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In naturally disturbed systems, harsh environmental conditions act as filters on the regional species pool, restricting the number of taxa able to form a local community to those with traits promoting resistance or resilience. Thus, communities in highly disturbed ecosystems may be less sensitive to a given disturbance than those in less disturbed ecosystems. We explored this idea by examining the response of aquatic invertebrate communities to flow intermittence in gravel-bed, braided rivers (BRs). Flow intermittence is considered a major driver of communities in rivers, but its influence on communities in BRs, which are recognized as naturally highly disturbed environments, is relatively unexplored. We used a multisite Before-After–Control-Impact (BACI) design to quantify the effects of drying events of different durations (moderate: 2–3 wk, severe: 1–3 mo) on invertebrate communities in 8 BRs in southeastern France. As predicted, no effects of flow intermittence were detected 1 to 4 mo after fl...

Climate change and human pressures are changing the global distribution and the extent of intermittent rivers and ephemeral streams (IRES), which comprise half of the global river network area. IRES are characterized by periods of flow cessation, during which channel substrates accumulate and undergo physico‐chemical changes (preconditioning), and periods of flow resumption, when these substrates are rewetted and release pulses of dissolved nutrients and organic matter (OM). However, there are no estimates of the amounts and quality of leached substances, nor is there information on the underlying environmental constraints operating at the global scale. We experimentally simulated, under standard laboratory conditions, rewetting of leaves, riverbed sediments, and epilithic biofilms collected during the dry phase across 205 IRES from five major climate zones. We determined the amounts and qualitative characteristics of the leached nutrients and OM, and estimated their areal fluxes from riverbeds. In addition, we evaluated the variance in leachate characteristics in relation to selected environmental variables and substrate characteristics. We found that sediments, due to their large quantities within riverbeds, contribute most to the overall flux of dissolved substances during rewetting events (56%–98%), and that flux rates distinctly differ among climate zones. Dissolved organic carbon, phenolics, and nitrate contributed most to the areal fluxes. The largest amounts of leached substances were found in the continental climate zone, coinciding with the lowest potential bioavailability of the leached OM. The opposite pattern was found in the arid zone. Environmental variables expected to be modified under climate change (i.e. potential evapotranspiration, aridity, dry period duration, land use) were correlated with the amount of leached substances, with the strongest relationship found for sediments. These results show that the role of IRES should be accounted for in global biogeochemical cycles, especially because prevalence of IRES will increase due to increasing severity of drying events., In this study we experimentally simulated under laboratory conditions rewetting of leaves, riverbed sediments, and epilithic biofilms collected during the dry phase from intermittent rivers and ephemeral streams covering global spatial scale. We determined the amounts and quality of the leached nutrients and dissolved organic matter, assessed their inter‐substrate and cross‐climate differences, and estimated areal fluxes from 1 m2 of riverbeds. In addition, we evaluated the variance in leachate characteristics related to selected environmental variables and substrate characteristics.

International audience; Perennial rivers and streams make a disproportionate contribution to global carbon (C) cycling. However, the contribution of intermittent rivers and ephemeral streams (IRES), which sometimes cease to flow and can dry completely, is largely ignored although they represent over half the global river network. Substantial amounts of terrestrial plant litter (TPL) accumulate in dry riverbeds and, upon rewetting, this material can undergo rapid microbial processing. We present the results of a global research collaboration that collected and analysed TPL from 212 dry riverbeds across major environmental gradients and climate zones. We assessed litter decomposability by quantifying the litter carbon-to-nitrogen ratio and oxygen (O2) consumption in standardized assays and estimated the potential short-term CO2 emissions during rewetting events. Aridity, cover of riparian vegetation, channel width and dry-phase duration explained most variability in the quantity and decomposability of plant litter in IRES. Our estimates indicate that a single pulse of CO2 emission upon litter rewetting contributes up to 10% of the daily CO2 emission from perennial rivers and stream, particularly in temperate climates. This indicates that the contributions of IRES should be included in global C-cycling assessments.

Intermittent rivers and ephemeral streams (IRES) can support a diverse and often abundant terrestrial and semiaquatic invertebrate (TSAI) fauna. This fauna can inhabit the shoreline, the surface of exposed gravel bars, unsaturated gravels, dry riverbeds, riparian zones, and floodplains. TSAI communities are typically dominated by ground-dwelling beetles, ants, and spiders, but many other taxonomic groups are represented. Much less is known about the species composition and ecological roles of TSAIs of IRES than their aquatic counterparts. In this chapter, we discuss the habitat requirements of TSAI fauna, as well as their taxonomic and functional diversity. We describe approaches to sampling them and the collection equipment required, the adaptations of IRES invertebrates to flow intermittence, and their roles in processing organic matter and nutrients. Finally, we identify threats to TSAI communities, outline existing knowledge gaps, and suggest how to manage IRES ecosystems to conserve or restore TSAI biodiversity. The explicit inclusion of IRES in management and legislation will help safeguard the diversity of TSAIs.

SUMMARY 1. River floodplains with their constituent biodiversity are among the most endangered ecosystems worldwide. These systems are conceptualised as mosaics of aquatic and terrestrial patches at different successional stages colonised by a wide range of aquatic, semi-aquatic and terrestrial species. 2. In three reaches in the active corridor of a French Mediterranean braided river, we tested whether community structure and composition of ground-dwelling terrestrial arthropods varied across the mosaic of terrestrial patches, and identified some of the underlying mechanisms. 3. We predicted that arthropod community structure and composition would vary consistently with the age and position of the patches in the mosaic, due to species habitat preferences and dispersal. We also predicted that between-patch species turnover would contribute to corridor diversity to a greater extent than within-patch diversity. 4. The natural flow regime of the Asse River promoted patch diversity in all the reaches considered. Arthropod communities differed according to patch age, indicating that successions had occurred over a decennial timescale. By contrast, the location of patches within the mosaics seemed not to affect communities, suggesting that dispersal was not limited. The contribution of between-patch diversity to corridor diversity far exceeded that of within-patch diversity, as predicted by mosaic theory. 5. These results put the onus on river ecosystem managers to preserve natural flow regimes in braided rivers, so as to foster various types of pioneer and mature habitat patches and maintain associated biodiversity.

SUMMARY 1. Rivers and their adjacent riparian zones are intimately linked by fluxes of water, air masses, nutri-ents and organisms. Intermittent rivers constitute ideal systems to examine these linkages becausethey are characterised by alternating wet and dry phases, and hence, natural and contrasting varia-tions occur in aquatic resources and environmental conditions.2. We addressed the effects of river drying on riparian communities by collecting ground-dwellingarthropods at three perennial sites and at four intermittent sites during a wet and a dry phase in theAlbarine River (France). We first predicted that during dry phases, declines in aquatic resources andloss of water would create harsh environmental conditions and alter community structure and com-position, due to decreases in predators and hygrophilous taxa. Second, we predicted that alternatingwet and dry phases would enhance cumulative taxonomic richness over time owing to an increasein species turnover.3. We found similar decreasing temporal patterns in taxonomic richness and abundance at intermit-tent and perennial sites, indicating that river drying had no effect on the structure of riparianarthropod communities. Community composition at intermittent and perennial sites differed onlyduring the dry phase, indicating that intermittent and perennial sites gained and lost different sets oftaxa as the river dried. Nevertheless, species turnover did not differ between sites, while taxonomicrichness was always higher at intermittent sites.4. Although some taxa were sensitive to river drying, the Albarine River supports rich and abun-dant riparian arthropod communities. While several studies have reported a strong dependenceof riparian populations on aquatic resources, our results indicate that the effects of river dryingon riparian arthropods at the community level are weak and discrete in time in the AlbarineRiver.Keywords: aquatic–terrestrial linkages, ground-dwelling arthropods, riparian zones, river drying, turnover

International audience; The distribution and ecological roles of ground-dwelling invertebrates colonizing dry beds of temporary rivers are poorly known, due in part to the lack of tested sampling methods. Sampling efficiency and completeness and detection biases associated with pitfall-trap and quadrat samples were compared at six sites in the dry beds of two New Zealand rivers. On average, pitfall-trap samples contained 3.5 times more taxa than quadrat samples, and pitfall traps required less time to collect as many taxa as quadrats. Among all taxa collected, 80% were exclusive to pitfall traps and 4% to quadrat samples. Among-sites differences in invertebrate composition were greater in pitfall-trap samples than in quadrat samples. Rarefaction curves indicated that eight to eleven pitfall traps/100 m2 were necessary to collect most of the common taxa in the study sites. Differences in the performance of pitfall traps and quadrats in dry riverbeds may be due to the absence of vegetation, to the large range of taxa considered, and to diel movements of some taxa. The large number of samples required at our study sites to reach a high level of sampling completeness suggests that the diversity of ground-dwelling invertebrates in dry riverbeds has probably been underestimated in previous studies.

Intermittent rivers and ephemeral streams (IRES) are highly heterogeneous systems, where habitat conditions range from fully aquatic to fully terrestrial. The dynamics of these habitats culminate in variation in connectivity between sites, which has multiple consequences for populations, communities, and ecosystems. IRES can thus be best understood in the context of metasystems, where local dynamics are influenced by regional processes. In this chapter, we synthesize recent work describing how metapopulation, metacommunity, and metaecosystem dynamics are driven by the hydrological variability of IRES. To date, very few studies have considered metapopulation issues in IRES, and the same is largely true for metacommunity and metaecosystem research. IRES are excellent arenas to study metasystems in highly dynamic systems. Research on metasystem issues is important for developing and testing ecological theory and also necessary to manage and conserve biodiversity and ecosystem services in IRES in the face of global change.

Summary 1. In temporary rivers, viewed as coupled terrestrial–aquatic ecosystems, spatial and temporal transition zones between aquatic and terrestrial conditions are common and occur simultaneously. 2. The effects of artificial rewetting on terrestrial and aquatic invertebrate assemblages were examined in dry sediments collected from the Albarine River, France. Rewetted sediments had previously been dry for between 0.1 and 142 days. Dry sediments were collected directly from the streambed (DS) and from riparian gravel bars (RGB). 3. We first predicted that invertebrate responses to rewetting would vary with the duration of the preceding dry period. Second, we predicted convergence of the invertebrate assemblages in DS and RGB sediments with increasing duration of the dry period. Third, we predicted that an aquatic ‘invertebrate seedbank’ (aquatic life stages that persist within streambed sediments during dry periods) would contribute substantially to the resilience of benthic assemblages. 4. Results indicated that the duration of the dry period was the primary driver of aquatic and terrestrial responses to artificial rewetting. The density and richness of aquatic taxa decreased with the duration of the dry period in both DS and RGB sediments, whereas the density of terrestrial invertebrates increased in DS sediments. 5. No convergence between DS and RGB assemblage composition was observed with an increasing dry period. Although there were more aquatic organisms in DS sediments than in RGB sediments, there was no difference in taxonomic richness between sediment types. Even after prolonged dry periods (142 days), there was typically a lower density and taxonomic richness of terrestrial invertebrates in DS sediments than in adjacent RGB sediments. 6. The results suggest that the aquatic invertebrate seedbank could contribute substantially to the resilience of benthic assemblages in the Albarine River, in addition to other mechanisms such as drift and oviposition. Of the taxa in the benthos before and after the summer dry period, 65% were also recovered from artificially rewetted DS sediments. The simultaneous presence of temporal and spatial terrestrial–aquatic transition zones in temporary rivers increases successional diversity (i.e. mosaics of dry and saturated streambed patches at various stages of terrestrial and aquatic succession). This contribution to biodiversity emphasises the need to protect dry reaches of temporary rivers.

International audience; The inputs and breakdown of terrestrial leaf litter in streams is a fundamental ecological process that sustains in-stream foodwebs and secondary production. In temporary rivers, litter breakdown is reduced during dry phases, but the long-term effect of alternating drying and wetting cycles on litter breakdown is still poorly understood. We tested the hypothesis that leaf litter breakdown (LLB) in temporary rivers is primarily controlled by flow permanence (the number of flowing days over a given period expressed in %), and that drying events affect LLB during leaf fall periods through reduction of microbial activity and the modification of aquatic invertebrate assemblages. LLB rates (k), microbial activity and invertebrate assemblages were determined in winter at ten crosssections scattered along a flow permanence gradient on the temporary Albarine River, France. Results demonstrated that summer drying events affected the breakdown process for up to 6 months after flow has resumed in the river. LLB rates decreased exponentially with decreasing flow permanence, and with increasing drying event duration and frequency. These exponential relationships were observed for flow permanence variables calculated for the river for both 24-years and 1-year time periods prior to the experiment. A decrease in flow permanence from 100 to 85% led to a four-fold decrease in leaf litter breakdown rate. Microbial activity, which typically did not differ between cross-sections, failed to explain the betweencross-section differences in k. By contrast, invertebrate assemblages and, shredders, in particular, decreased exponentially with decreasing flow permanence and with increasing drying event duration and frequency.

Correction to: Nature Geoscience https://doi.org/10.1038/s41561-018-0134-4, published online 21 May 2018. In the version of this Article originally published, the affiliation for M. I. Arce was incorrect; it should have been: 5Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany. This has now been corrected in the online versions of the Article.

International audience; Intermittent rivers are an increasing share of the world's river network, but current models don't include them. One research network is gathering knowledge about these rivers from around the world.

International audience; Temporary rivers are shifting mosaics of aquatic and terrestrial habitat driven by hydrologic variability. Advancing wetted fronts (AWFs) that rewet dry river beds are unpredictable events, and knowledge about their composition and role in habitat mosaics is scarce. We collected dead and living terrestrial invertebrates, aquatic invertebrates, and sestonic matter (i.e., suspended sediments and organic matter) entrained by flow in an AWF travelling downstream over a 7-km-long dry reach. We collected samples at 12 sites along the rewetting reach and at 3 sites in the upstream perennial reach of the Albarine River, France. Invertebrates in the AWF were mainly of terrestrial origin and organic matter was essentially coarse (>5 mm). Terrestrial invertebrate density and taxonomic richness and sestonic matter concentration were several orders of magnitude higher in the AWF than in the perennial reach. However, only terrestrial invertebrate taxonomic richness increased longitudinally. At least ⅓ of terrestrial taxa could have survived submersion, and the density and taxonomic richness of these taxa decreased downstream. These results indicate that terrestrially derived material is stored downstream during rewetting where it could greatly stimulate in-stream aquatic productivity and succession in terrestrial invertebrates along the riparian zone. In contrast, entrained aquatic taxa represented 15% of the benthic taxa collected 1 mo after rewetting, indicating a low contribution of AWFs to benthic invertebrate succession. In the context of global change and increasing appropriation of water resources by humans, our results suggest that conceptual models of invertebrate dynamic and organic matter processing in rivers should account for dry phases and transitional periods from dry to wet conditions.

National audience; Flow pulses that alternately immerse and expose benthic habitats are widely recognized as key determinants of biodiversity and ecosystem functioning in rivers. Despite terrestrial leaf litter input, colonisation, and breakdown are key processes in river ecosystems, little is known about the effects of alternating immersion and emersion on these processes. We examined Populus sp. litter breakdown, microbial activity, and colonisation by invertebrates along a natural gradient in immersion and emersion in a temporary river. We predicted that litter breakdown rate coefficient (k) would decrease as the frequency and duration of emersed periods increased, due to the negative effects on microbes and aquatic invertebrates. Moreover, we predicted that the respective importance of microbial activity on k will increase with emersion duration, because of elimination of shredder invertebrates. We also predicted that some terrestrial shredders originating in riparian zones could colonise and process leaf litter in riverine habitats following emersion. Results showed that k, colonisation by invertebrates and microbes were different among immersed, intermittent and emersed sediments, and decreased with increasing emersion duration. By contrast, the frequency of emersed periods had no detectable effects on these variables. k was correlated with shredder density on immersed sediments, with both microbial activity and shredder density on intermittent sediments, and only with microbial activity on emersed sediments. 15 terrestrial taxa did colonise leaf litter on emersed sediments. However, their role in the breakdown process was limited, likely because most were not shredders. These findings give support to recent conceptual views of temporary rivers, considered as punctuated biogeochemical reactors.