Marcel Liedermann

Zusammenfassung: Flüsse, Auen und ihre Grundwasserkörper gehören zu den artenreichsten, ökologisch bedeutsamsten und gesellschaftlich unverzichtbarsten Ökosystemen Europas. Zugleich gehören sie zu den am stärksten bedrohten Ökosystemen. Um den zunehmenden multiplen Belastungen durch Klimawandel, Landnutzung, Schadstoffeinträge und Habitatverlust wirksam zu begegnen und nachhaltige Nutzungsperspektiven sicherzustellen, sind integrierte, langfristige Beobachtungsansätze dringend erforderlich. Dieser Artikel stellt DANUBIUS-ERIC vor, die seit 2025 als „European Research Infrastructure Consortium“ formalisierte europäische Forschungsinfrastruktur für Fluss-Meer-Systeme, und beschreibt die österreichische Forschungs-Supersite „Upper Danube Austria“ als einen ihrer zentralen Standorte. Die Supersite umfasst zwei komplementäre Forschungsgebiete: die Donauauen östlich von Wien (Nationalpark Donau-Auen) und das Ybbs-Einzugsgebiet. Die Supersite hat auf mehreren Ebenen Messinfrastruktur bestehend aus stationären Langzeitmessstationen in beiden Forschungsgebieten, mobilen Feldmessgeräten und Laborinfrastruktur aufgebaut, die eng mit dem gesamten europäischen Netzwerk verbunden sind. Erste Anwendungsbeispiele illustrieren das thematische Spektrum der Supersite: von der integrativen Grundwasserökologie und der Kohlenstoff- und DOM-Dynamik in Auengewässern bis zur einzugsgebietsweiten Modellierung von PFAS-Transportprozessen. Die Infrastruktur ist explizit auf die Unterstützung naturbasierter Lösungen, Renaturierungsmaßnahmen, nachhaltige Nutzungsansätze sowie die Umsetzung der EU-Wasserrahmenrichtlinie sowie des EU-Renaturierungsgesetzes ausgerichtet und ergänzt die bestehende behördliche Gewässerüberwachung um ökologische, biogeochemische und hydromorphologische Dimensionen, für die bislang keine systematische Langzeiterfassung bestand.

Introduction: River regulation schemes generally led to changes in riverine assemblage composition and their trophic status, which often is reflected in a general loss or decrease of characteristic species and a concomitant increase or dominance of generalist species. Objectives: This study examines changes of the hydrodynamic conditions, plankton biomass as well as changes in feeding guilds and the assemblages of macrozoobenthos and of fishes before and after the re‐connection of a sidearm of the main stem in a free‐flowing section of a large river (Danube in Austria). Methods: In order to analyze the effects in the sidearm, a before–after control‐impact design was applied; a river section in the main stem served as a reference site. Results: The higher and increased connectivity had sizable effects on hydraulic parameters, with the expected shift of prevailing lentic to year‐round lotic conditions in the sidearm. The composition of suspended solids changed significantly. Likewise, chlorophyll‐ a concentrations and zooplankton abundance decreased markedly. The new conditions in turn modulated the pathways and the structure of the food web. Conclusions: The permanent flow‐through in the sidearm decreased the importance of detritus and forced the one of periphyton and biofilm as an energy and nutrient source for insect larvae which also affected higher trophic levels, that is, fishes.

Building on the methodological development carried out within the “Alplast” project, a newly developed isokinetic pump was successfully applied in combination with established net-based sampling methodology at several riverine measurement sites. This integrated approach allows for a comprehensive assessment of microplastic transport across a wide range of particle sizes, including the finest fractions that cannot be captured by net-based methods.The combined application of net sampling and isokinetic pump sampling has proven to be robust and operational under varying field conditions form small streams to large rivers. While net sampling continues to effectively target coarser microplastic particles and enables the filtration of large water volumes, the isokinetic pump has delivered reliable results for the smallest size fractions. First experiences with laboratory analysis and data evaluation indicate that these fine particles represent a significant proportion of the total microplastic mass, hence they contribute substantially to overall microplastic transport, also due to their high mobility and widespread spatial distribution within the flow. The isokinetic sampling principle ensured that flow conditions at the intake were representative of the ambient river velocity, thereby minimizing sampling bias and enabling direct weighting of transport across the cross-section. This proved especially advantageous for capturing the variability in microplastic concentrations while keeping the number of required samples manageable.Overall, the results confirm that the combination of net-based sampling and pump sampling with the isokinetic pump significantly enhances the representativeness of microplastic transport assessments in rivers. The methodology provides a sound basis for future studies aiming to quantify the role of fine microplastic fractions and contribute to standardized monitoring approaches.

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Since the introduction of the Sustainable Development Goals, and, in particular, with the goal of reducing marine pollution (SDG 14.1), riverine microplastics are attracting public and scientific attention. But standardized monitoring methods and comparable data are still missing. Therefore, the opportunity was taken to test three of the most common monitoring methods (multiple depths net-method, pressurized fractionated filtration and sedimentation-box) at seven sites in five countries along the Danube and the Tisza Rivers. Different boundary conditions (hydrological and morphological conditions, economic situation, equipment available, etc.) were considered for the evaluation, as well as different sampling methods and sample pre-treatments together with different methodologies for microplastic identification. The sampling methods were evaluated for their suitability to be used as a standard monitoring tool in the future. Only net sampling and pressurized fractionated filtration allow for the determination of microplastic concentration as well as load, and can therefore be recommended. The multi-depth net device, as a labor-intensive method, is recommended if the focus of the monitoring is on larger particles and it is important to calculate particle and mass concentrations. Pressurized fractionated filtration is a practical tool recommended for routine monitoring, having the advantage of less effort being required for sample preparation and simply considering small particle sizes below 500 µm. From a scientific perspective it is recommended to combine both the pump sampling and the net-based device.

Due to their persistent presence in the environment and the largely unknown long-term impacts on biota, plastic waste has increasingly become the focus of scientific research in recent years. Particularly in the field of microplastic monitoring in rivers, there remains a significant need for research. As methodologies are not yet standardized, and often not representative and comparable, efficient sampling poses a major challenge.Within the framework of the "Alplast" project, intensive efforts were made to develop a methodology capable of accurately capturing microplastic transport in rivers. Various carrier systems tailored to different river sizes were developed for net sampling, which targets the coarser fractions of microplastics (> 250 µm). In addition, a novel isokinetic pump was designed to analyze finer microplastic fractions from 50 to 250 µmGiven the spatial and temporal variability of plastic transport, multipoint sampling under varying hydrological conditions is strongly recommended. Larger microplastic particles occur less frequently, making it essential to sample large volumes of water to representatively capture this size range. Using net sampling, up to 1,000 m³ of water can be filtered at a single sampling point. However, finer particles, which cannot be captured by the limited mesh size of nets due to clogging especially under turbid boundary conditions, must be analyzed using pump samples. The number of sampling points across the river profile is often limited related to the high costs of analysis.The newly developed isokinetic pump addresses this gap by measuring flow velocity at the intake area of the sampling device. A control unit regulates the pump speed so that the pumped water volume matches the natural flow in the sampling cross-section. This isokinetic sampling approach offers two major advantages. First, the plastic flow is not disproportionately altered during sampling, and second, a direct weighting of the spatial distribution in flow and concentration is automatically accounted for. This significantly reduces the number of required samples, which is particularly beneficial given the high costs associated with sample analysis.The new methodology was successfully combined with net sampling and applied at multiple measurement sites. Results demonstrate that this approach enables efficient and representative monitoring of microplastic transport in rivers.

Zusammenfassung: Die Kiesmächtigkeit spielt eine entscheidende Rolle für die Stabilität der Flusssohle und den Geschiebehaushalt der Donau östlich von Wien. Durch anthropogene Eingriffe wie Begradigungen, Querbauwerke und Regulierungsmaßnahmen für die Schifffahrt kam es in diesem Bereich zu einem signifikanten Sedimentdefizit, das in weiterer Folge zu Sohlerosion führt. Besonders problematisch sind dabei Bereiche mit geringer Kiesüberdeckung, in denen feinere Sedimente tertiärer Ablagerungen anstehen und wo die Voraussetzungen für einen potenziellen Sohldurchschlag gegeben sind. In der vorliegenden Arbeit wurde ein systematisches Monitoring der Kiesmächtigkeit durchgeführt, um eine fundierte Datengrundlage für das Geschiebemanagement zu schaffen. Mithilfe von GIS-gestützten Interpolationsverfahren wurde aus 1097 Bohrkerndaten ein hochauflösendes digitales Geländemodell der Kiesunterkante erstellt und mit den Sohlgrundaufnahmen der letzten 18 Jahre verschnitten. So konnten kritische Bereiche mit geringer Kiesüberdeckung identifiziert und deren Entwicklung über die letzten Jahre analysiert werden. Die Ergebnisse zeigen, dass einige Kolkbereiche eine deutliche Verbesserung der Kiesüberdeckung aufweisen, während andere weiterhin kritisch bleiben. Zusätzlich wurden vier neue potenzielle Risikobereiche identifiziert. Die Untersuchungen bestätigen, dass gezielte Sedimentzugaben und Geschiebemanagementmaßnahmen positive Effekte haben, jedoch weiterhin ein präzises Monitoring erforderlich ist, um das Gefahrenpotenzial zu reduzieren. Das neu entwickelte GIS-basierte Monitoring-Tool bietet eine wertvolle Grundlage für die langfristige Überwachung der Flussmorphologie und ermöglicht eine effektive Planung zukünftiger Maßnahmen. Die gewonnenen Erkenntnisse tragen dazu bei, nachhaltige Strategien zur Stabilisierung der Flusssohle zu entwickeln und damit sowohl die ökologische Funktion als auch die Infrastrukturfunktion der Donau zu sichern.

Plastic waste as a permanent pollutant in the environment is of increasing concern due to its largely unknown long-term effects on biota. The occurrence in rivers, has, compared to research in the oceans, only become the focus of scientific investigations in the last few years. The Austrian Alps in particular are largely unexplored in this respect. Therefore, the Alplast project addresses microplastic transport from the glaciers at the summit over steep mountain torrents to the lowland rivers and aims in conducting a first inventory of the alpine area. Specifically, analyses of microplastic occurrences are being carried out from the Sonnblick glacier via the Rauriser Ache, the Salzach, the Inn and the Danube and are intended to expand the understanding of processes with regard to the behaviour of microplastics in the water cycle from the glacier to the valley. The influence of snowmelt as well as the temporal development, which can be determined from ice cores, are of great interest. In addition, questions regarding the origin and distribution of plastic in flowing waters as well as the possible biological degradation by microorganisms will be clarified.Since the sampling areas cover entire catchments at different altitudes, different methodologies and devices are used. For the studies on the glaciers, the snow cover as well as ice cores are sampled and analysed. In the rivers a multi-point method is used due to the spatial distribution of plastic particles in the river cross-section. But the net samples at different depths are combined with isokinetic pump sampling in order to detect the widest possible size range. Isokinetically taken pump samples have the great advantage that a weighting process takes place directly during sampling. This means that samples can be taken in different areas (high and low flow velocities) of the cross-section (together with the nets) and then a composite sample can be analysed for the profile. Particle counts, classification and the measurement of concentrations and loads are then used to determine quantities and the most common types of plastics in the alpine environment. The measuring stations were selected in such a way that more and more potential microplastic sources are added in the course of the catchment in order to achieve the best possible process understanding regarding the origin and fate of the plastic waste.

Plastic pollution in aquatic environments is a growing concern, with rivers recognized as major pathways. However, rivers themselves are also subject to pollution. Hence, understanding riverine plastic transport dynamics is essential for mitigating environmental impacts. Although plastic-related research focus has shifted from marine environments towards rivers, challenges remain in standardizing methods for monitoring and integrating spatio-temporal variabilities of riverine plastic occurrence into flux determination. This study addresses these challenges by adopting established methods from sediment research. Utilizing data from a net-based cross-sectional multi-point approach, it examines spatio-temporal and discharge-dependent variations. It comprehensively analyzes the complex dynamics of plastic transport in the Danube River, contrasting an impounded section near Aschach, Austria, with a free-flowing reach near Hainburg, Austria. The paper emphasizes the significance of applying these methodologies for accurate flux determination and underscores the risks of neglecting them. By incorporating average microplastic particle weights, we aim to overcome limitations in prior methodologies that solely emphasize qualitative aspects or rely on item numbers. Spatial distribution analysis revealed a pronounced stratification at low flow and a more variable distribution in the free-flowing section, attributed to higher turbulence. As discharge increased, vertical mixing occurred, along with distinct lateral patterns displaying increased concentrations toward the riverbanks. Encountering plastic particles throughout the river profile underscores their properties of both suspended and floating matter, emphasizing the importance of hydro-morphology and multi-point cross-sectional measurement approaches. Microplastic loads were calculated to be −1 in Aschach and −1 in Hainburg, compared to total plastic loads of −1 in Aschach and −1 in Hainburg. Consequently, plastic loads were doubled to tripled within the Austrian section of the Danube River. The study contributes valuable insights into the complex nature of plastic transport in river systems, emphasizing comprehensive spatial, temporal and discharge-dependent assessments for characterizing and managing plastic pollution. It suggests that rivers can function as sources, pathways and sinks of plastic pollution, contingent upon hydro-morphological conditions. This underscores the need for longitudinal, basin-wide assessments to accurately understand plastic transport dynamics.

Microplastics are already part of our environment - even in the most inaccessible and remote places we find plastic particles that also remain there due to their longevity. The finer the scale at which we analyse samples, the more particles we find. It also became clear in the recent years, that rivers transport microplastics and that they also represent a principal pathway for plastic transport from land to the sea.Initial measurements with benthic nets on the Austrian Danube have shown that the plastic particles are not only found on the surface, but are distributed unevenly with maximum concentrations partly near the bottom, partly on the surface and partly concentrated on one side of the river. Given the spatial and temporal distribution, a multipoint method seems inevitable and was chosen for further measurements, in which three measuring points each are distributed over the depths of several measuring verticals along a cross-section. Furthermore, it was found that the microplastic concentration also strongly depends on the discharge conditions and that by far the largest quantities of microplastics are mobilized and then transported during flood events.After the first comprehensive measurements on the Austrian Danube between 2014 and 2015, further samplings were carried out in Hungary, Serbia and at 3 locations in Romania over the recent years. Despite the limitations of the data set, regarding the longitudinal, cross-sectional and hydrological representation of the micro plastic transport in the Danube River, an attempt is made to describe its characteristics on a basin wide scale. The measurements, reaching from impounded sections in the Upper Danube all the way down to the Danube delta, have shown, that the concentrations fluctuate strongly longitudinally according also to the discharge level. Therefore, at least at certain times, the Danube – and river systems with their inundation areas in general – are not only pathways, but can also be regarded as a microplastic sink. However, to increases process understanding and derive reliable statements, more data are needed.