the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jun 2025
Microbial mats promote surface water retention in proglacial streams
Abstract. The retreat of glaciers opens up large proglacial areas which become available for colonization and primary succession. Yet, factors that contribute to habitability during early succession in proglacial areas remain poorly understood. In proglacial streams, biofilms, which are matrix-enclosed microbial communities, colonize the streambed and grow into millimeter thick mats. Particularly in proglacial streams draining relatively flat and stable lateral terraces, these biofilms may augment habitability by reducing water scarcity through clogging of the streambed. To quantitatively address this phenomenon, we performed streamside flume experiments and conceived the idealized terrace model, which models stream length elongation as a function of microbially induced clogging, sediment hydraulic properties, stream roughness, slope, width and inflow. Significant stream elongation, and hence habitabilization, occurs when clogging suffices to induce unsaturated conditions below the streambed. Considering multiple terrace configurations with educated parameter bounds, we found a wide range of possible elongation, ranging from none to 100-fold. Sensitivity analysis suggests that sediment hydraulic properties mostly contribute to variability in stream elongation due to biofilm induced clogging. Taken together, we here show that microbial communities can significantly extend the habitability of proglacial stream ecosystems by inducing streambed clogging and retaining water. This is relevant in light of the rapid glacier retreat.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-1664', Anonymous Referee #1, 21 Sep 2025
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RC2: 'Comment on egusphere-2025-1664', Heng Dai, 11 Oct 2025
This manuscript presents a compelling integration of flume experiments and an idealized terrace model to investigate how microbial mat-induced clogging enhances surface water retention in proglacial streams—a timely and ecologically significant topic in the context of accelerating glacier retreat. The authors convincingly demonstrate that biofilm development can substantially extend stream length through reduced infiltration, thereby promoting ecosystem habitability during early primary succession.
Overall, this is a strong and innovative contribution that advances our understanding of biogeomorphic interactions in nascent proglacial ecosystems. While the work is of high quality, I recommend that the authors address the following comments to further strengthen the manuscript:
Minor Comments:
- The "terrace model" is a useful simplification for exploring the effects of microbial clogging on stream elongation. However, the assumptions of steady-state flow, groundwater disconnection, and homogeneous clogging may not hold across all proglacial settings. The authors briefly acknowledge these limitations in Section 5.4, but a more systematic discussion—perhaps in a dedicated subsection or figure caption—would strengthen the manuscript.
- Several figures, particularly Figure 4, contain overlapping distributions and small fonts that hinder readability. I recommend increasing font sizes, and potentially separating panels for clarity.
- Although the authors note that the elongation factor is relatively insensitive to inflow variations, proglacial streams are inherently dynamic, with strong diurnal and seasonal discharge fluctuations. Given that clogging efficiency may vary with flow intensity (e.g., shear stress affecting mat integrity), could intermittent high flows reset or modulate the clogging process?
- The paper assumes constant Manning roughness (nM) for both clogged and unclogged scenarios. However, microbial mat development is known to alter bed roughness—often reducing it by smoothing grain surfaces or increasing it via filamentous structures. Since roughness affects flow depth and velocity, ignoring its change may bias estimates of ponding depth and, consequently, infiltration rates. The authors should explicitly acknowledge this limitation and, if possible, provide a sensitivity estimate (even qualitatively) of how variable roughness might influence the elongation factor.
- The model predicts stream elongation ranging from none to 100-fold, a striking result. To enhance the practical impact of the study, the authors could discuss how these predictions might be tested in the field. For example, could drone-based thermal imaging, time-lapse photography, or tracer experiments be used to detect clogging-induced changes in surface water extent?
Citation: https://doi.org/10.5194/egusphere-2025-1664-RC2
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Against the background of glacier retreat, the authors analyzed the potential effects of microbial mats on the habitability of proglacial stream ecosystems. This study improves the quantitative understanding of how microbial mats clog infiltration to retain surface water and thereby extend habitability, which is important for assessing the potential of proglacial stream lengthening under glacier retreat. The authors combined simulations with experimental measurements to obtain their results. The manuscript is well-written and well-structured, and the figures are of high quality. I only have a few minor comments for the authors’ consideration:
The authors evaluated the effect of different sediments on mat permeability and EF. Have you also considered different types of biofilms? Did you quantify organic matter in the samples? The procedure for preparing the microbial mats is not entirely clear.
Line 88: How did you account for water temperature? Could temperature affect the hydraulic conductivity of microbial mats? It seems no temperature measurements were included in the experiment.
Figure 4c: The black lines in the legend are confusing, as there is no solid black line visible in the figure. In addition, the figure is hard to read. If the solid lines indicate clogged conditions, why do they appear to show more drainage than the unclogged conditions?
Figure 4d: This figure is not cited in the main text and should be properly referenced.
I also encourage the authors to consider discussing a bit how microbial community structure and functional diversity may influence permeability and hydrological processes, as this could provide valuable ecological context for the findings.