| 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.
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.