This is a fascinating study about seasonal dynamics of snow microbes during the melting season at a location below canopy. Despite much research has been performed with blooms in snowpacks above timerline or on glaciers, the special situation of snow algae blooms under decidous trees in Japan still deserves more attention. The authors did an outstanding work in repeatatly sampling the same site throughout the season, developing a picture about temporal changes and vertical shifts within the community. The data are about snow algae, microinvertebrates and snow fungi.The new insight of this work is that birch buds fertilize snow and support phototrophlic microbes.

Bacterial abundances were not assessed because no molecular protocols were applied to describe the communty by metagenomics. Snow chemistry was evalated well, but changes in the snow-water-content, which are expected to influence vitality and migragion capacity of organisms within the snow, were not recorded. Please regard this comment as a suggestion for future studies. 

The results are very detailled and sufficient figures and literature is provided. The manuscript sometimes has repetitive elements in argumentation or conclusion and the text could be condensed here and there without effecting the scientific message. 

I suggest to alter the title in a way like "Temporal and vertical changes in microbial snow communities during the melting season below canopy in Northern Japan"

My comments in detail:

Line 10, start sentence with "During snowmelt, diverse ...

Line 23, delete "for the snowpack layers".

Line 32, algae bloom "in" snowpacks, not only "on"

Line 38, delete "during the melting season". Moreover, nutrients are not only deposited by snowfall and rain, but also airborne by winds.

Line 45, 46: Here, a very important reference is not cited (though later in the discussion): Hoham et al. 2008, who described in detail that balsam fir litter supports the growth of snow algae. Tuhs, in general, it has already been known that leaches from vascular plants support algae blooms.

Line 55: delete "to the snow surface", because it was never observed that flagellates swim actively to the surface, they stopp their migration earlier in time to reach a favourable layer below.

Line 56, insert "sometimes" before "concentrated", because this situation described by Hoham 1975b does occur all the times. 

Line 61, snow "particles", I guess you rather mean "crystals"?

Line 63, maybe you prefer to cite a much newer and well fitting reference about the swimming activity of snow algae instead? See: Détain, A., Suzuki, H., Wijffels, R. H., Leborgne-Castel, N., & Hulatt, C. J. (2025). Snow algae exhibit diverse motile behaviors and thermal responses.   io, 16(5), e02954-24. https://doi.org/10.1128/mbio.02954-24

Line 66, add to sentence "during the season."

Line 97, start sentence probably with "First surface green algal blooms ..."

Line 101, start sentence probably with "The remaining snow depth ..."

Line 106, so the two snow surfaces sampled adjacent to the blooms were your controls? If yes, state them as "white snow" control or similar.

Figure 1: Were all the photos taken at the same time of the day?

Line 112, please state the brand/type of ice auger

Line 122, which mesh size had the sieve?

Lines 138 - 140: It is a flaw that the classes of snow algae (A-D) are not described here! Ok, the authors perform no morphological identification based on frozen and rethawn material, which is difficult, but still, which would have allowed a tendative taxonomical addressing by LM even without marker sequencing. In this context, it is wrong to say that vegetative cells could be determined from the material, because Matsuzaki describred his key using laboratorial strain material. Contrary, with field material, the identification of non-vegetative cysts or spores would have been easier, even from frozen material. The reader does not need the information about exact species ID to understand the reported communty dynamics, but stillt the 4 morphological classes (in fact stages, because it does not mean 4 different species!) should be briefly presented. 

Line 149, elution "from" tree-derived litter?

Line 177, delete last word "season"?

Lines 195.196, here again, it is not acceptable just to cite a reference to describe which organisms were present. At least the supplement should provide a brief description of the snow algae stages A, B, C and D, ideally with a microphotograph. The tardigrada and the snow fungi information is clear.

Question: were any snow ciliates observed during the study?

Line 200, maybe you mean "at the first time of sampling" instead of "for the first time ..."

Line 215, maybe you mean "from which on" instead of "at which"?

Line 252, delite "chemical" in headline?

Line 306, microbial growth: of microinvertebrates? Because algae thrive already in the first month of snowmelt?

Line 315, "fatty acids and in the form of cysts", I guess you mean "fatty acids and other compounds like osmolytes, which increase during cyst formation" ? Another reference describes that the cyst morphology itself supports freezing tolerance: Ezzedine, J. A., Uwizeye, C., Si Larbi, G., Villain, G., Louwagie, M., Schilling, M., ... & Maréchal, E. (2023). Adaptive traits of cysts of the snow alga Sanguina nivaloides unveiled by 3D subcellular imaging. Nature Communications, 14(1), 7500.

Line 318, maybe insert "in the upper snow layer" after "growth"? The deeper snow pack is hardly influenced by air temperatures.

Line 322, the "upper" snowpack?

Line 326 - 329: this needs more explanation, are you talking about the local climatic conditions and composition of rainfall, which is here influenced by oceanic conditions? If yes, make this more clear to the reader and insert at line 328 "in this region" after "growth" and maybe give another reference about precipitation in Northern Japan?

Line 345, "was" instead of "is"

Line 346, which period? The study period?

Line 360, delete "forest" in the headline?

Line 375, 376: please discuss reference Hoham et al 2008 in more detail and compare your results. In their study, it was needle trees which fertilized, in your case buds of broadleaf trees. This reference should already be mentioned in the introduction.

Line 377: rather "below" instead of "in" the tree covered-area?

Line 378: here I dissagree with the authors. Why the increased shading in course of budburst should enhance algal growth due to decreased irradiation? In a low light scenario, photosynthesis is light limited. Flagellates have to overcome this by migration to the surface.

Line 381: But snow algae cope with high-light stress by the formation of astaxanthin and the bloom turns red. This does not happen below canopy?

Linie 382: I do not understand the argumentation. Wow can reduced light intensity contribute to the uptake of nutrients? In terms of physiology? Furthermore, increase in intracellular chlorophyll content is a side effect of long-term low light conditions, particuallary for chlorophyll b. 

Line 407: insert "or from soil" after "layers"?

Line 417, use "support" instead of "have triggered"?

Line 426: too general sentence. Which activity should be monitored in future?

This is a review of "Temporal and vertical changes in biological communities within snowpacks during melting season in Northern Japan".

First of all I wanted to note I really enjoyed reading this papers. I don't deal with green deciduous canopies over snow, so it's a fun problem to think about. For the authors, please note I am reviewing this as a snow hydrologist, and so that's my bias.

My first comment is that I think the title needs to be tightened up. Perhaps something like "Changes in biological communities within snowpacks during ablation in Northern Japan"?

My main concern is that the recent advances in understanding the complex snowpack-solute processes do not seem to be considered. Specifically, the ion exclusion that occurs can cause the first melt pulse to have substantially more than subsequent pulses, e.g., Costa, et al 2018, 2019. My concern is then that this complicates some of the correlative metrics used herein. I am hoping that the authors can better describe their framework within the context of these snowpack processes.

Secondly, the authors allude to air temperature during the melt, but the radiative processes are not noted. For example, the darkening of the snow decreases the albedo and increases the solar shortwave absorption. As well, as the trees heat up, they emit longwave radiation to the snowpack. The trees can be well above air temperature, even in sub-zero conditions. I realize this is not a melt dynamics paper, however these processes should be noted as the feedback is substantial, e.g.,

Aubry‐Wake, C., Bertoncini, A. & Pomeroy, J. W. Fire and Ice: The Impact of Wildfire‐Affected Albedo and Irradiance on Glacier Melt. _Earth’s Futur._ **10**, (2022).

Conway, H., Gades, A. & Raymond, C. F. Albedo of dirty snow during conditions of melt. _Water Resour Res_ **32**, 1713–1718 (1996).

I think the inclusion of these feedback is notable due to feedbacks with the biological communities themselves. E.g., the more the communities expand, the faster they can melt the snowpack, etc.

Otherwise this is a quite interesting study. Specific comments are below.

References:

Costa, D., Pomeroy, J. & Wheater, H. A numerical model for the simulation of snowpack solute dynamics to capture runoff ionic pulses during snowmelt: the PULSE model. _Advances in Water Resources_ (2018) doi:10.1016/j.advwatres.2018.09.008.

Costa, D., Baulch, H., Elliott, J., Pomeroy, J. & Wheater, H. Modelling nutrient dynamics in cold agricultural catchments: A review. _Environ Modell Softw_ **124**, 104586 (2019).

Specific comments:

L27 "its disappearance" this reference is fine, but perhaps adding a more regional specific citation would help with local context. Is this area seeing this disappearance faster or slower than global average?

L50 I assume dust and forest fire ash are important and should be added too.

L50 "flushed down" I suggest a different expression, e.g., percolating 

L61 "gradually deform" This description of snow crystal metamorphism can be tightened up and using terms more common in that literature

L63 "allow microbes to migrated within…" because of the increased pore space? or because of entrainment in advection of melt water?

L75 forest fire ash?

L83 "information regarding…" Is this the source for all the information below? It is not clear

L83 "heavy" in rate? density? amount? be specific

L85 "met conditions…" in the basin?

L88 "help of a probe" is this a ventilated thermocouple?

L89 "every 10min after" why the change?

L90 "this data logger" word choice

L93 "the hourly means" move this closer to the 10s sampling statement on L 89

L93 "records" observations?

L97 "Green algal […]" on ward there are a lot of results mixed in with the methodology. I would suggest splitting this up as much as a possible into methodology and results that are separate.  I understand some of the results informed the on going observations, but the way it's written it ends up being a bit unclear what is the result and what is the methodology.

L101 "rainfall" was there much rain on snow?

L102 It is a new pit each time, correct? Make this clear

L110 Figure 1. Great figure. The canopy over snow is very striking! Add something to help the reader know what direction is being viewed. A spatial scale would be excellent if possible

L113 "ice auger" I assume the authors mean a snow sampler liked an ESC30/federal sampler? were they weighed for density?

L120 "all the samples" including rain samples?

L121 "slowly melted" why?

L135, how were the cell counts done? manually? automatic?

L163 "1-2 days" why the variance and did it impact the results?

L190 Figure needs axis labels and titles

L215 Figure 4, legends are impossible to read

L280 Figure 6, red line is hard to see make it thicker. legend is impossible to read clearly

L300 "temperature fluctuations" air temp?