Leaf litter decomposition
To measure leaf litter decomposition rates, we enclosed 4 ± 0.05 g and 2 ± 0.05 g of dried Alder (Alnus glutinosa) leaves in 1 cm mesh (coarse-mesh) and 250 μm mesh (fine-mesh) bags, respectively. Where coarse-mesh bags allowed invertebrate to enter and consume leaves, fine-mesh bags only permitted microorganism-driven (i.e. by fungi and bacteria) decomposition (e.g. Datry et al. 2011, Foulquier et al. 2015). Alder leaves were collected in autumn 2020 from a single site, near Chambery, France (approx. 80 km from the Albarine), air-dried inside the laboratory and then oven-dried for 48h at 70°C prior to being weighed and enclosed into the bags. Six coarse- and six fine- mesh bags were incubated for 29 \(\pm\) 4 days (mean\(\ \pm\) SD) in the riparian (n=3 of each mesh-type) and instream (n=3) habitats at each site and campaign.
At the end of the incubation period, bags were taken to the laboratory where remaining leaves were cleaned from sediments and other organic matter (e.g. invertebrates, twigs, other leaves). Remaining leaves were then oven-dried and ashed as described in the “Leaf litter quantity” section. Decomposition rates were calculated for each litter bags based on the negative exponential decay model (K) corrected for the duration of incubation and cumulative temperatures (i.e. degree days; Swan et al. 2021). The decomposition rates were then averaged across the three bags exposed in each habitat (instream vs. riparian) at each site and in each season. Of the total 720 mesh bags incubated in the field nine and four fine-mesh and six and two coarse-mesh bags were lost in the instream and riparian habitats, respectively, due to floods mainly.