Large European Rivers Research Articles

ОСОБЛИВОСТІ ГІДРОГРАФІЇ ЄВРОПИ: РІЧКИ, ОЗЕРА, ВОДОСХОВИЩА

Europe has a dense hydrographic network, mainly related to the Atlantic Ocean basin. A certain part of the rivers of Eastern Europe belongs to the basin of the endorheic Caspian sea-lake. Only a few rivers carry their waters to the seas of the Arctic Ocean. Thus, 43 large rivers with a catchment area of more than 50 thousand km2, among which 8 are connected with the territory of Ukraine, are distributed as follows: a) the basin of the Caspian Sea-lake – 9 rivers (Volga, Ural, Kama, Oka, Belaya, Vyatka, Ufa, Sura, Moksha); b) the basin of the Arctic Ocean – 3 rivers (Pechora, Northern Dvina, Mezen); c) North Atlantic basin – 9 rivers (Rhine, Elbe, Vistula, Western Dvina, Neman, Oder, Warta, Seine, Narew); d) the basin of the South Atlantic – 6 rivers (Tagus, Loire, Douro, Guadiana, Guadalquivir, Garonne); e) the Mediterranean Sea basin (together with the Black and Azov Seas) – 16 rivers (Ebro, Rhone, Danube, Dnieper, Don, Dniester, Desna, Siverskyi Donets, Khoper, Tisza, Sava, Kuban, Southern Bug, Pripyat, Po, Maritsa ). With significant water content, the rivers of Northern, Western and Southern Europe are characterized by a small length and basin area. This is due to the relatively small area of the territory of Europe, its strong horizontal dissection and the frequent alternation of mountains and plains. On the expanses of the East European Plain, rivers get the opportunity to develop a catchment area and a channel network. Therefore, among the largest rivers in Europe, whose basin area exceeds 50 thousand km2, the rivers of Eastern Europe account for more than 60%. An important feature of European rivers is the transboundary nature of their basins. Among the huge rivers, with a catchment area of more than 50 thousand km2, transboundary rivers account for 51%. In Europe, there are about 16 thousand lakes with a surface area exceeding 1.0 km2. Three quarters of the continent’s lakes are located in Norway, Sweden, Finland and the Karelian-Kola part of Russia, and are of glacial-tectonic origin. The largest in Europe Ladoga (area 17700 km2) and Onega (9720 km2) lakes are located in Eastern Europe (north-western part of Russia) and significantly exceed other European lakes in size. The deepest lake in Europe is Lake Hornindalsvatnet (Norway) – 514 m. In Europe, about 7,000 large reservoirs have been created with a total area of more than 100 thousand km2, of which 50% is the area of reservoirs in the European part of Russia. Among the 13 European reservoirs with an area of more than 1000 km2, a dozen are located in Russia, two (Kremenchug Reservoir and Kakhovka Reservoir) are in Ukraine, and one (IJsselmeer) is in the Netherlands. The states with the largest number of reservoirs are Spain (about 1200) and Ukraine (about 1050). Other countries with a large number of reservoirs are Turkey (610), Great Britain (570), Italy (570), France (550), Norway (360), Russia (250) and Sweden (190)

Preface

The 7th Conference of the Environmental Problems of Large River Basins (EPLRB-7): PrefaceDear reader, I have the honor to bring to your kind attention a collection of proceedings of the International Scientific Conference “Environmental Problems of Large River Basins - 7”. Six previous conferences [1-5]1 were timed to coincide with anniversary dates in the history of the Institute - 10, 15, 20, etc. years from the date of its foundation. But time is moving faster and faster today. This made us hold the 7th Conference in the middle of the 5-year cycle and discuss the urgent environmental problems of the basins of large rivers (first and foremost, the Volga River, the largest river in Europe).

Impacts of vegetation and palaeohydrological changes on the n-alkane composition of a Holocene peat sequence from the Upper Vistula Valley (southern Poland)

PurposeThe analysis of n-alkanes in palaeoenvironmental studies of peatlands is mainly limited to ombrogenous peatlands which are a rare feature in the environment. Using n-alkane and plant macrofossil analysis, we have tracked changes in the environment in the valley of a large Central European river. We tested the possibilities for applying such studies to low-moor bogs which are the most commonly occurring type of bog in the world.Materials and methodsRepresentative peat samples were taken from two profiles in a peatland located in the Vistula Valley (southern Poland), and they were analysed for the occurrence and distribution of biomarkers. The distribution of n-alkanes was determined using gas chromatography-mass spectrometry (GC–MS). The botanical composition of the samples was determined by analysis of plant macrofossil remains, and the degree of peat decomposition was also determined.Results and discussionSamples of low-moor peat were characterised by a prevalence of medium- and long-chain n-alkane homologues, which is typical for higher plants. The variable values of the CPI, CPI (25–31), and the C23/C25 ratio have provided information on the stages of formation of the Zapadź bog. The source of n-alkane differentiation is vegetation change related to palaeohydrological dynamics and palaeoenvironmental conditions within the peatland.ConclusionsWe have shown that analysis of n-alkanes can be successfully used in low-moor bogs as a complement to palaeobotanical and palaeozoological methods. The bringing together of these types of research enables changes in the ecosystems of large river valleys to be tracked.

Nuisance species in lake constance revealed through eDNA

Biological invasions are a global threat to biodiversity especially for aquatic resources. The distribution of alien species is associated with human activities; therefore, exotic species tend to accumulate near big urban areas through different invasion vectors such as ballast water, hull fouling, aquarium and pet releases. The Rhine River region is one of the most economically important in Europe. Around 60 million people live in the river basin that is connected with other large European rivers via the Rhine–Main–Danube shipping canal. The Alpine Rhine flows to Lake Constance, which is the second largest subalpine lake in Europe. Here, eDNA metabarcoding was employed to inventory aquatic species from water samples in six riverine and four lake localities within Lake Constance region. A 313 bp fragment within cytochrome c oxidase subunit I gene was PCR amplified using generalist primers for metazoan and sequenced with MiSeq High-Throughput Sequencing platform. Seven invertebrate invasive species and the invasive fish Oncorhynchus mykiss were detected from eDNA. Species-specific primers were employed to confirm metabarcoded species. Most of the invasive species detected in this study correspond to samples from areas around lake ports, followed by other lake and degraded downstream river areas. Samples taken upstream of Lake Constance were free of invertebrate aliens. To establish common regulation and management actions regarding aquatic invasions in the three countries that share Lake Constance is recommended.

Round goby versus marbled crayfish: alien invasive predators and competitors

Aquatic biodiversity is threatened by spread of invasive alien species. Round goby Neogobius melanostomus is an invasive fish in large European rivers as well as in coastal waters near their mouths and marbled crayfish Procambarus virginalis is a highly invasive crustacean. Both are small, bottom-dwelling species occupying similar habitat and shelters and utilizing similar food sources. We hypothesized that goby presents a threat to both native and non-native astacofauna in invaded ecosystems. We tested this through laboratory experiments designed to determine aggressiveness and competitiveness of goby against marbled crayfish as a model for other North American cambarid crayfish, assessing goby prey size selection and competition with marbled crayfish for space and shelter. Gobies showed high aggressiveness and dominance over the crayfish. Goby predation on juvenile crayfish was limited by mouth gape size. In goby/crayfish pairs of similar weight, gobies were more aggressive, although each affected the behavior of the other.

Prirodnyye usloviya reki Dnestr v yeye nizhnem techenii

One of the largest rivers in Europe is the navigable Dniester. Today, it is extremely insufficiently used in the water transport system of Ukraine. In addition, it is the main river on the territory of Moldova and can be actively used for transport needs. The purpose of the study is an analytical characteristic of the navigation value of the Dniester. The main content of the analysis was reduced to graphic constructions: the location and planned location of courses, plotting optimal depths on the plan, drawing up morphometric profiles of river courses, etc. A navigation map of the Dniester estuary was built with a standard grid of depths, which were brought to the normal water level. The article also uses data on the natural conditions of the Dniester River based on research data obtained in the 1970s. The comparison of straight and curvilinear courses is carried out, the navigational importance of the Dniester estuary is studied. Recommendations for supporting favorable navigation conditions in the Dniester and in the estuary are determined. It is shown that within the lower course of the Dniester the greatest depths are found in straightened (7.4 m versus 5.8 m) parts. Minimum depths of 2–3 m are generally close to the banks, and maximum depths of 10–12 m are found in the middle parts of the course where they generally coincide with the midstream. Such depths facilitate movement of small and medium-sized river vessels. Recommendations for the updated navigation options for the state of the Dniester River and Dniester estuary are given. It is concluded that in case of resumption of navigation in the Dniester and Dniester estuary Ukraine will receive significant economic benefits.

Estimating salinity stress via hsp70 expression in the invasive round goby (Neogobius melanostomus): implications for further range expansion

Species invasions often occur on coasts and estuaries where abiotic conditions vary, e.g. salinity, temperature, runoff etc. Successful establishment and dispersal of non-indigenous species in many such systems are poorly understood, partially since the species tend to show genetic and ecological plasticity at population level towards many abiotic conditions, including salinity tolerance. Plasticity may be driven by shifting expression of heat shock proteins such as Hsp70, which is widely recognized as indicator of physical stress. In this study, we developed a qPCR assay for expression of the hsp70 gene in the invasive round goby (Neogobius melanostomus) and tested the expression response of fish collected from a brackish environment in the western Baltic Sea to three different salinities, 0, 10 and 30. hsp70 expression was highest in fresh water, indicating higher stress, and lower at brackish (ambient condition for the sampled population) and oceanic salinities, suggestive of low stress response to salinities above the population’s current distribution. The highest stress in fresh water was surprising since populations in fresh water exist, e.g. large European rivers and Laurentian Great Lakes. The results have implications to predictions for the species’ plasticity potential and possible range expansion of the species into other salinity regimes.

Multiple stressor effects on benthic macroinvertebrates in very large European rivers – A typology-based evaluation of faunal responses as a basis for future bioassessment

Two factors complicate the ecological status classification of very large rivers in Europe according to the EU Water Framework Directive: First, current assessment methods do not fully consider the specific ecology of very large rivers (such as lateral connectivity and the role of floodplains for ecological status). Second, most of Europe's very large rivers have been severely altered by human activities such as flood protection, damming and navigation. The aim of our study is to develop an assessment method for very large rivers by identifying suitable biological metrics as the basis for multi-metric bioassessment using benthic invertebrates. Based on the pan-European typology of very large rivers by Borgwardt et al. (2019), we established a river type-specific assessment approach using invertebrate samples from 25 European countries and 94 very large rivers. The frequency and intensity of eight pressures jointly acting on the sampling sites were described, and a selection of suitable invertebrate community metrics were correlated with the pressure intensities to establish pressure-response relationships. The very large river types differ in terms of relevant pressures and pressure combinations, with the invertebrate communities distinctly responding to these pressure patterns. Neozoa dominance correlated strongly with ‘navigation’, being a major pressure at very large rivers, which entails severe hydro-morphological alterations such as channelization, riparian vegetation alteration and impoundment. Under combined pressures, a critical community turnover became evident in terms of neozoa outnumbering EPT taxa and the ratio of hemilimnic invertebrates decreasing. We propose ten bioassessment metrics, including measures of biological diversity as well as newly generated indicators, for the development of a European type-specific assessment method for very large rivers.

The hydrogeological situation after salt-mine collapses at Solotvyno, Ukraine

Study regionThe study site is located in the south-western part of the Ukraine, in the area of the historical rock-salt mining town Solotvyno. The former mining area is situated in close vicinity to the River Tisza, the main tributary of the Danube River, the largest river in Europe. Study focusAfter uncontrolled flooding of several salt mines, a one month advisory mission was launched by the European Commission to estimate the impact of the abandoned salt mines (containing large quantities of salt water) on the environment. As a consequence of the flooding, dozens of sinkholes formed and sinkhole forming processes are ongoing, with sinkhole diameters reaching 250 m. As river contamination by the release of large quantities of saltwater would lead to an international disaster, hydrogeological measurements were taken on-site to study the system. New hydrological insights of the regionAt the study site, saturated (hyper-saline) water as well as fresh surface and groundwater were encountered in close vicinity to each other. Electrical conductivity, as a proxy for salinity, and temperature were measured on-site and water samples taken from surface-, ground- and mine waters were analyzed for chemistry and stable isotopes, providing new insights into groundwater flow dynamics. A conceptual model shows the salt dome, and potential flow paths from the mining area to the Tisza River, in the context of the mines and associated sinkholes potentially impacting the river water quality.

Comparative assessment of microplastics in water and sediment of a large European river

Aquatic ecosystems are globally contaminated with microplastics (MP). However, comparative data on MP levels in freshwater systems is still scarce. Therefore, the aim of this study is to quantify MP abundance in water and sediment of the German river Elbe using visual, spectroscopic (Fourier-transform infrared spectroscopy) and thermo analytical (pyrolysis gas chromatography mass spectrometry) methods. Samples from eleven German sites along the German part of the Elbe were collected, both in the water and sediment phase, in order to better understand MP sinks and transport mechanisms. MP concentrations differed between the water and sediment phase. Sediment concentrations (mean: 3,350,000 particles m−3, 125–5000 μm MP) were in average 600,000-fold higher than water concentrations (mean: 5.57 particles m−3, 150–5000 μm MP). The abundance varied between the sampling sites: In sediments, the abundance decreased in the course of the river while in water samples no such clear trend was observed. This may be explained by a barrage retaining sediments and limiting tidal influence in the upstream parts of the river. Particle shape differed site-specifically with one site having exceptionally high quantities of spheres, most probably due to industrial emissions of PS-DVB resin beads. Suspended MP consisted predominantly of polyethylene and polypropylene whereas sediments contained a higher diversity of polymer types. Determined MP concentrations correspond well to previous results from other European rivers. In a global context, MP levels in the Elbe relate to the lower (water) to middle section (sediment) of the global range of MP concentrations determined for rivers worldwide. This highlights that elevated MP levels are not only found in single countries or continents, but that MP pollution is an issue of global concern.

Danube Delta - a unique place in Europe

At the end of a flowing route that exceeds 2,860 km, through an impressive catchment area that covers more than 80% of Europe, the Danube, the second largest river in Europe, intersects the Black Sea forming the delta which is known as one of the great wetlands of the planet. The stretches of water and land formed here offer good living conditions to many species of plants and animals. The purpose of this study was to create several maps that present these aspects of the Danube Delta wetland using a set of GIS technologies. Since the GIS technology is an important tool in the analysis of geographic areas, we managed to obtain the following maps: hypsometric, slope, slope orientation and flood direction. Since September 1990, the Danube Delta Biosphere Reserve has been recognised internationally as a wetland; specifically, the Danube Delta Biosphere Reserve has been recognised internationally as a wetland, especially for water birds habitat. The value of the World Heritage Site of the Danube Delta Biosphere Reserve was recognised in December 1990 by including more than half of its surface into the World Cultural and Natural Heritage List. Keywords: Danube Delta, flood accumulation, natural heritage, slope, wetlands.

Morphological and molecular investigation on freshwater choanoflagellates (Craspedida, Salpingoecidae) from the River Rhine at Cologne (Germany)

Paramonosiga coloniensis sp. nov., Salpingoeca amphoriscia sp. nov., S. fluviatilis sp. nov. and S. loutrophoria sp. nov. are frequently found craspedid species in the River Rhine which have not yet been described, despite their high abundance. All new species are characterized based on a distinct morphology which is different from all up to now described species and on a molecular level based on transcriptome data. In addition, we give extended redescriptions of S. amphoridium and S. angulosa, based on SSU and LSU rDNA data and morphology. The six-gene phylogenetic analyses place all species into freshwater clades of the craspedids. The separation of the freshwater and marine clades of this group is becoming more distinct with every craspedid sequence added. The River Rhine is one of the largest rivers in Europe but its protist biodiversity is fairly undescribed, especially regarding choanoflagellates. We conclude that the biodiversity of craspedid choanoflagellates is broadly underestimated.

The potential natural vegetation of large river floodplains – From dynamic to static equilibrium

The potential natural vegetation (PNV) is a useful benchmark for the restoration of large river floodplains because very few natural reference reaches exist. Expert-based approaches and different types of ecological models (static and dynamic) are commonly used for its estimation despite the conceptual differences they imply. For natural floodplains a static concept of PNV is not reasonable, as natural disturbances cause a constant resetting of succession. However, various forms of river regulation have disrupted the natural dynamics of most large European rivers for centuries. Therefore, we asked whether the consideration of succession dynamics and time dependent habitat turnover are still relevant factors for the reconstruction of the PNV.To answer this we compared the results of a simulation of the vegetation succession (1872–2016) of a segment of the upper Rhine river after regulation (damming, straightening and bank protection) to different statistic and expert-based modelling approaches for PNV reconstruction. The validation of the different PNV estimation methods against a set of independent reference plots and the direct comparison of their results revealed very similar performances. We therefore conclude that due to a lack of large disturbances, the vegetation of regulated large rivers has reached a near-equilibrium state with the altered hydrologic regime and that a static perception of its PNV may be justified. Consequently, statistical models seem to be the best option for its reconstruction since they need relatively few resources (data, time, expert knowledge) and are reproducible.

ГИДРОЛОГИЧЕСКОЕ ОБОСНОВАНИЕ УПРАВЛЕНИЯ ВОДНЫМИ РЕСУРСАМИ В ВЕРХНЕЙ ЧАСТИ БАССЕЙНА ДНЕПРА

The Dnieper river is one of the largest rivers in Europe. It is the fourth largest river in terms of basin area and length after the Volga, Danube and Ural, and the second largest of the rivers that flow into the Black sea, after the Danube. From the point of view of territorial distribution of water resources, the Dnieper basin can be divided into two zones. The first zone includes the Russian and Belarusian parts of the basin, which is a zone of river flow formation. It is characterized by an insufficiently high level of water resources use. The second zone, which is a part of the Ukrainian part of the river basin, is characterized by a small inflow of water from tributaries and a high level of water resources use. Rivers, watercourses of which run through the territory of two or more states, are called as transboundary. Different sections of the same river are used by different states, and the use of water resources and catchment area in the upper reaches of the river will affect the quantitative and qualitative parameters of water downstream. This means that the unified environmental system is divided by a state border and is managed in each individual state at its discretion. This affects the river and surrounding areas. After the collapse of the Soviet Union, the problem of transboundary water bodies became very relevant for Russia. Neighboring countries have faced a number of environmental problems in border territories and transboundary river basins, where environmental problems have persisted and often worsened, but the institutional framework for their effective solution has been lost

Compositional dynamics of suspended sediment in the Rhine River: sources and controls

PurposeInformation on the geochemical composition of suspended sediments in rivers is crucial to identify sediment source type or area. In large river basins, however, the relation between sediment composition and its controlling factors is often obscured. This study aims to assess and improve the conceptual understanding on the factors and mechanisms that control the composition of suspended sediments in the River Rhine, one of the large European rivers, and to identify the dominant source types of elements.Materials and methodsWe performed log-linear regression analysis and principal component analysis (PCA) on bi-weekly monitoring data of suspended sediment composition, supplemented with daily measurements of suspended sediment concentrations (SSC) and discharge at the Lobith monitoring station near the German-Dutch border for the period 2011–2016.Results and discussionThe statistical analyses show a consistent grouping of elements that display contrasting temporal variation or different responses to increased discharge. The contrasting behaviour also becomes manifest in the results from the PCA. A first component that explains about the half of the total variance in the entire dataset reflects the variation in clay content in the suspended sediment. A second component reflects anthropogenic pollution and explains about a quarter of the total variance. A third component probably reflects variation in sediment provenance.ConclusionsThe majority of the temporal variation in suspended sediment composition can be attributed to variations in grain size (clay content), organic matter content, and anthropogenic pollution. Only a minority of the variation can be attributed to variations in the contributions from different upstream source areas. This variation represented by the third and higher components from the PCA can potentially be used for sediment provenance analysis.

Ex uno plures – Defining different types of very large rivers in Europe to foster solid aquatic bio-assessment

The sustainable management of very large rivers is based on the assessment of their environmental condition. However, unlike for smaller rivers, this assessment is yet less advanced due to the specific complexities of very large river systems. In Europe, this is reflected by the lack of an international large river typology classifying these ecosystems into units with homogenous environmental features. Focussing on European rivers of >10,000 km2 catchment size, we collated datasets describing orographic, geological, geomorphological, hydrological and climatic features of the riverine environment. These rivers were divided into segments of about 30 km length, and 40 environmental descriptors hardly modifiable by human activities were allocated to each segment. In a subsequent cluster analysis, we identified seven large river types (LRTs) across Europe: Alpine-Influenced rivers; Baltic rivers; Central Lowland rivers; Continental Lowland rivers; Mediterranean rivers; Nordic rivers; Atlantic rivers. These LRTs were compared with existing European bio-geographical classifications and tested using contemporary river sampling data of benthic invertebrates. Our study describes a systematic pan-European typology characterising different types of European very large rivers, reviewed and validated by biological data. This typology represents an essential step in fostering the implementation of bio-assessment methods tailored to very large rivers in Europe.

Fish diversity assessment in the headwaters of the Volga River using environmental DNA metabarcoding

Abstract The headwaters of the Volga River exhibit large reaches with near‐pristine conditions, and therefore long‐term biodiversity monitoring of this catchment can provide rare and valuable information on a European lowland river. More specifically, freshwater fish species assemblages are a good indicator of ecosystem status, as they are particularly sensitive to environmental changes and hydromorphological alterations. Historical records show that the fish fauna of the Upper Volga has changed over time, both in species composition and in abundance. The construction of the Volga–Kama cascade (a series of large dams) has specifically affected the migration of diadromous species. Environmental DNA metabarcoding offers a non‐invasive approach to determine the number of species in an aquatic ecosystem, as well as their identity and distribution. This approach is especially useful for fish fauna surveys along large rivers and long‐term biomonitoring, with the advantage of having no impact on the species and their habitats. To infer the current fish species diversity and the spatial distribution of each species in the free‐flowing section of the Upper Volga River, as well as in selected tributaries, an environmental DNA metabarcoding approach was applied, using three mitochondrial DNA markers. This method allowed the positive identification of 23 fish species and their respective distributions in the headwaters of the Volga. This assessment provides a valuable example of the application of environmental DNA metabarcoding in a large river system, and constitutes a starting point for future investigations and long‐term biomonitoring in the Upper Volga system. In addition, the results can also serve as a reference for fish diversity assessments of other large European lowland rivers, and can guide future conservation and management measures in the headwaters of the Volga.

Model‐based reconstruction of the succession dynamics of a large river floodplain

AbstractMost large rivers in Europe and North America suffered flow regulations and channelization in the 19th and 20th century. To study the effects of the altered site conditions on the development of floodplain vegetation and create a benchmark map for their restoration, we calibrated and applied a dynamic floodplain vegetation model that accounts for the processes recruitment as well as morphodynamic disturbance and physiologic stress on vegetation to reconstruct the succession dynamics of the floodplain vegetation of a segment of the Rhine River from shortly after it was channelized (1872) until today (2016). The model calibration was based on historical maps and hydrologic data.Our simulation demonstrated a steady, one‐way progression of the vegetation communities towards mature phases without regression to younger stages. It was possible to attribute this development to a lack of morphodynamic disturbances strong enough to reset succession and to identify physiological stress caused by long inundations periods as the most relevant controlling factor of succession. The resulting vegetation distribution (2016) can be considered an estimation of the potential natural vegetation (PNV) under altered site conditions.The good agreement of the model results with an expert‐based PNV map showed that our approach is a good alternative to create benchmark maps for floodplain conservation and restoration projects. From a research and practitioners' viewpoint, it has the big advantage over the traditional approach that it allows to analyse different points in time as well as to be comprehensive and reproducible.

Effects of large-river restoration measures on ecological fish guilds and focal species of conservation in a large European river (Danube, Austria)

River engineering measures of the last two centuries have severely impacted the fish coenoses of all larger rivers in Europe by creating river barriers, replacing natural shores with riprap-reinforced banks, and installing artificial structures that changed the hydrology of inshore habitats. As a result, fluvial specialists have declined and were replaced by generalists. Consequently, there is an urgent need to develop functioning means and measures to restore stream habitats and re-establish near-natural conditions. In this study, we analyze the effectiveness of four different restoration measures to increase the abundance and ratio of the rheophilic fish guild in general and of two fluvial focal species of conservation, the barbel and nase, in particular. The sampling took place in a free-flowing part of the Austrian Danube, during the monitoring program of a large hydro-engineering project with both economic and ecological targets. We applied a BACI design and used data from three extensive sampling years. The ecologically orientated measures included groynefield adaptations, a bank re-naturalization, and a side arm re-connection. All measures increased the abundance of nase and barbel and also provided suitable conditions for young-of-the-year fishes. The bank re-naturalization, significantly increased both the abundance and the ratio of rheophilic fishes. The side arm re-connection significantly improved the ratio of rheophilic fishes. Both groyne adaptation measures showed positive, but non-significant effects. Consequently, replacing riprap sections with near-natural gravel bars and re-connecting former side arms can be recommended as appropriate measures to support the rheophilic fish guild, whereas groyne adaptations appear to be of limited value. The evaluated restoration measures significantly improved conditions for local populations of target species and coenoses and can be used as building blocks in upscaled, river-wide restoration efforts. A sustainable restoration of the whole river ecosystem, however, requires a holistic approach that also addresses floodplains and catchment-wide issues.

Species and river specific effects of river fragmentation on European anadromous fish species

AbstractFragmentation is one of the major threats to riverine ecosystems and this is most explicitly expressed by the decline in numbers of migratory fish species. Yet each species has different migration requirements and their natural distribution can include several catchments with multiple dams. Hence, to prioritize candidate rivers for improving accessibility, differences between species and between catchments have to be taken into account. The aim of this study was therefore to analyse the species and river specific effects of river fragmentation on migratory fish on a European scale. The effect of river damming on migratory fish was quantified for all 16 European long‐ and mid‐distance anadromous species and for 33 large European rivers. The historical distribution was compared with the current upstream accessibility of the main river and the current distribution and population status of each species. The observed effects of reduced connectivity were further quantified using the Dendritic Connectivity Index for species and the Fragmentation Index for rivers. Our results showed that only very few rivers are still unaffected by dams in the main stem and that the few remaining viable migratory fish populations in Europe occur in these accessible rivers. Barriers were prioritized for making passable based on the potential accessibility gain and the number of benefitting species, showing that the main stems of the rivers Shannon and Nemunas are the best candidates. It was concluded that evaluating species and river specific effects of fragmentation strongly aids in prioritizing rivers for improving upstream accessibility.