Microplastics (MPs) represent an important threat to rivers and exhibit complex patterns of accumulation and transport that depend mainly on environmental characteristics, among which hydrology plays a key role. However, the effects of flow intermittence on MP transport and accumulation remain unexplored. In this study, the normalized abundance by total organic carbon (TOC) of the sediment as well as the characteristics (i.e. size and polymer type) of MPs present in sedimentation areas within intermittent and perennial reaches of a small river catchment were compared. The effects of other environmental variables known to affect MP abundance and characteristics were also considered, such as land cover, grain size distribution of the sediment, distance to the road and to the hydrological source of the sampled reaches. The results showed both a negative and positive correlation between TOC normalized MP abundance and median size of MPs with the distance to the nearest road, respectively. This was likely due to the presence of small illegal dumps located near roads, that led to localized hotspots of streambed MP pollution in nearby rivers. Flow intermittence affected the size of MPs, with a negative correlation between the median MP size and flow intermittence, but no effect was detected on TOC normalized MP abundance and polymer diversity. This study offers new insights regarding the effects of flow intermittence in combination with other factors on the abundance and characteristics of streambed MPs.

Extreme climatic events (ECEs) including floods, droughts and heatwaves are increasing in severity and frequency, fundamentally reshaping riverine ecosystems. In this Review, we synthesize global evidence of the impacts of ECEs on riverine biodiversity, revealing widespread and often compounding threats. ECEs affect biodiversity in diverse ways across scales; they can erode genetic diversity, alter community composition, reduce ecosystem function and disrupt population and community synchrony across the wider river meta-network. ECEs can also amplify the impact of, and be amplified by, other global stressors, and ECEs that occur in tandem or sequentially (compound events) have potentially strong but poorly understood biodiversity impacts. Several promising statistical and mechanistic modelling frameworks now enable prediction of the impacts of ECEs under non-stationary conditions. To adequately prepare for increasing and compounding ECEs, management strategies must shift from local, reactive interventions to catchment-scale, resilience-focused approaches. Top future research priorities include high-frequency and coordinated long-term monitoring, understanding legacies and biophysical feedbacks from extremes and deconstructing the impacts of compounding events. Our synthesis provides a roadmap for advancing science and practice to confront the ecological challenges posed by an increasingly extreme future. Extreme events — such as floods, droughts and heatwaves — are escalating in frequency, magnitude and duration. This Review discusses the implications of these global changes for biodiversity in rivers, across population, community and ecosystem scales.

Many inland waters are shrinking due to shifts in climate and water diversion for human uses. As they dry out, their exposed sediments emit large amounts of carbon dioxide (CO2) to the atmosphere. However, current global estimates of CO2 emissions from dry inland waters are derived exclusively from bare sediment dark-chamber measurements that do not account for the colonization of desiccated areas by vegetation. To understand the impact of vegetation on CO2 emissions from dry sediments, we analyzed 164 dry inland water bodies across five climatic regions and five inland water body types (lakes, ponds, reservoirs, streams and wetlands). On average, within vegetated zones, vegetation occupied 47 +/- 35% in measured biomass quadrants. Light-induced decreases in instantaneous CO2 emissions in vegetated dry sediments were lower (mean +/- SD = -3.7 +/- 12.9 mmol CO2 m-2 hr-1) than increases during dark conditions (14.7 +/- 20.1 mmol CO2 m-2 hr-1). Diel (24-hr) CO2 emissions from dry, vegetated sediments (mean +/- SD = 100 +/- 261 mmol CO2 m-2 d-1) were 25% lower than in bare sediments (133 +/- 245 mmol CO2 m-2 d-1). These results indicate that vegetation can partially off-set sediment respiration, although the magnitude of this effect is insufficient to switch dry beds from net sources to net sinks of carbon.

Drying river networks comprise intermittent reaches that cease to flow or dry, and these networks are becoming more prevalent with climate change. However, their distribution, constituent species, and ecological processes remain underexplored in the Neotropics. Drying is a strong environmental filter that reduces alpha-diversity through the loss of taxa that lack adaptations to resist and recover from drying. beta-diversity, however, can be high in river networks with intermittent reaches. In this study, we sought to understand the spatiotemporal dynamics of a fish metacommunity in a Neotropical drying river network through its annual hydrological cycle. To do so, we assessed the relationships of local and regional variables with fish metacommunity structural variability in the Choc & oacute; forests, a biodiversity hotspot in Northwestern Ecuador. Fish alpha-diversity was lower in intermittent reaches during the wet and dry seasons, whereas temporal beta-diversity was higher in perennial reaches. Distinct fish assemblages were indicative of intermittent and perennial reaches, with assemblage composition being stable in intermittent reaches but more variable in perennial reaches. Lastly, metacommunity analysis suggested that environmental filtering was the primary community assembly process, with species being replaced along the environmental gradient of the river network. Physical distance influenced community structure, but only for fish with limited inferred dispersal ability, indicating that for these species, community dynamics may be shaped by hydrological connectivity rather than environmental conditions alone. Our findings suggest that fish assemblage variability in this Neotropical river network resulted from seasonal drying, species-specific habitat preferences, and dispersal abilities, partially confirming previous hypotheses on fish diversity in drying river networks. The results emphasize that preserving both intermittent and perennial habitats while maintaining longitudinal connectivity is essential to safeguard the ecological processes that sustain fish metacommunities in drying river networks.

Abstract Context . Worldwide, more than 50% of river reaches experience flow intermittence and this proportion is increasing due to climate change and water abstraction, with foreseeable impacts on ecosystem function and structure. Concomitantly, multiple anthropogenic impacts associated with land‐use changes are impairing river ecosystems. Evaluating whether and how anthropogenic impacts and flow intermittence interact is paramount to guide ecosystem management in a rapidly changing world. Goal . First, we aimed at testing whether and how the effects of intermittence and anthropogenic stressors on biodiversity interact and secondly to evaluate whether such interactions change depending on the biomonitoring metric employed. Methods . We used a large dataset of French river biomonitoring networks ( n = 545 sites) in which macroinvertebrates were sampled along wide gradients of flow intermittence and multiple anthropogenic stressors. We ran generalized linear models to test the independent and interactive effects of flow intermittence and anthropogenic stressors on both traditional and functional‐based biomonitoring metrics. Main results . We found that most metrics responded interactively to the effects of flow intermittence and anthropogenic stressors, except functional redundancy, which responded only to anthropogenic impacts, showing a single effect. In addition, we observed that both taxonomic and functional diversity metrics of macroinvertebrates are particularly vulnerable to anthropogenic stressors in non‐perennial reaches compared with perennial ones. Synthesis and applications . Our results show that functional redundancy provides a consistent indicator of anthropogenic impact across both perennial and non‐perennial rivers. We suggest that incorporating this metric into biomonitoring frameworks could improve the detection of ecological degradation in increasingly intermittent river networks, offering a more robust basis for management and conservation decisions under global change.

River drying is intensifying worldwide due to climate change and increasing water abstraction, with major consequences for riverine biodiversity and ecosystem functioning. In river networks, drying not only alters local environmental conditions but also disrupts hydrological connectivity, reshaping the movement of organisms and resources at the river network scale. Leaf litter decomposition-a key ecosystem function in freshwater systems-is particularly sensitive to changes in the structure of decomposer communities. We hypothesized that spatiotemporal patterns of drying regulate decomposition by altering the diversity and composition of detritivorous macroinvertebrates, bacteria and fungi. We combined data from six European river networks spanning a wide latitudinal gradient to assess how local drying intensity and regional hydrological connectivity affect decomposition through changes in these decomposer groups. We found that short drying events (≤ 6 dry days) reduced decomposition rates by up to 50% mainly mediated by changes in the composition and diversity of decomposer communities. Drying decreased the diversity of groups contributing to decomposition, shifting the control of this function from a balanced contribution of fungi, bacteria and detritivores in perennial rivers to dominance by dry-tolerant but less efficient bacteria in drying rivers. These community shifts persisted for months after flow resumption, causing sustained reductions in decomposition also in flowing conditions. Regional hydrological connectivity alleviated these negative effects of local drying by facilitating the recovery of more efficient aquatic decomposers. However, this effect depended on the river network. In more arid networks, stronger fragmentation hindered the recovery of decomposer communities after flow resumption. Overall, our results evidence that spatiotemporal patterns of drying can regulate the linkages between community structure and ecosystem functioning in river networks. As drying events become more frequent and prolonged, shifts in decomposers' diversity are likely to alter carbon cycling and energy fluxes in freshwater ecosystems under global change.

Global change is increasing river drying and transforming surrounding landscapes through human land use, with important consequences for freshwater biodiversity. Yet how river drying and land use jointly influence biodiversity across spatial scales remains poorly understood. Using macroinvertebrate data from 49 riverine metacommunities in southeastern France, we examined how river drying and land use affect biodiversity across spatial scales and the metacommunity properties underlying these patterns. We estimated species-area relationship (SAR), with intercept and slope of SAR curve representing patterns analogous to alpha- and beta-diversity, respectively, and fitted structural equation models to assess how flow intermittence, land use (% of non-natural surface), and network size affected the metacommunity properties underlying these patterns, including intraspecific aggregation, species abundance distribution, total abundance, and regional species pool. Flow intermittence increased intraspecific aggregation and reduced evenness, leading to a lower SAR intercept and a steeper slope, reflecting lower local diversity but greater compositional differentiation among non-perennial rivers, and revealing strong scale-dependent responses of biodiversity to drying. Trait analyses further showed that flow intermittence favored aerial passive dispersers while reducing the relative abundance of aquatic active dispersers, suggesting that altered dispersal dynamics in non-perennial river networks contribute to biodiversity organization. In addition, land use primarily influenced biodiversity through nonlinear effects on the metacommunity species pool, with intermediate levels of land use associated with the highest regional diversity. Moreover, land use weakened the positive effect of flow intermittence on aggregation, indicating that anthropogenic pressures can homogenize metacommunities in drying regimes. Our findings demonstrate that flow intermittence strongly influences biodiversity across spatial scales through different mechanisms rather than uniformly reducing it. As climate change is forecast to increase the proportion of non-perennial rivers worldwide, our results highlight the importance of incorporating metacommunity processes and multi-scale diversity patterns into the conservation and management of these ecosystems.

The role of streams in dissolved organic matter (DOM) fluxes is widely acknowledged, yet the contribution of the hyporheic zone (Hz) to these dynamics remains unclear. For inorganic species of nitrogen, the Hz has been reported to be a sink, but what could be expected for DOM? We propose that the contribution of the Hz to stream DOM dynamics is conditioned by the water connectivity between surface and Hz (i.e., hyporheic flow). As hyporheic flow increases, DOM will tend to be removed by the microbial activity associated to the sediments in the Hz. While as hyporheic flow decreases, the microbial community will release DOM, acting as a source. We tested this hypothesis in two reaches, one with high hyporheic flow (connected reach) and another without hyporheic flow (disconnected reach), combining measurements at reach- and patch-scale of relative hyporheic flow with pore water sampling to determine DOM quantity and properties. We observed that at the reach scale, the connected reach tended to be a sink while the connected one was rather a source. Within the hyporheic zone, at the patch-scale, the areas with low hyporheic flow tended to have higher production of DOM than those more connected. Our results suggest that the contribution of the HZ to reach-scale DOM dynamics may be driven by hyporheic flow, and whether it is a source or a sink will result from the interplay among hyporheic areas with different degree of hyporheic flow.

There is an urgent need for planning actions to mitigate biodiversity loss worldwide, which involves developing assessment methods to help decision-makers identifying areas most at risk and prioritizing action. This requires robust data and analyses but it also implies thinking about realistic and cost-effective measures. Fresh waters host an important part of global biodiversity but freshwater organisms are expected to be profoundly impacted by the predicted increase in water temperatures and discharge alterations associated with climate change. However, available models focus mostly on changes in air temperature, potentially failing to incorporate these impacts. Given that freshwater biodiversity is declining at an alarming and exponentially increasing rate, there is an urgent need to monitor the potential effects of climate change. Here, we modeled the distribution of freshwater macroinvertebrates across Europe for present and future conditions including recently available data on water temperature and discharge. We also included other environmental variables that might be relevant in understanding the current spatial distribution of invertebrates (e.g. geology, adjacent land use). We used 40 datasets of standardized monitoring protocols of freshwater invertebrates spanning 23 years. Then a score of the vulnerability to climate change was attributed to each taxon based on the models. Finally, the average community indicator calculated for all European rivers allowed us to identify relevant regions for monitoring climate change using a planning conservation tool.

Climate and land use changes, as well as human water use and flow alteration, are causing worldwide shifts in river flow dynamics. During the last decades, low flows, flow intermittence, and drying have increased in many regions of the world, including Europe. This trend is projected to continue and amplify in the future, resulting in more frequent and intense hydrological droughts. However, due to a lack of data and studies on temporary rivers in the past, little is known about the processes governing the development of flow intermittence and drying, their timing and frequency, or their long-term evolution under climate change. Moreover, understanding the impact of climate change on the drying up of rivers is crucial to assess the impact of climate change on aquatic ecosystems, including the biodiversity and functional integrity of freshwater systems. This study is one of the first to present future projections of drying in intermittent river networks and to analyse future changes in the drying patterns at a high spatial and temporal resolution. Flow intermittence projections were produced using a hybrid hydrological model forced with climate projection data from 1985 until 2100 under three climate scenarios in six European drying river networks. The studied watershed areas are situated in different biogeographic regions, located in Spain, France, Croatia, Hungary, Czechia, and Finland, and their areas range from 150 to 350 km2. Additionally, flow intermittence indicators were developed and calculated to assess (1) changes in the characteristics of the drying spells at the reach scale and (2) changes in the spatial extent of drying in the river network at various time intervals. The results for all three climate scenarios show that drying patterns are projected to increase and expand in time and space, despite differences in the amplitude of changes. Temporally, in addition to the average frequency of drying events, the duration increases over the year. Seasonal changes are expected to result in an earlier onset and longer persistence of drying throughout the year. Summer drying maxima are likely to shift to earlier in the spring, with extended drying periods or additional maxima occurring in autumn and extending into the winter season in some regions. A trend analysis of extreme events shows that the extreme dry spells observed in recent years could become regular by the end of the century. Additionally, we observe transitions from perennial to intermittent reaches in the future.

Shredder organisms play a key role in rivers by feeding and fragmenting coarse organic matter that will then be exploited by other consumers. The effects of microplastics (MPs) on Gammarus sp., an ubiquitous genus of freshwater amphipods, and its shredding activity have been broadly investigated. However, the potential behavioral and physiological effects of different sizes of MPs on Gammarus sp. remain overlooked despite the recognized influence of MP size on MP toxicity. This study investigated the effects of a 28-day exposure to four different concentrations of two size fractions of PVC-microplastics (PVC-MPs), on Gammarus fossarum mortality rate, feeding rate, assimilation efficiency, and expression of proteins involved in key processes. Increased mortality was observed for all treatments exposed to PVC-MPs, with higher mortality in the presence of smaller PVC-MPs at the highest concentration. No protein biomarker modulation was observed in presence of PVC-MPs, suggesting that no metabolic stress but direct physical damages of PVC-MPs might have led to the observed mortalities. No difference was observed for feeding rates, but a higher assimilation efficiency was measured for individuals exposed to PVC-MPs, regardless of the concentration. This could be due to energy reallocation towards defense mechanisms or indicate a potential shift in digestive microbiota. This study highlighted the toxicity of PVC-MPs, particularly of smaller sizes and even at relatively low concentration, for Gammarus fossarum. PVC-MP pollution may therefore alter the functional integrity of river ecosystems by reducing the abundance of shredder organisms and, subsequently, the process of leaf litter decomposition.

AimUnderstanding the joint influence of natural disturbance regime, connectivity and biogeography on the seasonal variation of community structure.LocationDrying river networks (DRN) in Europe.Time PeriodPresent.Major Taxa StudiedAquatic macroinvertebrates.MethodsWe analyse the taxonomic and trait structure of 638 macroinvertebrate communities sampled across 125 reaches with perennial and intermittent streamflow, surveyed in six DRNs across Europe, up to six times over 1 year.ResultsRichness and trait diversity of macroinvertebrate communities decreased with increasing drying frequency, but increased with spatio-temporal connectivity in reaches with long drying events. Communities experiencing frequent drying events had higher relative abundance of taxa with a long lifecycle and drying resistance traits. Communities experiencing long drying events compensated by high spatio-temporal connectivity, and had more taxa with high fecundity and high dispersal ability. Taxa richness peaked in summer but that pattern was more prominent when drying frequency was high. Trait diversity decreased throughout the year, showing increasing abiotic stress as the year progressed. Communities changed from communities of mobile, fecund, short-lived taxa in spring and autumn to communities of long-lived taxa in summer. However, when drying frequency increased, autumn communities shifted towards communities of long-lived taxa. Macroinvertebrate community trait structure changed across Europe. It opposed communities from Mediterranean and/or upland DRNs (with more fecund and mobile taxa) to lowland DRNs (with more long-lived taxa).Main ConclusionsFrequency and duration of drying events and spatio-temporal connectivity drive divergent macroinvertebrate community structures, suggesting the presence of an ecological threshold that explains the variability of disturbed ecosystems across broad spatial scales. These factors also influence seasonal variations, with macroinvertebrate communities shaped by distinct trait-filtering processes throughout the year based on drying frequency. Ultimately, spatio-temporal connectivity plays a crucial role in sustaining species richness and trait diversity in reaches experiencing intense drying.

River drying is intensifying worldwide due to climate change and water abstraction, with major consequences for biodiversity and ecosystem functioning. In river networks, drying not only alters local environmental conditions but also disrupts hydrological connectivity, reshaping the movement of organisms and resources at the network scale. Leaf litter decomposition—a key ecosystem function in freshwater systems—is particularly sensitive to changes in the structure of decomposer communities. We hypothesized that spatiotemporal patterns of drying regulate decomposition by altering the diversity and composition of detritivore macroinvertebrates, bacteria and fungi. We combined data from six European drying river networks (DRNs) spanning a wide latitudinal gradient to assess how local drying intensity and regional hydrological connectivity affect decomposition through changes in these decomposer groups. We found that short drying events (≤ six dry days) reduced decomposition rates by up to 50% by shifting the control of decomposition from a balanced contribution of fungi, bacteria, and detritivores to one dominated by dry-tolerant but less efficient bacteria. These community shifts persisted after flow resumption, leading to sustained reductions in decomposition even under flowing conditions. Regional connectivity alleviated these negative effects of local drying by facilitating the recovery of more efficient aquatic decomposers through dispersal. However, this effect depended on DRN context. In particular, in southern, more arid DRNs, stronger fragmentation hindered the recovery of decomposer communities after flow resumption. Overall, our results provide mechanistic evidence that spatiotemporal patterns of drying can regulate the linkages between community structure and ecosystem functioning in river networks. As drying events become more frequent and prolonged, increasing disruption of these linkages will impact carbon cycling and energy fluxes in freshwater ecosystems under global change. ### Competing Interest Statement The authors have declared no competing interest. European Union Horizon 2020 research and innovation program, 869226

Abstract While collaborative science is becoming the norm in ecology, many ecologists participating in collaborations are less aware of the body of research that studies the processes by which collaborative teams organize and communicate. Here, we discuss how we successfully used a shared leadership model in the Dry Rivers Research Coordination Network. We discuss how this model promoted our success in different stages of the project, using the Tuckman model of team development: forming, storming, norming, performing and adjourning. Shared leadership in the forming phase helped us recruit a diverse membership from different scientific disciplines. In the storming and norming phases, shared leadership was especially useful in ensuring that all voices were heard in establishing group norms that promoted adhesion among and investment by RCN members. Shared leadership in the performing phase was crucial in providing opportunities for early career members to lead projects, and in the adjourning phase we reflected upon our entire collaboration to identify that shared leadership was crucial to our success, generating the thesis for this commentary. It is our hope that others may find this discussion of our experience in implementing a shared leadership model useful in developing their own fruitful collaborations. Read the free Plain Language Summary for this article on the Journal blog.

River networks are meta-ecosystems in which resources, such as leaf-litter, and their consumers are exchanged across riparian and instream ecosystems. Consumers and leaf-litter quality and decomposition vary depending on riparian land use and instream hydrological conditions, including intermittent drying. However, limited evidence of the mechanisms driving leaf-litter decomposition across aquatic-terrestrial ecosystems hinders our understanding of carbon transfer across river networks. We exposed alder (Alnus glutinosa) leaves to seven preconditioning treatments, including different riparian land uses (i.e., coniferous and deciduous forest, cattle-grazed grassland and urban) and instream habitats (i.e., buried in sediments, on a dry riverbed, in an anoxic pool), mimicking environmental conditions litter can be exposed to before entering flowing waters. We determined leaf-litter chemical composition, decomposition rates and microbial community composition in each preconditioning treatment. Then, the same litter was incubated in two flowing rivers with different flow regimes (i.e., perennial and intermittent) to monitor how previous preconditioning affected subsequent microbial succession and decomposition dynamics. During preconditioning, leaf chemical diversification, decomposer community composition and leaf-litter decomposition differed among preconditioning treatments and were mostly influenced by the presence of water, with stronger responses observed in instream than in land-use habitats. Preconditioning mediated subsequent decomposition in the flowing rivers through the alteration of leaf chemical composition-for example, depletion of carbon compounds in litter exposed to forested environments-and the turnover of bacterial and fungal taxa, likely driven by priority effects. The effects of preconditioning on aquatic decomposition differed among flow regimes, with changes in microbial community composition explaining a greater proportion of the variance in decomposition rates in the perennial river. Invertebrate-driven decomposition was two to four times faster in the intermittent than in the perennial river, reflecting the context-dependent effects of flow regimes on shredder communities. Our results demonstrate how riparian land uses and instream conditions affect river leaf-litter decomposition through cascading effects of leaf preconditioning on microbial communities and their activity. Understanding the dynamics of leaf-litter decomposition across terrestrial-aquatic boundaries is key to better predicting how global changes, including hydrological and land-use changes, may affect ecosystem functioning at the river-network meta-ecosystem scale. As the proportion of intermittent rivers is increasing globally and riparian land use is changing quickly, our conclusions indicate that the transition from perennial to intermittent flow regimes, along with riparian forest loss, could significantly alter carbon cycling across river networks.

Sociohydrosystems provide vital ecosystem services such as water purification, flood regulation, and climate regulation. However, understanding the complex relationships among ecosystem processes, hydrosystem dynamics, and ecosystem services is crucial. The metaecosystem framework, which is focused on the flows of organisms, matter, and energy among ecosystems across spatial and temporal scales, provides valuable tools for more holistic ecosystem assessments. The present article shows that a unified framework is crucial for understanding how ecosystem services are influenced by spatiotemporal dynamics of organisms, matter, and energy, advancing our understanding of how changes in one area of the hydrosystem translate into changes in other areas. The proposed framework also helps identify synergies and trade-offs among ecosystem services and the influence of environmental conditions. Finally, this unified framework enhances our ability to inform decision-making; design adaptive management strategies that consider changing environmental conditions, especially in the context of climate change; and mitigate social inequalities.

The cross-scale resilience model suggests that resilience, the amount of disturbance an ecosystem can absorb before collapsing and reorganizing, can be measured by evaluating the diversity and redundancy of functions performed by species at different spatiotemporal scales. Yet, little is known about the effects of flow intermittence and associated hydrological connectivity on the resilience capacity of instream communities, and the ecosystem functions they perform. We expected lower resilience capacity in non-perennial and isolated reaches. Here, we used fish and invertebrate community data and litter decomposition rates across 20 sites in a river network naturally fragmented by drying to characterize the drivers of resilience at the river-network scale. Using discontinuity analysis, a set of resilience indicators were calculated from body size distribution and species traits, and related to flow intermittence, network fragmentation and position in the stream network. We found that non-perennial reaches were characterized by lower resilience with fewer species, lower levels of functional redundancy of five out of eight functional feeding groups. Functional redundancy and response diversity in shredders were decoupled, translating into low litter decomposition rates in non-perennial reaches. Upstream reaches were characterized by low community resilience, likely reflecting their isolated position in the river network, but flow intermittence affected more strongly the resilience of downstream than upstream reaches. Cross-scale functional redundancy and grazer response diversity were driven by network fragmentation, meaning that the functions these groups perform might be at risk when facing other anthropogenic pressures. Finally our study suggests that reliable resilience assessments need to be based on several standardized indicators and call for more studies comparing these indicators in diverse ecosystems. ### Competing Interest Statement The authors have declared no competing interest. H2020 Marie Skłodowska-Curie, 891090