Climate change and the biodiversity of alpine ponds: Challenges and perspectives.

Inland waters are among the most threatened biodiversity hotspots. Ponds located in alpine areas are experiencing more rapid and dramatic water temperature increases than any other biome. Despite their prevalence, alpine ponds and their biodiversity responses to climate change have been poorly explored, reflecting their small size and difficult access. To understand the effects of climate change on alpine pond biodiversity, we performed a comprehensive literature review for papers published since 1955. Through analysis of their geographic distribution, environmental features, and biodiversity values, we identified which environmental factors related to climate change would have direct or indirect effects on alpine pond biodiversity. We then synthesized this information to produce a conceptual model of the effects of climate change on alpine pond biodiversity. Increased water temperature, reduced hydroperiod, and loss of connectivity between alpine ponds were the main drivers of biodiversity geographic distribution, leading to predictable changes in spatial patterns of biodiversity. We identified three major research gaps that, if addressed, can guide conservation and restoration strategies for alpine ponds biodiversity in an uncertain future.

Understanding temporal variability across trophic levels and spatial scales in freshwater ecosystems.

A tenet of ecology is that temporal variability in ecological structure and processes tends to decrease with increasing spatial scales (from locales to regions) and levels of biological organization (from populations to communities). However, patterns in temporal variability across trophic levels and the mechanisms that produce them remain poorly understood. Here we analyzed the abundance time series of spatially structured communities (i.e., metacommunities) spanning basal resources to top predators from 355 freshwater sites across three continents. Specifically, we used a hierarchical partitioning method to disentangle the propagation of temporal variability in abundance across spatial scales and trophic levels. We then used structural equation modeling to determine if the strength and direction of relationships between temporal variability, synchrony, biodiversity, and environmental and spatial settings depended on trophic level and spatial scale. We found that temporal variability in abundance decreased from producers to tertiary consumers but did so mainly at the local scale. Species population synchrony within sites increased with trophic level, whereas synchrony among communities decreased. At the local scale, temporal variability in precipitation and species diversity were associated with population variability (linear partial coefficient, ß = 0.23) and population synchrony (ß = -0.39) similarly across trophic levels, respectively. At the regional scale, community synchrony was not related to climatic or spatial predictors, but the strength of relationships between metacommunity variability and community synchrony decreased systematically from top predators (ß = 0.73) to secondary consumers (ß = 0.54), to primary consumers (ß = 0.30) to producers (ß = 0). Our results suggest that mobile predators may often stabilize metacommunities by buffering variability that originates at the base of food webs. This finding illustrates that the trophic structure of metacommunities, which integrates variation in organismal body size and its correlates, should be considered when investigating ecological stability in natural systems. More broadly, our work advances the notion that temporal stability is an emergent property of ecosystems that may be threatened in complex ways by biodiversity loss and habitat fragmentation.

The recovery of European freshwater biodiversity has come to a halt.

Owing to a long history of anthropogenic pressures, freshwater ecosystems are among the most vulnerable to biodiversity loss1. Mitigation measures, including wastewater treatment and hydromorphological restoration, have aimed to improve environmental quality and foster the recovery of freshwater biodiversity2. Here, using 1,816 time series of freshwater invertebrate communities collected across 22 European countries between 1968 and 2020, we quantified temporal trends in taxonomic and functional diversity and their responses to environmental pressures and gradients. We observed overall increases in taxon richness (0.73% per year), functional richness (2.4% per year) and abundance (1.17% per year). However, these increases primarily occurred before the 2010s, and have since plateaued. Freshwater communities downstream of dams, urban areas and cropland were less likely to experience recovery. Communities at sites with faster rates of warming had fewer gains in taxon richness, functional richness and abundance. Although biodiversity gains in the 1990s and 2000s probably reflect the effectiveness of water-quality improvements and restoration projects, the decelerating trajectory in the 2010s suggests that the current measures offer diminishing returns. Given new and persistent pressures on freshwater ecosystems, including emerging pollutants, climate change and the spread of invasive species, we call for additional mitigation to revive the recovery of freshwater biodiversity.

Clogging modulates the copper effects on microbial communities of streambed sediments.

The hyporheic zone, i.e. the water-saturated sediment beneath and alongside the riverbed, is exposed to multiple stressors. Agricultural-watershed rivers are frequently exposed to two concomitant stressors: clogging and copper contamination. However, one stressor exposure can increase sensitivity to a second stressor. The aim of this study was to experimentally test the cumulative effects of these two stressors on copper distribution and structural and functional microbial communities responses in the hyporheic zone. A slow filtration column experiment was conducted to compare the effects of 3 treatments of increasing complexity: 'Reference', 'Copper-contaminated' (dissolved copper added at 191 µg L-1), and 'Clogging+Copper' (dissolved copper + addition of 2 cm of fine sediment). Microbial community structure and activities were studied at 4 column sediment depths. The results showed that clogging did not modify the distribution of copper, which remained fixed in the first few centimetres. In the first few centimetres, clogging had a stimulating effect on microbial activities whereas copper had limited effects mainly on leucine aminopeptidase activity and microbial community tolerance to copper. The subsurface zone thus hosts significant different microbial communities from the communities in the deeper zones that were protected from surface stressors. This experiment confirms the valuable filtering role played by the hyporheic zone and shows that microbial responses are strongly correlated to microhabitat-scale physicochemical conditions in sediment.

Long-term trends in crayfish invasions across European rivers.

Europe has experienced a substantial increase in non-indigenous crayfish species (NICS) since the mid-20th century due to their extensive use in fisheries, aquaculture and, more recently, pet trade. Despite relatively long invasion histories of some NICS and negative impacts on biodiversity and ecosystem functioning, large spatio-temporal analyses of their occurrences are lacking. Here, we used a large freshwater macroinvertebrate database to evaluate what information on NICS can be obtained from widely applied biomonitoring approaches and how usable such data is for descriptions of trends in identified NICS species. We found 160 time-series containing NICS between 1983 and 2019, to infer temporal patterns and environmental drivers of species and region-specific trends. Using a combination of meta-regression and generalized linear models, we found no significant temporal trend for the abundance of any species (Procambarus clarkii, Pacifastacus leniusculus or Faxonius limosus) at the European scale, but identified species-specific predictors of abundances. While analysis of the spatial range expansion of NICS was positive (i.e. increasing spread) in England and negative (significant retreat) in northern Spain, no trend was detected in Hungary and the Dutch-German-Luxembourg region. The average invasion velocity varied among countries, ranging from 30 km/year in England to 90 km/year in Hungary. The average invasion velocity gradually decreased over time in the long term, with declines being fastest in the Dutch-German-Luxembourg region, and much slower in England. Considering that NICS pose a substantial threat to aquatic biodiversity across Europe, our study highlights the utility and importance of collecting high resolution (i.e. annual) biomonitoring data using a sampling protocol that is able to estimate crayfish abundance, enabling a more profound understanding of NICS impacts on biodiversity.

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.

Assessing placement bias of the global river gauge network.

Knowing where and when rivers flow is paramount to managing freshwater ecosystems. Yet stream gauging stations are distributed sparsely across rivers globally and may not capture the diversity of fluvial network properties and anthropogenic influences. Here we evaluate the placement bias of a global stream gauge dataset on its representation of socioecological, hydrologic, climatic and physiographic diversity of rivers. We find that gauges are located disproportionally in large, perennial rivers draining more human-occupied watersheds. Gauges are sparsely distributed in protected areas and rivers characterized by non-perennial flow regimes, both of which are critical to freshwater conservation and water security concerns. Disparities between the geography of the global gauging network and the broad diversity of streams and rivers weakens our ability to understand critical hydrologic processes and make informed water-management and policy decisions. Our findings underscore the need to address current gauge placement biases by investing in and prioritizing the installation of new gauging stations, embracing alternative water-monitoring strategies, advancing innovation in hydrologic modelling, and increasing accessibility of local and regional gauging data to support human responses to water challenges, both today and in the future.

From meta-system theory to the sustainable management of rivers in the Anthropocene.

Regional-scale ecological processes, such as the spatial flows of material, energy, and organisms, are fundamental for maintaining biodiversity and ecosystem functioning in river networks. Yet these processes remain largely overlooked in most river management practices and underlying policies. Here, we propose adoption of a meta-system approach, where regional processes acting at different levels of ecological organization - populations, communities, and ecosystems - are integrated into conventional river conservation, restoration, and biomonitoring. We also describe a series of measurements and indicators that could be assimilated into the implementation of relevant biodiversity and environmental policies. Finally, we highlight the need for alternative management strategies that can guide practitioners toward applying recent advances in ecology to preserve and restore river ecosystems and the ecosystem services they provide, in the context of increasing alteration of river network connectivity worldwide.

Reconceptualizing the hyporheic zone for nonperennial rivers and streams.

Nonperennial streams dominate global river networks and are increasing in occurrence across space and time. When surface flow ceases or the surface water dries, flow or moisture can be retained in the subsurface sediments of the hyporheic zone, supporting aquatic communities and ecosystem processes. However, hydrological and ecological definitions of the hyporheic zone have been developed in perennial rivers and emphasize the mixing of water and organisms, respectively, from both the surface stream and groundwater. The adaptation of such definitions to include both humid and dry unsaturated conditions could promote characterization of how hydrological and biogeochemical variability shape ecological communities within nonperennial hyporheic zones, advancing our understanding of both ecosystem structure and function in these habitats. To conceptualize hyporheic zones for nonperennial streams, we review how water sources and surface and subsurface structure influence hydrological and physicochemical conditions. We consider the extent of this zone and how biogeochemistry and ecology might vary with surface states. We then link these components to the composition of nonperennial stream communities. Next, we examine literature to identify priorities for hydrological and ecological research exploring nonperennial hyporheic zones. Lastly, by integrating hydrology, biogeochemistry, and ecology, we recommend a multidisciplinary conceptualization of the nonperennial hyporheic zone as the porous subsurface streambed sediments that shift between lotic, lentic, humid, and dry conditions in space and time to support aquatic-terrestrial biodiversity. As river drying increases in extent because of global change, we call for holistic, interdisciplinary research across the terrestrial and aquatic sciences to apply this conceptualization to characterize hyporheic zone structure and function across the full spectrum of hydrological states.

Ecological drivers of macroinvertebrate metacommunity assembly in a subtropical river basin in the Yangtze River Delta, China.

Identifying the underlying ecological drivers of macroinvertebrate community assembly is fundamental to metacommunity ecology. Comparably, determining the influence of different drivers on beta diversity patterns can provide insight into processes governing community organization. Exploring the ecological drivers of metacommunity and beta diversity are major avenues to improve bioassessment, restoration, and river management, which are still poorly explored in China, especially in subtropical highly developed river networks. To address this gap, we use a dataset (macroinvertebrate communities and environmental variables) collected from the Yangtze River Delta, China to test the above ideas. We used the K-means clustering method to divide 405 river sites into three anthropogenic impacted groups, nearly pristine sites, moderately impacted sites, and heavily impacted sites, and subsequently used partial Mantel tests to investigate how species sorting and dispersal shaped the metacommunity that varied with the levels of anthropogenic impacts and to explore the responses of different components of beta diversity to environmental and spatial distances among sites for each group. Our results revealed that both species sorting and dispersal shape communities, but the importance of species sorting and dispersal varied with the levels of anthropogenic impacts. Nearly pristine sites were mostly shaped only by species sorting, while heavily impacted sites were shaped by dispersal. We also found that turnover was by far the dominant component of beta diversity across all levels of impact. Therefore, we encourage that environmental variables and spatial processes should be considered in bioassessment approaches. In addition, it is essential to focus on maintaining habitat heterogeneity and identifying and protecting regional species pools that could improve local biodiversity through dispersal for ecosystem management of the Yangtze River Delta of China.

Invasion impacts and dynamics of a European-wide introduced species.

Globalization has led to the introduction of thousands of alien species worldwide. With growing impacts by invasive species, understanding the invasion process remains critical for predicting adverse effects and informing efficient management. Theoretically, invasion dynamics have been assumed to follow an "invasion curve" (S-shaped curve of available area invaded over time), but this dynamic has lacked empirical testing using large-scale data and neglects to consider invader abundances. We propose an "impact curve" describing the impacts generated by invasive species over time based on cumulative abundances. To test this curve's large-scale applicability, we used the data-rich New Zealand mud snail Potamopyrgus antipodarum, one of the most damaging freshwater invaders that has invaded almost all of Europe. Using long-term (1979-2020) abundance and environmental data collected across 306 European sites, we observed that P. antipodarum abundance generally increased through time, with slower population growth at higher latitudes and with lower runoff depth. Fifty-nine percent of these populations followed the impact curve, characterized by first occurrence, exponential growth, then long-term saturation. This behaviour is consistent with boom-bust dynamics, as saturation occurs due to a rapid decline in abundance over time. Across sites, we estimated that impact peaked approximately two decades after first detection, but the rate of progression along the invasion process was influenced by local abiotic conditions. The S-shaped impact curve may be common among many invasive species that undergo complex invasion dynamics. This provides a potentially unifying approach to advance understanding of large-scale invasion dynamics and could inform timely management actions to mitigate impacts on ecosystems and economies.

The terrestrial and semi-aquatic invertebrates of intermittent rivers and ephemeral streams.

Intermittent rivers and ephemeral streams (IRES), which cease flow and/or dry at some point, are the most abundant waterways on earth, and are found on every continent. They can support a diverse, and often abundant, terrestrial and semi-aquatic invertebrate (TSAI) fauna, which has been poorly explored due to its position at the fringe between aquatic and terrestrial disciplines. TSAIs can inhabit a variety of habitat types, including the shoreline, the surface of exposed gravel bars, unsaturated gravels, dry riverbeds, riparian zones, and floodplains. Much less is known about the species composition and ecological roles of TSAIs of IRES than their aquatic counterparts, with TSAIs being largely overlooked in conceptual models, legislation, policy, and ecological monitoring. Herein we review the TSAI literature that has increased substantially over the last decade and present conceptual models describing how TSAIs respond to hydrological changes in IRES. Then, we test these models with data collected during wet and dry phases in IRES from Australia and France. These generic models can be utilised by water managers and policy makers, ensuring that both wet and dry phases are considered in the management and protection of IRES. IRES should be viewed as a habitat continuum through time, with taxa from a pool of aquatic, semi-aquatic and terrestrial invertebrates inhabiting at any hydrological stage. We call for collaboration among terrestrial and aquatic ecologists to explore these invertebrates and ecosystems further.

Passive sampling of environmental DNA in aquatic environments using 3D-printed hydroxyapatite samplers.

The study of environmental DNA (eDNA) released by aquatic organisms in their habitat offers a fast, noninvasive and sensitive approach to monitor their presence. Common eDNA sampling methods such as water filtration and DNA precipitation are time-consuming, require difficult-to-handle equipment and partially integrate eDNA signals. To overcome these limitations, we created the first proof of concept of a passive, 3D-printed and easy-to-use eDNA sampler. We designed the samplers from hydroxyapatite (HAp samplers), a natural mineral with a high DNA adsorption capacity. The porous structure and shape of the samplers were designed to optimize DNA adsorption and facilitate their handling in the laboratory and in the field. Here we show that HAp samplers can efficiently collect genomic DNA in controlled set-ups, but can also collect animal eDNA under controlled and natural conditions with yields similar to conventional methods. However, we also observed large variations in the amount of DNA collected even under controlled conditions. A better understanding of the DNA-hydroxyapatite interactions on the surface of the samplers is now necessary to optimize eDNA adsorption and to allow the development of a reliable, easy-to-use and reusable eDNA sampling tool.

Taxon-specific sensitivities to flow intermittence reveal macroinvertebrates as potential bioindicators of intermittent rivers and streams.

As complex mosaics of lotic, lentic, and terrestrial habitats, intermittent rivers and ephemeral streams (IRES) support high biodiversity. Despite their ecological importance, IRES are poorly represented in routine monitoring programs, but recent recognition of their considerable-and increasing-spatiotemporal extent is motivating efforts to better represent IRES in ecological status assessments. We examine response patterns of aquatic macroinvertebrate communities and taxa to flow intermittence (FI) across three European climatic regions. We used self-organizing map (SOM) to ordinate and classify sampling sites based on community structure in regions with continental, Mediterranean and oceanic climates. The SOM passively introduced FI, quantified as the mean annual % flow, and visualized its variability across classified communities, revealing a clear association between community structure and FI in all regions. Indicator species analysis identified taxa indicative of low, intermediate and high FI. In the continental region, the amphipod Niphargus was indicative of high FI and was associated with groundwater-fed IRES, whereas indicators of Mediterranean IRES comprised Odonata, Coleoptera and Heteroptera taxa, which favor lentic conditions. In the oceanic region, taxa indicative of relatively high FI included leuctrid stoneflies and a limnephilid caddisfly, likely reflecting the colonization of IRES by aerial adults from nearby perennial reaches. The Diptera families Chironomidae and Simuliidae showed contrasting FI preferences among regions, reflecting environmental heterogeneity between regions and the coarse taxonomic resolution to which these organisms were identified. These region-specific community and taxon responses of aquatic biota to FI highlight the need to adapt standard biotic indices to enable effective ecological status assessments in IRES.

Ecological values of intermittent rivers for terrestrial vertebrate fauna.

Rivers are generally considered critical habitats for biodiversity; however, this often ignores the fact that many rivers may run dry and support terrestrial as well as aquatic fauna. Here, we investigated the ecological value of intermittent rivers for terrestrial vertebrates by installing camera traps along rivers subject to varying dry periods in two contrasting European climatic zones. We then analysed i) species presence and behaviours (as a proxy of ecological functions) on perennial and intermittent streams; ii) environmental (hydrological and geomorphological) and anthropogenic factors affecting the frequency of occurrence and number of species recorded; and iii) the importance of hydrological factors as regards ecological functioning. In both study areas, we recorded a higher number of species and individuals along intermittent streams than perennial streams, with highest values in intermittent reaches exhibiting shorter dry periods. Both abundance and species richness were strongly affected by hydrological factors in both study areas, including not only the occurrence but also the duration of the dry period. Dry channels played a key role as migration corridors and as a source of food, being used more frequently than riparian habitats when the river ran dry. Our findings indicate that terrestrial vertebrate fauna benefit from dry phases in rivers. Intermittent rivers, supporting a high abundance and diversity of fauna, should be considered as target ecosystems for wildlife conservation. Not doing so will jeopardise urgently needed conservation strategies in the face of accelerating global climate change.

A global agenda for advancing freshwater biodiversity research.

Global freshwater biodiversity is declining dramatically, and meeting the challenges of this crisis requires bold goals and the mobilisation of substantial resources. While the reasons are varied, investments in both research and conservation of freshwater biodiversity lag far behind those in the terrestrial and marine realms. Inspired by a global consultation, we identify 15 pressing priority needs, grouped into five research areas, in an effort to support informed stewardship of freshwater biodiversity. The proposed agenda aims to advance freshwater biodiversity research globally as a critical step in improving coordinated actions towards its sustainable management and conservation.

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.

Global prevalence of non-perennial rivers and streams.

Flowing waters have a unique role in supporting global biodiversity, biogeochemical cycles and human societies1-5. Although the importance of permanent watercourses is well recognized, the prevalence, value and fate of non-perennial rivers and streams that periodically cease to flow tend to be overlooked, if not ignored6-8. This oversight contributes to the degradation of the main source of water and livelihood for millions of people5. Here we predict that water ceases to flow for at least one day per year along 51-60 per cent of the world's rivers by length, demonstrating that non-perennial rivers and streams are the rule rather than the exception on Earth. Leveraging global information on the hydrology, climate, geology and surrounding land cover of the Earth's river network, we show that non-perennial rivers occur within all climates and biomes, and on every continent. Our findings challenge the assumptions underpinning foundational river concepts across scientific disciplines9. To understand and adequately manage the world's flowing waters, their biodiversity and functional integrity, a paradigm shift is needed towards a new conceptual model of rivers that includes flow intermittence. By mapping the distribution of non-perennial rivers and streams, we provide a stepping-stone towards addressing this grand challenge in freshwater science.

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.

What's in a Name? Patterns, Trends, and Suggestions for Defining Non-Perennial Rivers and Streams.

Rivers that cease to flow are globally prevalent. Although many epithets have been used for these rivers, a consensus on terminology has not yet been reached. Doing so would facilitate a marked increase in interdisciplinary interest as well as critical need for clear regulations. Here we reviewed literature from Web of Science database searches of 12 epithets to learn (Objective 1-O1) if epithet topics are consistent across Web of Science categories using latent Dirichlet allocation topic modeling. We also analyzed publication rates and topics over time to (O2) assess changes in epithet use. We compiled literature definitions to (O3) identify how epithets have been delineated and, lastly, suggest universal terms and definitions. We found a lack of consensus in epithet use between and among various fields. We also found that epithet usage has changed over time, as research focus has shifted from description to modeling. We conclude that multiple epithets are redundant. We offer specific definitions for three epithets (non-perennial, intermittent, and ephemeral) to guide consensus on epithet use. Limiting the number of epithets used in non-perennial river research can facilitate more effective communication among research fields and provide clear guidelines for writing regulatory documents.