Disturbance and connectivity control biodiversity, ecosystem functioning and their interactions across connected aquatic and terrestrial ecosystems, that form a meta-ecosystem. In rivers, detrital organic matter (OM) is transported across terrestrial-aquatic boundaries and along the river network and decomposed on the way by diverse communities of organisms, including microorganisms and invertebrates. Drying naturally fragments most river networks and thereby modify organism dispersal and OM transfers across ecosystems. This may prevent organisms from reaching and consuming OM, generating mismatches between community composition and decomposition. However, little evidence of the effects of drying on river network-scale OM cycling exists. Here, we aim to examine the effects of fragmentation by drying on the structure of consumer communities and ecosystem functioning within interacting aquatic-terrestrial river ecosystems. We monitored leaf resource stocks, invertebrate communities and decomposition rates in the instream and riparian habitats of 20 sites in a river network naturally fragmented by drying. Although instream resource quantity and quality increased with drying severity, decomposition decreased due to changes in invertebrate communities and particularly leaf-decomposer abundance. Invertebrate-driven decomposition peaked at intermediate levels of upstream connectivity, suggesting that intermediate levels of fragmentation can promote the functioning of downstream ecosystems. We found that the variability in community composition was unrelated to variability in decomposition at sites with low connectivity and high drying severity, suggesting that such conditions can promote mismatches between community composition and decomposition. Decomposition instream was correlated to decomposition in the riparian area, revealing one of the first network-scale evidence of the links between ecosystem functions across terrestrial-aquatic boundaries. Our river network-scale study thus demonstrates the paramount effect of drying on the dynamics of resources, communities and ecosystem functioning in river networks, with crucial implications for the adaptive management of river networks and preservation of their functional integrity.

The dry phase of intermittent rivers promotes the emergence of diverse terrestrial and aquatic habitats where large amounts of leaf litter can accumulate. This environmental heterogeneity can cause diverse chemical alterations in leaf litter by the co-occurrence of multiple physical and biological degradation processes across these different habitats. After flow resumption, these chemically diversified leaves are mixed and continue decomposition downstream in fully aquatic conditions. We hypothesized that environmental heterogeneity during the dry phase can translate into a chemical diversification of leaf litter, that may affect leaf litter decomposition in re-established lotic conditions. Our laboratory treatments mimicking dry-phase habitats caused a strong chemical diversification of leaf litter, which – upon combination in mixed litter bags – accelerated its decomposition in a perennial river reach. Intermittent river reaches may act as hotspots of organic matter diversification, with potential implications on C processing at river-network scale.