Martin Schletterer

Freshwater systems, including urban streams and groundwater, are increasingly affected by climate change through altered precipitation, rising temperatures, droughts, floods, and changing water demand. This narrative review synthesizes evidence on how these pressures modify groundwater–stream connectivity and the resulting ecological, conservation, and sustainability implications in urban environments. The literature shows that groundwater supports baseflow, thermal buffering, hyporheic exchange, and habitat persistence, but that urbanization can simultaneously reduce natural recharge and create new anthropogenic recharge pathways. Studies of urban streams document flashier hydrographs and ecological degradation under conventional urbanization, whereas infiltration-focused low-impact development has been shown in some catchments to increase baseflow and reduce seasonal variability. Climate change adds further pressure: global modelling projects substantial warming of shallow groundwater by 2100, while long-term stream studies show taxon-specific community responses to warming and extreme events. Major knowledge gaps include limited long-term paired groundwater–stream observations, scale mismatches among hydrology, ecology, and governance, and underrepresentation of arid, semi-arid, tropical, and Global South cities. Addressing these gaps requires coupled hydrological–ecological research, functional indicators, and context-specific nature-based and hybrid management strategies that maintain or restore groundwater–stream connectivity as a component of urban freshwater resilience.

Freshwater biodiversity in Central Asia faces increasing pressure from anthropogenic impacts. A group of fish species of ecological and economical importance in the area are snow trout, cyprinids of the genus Schizothorax . However, fundamental knowledge to aid species conservation is limited, such as information about their seasonal movement patterns and habitat preferences. Consequently, in this study we tracked 27 snow trout ( Schizothorax eurycephalus ) equipped with radio transmitters for almost two years in the transboundary Shakhimardan River basin in the Alay mountain range, Uzbekistan and Kyrgyzstan. Movement distances and directions were analyzed using descriptive statistics and linear mixed models, and habitat use were assessed using ordinal logistic regression. Our results show that snow trout had a movement range up to 18 km and exhibited a seasonal migration pattern, moving downstream in spring and upstream in autumn. We observed that many fish migrated upstream into the groundwater-dominated Koksu River for wintering, potentially highlighting the role of thermal refugia. They mostly used habitats in medium-to-fast flowing sections with large substrates. This study is the first biotelemetry-based research of fish in Central Asia, revealing details on seasonal migration and habitat use of Schizothorax eurycephalus . Our findings underline the need to preserve aquatic connectivity and structurally diverse habitats in mountain rivers. Incorporating these insights into hydropower development, design and river restoration programs can help maintain ecological integrity while supporting sustainable development.

Introduction: This study examines the dynamics and characteristics of seasonal fish migration into the floodplain of a large lowland river. Methods: For continuous recording of the number of fish migrating upstream and downstream, and their size composition, the hydroacoustic complex “NetCor” was used. The system was permanently installed in spring 2022 on a floating platform in the Varpak River (a small tributary of the Irtysh River), mounted in a side-scan setup from the left bank. Results: The dominant role of water level and temperature for abundance dynamics of fish migrating into the floodplain is demonstrated. The earlier onset of high values for these factors positively correlate with the abundance of migratory fish. Their influence accounts for approximately 70%. On a daily basis, migration dynamics are determined by the daily variation in illumination. The abundance of upstream migrant fish correlated with diurnal variations in illumination. Migration intensity increases during periods preceding morning and, to a lesser extent, evening twilight. The lowest proportion of migratory fish was observed during the daytime, when illumination was highest. Discussion: The influence of water temperature, water level and light on the dynamics of seasonal fish migration is revealed. At the same time, a number of specific features have been noted for seasonal migration to the floodplain: synchronous increases and decreases in the intensity of fish movements up and downstream are simultaneously recorded, while the general trend of migration to the floodplain is maintained; in the initial phase of the flood, the largest fish are the first to migrate into the floodplain, which is an adaptive behavioral response. The diurnal distribution pattern of migratory fish is a strategy which they use to counter visually oriented predatory fish species—zander, perch, and pike—to avoid optimal prey search conditions.

Freshwater biodiversity is declining at a pace that outstrips the capacity of existing monitoring approaches both in temporal and spatial dimensions, highlighting the urgent need for rapid and scalable assessment and attribution of biodiversity states and changes. Here we present a global assessment and unified analysis of riverine fish biodiversity using environmental DNA collected from 1,818 sites across 113 river systems spanning 5 continents. We quantified species richness, functional redundancy, phylogenetic diversity and genetic sequence diversity, and related them to drainage characteristics. Our results showed that environmental DNA effectively captured global patterns of multi-faceted riverine fish biodiversity and disentangled the roles of climate and different aspects of human activities in shaping biodiversity–area relationships. The accumulation of biodiversity with catchment area was consistently enhanced in warmer climates, while human activities weakened this scaling. Further, species richness, functional and genetic sequence diversity exhibited stronger negative responses to human activities in larger catchments. In contrast, phylogenetic diversity was negatively affected by human activities, yet this effect diminished as catchment area increased, highlighting the facet-dependent nature of biodiversity responses to environmental gradients and catchment characteristics. Our findings demonstrate the power of environmental-DNA-based datasets for harmonized, multi-faceted biodiversity assessments, offering a scalable approach for detecting and attributing biodiversity change and informing conservation strategies under accelerating global change.

The mayfly Prosopistoma Latreille, 1833 has an outstanding research history. Discovered by Geoffroy (1762) in streams near Paris (France), for more than 100 years its larvae were considered сrustac...

Two-dimensional (2-D) hydrodynamic models based on high resolution bathymetric data from Airborne Lidar Bathymetry (ALB) surveys and cross-profile surveys are widely used to describe spatial flow variability in river systems and are particularly important in restored and widened river reaches, where flow patterns are highly heterogeneous. In parallel, environmental DNA (eDNA) has emerged as a cost-effective and non-invasive tool for biodiversity monitoring. However, linking eDNA signals to hydraulic processes in complex river geometries remains a key challenge, limiting its application for assessing restoration success.This study investigates the relationship between hydrodynamic conditions and fish eDNA dispersion and occurrence in a restored alpine river reach. The study site is located on the Inn River between Stams and Rietz in Tyrol, Austria, where as part of river restoration measures, bank protections were removed to widen the river and side arms were created to provide hydraulically diverse conditions for habitats. A controlled eDNA source, consisting of a cage containing dead eels (Anguilla anguilla), was placed in a side arm of the restored reach. Water samples were collected at multiple locations downstream and across the river and filtered on site. The eDNA was extracted from the filters and analysed for fish DNA using metbarcoding and general fish primers. Additionally, all samples were screed for eDNA of eel using qPCR and the total amount of overall fish eDNA from was determined for each sample. The spatial distribution and occurrence of eDNA from fish is analysed in relation to simulated hydraulic parameters, including flow velocity and water depth, derived from a 2-D hydrodynamic model. The results provide insights into how local flow conditions influence eDNA transport and dilution in restored river sections. This improved process understanding supports the use of eDNA as a monitoring tool for river restoration projects and contributes to the ecological assessment of restored reaches in line with the European Water Framework Directive.

The Volga is the largest river in Europe and its headwaters comprise reference or least disturbed conditions. In the headwaters of the Volga (445 km) upstream of Tver, 27 diatom samples (18 from the Volga and nine from selected tributaries) were collected in shallow water along the banks from different habitats, wherefrom 270 taxa (244 pennate and 26 centric) were identified. Most benthic taxa were found within Naviculaceae (40 taxa), Fragilariaceae (29 taxa), Bacillariaceae (27 taxa), Achnanthaceae (23 taxa), Gomphonemataceae (19 taxa), Cymbellaceae (17 taxa), and 16 taxa belong to the Amphora-complex. Species richness in the Volga and its tributaries was comparable; the mean value was 46 in the Volga and 50 in the tributaries. Regarding the saprobic index, the source region (reach R1) was characterized by a large proportion of xeno- and oligo-saprobic species, with the proportion of oligo-beta and beta-meso-saprobic species increasing along the continuum. This study provides a first comprehensive checklist of benthic diatoms for the Volga headwaters and analyzes longitudinal changes as well as the interplay between abiotic parameters and the diatom community in the headwaters of the Volga.

Freshwater biodiversity is declining at a pace that outstrips the capacity of existing monitoring approaches both in temporal and spatial dimensions, highlighting the urgent need for rapid and scalable assessment and attribution of biodiversity states and changes. Here, we present one of the first global assessments and unified analyses of riverine fish biodiversity using environmental DNA (eDNA) collected from 1818 sites across 113 river systems. We quantified species richness, functional redundancy, phylogenetic diversity, and genetic sequence diversity, and related them to drainage characteristics. Our results showed that eDNA effectively captured global patterns of multi-faceted riverine fish biodiversity and disentangled the roles of climate and human activities in shaping biodiversity–area relationships. Catchments in warmer climates consistently enhanced biodiversity accumulation with area, while higher human activity intensity weakened this scaling. Species richness, functional, and genetic sequence diversity exhibited stronger negative responses to human activities in larger catchments. In contrast, phylogenetic diversity showed the strongest negative effects in smaller catchments with these impacts diminishing as catchment area increased, highlighting the facet-dependent nature of biodiversity responses to environmental gradients. Our findings demonstrate the power of eDNA-based datasets for harmonized, multi-faceted biodiversity assessments, offering a scalable approach for detecting and attributing biodiversity change and informing conservation strategies under accelerating global change.

The analysis of environmental DNA (eDNA) has revolutionized biodiversity assessments in aquatic ecosystems, enabling non-invasive monitoring of fish communities across diverse regions. However, the global comparability of these eDNA datasets remains ambiguous due heterogeneous sampling protocols and bioinformatic workflows across studies, particularly regarding the robustness of their conclusions on biodiversity assessments. Here, we conducted a meta-analysis of 58 riverine fish eDNA metabarcoding datasets, covering 1,818 sampling sites worldwide, to evaluate the robustness of eDNA-derived biodiversity patterns. We found that species richness estimates and metrics of community structure derived under a common bioinformatic workflow were overall consistent with those of original analyses, despite the relatively high variability in bioinformatic analyses in the respective original studies. Contrastingly, congruence of species identity varied more extensively across datasets, mostly reflecting different completeness and regional relevance of reference databases. Restricting taxonomic assignment to basin-specific species pools improved species identification accuracy, while datasets lacking publicly accessible or well-curated reference data were more prone to mismatches. Year of sampling had a positive effect on taxonomic congruence, such that more recent studies showed increased robustness, also reflecting improved reference database coverage and enhanced species-level identification over time and overall method congruence in more recent years. Overall, the suitability and potential of eDNA for global biodiversity monitoring is corroborating overall robust biodiversity estimates, irrespective of the bioinformatic approaches. Our study underlines the effectiveness and need of further harmonization of bioinformatic workflows and strengthened region-specific reference databases for improved taxonomic resolution and comparability across studies.

Morphodynamic models are essential tools for estimating sediment dynamics along river reaches. A wide range of bedload transport equations is available, and applying these models to river restoration projects and river widenings requires simplifying assumptions about slope effects, secondary currents, and bank failure, introducing significant uncertainties. This study evaluates morphological changes during a flood event in the restored section between Stams and Rietz on the Inn River in Tyrol, Austria, using the 2D hydrodynamic model Telemac2D coupled with the morphodynamic model Gaia. As the flood occurred shortly after the completion of the restoration works, vegetation had not yet developed, allowing the influence of vegetation to be neglected in the numerical model. To enhance the applicability to similar restoration projects, the simulations were solely hydraulically calibrated and only relied on default morphodynamic calibration parameters. A comprehensive sensitivity analysis tested multiple bedload transport equations and key model parameters. The results suggest that the Meyer‐Peter & Müller and Hunziker equations effectively replicate erosional and depositional patterns, with volume estimates within a factor of two compared to the survey data. Transport rates varied significantly across all tested equations, underscoring the limitations of existing formulas for rivers characterized by coarse and broad gravel‐dominated particle size distributions and immobile coarse particle fractions. The sensitivity analysis also highlights the critical role of slope effects. While accounting for secondary currents and bank failure improved the model results, their impact was comparatively smaller. Overall, this study demonstrates that morphodynamic modeling can be a valuable tool for planning river restoration projects, particularly in estimating sediment budgets and predicting erosional and depositional patterns.