River drying is intensifying worldwide due to climate change and increasing water
abstraction, with major consequences for riverine biodiversity and ecosystem functioning.
In river networks, drying not only alters local environmental conditions but also
disrupts hydrological connectivity, reshaping the movement of organisms and resources
at the river network scale. Leaf litter decomposition—a key ecosystem function in
freshwater systems—is particularly sensitive to changes in the structure of decomposer
communities. We hypothesized that spatiotemporal patterns of drying regulate decomposition
by altering the diversity and composition of detritivorous macroinvertebrates, bacteria
and fungi. We combined data from six European river networks spanning a wide latitudinal
gradient to assess how local drying intensity and regional hydrological connectivity
affect decomposition through changes in these decomposer groups. We found that short
drying events (≤ 6 dry days) reduced decomposition rates by up to 50% mainly mediated
by changes in the composition and diversity of decomposer communities. Drying decreased
the diversity of groups contributing to decomposition, shifting the control of this
function from a balanced contribution of fungi, bacteria and detritivores in perennial
rivers to dominance by dry‐tolerant but less efficient bacteria in drying rivers.
These community shifts persisted for months after flow resumption, causing sustained
reductions in decomposition also in flowing conditions. Regional hydrological connectivity
alleviated these negative effects of local drying by facilitating the recovery of
more efficient aquatic decomposers. However, this effect depended on the river network.
In more arid networks, stronger fragmentation hindered the recovery of decomposer
communities after flow resumption. Overall, our results evidence that spatiotemporal
patterns of drying can regulate the linkages between community structure and ecosystem
functioning in river networks. As drying events become more frequent and prolonged,
shifts in decomposers' diversity are likely to alter carbon cycling and energy fluxes
in freshwater ecosystems under global change.
