Microbial mats promote surface water retention in proglacial streams

Analysis of short-term spatial and temporal patterns of glacio-fluvial sedimentation associated with recent glacier retreat at Feegletscher, Swiss Alps, was undertaken. Data was collected through glacial imagery, ablation, meteorological, and proglacial channel monitoring to understand how sediment transport in a glaciated catchment with two geomorphologically contrasting proglacial zones functions during, and is responding to, recent glacier retreat. The research was performed during the 2019 ablation season on the Feegletscher Sud and Feegletscher Nord, Switzerland, whose proglacial streams are situated in geomorphologically contrasting proglacial zones. To the best of this investigation's knowledge, this is the first study to identify short-term intra-basin spatial and temporal patterns of glacio-fluvial sedimentation of two geomorphologically contrasting proglacial zones. The Feegletscher Nord's upper proglacial area is characterised by a narrow gorge and a waterfall that leads into a braided proglacial source of unstable, unconsolidated fine glacio-fluvial sediment deposits, and the lower proglacial area displayed consolidated morainic sediment deposits and reworked paraglacial debris. The Feegletscher Sud's upper proglacial area is characterised by a clean bedrock forefield with minimal quantities of glacio-fluvial sediment deposits, followed by a proglacial channel that is constrained by incised lateral moraines. Examples of data analysis techniques that were used are the cross correlation of meteorological and proglacial channel data to determine links between all variables, and hysteresis plots of suspended sediment concentration-discharge to indicate glacio-fluvial dynamics. This research showed that there was variability in fluvio-glacial sedimentation between the multiple monitoring stations along the Feegletscher Nord's proglacial stream, and between the Feegletscher Nord's and Sud's proglacial stream. Both clockwise and anti-clockwise hysteresis was observed at all monitoring sites. The Feegletscher Sud showed an overall clockwise hysteresis, the upper monitoring site on the Feegletscher Nord showed anti-clockwise hysteresis, and the lower monitoring site on the Feegletscher Nord showed equal amounts of clockwise and anti-clockwise. The variability in hysteresis between the monitoring stations is attributed to sediment exhaustion, and the overall availability and stability of fine grained sediment within each proglacial area, which is controlled by variations in sedimentary and geomorphological characteristics. The differences in glacio-fluvial sedimentation at Feegletscher demonstrate that even within a relatively small glaciated catchment a range of spatial and temporal patterns exist, and a major control on determining these patterns is the location and quantity of the monitoring stations within the proglacial forefield. The additional complexity of comparing geomorphologically contrasting proglacial zones showed the importance of considering the geomorphological history of the proglacial zone…

ABSTRACTIn proglacial floodplains, glacier recession promotes biogeochemical and ecological gradients across relatively small spatial scales. The resulting environmental heterogeneity induces remarkable microbial biodiversity among proglacial stream biofilms. Yet the relative importance of environmental constraints in forming biofilm communities remains largely unknown. Extreme environmental conditions in proglacial streams may lead to the homogenizing selection of biofilm-forming microorganisms. However, environmental differences between proglacial streams may impose different selective forces, resulting in nested, spatially structured assembly processes. Here, we investigated bacterial community assembly processes by unraveling ecologically successful phylogenetic clades in two stream types (glacier-fed mainstems and non-glacier-fed tributaries) draining three proglacial floodplains in the Swiss Alps. Clades with low phylogenetic turnover rates were present in all stream types, including Gammaproteobacteria and Alphaproteobacteria, while the other clades were specific to one stream type. These clades constituted up to 34.8% and 31.1% of the community diversity and up to 61.3% and 50.9% of the relative abundances in mainstems and tributaries, respectively, highlighting their importance and success in these communities. Furthermore, the proportion of bacteria under homogeneous selection was inversely related to the abundance of photoautotrophs, and these clades may therefore decrease in abundance with the future “greening” of proglacial habitats. Finally, we found little effect of physical distance from the glacier on clades under selection in glacier-fed streams, probably due to the high hydrological connectivity of our study reaches. Overall, these findings shed new light on the mechanisms of microbial biofilm assembly in proglacial streams and help us to predict their future in a rapidly changing environment.IMPORTANCE Streams draining proglacial floodplains harbor benthic biofilms comprised of diverse microbial communities. These high-mountain ecosystems are rapidly changing with climate warming, and it is therefore critical to better understand the mechanisms underlying the assembly of their microbial communities. We found that homogeneous selection dominates the structuring of bacterial communities in benthic biofilms in both glacier-fed mainstems and nonglacier tributary streams within three proglacial floodplains in the Swiss Alps. However, differences between glacier-fed and tributary ecosystems may impose differential selective forces. Here, we uncovered nested, spatially structured assembly processes for proglacial floodplain communities. Our analyses additionally provided insights into linkages between aquatic photoautotrophs and the bacterial taxa under homogeneous selection, potentially by providing a labile source of carbon in these otherwise carbon-deprived systems. In the future, we expect a shift in the bacterial communities under homogeneous selection in glacier-fed streams as primary production becomes more important and streams become “greener”.

Characteristics and scientific values of ecological succession in Danxia Landform of China

Here for the first time, we analyze the concentration of dissolved (DOC) and particulate organic carbon (POC), as well as its optical properties (absorbance and fluorescence) from several proglacial streams across Iceland, the location of Europe’s largest non-polar ice cap. We found high spatial variability of DOC concentrations and dissolved organic matter (DOM) composition during peak melt, sampling 13 proglacial streams draining the 5 main Icelandic glaciers. Although glacial-derived organic matter (OM) was dominated by proteinaceous florescence, organic matter composition was variable among glaciers, often exhibiting relatively higher aromatic content and increased humification (based on absorbance and fluorescence measurements) closer to the glacier terminus, modulated by the presence of glacial lakes. Additional sampling locations the in flow path of the river Hvitá revealed that while POC concentrations decreased downstream, DOC concentrations and the autochthonous fraction of OM increased, suggesting the reworking of the organic carbon by microbial communities, with likely implications for downstream ecosystems as glaciers continue to melt. Based on our measured DOC concentrations ranging from 0.11 mg·L−1 to 0.94 mg·L−1, we estimate a potential annual carbon release of 0.008 ± 0.002 Tg·C·yr−1 from Icelandic glaciers. This non-conservative first estimate serves to highlight the potentially significant contribution of Icelandic pro-glacial streams to the global carbon cycle and the need for the quantification and determination of the spatio-temporal variation of DOC and POC fluxes and their respective drivers, particularly in light of increased rates of melting due to recent trends in climatic warming.

Community assembly processes underlying the temporal dynamics of glacial stream and lake bacterial communities

The gut microbiota plays a critical role in human health and disease. Microbial community assembly and succession early in life are influenced by numerous factors. In turn, assembly of this microbial community is known to influence the host, including immune system development, and has been linked to outcomes later in life. To date, the role of host-mediated nutritional immunity and metal availability in shaping microbial community assembly and succession early in life has not been explored in depth. Using a human infant cohort, we show that the metal-chelating protein calprotectin is highly abundant in infants. Taxa previously shown to be successful early colonizers of the infant gut, such as Enterococcus, Enterobacteriaceae, and Bacteroides, are highly resistant to experimental metal starvation in culture. Lactobacillus, meanwhile, is highly susceptible to metal restriction, pointing to a possible mechanism by which host-mediated metal limitation shapes the fitness of early colonizing taxa in the infant gut. We further demonstrate that formula-fed infants harbor markedly higher levels of metals in their gastrointestinal tract compared to breastfed infants. Formula-fed infants with high levels of metals harbor distinct microbial communities compared to breastfed infants, with higher levels of Enterococcus, Enterobacter, and Klebsiella, taxa which show increased resistance to the toxic effects of high metal concentrations. These data highlight a new paradigm in microbial community assembly and suggest an unappreciated role for nutritional immunity and dietary metals in shaping the earliest colonization events of the microbiota.IMPORTANCEEarly life represents a critical window for microbial colonization of the human gastrointestinal tract. Surprisingly, we still know little about the rules that govern the successful colonization of infants and the factors that shape the success of early life microbial colonizers. In this study, we report that metal availability is an important factor in the assembly and succession of the early life microbiota. We show that the host-derived metal-chelating protein, calprotectin, is highly abundant in infants and successful early life colonizers can overcome metal restriction. We further demonstrate that feeding modality (breastmilk vs formula) markedly impacts metal levels in the gut, potentially influencing microbial community succession. Our work suggests that metals, a previously unexplored aspect of early life ecology, may play a critical role in shaping the early events of microbiota assembly in infants.

Initial stages of pedogenesis are particularly slow on serpentinite. This implies a slow accumulation of available nutrients and leaching of phytotoxic elements. Thus, a particularly slow plant primary succession should be observed on serpentinitic proglacial areas. The observation of soil-vegetation relationships in such environments should give important information on the development of the “serpentine syndrome”. Plant-soil relationships have been statistically analysed, comparing morainic environments on pure serpentinite and serpentinite with small sialic inclusions in the North-western Italian Alps. Pure serpentinite supported strikingly different plant communities in comparison with the sites where the serpentinitic till was enriched by small quantities of sialic rocks. While on the former materials almost no change in plant species composition was observed in 190 years, 4 different species associations were developed with time on the other. Plant cover and biodiversity were much lower on pure serpentinite as well. Extremely low P and high Ni contents in soil were strongly related with these differences, but none of them could be interpreted as the actual limiting factor for plant development on pure serpentinite. Other nutrients or bases were not related with the different primary succession speed and species composition.

Plant colonization can be limited by lack of seeds or by factors that reduce establishment. The role of seed limitation in community assembly is being increasingly recognized, but in early primary succession, establishment failure is still considered more important. We studied the factors limiting colonization on the foreland of Coleman Glacier, Washington, USA, to determine the importance of seed and establishment limitation during primary succession. We also evaluated the effects of seed predation, drought, and existing vegetation on establishment. We planted seeds of seven species into plots of four different ages and found evidence that both seed and establishment limitation are strong in early succession. We also found that seed and establishment limitation both remained high in later stages of succession. Seed predation reduced establishment for most species and some evidence suggested that drought and existing vegetation also limit establishment. Because both dispersal and establishment failure restrict colonization in recently exposed habitat, late-seral forest species may have a difficult time migrating upward in response to global climate change.

Bacterial community structure and function change in association with colonizer plants during early primary succession in a glacier forefield

Sediment export, transient landscape response and catchment-scale connectivity following rapid climate warming and Alpine glacier recession

An important consequence of the rapid retreat of Alpine glaciers associated with global warming is the increasing extent of proglacial areas. These environments are dominated by a heterogeneous and dynamic fluvial system, whose evolution mostly depends on the interplay between the varying water discharge and coarse sediment supply coming from the glacier terminus. Although understanding the impact of glacier retreat on bedload yield is essential for the preservation of high-mountain regions, long-lasting investigations on the processes occurring in proglacial areas are lacking. In this context, seismic sensors recording river-induced ground vibrations have been shown to constitute a valid monitoring technique (Mancini et al., 2023; Corte et al., 2024).Here, we present the results of monitoring campaigns carried out in the proglacial area of the Rutor Glacier (Aosta Valley, Italy) during the ablation seasons of the last three years. Ground vibrations have been monitored using a network of three geophones installed next to a stable reach of the main proglacial torrent  ∼150 m downstream of the glacier mouth. Direct measurements of bedload transport have been made in 2022 and 2023 by deploying portable bedload traps at the glacier mouth. In addition to meteorological data gathered at a weather station, water discharge has been estimated by means of a downstream gauge station. We have found that a varying and non-trivial relationship exists between the direct bedload measurements and the recorded seismic signals, indicating a potential strong buffering of sediment export exerted by the proglacial area. Moreover, for all the three monitoring campaigns but starting at different moments of the ablation season, we have observed quasi-periodic peaks of seismic power occurring at a sub-hourly scale during the afternoon. We advance that they could be related to water discharge fluctuations resulting from the dynamics of the subglacial drainage system. These observations show the effectiveness of using seismic methods to shed some light on the complex feedback mechanisms existing between glacier dynamics and the natural processes of proglacial areas.ReferencesMancini, D., Dietze, M., Müller, T., Jenkin, M., Miesen, F., Roncoroni, M., et al. (2023). Filtering of the signal of sediment export from a glacier by its proglacial forefield. Geophysical Research Letters, 50, e2023GL106082. https://doi.org/10.1029/2023GL106082Corte, E., Ajmar, A., Camporeale, C., Cina, A., Coviello, V., Giulio Tonolo, F., Godio, A., Macelloni, M. M., Tamea, S., and Vergnano, A. (2024): Multitemporal characterization of a proglacial system: a multidisciplinary approach, Earth Syst. Sci. Data, 16, 3283–3306, https://doi.org/10.5194/essd-16-3283-2024

The effects of isolation on primary succession are poorly documented. I monitored vegetation recovery on two Mount St. Helens lahars (mud flows) with different degrees of isolation using contiguous plots. Seventeen years after the eruption, species richness was stable, but cover continued to increase. That isolation affects community structure was confirmed in several ways. The dominance hierarchies of the lahars differed sharply. Detrended correspondence analysis on Lahar I showed a trend related to distance from an adjacent woodland, whereas vegetation on Lahar II was relatively homogeneous. Spectra of growth forms and dispersal types also differed. Lahar I was dominated by species with modest dispersal ability, while Lahar II was dominated by species with better dispersal. Variation between plots should decline through time, a prediction confirmed on Lahar II. Lahar I remained heterogeneous despite having developed significantly higher cover. Here, the increasing distance from the forest has prevented plots from becoming more homogeneous. At this stage of early primary succession, neither lahar is converging towards the species composition of adjacent vegetation. This study shows that isolation and differential dispersal ability combine to determine initial vegetation structure. Stochastic effects resulting from dispersal limitations may resist the more deterministic effects of competition that could lead to floristic convergence.

Plant-microbe interactions are crucial components of ecosystem development but are understudied during early succession. The goal of this study was to investigate species-specific effects of plants on unvegetated soils being colonized by plants as climate changes, and assess how plant-soil feedbacks influence plant succession. We used lab and field litter additions in an early successional unvegetated ecosystem in the Front Range of the Colorado Rocky Mountains to examine litter-driven changes in soil bacterial and fungal communities. We then used plant litter-trained soil as inocula in a greenhouse experiment to test plant-soil feedbacks. We found species-specific effects of litter additions on bacterial and fungal communities in unvegetated soils, which are likely due to both differences in tissue litter chemistry and differences in the litter microbiome. We identified a negative effect of soil trained by litter from the conservative forb Silene acaulis on the growth of the fast-growing bunchgrass Deschampsia cespitosa, likely due to changes in microbial communities that resulted in lowered nitrification rates or to a litter-driven increase in N-immobilization. Our study demonstrates the importance of plant specificity and potential negative litter-driven feedbacks in primary succession, which could lead to patchy distribution of plant colonists as climate change allows colonization of these areas.

Abstract. Climate change and the associated glacier retreat lead to considerable enlargement and alterations of the proglacial systems. The colonisation of plants in this ecosystem was found to be highly dependent on terrain age, initial site conditions and geomorphic disturbances. Although the explanatory variables are generally well understood, there is little knowledge on their collinearities and resulting influence on proglacial primary succession. To develop a sphere-spanning understanding of vegetation development, a more interdisciplinary approach was adopted. In the proglacial areas of Fürkeleferner, Zufallferner and Langenferner (Martell Valley, Eastern Italian Alps), in total 65 plots of 5×2 m were installed to perform the vegetation analysis on vegetation cover, species number and species composition. For each of those, 39 potential explanatory variables were collected, selected through an extensive literature review. To analyse and further avoid multicollinearities, 33 of the explanatory variables were clustered via principal component analysis (PCA) to five components. Subsequently, generalised additive models (GAMs) were used to analyse the potential explanatory factors of primary succession. The results showed that primary succession patterns were highly related to the first component (elevation and time), the second component (solar radiation), the third component (soil chemistry), the fifth component (soil physics) and landforms. In summary, the analysis of all explanatory variables together provides an overview of the most important influencing variables and their interactions; thus it provides a basis for the debate on future vegetation development in a changing climate.

Supplementary material to "Microbial mats promote surface water retention in proglacial streams"