finland a larger or a smaller sink than the pond studied here?

In conclusions, line 416, what is the boundary model approach? This is the first mention of it in the manuscript.

Line 429, what does this mean? How is data availability "not applicable"?

Author Response

Please see the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have improved the manuscript somewhat and reacted to some of my previous comments; especially the most confusing original paragraphs have been made significantly clearer. However, there are still items missing. Perhaps my review style was not clear; when I make a question, I like to see it answered in the manuscript rather than in the author's reply. Similarly, if I state that I can't see something in the manuscript, I'd like to see it added into the manuscript rather than just explained in the reply.

Specifically,

1. there is still no information in the manuscript on what type of microsite the measurement plots were placed (hummock/string, lawn, hollow)

2. the method of estimating plant biomass is missing, and the relative abundance of the species is not presented (this is not so important if there is no upscaling or any effort to explain the fluxes by the vegetation)

3. the number of accepted/rejected measurements is not presented, and my concern that low fluxes might be improperly discarded is not addressed

4. there is still no S.E. presented for the points (figs. 4b, 6) or bars (fig. 4a) that represent more than one measurement. This makes one wonder whether the deviations between measurements were taken into account when calculating the fits presented in the scatter plots?

5. my concern that the values presented as CO2 fluxes are actually respiration measurements was not seriously taken into account

Author Response

Please see the attachment.

Author Response File: Author Response.docx