Flow Intermittency Affects Leaf Decomposition and Benthic Consumer Communities of Alpine Streams: A Case Study along the Po River

Abstract

Streams and rivers are becoming increasingly intermittent in Alpine regions due to the global climate change and related increases of local water abstractions, making it fundamental to investigate the occurrence of supraseasonal drying events and their correlated effects. We aimed to investigate leaf litter decomposition, the C:N ratio of the litter, and changes in associated macroinvertebrate communities in three reaches of the Po River: One upstream, consistently perennial, a perennial mid-reach with high hydrological variability, and an intermittent downstream reach. We placed leaf litter bags of two leaf types—chestnut and oak; both showed comparable decomposition rates, but the remaining litter mass was different and was attributed to the C:N ratio and palatability. Furthermore, (1) in perennial reaches, leaf litter decomposed faster than in the intermittent ones; (2) in intermittent reaches, the C:N ratio showed a decreasing trend in both leaf types, indicating that drying affected the nitrogen consumption, therefore the conditioning phase; (3) associated macroinvertebrate communities were richer and more stable in perennial reaches, where a higher richness and abundance of EPT taxa and shredders was observed. Our results suggest that the variations in the hydrology of mountain streams caused by global climate change could significantly impact on functional processes and biodiversity of benthic communities.

Hidden Decomposers: the Role of Bacteria and Fungi in Recently Intermittent Alpine Streams Heterotrophic Pathways

“…The higher bacterial and fungal diversity (increased evenness) [ 65 , 84 ] of oak leaf communities suggests that recalcitrance and high secondary compound content may require a more complex microbial community for initial decomposition and mineralization [ 85 ]. In our previous work of leaf litter decomposition along the Po river [ 42 , 43 ], we observed that chestnut leaves decomposed faster than oak, where the process continued, but delayed, during riverbed drying. Given the previous findings, combined with these microbial community changes, we suggest that terrestrial leaf-associated fungal taxa, along with early aquatic colonizers of in-stream litter accelerate decomposition for less recalcitrant leaves.…”

“…To measure leaf litter decomposition associated microbial communities, leaf litter bags were employed using two leaf species abundant in the watersheds and with distinct characteristics of quality/recalcitrance: chestnut ( Castanea sativa ) with 47.64 ± 0.47 C:N content at the beginning of the experiment, and oak ( Quercus robur ), 38.98 ± 0.47 initial C:N content, both collected in autumn after abscission. For more details regarding the experimental setting, including additional data on leaf litter decomposition rates and associated macroinvertebrates, please see previous publications by Gruppuso et al [ 42 , 43 ]. Three leaf bags per leaf species were collected on each sampling date, placed into individual plastic bags, and stored at −20°C until DNA extraction and microbial community sequencing.…”

Hidden Decomposers: the Role of Bacteria and Fungi in Recently Intermittent Alpine Streams Functional Processes

“…To measure leaf litter decomposition rates and associated microbial and invertebrate communities, leaf litter bags were employed using two leaf species abundant in the watersheds and with distinct differences in quality/recalcitrance: chestnut (Castanea sativa) with 47.64 ± 0.47 C:N content at the beginning of the experiment, and oak (Quercus robur), 38.98 ± 0.47 initial C:N content, both collected in autumn after abscission. For more details regarding experimental setting please see Gruppuso et al [28,29]. Three leaf bags per leaf species were collected on each sampling date, placed into individual plastic bags and stored at -20°C until DNA extraction and microbial community sequencing.…”

Effects of Climate Change on Freshwater Biodiversity

“…Gruppuso et al [8] provided new insights into the increasing climate-induced phenomena of riverbed drying in mountain streams and their correlated effects on leaf litter (in terms of decomposition rates and the C:N ratio) and the associated macroinvertebrate communities. Using leaf litter decomposition as a tool to assess river ecosystem quality and functional stream integrity, the authors compared this process in perennial and intermittent mountain stream reaches of the Po river (Northwest Italy) and evaluated the benthic community metrics (taxa richness, feeding group abundances).…”