Drying and fragmentation drive the dynamics of resources, consumers and ecosystem functions across aquatic-terrestrial habitats in a river network

Rivers form meta-ecosystems, in which disturbance and connectivity control
biodiversity, ecosystem functioning, and their interactions – across the
river network, but also across connected instream and riparian ecosystems.
This aquatic-terrestrial linkage is modified by drying; a disturbance that
also naturally fragments river networks and thereby modifies organism
dispersal and organic matter (OM) transfers across the river network.
However, little evidence of the effects of drying on river network-scale
OM cycling exists. Here, we assessed the effects of fragmentation by
drying at the river meta-ecosystem scale by monitoring, leaf resource
stocks, invertebrate communities, and decomposition rates across three
seasons and 20 sites, in the instream and riparian habitats of a river
network naturally fragmented by drying. Although instream leaf resource
quantity and quality increased, leaf-shredder invertebrate richness and
abundance decreased with flow intermittence. Decomposition was however
mainly driven by network-scale fragmentation and connectivity. Shredder
richness and invertebrate-driven decomposition both peaked at sites with
an intermediate amount of intermittent reaches upstream, suggesting that
upstream drying can promote the biodiversity and functioning of downstream
ecosystems. Shredder richness had however a negative effect on
decomposition in perennial sites, likely due to interspecific competition.
Leaf quantity, invertebrate communities, and invertebrate-driven
decomposition became more similar between instream and riparian habitats
as drying frequency increased, likely due to homogenization of
environmental conditions between both habitats as the river dries. Our
study demonstrates the paramount effects of drying on the dynamics of
resources, communities, and ecosystem functioning in rivers and represents
one of the first network-scale evidence of the co-drivers of ecosystem
functions across terrestrial-aquatic boundaries.