Stieg review BioGeosciences

1. Does the paper address relevant scientific questions within the scope of BG?

BG seeks interdisciplinary studies that look at the interactions between e.g. biological, chemical and physical processes. Steig et al. do this by taking a palaeolimnological approach to investigating biological and organic geochemical records in a relatively remote region of eastern Siberia.

1. Does the paper present novel concepts, ideas, tools, or data?

High resolution diatom analyses and organic geochemical analyses are well established in palaeolimnology, including multiproxy studies. However, there is novelty in these data in the presentation ofd ³⁰Si of the diatom silica in the Lake Khamra sediment record. Records of d ³⁰Si_diatom are still relatively uncommon in lake sediment records, yet they have the potential provide much needed information on e.g. lake biogeochemistry, dissolved silica utilisation etc. What makes their record stand out even more, in my opinion, is that the data are presented at a decadal resolution for the past 200 years.

I think that there is also a missed potential of presenting these d ³⁰Si_diatom  records alongside their d¹⁸O_diatom records from a previous publication – having joint isotopes from the same diatom material, alongside the diatoms themselves, has great potential for quite deep lake-catchment ecosystem understanding.

Moreover, although d¹⁵N data are presented, they are barely discussed at all, which I found surprising given the discussion on potential acidification of the lake, changes in atmospheric reactive nitrogen etc.

Just thinking about it more, their samples actually have 4 different types of stable  isotopes investigated but little is made of this.

1. Are substantial conclusions reached?

Substantial conclusions are reached, but I don’t agree with one of the main findings that their record provides evidence for lake acidification.

1. Are the scientific methods and assumptions valid and clearly outlined?

Largely yes, except for the use of chrysophyte scales to create an index to be interpreted as a record for lake acidification.

1. Are the results sufficient to support the interpretations and conclusions?

Largely yes, but not for lake acidification

1. Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)?

Yes

1. Do the authors give proper credit to related work and clearly indicate their own new/original contribution?

Yes

1. Does the title clearly reflect the contents of the paper?

I don’t think that their study really does focus on anthropogenic pollution. No pollutant data are actually shown (only referred to) and I question the validity of using a chrysophyte index as their evidence for lake acidification.

1. Does the abstract provide a concise and complete summary?

It will need to be revised

1. Is the overall presentation well structured and clear?

It makes sense to present data in order that these data are then discussed.

1. Is the language fluent and precise?

Overall yes, but the authors needs to be more careful in use of e.g. ‘statistical’ when no statistical tests have been done, and ‘various’ which leaves the reader to make guesses. And sometimes statements are rather vague, or need further supporting evidence. I have highlighted these where relevant below and in the annotated PDF

1. Are mathematical formulae, symbols, abbreviations, and units correctly defined and used?

I think I found one mistake in how the diatom fluxes are presented – details are given below.

1. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?

I think that the figures need to be re-ordered so that they are presented in the order that they are subsequently discussed in. The discussion is overly long and could be more focussed

1. Are the number and quality of references appropriate?

Largely yes, although I think that there are many more studies from Siberian lakes that have looked at recent environmental change. Also, I’d like to see better use made of studies that detail diatom autecologies -these will help in the diatom interpretations.

1. Is the amount and quality of supplementary material appropriate?

If my suggestions for corrections are found useful, then I don’t think any SM is necessary

Diatom shifts and limnological changes in a Siberian boreal lake: impacts of climate warming and anthropogenic pollution

Overall review

This is an interesting palaeolimnology study that provides much needed information in lake ecosystem changes in a relatively remote region of eastern Siberia. The authors take a multiproxy approach to understand how environmental change has impacted Lake Khamra over the past 200 years. The datasets are excellent, with radiometric dating suggesting that a decadal resolution has been reached throughout the time-period investigated. This is no mean feat, as lakes in these cold regions often have very slow accumulation rates, making records of recent change difficult to decipher. This is not the case here.

The main datasets presented include biology (diatoms) and organic geochemistry. These are palaeolimnological mainstays. However, there is also an excellent dataset of diatom silicon isotope analyses d³⁰Si_diatom, that is of decadal resolution. There is also a record of Mallomonas chrysophyte scales, but more on that below. The study details ecological and geochemical changes over the past 200 years that are concurrent with other studies in the region and further afield, even in different continents (e.g. North America, northern Europe) of the impact of warming on ice-covered lakes. A convincing key finding from Steig et al. is a shift in geochemical properties of the lake (ca. 1950 CE) that precede shifts in diatom communities a couple of decades later (ca. 1970 CE), trends that are exhibited in other cold regions of the northern hemisphere.

Less convincing however, is the suggested impact from pollution, causing the lake Khamra to acidify in recent decades – here I think the study is not critical enough in the evidence presented. Indeed, as detailed below, there a few parts of the study where a greater and more critical examination of the literature is required.

Overall, I recommend publication, but major revisions will need to be made. I provide more detailed comments below, and in the attached PDF.

Specifics

Title: As will be clear below, I’m not sure that the title is appropriate with regards to impacts of anthropogenic pollution. I didn’t see any keywords on the manuscript, but I think the term ‘multiproxy’ would be a good one to add, either to the title or as a keyword.

Introduction:

Line 49- : the statement “These lake observations align with the proposed onset of the 'Anthropocene', a geological epoch beginning in the mid–20th century, introduced by Crutzen and Stoermer (2000) and further supported by Zalasiewicz et al. (2017).” Is rather out of place, now that the Anthropocene has not been retified as a formal Epoch. Use of the term Anthropocene is still of course valid, but requires a more nuanced context.

Lines 56-57: I want to push back a bit on the statement “However, there is a notable gap in understanding how these global changes impacted remote and less studied regions like Siberia.”. I think that there are a considerable number of studies showing evidence of recent environmental change over the past 150 years in Siberia that could be used to here, but I do agree that much more research is needed for this region.

Lines 65-66: you attribute “reduced lake ice cover duration” and “stronger thermal

Stratification” since the 1950s to Roberts et al. 2018, but both those statements are linked to work done by other studies that Roberts et al. have cited. I think my point here is to make sure that when you cite a study, you cite it for the research it itself has done.

Line 75: Leng and Barker reviewed oxygen isotopes in diatom silica, not silicon isotopes.

Line 82: I don’t think I would call Hg a biogeochemical proxy

Line 90: “…the lakes investigated are either not located in Siberia…” but this is not really true. For example, even from my own (and collaborators) and other Russian-led work, there are many studies looking at palaeo records in Siberia for past 150 years.

In this section of the Introduction, in providing a setting for their study, the authors justify why their study needs to be done by saying where other studies have been done, with few in Siberia. But the case made is not convincing as sources used are rather old and not comprehensive. I think the authors should have confidence that their own study importantly adds to a growing body of work from Siberia and the Yakutian region especially. Maybe a route through this would be to focus in on the Yakutian region more, as Siberia is vast, over many eco-regions N-S, E-W etc, with many competing drivers of environmental change.

Lines 94-98: With the sentence starting “Beside oxygen isotopes, silicon isotopes of diatoms…” the rationale for Si can be better expressed as an indicator of changing productivity and nutrient utilization etc, which has been found in other lakes, including Baikal (see Virginia Panizzo-led articles).

Line 99: be careful using terms such as “significant” when significant tests were not undertaken. This is done many times throughout the manuscript

Lines 104-107: with the objectives, I’m not sure that erosion input linked to climate variability has been shown, as no proxy for erosion has been specifically measured (possibly % dry weight might work here, but something like magnetic susceptibility would have been a better indicator). Overall, a much more critical approach needs to be taken on the impact of pollution on diatoms in the lake.

Section 2: Overall, the methods section is excellent – really well written.

Line 159: state  / summarise how water content and DBD were determined, (with full details given in Stieg et al. 2024a). For example were these determined through a scanner or through physical drying in an oven). I think it would be good to have MAR shown in one of the profiles.

Section 2.4: This really is an impressive highly-resolved dataset

Lines 222-223: the range of flora used are quite European focussed. Were any other guides used, especially Russian flora?

The only approach I have an issue with is the use of a Mallomonas index as a proxy for lake acidification. Greater justification for lake acidification needs to be made, because I was quite surprised to hear that acidification could occur give that the catchment consists of  “Cambrian bedrock composed of  alternating dolomite and limestone”, rocks that are rich in acid-neutralising cations. In the introduction to the study, while the threats of climate change are well set out, it is not made clear if atmospheric pollution and lake acidification are real threats to the region.

Section 2.6: lines 261-264, is there a mistake with the units?

If MAR = g/cm2/a, and concentrations are 10^7 valves/g, then that will mean DVAR = 10^7 valves /g/cm2/a. I assume the authors are multiplying by 100 to give 10^9 valves /g/cm2/a (although I’m not sure why!). But DVARs are then being expressed as 10^9 valves /m2/a and this is wrong - it should be 10^9 valves /cm2/a. The /m2 was calculated for reporting the changes in carbon fluxes. If you were to go from /cm2 to /m2 you'd need to multiply the diatoms by 10,000? Or am I missing something here?  

Line 263+ 266: as the data being correlated are from a sediment core, they are majourly affected by temporal autocorrelation and not truly independent. Therefore, Pearson correlation is not appropriate here. Moreover, as mixed data types are being correlated (and data not tested for normality), a Spearman-Rank correlation would be more appropriate. Lastly, I'd suggest a more rigorous p value as your data are not independent e.g. 0.005 at least. I’d actually question of correlating these data to such an extent is even warranted as I don’t think this analyses adds to the study.

Lines 288-290: There is an interesting decline d¹⁵N since the 1960s -is this similar to studies of remote cold lakes in e.g. North America where a decline in also observed, and if so, is it related to increase in isotopically lower nitrogen from anthropogenic sources? (I note however that the magnitude of decline in Khamra is much lower than in other studies).

Figure 2: an effective way of comparing geochemical trends with the diatoms would be to plot here the PCA axis 1 and axis 2 sample scores. I see that these are presented in Figure 4.

Figure 3: Are the authors certain Fragilaria cf. gracilis is tychoplanktonic (where was this information obtained from)?  Fragilaria cf. gracilis has recently been suggested to actually be F. radians. If that is truly the case, then this would mean F. gracilis/radians a planktonic taxa not tychoplanktonic. https://fottea.czechphycology.cz/artkey/fot-202202-0009_fragilaria_radians_k_tzing_d_m_williams_et_round_the_correct_name_for_f_gracilis_fragilariaceae_bacillari.php#:~:text=Fragilaria%20gracilis%20is%20one%20of,rivers%20to%20even%20eutrophic%20lakes.

Overall a succinct account is given of the main changes in the diatom zones.

Figure 5: This is a nice figure. But I think the colours for the triangles for the two time periods could be more clearly contrasted (even with different shapes)

Discussion

The Discussion is overly long at c. 5500 words alone. Perhaps my comments below will provide some scope for a reduction in length and a clearer focus. I think there is too much repetition between the results and discussion, and a stronger discussion would be one that links together the different proxies more effectively.

Line 392: Aulacoseira are also common in lakes that stratify for part of the year, e.g. during ice cover or in warmer summer months. Not all Aulacoseira species are large, but there different sizes does have implications for resource competition for e.g. dissolved silica

Line 399: “…and a seasonal ice cover does not necessarily reduce its abundance” – indeed! I don’t think diatomists would ever think this the case because the majority of lakes are dimictic.

Line 400-402: I'd agree with this statement, but changes in temperature between air and surface waters, and wind strength & direction are also very important in causing overturn, not just snow-melt.

Line 415:  The authors suggest that diatom concentrations “likely indicates favourable growth conditions at Lake Khamra that enhance diatom productivity”. Possibly, but caution is needed here.

Concentrations are dependent on SARs, so if accumulation rates increase while diatom production remains constant, then diatom concentrations will decline without without needing to infer low-productivity lake. The authors know this, as elsewhere it is stated that DVC vary with SAR. Also, it looks like you are comparing DVC with an average of your whole core, with surface sediments from other cores, which will likely have higher SAR as sediments / core compaction is minimised at the surface layers.

I think it is important to make the decision early on in the study if you are going to present and discuss DVCs or DVARs. I’d recommend the latter, and DVCs do not therefore need to be discussed at all beyond the calculation being an intermediate stage to get to DVAR.

Section 4.2:

I found this section quite difficult to keep track off. There is repetition with the results section, when changes in diatom abundances are described by zone.

There are a lot of inferences being made with regard to changes in diatom relative abundances, and it's hard to decipher which of these are likely to be real or simply a function of having species expressed as relative abundances (as one species increases, others must decline etc).

Rather than having a discussion based on the zones (which is rather descriptive), a higher-level discussion that just focuses on the main trends would be better.

When I look at the stratigraphy in Fig 3 in relation to your discussion I’d recommend that authors consider:

1. what are the ecology and habitat preferences of each of the taxa shown - draw on information from papers that look at their ecologies (lab, modelling, field).

1. provide some summary plots on the stratigraphy: total planktonic, total tychoplanktonic; total benthic; PCA plots of axis 1 and axis 2 sample scores. These will all help with the interpretation and could help ensure over-interpretation of the percentage changes is not being done.

1. As you have calculated total diatom fluxes, how might the stratigraphy look if all the major taxa were shown as fluxes. This means that changes in their abundance are independent from each other.

Lines 431-434: Not sure if this is a good comparison to make. Stephanodiscus species normally need less silica and do better in waters that are sufficient in nutrients - e.g. see classic Kilham et al. 1986 paper (and others) https://aslopubs.onlinelibrary.wiley.com/doi/abs/10.4319/lo.1986.31.6.1169

Lines 459-460: Be really careful of interpretation. I would not rely on a PCA biplot to inform competition ecology between your species. Go to source material for those species ecologies.

Lines 499-503: There is far too much conjecture here for so many reasons. The authors have not provided the evidence to support the claims being made as they are not comparing like (ecosystem) with like (ecosystem).

Lines 527-529: A lot of caution is required here when making comparison to Biskaborn et al. 2021b. for a couple of reasons:

First, the resolution of the stratigraphies in Biskaborn is *much* lower than for Khamra, and so there are major uncertainties in tying together what look to be similar patterns.

Second, their interpretation requires greater critical consideration when interpreting the data presented. The two shifts between major taxa - one around 1850 CE or so, and one in the 1970s - must be responses to different drivers.

• The shift at c. 1850 CE I think will be climate related, with the northern hemisphere broadly moving out of the cooler Little Ice Age. While many countries were industrialising at that time, you would need to provide economic evidence that this was also occurring in your region to cause acid rain. I’m not an expert in this region, but I don't think this exists, and no evidence is presented to show that it does exist. We see similar shifts in diatoms across Lake Baikal for example, that had little industry around its catchment.
• The shift in the 1970s is likely related to global warming and possibly increased reactive nitrogen from pollution. You have perhaps d¹⁵N evidence of this, although the decline is rather small.

Section 4.3: I wonder if it would be better to layout the changes in organic geochemistry in the lake / catchment system before the discussion of the diatoms. Then relate the diatoms to changes in the broader organic geochemistry environment?

Lines 571-572: The authors make the assumption that because climate is inferred to be drier at this time period, then erosional input from the catchment is reduced, but no evidence is produced for a decline in erosional material. While it may be the case that slower accumulation rates are linked to less catchment input, given the planktonic diatoms dominate the profile it could be as likely that sediment accumulation is linked to primary production in the lake, and if conditions are cooler and drier, then there may be less primary production.

Line 592: I think that Fig A5 should actually be shown in the main body of the paper.

Lines 604-606: again, be careful of over-interpreting data. At this site, ice cover, snow cover, depth of mixing etc are all going to be much bigger determinants of the amount of light the planktonic diatoms can use than changes in solar variability!

Line 669: Fig 2: It's odd having the figure for geochemistry being so far away from its discussion. I'd recommend re-ordering the figures, or reordering the structure of the discussion.

Line 670-671: The Suess effect can be detected on d¹³C values from c. 1850CE and the start of the industrial revolution....

Lines 687-690: This is quite a conclusion to make. If we look at the d³⁰Si profile and e.g. A. subarctica, they both show a concurrent response at 1830 CE for example. d³⁰Si  values do not need to change very much to infer quite substantial changes in resource availability and nutrient cycling.

Lines 700-701: A statement is needed here as to what this level of (Hg) pollution is, contextual wise. Is it similar to background increases across the northern hemisphere? Or does it indicate local origin? Are the concentrations high enough to indicate major sources of pollution that could have caused the suggested impact on your lake?

Lines 703 - : I do have a problem with the use of the Mallomonas scales as part of an acidification index in this lake.

• While lakes in base-poor catchments may acidify due to acidifying pollutants, the catchment has dolomite in it, which surely would buffer against potential acidification
• if increasing Mallomonas scales are also indicative of increasing nutrients, then would freshwater acidification also occur?
• the increase in stelligera is not a diatom I recognise from an acid flora.
• pH 6.07 is not a low pH; lake acidification becomes problematic when pH values fall below a critical level of 5.6.
• Remember a correlation between Hg and Mallomonas scales does not mean cause and effect. Is the amount of Hg increasing in the lake likely to be an indicator of enough acid rain to cause lake acidification?
• A lot of this discussion of acidification is based on the conclusions drawn by Biskaborn et al. 2021b. Biskaborn show that Mallomonas scales are actually only found at depth in two of their four cores. And in the other two cores, scales are found only in the surface sediments and therefore cannot represent any acidification trend (for example in their dated PG2203). So during their 'industrial period' (their term), there are no Mallomonas scales apart from the very surface layer. In PG2208, there are scales down-core, but these start to increase before the period of 'industrialisation', and therefore cannot be attributed to acidification either.

Personally, I’m not convinced that the Briskaborn study shows (indirect) evidence of acidification at all. But regardless of my opinion on a separate study, the authors of this current study need to be more critical in the evidence presented for lake acidification.

Line 721-724: The increase in A. formosa is very small (c. 4 to 6%?) - this is likely just natural fluctuation. Moreover, it is certainly smaller than the increase around 1810 - 1820 CE. What is the explanation for the larger fluctuation here - it can't be acid rain

The authors provide a multi-proxy paleolimnological study from an understudied region in eastern Siberia, providing important limnological data on how lakes from this part of the world are responding to climate and other environmental change. The methodology and study design are sound and their arguments are generally well supported with literature (but see below my comment with respect to how chrysophyte scales were interpreted incorrectly as well as  N deposition). The paper is nicely organized and generally well-written. Some grammatical fixes are needed and I’ve tried to help where I can. The manuscript should be acceptable for publication with minor revisions.

The 2 issues I note above are detailed below: (but result in only minor changes in writing)

• The authors mis-read one of my early paper on chrysophyte scale fossils and acidification. The authors cite my 1984 Nature paper – this showed how chrysophyte scales (well 2 species of chrysophytes) could be used for indicating lake acidification and led to many other papers. BUT almost all chrysophytes thrive in circumneutral and alkaline waters – it was mainly only 2 taxa that replaced the many circumneutral and alkaline chrysophytes) that indicated acidification.  There are many papers from my lab and elsewhere documenting this.  So, the first issue:  there is no evidence for acidification from chrysophytes or any other indicator.

• Second issue from the chrysophyte interpretation is the incorrect statement and interpretation that chrysophytes indicate higher nutrients. The authors cite a 40 or so year old paper by Munch in support.  I have not re-read that old paper but if Munch wrote this over 40 years ago, she was incorrect.  Chrysophytes thrive especially in oligotrophic waters – and thrive with declines (not increases) with N and P additions.  Opposite to what was said in paper.   Chrysophytes have diverse nutritional strategies and are flagellated…  allowing them to thrive in well-stratified and very nutrient-poor waters.  In fact, back when I was a student, I even suggested an index of diatom frustules to chrysophyte cysts to indicate eutrophication  (i.e. higher chrysophytes indicating more nutrient-poor waters, not eutrophication). See Smol, J.P. 1985. The ratio of diatom frustules to chrysophycean statospores: a useful paleolimnological index.  Hydrobiologia 123: 199‑  But there are many other papers and reviews documenting this.  So, the chrysophyte indicate lower nutrient, not higher.

• The above are actually minor fixes in the paper. And in fact the chrysophytes STRONLY support the diatom and other proxies in that there was NO nutrient nor pH additions from deposition, and the changes you are seeing are solidly linked to declining ice cover and increased thermal stratification.  We (and many others) have ben using the rise of chrysophyte scales in sediments to indicate warming and especially increased thermal stratification.  Scaled chrysophytes thrive in oligotrophic, and well stratified waters.  They are flagellated and are especially competitive in stratified waters since they can control their position in the photic zone etc.  So again, the chrysophytes strengthen your argument that this is a climate (ice cover and thermal stratification story) and argues against any aerial deposition story.

We have many papers on the above, showing chrysophyte scales increase with stratification – but here is one very recent one just published:

Favot, E.J., Rühland, K.M., Paterson, A.M., and Smol, J.P. 2024. Sediment records from Lake Nipissing (ON, Canada) register a lake-wide multi-trophic response to climate change and its possible role for increased cyanobacterial blooms. International Journal of Great Lakes Research 50: 102268.

Or go back earlier and see:

Ginn, B.K., Rate, M., Cumming, B.F., and Smol, J.P. 2010. Ecological distribution of scaled-chrysophyte assemblages from the sediments of 54 lakes in Nova Scotia and southern New Brunswick, Canada.  J. Paleolimnology 43: 293-308.

In summary, then, it seems that the profiles you have match perfectly with your climate interpretations. What we have been seeing and publishing in many lakes is an increase in scaled chrysophytes, like Mallomonas, with increased thermal stratification and other climate-related variables.  Being planktonic, they can thrive in well-stratified waters, controlling their position in the photic zone. Similar to the Discostella change, we often see a rise in chrysophyte scales with warming.  It seems this matches your interpretations perfectly?  So, warming seems to be the driver, but not acidification nor eutrophication.

I address some of this further below with some minor suggestions, especially with respect to the discussion.

Minor comments

Line 19  remove comma after Siberia

Line 23 and elsewhere in text.  I think you should put approximate signs ( ~) whenever you say  ~xx year ago… and ca. before all dates.  All our dates are approximate.

Line 25. Change less-examined to understudied

Line 29 spell out Discostella as first use

Line 38 same for Asterionella (but see my comments on Asterionella Formosa and climate and not nutrients – Sivarajah et al – as one example)

Lines 38 to 39 – see my comments that an increase in Mallomonas does NOT indicate acidification and nutrients, but oligotrophy and warming

Line 58. Change experiences to has experienced

Line 62. Unclear what is meant by anthropogenic alteration on the ecosystems. Can you provide examples?

Line 64. Change There are evidences to There is evidence

Line 95. Change form to from

Line 99. Suggest, In this study, we examine subfossil diatom assemblages in Lake Khamra and explore whether any changes are consistent with recent climate warming as has been documented in many temperate lakes throughout North America and Europe.

Line 105. Suggest adding the timeframe of the study to objective 1. (I) identify historical lake ecosystem changes within a continuous diatom assemblage record spanning the past ~220 years,

Fig. 1 caption. Change “drilling position of sediment core” to “coring location of”

Methods. The core sectioning details are unclear to me.  When it is stated that “The rim material (<0.5 cm) was removed to avoid possible contamination due to mixing”, does this mean the core was split and sectioned horizontally? Was there no loss of the surface-most sediments? Was anything done to preserve the sediment-water interface (gel seal? etc) when transporting the core back the lab?

Line 213. Change The slide preparation followed the common procedure to Preparation of slides for siliceous microfossils followed common procedures

Lines 228 to 229 – good to have this index BUT it does not indicate acidification and increased nutrients, but increased thermal stratification etc… see comments above.

Line 309, you write “cyclotelloid genus Aulacoseira”  -- Aulacoseira is not a cyclotelloid diatom --  I think you meant centric or colonial or centric colonial diatom

Lines 358 to 359 – yes, clear indication of warming and stratification in your Mallomonas index.

Line 435. Thrives instead of thrive ----    but true you can have A Formosa in higher nutrient waters, but also thrives in oligotrophic.  See for example:

Sivarajah, B., Rühland, K.R., Labaj, A.L., Paterson, A.M., and Smol, J.P. 2016. Why is the relative abundance of Asterionella formosa increasing in Boreal Shield lakes as nutrient levels decline? J. Paleolimnology 55: 357-367.

This taxon has been increasing strikingly in an area of known declining N deposition and in ultra-oligotrophic waters. We had long-term N deposition in this area showing striking declines as well as in-lake N water chemistry data– and that is when A Formosa thrived and increased.

Line 458. Hill’s N2

Lines 443, 509. Basionym rather than synonym

Line 513. From the northern hemisphere

Line 543. Italicize Fragilaria

Line 568. Replace broader with “most ecologically significant”

Lines 656, 698. Besides, not beside

Lines 703-704 – incorrect interpretation of Mallomonas and acidification as noted above.

Lines 709-714 – same error (see previous comments)

Line 720 – see Sivarajah et al paper on A Formosa and N that I discussed earlier

Line 720. As noted at start of review, an argument is made that atmospheric N enrichment may have caused diatom assemblage shifts and limnological changes. However, the authors do not discuss their d15N profile, which is the primary proxy to either support or refute this hypothesis. The d15N profile in Fig 2 shows no trend of depletion that is outside natural variability over the past 220 years. This is important because it allows the authors to conclude that the increase in D. stelligera and A. formosa in recent years is not related to atmospherically-derived nutrients, as is often questioned with these types of diatom shifts. It is fine to cite these other studies, but your data clearly show no influence of atmospherically-derived N deposition. Also worth reiterating here is that long-range transport of contaminants, eg Hg, is noted to occur at this lake.

As noted above, the author also incorrectly used the increase in Mallomonas scales as indicating aerial transport, but the chrysophyte results indicate exactly opposite interpretations to this conclusion.  The important change in chrysophytes near the surface indicates low nutrients and do not indicate acidification.  In fact, we and many others, see changes like this is scaled chrysophytes indicating warming – these taxa thrive in thermally stratified waters (their flagella etc give them important advantages in oligotrophic and stratified waters).

The authors have a much simpler (but important) story here – a clear indication of the lake changing markedly with less ice and stronger and longer thermal stratification.

Nice contribution.

John Smol