| 07 Nov 2025
Bioreactivity of dissolved organic carbon in ponds of the ice-wedge polygonal tundra
Abstract. The role of ponds in transforming laterally exported dissolved organic matter (DOM) within polygonal landscapes affected by degrading ice-wedges remains poorly understood, despite their potential importance in carbon cycling. We hypothesized that the morphological and limnological diversity of ponds–driven by permafrost erosion and soil subsidence–generates DOM of varying bioreactivity. To test this, we conducted a 188-day bioassay using water from 15 ponds representing the main geomorphological pond types in a polygonal landscape in northeastern Canada. Using optical spectroscopy, we examined the relationship between DOM properties and its bioreactivity. We also conducted a parallel bioassay experiment with nutrient additions to assess potential inorganic nitrogen and phosphorus limitations. Results show that a significant proportion of dissolved organic carbon (DOC) is available to bacterioplankton in these shallow lentic systems during summer (33 % decomposed after 97 days). Contrary to our hypothesis, and despite variations in DOM composition, no difference in DOC loss was observed among the three pond categories defined in this study, suggesting comparable bioavailable DOC pools. Moreover, nutrient addition did not significatively enhance DOC loss or decay rates, suggesting that bacterial decomposition depends mainly on organic matter bioavailability. This is further supported by a positive correlation between DOC loss and tryptophan-like fluorophores, a marker of bioavailable DOM. This suggests that DOM released by cyanobacterial mats and other autochthonous producers may be more readily utilized by bacteria than DOM derived from peaty soils. These findings highlight the importance of freshly produced organic matter in regulating carbon cycling in ponds of the ice-wedge polygonal tundra, with consequences on the fate of carbon released from thawing soils.
Review of the manuscript titled “Bioreactivity of dissolved organic carbon in ponds of the ice-wedge polygonal tundra”, proposed by Thomas Pacoureau, Milla Rautio and Isabelle Laurion.
This study tried to describe processes surrounding the dynamics of the dissolved organic carbon (quantity and quality), specifically for a variety of defrosted Arctic ponds under the short summer season. For this matter, Pacoureau et al. have associated a sampling of environmental reference forcing conditions with a controlled laboratory experiment to reproduce DOC loss over the season under heterotrophic conditions. The experiment is doubled with a nutrient-enriched essay to test the second hypothesis of work on the drivers limiting microbial biomass growth. The dissolved organic carbon is studied under the spectrum of its concentration for the quantitative part and under the spectrum of its optical properties for the qualitative part. Finally, Pacoureau et al. concluded that beyond a homogeneous loss of DOC over ponds and to the end of the season, it is the lability of the DOC that showed correlations with pond-types and the environmental conditions, opening the discussion over global change effects on such ecosystems.
Global critic
Globally, I enjoyed discovering this study, the scientific strategy, its results and the biogeochemical processes finally highlighted. Besides, I found a consistent and well-written text (almost all the time) easy to follow and understand. However, there is for me one structural issue that I will detail below, in addition to minor questions or remarks. To resume the minor remarks, I mainly found terms or phrases vague or ambiguous, needing more precision and justification.
Autotrophic processes and the global carbon cycle
As a scientific strategy, you chose to minimise the autotrophic processes in your experiments, but it must be acknowledged that, finally, the labile OM provided is essential in explaining the DOC loss dynamics. There are structural gaps in its integration of it, from the beginning to the end. As you saw, chlorophyll a levels are quite high, meaning an active local living biomass of phytoplankton, maybe also the mat of cyanobacteria, providing protein-like DOC, and the microbial compartments consumed it preferentially until 90 days (why the plateau?). One parameter, phaeopigments, would have really tested all your grey areas. As a proxy of the decayed photoautotroph cells, it would have better accounted for the cyanobacteria mats (more phaeo than chla since it is not in the water), and quantified the labile POM pool besides the P1-P2 dynamics. Maybe as a hypothesis, all the chla+phaeo have been consumed at D+90 (or is it another nutrient that is limiting? Unfortunately, it's not discussed). Also, there is no discussion about the link between phytoplankton (and microphytobenthos including cyanobacteria) and the bacterial compartment, known to be strongly intricated in quantity and quality (Costas-Selas et al., 2024; Liénart et al., 2020). Finally, I craved for a better discussion integrating the relationships of decreasing decay while a360 increasing, DOC loss and P1 intensity in terms of the global carbon cycle. What do your (great) findings tell us about the carbon qualitative dynamics of the region (4.6 to reshape)? I do not see a clear statement to conclude strongly the discussion, where flux biogeochemists will use your paper(s) to clearly state: What does this new understanding about heterotrophic consumption of DOC tell us about the exchanges between compartments? What is the specific role of such aquatic ecosystems in the global carbon cycle (in view of the current knowledge, Chaplot and Mutema, 2021)? Maybe a conceptual synthesis figure is the only answer to this last point.
Abstract
l.11-12: Maybe the concept of “ice-wedge polygonal tundra” is a little too niche to not be defined even in the abstract. For you to see.
l.18: “in these shallow lentic systems” seems awkward to be recalled here.
l.20: The pond types have to be called at least, and they do not originate from this study, so just call them.
Introduction
l.32-33: Are 5 references really necessary?
l.34: Vegetation has not been introduced yet; it has to be done earlier to understand what primary producers (living or decayed) exist in such a particular ecosystem.
l.47: I suppose you used “watershed” for its American meaning of surface of catchment. I suggest that for all English users, you use a less ambiguous term (catchment, basin, drainage area, etc.).
l.63-64: The description of the three types of ponds is simple and clear, but introduced ambiguously. I don't understand what "representative" means here. You have already established that there are only three types of ponds in these systems, so it seems clearer to me to say "the three types of ponds that can be found in ...".
Material and methods
Study site: I found in Pacoureau et al. 2025 the study site figure I wanted, firstly to understand personally what a polygonal ice-wedge tundra is and what the ponds look like, and secondly to check why there is nothing graphical in this Method. I know that after an analogous paper about the same site, you are tempted to resume the Material and Methods for the next one, but as proof, I was not able to understand this article without seeing the previous one. I would like to see a more detailed description of the study site, something between the actual version and the Pacoureau et al. 2025 one.
MLR: I have some questions about the statistics. Why did you choose to perform only multiple linear regressions, and not general linear models, that would have permitted testing more distributions than the simple Gaussian one, also avoiding the log transformation for scaling? For the multicollinearity, why Spearman and not the Variance Inflation Factor (Borcard et al., 2011)? Why the AIC and not the BIC? You do not mention whether you checked that you retained models only if all the variables were significant. I suggest testing those, or justify why not.
l.83-84: I understand that there is 78 mm of precipitation on average in total over the 3 months. This should be marked more clearly to avoid confusion (with monthly precipitation).
l.106-108: I don’t understand. Smaller than what? I understood above that you used a greater mesh size than usual, to retain more bacteria, but always excluding bacterivores. Either a word is false, or the paragraph should be clearer.
l.112-115: Unless I am mistaken, you are not taking this bias into account in the discussion on the DOC loss.
l.126: naïve question, why a 29-day basis for a month and not 30 or 31?
l.127-128: Please specify the GF75 grade, not to mistake the filter properties with the GF/F one
l.133-135: I suggest displaying the equations as a synthesis, at least for the complex exponential one (which is prominent later).
Results
3.1: Chlorophyll a levels are quite interesting; you should acknowledge them to discuss more about the trophic level later (associated with the nutrient-based part of the discussion). Fmax of microbial-like and protein-like are at the same level, is not it interesting to note it in view of the discussion?
3.2: Good
3.3: Good. I am just wondering if the information carried by the figure 4 is sufficient to be a whole figure, or if it cannot be mixed with fig. 3 or just put as a table.
3.4: As it stands, the figure 6 is badly exploited. I had to go to Pacoureau et al. 2025 to figure out what the EE signature of P1 was, finally to see that the scales are not the same for each pond type. It is not correct. For me, EEMs have a quantitative lecture, so you have to homogenise the scales. Then you will be able to describe it, comparing the ΔRU but also between the ponds.
l.232-233: The form could be better.
Discussion and conclusion
I found the discussion pleasant, well-structured and written, outside of the global criticism.
L389-390: You should check your writing around the nutrient mentions (here the C:N and C:P ratios), where you forget to mention “dissolved”, I know that it seems obvious for you, but not for those who juggle between dissolved and particulate.
4.5: I find this part of the discussion a bit too advanced (l.410-411), as you voluntarily focused your experiments on the heterotrophic processes; even if you have found a great residual DOC pool, you don’t know the amplitude of action of photodegradation and primary production, for example. Independently, some insights about what forcings can be responsible for resuspension (l.414-415) will deepen this paragraph.
l.426-427: the reverse is also true from POM to DOM (Hu et al., 2022).
l.439: The first sentence of the conclusion is decisive, and it should be more precise. I suggest either adding a bio-essay (or experiment) around or replacing “estimation”, and/or adding heterotrophic or microbial to “DOM decomposition”.
l.420: As for the abstract, I do not understand your use of “morphological” and “limnological”, terms that are vague to me. I found in your article a comparison of pond types, led by their hydro(geo)morphology, and a comparison of nutrient levels, so biogeochemistry.
Suggestion to the editor
As a synthesis, I found that the science carried by this manuscript, after intermediary corrections, will be a matter of interest and advances and should be disseminated to the scientific community in this journal.
References
Borcard, D., Legendre, P., and Gillet, F.: Numerical Ecology with R, Springer, 315 pp., 2011.
Chaplot, V. and Mutema, M.: Sources and main controls of dissolved organic and inorganic carbon in river basins: A worldwide meta-analysis, Journal of Hydrology, 603, 126941, https://doi.org/10.1016/j.jhydrol.2021.126941, 2021.
Costas-Selas, C., Martínez-García, S., Delgadillo-Nuño, E., Justel-Díez, M., Fuentes-Lema, A., Fernández, E., and Teira, E.: Linking the impact of bacteria on phytoplankton growth with microbial community composition and co-occurrence patterns, Marine Environmental Research, 193, 106262, https://doi.org/10.1016/j.marenvres.2023.106262, 2024.
Hu, B., Wang, P., Wang, C., and Bao, T.: Photogeochemistry of particulate organic matter in aquatic systems: A review, Science of The Total Environment, 806, 150467, https://doi.org/10.1016/j.scitotenv.2021.150467, 2022.
Liénart, C., Savoye, N., Conan, P., David, V., Barbier, P., Bichon, S., Charlier, K., Costes, L., Derriennic, H., Ferreira, S., Gueux, A., Hubas, C., Maria, E., and Meziane, T.: Relationship between bacterial compartment and particulate organic matter (POM) in coastal systems: An assessment using fatty acids and stable isotopes, Estuarine, Coastal and Shelf Science, 239, 106720, https://doi.org/10.1016/j.ecss.2020.106720, 2020.