Introduction

Accelerated glacier retreat in alpine regions and the European Alps is reducing melt-water contributions to mountain rivers and is ultimately changing the habitat conditions for aquatic life in these high-altitude waters. The retreat of glaciers is linked to a reduced hydrological variability, a decrease in sediment load, or altered water chemistry (Milner et al. , 2017), but also rising water temperatures (Niedrist, 2023). Based on their preferences and traits, invertebrates seem to be differently prepared to such climate change effects (Niedrist & Füreder, 2023). While overall changes of specialized glacier-fed river communities (i.e., their community structure) and effects of glacier retreat on freshwater biodiversity are well studied (Jacobsenet al. , 2012; Füreder, 2012; Hotaling et al. , 2017; Cauvy-Fraunié & Dangles, 2019), the quantitative response of different biotic components is still poorly understood. In addition, effects of habitat ameliorations on population structures, as observed in other extreme ecosystems (Baum et al. , 2005; Bærum et al. , 2021), have not been examined yet.
Periphyton, or benthic biofilm, is the dominant food source for invertebrates in alpine streams (Niedrist & Füreder, 2017) and allochthonous input plays a minor role above the tree line (Vannoteet al. , 1980). Given the dependency of first invertebrate consumers upon sufficient basal production, the dietary availability and quantity of periphyton may regulate grazing invertebrate abundance with subsequent cascading effects on predatory insects and cold-water fish populations in stream food webs. While the occurrence of diatoms, or chrysophytes in mountain and alpine streams (Hieber et al. , 2001; Rott et al. , 2006; Uehlinger et al. , 2010) and their response to deglaciation (Rott et al. , 2006; Fell et al. , 2018), but also the detailed composition of biofilms in glacier-fed streams (Brandani et al. , 2022), has been studied thoroughly, quantitative effects have not yet been estimated and consequences of glacier retreat on periphyton availability remain thus difficult to predict.
Different algal and bacterial groups that form periphyton (e.g., diatoms, green algae, brown algae, cyanobacteria) contain distinct lipids and fatty acids (i.e., highly nutritious polyunsaturated fatty acids) at different levels (Galloway & Winder, 2015; Twining et al. , 2016) that are highly nutritional and required by invertebrate grazers, so that the nutritional status of periphyton is largely linked to its lipid composition. Since deglaciation leads to tremendous abiotic habitat changes (Milner et al. 2017) and shifting niches for algal and bacterial groups (Uehlinger et al. , 2010), changes in periphyton biomass and lipid composition, thus in the provision of dietary nutrients are expected.
In this study we quantify and characterize benthic periphyton in mountain streams that drain glaciated catchments in the eastern European Alps. These high-altitude catchments vary in the extent of glaciation and form a gradient of catchment ice cover that served as a proxy for different stages of glacier retreat for this study.
The aim of this field study was to; a) quantify habitat-specific environmental changes relevant for the growth of primary producers (i.e., turbidity, water temperature, nutrients) that are linked to differences in catchments’ glacier coverage; b) quantify the periphyton biomass along the gradient of glaciated catchment cover, and; c) identify group-specific responses to changing habitat conditions linked to glacier retreat. This study tested the hypothesis that reduced glaciation in mountain stream catchments is negatively linked with the harshness of physical habitat conditions for primary producers (i.e., water temperature, sediment load), resulting in higher periphyton biomass availability. Further, we hypothesized that the amelioration of physical habitat conditions might also provide opportunities for previously non-dominant primary producers to thrive and increase their abundance. By testing these assumptions, this research provides insight into the effects of glacier retreat on habitat conditions and the biomass and composition of primary producers in high-altitude rivers, thus contributing to a deeper understanding of the impacts of climate change-driven glacier retreat on primary producers in glacier-fed streams.