Discussion

Glacier-retreat in Alpine catchments is progressing fast with accelerating rates (Hugonnet et al. , 2021), altering the geomorphological and hydrological characteristics of downstream rivers (Milner et al. , 2017). While consequences for invertebrates (esp. cold-water specialists) have been extensively studied (e.g., Jacobsenet al. , 2012; Cauvy-Fraunié & Dangles, 2019) and are well predicted (Wilkes et al. , 2023), impacts on primary producers remain poorly understood (but see Fell et al. (2018) on expected decrease of diatom richness as glaciers retreat). In this study we quantified how glacial contribution regulates periphyton biomass in glacier-fed mountain streams, which expands our understanding of glacier loss effects on primary production in these fast transforming headwaters (Sudlow, Tremblay & Vinebrooke, 2023). Abiotic shifts appear to ameliorate habitat conditions that favor periphyton growth, as evidenced by higher biomass in less glaciated catchments. Also, a reduced glacial influence seems to promote the colonization and establishment of less nutritious basal resources, such as cyanobacteria, demonstrating that the provision of dietary primary producers to stream consumers is fundamentally linked with flow regimes of glacial streams.

Ameliorating habitat conditions

Contrary to the general assumption that stream water temperatures increase in catchments with lower glacial cover (Füreder & Niedrist, 2020), our findings revealed that waters from catchments with larger glaciers exhibited higher summer temperatures compared to those fed by smaller glaciers. Sites with higher glacial coverage also had the highest daily minimum temperatures. These temperature patterns suggest that glacier-fed river temperatures are not simply driven by the quantity of ice in the catchment, but indicate to a more complex relationship with glacier cover, discharge, and river morphology (Williamson, Entwistle & Collins, 2019). Flat and broad floodplains might lead to larger river surface areas and allow water to warm faster (Williamson et al. , 2019; O’Sullivan, Devito & Curry, 2019), as likely the case in this study. Since periphyton growth is generally enhanced by increasing temperatures through its direct influence on metabolic rates and nutrient uptake (Demars et al. , 2011), considering such antagonistic temperature dynamics is important to predict periphyton shifts as glaciers keep retreating. During summer, however, water temperature and nutrients may not be the limiting factor for periphyton growth in glacier-fed streams, but physical habitat conditions are. In particular, this study confirmed that the effects of physical limitation outcompetes a potential effect of nutrients on the periphyton growth in glacier-fed rivers (as hypothesized by (Sudlowet al. , 2023).
This field study across catchments with varying glacier coverage revealed an exponential decrease in meltwater turbidity and sediment load as glaciers recede. This milder physical habitat condition enables periphyton to thrive even during high flow conditions. The reduced sediment concentrations in the waters reduce drag forces, facilitating the colonization of habitats by species with outward growth forms (Rottet al. , 2006), rather than solely early colonizers with small cells (Biggs, Goring & Nikora, 1998). Furthermore, the study indicates that the consequences of glacier retreat for sediment load and water turbidity are significant. These effects can be partially predicted based on the degree of glaciation in the catchment, with sediment concentration and load decreasing exponentially when the degree of glaciation drops <30%. On average, sediment concentration and load decreased by 12% and 21% per 1% decrease in relative glacier cover, indicating a disproportionally higher decrease of sedimented substrates in glacial streams with respect to discharge (-8.9% per 1% decrease in relative glacier cover). However, it is important to note that sediment concentration in glacier-fed rivers can vary considerably and may be influenced by factors such as glacial activity, river slope, and flow velocity (Mao et al. , 2019).
Reduced sediment concentrations in less glaciated catchments generally improve the underwater photosynthetically active radiation (PAR) and can promote periphyton growth to certain extend (Boix Canadell et al. , 2021). In return, sediment transport can, depending on the flow velocity, abrade and scour periphyton biomass. Despite the potential photoinhibition by high UV-radiation in clear waters (Martyniuk, Modenutti & Balseiro, 2014; Jacobsen & Dangles, 2017; Elser et al. , 2020), we detected highest periphyton biomasses in rivers with no or little turbidity and less in waters with higher glacial contributions. We thus suggest that the light regime and the UV-stress in the studied sites was of minor importance for primary productivity than the physical abrasion by high discharge or transported sediment (Rinke, Robinson & Uehlinger, 2001; Hoyle et al. , 2017; Sudlowet al. , 2023).
Glacial water typically carries higher nutrient concentrations, particularly nitrogen and phosphorous, which can influence periphyton growth, as summarized by Sudlow et al. (2023). The nitrogen delivered by glacier melt originates from atmospheric deposition (Saroset al. , 2010). Unlike non-glaciated catchments, glacial streams still exhibit higher nitrogen concentrations (Hood & Scott, 2008; Slemmons et al. , 2013; Sudlow et al. , 2023), but with glacier retreat, a general decline in nutrient levels is anticipated to occur across the river network level. However, these assumptions were only partly confirmed in this study (i.e., we observed a weak negative link of nutrients with %glaciation in the catchments), assuming area-specific (and geology-based) differences. This potential interplay of factors during glacier retreat underscores the importance of understanding glacial stream nutrient dynamics to evaluate the (future) productivity of these ecosystems.

Increasing and diversifying periphyton biomass with declining glacial influence

Decreasing glacier-cover in the catchment has been linked to increasing biomass of benthic biofilms in glacial streams. Thus, warmer, slower, and less turbid waters seem to promote periphyton growth as suggested by (Sudlow et al. , 2023), suggesting that the suppression by physical disturbance in glacial streams appears to be greater than the inhibition by UV radiation in clear mountain waters (Hoyle et al. , 2017). Depending on how periphyton is quantified (dry weight vs. ash-free dry mass), our analysis reveals different biomass patterns across the rivers with different glacial influence. We attribute such difference to the accumulation of glacial flour in the biofilm because silt accumulation in epilithon that has been reported previously (Graham, 1990). Thus, this underscores the importance of quantifying periphyton biomass using ash-free dry mass, also in slightly turbid rivers.
Generally, the screened periphyton in all study streams was dominated by the pigment-group B (diatoms and chrysophytes). Although cyanobacteria and green algae were present at most sites, diatoms and chrysophytes contributed between 62 and 74% to the total chlorophyll a content, similar to the reported dominance of species in such ecosystems (Rottet al. , 2006; Fell et al. , 2018) or previous pigment screenings (Niedrist, Cantonati & Füreder, 2018). The quantifications of this pigment group correlated with the content of the diatom-specific palmitoleic acid in the same samples, which generally verified the quantifications based on fluorescence measurements. Along the gradient of glaciation, we found increases of diatoms and chrysophyte densities. We attribute this to the more benign habitat conditions in less glaciated catchments (i.e., decreased turbidity, calmed discharge, elevated temperature) that generally promotes and allows growth of present periphyton groups (Cauvy-Fraunié et al. , 2016; Sudlowet al. , 2023). The more favorable habitat conditions also support other periphyton groups, such as cyanobacteria. We observed an increasing cyanobacteria pigment content in the periphyton as glaciation decreased. Although densities remain below those of diatoms or chrysophytes even in unglaciated catchments, this confirms the general advance of cyanobacteria also in oligotrophic waters (e.g., Reinlet al. , 2023) and the known positive growth response of the cells to increasing temperatures (Lürling et al. , 2013). However, the share of immigrating species in this increase of biomass remains unknown. To estimate and compare the many species quantities at taxonomically higher level, methods such as qPCR will be necessary.