Fig. 2. Relationship between the suspended sediment concentration (left) and the sediment load (right) with the relative glacier cover in the catchment (% GCC) in the study streams during July 2021 and 2022 with indications of relationships (polynomial regressions in orange, linear model as grey text). Dashed lines indicate 95% confidence intervals, vertical dotted line indicates the breakpoint of the relationship. Y-axis has a log-scale.

Periphyton biomass in differently glaciated catchments

The mean dry mass of biofilm in the study streams ranged from 1710 to 5740 mg m-2, whereas the mean organic fraction (ash-free dry mass, AFDM) was significantly lower (ranging from 130 to 693 mg m-2; mean values). Single highest AFDM densities reached 1181 mg m-2 in the least glaciated catchments. The periphyton dry mass (g m-2) was not related to the degree of glaciation in the studied catchments (no significant linear relationship, p=0.78, R²=0.01, Fig. 3 A). Instead, AFDM was significantly negatively linked to glaciation in the catchment (F1,13=28, p<0.001, R²=0.68, Fig. 3 B) in all sampled months. In streams with larger glacial influence, the AFDM reached 300 mg m-2, whereas the AFDM of riverbeds with little (10%) or no glacial input reached mean values of 500-600 mg m-2. Compared to the potential link between nutrients (i.e., NO3-N) and AFDM (no significant relationship, p=0.062), the effect size of the relative glacier cover in the catchment exceeded the one of nitrate concentration (R²partial= 0.45 vs. 0.10).
The biofilm dry mass per study stream varied among transects, with large deviations found for the total dry mass (estimates within same streams deviate between 29-48%, Fig. S2) and smaller variability for the AFDM (organic fraction; 14 – 35% (95% CI) (Fig. S2). Chlorophyll a
contents (µg cm-2) were negatively related to glaciation (polynomial model, R²=0.63, p=0.002, Fig. 3).