Spatial patterns of benthic biofilm diversity among streams draining proglacial floodplains

Abstract

Glacier shrinkage opens new proglacial terrain with pronounced environmental gradients along longitudinal and lateral chronosequences. Despite the environmental harshness of the streams that drain glacier forelands, their benthic biofilms can harbor astonishing biodiversity spanning all domains of life. Here, we studied the spatial dynamics of prokaryotic and eukaryotic photoautotroph diversity within braided glacier-fed streams and tributaries draining lateral terraces predominantly fed by groundwater and snowmelt across three proglacial floodplains in the Swiss Alps. Along the lateral chronosequence, we found that benthic biofilms in tributaries develop higher biomass than those in glacier-fed streams, and that their respective diversity and community composition differed markedly. We also found spatial turnover of bacterial communities in the glacier-fed streams along the longitudinal chronosequence. These patterns along the two chronosequences seem unexpected given the close spatial proximity and connectivity of the various streams, suggesting environmental filtering as an underlying mechanism. Furthermore, our results suggest that photoautotrophic communities shape bacterial communities across the various streams, which is understandable given that algae are the major source of organic matter in proglacial streams. Overall, our findings shed new light on benthic biofilms in proglacial streams now changing at rapid pace owing to climate-induced glacier shrinkage.

Catchment characteristics and seasonality control the composition of microbial assemblages exported from three outlet glaciers of the Greenland Ice Sheet

“…Upstream ponding may reduce stream velocity, allowing some of the glacial sediment to settle, thus potentially depositing a subset of the sediment-associated microbes. Proglacial freshwater habitats furthermore give rise to numerous ecological niches ( Brandani et al, 2022 ), the microbial cells from which may be subsequently entrained and transported downstream. The RG and IS rivers serve as a contrast to the LG river, which by comparison has a streambank armored by bedrock and boulder scree, with little lateral erosion and no significant tributaries above the sampling site, and may help explain the lower observed diversity at this site.…”

“…For IS and RG, the increase in richness may be attributed to greater hydrological connectivity within the proglacial catchment over time. Adjacent freshwater ecosystems become hydrologically activated with snowmelt, and likely undergo succession throughout the summer, bearing influence on the bacterial assemblages that they contribute to the streams ( Brandani et al, 2022 ).…”

“…At the same time, “autotroph” relative abundances increased in RG and IS (both p < 0.001), but not LG. This increase in autotroph relative abundance over time might indicate both an increase in connectivity with non-glacial tributaries which may be more likely to harbor such taxa ( Brandani et al, 2022 ), but also growth and successional patterns within communities inhabiting proglacial areas in general ( Kong et al, 2019 ). Alternatively, it might also be caused by the growing connection between the supra- and subglacial systems and the higher input of the surface autotrophs specifically to the proglacial stream ( Dubnick et al, 2017 ).…”

“…While glaciers are a significant source of suspended cells in the water column of proglacial streams, other sources within the hydrological catchment may gain influence downstream. For example, eroding stream banks, inlets of tributary streams, and thawing permafrost may also contribute to the transported “seed bank” ( Battin et al, 2016 ; Hauptmann et al, 2016 ; Brandani et al, 2022 ). Over the course of the melt season, the relative importance of these sources may increase as hydrological connectivity intensifies within the catchment ( Crump et al, 2003 ; Hauptmann et al, 2016 ; Cavaco et al, 2019 ).…”

Cross-domain interactions induce community stability to benthic biofilms in proglacial streams

“…To study potential interactions between pro-and eukaryotes, co-occurrence network analyses were performed with samples meeting specific criteria. These included: 1) the presence of both 16S and 18S sequence data for each sample, and 2) samples had to be categorised the same way across both samplings to ensure replicability (i.e., either designated as GFS or TRIB for both samplings as described by Brandani et al 8 ). Due to the dynamic nature of proglacial streams, GFS tend to migrate, leaving some sites dry or under the influence of TRIB, or even flood samples previously under the influence of TRIB streams.…”