Extreme drought pushes stream invertebrate communities over functional thresholds

Interactive effects of aridity and local environmental factors on the functional trait composition and diversity of macroinvertebrate assemblages in dryland rivers

Drought and local habitat alteration are major environmental stressors shaping the aquatic biota in dryland rivers. However, the combined effects of these factors on aquatic biodiversity remain poorly understood. We collected macroinvertebrate data from Central Asian dryland rivers in Xinjiang, China, from 2012 to 2022, to investigate the individual and interactive effects of drought (as indicated by increasing values of Aridity, AI) and local habitat conditions (fine sediments, velocity and pH) on aquatic macroinvertebrate functional trait composition and diversity. We found that interactions of the selected environmental stressors exhibited more frequent additive than synergistic or antagonistic effects, leading to shifts in macroinvertebrate functional trait composition and diversity accordingly. Interaction of AI and fine sediments showed more pronounced synergistic effects (positive or negative) compared to others and had positive influences on traits like small body size, ovoviviparity, etc. Functional diversity metrics responded differently to stressor interactions, with FRic and FDis being negatively affected, whereas FEve was positively correlated to stressor interaction, suggesting the complementary roles of functional diversity metrics to diagnose impacts of stressor interactions. Overall, our study provides new insights into macroinvertebrate assemblage-stressor relationships in dryland rivers and can help better assess, predict and manage aquatic biodiversity in these rivers under ongoing environmental change.

Bending the curve of global freshwater biodiversity loss: what are the prospects?

Freshwater biodiversity conservation has received substantial attention in the scientific literature and is finally being recognized in policy frameworks such as the Global Biodiversity Framework and its associated targets for 2030. This is important progress. Nonetheless, freshwater species continue to be confronted with high levels of imperilment and widespread ecosystem degradation. An Emergency Recovery Plan (ERP) proposed in 2020 comprises six measures intended to "bend the curve" of freshwater biodiversity loss, if they are widely adopted and adequately supported. We review evidence suggesting that the combined intensity of persistent and emerging threats to freshwater biodiversity has become so serious that current and projected efforts to preserve, protect and restore inland-water ecosystems may be insufficient to avert substantial biodiversity losses in the coming decades. In particular, climate change, with its complex and harmful impacts, will frustrate attempts to prevent biodiversity losses from freshwater ecosystems already affected by multiple threats. Interactions among these threats will limit recovery of populations and exacerbate declines resulting in local or even global extinctions, especially among low-viability populations in degraded or fragmented ecosystems. In addition to impediments represented by climate change, we identify several other areas where the absolute scarcity of fresh water, inadequate scientific information or predictive capacity, and a widespread failure to mitigate anthropogenic stressors, are liable to set limits on the recovery of freshwater biodiversity. Implementation of the ERP rapidly and at scale through many widely dispersed local actions focused on regions of high freshwater biodiversity and intense threat, together with an intensification of ex-situ conservation efforts, will be necessary to preserve native freshwater biodiversity during an increasingly uncertain climatic future in which poorly understood, emergent and interacting threats have become more influential. But implementation of the ERP must be accompanied by measures that will improve water, energy and food security for humans - without further compromising the condition of freshwater ecosystems. Unfortunately, the inadequate political implementation of policies to arrest widely recognized environmental challenges such as climate change do not inspire confidence about the possible success of the ERP. In many parts of the world, the Anthropocene future seems certain to include extended periods with an absolute scarcity of uncontaminated surface runoff that will inevitably be appropriated by humans. Unless there is a step-change in societal awareness of - and commitment to - the conservation of freshwater biodiversity, together with necessary actions to arrest climate change, implementation of established methods for protecting freshwater biodiversity may not bend the curve enough to prevent continued ecosystem degradation and species loss.

The spatial extent and the dispersal strategy of species shape the occupancy frequency distribution of stream insect assemblages

Several theoretical models have been proposed as the underlying mechanisms behind occupancy frequency distribution (OFD) patterns. For instance, the metapopulation dynamic model predicts bimodal OFD pattern indicating the dominance of dispersal processes in structuring the assemblages, while the niche-based model predicts unimodal right-skewed OFD pattern, and thus assemblages are driven mostly by niche processes. However, it is well known that the observed OFD pattern reflects the interplay of several other factors (e.g. habitat heterogeneity, species specificity and sampling protocol parameters). It follows that the individual contribution of each factor to the OFD pattern is rather complicated to explore. Our main objective was to examine the role of the spatial extent of the sampling and the dispersal strategies of species in shaping OFD pattern. For this, we collected samples of stream insect assemblages inhabiting near-natural streams in the Pannon Ecoregion. We formed groups of species representing contrasting dispersal strategies (referred to as dispersal groups). Applying a computer program algorithm, we produced samples with different spatial extent. We found that with increasing spatial extent, the OFD pattern changed from bimodal to unimodal for active dispersers. Insect groups with different dispersal strategies differed in the strength of support for OFD patterns within all spatial extent. Furthermore, the strength of support for OFD patterns varied across dispersal groups differently as the spatial extent increased. Our results reflected underlying changes in mechanisms structuring assemblages along an increasing spatial extent. We also assumed that the stream insect dispersal strategy influences the relative role of dispersal and niche processes particularly as spatial extent increases from stream reaches to the extent of adjacent valleys. We could define spatial extents and dispersal strategies within which unique metacommunity processes could underlie the organisation of assemblages.

Ecosystem-size relationships of river populations and communities

Knowledge of ecosystem-size influences on river populations and communities is integral to the balancing of human and environmental needs for water. The multiple dimensions of dendritic river networks complicate understanding of ecosystem-size influences, but could be resolved by the development of scaling relationships. We highlight the importance of physical constraints limiting predator body sizes, movements, and population sizes in small rivers, and where river contraction limits space or creates stressful conditions affecting community stability and food webs. Investigations of the scaling and contingency of these processes will be insightful because of the underlying generality and scale independence of such relationships. Doing so will also pinpoint damaging water-management practices and identify which aspects of river size can be most usefully manipulated in river restoration.

Interactive effects of drought and deforestation on multitrophic communities and aquatic ecosystem functions in the Neotropics-a test using tank bromeliads

Background

Together with the intensification of dry seasons in Neotropical regions, increasing deforestation is expected to exacerbate species extinctions, something that could lead to dramatic shifts in multitrophic communities and ecosystem functions. Recent studies suggest that the effects of habitat loss are greater where precipitation has decreased. Yet, experimental studies of the pure and interactive effects of drought and deforestation at ecosystem level remain scarce.

Methods

Here, we used rainshelters and transplantation from rainforest to open areas of natural microcosms (the aquatic ecosystem and microbial-faunal food web found within the rainwater-filled leaves of tank bromeliads) to emulate drought and deforestation in a full factorial experimental design. We analysed the pure and interactive effects of our treatments on functional community structure (including microorganisms, detritivore and predatory invertebrates), and on leaf litter decomposition in tank bromeliad ecosystems.

Results

Drought or deforestation alone had a moderate impact on biomass at the various trophic level, but did not eliminate species. However, their interaction synergistically reduced the biomass of all invertebrate functional groups and bacteria. Predators were the most impacted trophic group as they were totally eliminated, while detritivore biomass was reduced by about 95%. Fungal biomass was either unaffected or boosted by our treatments. Decomposition was essentially driven by microbial activity, and did not change across treatments involving deforestation and/or drought.

Conclusions

Our results suggest that highly resistant microorganisms such as fungi (plus a few detritivores) maintain key ecosystem functions in the face of drought and habitat change. We conclude that habitat destruction compounds the problems of climate change, that the impacts of the two phenomena on food webs are mutually reinforcing, and that the stability of ecosystem functions depends on the resistance of a core group of organisms. Assuming that taking global action is more challenging than taking local-regional actions, policy-makers should be encouraged to implement environmental action plans that will halt habitat destruction, to dampen any detrimental interactive effect with the impacts of global climate change.

Effects of different hydrological conditions on the taxonomic structure and functional traits of mollusk communities in a large floodplain wetland

Floodplain wetlands are critical to the conservation of aquatic biodiversity and the ecological integrity of river networks. However, increasing drought severity and frequency caused by climate change can reduce floodplain wetlands' resistance and recovery capacities. Mollusks, which are common inhabitants of floodplain wetlands, are among the most vulnerable species to drought. However, the response of mollusk communities to drought has received little attention. Here, we investigated how the structure and functional traits of mollusk communities changed in response to varying hydrological conditions, including a flash drought (FD) in the Poyang Lake floodplain wetland. Our findings showed that FD strongly reduced mollusk abundance and biomass, decreased both α- and β-diversity, and resulted in the extinction of bivalve taxa. A sudden shift in community trait structure was discovered due to the extinction of many species. These traits, which include deposit feeding, crawling, scraping, aerial respiration, and dormancy, help mollusks survive in FD and tolerate completely dry out of their Changhuchi habitat. Finally, we discovered that dissolved oxygen was an important controlling variable for mollusk communities during drought. Our findings provide a scientific basis for the management and conservation of floodplain wetland biodiversity in the context of increasing drought frequency and intensity.

Analysis of water quality and the response of phytoplankton in the low-temperature environment of Majiagou Urban River, China

Majiagou River, a crucial urban river in Harbin, traverses densely populated areas including agricultural, suburban, and main urban areas, presenting highly intricate habitat characteristics. In recent years, urbanization has significantly intensified human interference, fundamentally reshaping the phytoplankton community. Understanding the response mechanism of phytoplankton to environmental factors is of paramount importance as they serve as primary producers in aquatic ecosystems. To investigate this, we established 25 sampling sites to analyze the phytoplankton community and 14 key physicochemical parameters, such as total phosphorus (TP) and total nitrogen (TN). Utilizing hierarchical clustering analysis (HCA) and One-way Analysis of Variance (ANOVA), we identified distinct river segments, revealing spatial distribution differences and environmental factor variations among phytoplankton species across segments. By adopting redundancy analysis (RDA), we pinpointed the primary environmental factors impacting phytoplankton communities and examined the correlation between phytoplankton and these factors to elucidate the driving mechanisms governing phytoplankton dynamics. The outcomes demonstrated that the phytoplankton community in Majiagou River was predominantly composed of Bacillariophyta and Chlorophyta, however, notable disparities in spatial distribution and species composition resulting from human interference were evident. Areas with intense human disturbance were dominated by diatoms and exhibited trends of homogenization and reduced biodiversity. RDA showed that pH, NH4+-N, NH3-N, chemical oxygen demand (COD), and TP were key environmental factors influencing phytoplankton communities. We have confirmed that due to variations in environment conditions and different levels of human disturbance, there will be some differences in the critical limiting factors affecting phytoplankton. Our study offers valuable insights for governing urban rivers during the low-temperature period.

Non-perennial segments in river networks

Non-perennial river segments - those that recurrently cease to flow or frequently dry - occur in all river networks and are globally more abundant than perennial (always flowing) segments. However, research and management have historically focused on perennial river segments. In this Review, we outline how non-perennial segments are integral parts of river networks. Repeated cycles of flowing, non-flowing and dry phases in non-perennial segments influence biodiversity and ecosystem dynamics at different spatial scales, from individual segments to entire river networks. Varying configurations of perennial and non-perennial segments govern physical, chemical and ecological responses to changes in the flow regimes of each river network, especially in response to human activities. The extent of non-perennial segments in river networks has increased owing to warming, changing hydrological patterns and human activities, and this increase is predicted to continue. Moreover, the dry phases of flow regimes are expected to be longer, drier and more frequent, albeit with high regional variability. These changes will likely impact biodiversity, potentially tipping some ecosystems to compromised stable states. Effective river-network management must recognize ecosystem services (such as flood risk management and groundwater recharge) provided by non-perennial segments and ensure their legislative and regulatory protection, which is often lacking.

Extra terrestrials: drought creates niche space for rare invertebrates in a large-scale and long-term field experiment

Freshwater habitats are drying more frequently and for longer under the combined pressures of climate change and overabstraction. Unsurprisingly, many aquatic species decline or become locally extinct as their benthic habitat is lost during stream droughts, but less is known about the potential 'winners': those terrestrial species that may exploit emerging niches in drying riverbeds. In particular, we do not know how these transient ecotones will respond as droughts become more extreme in the future. To find out we used a large-scale, long-term mesocosm experiment spanning a wide gradient of drought intensity, from permanent flows to full streambed dewatering, and analysed terrestrial invertebrate community assembly after 1 year. Droughts that caused stream fragmentation gave rise to the most diverse terrestrial invertebrate assemblages, including 10 species with UK conservation designations, and high species turnover between experimental channels. Droughts that caused streambed dewatering produced lower terrestrial invertebrate richness, suggesting that the persistence of instream pools may benefit these taxa as well as aquatic biota. Particularly intense droughts may therefore yield relatively few 'winners' among either aquatic or terrestrial species, indicating that the threat to riverine biodiversity from future drought intensification could be more pervasive than widely acknowledged.

Multiple anthropogenic stressors influence the taxonomic and functional homogenization of macroinvertebrate communities on the mainstream of an urban-agricultural river in China

Biodiversity loss is caused by intensive human activities and threatens human well-being. However, less is known about how the combined effects of multiple stressors on the diversity of internal (alpha diversity) and multidimensional (beta diversity) communities. Here, we conducted a long-term experiment to quantify the contribution of environmental stressors (including water quality, land use, climate factors, and hydrological regimes) to macroinvertebrate communities alpha and beta diversity in the mainstream of the Songhua River, the third largest river in China, from 2012 to 2019. Our results demonstrated that the alpha and beta diversity indices showed a decline during the study period, with the dissimilarity in community composition between sites decreasing significantly, especially in the impacted river sections (upper and midstream). Despite overall improvement in water quality after management intervention, multiple human-caused stressors still have led to biotic homogenization of macroinvertebrate communities in terms of both taxonomic and functional diversities in the past decade. Our study revealed the increased human land use explained an important portion of the variation of diversities, further indirectly promoting biotic homogenization by changing the physical and chemical factors of water quality, ultimately altering assemblage ecological processes. Furthermore, the facets of diversity have distinct response mechanisms to stressors, providing complementary information from the perspective of taxonomy and function to better reflect the ecological changes of communities. Environmental filtering determined taxonomic beta diversity, and functional beta diversity was driven by the joint efforts of stressors and spatial processes. Finally, we proposed that traditional water quality monitoring alone cannot fully reveal the status of river ecological environment protection, and more importantly, we should explore the continuous changes in biodiversity over the long term. Meanwhile, our results also highlight timely control of nutrient input and unreasonable expansion of land use can better curb the ecological degradation of rivers and promote the healthy and sustainable development of floodplain ecosystems.

Causes, Responses, and Implications of Anthropogenic versus Natural Flow Intermittence in River Networks

Rivers that do not flow year-round are the predominant type of running waters on Earth. Despite a burgeoning literature on natural flow intermittence (NFI), knowledge about the hydrological causes and ecological effects of human-induced, anthropogenic flow intermittence (AFI) remains limited. NFI and AFI could generate contrasting hydrological and biological responses in rivers because of distinct underlying causes of drying and evolutionary adaptations of their biota. We first review the causes of AFI and show how different anthropogenic drivers alter the timing, frequency and duration of drying, compared with NFI. Second, we evaluate the possible differences in biodiversity responses, ecological functions, and ecosystem services between NFI and AFI. Last, we outline knowledge gaps and management needs related to AFI. Because of the distinct hydrologic characteristics and ecological impacts of AFI, ignoring the distinction between NFI and AFI could undermine management of intermittent rivers and ephemeral streams and exacerbate risks to the ecosystems and societies downstream.

Review: An integrated framework for understanding ecological drought and drought resistance

Droughts are a frequent natural phenomenon that has amplified globally in the 21st century and are projected to become more common and extreme in the future. Consequently, this affects the progress of drought indices and frameworks to categorize drought conditions. Several drought-related indices and variables are required to capture different features of complex drought conditions. Therefore, we explained the signs of progress of ecological drought that were ecologically expressive to promote the integration between the research on and identification of water scarcity situations and analyzed different frameworks to synthesize the drought effects on species and ecosystems. Notably, we present an inclusive review of an integrated framework for an ecological drought. The ecological drought framework affords the advantage of improved methodologies for assessing ecological drought. This is supported by research on water-limited ecosystems that incorporated several drought-related elements and indicators to produce an integrated drought framework. In this framework, we combined multiple studies on drought recovery, early warning signs, and the effects of land management interferences, along with a schematic representation of a new extension of the framework into ecological systems, to contribute to the success and long-term sustainability of ecological drought adaptation, as well as on-the-ground examples of climate-informed ecological drought management in action for an integrated framework for ecological drought. This study provides an integrated approach to the understanding of ecological drought in line with accelerated scientific advancement to promote persistence and plan for a future that irretrievably exceeds the ecosystem thresholds and new multivariate drought indices.

Human disturbances affect the topology of food webs

Networks describe nodes connected by links, with numbers of links per node, the degree, forming a range of distributions including random and scale-free. How network topologies emerge in natural systems still puzzles scientists. Based on previous theoretical simulations, we predict that scale-free food webs are favourably selected by random disturbances while random food webs are selected by targeted disturbances. We assume that lower human pressures are more likely associated with random disturbances, whereas higher pressures are associated with targeted ones. We examine these predictions using 351 empirical food webs, generally confirming our predictions. Should the topology of food webs respond to changes in the magnitude of disturbances in a predictable fashion, consistently across ecosystems and scales of organisation, it would provide a baseline expectation to understand and predict the consequences of human pressures on ecosystem dynamics.

Land conversion induced by urbanization leads to taxonomic and functional homogenization of a river macroinvertebrate metacommunity

Conversion of forests to urban land-use in the processes of urbanization is one of the major causes of biotic homogenization (i.e., decline in beta diversity) in freshwater ecosystems, threating ecosystem functioning and services. However, empirical studies exploring urban land-use shaping patterns of taxonomic and functional beta diversities and their components in subtropical urban rivers are limited. Here, by leveraging data for 43 sampling sites from urban and forest rivers in Shenzhen, a megacity showing rapid urbanization, we determined the spatio-temporal dynamics and associated drivers of taxonomic and functional beta diversities of river macroinvertebrates. Our results showed that, from the forest to urban rivers, taxonomic beta diversity (wet: 32.9%; dry: 17.1%) declined more significantly than functional beta diversity (wet: 17.4%; dry: 9.5%) in different seasons. We further found that these compositional changes were largely driven by decreased roles of species/traits replacement. Although replacement was also dominant for taxonomic beta diversity (60.4%-68.4%) in two sets of rivers, richness difference contributed more to functional beta diversity in the urban river (52.6%-60.5%). Both deterministic and stochastic processes simultaneously affected beta diversity, with stochastic processes being more important in the urban (3.0-19.0%) than forest rivers (0.0%-3.0%). Besides, db-RDA and variation partitioning results showed that local-scale environmental variables explained considerably large fractions of variation in beta diversity. We hence recommended that biodiversity conservation should focus on improving and restoring local environmental conditions. Despite no significant seasonal differences in beta diversity were detected in this study, we found that the roles of deterministic (i.e., local-scale and land-use variables) and stochastic processes varied considerately across seasons. This result highlights the viewpoint that urban river biodiversity monitoring should go beyond one-season snapshot surveys. As the ongoing trend of urbanization in developing countries, the findings of this study are relevant in guiding urban river environmental monitoring, biodiversity conservation and land-use planning.

Cross-Latitude Behavioural Axis in an Adult Damselfly Calopteryx splendens (Harris, 1780)

Simple Summary

Animals adapt to the environment they live in. If the environment changes, animals usually adapt behaviourally as a first response. By studying behavioural profiles across long distances, we can detect environmental change reflected in shifts in behavioural profiles. This study examined variation in three behavioural axes: activity, courtship and boldness, and the association between these behaviours, i.e., behavioural syndromes, across three damselfly populations along a latitudinal gradient (i.e., climatic gradient). Our study organism was the temperate damselfly Calopteryx splendens. We predicted that behavioural expressions would gradually increase from southern to northern regions. This is because northern animals should compensate behaviourally for a brief and cold breeding season (i.e., time constraint). Activity was the only behaviour feature positively associated with latitudinal gradient. Courtship effort was highest in the central region, whereas boldness values were highest in the north but did not differ between central and south. In the southern region, an activity–boldness and a courtship—boldness syndrome were present, and in the northern region, only an activity–boldness syndrome was found. Our results confirm that environmental variability in biotic and abiotic factors across studied latitudes generates regional differences in behavioural profiles, which do not always follow latitudinal gradient.

Abstract

Behavioural variation is important for evolutionary and ecological processes, but can also be useful when predicting consequences of climate change and effects on species ranges. Latitudinal differences in behaviour have received relatively limited research interest when compared to morphological, life history and physiological traits. This study examined differences in expression of three behavioural axes: activity, courtship and boldness, and their correlations, along a European latitudinal gradient spanning ca. 1500 km. The study organism was the temperate damselfly Calopteryx splendens (Harris). We predicted that the expression of both behavioural traits and behavioural syndromes would be positively correlated to latitude, with the lowest values in the southern populations, followed by central and the highest in the north, because animals usually compensate behaviourally for increasing time constraints and declining environmental conditions. We found that behavioural expression varied along the latitudinal cline, although not always in the predicted direction. Activity was the only behaviour that followed our prediction and gradually increased northward. Whereas no south-to-north gradient was seen in any of the behavioural syndromes. The results, particularly for activity, suggest that climatic differences across latitudes change behavioural profiles. However, for other traits such as courtship and boldness, local factors might invoke stronger selection pressures, disrupting the predicted latitudinal pattern.

A global perspective on the functional responses of stream communities to flow intermittence

The current erosion of biodiversity is a major concern that threatens the ecological integrity of ecosystems and the ecosystem services they provide. Due to global change, an increasing proportion of river networks are drying and changes from perennial to non-perennial flow regimes represent dramatic ecological shifts with potentially irreversible alterations of community and ecosystem dynamics. However, there is minimal understanding of how biological communities respond functionally to drying. Here, we highlight the taxonomic and functional responses of aquatic macroinvertebrate communities to flow intermittence across river networks from three continents, to test predictions from underlying trait-based conceptual theory. We found a significant breakpoint in the relationship between taxonomic and functional richness, indicating higher functional redundancy at sites with flow intermittence higher than 28%. Multiple strands of evidence, including patterns of alpha and beta diversity and functional group membership, indicated that functional redundancy did not compensate for biodiversity loss associated with increasing intermittence, contrary to received wisdom. A specific set of functional trait modalities, including small body size, short life span and high fecundity, were selected with increasing flow intermittence. These results demonstrate the functional responses of river communities to drying and suggest that on-going biodiversity reduction due to global change in drying river networks is threatening their functional integrity. These results indicate that such patterns might be common in these ecosystems, even where drying is considered a predictable disturbance. This highlights the need for the conservation of natural drying regimes of intermittent rivers to secure their ecological integrity.

Drought effects on invertebrate metapopulation dynamics and quasi-extinction risk in an intermittent river network

Ecological communities can remain stable in the face of disturbance if their constituent species have different resistance and resilience strategies. In turn, local stability scales up regionally if heterogeneous landscapes maintain spatial asynchrony across discrete populations-but not if large-scale stressors synchronize environmental conditions and biological responses. Here, we hypothesized that droughts could drastically decrease the stability of invertebrate metapopulations both by filtering out poorly adapted species locally, and by synchronizing their dynamics across a river network. We tested this hypothesis via multivariate autoregressive state-space (MARSS) models on spatially replicated, long-term data describing aquatic invertebrate communities and hydrological conditions in a set of temperate, lowland streams subject to seasonal and supraseasonal drying events. This quantitative approach allowed us to assess the influence of local (flow magnitude) and network-scale (hydrological connectivity) drivers on invertebrate long-term trajectories, and to simulate near-future responses to a range of drought scenarios. We found that fluctuations in species abundances were heterogeneous across communities and driven by a combination of hydrological and stochastic drivers. Among metapopulations, increasing extent of dry reaches reduced the abundance of functional groups with low resistance or resilience capacities (i.e. low ability to persist in situ or recolonize from elsewhere, respectively). Our simulations revealed that metapopulation quasi-extinction risk for taxa vulnerable to drought increased exponentially as flowing habitats contracted within the river network, whereas the risk for taxa with resistance and resilience traits remained stable. Our results suggest that drought can be a synchronizing agent in riverscapes, potentially leading to regional quasi-extinction of species with lower resistance and resilience abilities. Better recognition of drought-driven synchronization may increase realism in species extinction forecasts as hydroclimatic extremes continue to intensify worldwide.

Low flow and heatwaves alter ecosystem functioning in a stream mesocosm experiment

Climate change is expected to intensify the effect of environmental stressors on riverine ecosystems. Extreme events, such as low flow and heatwaves, could have profound consequences for stream ecosystem functioning, but research on the impact of these stressors and their interaction across multiple processes, remains scarce. Here, we report the results of a two-month stream mesocosm experiment testing the effect of low flow (66% water level reduction, without gravel exposure) and heatwaves (three 8-d episodes of +5 °C above ambient with 10-15 days recovery between each episode) on a suite of ecosystem processes (i.e. detrital decomposition, biofilm accrual, ecosystem metabolism and DOC quantity and quality). Low flow reduced whole system metabolism, suppressing the rates of gross primary production (GPP) and ecosystem respiration (ER), but elevated DOC concentration. Overall, habitat contraction was the main driver of reduced ecosystem functioning in the low flow treatment. By contrast, heatwaves increased decomposition, algal accrual, and humic-like DOC, but reduced leaf decomposition efficiency. Net ecosystem production (NEP) generally decreased across the experiment but was most pronounced for low flow and heatwaves when occurring independently. Assessment of NEP responses to the three successive heatwave events revealed that responses later in the sequence were more reduced (i.e. more similar to controls), suggesting biofilm communities may acclimate to autumn heatwaves. However, when heatwaves co-occurred with low flow, a strong reduction in both ER and GPP was observed, suggesting increased microbial mortality and reduced acclimation. Our study reveals autumn heatwaves potentially elongate the growth season for primary producers and stimulate decomposers. With climate change, river ecosystems may become more heterotrophic, with faster processing of recalcitrant carbon. Further research is required to identify the impacts on higher trophic levels, meta-community dynamics and the potential for legacy effects generated by successive low flows and heatwaves.

Energy pathways modulate the resilience of stream invertebrate communities to drought

While climate change is altering ecosystems on a global scale, not all ecosystems are responding in the same way. The resilience of ecological communities may depend on whether food webs are producer- or detritus-based (i.e. 'green' or 'brown' food webs, respectively), or both (i.e. 'multi-channel' food web). Food web theory suggests that the presence of multiple energy pathways can enhance community stability and resilience and may modulate the responses of ecological communities to disturbances such as climate change. Despite important advances in food web theory, few studies have empirically investigated the resilience of ecological communities to climate change stressors in ecosystems with different primary energy channels. We conducted a factorial experiment using outdoor stream mesocosms to investigate the independent and interactive effects of warming and drought on invertebrate communities in food webs with different energy channel configurations. Warming had little effect on invertebrates, but stream drying negatively impacted total invertebrate abundance, biomass, richness and diversity. Although resistance to drying did not differ among energy channel treatments, recovery and overall resilience were higher in green mesocosms than in mixed and brown mesocosms. Resilience to drying also varied widely among taxa, with larger predatory taxa exhibiting lower resilience. Our results suggest that the effects of drought on stream communities may vary regionally and depend on whether food webs are fuelled by autochthonous or allochthonous basal resources. Communities inhabiting streams with large amounts of organic matter and more complex substrates that provide refugia may be more resilient to the loss of surface water than communities inhabiting streams with simpler, more homogeneous substrates.

Long-term river invertebrate community responses to groundwater and surface water management operations

River flow regimes have been transformed by groundwater and surface water management operations globally, prompting widespread ecological responses. Yet, empirical evidence quantifying the simultaneous effects of groundwater and surface water management operations on freshwater ecosystems remains limited. This study combines a multi-decadal freshwater invertebrate dataset (1995-2016) with groundwater model outputs simulating the effects of different anthropogenic flow alterations (e.g. groundwater abstraction, effluent water returns) and river discharges. A suite of flow alteration- and flow-ecology relationships were modelled that tested different invertebrate community responses (taxonomic, functional, flow response guilds, individual taxa). Most flow alteration-ecology relationships were not statistically significant, highlighting the absence of consistent, detectable ecological responses to long-term water management operations. A small number of significant statistical models provided insights into how flow alterations transformed specific ecological assets; including Ephemeroptera, Plecoptera and Trichoptera taxa which are rheophilic in nature being positively associated with groundwater abstraction effects reducing river discharges by 0-15%. This represents a key finding from a water resource management operation perspective given that such flow alteration conditions were observed on average in over two-thirds of the study sites examined. In a small number of instances, specific invertebrate responses displayed relative declines associated with the most severe groundwater abstraction effects and artificial hydrological inputs (predominantly effluent water returns). The strongest flow-ecology relationships were recorded during spring months, when invertebrate communities were most responsive to antecedent minimum and maximum discharges, and average flow conditions in the preceding summer months. Results from this study provide new evidence indicating how groundwater and surface water resources can be managed to conserve riverine ecological assets. Moreover, the ensemble of flow alteration- and flow-ecology relationships established in this study could be used to guide environmental flow strategies. Such findings are of global importance given that future climatic change and rising societal water demands are likely to further transform river flow regimes and threaten freshwater ecosystems.

Stochastic processes and ecological connectivity drive stream invertebrate community responses to short-term drought

Community responses to and recovery from disturbances depend on local (e.g. presence of refuges) and regional (connectivity to recolonization sources) factors. Droughts are becoming more frequent in boreal regions, and are likely to constitute a severe disturbance for boreal stream communities where organisms largely lack adaptations to such hydrological extremes. We conducted an experiment in 24 semi-natural stream flumes to assess the effects of local and regional factors on the responses of benthic invertebrate communities to a short-term drought. We manipulated flow (drought vs. constant-flow), spatial arrangement of leaf litter patches (aggregated vs. evenly distributed) and colonization from regional species pool (enhanced vs. ambient connectivity) to test the combined effects of disturbance, resource arrangement and connectivity on the structural and functional responses of benthic invertebrate communities. We found that a drought as short as 1 week reduced invertebrate taxonomic richness and abundance, mainly through stochastic extinctions. Such changes in richness were not reflected in functional diversity. This suggests that communities were characterized by a high degree of functional redundancy, which allowed maintenance of functional diversity despite species losses. Feeding groups responded differently to drought, with organic matter decomposers responding more than scrapers and predators. Three weeks were insufficient for complete invertebrate community recovery from drought. However, recovery was greater in channels subjected to enhanced connectivity, which increased taxonomic diversity and abundance of certain taxa. Spatial configuration of resources explained the least variation in our response variables, having a significant effect only on invertebrate abundance and evenness (both sampling occasions) and taxonomic richness (end of recovery period). Even a short drought, if occurring late in the season, may not allow communities to recover before the onset of winter, thus having a potentially long-lasting effect on stream communities. For boreal headwaters, extreme dewatering poses a novel disturbance regime that may trigger substantial and potentially irreversible changes. An improved understanding of such changes is needed to underpin adaptive management strategies in these increasingly fragmented and disturbed ecosystems.

Reservoir trophic state confounds flow-ecology relationships in regulated streams

Cultural eutrophication and river regulation have been identified as two of the most pressing threats to global freshwater biodiversity. However, we know little about their combined ecological effects, raising questions over biomonitoring practices that typically consider such stressors in isolation. To address this inconsistency, we examined a decade-long dataset of macroinvertebrate samples and environmental data collected downstream of three reservoirs spanning a broad gradient of trophic states, from mesotrophic to hypereutrophic. We analysed the responses of routine macroinvertebrate biomonitoring indices and community composition to antecedent flow, temperature and water quality, including parameters associated with eutrophication (total phosphorous, nitrate, nitrite, ammonia, chlorophyll a). Multi-model comparisons of linear regressor combinations, variation partitioning and distance-based redundancy analyses all revealed shifts in the relative significance of flow and water quality predictors across the trophic state gradient. At the mesotrophic site biomonitoring indices and community composition were most sensitive to seasonal flow variability- particularly high-flow conditions- whereas in the hypereutrophic system stronger associations with nutrient concentrations emerged, notably nitrite and nitrate. Patterns at the eutrophic site were broadly intermediate between these, with significant biotic responses to antecedent flows mediated by water quality. Based on these results we suggest that nutrient regimes should be regarded as an integral component of environmental flows science. We therefore call on practitioners to look beyond the stressor-specific indices widely used to assess ecological status in rivers to consider the interactive effects of flow and water quality.

Increasing climate-driven taxonomic homogenization but functional differentiation among river macroinvertebrate assemblages

Global change is increasing biotic homogenization globally, which modifies the functioning of ecosystems. While tendencies towards taxonomic homogenization in biological communities have been extensively studied, functional homogenization remains an understudied facet of biodiversity. Here, we tested four hypotheses related to long-term changes (1991-2016) in the taxonomic and functional arrangement of freshwater macroinvertebrate assemblages across space and possible drivers of these changes. Using data collected annually at 64 river sites in mainland New Zealand, we related temporal changes in taxonomic and functional spatial β-diversity, and the contribution of individual sites to β-diversity, to a set of global, regional, catchment and reach-scale environmental descriptors. We observed long-term, mostly climate-induced, temporal trends towards taxonomic homogenization but functional differentiation among macroinvertebrate assemblages. These changes were mainly driven by replacements of species and functional traits among assemblages, rather than nested species loss. In addition, there was no difference between the mean rate of change in the taxonomic and functional facets of β-diversity. Climatic processes governed overall population and community changes in these freshwater ecosystems, but were amplified by multiple anthropogenic, topographic and biotic drivers of environmental change, acting widely across the landscape. The functional diversification of communities could potentially provide communities with greater stability, resistance and resilience capacity to environmental change, despite ongoing taxonomic homogenization. Therefore, our study highlights a need to further understand temporal trajectories in both taxonomic and functional components of species communities, which could enable a clearer picture of how biodiversity and ecosystems will respond to future global changes.

In situ resistance, not immigration, supports invertebrate community resilience to drought intensification in a Neotropical ecosystem

While future climate scenarios predict declines in precipitations in many regions of the world, little is known of the mechanisms underlying community resilience to prolonged dry seasons, especially in 'naïve' Neotropical rainforests. Predictions of community resilience to intensifying drought are complicated by the fact that the underlying mechanisms are mediated by species' tolerance and resistance traits, as well as rescue through dispersal from source patches. We examined the contribution of in situ tolerance-resistance and immigration to community resilience, following drought events that ranged from the ambient norm to IPCC scenarios and extreme events. We used rainshelters above rainwater-filled bromeliads of French Guiana to emulate a gradient of drought intensity (from 1 to 3.6 times the current number of consecutive days without rainfall), and we analysed the post-drought dynamics of the taxonomic and functional community structure of aquatic invertebrates to these treatments when immigration is excluded (by netting bromeliads) or permitted (no nets). Drought intensity negatively affected invertebrate community resistance, but had a positive influence on community recovery during the post-drought phase. After droughts of 1 to 1.4 times the current intensities, the overall invertebrate abundance recovered within invertebrate life cycle durations (up to 2 months). Shifts in taxonomic composition were more important after longer droughts, but overall, community composition showed recovery towards baseline states. The non-random patterns of changes in functional community structure indicated that deterministic processes like environmental filtering of traits drive community re-assembly patterns after a drought event. Community resilience mostly relied on in situ tolerance-resistance traits. A rescue effect of immigration after a drought event was weak and mostly apparent under extreme droughts. Under climate change scenarios of drought intensification in Neotropical regions, community and ecosystem resilience could primarily depend on the persistence of suitable habitats and on the resistance traits of species, while metacommunity dynamics could make a minor contribution to ecosystem recovery. Climate change adaptation should thus aim at identifying and preserving local conditions that foster in situ resistance and the buffering effects of habitat features.

Unravelling the effects of multiple stressors on diatom and macroinvertebrate communities in European river basins using structural and functional approaches

Rivers suffer from more severe decreases in species diversity compared to other aquatic and terrestrial ecosystems due to a variety of pressures related to human activities. Species provide different roles in the functioning of the ecosystem, and their loss may reduce the capacity of the ecosystems to respond to multiple stressors. The effects on diversity will differ based on the type, combination and severity of stressors, as well as on the characteristics of the community composition and tolerance. Multiple trait-based approaches (MTBAs) can help to unravel the effects of multiple stressors on communities, providing a mechanistic interpretation, and, thus, complementing traditional biodiversity assessments using community structure. We studied the relationships between diversity indexes and trait composition of macroinvertebrate and diatom communities, as well as environmental variables that described the hydrological and geomorphological alterations and toxic pollution (pesticides and pharmaceuticals) of three different European river basins: the Adige, the Sava, and the Evrotas. These river basins can be considered representative cases of different situations in European freshwater systems. Hydrological variables were the main drivers determining the community structure and function in the rivers, for both diatoms and macroinvertebrates. For diatom communities, pharmaceutical active compound (PhAC) toxic units were also identified as a very important driver of diversity changes, explaining up to 57% of the variance in taxonomic richness. For macroinvertebrates, river geomorphology was an important driver of structural changes, particularly affecting Plecoptera richness. In addition, PhAC and pesticide toxic units were also identified as stressors for macroinvertebrate communities. MTBA provided a detailed picture of the effects of the stressors on the communities and confirmed the importance of hydrological variables in shaping the functional attributes of the communities.

River ecosystem conceptual models and non-perennial rivers: A critical review

Conceptual models underpin river ecosystem research. However, current models focus on continuously flowing rivers and few explicitly address characteristics such as flow cessation and drying. The applicability of existing conceptual models to nonperennial rivers that cease to flow (intermittent rivers and ephemeral streams, IRES) has not been evaluated. We reviewed 18 models, finding that they collectively describe main drivers of biogeochemical and ecological patterns and processes longitudinally (upstream-downstream), laterally (channel-riparian-floodplain), vertically (surface water-groundwater), and temporally across local and landscape scales. However, perennial rivers are longitudinally continuous while IRES are longitudinally discontinuous. Whereas perennial rivers have bidirectional lateral connections between aquatic and terrestrial ecosystems, in IRES, this connection is unidirectional for much of the time, from terrestrial-to-aquatic only. Vertical connectivity between surface and subsurface water occurs bidirectionally and is temporally consistent in perennial rivers. However, in IRES, this exchange is temporally variable, and can become unidirectional during drying or rewetting phases. Finally, drying adds another dimension of flow variation to be considered across temporal and spatial scales in IRES, much as flooding is considered as a temporally and spatially dynamic process in perennial rivers. Here, we focus on ways in which existing models could be modified to accommodate drying as a fundamental process that can alter these patterns and processes across spatial and temporal dimensions in streams. This perspective is needed to support river science and management in our era of rapid global change, including increasing duration, frequency, and occurrence of drying.

Role of the Hyporheic Zone in Increasing the Resilience of Mountain Streams Facing Intermittency

We investigated the impact of intermittence in previously-perennial Alpine stream reaches, targeting the role of the hyporheic zone in increasing the resilience of these aquatic systems. We selected a perennial and an intermittent site in a reach of the Po River (North-Western Italy). We installed piezometers reaching −1 m (permanent and intermittent site), and −3 m (intermittent site) and monitored three supraseasonal droughts over a period of three years. We classified the hyporheic fauna into three categories of increasing affinity to life in the hyporheic (stygoxene, stygophile, stygobite), and used communities composition, abundance, beta-diversity and functional groups: (1) to compare assemblages at the same depth but with different hydrological characteristics, as well as assemblages from two depths at the intermittent site, and (2) to assess how the connection with surface water and the direction of the vertical aquifer flow determined the faunistic assemblages. Different taxonomic groups responded differently to intermittence, the hyporheic zone acted as a refuge increasing the resilience of the system, but resilience decreased with increasing degree of affinity to hyporheic life. Disentangling the effects of intermittence on the different faunistic component in the hyporheic zone can help guiding effective protection and restoration measures of river systems with temporary reaches.

Assessment of Ecological and Hydro-Geomorphological Alterations under Climate Change Using SWAT and IAHRIS in the Eo River in Northern Spain

Magnitude and temporal variability of streamflow is essential for natural biodiversity and the stability of aquatic environments. In this study, a comparative analysis between historical data (1971–2013) and future climate change scenarios (2010–2039, 2040–2069 and 2070–2099) of the hydrological regime in the Eo river, in the north of Spain, is carried out in order to assess the ecological and hydro-geomorphological risks over the short-, medium- and long-term. The Soil and Water Assessment Tool (SWAT) model was applied on a daily basis to assess climate-induced hydrological changes in the river under five general circulation models and two representative concentration pathways. Statistical results, both in calibration (Nash-Sutcliffe efficiency coefficient (NSE): 0.73, percent bias (PBIAS): 3.52, R2: 0.74) and validation (NSE: 0.62, PBIAS: 6.62, R2: 0.65), are indicative of the SWAT model’s good performance. The ten climate scenarios pointed out a reduction in rainfall (up to −22%) and an increase in temperatures, both maximum (from +1 to +7 °C) and minimum ones (from +1 to +4 °C). Predicted flow rates resulted in an incrementally greater decrease the longer the term is, varying between −5% (in short-term) and −53% (in long-term). The free software IAHRIS (Indicators of Hydrologic Alteration in Rivers) determined that alteration for usual values remains between excellent and good status and from good to moderate in drought values, but flood values showed a deficient regime in most scenarios, which implies an instability of river morphology, a progressive reduction in the section of the river and an advance of aging of riparian habitat, endangering the renewal of the species.

Resilience of aquatic systems: Review and management implications

Our understanding of how ecosystems function has changed from an equilibria-based view to one that recognizes the dynamic, fluctuating, nonlinear nature of aquatic systems. This current understanding requires that we manage systems for resilience. In this review, we examine how resilience has been defined, measured and applied in aquatic systems, and more broadly, in the socioecological systems in which they are embedded. Our review reveals the importance of managing stressors adversely impacting aquatic system resilience, as well as understanding the environmental and climatic cycles and changes impacting aquatic resources. Aquatic resilience may be enhanced by maintaining and enhancing habitat connectivity as well as functional redundancy and physical and biological diversity. Resilience in aquatic socioecological system may be enhanced by understanding and fostering linkages between the social and ecological subsystems, promoting equity among stakeholders, and understanding how the system is impacted by factors within and outside the area of immediate interest. Management for resilience requires implementation of adaptive and preferably collaborative management. Implementation of adaptive management for resilience will require an effective monitoring framework to detect key changes in the coupled socioecological system. Research is needed to (1) develop sensitive indicators and monitoring designs, (2) disentangle complex multi-scalar interactions and feedbacks, and (3) generalize lessons learned across aquatic ecosystems and apply them in new contexts.

Ecological response to altered rainfall differs across the Neotropics

There is growing recognition that ecosystems may be more impacted by infrequent extreme climatic events than by changes in mean climatic conditions. This has led to calls for experiments that explore the sensitivity of ecosystems over broad ranges of climatic parameter space. However, because such response surface experiments have so far been limited in geographic and biological scope, it is not clear if differences between studies reflect geographic location or the ecosystem component considered. In this study, we manipulated rainfall entering tank bromeliads in seven sites across the Neotropics, and characterized the response of the aquatic ecosystem in terms of invertebrate functional composition, biological stocks (total invertebrate biomass, bacterial density) and ecosystem fluxes (decomposition, carbon, nitrogen). Of these response types, invertebrate functional composition was the most sensitive, even though, in some sites, the species pool had a high proportion of drought-tolerant families. Total invertebrate biomass was universally insensitive to rainfall change because of statistical averaging of divergent responses between functional groups. The response of invertebrate functional composition to rain differed between geographical locations because (1) the effect of rainfall on bromeliad hydrology differed between sites, and invertebrates directly experience hydrology not rainfall and (2) the taxonomic composition of some functional groups differed between sites, and families differed in their response to bromeliad hydrology. These findings suggest that it will be difficult to establish thresholds of "safe ecosystem functioning" when ecosystem components differ in their sensitivity to climatic variables, and such thresholds may not be broadly applicable over geographic space. In particular, ecological forecast horizons for climate change may be spatially restricted in systems where habitat properties mediate climatic impacts, and those, like the tropics, with high spatial turnover in species composition.

Linkages between flow regime, biota, and ecosystem processes: Implications for river restoration

River ecosystems are highly biodiverse, influence global biogeochemical cycles, and provide valued services. However, humans are increasingly degrading fluvial ecosystems by altering their streamflows. Effective river restoration requires advancing our mechanistic understanding of how flow regimes affect biota and ecosystem processes. Here, we review emerging advances in hydroecology relevant to this goal. Spatiotemporal variation in flow exerts direct and indirect control on the composition, structure, and dynamics of communities at local to regional scales. Streamflows also influence ecosystem processes, such as nutrient uptake and transformation, organic matter processing, and ecosystem metabolism. We are deepening our understanding of how biological processes, not just static patterns, affect and are affected by stream ecosystem processes. However, research on this nexus of flow-biota-ecosystem processes is at an early stage. We illustrate this frontier with evidence from highly altered regulated rivers and urban streams. We also identify research challenges that should be prioritized to advance process-based river restoration.