Highlights on periphyton in a recently deglaciated floodplain

Glaciers are retreating worldwide due to climate change, creating extensive proglacial margins exposed to solar radiation and hence colonization by phototrophic organisms. The extremely dynamic nature of proglacial margins makes ecological colonization difficult. Whilst proglacial margins have received significant attention from the geomorphology community, their ecological functioning remains less intensively investigated. Classic research has shown that colonization depends on distance from the glacier terminus and on season. However, with current rates of glacier retreat, long downstream distances are becoming exposed in a relatively short time, questioning the validity of this longitudinal chronosequence model. In this research, we decrypt the physical habitat of periphyton in recently deglaciated floodplains and we demonstrate the role that periphyton plays in favoring embryonic ecosystem development.

 

First, we combine UAV based remote sensing with characterization of local environmental conditions (e.g., inundation extent, rates of disturbance). We show that in proglacial margins periphyton effectively develop extensively during windows of opportunity (i.e., spring and autumn) but they can also develop less extensive but still important extents in summer, during the season of most intense glacial melt. Such development may occur rapidly (timescale of days) in the active zone of the braidplain as access to water is secured. But high rates of morphodynamic reworking means that the periphyton are emphemeral. However, in smaller channels, often fed by hillslope tributaries and/or groundwater, away from the active zone, that are more stable, extensive perennial periphyton cover may develop. As the probability of access to water tends to be positively correlated with the probability of disturbance, extensive perennial periphyton development is spatially restricted.

 

Second, we deploy in-situ flume experiments to mimic the conditions of stable channels and use close-range photogrammetry and 3D hydraulic analysis. We show that periphyton development strongly modifies the streambed morphology but much less so the near-bed hydraulics. Most importantly, it reduces water vertical infiltration by clogging the streambed interstices. This autogenic response, a form of ecosystem engineering, explains why pioneer vegetation tends to develop in specific locations of a glacial floodplain, and reveals new patterns in primary succession in deglaciated terrains and the important role played by periphyton. However, whilst periphyton can improve local hydrological conditions, they do not appear to be able to counter the potential risks of geomorphic disturbance and it is that which determines the patterns of ecological succession.