Long-term response of diatoms in high-elevation streams influenced by rock glaciers

In this study, we monitored diatom assemblages at various sites along a stream-lake network draining a glaciated alpine cirque landscape (2600 m asl) in the Swiss National Park over 19 years (2001-2020). The primary goal of this study was to examine the long-term changes in diatom assemblages in the running waters at high-elevation influenced by rock glaciers, and particularly the use of diatoms as indicators of environmental change. Because streams in the catchment are interconnected with lakes, another objective was to compare diatom assemblages between lake inlets and lake outlets as well as along the chain of lakes. The Macun cirque comprises two main drainage basins with one influenced by waters from rock glaciers. Lakes are interconnected by stream segments in both basins before merging at an outflow lake. We expected changes in diatom assemblages would reflect changes in long-term physico-chemistry of surface waters between basins, between lake inlets and outlets, and along lake chains. The novelty of this work is that covers a unique heterogeneous aquatic environment in the Alps affected by climate change, an area where the temperature has increased over twofold more than other regions in the northern hemisphere. This long-term study provides the opportunity of forecasting how other alpine freshwater ecosystems globally may change in the future. We combined a classical taxonomic approach and scanning electron microscopy (SEM) along with physico-chemical data across a broad spatio-temporal scale. Diatom assemblages showed distinct spatial differences between basins, reflecting basin-wide differences in water physico-chemistry. Diatoms in both basins displayed a major temporal shift in composition between samples collected pre-2010 with those collected post-2010 that also followed temporal changes in physico-chemistry, suggesting a tipping point occurred in the system. There were subtle differences in diatom assem-blages between lake inlets and outlets, most evident in the precipitation/groundwater fed north basin. Lastly, a clear lake order effect was detected in both basins with upper basin sites differing from lower basin sites at similar altitude. We conclude that diatoms were effective in documenting the environmental changes that occurred in these alpine waters, specifically reflecting the abrupt change in physico-chemistry (concurrent tipping point in diatom assemblages) and the concomitant increase in surface water homogeneity and similarity of diatom assemblages over the study period.