Discussion
Recent microcosm experiments and model simulations have unravelled how
lateral (Harvey et al. 2020) and longitudinal (Jacquet et al. 2022a)
exchanges of resources can shape network-scale biodiversity and how
drying patterns may influence the dynamic of resource processing
(Catalàn et al. 2022) and communities (Jacquet et al. 2022b) within
river networks. However, comprehensive field-based evidence is missing.
Our network-scale field study confirms the paramount effect of drying in
governing resource stocks, community composition, ecosystem functioning
and their relationships in river meta-ecosystems. As expected in H1,
flow intermittence had a positive effect on instream leaf quantity, but
a negative effect on shredder abundance and hence on organic matter
decomposition rates. Partially agreeing with H2, invertebrate
communities and decomposition rates changed throughout the river network
in response to upstream connectivity, but not always linearly.
Interestingly, invertebrate-driven decomposition peaked at intermediate
levels of upstream fragmentation, revealing the potential positive
effects of upstream drying on the functioning of downstream ecosystems.
Instream and riparian responses were generally weakly related but the
relationship between riparian and instream decomposition rates changed
with flow permanence (FK; H3) and network location (CK; H4), suggesting
that the links between riparian and instream ecosystem functioning can
change depending on the disturbance regime and network-scale
connectivity. Although invertebrate richness did not directly relate to
decomposition, community composition and shredder abundance did, and the
strength of these relationships was lower among intermittent and
headwater sites (H5), suggesting that flow intermittence and low
connectivity may induce mismatches between community composition and
ecosystem functioning.
Although the negative effects of flow intermittence on decomposition can
be linked to changes in leaf resource nutritional quality (del Campo et
al. 2021a), most field studies attribute these negative effects to
losses of primary consumers such as shredders (Datry et al. 2011,
Schlief and Mutz 2011, Abril et al. 2015, Northington and Webster 2017).
We found that increasing flow intermittence had a seasonal positive
effect on instream resource quality and quantity, but the negative
effects on invertebrate richness and shredder abundances drove the
associated decreases in decomposition. Our results echo those from
Catalàn et al. (2022), who demonstrated through simulations that coarse
particulate organic matter (e.g. leaves) tend to accumulate and remain
unprocessed – thus conserving a higher reactivity – on dry riverbeds.
Such accumulation can result from decreases in transport caused by low
or null water discharges and the absence or low abundance of efficient
decomposers such as aquatic shredders (Northington and Webster 2017). In
our study, decomposition was associated to invertebrate community
variability among perennial but not intermittent sites, suggesting that
flow intermittence may create mismatches between community structure and
ecosystem functions, here decomposition. Such mismatches may result from
the variable levels and paths of recovery communities are on in these
frequently disturbed environments, preventing communities from reaching
equilibrium and thus use available resources optimally (Fukami et al.
2010, Brose and Hillebrand 2016, Datry et al. 2016). Conversely, in
hydrologically more stable, perennial sites, communities are more likely
to be at equilibrium with their environment and their composition may
better reflect resource availability and uses. As commonly found in the
literature, microbially driven decomposition was little affected by flow
intermittence, likely as a result of the microorganisms’ capacities to
(1) sustain activity during dry phases, especially if moisture is
preserved and (2) to recover their activity within days to weeks of flow
resumption (Foulquier et al. 2015, Arroita et al. 2018, Truchy et al.
2020).
River dendritic structure, i.e. multiple small streams branching into
larger rivers, promotes gradual increases in biodiversity (Finn et al.
2011, Altermatt 2013) and metabolic activity (Diamond et al. 2021) as
influx of organisms and resources (e.g. nutrients and dissolved organic
matter) increase with water discharge from up to downstream. Because of
this structure, communities in headwaters typically respond more
strongly to disturbance than in the mainstem where mass dispersal can
override the negative effect of disturbance (Tornwall et al. 2017).
However, responses to natural drying can contrast with this pattern due
to the fragmenting effect of drying, reducing connectivity and dispersal
even in the mainstem (Crabot et al. 2020, Gauthier et al. 2020). We
found that invertebrate richness responded more strongly to drying in
the mainstem than in headwaters. This bigger loss of species with
increasing intermittence in the mainstem sites may thus result from
reduced mass effects in this fragmented network and from the higher
richness typical of low-order perennial rivers, where communities have
more to lose when facing a disturbance than already species-poor
headwaters. We also found that invertebrate-driven decomposition
increased with the distance to the source and was related to shredder
abundance in headwaters only, agreeing with the theory that ecosystem
functioning and BEF relationships should peak at intermediate levels of
network connectivity (Leibold et al. 2017). Stronger relationship
between decomposition and community multidimensional variability in
headwaters than in mainstems may also reflect the high dependence of
smaller, shaded streams on leaf resources and the stronger community
specialization towards the use of leaf resource in the former.
Contrastingly, microorganism-driven decomposition and leaf litter
quality increased linearly with distance to the source, suggesting that
microorganisms activity may increase from upstream to downstream, at
least during the summer season, when microbial activity is likely
boosted by higher temperature (Friberg et al. 2009). This may in turn
increase leaf litter quality through microbial nitrogen immobilization
from the water column.
The effects of drying on communities and ecosystem functions are
relatively well documented at local reach scales (Datry et al. 2011,
Foulquier et al. 2015), whereas network-scale responses owing to
fragmentation remain overlooked, although evidence is emerging for fish
(Jaeger et al. 2014) and invertebrate communities (Gauthier et al. 2020,
Sarremejane et al. 2021). The proportion of perennial reaches upstream
had a positive effect on leaf stocks, shredder abundance and
invertebrate-driven decomposition, suggesting that connectivity to
upstream perennial habitats is key in determining downstream leaf
transfer (Catalàn et al. 2022), communities (Pineda-morante et al. 2022,
Fournier et al. 2023) and ecosystem functions (Harvey et al. 2017).
Interestingly, decomposition was higher when upstream connectivity was
intermediate, suggesting that intermediate extents of flow intermittence
upstream may promote higher functional rate in downstream habitats. In
rivers, where the flux of resources and organisms is directional (i.e.
dictated by the unidirectional flow of water), downstream ecosystem
functions may thus depend on the disturbance regime in – and
connectivity to – upstream habitats. An intermediate amount of upstream
intermittence could therefore promote downstream decomposition rates by
providing 1) pulses of high quality leaf resources and 2) influx of
diverse species best adapted to resource use, from a variety of upstream
habitats, following rewetting events (Northington and Webster 2017, del
Campo et al. 2021a, Catalàn et al. 2022).
Decomposition is usually faster in aquatic than in riparian habitats due
to moisture limitation hindering microbial activity and the presence of
less efficient and abundant detritivore invertebrate communities in the
latter (Hutchens and Wallace 2002, Tiegs et al. 2019, Lohse et al. 2020,
Simões et al. 2021). Accordingly, we found lower microorganism- and
invertebrate-driven decomposition, shredder abundance and invertebrate
richness in network-wide riparian habitats. We found weak evidence of
relationships between instream and riparian invertebrate communities and
changes along flow permanence gradients, suggesting that community
overlap between these habitats was limited. Although long dry phases
(i.e. > year) may lead to invertebrate community overlap
between riparian and instream habitats due to convergences in
environmental characteristics (Steward et al. 2022), the dry phases the
Albarine experienced in 2021 were likely too short (< 2
months) to promote such community homogenization. However, the
relationship between instream and riparian invertebrate-driven
decomposition, and more weakly so instream and riparian invertebrate
richness, changed with the distance to the source. The negative
relationships observed in headwaters may suggest a compensation
phenomenon where high invertebrate community richness and decomposing
activity in riparian habitat may promote low richness and decomposition
in adjacent instream habitats, and vice versa (Abelho and Descals 2019).
This may occur if riparian decomposers use high quality compounds,
leaving instream communities with lower quality resource entering
through runoff, though we did not observe a negative relationship
between leaf litter quality and quantity across both habitats.
Conversely, microbial-led decomposition instream and in riparian
habitats were positively related and this relationship was stronger
among intermittent sites, suggesting that the microbial decomposer
communities in both habitats may be related and more strongly so as flow
intermittence increases. Instream and riparian microbial communities are
likely to interact as suggested by compositional overlap between
riparian soil and instream bacterial communities (Ruiz-González et al.
2015) and between senescent leaf and instream leaf-litter fungal
communities (Koivusaari et al. 2019). Site-specific homogeneity in
environmental characteristics such as humidity may also explain
co-variability between riparian and instream microbial decomposition,
particularly in headwaters where tree canopy may help preserving
humidity in both the riparian and instream area.