Comment on bg-2021-235

Lines 21-25: this info is correct but as a 'textbook knowledge' is unsuitable for the

I have several general minor-to-moderate comments which (I hope) will improve the readability and the reception of the manuscript.
The language -please try to be as specific and consistent as possible. Dealing with isotopes and environmental controls, the vocabulary can be daunting, especially for less familiar readers. Please, when talking about 'precipitation' note each time if you refer to atmospheric (rainfall) or carbonate precipitation. Also, perhaps it is worth to explain once and upfront (but not as in the present version in the abstract) all the environmental factors influencing isotopic composition of carbonates and their direction. As of yet, provided explanation is correct but condensed to two long and complex sentences in the abstract. Again, please keep in mind readers less familiar with principles of stable isotope geochemistry and shrieking when 'fractionation' is mentioned. The fact that oxygen isotope fractionation is temperature-dependant, but the process happens (1) in the atmosphere and (2) in the ambient water, and drives the isotopic composition of water/ carbonate in two different directions is probably best explained using a simple sketch? I do agree that a picture is worth a thousand words, and in this case a well-designed but simple figure could improve the clarification of processes influencing d18O in lacustrine carbonates. Such figure would be a great asset in the introduction. Shall you decide to leave out the sketch option, please explain the processes consequently starting with atmospheric temperature effect on rainfall oxygen composition and lake water composition (additionally through evaporation) and only then move to ambient water temperature influence on carbonate precipitation (modified by vital offsets).
In the chapter 'Material and methods' the 'material' is actually not described. An SEM image of Candona, an SEM or macro image of Chara elements and perhaps a macro image of Pisidium would be a good addition. Also, I would welcome a sketch of Chara components (branchlet and internote) as I am familiar mostly with oospores and it took me a while to understand what to you refer to as 'encrustation'.
Field sampling. I wish to see a more detailed information on field sampling. How do one take a less than 1 cm surface sediment (with a small shovel) from a water depth of more than 1 m? I imagine that one needs to employ a diver? How was the water sampling in 2013 and 2014 done? With Niskin Bottles? How was the Chara sampled? I see no justification for sampling Lake Blaktjärnen -its Chara results are not well incorporated into the rest of the paper. Please, if you want to keep them make sure that the reader knows why they are relevant and how they fit into the general picture. Having said that I find the conclusion misleadingly presented. I agree that differences in vital offset -corrected d18O values of different carbonate components suggest different periods of formation and might point to the amplitude of seasonal temperature contrasts. This holds true only if several components are extracted from the same sedimentary layer and their isotopic composition is compared and contrasted (conclusion 1). However, this information is interwoven with influences of lake water d18O and temperature. By the time the reader reaches conclusion 2, the essential notion of comparison is already forgotten, and it reads like any seasonal change in water temperature is clearly reflected in d18O of any biogenic carbonate, and I cannot agree with this statement. The order of arguments provided in conclusion 2 does not strengthen it either. Please, streamline the arguments towards the conclusion, not away from it. Again, a well-designed sketch in the introduction, could help in making this conclusion more succinct. Conclusion 3, while correct, is very loosely formulated and, in its present form reiterates the findings of McCormack & Kwiecien 2021. Your work deals with a more complex example and is the first such comprehensive attempt of comparing carbonate components from shallow water, above the thermocline of an open lake (as explained in conclusion 2). I think that focusing conclusions on this particular case and making them more specific will be very beneficial.

Introduction
Line 36: 'depending on the local context' Lines 308-313: information provided here is correct, but it is not a result Lines 326-329: information provided here is correct, but it is not a result Line 326: exobiotic mentioned for the first time without explanation Discussion Lines 344-346: correct information but should be better explained in the introduction (see general comments) Lines 449-453: without clear reference to a figure, I cannot see how your results demonstrate these two points. Also, the points are very vague -what do you mean by 'sufficiently large'? How do you know or how can you test what is an 'representative average'? Please, try to rethink this argument.

Figures
All figures are informative but with small adjustment they could convey the message more efficiently. Fig. 2: Please, indicate clearly 8 m water depth mark (the deepest sampling point). If the grid is necessary in the figures, please, align the legend within the grid boxes. Also, please put the data points in the foreground not in the background. The present effect is visually unsettling.    Table 1: The species, instar and the no. samples are the same for both panels, I suggest merging them into one. For the consistency, I would suggest adding all data presented in figure 7 (including 'fine calcite', 'fragmented encrustation from surface sediments' and Chara samples from Lake Blaktjärnen). Please, also indicate if these are measured or vital offset -corrected data. Last comment here -please try to keep the terminology consistent throughout the main text, figures and figure captions and the table.
To wrap up, I think this is a really valuable contribution showing pitfalls of using single carbonate component and highlighting the interpretational difficulties but, also benefits of multi-component analyses, and I very much wish to see it published. I hope that authors will find my feedback helpful.