Decrypting the stream periphyton physical habitat of recently deglaciated floodplains

The rapid recession of glaciers is exposing large zones to the development of embryonic phototrophic ecosystems and eventual ecological succession. Traditionally, succession patterns in glacial forefields have been seen as a response to time since deglaciation, but nowadays forefield exposure is so rapid that this theory may be less applicable. In this succession process, periphyton are potential pioneer organisms because of their role in modifying the local environment (e.g. access to water) to create conditions conducive to plant colonization. In this paper, we aim to decrypt the physical properties of the habitats that define the spatial and temporal assemblage of periphyton during the melt-season of an Alpine temperate glacier in the context of rapid climate change. We show that periphyton develop in glacial floodplains throughout the melt-season and could extend to cover significant surfaces. However, development is only possible when the combined conditions of stability and water accessibility are met. In glacial floodplains, stable zones exist and are typically located on terraces; but they can also be locally found for much shorter periods in the more active, glacial-stream reworked zone. On terraces, water accessibility can be a limit due to well-drained sediments, but when present, often aided by the role that biofilms play in creating an impermeable layer, it provides a stable and clear water source that biofilms could exploit. In the active part of the braid plain, whilst water availability is very high, the water is harsh (low temperature, high turbidity) and erosive. Therein, periphyton can rapidly exploit short windows of opportunity but the habitat conditions rarely remain stable for long enough for continuous periphyton cover to develop. Thus, the role of periphyton in ecosystem succession is strongly conditioned by the spatial extent of the active zone, itself a function of high rates of glacier melt and sediment supply associated with rapid glacier retreat.