Introduction
Aquatic and terrestrial ecosystems are connected through fluxes of
resources (e.g. carbon and nutrients) and organisms, mediated by
physical forces (water, wind) and dispersal, as part of a meta-ecosystem
(Loreau et al. 2003). River networks are typical meta-ecosystems, in
which resources are exchanged across longitudinal (i.e. from headwaters
to downstream sections) and lateral (i.e. between riparian and instream
habitats) dimensions (Gounand et al. 2018, Harvey et al. 2020, Cid et
al. 2022) and transformed by organisms through ecosystem functions such
as primary production or decomposition (Hotchkiss et al. 2015, Tiegs et
al. 2019). Ecosystem functioning and community composition naturally
change along the river network as the basal trophic resource switches
from brown – i.e. based on coarse detrital organic matter riparian
input – to green – i.e. based on instream algal production – from
upstream to downstream (Vannote et al. 1980, Finlay 2001). Although the
structuring of resources, communities and their functioning along the
river network have been theorized > 40 years ago (Vannote
et al. 1980, Gounand et al. 2018), limited empirical evidence of the
mechanisms driving such structuring exist, apart from simulations
(Jacquet et al. 2022a).
Leaf-litter is a key source of organic carbon and nutrients in rivers,
exchanged in both lateral (through leaf fall and runoff) and
longitudinal (up to downstream flow) dimensions (Catalàn et al. 2022,
Scherer-lorenzen et al. 2022). The decomposition rate of leaf litter in
both aquatic and terrestrial habitats depends on several factors
including, leaf chemical quality and palatability (Pastor et al. 2014,
del Campo et al. 2021a), environmental factors such as humidity and
temperature (Tiegs et al. 2019, Annala et al. 2022) and decomposer
community (i.e. microorganisms and invertebrates) composition and
activity (Abelho and Descals 2019, Boyero et al. 2021). Although
leaf-decomposer (shredder) invertebrate communities typically differ
between instream and riparian habitats, terrestrial and aquatic
decomposers may colonize riverbed and riparian habitats during streambed
drying (Corti and Datry 2016) and overbank flooding events (Hutchens and
Wallace 2002, Steward et al. 2022), respectively. Thus, lateral movement
of consumers could link aquatic and terrestrial exchanges of organic
matter (Abelho and Descals 2019, Barthélémy et al. 2022,
Scherer-lorenzen et al. 2022), but little evidence of such links exists
at the river network scale (Harvey et al. 2020). As a result of
decreasing leaf input from headwaters to mainstem, decomposer abundances
and their resource use also typically decrease from up to downstream
(Collins et al. 2016, Jacquet et al. 2022a). However, dispersal
limitation mechanism preventing species from reaching their favoured
habitat or resource because of low connectivity, could lead to
sub-optimal functioning in isolated headwaters, despite abundant
resources (Leibold et al. 2017, Thompson et al. 2017).
Drying, i.e. the loss of surface water, is one of the most fundamental
types of disturbance in river ecosystems, triggering drastic changes in
environmental conditions that induce shifts in community composition
(Boulton 2003, Datry et al. 2014) and carbon-related biogeochemical
processes (Datry et al. 2018). By fragmenting the river continuum,
drying can also alter the transport and dispersal of resources and
organisms, limiting exchanges between populations (i.e.
metapopulations), communities (metacommunities) and ecosystems across
the network (Gauthier et al. 2020, Cid et al. 2022). Typically, the
diversity and abundance of aquatic organisms such as invertebrates
decreases as drying frequency and duration increase (Datry et al. 2014,
Soria et al. 2017, Sarremejane et al. 2020) but the colonization of dry
riverbeds by terrestrial organisms can counteract those diversity losses
(Corti and Datry 2016, Steward et al. 2022). As a result of the negative
effect of drying on shredder invertebrates, decomposition rates are
usually lower in rivers that experience drying, particularly during
their dry phase (Corti et al. 2011) but also during their flowing phase
due to legacy effects (Datry et al. 2011, del Campo et al. 2021b).
Fungal and bacterial decomposition may however be little affected by
drying, if sufficient moisture preserves microbial activity (Riedl et
al. 2013, Foulquier et al. 2015). Drying may also affect leaf-litter
quality, altering labile carbon contents and nutrient immobilization by
microbial communities, which often reduce palatability for invertebrates
(Pastor et al. 2014, del Campo et al. 2021a). At least 50% of the
world’s river networks dry, on average, for one day per year due to
geological and/or climatic features (Messager et al. 2021), and this
proportion is likely to increase due to climate change and other
anthropogenic alterations (e.g. water abstraction, irrigation; Datry et
al. 2023). We urgently need to understand how fragmentation by drying
modifies spatial and temporal interactions between organisms and
resources at the river meta-ecosystem scale, including across
aquatic-terrestrial boundaries (Scherer-lorenzen et al. 2022) to be able
to predict when and where adaptive management strategies may be needed
to preserve functional river ecosystems under global changes.
Drying and fragmentation may create mismatches between resource and
consumer availability if drying and low connectivity prevent efficient
aquatic consumers from being present and use resources (Thompson et al.
2017). Such mismatches could result in changes in the biodiversity –
ecosystem functioning (BEF) relationship as communities among highly
disturbed or isolated environments may not be composed of the most
efficient consumers; hence translating into a weak or null BEF
relationship across networks fragmented by drying (Brose and Hillebrand
2016, Leibold et al. 2017). However, intermediate levels of disturbances
and connectivity may strengthen the BEF by 1) allowing selection of the
species best adapted to the environment and resource use and 2)
preventing the dominance of functionally inefficient, highly competitive
species occurring under low levels of disturbance and high connectivity
(Cardinale and Palmer 2002, Leibold et al. 2017).
Here, we aim to examine how resource quality and quantity, the structure
of consumer communities and ecosystem functioning respond to drying
across lateral and longitudinal dimensions. To do so, we monitored
across three seasons leaf resource stocks, invertebrate communities and
decomposition rates in the instream and riparian habitats of 20 sites in
a river network fragmented by drying. We hypothesize (H1, Fig. 1) that
increasing flow intermittence promotes accumulation of leaves of low
chemical quality (low conditioning by microorganisms), decreases in
invertebrate abundance and diversity and thus lower decomposition rates
instream. Instream responses to drying may also change with connectivity
and effects may be stronger in less connected headwaters that typically
have lower diversity and post-drying recolonization capacity, likely
reducing functioning rates there (H2, Fig. 1). Drying may strengthen
relationships between instream and riparian resource community and
functioning due to homogenization of environmental conditions among
these two habitats as the riverbed dries (H3, Fig. 1). Relationship
between riparian and instream communities and functioning may also be
stronger in isolated headwaters where riparian and instream habitats are
more environmentally similar (e.g. shading, moisture) and edge effects
more dominant than in more connected downstream sections (H4; e.g.
Harvey et al. 2019). We also hypothesised that the relationship between
communities and functioning may weaken with increasing flow
intermittence and isolation, potentially driven by mismatches (H5).