the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Nov 2025
Extreme drought–accelerated dissolved carbon metabolism triggers pulsed CO2 outgassing in karst lakes
Abstract. Karst aquatic ecosystems are important reservoirs of dissolved carbon (C), supporting dynamic CO2 fluxes through the biological C pump. However, our current understanding of how sophisticated interactions between aquatic microbiomes and dissolved C turnover constrain the timing of CO2 sequestration and emission remains limited. Here we capture an extreme drought event and the ensuing relatively wet conditions from systematic investigations in karst lakes, demonstrating that temporally distinct microbiomes are tuned to the metabolic patterns of dissolved C and thereby modulate CO2 emissions. Specifically, we find that the extreme drought accelerates respiration of dissolved organic C, sharply increasing the CO2 evasion rate. Wet conditions stimulate photosynthetic uptake of dissolved inorganic C, consuming lake CO2 while promoting labile organic C formation. We therefore propose that pulses of CO2 emissions from karst lakes occur after wet conditions end, as a consequence of rapid remineralization of newly produced bioavailable organic C, especially during extreme droughts. Our findings highlight the crucial importance of managing periodic CO2 outgassing from karst waters under drought conditions for the implementation of region-specific C neutrality strategies.
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Status: open (until 08 Feb 2026)
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RC1: 'Comment on egusphere-2025-4284', Anonymous Referee #1, 29 Dec 2025
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CC1: 'Reply on RC1', Maofei Ni, 02 Jan 2026
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We thank you for your positive comments and constructive suggestions. Based on your suggestion, we separated the map and the boxplots in Figure 1 and harmonized the boxplot style between Figures 1 and 2, especially the display of statistical significance brackets. These revisions have been incorporated into the revised manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-4284-CC1
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CC1: 'Reply on RC1', Maofei Ni, 02 Jan 2026
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RC2: 'Comment on egusphere-2025-4284', Anonymous Referee #2, 15 Jan 2026
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This preprint reports a 2-year survey of three karst lakes (Guizhou, China) spanning an extreme drought and subsequent wetter periods, linking dissolved carbon (DOC/DIC) dynamics and aquatic microbiome shifts to CO₂ outgassing. Using carbonate-system calculations (CO2SYS) and a thin boundary-layer gas exchange model, they show drought conditions coincide with much higher pCO₂ and CO₂ efflux than wet periods, alongside DOC and community changes consistent with stronger heterotrophy during drought and more photosynthetic signatures during wet conditions. I find the manuscript interesting and the methodology sound.
General comments:
- I find the hypothesis rather shallow and suggest its revision. As the study is focused on drought and wetting events, I would expect the main hypothesis to address this.
- The “extreme drought pulse” conclusion is based on capturing one major drought episode; it’s compelling, but the manuscript should be careful about extrapolating frequency/magnitude of pulsed emissions without additional years/events or external validation.
- I think the authors should at least discuss the limitations of surface water sampling.
- The English throughout the manuscript needs some improvement. Many sentences have mistakes that harm the readers understanding and ultimate flow of the text.
Specific comments:
I think that the graphical abstract should be simplified a little, particularly the left side. What do the arrow colors and formatting represent? What are the numbers? It is all so small and packed together that the differences between conditions are unclear. Furthermore, there are some grammar mistakes.
Examples of unclear sentences:
Line 64 – “heterotrophic respiration of DOC”? Perhaps rephrase?
Lines 80-83 – “related to aquatic biology” seems very general and vague…
Lines 84-85 – “Theoretically, microbiome is anticipated to govern internal cycling between DIC
and DOC, and this process, in turn, may affect specific CO2 pathways”. This sentence is very unclear. Is this the lake aquatic microbiome? What is internal C cycling? And which specific pathways are the authors referring to?
Citation: https://doi.org/10.5194/egusphere-2025-4284-RC2 -
CC2: 'Reply on RC2', Maofei Ni, 31 Jan 2026
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Dear Referee
Thank you very much for your comments. We are able to accommodate all comments and suggestions, and also updated and re-formatted the references. Specifically, we carefully checked and updated the information, structure and language presented in the manuscript. On separated pages, we have a point-by-point response to your comments. I hope you will find the manuscript publishable in the Journal.
COMMENTS AND SUGGESTIONS:
This preprint reports a 2-year survey of three karst lakes (Guizhou, China) spanning an extreme drought and subsequent wetter periods, linking dissolved carbon (DOC/DIC) dynamics and aquatic microbiome shifts to CO₂ outgassing. Using carbonate-system calculations (CO2SYS) and a thin boundary-layer gas exchange model, they show drought conditions coincide with much higher pCO₂ and CO₂ efflux than wet periods, alongside DOC and community changes consistent with stronger heterotrophy during drought and more photosynthetic signatures during wet conditions. I find the manuscript interesting and the methodology sound.
Response: We thank you for your constructive comments. We have thoroughly revised the manuscript in response to your suggestions, which have helped improve the clarity of data interpretation as well as the overall structure and language.
General comments:
1. I find the hypothesis rather shallow and suggest its revision. As the study is focused on drought and wetting events, I would expect the main hypothesis to address this.
Response: Based on your suggestion, we removed the overly shallow/general hypothesis and updated the main hypothesis in the Introduction section to focus on event-driven wetting and drought. The revised hypothesis now explicitly links these events to shifts in microbial metabolic strategies, dissolved carbon (DIC-DOC) turnover, and CO2 dynamic as follows:
“Because hydrologic wet-dry shifts can restructure aquatic habitats and substrate availability for microorganisms, we thus hypothesized that wet and drought conditions will shift microbial metabolic strategies, thereby modulating dissolved C turnover and CO2 dynamics in karst lakes.”
The following text was added in the section of Discussion to address our hypothesis:
“Overall, consistent with our hypothesis, wet-dry seasonality shifted microbial metabolic strategies and altered temporally structured turnover between DIC and DOC, thereby regulating CO2 uptake and evasion in the karst lakes.”
2. The “extreme drought pulse” conclusion is based on capturing one major drought episode; it’s compelling, but the manuscript should be careful about extrapolating frequency/magnitude of pulsed emissions without additional years/events or external validation.
Response: Thank you for this helpful comment. Based on your comment, we realized that drought-driven responses of aquatic CO2 fluxes can vary across spatial and temporal scales (Leng et al., 2025; Zhao et al., 2025), and therefore estimating the frequency / magnitude of pulsed emissions requires additional years/events or external validation. Therefore, we now explicitly clarify that the “extreme drought pulse” conclusion is based on a single major drought episode observed during the study period. Accordingly, we refrain from extrapolating the frequency/ magnitude of pulsed emissions beyond this event. The relevant information in the sections of Abstract, Results, Discussion and Conclusion was thoroughly revised as follows:
Abstract section:
“We therefore propose that during the observed extreme drought, pulses of CO2 emissions from the study karst lakes occur after wet conditions end, as a consequence of rapid remineralization of newly produced bioavailable organic C.”
Results section:
“We show that the extreme drought increased areal CO2 efflux sharply (111 ± 104 mmol m-2 d-1), relative to initial- (6 ± 22 mmol m-2 d-1) and post-wet periods (10 ± 26 mmol m-2 d-1) in our observed episode (p < 0.001).”
Discussion section:
“In this study, we propose that the observed extreme-drought event yielded large CO2 emissions from the karst lakes, increasing >10 times on average relative to wet conditions (Fig. 1d).”
Conclusion section:
“These specific microbial strategies indicate that in our observed episode, pulsed CO2 outgassing from karst lakes may occur after periods of high photosynthetic activity, with the magnitude being significantly amplified during extreme droughts.”
3. I think the authors should at least discuss the limitations of surface water sampling.
Response: Based on your comment, we added text discussing the spatial and temporal limitations of our surface water sampling in the “Data Quality” section as follows:
“Water collection and analysis followed the standard procedures, as proposed by the American Public Health Association (1985). However, we acknowledge that (1) temporally, discrete surface sampling may omit short-lived C dynamics driven by diel variability and episodic event (e.g., flooding and thermal fluctuations); and (2) spatially, surface measurements at limited stations may not fully represent horizontal heterogeneity (e.g., differences in macrophytes and local human activities). Therefore, we interpret our results as evidence for surface water patterns during the observed period and avoid extrapolating the frequency and magnitude of CO2 pulses without higher-frequency and multi-site observations.”
4. The English throughout the manuscript needs some improvement. Many sentences have mistakes that harm the readers understanding and ultimate flow of the text.
Response: We have carefully revised the English throughout the manuscript to improve clarity and readability, corrected grammatical errors and awkward phrasing, and improved the overall flow. The English was polished by a native speaker throughout the manuscript. All changes have been incorporated in the revised manuscript.
Specific comments:
5. I think that the graphical abstract should be simplified a little, particularly the left side. What do the arrow colors and formatting represent? What are the numbers? It is all so small and packed together that the differences between conditions are unclear.
Response: We realized that the previous Graphical Abstract was too crowded, especially on the left-hand side (structural equation model, SEM), and that the arrow colors (red: positive effect; blue: negative effect), formatting, and the numbers (path coefficients) were difficult to interpret at the reduced size. To improve readability, we simplified the Graphical Abstract by removing the left-hand SEM panel and replacing it with clear visual symbols and brief text that convey the same key message. Please see the uploaded Supplement for the new version of Graphical Abstract.
6. Furthermore, there are some grammar mistakes. Examples of unclear sentences:
Response: Thank you for your careful review. We revised the sentences highlighted in your examples and carefully checked the similar mistakes to improve clarity and readability throughout the manuscript.
Line 64 – “heterotrophic respiration of DOC”? Perhaps rephrase?
Response: Changed “heterotrophic respiration of DOC” to “heterotrophic respiration fuelled by DOC”. The similar phrasing was also revised throughout the manuscript.
Lines 80-83 – “related to aquatic biology” seems very general and vague…
Response: We clarified the general and vague term “aquatic biology” by specifying it as “microbially mediated carbon fixation and mineralization”. The sentence has been revised as follows:
“These critical processes are tightly linked to microbially mediated C fixation and mineralization, but little is known about how lake microorganisms drive dissolved C turnover and ultimately modulate CO2 emissions in karst lakes.”
Lines 84-85 – “Theoretically, microbiome is anticipated to govern internal cycling between DIC and DOC, and this process, in turn, may affect specific CO2 pathways”. This sentence is very unclear. Is this the lake aquatic microbiome? What is internal C cycling? And which specific pathways are the authors referring to?
Response: Thank you for helping improve the writing and organization of the paper. We now specify that “microbiome” refers to the lake aquatic microbial community, define “internal carbon cycling” as in-lake transformation between DIC and DOC, and clarify that the “CO2 pathways” refer to CO2 production and consumption associated with microbial carbon fixation and mineralization. The sentence was revised as follows:
“Theoretically, aquatic microbial community in karst lakes is expected to regulate DIC-DOC transformation, whereby microbially mediated C fixation and mineralization shift the balance between CO2 production and consumption”.
References
Leng, P., Rode, M. and Koschorreck, M. 2025. Summer drought enhances diurnal amplitude of CO2 in two German rivers of different size. Water Research 271, 122870.
Zhao, L., Zhao, X., Fan, X., Zhang, J., Gan, G., Tan, Z., Yao, J., Wan, R., Xu, L. and Yang, G. 2025. Moderate Hydrological Droughts Maximized CO2 Sink in China’s Largest Floodplain Lake. Environ Sci Technol 59(33), 17606-17616.
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Overall, the manuscript is very well prepared, presents a clear structure, and effectively integrates biogeochemical and microbiological approaches to investigate dissolved carbon dynamics and CO₂ emissions in karst lakes. The study addresses a highly relevant and timely topic, particularly in the context of climate change and the increasing frequency of extreme events such as droughts.
I particularly appreciate that the study addresses processes that remain poorly documented in tropical and subtropical regions, where our understanding of aquatic carbon cycling and greenhouse gas emissions is still limited. The results significantly improve our understanding of the role of karst lakes in the broader carbon cycle.
In general, the manuscript is very solid, and I only have minor suggestions regarding figure presentation. Specifically, separating the map from the boxplots in Figure 1 and harmonizing the boxplot style between Figures 1 and 2 (especially the way statistical significance brackets are displayed) would improve visual clarity and consistency. These are minor points and do not affect the overall conclusions of the study.