the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Nov 2024
Thermokarst lake change and lake hydrochemistry: A snapshot from the Arctic Coastal Plain of Alaska
Abstract. The rapid climate warming is affecting the Arctic which is rich in aquatic systems. As a result of permafrost thaw, thermokarst lakes and ponds are either shrinking due to lake drainage or expanding due to lake shore erosion. This process in turn mobilizes organic carbon, which is released by permafrost deposits and active layer material that slips into the lake. In this study, we combine hydrochemical measurements and remote sensing data to analyze the influence of lake change processes, especially lake growth, on lake hydrochemical parameters such as DOC, EC, pH as well as stable oxygen and hydrogen isotopes in the Arctic Coastal Plain. For our entire dataset of 97 water samples from 82 water bodies, we found significantly higher CH4 concentrations in lakes with a floating-ice regime and significantly higher DOC concentrations in lakes with a bedfast-ice regime. We show significantly lower CH4 concentrations in lagoons compared to lakes as a result of an effective CH4 oxidation that increased with a seawater connection. For our detailed lake sampling of two thermokarst lakes, we found a significant positive correlation for lake shore erosion and DOC for one of the lakes. Our detailed lake sampling approach indicates that the generally shallow thermokarst lakes are overall well mixed and that single hydrochemical samples are representative for the entire lake. Finally, our study confirms that DOC concentrations correlates with lake size, ecoregion type and underlying deposits.
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RC1: 'Comment on egusphere-2024-2822', Anonymous Referee #1, 12 Dec 2024
The authors report a novel dataset on hydrochemistry and CH4 regime in lakes from highly remote region, strongly understudied, which has high environmental importance.
This is consistent dataset for large number of variable lakes. It is a pity that CO2 concentrations were not assessed; however, the data are adequately interpreted and the available literature is well captured.
I can recommend moderate revision of this manuscript.
Specific comments
L171 Reporting median E.C. for this dataset does not make sense – coastal lagoons and thermokarst lakes are incompatible categories
L230 edit ‘with elevation’
L249 for cations; L 250 for anions
L326-329 Note also that permafrost thaw an active layer deepening can liberate low molecular weight, potentially highly biodegradable OC from dispersed peat ice (i.e., https://doi.org/10.1016/j.geoderma.2022.116256; DOI: 10.1039/D1EM00547B; https://doi.org/10.1016/j.chemosphere.2020.128953)
L341-343 This is important result, that should be stated in the Abstract
L379-381 Note that Zabelina et al (2021, doi: 10.1002/lno.11560) also reported a decrease in CH4 concentrations and emissions in large (>100,000 m²) lakes compared to small thaw ponds and lakes (100-10,000 m²).
L394-395 This sentence is not necessary for Conclusions
Citation: https://doi.org/10.5194/egusphere-2024-2822-RC1 -
RC2: 'Comment on egusphere-2024-2822', Anonymous Referee #2, 28 Mar 2025
This manuscript presents hydrochemical data from 82 water bodies in the Arctic coastal plain of Alaska. The authors collected 97 surface water samples and analysed them for concentrations of DOC, CH4, various cations and anions, and water stable isotopes. Two large lakes were studied "in detail", i.e. the authors took seven and ten surface samples, respectively. These hydrochemical data are placed in the context of lake shore erosion rates derived from a previous study by the authors. The topic of the study is indeed highly relevant, as we are witnessing dramatic changes in Arctic landscapes due to rapidly rising temperatures, and we do not know what impact these changes will have on global climate. Furthermore, due to the remoteness of the Arctic, field observations are limited and we urgently need more observations to better understand the current state and improve simulations of future development. The manuscript is well written and the data are clearly presented. On the other hand, much more could have been gained from this study by going beyond standard analyses. A 14C analysis of DOC would have helped to assess whether the DOC originates from surface active layer material or from permafrost deepening, and a d13C analysis of dissolved methane would have helped to assess whether low CH4 concentrations in the lagoons are due to low CH4 production or increased CH4 oxidation, to name only a few examples.
The authors' aim was 'improving our understanding of the direct impacts of lake change processes on hydrochemical parameters'. However, due to the sampling design, this study does not contribute much to this goal. More than 7-10 surface water samples should have been taken per lake and, more importantly, the samples should have been taken according to the lake shore erosion rates. In particular, at lake TOL18_12, samples were taken in areas with similar shoreline erosion rates, although the rates vary greatly, and data from TLO18_13 indicate a relationship between erosion rate and DOC concentrations.
As it stands, the study mainly confirms what was already known, such as that surface water DOC concentrations in well mixed lakes do not differ very much (which seem obvious, otherwise they would not be well mixed), that DOC concentrations are higher in lakes overlying peaty sediments than overlying sandy sediments, that lakes with year-round unfrozen sediments (taliks) produce more methane than those that freeze to the ground in winter, or that CH4 concentrations in lagoons are lower than in freshwater lakes. Furthermore, some of the data presented are not discussed at all, such as anion and cation concentrations, or only very briefly, such as water stable isotopes. If these data are not important for the study, I suggest not presenting them.
To improve the manuscript, authors should focus on results that surprised them rather than on results that confirm previous knowledge.
Specific comments:
L9: I think taking seven or ten surface water samples from two lakes larger than 100 ha may not be called 'detailed lake sampling'.
L78: Refer to Figure 1
L165f: Both tests are used to test for significant differences between groups?
L172: Table 1
L248: You are confusing cations with anions (also in Table 2 and the following text). Also, how do you measure phosphorus and phosphate? Phosphorus was probably only measured as phosphate. If elements were below the detection limit, I suggest just mentioning this without writing that they were measured.
L324: I do not see that the data from TOL18_12 are in contrast to the data from TOL18_13, since the samples from the shore at TOL18_12 are from sites with similar shore erosion rates, and therefore a dependence between shore erosion rate and DOC concentration cannot be evaluated. Unfortunately, areas with higher shore erosion rates were not sampled at TOL18_12. I would shorten the discussion here, as the limited data and sampling sites give only limited insight into the effect of shore erosion on lake hydrochemistry.
L377ff: It should be made clear that these statements about CH4 production and CH4 oxidation refer to the anoxic sediments of the lagoons, not the water columns from which the samples were taken.
L388: I have not seen any mention of pH in the discussion.
Citation: https://doi.org/10.5194/egusphere-2024-2822-RC2
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