Spatio-temporal changes in cryoconite community, isotopic, and elemental composition over the ablation season of an alpine glacier

Novotná Jaroměřská, Tereza; Ambrosini, Roberto; Richter, Dorota; Pietryka, Miroslawa; Niedzielski, Przemyslaw; Souza-Kasprzyk, Juliana; Klimaszyk, Piotr; Franzetti, Andrea; Pittino, Francesca; Vondrovicová, Lenka; Kohler, Tyler; Zawierucha, Krzysztof

doi:https://doi.org/10.5194/bg-2022-198

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https://doi.org/10.5194/bg-2022-198

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11 Oct 2022

| 11 Oct 2022

Status: this preprint has been withdrawn by the authors.

Spatio-temporal changes in cryoconite community, isotopic, and elemental composition over the ablation season of an alpine glacier

Tereza Novotná Jaroměřská, Roberto Ambrosini, Dorota Richter, Miroslawa Pietryka, Przemyslaw Niedzielski, Juliana Souza-Kasprzyk, Piotr Klimaszyk, Andrea Franzetti, Francesca Pittino, Lenka Vondrovicová, Tyler Kohler, and Krzysztof Zawierucha

Abstract. Cryoconite holes (water-filled reservoirs) on glacier surfaces are important biodiversity hotspots and biogeochemical factories within terrestrial cryosphere. In this study, we collected cryoconite from the ablation zone of the Forni Glacier (Central Italian Alps) over the whole ablation season. We aimed to describe spatial and temporal patterns in: (i) biomass and community structure of photoautotrophs (cyanobacteria, diatoms, and eukaryotic green algae) and invertebrates; (ii) carbon and nitrogen stable isotopic composition of invertebrates and their potential food; and (iii) the organic matter content and general elemental composition of cryoconite. Structure and biomass of cryoconite biota showed spatio-temporal changes over the season. Dominant cyanobacteria were Oscillatoriaceae and Leptolyngbyaceae, while dominant eukaryotic green algae were Mesotaeniaceae and Chlorellaceae. Eukaryotic green algae dominated in the upper part of the ablation zone, while a seasonal shift from algae- to cyanobacteria-dominated communities was observed in the lower part. Some taxa of photoautotrophs appeared only during specific sampling days. Dominant grazers were tardigrades (Cryobiotus klebelsbergi). The biomass of tardigrades in the upper part was significantly related to the biomass of eukaryotic green algae indicating that algal communities are likely controlled by grazing. The δ¹³C of tardigrades followed fluctuations of δ¹³C in organic matter. We did not observe spatial and temporal changes in the general elemental composition of cryoconite. Thus, changes in community structure and biomass are likely dependent on the interplay between phenology, stochastic events (e.g. rainfall), top-down, or bottom-up controls. Our study shows that understanding the ecology of biota in cryoconite holes requires a spatially explicit and seasonal approach.

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Received: 23 Sep 2022 – Discussion started: 11 Oct 2022

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Interactive discussion

Status: closed

CC1:
'Comment on bg-2022-198', Łukasz Kaczmarek, 02 Nov 2022

I want to ask if you studied also other microscopic animals living in cryoconite holes like rotifers. Did you find any?

Citation: https://doi.org/10.5194/bg-2022-198-CC1
- AC1: 'Reply on CC1', Krzysztof Zawierucha, 24 Nov 2022
  
  Thank you for the comments on our manuscript. Contrary to Arctic cryoconite holes where tardigrades and rotifers are coexisting, on the Forni Glacier only tardigrades, specifically Cryobiotus klebelsbergi, exclusively dominate cryoconite holes environment. Among 846 tardigrades, we found only few rotifers. We have exactly the same observations from previous seasons. Although rotifers were present, their numbers did not allow any statistical comparison or analyses (e.g. stable istopes). This information is already provided in the manuscript in the section Results, "Dominant invertebrates found in cryoconite were tardigrades, represented by a single species, Cryobiotus klebelsbergi. Among hundreds of tardigrades, only a few bdelloid rotifers were detected.".
  
  Citation: https://doi.org/10.5194/bg-2022-198-AC1
CC2:
'Comment on bg-2022-198', Łukasz Kaczmarek, 02 Nov 2022

Your results are interesting but.... Did you experimentally checked if tardigrades feed on these algae you studied?

Citation: https://doi.org/10.5194/bg-2022-198-CC2
- AC2: 'Reply on CC2', Krzysztof Zawierucha, 24 Nov 2022
  
  Thank you for this comment. In fact we have never cultured cryoconite eukaryotic green algae. However, We observed under laboratory conditions that C. klebelsbergi feeds on Chlorella sp. which belongs to Chlorophyta. The same observation about feeding behaviour of tardigrades, with similar buccal tube morphology, are already presented in the literature (Kosztyła et al. 2016, Altiero et al. 2011, Bryndova et al. 2019, Roszkowska et al. 2021). We will cite appropriate references confirming our observation in the section Discussion. Moreover, in other work we used molecular approach for the identification of food of C. klebelsbergi, sequences of bacteria but also sequences of algae were detected among potential food sources which corroborate our assumption.
  
  Citation: https://doi.org/10.5194/bg-2022-198-AC2
RC1:
'Comment on bg-2022-198', Anonymous Referee #1, 03 Nov 2022

General comments:

An interesting and generally well written study on invertebrates (particularly tardigrades) and their relationship to photoautotrophic species in cryoconite on the glacier surface.

Specific comments:

Abstract

Line 23: “the” terrestrial cryosphere

Line 24: how many cryoconites were sampled?

Line 33: can be reduced to “Some phototrophic taxa”

Line 36: give statistic

Introduction

Line 47-52: I would actually begin with a topic sentence on glaciers, and then how quickly they are changing, to what diversity glacier host on their surfaces, and then why this matters.

Line 84-85: a little unclear, suggest rewording to: “together with how tardigrades interact with the food web”.

Materials and Methods:

Line 126: define ablation zone where it is first mentioned in the intro

Line 132: “stainless steel” spoon

Line 130: how many cryoconites were sampled?

Results:

Line 286: Is there a percentage or number of rotifers you can include here for reference?

Line 287-289: Would be useful to denote which species in the table belong to which family

Line 309: change to “however, no seasonal trend in the lower part was observed” for clarity

Discussion:

Line 249: term “wash up” is a little vague- do you mean that ice marginal melt and other melt dynamics result in washing away of DOM?

Line 472: due to less stability

Figures/Tables:

Figure 1: Would be nice to see where each sample lies within the upper and lower part- would it be possible to add their locations on the map?

Table 1: to clarify, this table is showing in which samples there is at least one of the species shown?

Table 2: Unsure if the repeated line “upper sampling area” is meant to be there, and why is log (Chlorophyta biomass) present twice? Suggest showing results from modelling season as well.

Figure 5: would it be possible to include a legend with the figure denoting upper and lower zone samples?

Figure 7: Like Figure 5, I think a legend underneath each RDA would be nice, just to cut down on text in the caption

Citation: https://doi.org/10.5194/bg-2022-198-RC1
- AC3: 'Reply on RC1', Krzysztof Zawierucha, 24 Nov 2022
  
  General comments:
  An interesting and generally well written study on invertebrates (particularly tardigrades) and their relationship to photoautotrophic species in cryoconite on the glacier surface.
  Reply: Thank you for reviewing our paper. We really appreciate all comments and remarks. We provided responses to each comment below.
  Specific comments:
  Abstract
  Line 23: “the” terrestrial cryosphere
  Reply: We agree. A corrected version will include this change.
  Line 24: how many cryoconites were sampled?
  Reply: It is already specified in the section Methods: "Cryoconite was collected from the bottom of cryoconite holes in five sampling campaigns during the 2019 ablation season (July 4th and 26th, August 15th and 30th, and September 19th). Samples were collected with an aseptic stainless spoon and transferred into 50 mL plastic test tubes. During each sampling campaign, cryoconite was collected from at least 5 holes to create one pooled sample at each part of the ablation zone.". We believe that this information is not crucial in the abstract.
  Line 33: can be reduced to “Some phototrophic taxa”
  Reply: A corrected version will include this change.
  Line 36: give statistic
  Reply: All statistics are provided in the text and we would prefer to avoid including only significant results to the abstract, we believe all results are equally important. Including all statistics would make the abstract hard to follow.
  Introduction
  Line 47-52: I would actually begin with a topic sentence on glaciers, and then how quickly they are changing, to what diversity glacier host on their surfaces, and then why this matters.
  Reply: A corrected version will include this change.
  Line 84-85: a little unclear, suggest rewording to: “together with how tardigrades interact with the food web”.
  Reply: This part will be rewritten to “Even though some studies have investigated the ecology, the community structure of biota in cryoconite, and the potential role of tardigrades in the food web by various methods (Vonnahme et al., 2016; Zawierucha et al., 2022), evidence related to their potential ecological and trophic roles, including possible top down control of the cryoconite ecosystem, remains limited.”
  
  Materials and Methods:
  Line 126: define ablation zone where it is first mentioned in the intro
  Reply: A corrected version will include this change.
  Line 132: “stainless steel” spoon
  Reply: A corrected version will include this change.
  Line 130: how many cryoconites were sampled?
  Reply: This information is already provided in the line below, "During each sampling campaign, cryoconite was collected from at least 5 holes to create one pooled sample at each part of the ablation zone.". In total we collected material from over 50 cryoconite holes (5 sampling campaigns × (5 replicates at lower part of glacier + 5 replicates at upper part of glacier)).
  Results:
  Line 286: Is there a percentage or number of rotifers you can include here for reference?
  Reply: Regrettably, we do not have precise number of rotifers from all samples together. However, in all samples for isotopic analyses together, we found approx. 7 individual rotifers. Each season (2012, 2017, 2018, 2019, 2020), we found few rotifers in cryoconite among thousands of tardigrades.
  Line 287-289: Would be useful to denote which species in the table belong to which family
  Reply: Yes, we agree. A corrected version will include this change.
  Line 309: change to “however, no seasonal trend in the lower part was observed” for clarity
  Reply: A corrected version will include this change.
  Discussion:
  Line 249: term “wash up” is a little vague- do you mean that ice marginal melt and other melt dynamics result in washing away of DOM?
  Reply: A corrected version will include this change. Since the ablation zone of the Forni Glacier has some slope and some cryoconite holes are shallow, when the glacier surface melts, meltwater can take cryoconite (incl. DOM) from these holes and transport it to lower parts of the glacier. The same can occur during rain events when the amount of water flow (water from rain + meltwater) down the glacier surface is even higher.
  Line 472: due to less stability
  Reply: A corrected version will include this change.
  Figures/Tables:
  Figure 1: Would be nice to see where each sample lies within the upper and lower part- would it be possible to add their locations on the map?
  Reply: Unfortunately, providing a precise location is impossible. During each sampling campaign, we collected one pooled sample (containing at least 5 subsamples pooled together) per elevation. Taking into account that cryoconite holes on the Forni Glacier are very dynamic and melt and disappear within one or few days, we have never collected samples from precisely the same location.
  Table 1: to clarify, this table is showing in which samples there is at least one of the species shown?
  Reply: Precisely. The raw data are provided in supplementary material 1.
  Table 2: Unsure if the repeated line “upper sampling area” is meant to be there, and why is log (Chlorophyta biomass) present twice? Suggest showing results from modelling season as well.
  Reply: The descriptions in Table 2 are correct. The table include results of linear model of tardigrade biomass on chlorophyta biomass, sampling area and their interaction.
  Figure 5: would it be possible to include a legend with the figure denoting upper and lower zone samples?
  Reply: Yes, we agree. A corrected version will include this change.
  Figure 7: Like Figure 5, I think a legend underneath each RDA would be nice, just to cut down on text in the caption
  Reply: Yes, we agree. A corrected version will include this change.
  
  Citation: https://doi.org/10.5194/bg-2022-198-AC3
CC3:
'Comment on bg-2022-198', Ewa Poniecka, 03 Jan 2023
It is an interesting study targeting the basic ecological questions regarding glacial ecosystems. I believe that tested hypotheses are crucial for fundamental understanding of glacial biology and will allow to better predict changes of the ecosystem functioning under the future melt scenarios. Overall, the article presents novel data and contributes to the general knowledge. It is also a valuable resource for the community. For example, morphological identification of phototrophs is less common now than use of molecular biology tools, but is immensely valuable for cross validation of DNA studies.
Specific comments and questions:
There is an interesting trend in the biomass of tardigrades and algae in the upper part of the glacier, which is not observed in the lower part – do you think it is affected by the presence of cyanobacteria?

Why do you think there is no correlation with all the producers, only with algae?

Cyanobacteria seem to be more abundant in the lower part of the glacier – why?

Line 371-372: “cryoconite and tardigrades appeared to have similarities in fluctuations of their δ13C values” – I assume there is no significant correlation?

Line 428: I think you meant “present” not “representing”

You mention that you do not observe standard fractionation between consumer and food, likely because of unconsumed food – why do you think there is so much unconsumed food?

Line 482-485: A little bit unclear. I would rephrase. For example: However, unlike in microbial mats, since organic matter in cryoconite holes on the Forni Glacier was depleted in heavy carbon (13C) and nitrogen (15N) isotopes and the differences between δ13C and δ15N of organic matter and consumers were higher.
Citation: https://doi.org/10.5194/bg-2022-198-CC3
- AC4: 'Reply on CC3', Krzysztof Zawierucha, 13 Jan 2023
  
  It is an interesting study targeting the basic ecological questions regarding glacial ecosystems. I believe that tested hypotheses are crucial for fundamental understanding of glacial biology and will allow to better predict changes of the ecosystem functioning under the future melt scenarios. Overall, the article presents novel data and contributes to the general knowledge. It is also a valuable resource for the community. For example, morphological identification of phototrophs is less common now than use of molecular biology tools, but is immensely valuable for cross validation of DNA studies.
  Dear Dr. Poniecka,
  thank you very much for your for very useful comments and questions. Below we provided point by point responses.
  Specific comments and questions:
  1. There is an interesting trend in the biomass of tardigrades and algae in the upper part of the glacier, which is not observed in the lower part – do you think it is affected by the presence of cyanobacteria?
  Reply: Thank you for this comment. We rather think it is related to more dynamic conditions in the lower part of the glacier (e.g., frequent meltwater overflow) and consequent shorter retention of material on the glacier surface or in cryoconite holes. However, we cannot exclude specific effects of cyanobacteria as well. In a new version of the manuscript, we will add a new sentence to the discussion on the relation between algae and tardigrades, “The negative effect of cyanobacterial biofilms and toxins on tardigrades cannot be excluded. Therefore, the lower biomass of tardigrades at the lower part of the Forni Glacier may be affected by both specific metabolic products of cyanobacteria and high washing out effect.”
  2. Why do you think there is no correlation with all the producers, only with algae?
  Reply: Our data indicate that significant correlation exists only between eukaryotic green algae and tardigrades. Considering that green algae dominated in terms of biomass, most likely tardigrades feed mostly on them. Therefore, we did not find relation with other groups. If any process/relation existed between tardigrades and other algae, it is most likely hidden by the significant amount of biomass of eukaryotic green algae.
  3. Cyanobacteria seem to be more abundant in the lower part of the glacier – why?
  Reply: We are aware that cyanobacteria are known as cryoconite ecosystems engineers. However, most of studies on glacier cyanobacteria were conducted on polar glaciers. Thus, mountains valley glaciers could be characterized by different organisms and their spatial distribution. We already discussed this phenomenon in the section Discussion where we paid attention that in the lower part of the Forni Glacier small mineral grains could play a role as a substratum for cyanobacterial growth.
  4. Line 371-372: “cryoconite and tardigrades appeared to have similarities in fluctuations of their δ13C values” – I assume there is no significant correlation?
  Reply: Indeed, there is an obvious trend in δ13C of tardigrades and cryoconite. However, the number of tardigrade samples was too low to make further statistical analyses. Thus, correlation was not calculated.
  5. Line 428: I think you meant “present” not “representing”
  Reply: Yes, it should be “present”. We will correct the sentence accordingly.
  6. You mention that you do not observe standard fractionation between consumer and food, likely because of unconsumed food – why do you think there is so much unconsumed food?
  Reply: Cryoconite is highly heterogenous material with a significant amount of organic matter (OM) of various origin. Thus, very likely microinvertebrates randomly or preferentially consume only some compartments of cryoconite OM. Also, differences in δ13C of consumers and their potential food (OM in cryoconite) on the Forni Glacier could be influenced by the specific fractionation of potentially consumed microbes. However, since there is a lack of stable isotopic data on glacier microbes, we did not include such speculation into the discussion.
  7. Line 482-485: A little bit unclear. I would rephrase. For example: However, unlike in microbial mats, since organic matter in cryoconite holes on the Forni Glacier was depleted in heavy carbon (13C) and nitrogen (15N) isotopes and the differences between δ13C and δ15N of organic matter and consumers were higher.
  Reply: We will correct the sentence as suggested with a slight modification: “However, unlike microbial mats, organic matter in cryoconite holes on the Forni Glacier was depleted in heavy carbon (13C) and nitrogen (15N) isotopes and the differences between δ13C and δ15N of organic matter and consumers were higher.”
  
  Citation: https://doi.org/10.5194/bg-2022-198-AC4
RC2:
'Comment on bg-2022-198', Anonymous Referee #2, 05 Mar 2023

General comment

Very interesting study integrating C and N isotopic composition of the cryoconites and the tardigrades, with elaborative analysis of the composition of their potentional feed source (algae and cyanos), in comparative approach (lower and slightly upper part of the glacier tongue) during several visits in one season.

Specific comments
Abstract

line 26, 31 and others - eukaryotic algae as object of this study are not only green algae but also diatoms, so I suggest to omit the word "eukaryotic" here. Furthermore, there exist no prokaryotic green algae, so the expression "eukaryotic green algae" sounds weird to me
Material and Methods

lines 132-134 and watching Table 3 and - to be truly able to quantify the variability between the sampled cryoconites authors may consider in future to investigate each cryoconite separately (i.e. without pooling), then it will be possible to mention +/-SD. Aparently there were very dramatic changes in biomass of cyanobacteria and green algae from one sampling event to another sampling event in the same ablation zone (even uo to the level of two magnitudes when speaking about the biomass in absolute numbers)
line 156-158 Not clear why you assummed that the algal cell density is 1.0 g/cm3?

Results

Table 1 - error in spelling - there should be "Chroococcus" (not Chrococcus)
Table 1 - errors in spelling in species epithet and the author - there should be "Cylindrocystis brebissonii f. cryophila Kol" (not "Cylindrocystis brebbissoni (Ralfs) De Bary")
Table 1 - Mesotaenium sp. - I wonder how these cells looked like (e.g. chloroplast shape and vacuole colour)? You may compare your findings with the studies focused on glacier ice algae, e.g. in Procházková et al. 2021 https://www.mdpi.com/2076-2607/9/5/1103 or Remias et al. 2009 https://doi.org/10.2216/08-13.1 It is said that glacier ice algae of Ancylonema, e.g. Ancylonema alaskanum (basionym Mesotaenium berggrenii var. alaskanum), are not typical for cryoconite environment, but still it would be appretiated if the authors provide a few represenative LM pictures of majority (ideally) all the specific authotrophic taxa they found in studied cryoconite holes. It can be included in the Supplementary material. These LM pics will be of a big help for any latter studies working mainly with DNA material.
Table 2 - please explain "*" by writing it in the legend: "and their interaction (*)"
Figure 4, 5, Table 1 - omit "eukaryotic" from the legend and figure/Table, diatoms are also eukaryota
Discussion

line 434 - errors in spelling in species epithet - there should be "Cylindrocystis brebissonii" (not Cylindrocystis brebbissoni)
line 470-473 - "most probably due to less of stability" I assume this is relevant to lower part of the ablation zone, which has steeper slope and thus theoretically shallower cryoconites holes? Please ideally state in the text. The statement of Scheffer et al. 2008 is not fully clear - "if organisms are slowly growing, they are much less affected by episodic pulses" - is it possible to specify what kind of episodic pulses - nutrient input? "e.g. mirroring the lower part of the ablation zone" - not clear in which way it is mirroring, I would assume that the lower part of ablation zone is constantly (not episodically) receiving material from the above part of ablation zone due to surface "streams" of glacier meltwater. If you mean episodic via "rainfall" (?), maybe it is good to be mentioned.
line 485-487 - Is this the first study bringing (indirect) evidence that tardigrades are feeding on green algae?
line 509 - I would prefer keeping the same taxonomic rank (i.e. family) as it was used in the previous parts in the text, e.g. Chlorellaceae and Mesotaeniaceae
Supplementary Material
Table S1 - no need to specify numbers for so many decimal positions, just the first decimal positions should be sufficient

Citation: https://doi.org/10.5194/bg-2022-198-RC2
- AC5: 'Reply on RC2', Krzysztof Zawierucha, 25 Mar 2023
  
  General comment
  
  Very interesting study integrating C and N isotopic composition of the cryoconites and the tardigrades, with elaborative analysis of the composition of their potentional feed source (algae and cyanos), in comparative approach (lower and slightly upper part of the glacier tongue) during several visits in one season.
  Reply: Thank you for reviewing our paper. We really appreciate all comments and remarks. We provided responses to each comment below.
  
  Specific comments
  Abstract
  
  line 26, 31 and others - eukaryotic algae as object of this study are not only green algae but also diatoms, so I suggest to omit the word "eukaryotic" here. Furthermore, there exist no prokaryotic green algae, so the expression "eukaryotic green algae" sounds weird to me.
  Reply: Corrected as suggested in the whole text and all figures and tables.
  Material and Methods
  
  lines 132-134 and watching Table 3 and - to be truly able to quantify the variability between the sampled cryoconites authors may consider in future to investigate each cryoconite separately (i.e. without pooling), then it will be possible to mention +/-SD. Aparently there were very dramatic changes in biomass of cyanobacteria and green algae from one sampling event to another sampling event in the same ablation zone (even uo to the level of two magnitudes when speaking about the biomass in absolute numbers)
  Reply: Thank you for this comment. According to our field experience, cryoconite holes can disappear within few hours and change into shallow puddles. Sometimes, ablation is so abrupt and intense that cryoconite hole is completely flushed away. We feel that pooling is more representative, however, it would be great to compare results of both approaches, pooled samples and sampling of separate cryoconite holes.
  line 156-158 Not clear why you assummed that the algal cell density is 1.0 g/cm3?
  Reply: Methods of calculating biomass of plankton, benthos and soil algae varied. In this paper, the calculation of biomass is conducted by using the methods devoted to determine phytoplankton biomass. Cryoconite holes are inhabited by microorganisms floating mid-water. The methodology of phytoplankton biomass research assumes that total algal biomass is the sum of the total plankton biovolume values. This is due to the assumption that plasma volume equals cell volume (Smayda 1978, Hillebrand et all. 1999).
  Hillebrand, H., Dürselen, C.-D., Kirschtel, D., Pollingher, U. and Zohary, T. 1999, BIOVOLUME CALCULATION FOR PELAGIC AND BENTHIC MICROALGAE. Journal of Phycology, 35: 403-424. https://doi.org/10.1046/j.1529-8817.1999.3520403.x
  Smayda T. J. 1978. From phytoplankters to biomass. Phytoplankton manual.
  
  Results
  
  Table 1 - error in spelling - there should be "Chroococcus" (not Chrococcus)
  Reply: Corrected as suggested.
  Table 1 - errors in spelling in species epithet and the author - there should be "Cylindrocystis brebissonii f. cryophila Kol" (not "Cylindrocystis brebbissoni (Ralfs) De Bary")
  Reply: Corrected as suggested.
  Table 1 - Mesotaenium sp. - I wonder how these cells looked like (e.g. chloroplast shape and vacuole colour)? You may compare your findings with the studies focused on glacier ice algae, e.g. in Procházková et al. 2021 https://www.mdpi.com/2076-2607/9/5/1103 or Remias et al. 2009 https://doi.org/10.2216/08-13.1 It is said that glacier ice algae of Ancylonema, e.g. Ancylonema alaskanum (basionym Mesotaenium berggrenii var. alaskanum), are not typical for cryoconite environment, but still it would be appretiated if the authors provide a few represenative LM pictures of majority (ideally) all the specific authotrophic taxa they found in studied cryoconite holes. It can be included in the Supplementary material. These LM pics will be of a big help for any latter studies working mainly with DNA material.
  Reply: We added LM pictures of few species as Fig. S1.
  Table 2 - please explain "*" by writing it in the legend: "and their interaction (*)"
  Reply: Corrected as suggested.
  Figure 4, 5, Table 1 - omit "eukaryotic" from the legend and figure/Table, diatoms are also eukaryote
  Reply: Corrected as suggested.
  Discussion
  
  line 434 - errors in spelling in species epithet - there should be "Cylindrocystis brebissonii" (not Cylindrocystis brebbissoni)
  Reply: Corrected as suggested.
  line 470-473 - "most probably due to less of stability" I assume this is relevant to lower part of the ablation zone, which has steeper slope and thus theoretically shallower cryoconites holes? Please ideally state in the text. The statement of Scheffer et al. 2008 is not fully clear - "if organisms are slowly growing, they are much less affected by episodic pulses" - is it possible to specify what kind of episodic pulses - nutrient input? "e.g. mirroring the lower part of the ablation zone" - not clear in which way it is mirroring, I would assume that the lower part of ablation zone is constantly (not episodically) receiving material from the above part of ablation zone due to surface "streams" of glacier meltwater. If you mean episodic via "rainfall" (?), maybe it is good to be mentioned.
  Reply: We agree with the reviewer. The sentence is rewritten, now it reads: “Although, Scheffer et al. (2008) suggested that if organisms are slow-growing, they are much less affected by episodic pulses (e.g. strong water flow at the lower part of the ablation zone during the rain), the biomass in the lower part, of both algae and grazers, did not build up so fast as in the upper part, most probably due to lower stability of the lower part (including steepness and abrupt pulses of water).”
  line 485-487 - Is this the first study bringing (indirect) evidence that tardigrades are feeding on green algae?
  Reply: Based on the evidence from the field sampling on glaciers or in polar/high mountain regions, our study is the first of its kind. Other studies (laboratory) where tardigrades feed on green algae are already cited in the text.
  line 509 - I would prefer keeping the same taxonomic rank (i.e. family) as it was used in the previous parts in the text, e.g. Chlorellaceae and Mesotaeniaceae
  Reply: Corrected as suggested. We also replaced Mesotaeniaceae by Zygnemataceae in the whole text since Zygnemataceae not Mesotaeniaceae dominated during the season.
  Supplementary Material
  Table S1 - no need to specify numbers for so many decimal positions, just the first decimal positions should be sufficient
  Reply: Corrected as suggested.
  
  Citation: https://doi.org/10.5194/bg-2022-198-AC5

Interactive discussion

Status: closed

CC1:
'Comment on bg-2022-198', Łukasz Kaczmarek, 02 Nov 2022

I want to ask if you studied also other microscopic animals living in cryoconite holes like rotifers. Did you find any?

Citation: https://doi.org/10.5194/bg-2022-198-CC1
- AC1: 'Reply on CC1', Krzysztof Zawierucha, 24 Nov 2022
  
  Thank you for the comments on our manuscript. Contrary to Arctic cryoconite holes where tardigrades and rotifers are coexisting, on the Forni Glacier only tardigrades, specifically Cryobiotus klebelsbergi, exclusively dominate cryoconite holes environment. Among 846 tardigrades, we found only few rotifers. We have exactly the same observations from previous seasons. Although rotifers were present, their numbers did not allow any statistical comparison or analyses (e.g. stable istopes). This information is already provided in the manuscript in the section Results, "Dominant invertebrates found in cryoconite were tardigrades, represented by a single species, Cryobiotus klebelsbergi. Among hundreds of tardigrades, only a few bdelloid rotifers were detected.".
  
  Citation: https://doi.org/10.5194/bg-2022-198-AC1
CC2:
'Comment on bg-2022-198', Łukasz Kaczmarek, 02 Nov 2022

Your results are interesting but.... Did you experimentally checked if tardigrades feed on these algae you studied?

Citation: https://doi.org/10.5194/bg-2022-198-CC2
- AC2: 'Reply on CC2', Krzysztof Zawierucha, 24 Nov 2022
  
  Thank you for this comment. In fact we have never cultured cryoconite eukaryotic green algae. However, We observed under laboratory conditions that C. klebelsbergi feeds on Chlorella sp. which belongs to Chlorophyta. The same observation about feeding behaviour of tardigrades, with similar buccal tube morphology, are already presented in the literature (Kosztyła et al. 2016, Altiero et al. 2011, Bryndova et al. 2019, Roszkowska et al. 2021). We will cite appropriate references confirming our observation in the section Discussion. Moreover, in other work we used molecular approach for the identification of food of C. klebelsbergi, sequences of bacteria but also sequences of algae were detected among potential food sources which corroborate our assumption.
  
  Citation: https://doi.org/10.5194/bg-2022-198-AC2
RC1:
'Comment on bg-2022-198', Anonymous Referee #1, 03 Nov 2022

General comments:

An interesting and generally well written study on invertebrates (particularly tardigrades) and their relationship to photoautotrophic species in cryoconite on the glacier surface.

Specific comments:

Abstract

Line 23: “the” terrestrial cryosphere

Line 24: how many cryoconites were sampled?

Line 33: can be reduced to “Some phototrophic taxa”

Line 36: give statistic

Introduction

Line 47-52: I would actually begin with a topic sentence on glaciers, and then how quickly they are changing, to what diversity glacier host on their surfaces, and then why this matters.

Line 84-85: a little unclear, suggest rewording to: “together with how tardigrades interact with the food web”.

Materials and Methods:

Line 126: define ablation zone where it is first mentioned in the intro

Line 132: “stainless steel” spoon

Line 130: how many cryoconites were sampled?

Results:

Line 286: Is there a percentage or number of rotifers you can include here for reference?

Line 287-289: Would be useful to denote which species in the table belong to which family

Line 309: change to “however, no seasonal trend in the lower part was observed” for clarity

Discussion:

Line 249: term “wash up” is a little vague- do you mean that ice marginal melt and other melt dynamics result in washing away of DOM?

Line 472: due to less stability

Figures/Tables:

Figure 1: Would be nice to see where each sample lies within the upper and lower part- would it be possible to add their locations on the map?

Table 1: to clarify, this table is showing in which samples there is at least one of the species shown?

Table 2: Unsure if the repeated line “upper sampling area” is meant to be there, and why is log (Chlorophyta biomass) present twice? Suggest showing results from modelling season as well.

Figure 5: would it be possible to include a legend with the figure denoting upper and lower zone samples?

Figure 7: Like Figure 5, I think a legend underneath each RDA would be nice, just to cut down on text in the caption

Citation: https://doi.org/10.5194/bg-2022-198-RC1
- AC3: 'Reply on RC1', Krzysztof Zawierucha, 24 Nov 2022
  
  General comments:
  An interesting and generally well written study on invertebrates (particularly tardigrades) and their relationship to photoautotrophic species in cryoconite on the glacier surface.
  Reply: Thank you for reviewing our paper. We really appreciate all comments and remarks. We provided responses to each comment below.
  Specific comments:
  Abstract
  Line 23: “the” terrestrial cryosphere
  Reply: We agree. A corrected version will include this change.
  Line 24: how many cryoconites were sampled?
  Reply: It is already specified in the section Methods: "Cryoconite was collected from the bottom of cryoconite holes in five sampling campaigns during the 2019 ablation season (July 4th and 26th, August 15th and 30th, and September 19th). Samples were collected with an aseptic stainless spoon and transferred into 50 mL plastic test tubes. During each sampling campaign, cryoconite was collected from at least 5 holes to create one pooled sample at each part of the ablation zone.". We believe that this information is not crucial in the abstract.
  Line 33: can be reduced to “Some phototrophic taxa”
  Reply: A corrected version will include this change.
  Line 36: give statistic
  Reply: All statistics are provided in the text and we would prefer to avoid including only significant results to the abstract, we believe all results are equally important. Including all statistics would make the abstract hard to follow.
  Introduction
  Line 47-52: I would actually begin with a topic sentence on glaciers, and then how quickly they are changing, to what diversity glacier host on their surfaces, and then why this matters.
  Reply: A corrected version will include this change.
  Line 84-85: a little unclear, suggest rewording to: “together with how tardigrades interact with the food web”.
  Reply: This part will be rewritten to “Even though some studies have investigated the ecology, the community structure of biota in cryoconite, and the potential role of tardigrades in the food web by various methods (Vonnahme et al., 2016; Zawierucha et al., 2022), evidence related to their potential ecological and trophic roles, including possible top down control of the cryoconite ecosystem, remains limited.”
  
  Materials and Methods:
  Line 126: define ablation zone where it is first mentioned in the intro
  Reply: A corrected version will include this change.
  Line 132: “stainless steel” spoon
  Reply: A corrected version will include this change.
  Line 130: how many cryoconites were sampled?
  Reply: This information is already provided in the line below, "During each sampling campaign, cryoconite was collected from at least 5 holes to create one pooled sample at each part of the ablation zone.". In total we collected material from over 50 cryoconite holes (5 sampling campaigns × (5 replicates at lower part of glacier + 5 replicates at upper part of glacier)).
  Results:
  Line 286: Is there a percentage or number of rotifers you can include here for reference?
  Reply: Regrettably, we do not have precise number of rotifers from all samples together. However, in all samples for isotopic analyses together, we found approx. 7 individual rotifers. Each season (2012, 2017, 2018, 2019, 2020), we found few rotifers in cryoconite among thousands of tardigrades.
  Line 287-289: Would be useful to denote which species in the table belong to which family
  Reply: Yes, we agree. A corrected version will include this change.
  Line 309: change to “however, no seasonal trend in the lower part was observed” for clarity
  Reply: A corrected version will include this change.
  Discussion:
  Line 249: term “wash up” is a little vague- do you mean that ice marginal melt and other melt dynamics result in washing away of DOM?
  Reply: A corrected version will include this change. Since the ablation zone of the Forni Glacier has some slope and some cryoconite holes are shallow, when the glacier surface melts, meltwater can take cryoconite (incl. DOM) from these holes and transport it to lower parts of the glacier. The same can occur during rain events when the amount of water flow (water from rain + meltwater) down the glacier surface is even higher.
  Line 472: due to less stability
  Reply: A corrected version will include this change.
  Figures/Tables:
  Figure 1: Would be nice to see where each sample lies within the upper and lower part- would it be possible to add their locations on the map?
  Reply: Unfortunately, providing a precise location is impossible. During each sampling campaign, we collected one pooled sample (containing at least 5 subsamples pooled together) per elevation. Taking into account that cryoconite holes on the Forni Glacier are very dynamic and melt and disappear within one or few days, we have never collected samples from precisely the same location.
  Table 1: to clarify, this table is showing in which samples there is at least one of the species shown?
  Reply: Precisely. The raw data are provided in supplementary material 1.
  Table 2: Unsure if the repeated line “upper sampling area” is meant to be there, and why is log (Chlorophyta biomass) present twice? Suggest showing results from modelling season as well.
  Reply: The descriptions in Table 2 are correct. The table include results of linear model of tardigrade biomass on chlorophyta biomass, sampling area and their interaction.
  Figure 5: would it be possible to include a legend with the figure denoting upper and lower zone samples?
  Reply: Yes, we agree. A corrected version will include this change.
  Figure 7: Like Figure 5, I think a legend underneath each RDA would be nice, just to cut down on text in the caption
  Reply: Yes, we agree. A corrected version will include this change.
  
  Citation: https://doi.org/10.5194/bg-2022-198-AC3
CC3:
'Comment on bg-2022-198', Ewa Poniecka, 03 Jan 2023
It is an interesting study targeting the basic ecological questions regarding glacial ecosystems. I believe that tested hypotheses are crucial for fundamental understanding of glacial biology and will allow to better predict changes of the ecosystem functioning under the future melt scenarios. Overall, the article presents novel data and contributes to the general knowledge. It is also a valuable resource for the community. For example, morphological identification of phototrophs is less common now than use of molecular biology tools, but is immensely valuable for cross validation of DNA studies.
Specific comments and questions:
There is an interesting trend in the biomass of tardigrades and algae in the upper part of the glacier, which is not observed in the lower part – do you think it is affected by the presence of cyanobacteria?

Why do you think there is no correlation with all the producers, only with algae?

Cyanobacteria seem to be more abundant in the lower part of the glacier – why?

Line 371-372: “cryoconite and tardigrades appeared to have similarities in fluctuations of their δ13C values” – I assume there is no significant correlation?

Line 428: I think you meant “present” not “representing”

You mention that you do not observe standard fractionation between consumer and food, likely because of unconsumed food – why do you think there is so much unconsumed food?

Line 482-485: A little bit unclear. I would rephrase. For example: However, unlike in microbial mats, since organic matter in cryoconite holes on the Forni Glacier was depleted in heavy carbon (13C) and nitrogen (15N) isotopes and the differences between δ13C and δ15N of organic matter and consumers were higher.
Citation: https://doi.org/10.5194/bg-2022-198-CC3
- AC4: 'Reply on CC3', Krzysztof Zawierucha, 13 Jan 2023
  
  It is an interesting study targeting the basic ecological questions regarding glacial ecosystems. I believe that tested hypotheses are crucial for fundamental understanding of glacial biology and will allow to better predict changes of the ecosystem functioning under the future melt scenarios. Overall, the article presents novel data and contributes to the general knowledge. It is also a valuable resource for the community. For example, morphological identification of phototrophs is less common now than use of molecular biology tools, but is immensely valuable for cross validation of DNA studies.
  Dear Dr. Poniecka,
  thank you very much for your for very useful comments and questions. Below we provided point by point responses.
  Specific comments and questions:
  1. There is an interesting trend in the biomass of tardigrades and algae in the upper part of the glacier, which is not observed in the lower part – do you think it is affected by the presence of cyanobacteria?
  Reply: Thank you for this comment. We rather think it is related to more dynamic conditions in the lower part of the glacier (e.g., frequent meltwater overflow) and consequent shorter retention of material on the glacier surface or in cryoconite holes. However, we cannot exclude specific effects of cyanobacteria as well. In a new version of the manuscript, we will add a new sentence to the discussion on the relation between algae and tardigrades, “The negative effect of cyanobacterial biofilms and toxins on tardigrades cannot be excluded. Therefore, the lower biomass of tardigrades at the lower part of the Forni Glacier may be affected by both specific metabolic products of cyanobacteria and high washing out effect.”
  2. Why do you think there is no correlation with all the producers, only with algae?
  Reply: Our data indicate that significant correlation exists only between eukaryotic green algae and tardigrades. Considering that green algae dominated in terms of biomass, most likely tardigrades feed mostly on them. Therefore, we did not find relation with other groups. If any process/relation existed between tardigrades and other algae, it is most likely hidden by the significant amount of biomass of eukaryotic green algae.
  3. Cyanobacteria seem to be more abundant in the lower part of the glacier – why?
  Reply: We are aware that cyanobacteria are known as cryoconite ecosystems engineers. However, most of studies on glacier cyanobacteria were conducted on polar glaciers. Thus, mountains valley glaciers could be characterized by different organisms and their spatial distribution. We already discussed this phenomenon in the section Discussion where we paid attention that in the lower part of the Forni Glacier small mineral grains could play a role as a substratum for cyanobacterial growth.
  4. Line 371-372: “cryoconite and tardigrades appeared to have similarities in fluctuations of their δ13C values” – I assume there is no significant correlation?
  Reply: Indeed, there is an obvious trend in δ13C of tardigrades and cryoconite. However, the number of tardigrade samples was too low to make further statistical analyses. Thus, correlation was not calculated.
  5. Line 428: I think you meant “present” not “representing”
  Reply: Yes, it should be “present”. We will correct the sentence accordingly.
  6. You mention that you do not observe standard fractionation between consumer and food, likely because of unconsumed food – why do you think there is so much unconsumed food?
  Reply: Cryoconite is highly heterogenous material with a significant amount of organic matter (OM) of various origin. Thus, very likely microinvertebrates randomly or preferentially consume only some compartments of cryoconite OM. Also, differences in δ13C of consumers and their potential food (OM in cryoconite) on the Forni Glacier could be influenced by the specific fractionation of potentially consumed microbes. However, since there is a lack of stable isotopic data on glacier microbes, we did not include such speculation into the discussion.
  7. Line 482-485: A little bit unclear. I would rephrase. For example: However, unlike in microbial mats, since organic matter in cryoconite holes on the Forni Glacier was depleted in heavy carbon (13C) and nitrogen (15N) isotopes and the differences between δ13C and δ15N of organic matter and consumers were higher.
  Reply: We will correct the sentence as suggested with a slight modification: “However, unlike microbial mats, organic matter in cryoconite holes on the Forni Glacier was depleted in heavy carbon (13C) and nitrogen (15N) isotopes and the differences between δ13C and δ15N of organic matter and consumers were higher.”
  
  Citation: https://doi.org/10.5194/bg-2022-198-AC4
RC2:
'Comment on bg-2022-198', Anonymous Referee #2, 05 Mar 2023

General comment

Very interesting study integrating C and N isotopic composition of the cryoconites and the tardigrades, with elaborative analysis of the composition of their potentional feed source (algae and cyanos), in comparative approach (lower and slightly upper part of the glacier tongue) during several visits in one season.

Specific comments
Abstract

line 26, 31 and others - eukaryotic algae as object of this study are not only green algae but also diatoms, so I suggest to omit the word "eukaryotic" here. Furthermore, there exist no prokaryotic green algae, so the expression "eukaryotic green algae" sounds weird to me
Material and Methods

lines 132-134 and watching Table 3 and - to be truly able to quantify the variability between the sampled cryoconites authors may consider in future to investigate each cryoconite separately (i.e. without pooling), then it will be possible to mention +/-SD. Aparently there were very dramatic changes in biomass of cyanobacteria and green algae from one sampling event to another sampling event in the same ablation zone (even uo to the level of two magnitudes when speaking about the biomass in absolute numbers)
line 156-158 Not clear why you assummed that the algal cell density is 1.0 g/cm3?

Results

Table 1 - error in spelling - there should be "Chroococcus" (not Chrococcus)
Table 1 - errors in spelling in species epithet and the author - there should be "Cylindrocystis brebissonii f. cryophila Kol" (not "Cylindrocystis brebbissoni (Ralfs) De Bary")
Table 1 - Mesotaenium sp. - I wonder how these cells looked like (e.g. chloroplast shape and vacuole colour)? You may compare your findings with the studies focused on glacier ice algae, e.g. in Procházková et al. 2021 https://www.mdpi.com/2076-2607/9/5/1103 or Remias et al. 2009 https://doi.org/10.2216/08-13.1 It is said that glacier ice algae of Ancylonema, e.g. Ancylonema alaskanum (basionym Mesotaenium berggrenii var. alaskanum), are not typical for cryoconite environment, but still it would be appretiated if the authors provide a few represenative LM pictures of majority (ideally) all the specific authotrophic taxa they found in studied cryoconite holes. It can be included in the Supplementary material. These LM pics will be of a big help for any latter studies working mainly with DNA material.
Table 2 - please explain "*" by writing it in the legend: "and their interaction (*)"
Figure 4, 5, Table 1 - omit "eukaryotic" from the legend and figure/Table, diatoms are also eukaryota
Discussion

line 434 - errors in spelling in species epithet - there should be "Cylindrocystis brebissonii" (not Cylindrocystis brebbissoni)
line 470-473 - "most probably due to less of stability" I assume this is relevant to lower part of the ablation zone, which has steeper slope and thus theoretically shallower cryoconites holes? Please ideally state in the text. The statement of Scheffer et al. 2008 is not fully clear - "if organisms are slowly growing, they are much less affected by episodic pulses" - is it possible to specify what kind of episodic pulses - nutrient input? "e.g. mirroring the lower part of the ablation zone" - not clear in which way it is mirroring, I would assume that the lower part of ablation zone is constantly (not episodically) receiving material from the above part of ablation zone due to surface "streams" of glacier meltwater. If you mean episodic via "rainfall" (?), maybe it is good to be mentioned.
line 485-487 - Is this the first study bringing (indirect) evidence that tardigrades are feeding on green algae?
line 509 - I would prefer keeping the same taxonomic rank (i.e. family) as it was used in the previous parts in the text, e.g. Chlorellaceae and Mesotaeniaceae
Supplementary Material
Table S1 - no need to specify numbers for so many decimal positions, just the first decimal positions should be sufficient

Citation: https://doi.org/10.5194/bg-2022-198-RC2
- AC5: 'Reply on RC2', Krzysztof Zawierucha, 25 Mar 2023
  
  General comment
  
  Very interesting study integrating C and N isotopic composition of the cryoconites and the tardigrades, with elaborative analysis of the composition of their potentional feed source (algae and cyanos), in comparative approach (lower and slightly upper part of the glacier tongue) during several visits in one season.
  Reply: Thank you for reviewing our paper. We really appreciate all comments and remarks. We provided responses to each comment below.
  
  Specific comments
  Abstract
  
  line 26, 31 and others - eukaryotic algae as object of this study are not only green algae but also diatoms, so I suggest to omit the word "eukaryotic" here. Furthermore, there exist no prokaryotic green algae, so the expression "eukaryotic green algae" sounds weird to me.
  Reply: Corrected as suggested in the whole text and all figures and tables.
  Material and Methods
  
  lines 132-134 and watching Table 3 and - to be truly able to quantify the variability between the sampled cryoconites authors may consider in future to investigate each cryoconite separately (i.e. without pooling), then it will be possible to mention +/-SD. Aparently there were very dramatic changes in biomass of cyanobacteria and green algae from one sampling event to another sampling event in the same ablation zone (even uo to the level of two magnitudes when speaking about the biomass in absolute numbers)
  Reply: Thank you for this comment. According to our field experience, cryoconite holes can disappear within few hours and change into shallow puddles. Sometimes, ablation is so abrupt and intense that cryoconite hole is completely flushed away. We feel that pooling is more representative, however, it would be great to compare results of both approaches, pooled samples and sampling of separate cryoconite holes.
  line 156-158 Not clear why you assummed that the algal cell density is 1.0 g/cm3?
  Reply: Methods of calculating biomass of plankton, benthos and soil algae varied. In this paper, the calculation of biomass is conducted by using the methods devoted to determine phytoplankton biomass. Cryoconite holes are inhabited by microorganisms floating mid-water. The methodology of phytoplankton biomass research assumes that total algal biomass is the sum of the total plankton biovolume values. This is due to the assumption that plasma volume equals cell volume (Smayda 1978, Hillebrand et all. 1999).
  Hillebrand, H., Dürselen, C.-D., Kirschtel, D., Pollingher, U. and Zohary, T. 1999, BIOVOLUME CALCULATION FOR PELAGIC AND BENTHIC MICROALGAE. Journal of Phycology, 35: 403-424. https://doi.org/10.1046/j.1529-8817.1999.3520403.x
  Smayda T. J. 1978. From phytoplankters to biomass. Phytoplankton manual.
  
  Results
  
  Table 1 - error in spelling - there should be "Chroococcus" (not Chrococcus)
  Reply: Corrected as suggested.
  Table 1 - errors in spelling in species epithet and the author - there should be "Cylindrocystis brebissonii f. cryophila Kol" (not "Cylindrocystis brebbissoni (Ralfs) De Bary")
  Reply: Corrected as suggested.
  Table 1 - Mesotaenium sp. - I wonder how these cells looked like (e.g. chloroplast shape and vacuole colour)? You may compare your findings with the studies focused on glacier ice algae, e.g. in Procházková et al. 2021 https://www.mdpi.com/2076-2607/9/5/1103 or Remias et al. 2009 https://doi.org/10.2216/08-13.1 It is said that glacier ice algae of Ancylonema, e.g. Ancylonema alaskanum (basionym Mesotaenium berggrenii var. alaskanum), are not typical for cryoconite environment, but still it would be appretiated if the authors provide a few represenative LM pictures of majority (ideally) all the specific authotrophic taxa they found in studied cryoconite holes. It can be included in the Supplementary material. These LM pics will be of a big help for any latter studies working mainly with DNA material.
  Reply: We added LM pictures of few species as Fig. S1.
  Table 2 - please explain "*" by writing it in the legend: "and their interaction (*)"
  Reply: Corrected as suggested.
  Figure 4, 5, Table 1 - omit "eukaryotic" from the legend and figure/Table, diatoms are also eukaryote
  Reply: Corrected as suggested.
  Discussion
  
  line 434 - errors in spelling in species epithet - there should be "Cylindrocystis brebissonii" (not Cylindrocystis brebbissoni)
  Reply: Corrected as suggested.
  line 470-473 - "most probably due to less of stability" I assume this is relevant to lower part of the ablation zone, which has steeper slope and thus theoretically shallower cryoconites holes? Please ideally state in the text. The statement of Scheffer et al. 2008 is not fully clear - "if organisms are slowly growing, they are much less affected by episodic pulses" - is it possible to specify what kind of episodic pulses - nutrient input? "e.g. mirroring the lower part of the ablation zone" - not clear in which way it is mirroring, I would assume that the lower part of ablation zone is constantly (not episodically) receiving material from the above part of ablation zone due to surface "streams" of glacier meltwater. If you mean episodic via "rainfall" (?), maybe it is good to be mentioned.
  Reply: We agree with the reviewer. The sentence is rewritten, now it reads: “Although, Scheffer et al. (2008) suggested that if organisms are slow-growing, they are much less affected by episodic pulses (e.g. strong water flow at the lower part of the ablation zone during the rain), the biomass in the lower part, of both algae and grazers, did not build up so fast as in the upper part, most probably due to lower stability of the lower part (including steepness and abrupt pulses of water).”
  line 485-487 - Is this the first study bringing (indirect) evidence that tardigrades are feeding on green algae?
  Reply: Based on the evidence from the field sampling on glaciers or in polar/high mountain regions, our study is the first of its kind. Other studies (laboratory) where tardigrades feed on green algae are already cited in the text.
  line 509 - I would prefer keeping the same taxonomic rank (i.e. family) as it was used in the previous parts in the text, e.g. Chlorellaceae and Mesotaeniaceae
  Reply: Corrected as suggested. We also replaced Mesotaeniaceae by Zygnemataceae in the whole text since Zygnemataceae not Mesotaeniaceae dominated during the season.
  Supplementary Material
  Table S1 - no need to specify numbers for so many decimal positions, just the first decimal positions should be sufficient
  Reply: Corrected as suggested.
  
  Citation: https://doi.org/10.5194/bg-2022-198-AC5

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Changes in the composition and biomass of photoautotrophs and consumers on glacier indicated phenological or ecological controls over their communities. We demonstrated that the recognition of the community structure of cryoconite holes requires a broad-scale and seasonal approach since biological communities vary in time and space on the glacier surface.


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