Conclusion

The rapid retreat of glaciers in mountain catchments (Sommer et al. , 2020; Hugonnet et al. , 2021) is causing significant changes in the hydrological and geomorphological characteristics of downstream rivers (Milner et al. , 2017). Besides the well-studied effects on invertebrates, especially cold-water specialists, the impacts on primary producers are only partly understood (e.g., Niedrist et al. , 2018; Fell et al. , 2018). This study focused on quantifying the regulation of periphyton biomass and the composition by glacial contribution, expanding our understanding of glacier loss on primary production in these rapidly transforming headwaters. We revealed that reduced glacial influence leads to ameliorated habitat conditions favoring periphyton growth, with higher biomass content observed in less glaciated catchments. Thereby, reductions in sediment load have been identified most beneficial for periphyton growth, whereas summer water temperature alone may not be the limiting factor for periphyton growth in these systems. Given the regulatory role of sediment for biota in glacier-fed rivers, predictions of sediment dynamics are crucial for understanding future habitat developments in mountain streams. However, developing such predictive models requires data from diverse glacial streams with varying topographical characteristics. Therefore, we recommend a collaborative approach that utilizes existing data from observed glacial streams to improve the accuracy of predictions.
Additionally, the colonization and establishment of groups previously underrepresented in highly glaciated catchments, such as cyanobacteria, are promoted by the reduced glacial influence. This study also highlights the importance of quantifying periphyton biomass using ash-free dry mass, even in slightly turbid rivers, since sediments might blur the estimations using dry mass only. Overall, this research contributes to our understanding of the complex interactions between glacier retreat, periphyton communities, and their habitats in mountain rivers, emphasizing the need for collaborative efforts and further investigation to predict future habitat developments in the face of ongoing glacier loss across the globe.