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.