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.