Restoration strategies for river floodplains along large lowland rivers in Europe

1. Most temperate rivers are heavily regulated and characterised by incised channels, aggradated floodplains and modified hydroperiods. As a consequence, former extensive aquatic/terrestrial transition zones lack most of their basic ecological functions.
2. Along large rivers in Europe and North America, various floodplain restoration or rehabilitation projects have been planned or realised in recent years. However, restoration ecology is still in its infancy and the literature pertinent to river restoration is rather fragmented. (Semi-) aquatic components of floodplains, including secondary channels, disconnected and temporary waters as well as marshes, have received little attention, despite their significant contribution to biological diversity.
3. Many rehabilitation projects were planned or realised without prior knowledge of their potential for success or failure, although, these projects greatly contributed to our present understanding of river–floodplain systems.
4. River rehabilitation benefits from a consideration of river ecosystem concepts in quantitative terms, comparison with reference conditions, historical or others, and the establishment of interdisciplinary partnerships.
5. We present examples from two large European rivers, the Danube and the Rhine, in which the role of aquatic connectivity has been extensively studied. The Danube delta with its diversity of floodplain lakes across an immense transversal gradient (up to 10 km) serves as a reference system for restoration projects along lowland sections of large rivers such as the Rhine in the Netherlands.     

Riverine landscape diversity

1. This review is presented as a broad synthesis of riverine landscape diversity, beginning with an account of the variety of landscape elements contained within river corridors. Landscape dynamics within river corridors are then examined in the context of landscape evolution, ecological succession and turnover rates of landscape elements. This is followed by an overview of the role of connectivity and ends with a riverine landscape perspective of biodiversity. 2. River corridors in the natural state are characterised by a diverse array of landscape elements, including surface waters (a gradient of lotic and lentic waterbodies), the fluvial stygoscape (alluvial aquifers), riparian systems (alluvial forests, marshes, meadows) and geomorphic features (bars and islands, ridges and swales, levees and terraces, fans and deltas, fringing floodplains, wood debris deposits and channel networks). 3. Fluvial action (erosion, transport, deposition) is the predominant agent of landscape evolution and also constitutes the natural disturbance regime primarily responsible for sustaining a high level of landscape diversity in river corridors. Although individual landscape features may exhibit high turnover, largely as a function of the interactions between fluvial dynamics and successional phenomena, their relative abundance in the river corridor tends to remain constant over ecological time. 4. Hydrological connectivity, the exchange of matter, energy and biota via the aqueous medium, plays a major though poorly understood role in sustaining riverine landscape diversity. Rigorous investigations of connectivity in diverse river systems should provide considerable insight into landscape‐level functional processes. 5. The species pool in riverine landscapes is derived from terrestrial and aquatic communities inhabiting diverse lotic, lentic, riparian and groundwater habitats arrayed across spatio‐temporal gradients. Natural disturbance regimes are responsible for both expanding the resource gradient in riverine landscapes as well as for constraining competitive exclusion. 6. Riverine landscapes provide an ideal setting for investigating how complex interactions between disturbance and productivity structure species diversity patterns.     

The rivers Rhine and Meuse in The Netherlands: present state and signs of ecological recovery

The ecosystems of the rivers Rhine and Meuse have suffered drastic environmental changes, for example because of the regulation of the stream bed and the construction of weirs and dams. Furthermore, discharges of industrial and municipal waste water have caused the water quality of these rivers to deteriorate; this problem became acute in the sixties and seventies. Recently some chemical parameters of water quality have improved in the Rhine, and as a consequence some aquatic communities are showing signs of recovery after decades of severe degradation. This paper describes the present state of the aquatic communities in the Dutch part of the rivers Rhine and Meuse, using published observations on plankton, macrophytes, invertebrates, and fish. The sparse information on the food chain in these rivers is summarized. The main channels of the Rhine and Meuse are characterized by a dense plankton that develops rapidly in the nutrient-rich river waters. The stream beds, now dominated by wave-exposed sand and gravel, have a sparse fauna and flora. The river banks, mostly consolidated by blocks of stone, offer a substratum for numerous benthic organisms, particularly now that the water quality has improved. The floodplain waters and old river channels harbour a flora and fauna rich in species. The degree of water exchange with the river is crucial for the ecological development of the river and its backwaters. Today the freshwater tidal reaches of the rivers occupy a very restricted area, and only remnants of the previously abundant vegetation of rushes are found. Losses in the numbers of animal and plant species, notably those specific to rivers, are evident, but over the last 15 years several species have returned. Allochthonous species (exotics), including crustaceans and molluscs, have also settled in the Rhine and Meuse. Fish species characteristic of these rivers, such as river lamprey, sea trout, barbel, and flounder, have recently been observed in appreciable numbers. The Rhine Action Programme provides a framework for the large-scale rehabilitation of the Rhine. Experiments on re-stocking the Rhine with Atlantic salmon and on the ecological rehabilitation of floodplains are being carried out on the assumption that there will be a further reduction of the pollution load. A similar programme is required for the Meuse.     

Comparing the fish assemblages and food‐web structures of large floodplain rivers

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Spatiotemporal heterogeneity of soil and sediment respiration in a river‐floodplain mosaic (Tagliamento,NE Italy)

1. In their natural state, river floodplains are composed of a complex mosaic of contrasting aquatic and terrestrial habitats. These habitats are expected to differ widely in their properties and corresponding ecological processes, although empirical data on their capacity to produce, store and transform organic matter and nutrients are limited. 2. The objectives of this study were (i) to quantify the spatiotemporal variation of respiration, a dominant carbon flux in ecosystems, in a complex river floodplain, (ii) to identify the environmental drivers of respiration within and among floodplain habitat types and (iii) to calculate whole‐floodplain respiration and to put these values into a global ecosystem context. 3. We measured soil and sediment respiration (sum of root and heterotrophic respiration; SR) throughout an annual cycle in two aquatic (pond and channel) and four terrestrial (gravel, large wood, vegetated island and riparian forest) floodplain habitat types in the island‐braided section of the near‐natural Tagliamento River (NE Italy). 4. Floodplain habitat types differed greatly in substratum composition (soil to coarse gravel), organic matter content (0.63 to 4.1% ash‐free dry mass) and temperature (seasonal range per habitat type: 8.6 to 33.1 °C). Average annual SR ranged from 0.54 ± 1.56 (exposed gravel) to 3.94 ± 3.72 μmol CO₂ m^?2 s^?1 (vegetated islands) indicating distinct variation in respiration within and among habitat types. Temperature was the most important predictor of SR. However, the Q₁₀ value ranged from 1.62 (channel habitat) to 4.57 (riparian forest), demonstrating major differences in habitat‐specific temperature sensitivity in SR. 5. Total annual SR in individual floodplain habitats ranged from 160 (ponds) to 1205 g C m^?2 (vegetated islands) and spanned almost the entire range of global ecosystem respiration, from deserts to tropical forests.     

Endangered ecosystems: a review of the conservation status of tropical Asian rivers

Tropical Asian rivers are characterized by their flow seasonality. One (sometimes two) peaks in discharge cause temporary declines in phytoplankton, zooplankton and zoobenthos biomass, but lead to inundation of river floodplains and significant land-water interactions. Fishes undertake lateral or longitudinal breeding migrations within the river system during the flood season, which is marked also by intensive feeding upon allochthonous inputs.Among the diverse human influences upon tropical Asian rivers, three threats stand out. Firstly, degradation of drainage basins (particularly through deforestation and overgrazing) leads to increased suspended sediment loads and extensive flooding. Excessive floodplain siltation alters habitats causing species decline or disappearance. The second threat — river regulation and control — has been practised widely in the region for centuries but, with the planned development of massive projects on the Yangtze and Mekong Rivers, the potential for environmental damage has increased. Flow regulation reduces flood-season peaks, changing the magnitude and extent of floodplain inundation and land-water interactions. Fish breeding migrations may be disrupted, because dams block migration routes or changed flow regimes fail to stimulate reproduction. River pollution is pervasive throughout the region, and constitutes the third threat. Untreated sewage is a particular problem in densely-populated areas, and pollution by industrial effluents and mining wastes is becoming more important. The effects of pollution in tropical Asian rivers are essentially the same as those recorded in north-temperate regions. However, biological understanding has yet to be matched by an ability to halt or limit river degradation.Together, the three threats have led to declines and range constrictions of aquatic animals and those terrestrial species associated with riparian corridors and floodplains. River dolphins and certain crocodilians are particularly threatened, but declines in species of waterfowl, floodplain deer, a host of fishes, macrophytes, and invertebrates have been documented. Reversing the trend is difficult as pollution, flow regulation, and drainage-basin degradation have non-additive detrimental effects on river ecosystems, and enhance the success of exotic invasive species. Moreover, our ability to predict the outcome of man-made changes is hampered by a lack of knowledge of species' life histories and a paucity of data on the trophic basis of production. Despite a lack of detailed information, conservation of tropical Asian rivers will be effected only if limnologists move beyond the bailiwick of science. Ecologically viable management strategies for tropical Asian rivers will succeed only if the socioeconomic context of development plans is taken into account. A failure to rise this challenge will result in the further degradation of these endangered ecosystems.     

Archaic and contemporary topographic diversification of Upper Mississippi River forests

Topographic diversity is an important component of environmental heterogeneity. Topographic diversity within the Upper Mississippi River floodplain has been degraded because of modifications for navigation improvement. Efforts aimed at restoring topographic diversity in the Upper Mississippi River floodplain have been extensive but have not focused on reversing the effects of forest loss and degradation. We investigated habitat features associated with Cerulean Warbler (Setophaga cerulea) locations both within and outside of river floodplains and hypothesized this species would select topographically diverse habitats. Both topographic diversity and the distance to the upland forest/floodplain forest interface were useful predictors of Cerulean Warbler presence. We conclude that incorporation of topographic diversity into floodplain forest restoration planning would likely benefit Cerulean Warblers and the other species with similar habitat requirements. Incorporating topographic diversity into floodplain forest conservation planning will be challenging on major rivers that serve multiple purposes.     

Fish zonations and guilds as the basis for assessment of ecological integrity of large rivers

Longitudinal zonation concepts describe the downstream changes in chemico-physical and biological properties of rivers. Including information on ecological fish guilds can enhance the usefulness of fish zonation concepts, in a way that they can be used as tools for assessment and management of the ecological integrity of large rivers. We present an ecological characterization of fish zones and fish communities in near-natural and in regulated large rivers in Europe (the River Doubs in France and the Rivers Rhine and Meuse in the Netherlands), using guild classifications of several life-history traits of fish and national Red Lists of threatened species. The Doubs data set was also analyzed using indices of the sensitivity of fish species to environmental degradation and indices for eurytopy. In these rivers, the number of ecological guilds per zone increases downstream, and there are clear shifts in the structure of the guilds. Flow preference and reproduction ecology of river fish are closely linked. The proportion of rheophilic species in the fish community decreases downstream, and the proportions of limnophilic and eurytopic species increase. Lithophilic and psammophilic spawners are dominant in the upper zones, whereas the lower zones are dominated by phytophilic and phytolithophilic spawners. The proportion of zoobenthivorous and periphytivorous species decreases downstream, and the proportion of zooplanktivorous and phytivorous species increases. However, because the European fish fauna mainly consists of feeding generalists, the discriminative abilities of simplistic feeding guild classifications are not very high. Guilds of sensitive, stenoecious species that share life history strategies that are highly adapted to specific riverine conditions (rheophils and limnophils) have declined far more than generalist species that can survive in a wide range of habitats that are not characteristic of natural river ecosystems. Because of the subsequent over-abundance of the eurytopic species the original longitudinal fish zonations are hardly recognizable anymore in heavily impacted large rivers such as the River Rhine. Hence these rivers do not meet the criteria for ecological integrity. Within a specific fish region, a suitable way of analyzing and monitoring the impact of human disturbance on the structure of the fish community is by comparing the guild structure of the present state of a fish zone with that of the reference situation.     

Distribution and invasion genetics of the quagga mussel (<Emphasis Type="Italic">Dreissena rostriformis bugensis</Emphasis>) in German rivers

The quagga mussel Dreissena rostriformis bugensis, native to the Dnieper and the northern Black Sea, has become a major invasive species in both the Volga River and the North American Great Lakes since the early 1990s. Findings in the Netherlands (2006) and Germany (2007) mark the start of its establishment in Western Europe. We investigated the current distribution, time of first arrival and population structure of D. rostriformis bugensis from the rivers Rhine, Main and in the Main-Danube canal in Germany. Two putative sources of the German populations were analysed by genetically comparing these populations to older invasive populations from North America and the southeast Danube. Dreissena rostriformis bugensis was abundant in the Main and in three Rhine harbours, but rare in the actual Rhine river and absent south of the Main-Danube canal. Mussels found in the Rhine harbours were significantly smaller than in the Main. Population genetic analyses found no sign of founder effects and minimal differentiation between German, North American and southeast Danube populations. The genetic data suggest that these invasive populations derive from a common and rapidly expanding source. Based on the non-continuous distribution and shell size differences of Rhine harbour and Main populations, our results indicate that expansion in Germany involved at least two independent settling events, one of which happened before 2005, and most likely was caused by jump dispersal.     

The river Rhine: a global highway for dispersal of aquatic invasive species

The river Rhine is heavily influenced by human activities and suffers from a series of environmental constraints which hamper a complete recovery of biodiversity. These constraints comprise intensive navigation and habitat modification by hydraulic engineering. Improving water quality while these constraints remain in place has led to increased colonization by aquatic invasive species. This tendency has been accelerated by the construction of canals connecting river basins. Over the last two centuries, the total surface area of river catchments connected to the river Rhine via inland waterways has been increased by a factor 21.6. Six principal invasion corridors for aquatic species to the river Rhine are discerned. The extensive network of inland waterways has allowed macroinvertebrate species from different bio-geographical regions to mix, changing communities, affecting the food webs and forming new constraints on the recovery of the native biodiversity. From the eighteenth century onward, in the freshwater sections of the river Rhine, a total of 45 non-indigenous macroinvertebrate species have been recorded. The average number of invasions per decade shows a sharp increase from <1 to 13 species. Currently, the contribution of non-indigenous species to the total species richness of macroinvertebrates in the river Rhine is 11.3%. The Delta Rhine and Upper Rhine exhibit higher numbers of non-indigenous species than other river sections, because the sea ports in the Delta Rhine and the Main-Danube canal function as invasion gateways. Important donor areas are the Ponto-Caspian area and North America (44.4 and 26.7% of the non-indigenous macroinvertebrate species, respectively). Transport via shipping and dispersal via man made waterways are the most important dispersal vectors. Intentional and unintentional introductions are highest for the period 1950–1992. The cumulative number of non-indigenous species in time is significantly correlated with the increase in total surface area of other river catchments connected to the river Rhine by means of networks of canals. The species richness of non-indigenous macroinvertebrates is strongly dominated by crustaceans and molluscs. Invasive species often tolerate higher salt content, temperature, organic pollution and current flow than native species. Spatiotemporal analyses of distribution patterns reveal that average and maximum dispersal rates of six invasive species vary between 44–112 and 137–461 km year⁻¹, respectively. Species arriving in upstream sections first show a shorter time lag between colonisation of the Delta and Upper Rhine than species initially arriving in downstream areas. Temporal analyses of macroinvertebrate assemblages in the littoral zones indicate that native species are displaced by non-indigenous species. However, established non-indigenous species are also displaced by more recent mass invaders.     

Estimation of minimum area requirement of river‐connected lakes for fish diversity conservation in the Yangtze River floodplain

Aim Hydrological disconnection of floodplains from rivers is among the top factors threatening river‐floodplain ecosystems. To keep enough floodplain area is of great importance to biodiversity conservation. In the Yangtze River floodplain, most lakes were disconnected from the mainstream by dams in 1950–1970s. By analysing fish diversity data, we aim at determining the effects of river‐lake disconnection on fish diversity, at estimating the minimum protected area of river‐connected lakes and at proposing a holistic strategy for fish conservation in the mid‐lower reaches of the river. Location The Yangtze River floodplain, China. Methods We collected recorded data of fish diversity of 30 Yangtze floodplain lakes. Species–area relationships were analysed and compared between river‐connected and river‐disconnected lakes. Cumulative species–area models were constructed to estimate the minimum protected area of river‐connected lakes. Results River‐lake disconnection reduced fish diversity of Yangtze lakes by 38.1%, so that the river‐connected lakes play an important role in maintaining the floodplain biodiversity. The minimum protected area of river‐connected lakes was estimated to be 14,400 km². Therefore, we should not only protect the existent connected lakes of 5500 km², but also reconnect disconnected lakes of at least 8900 km² in the Yangtze basin. Main conclusions Species–area relationships are of importance in reserve design. We suggest that cumulative species–area model might be more suitable for ecosystems with high connectivity among regions such as floodplains. As the Yangtze River floodplain is an integrative ecosystem, we suggest establishing a holistic nature reserve in the mid‐lower basin for effective conservation of biodiversity.     

The implementation of an environmental flow regime results in ecological recovery of regulated rivers

Environmental flows are the main restoration technique used to ameliorate the ecological effects of regulation in rivers. However, their effectiveness has yet to be unanimously accepted. This study assessed the potential ecological benefits of the application of an environmental flow regime downstream of four dams and two weirs in the Upper Nepean River system, Sydney, Australia. Aquatic macroinvertebrates in three habitat types were sampled at water‐supply and low‐flow sites and unregulated sites in 1995 and 1996, prior to the environmental flows and in 2013 and 2014, approximately 13 years following the environmental flows. The macroinvertebrate assemblage structure was significantly different between regulated and reference sites and the number of taxa lower at water‐supply sites prior to the implementation of the environmental flows. Following the environmental flows, the assemblage structure became more similar to, although still significantly different from, the unregulated sites and the number of taxa was not significantly different between regulated and unregulated sites. Thirteen or approximately 30% of taxa indicative of unregulated rivers increased in frequency at regulated sites following the environmental flows. Despite potentially similar dispersal capabilities the remainder of the taxa failed to respond the new flow regimes. The mechanisms resulting in recovery of some taxa but not others remain unclear and require further investigation as the basis of future research and monitoring. Such information and knowledge would support the application of future environmental flow regimes as the primary mechanism to ameliorate the ecological effects of river regulation.     

Rapid range expansion of the invasive quagga mussel in relation to zebra mussel presence in The Netherlands and Western Europe

Since its appearance in 2006 in a freshwater section of the Rhine–Meuse estuary (Hollandsch Diep, The Netherlands), the non-indigenous quagga mussel has displayed a rapid range expansion in Western Europe. However, an overview characterising the spread and impacts of the quagga mussel in this area is currently lacking. A literature study, supplemented with field data, was performed to gather all available data and information relating to quagga mussel dispersal. Dispersal characteristics were analysed for rate and direction and in relation to hydrological connectivity and dispersal vectors. To determine ranges of conditions suitable for quagga mussel colonisation, physico-chemical characteristics of their habitats were analysed. After its initial arrival in the freshwater section of the Rhine-Meuse estuary and River Danube, the quagga mussel demonstrated a rapid and continued range expansion in Western Europe. Quagga mussels have extended their non-native range to the network of major waterways in The Netherlands and in an upstream direction in the River Rhine (Germany), its tributaries (rivers Main and Moselle) and the River Meuse (Belgium and France). The calculated average quagga mussel dispersal rate in Europe was 120 km year^?1 (range 23–383 km year^?1). Hydrological connectivity is important in determining the speed with which colonisation occurs. Dispersal to water bodies disconnected from the freshwater network requires the presence of a suitable vector e.g. pleasure boats transferred over land. Upstream dispersal is primarily human mediated through the attachment of mussels to watercraft. The relative abundance of quagga mussel to zebra mussel has greatly increased in a number of areas sampled in the major Dutch rivers and lakes and the rivers Main and Rhine and the Rhine–Danube Canal leading to a dominance shift from zebra mussels to quagga mussels. However, evidence for displacement of the zebra mussel is limited due to the lack of temporal trends relating to the overall density of zebra and quagga mussel.     

Learning from the past: palaeohydrology and palaeoecology

Attempts to increase European biodiversity by restoring rivers and floodplains are based on inadequate data on natural systems. This is particularly the case for NW European rivers because all catchments have been impacted by agriculture and river engineering. If river restoration is to have an ecological, as opposed to `cosmetic' design basis then baseline models are required. However, this poses three questions; (a) what is the natural river‐floodplain state, (b) how can it be defined and modelled and (c) can this state be recreated today? The first two questions can only be addressed by using palaeohydrological and palaeoecological data. A second and equally vital consideration is the stability/instability of any restored system to change in external forcing factors (e.g. climate) and in this context it may not be realistic to expect baseline models to provide equilibrium solutions but instead to define process‐form domains. Over the last two decades evidence has accumulated that the natural state of lowland rivers in much of NW Europe was multi rather than single thread‐braided, anastomosing or anabranching. Until recently our knowledge of floodplain palaeoecology was generally derived from pollen diagrams, which have source‐area of problems and lack of taxonomic specificity. The precision and breadth of palaeoecological reconstruction (including richness and structure) has been greatly increased by the use of multiple palaeo‐indicators including macrofossils, diatoms and beetles. The dynamics of small to medium sized, low‐energy, predeforestation floodplains were dominated by disturbance (windthrow, beavers, etc.) and large woody debris. In order to compare the hydrogeomorphological basis of floodplain ecology, both temporally and spatially, a simple index of fluvial complexity is presented. Palaeoecological and geomorphological investigations have the potential to provide in‐depth models of the natural range of channel conditions and sensitivity to external change that can be used to provide a scientific basis for floodplain restoration. There is also the possibility that floodplain‐channel restoration may be a valuable tool in the mitigation of future geomorphological change forced by climatic instability.     

Effects of desiccation on native and non‐native molluscs in rivers

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Helminth communities of the barbel Barbus barbus from large river systems in Austria

The composition and diversity of the total and intestinal component endohelminth communities were determined in the cyprinid barbel from three study areas in two large river systems in Austria. Two sample sites in the Danube and one site in the River Drau are the only free flowing stretches of these rivers in Austria. Nine helminth species were identified, with the acanthocephalan Pomphorhynchus laevis being dominant in the Danube, with up to 100% prevalence. In the Drau, where the dominant species was the cestode Bathybothrium rectangulum, species diversity was higher than in the Danube.     

Functional diversity of phytoplankton highlights long‐term gradual regime shift in the middle section of the Danube River due to global warming,human impacts and oligotrophication

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Differences in levels of heterozygosity in populations of the common gudgeon (Gobio gobio,Cyprinidae) among adjacent drainages in Central Europe: an effect of postglacial range dynamics?

Twenty-nine allozyme loci analyzed in 295 common gudgeons (Gobio gobio) from the Rhine, the upper Danube and the Elbe river systems revealed variability measures of P = 0.590, H(e) = 0.066, and G(ST) = 0.1415. Gene flow was estimated at N(e)m = 1.88 over 223 river km in the Danube basin, and at N(e)m = 1.96 over 300 river km in the Rhine system. Isolation-by-distance was not observed. Danubian gudgeons proved significantly more heterozygous (H(e) = 0.106) than those from the Rhine (H(e) = 0.057) or the Elbe drainages (H(e) = 0.029). Nine polymorphic enzymes contributed to this difference, which probably indicates dispersal bottlenecking of the postglacial immigrants into the Atlantic drainages of the study area. Refugial bottlenecking of gudgeons from Atlantic drainages in a Pleistocene refuge located in oceanic northwest Europe also seems possible. Slightly deeper genetic lineages in the Danube (G(ST) = 0.0859) than in the Rhine (G(ST) = 0.0793) agree with this explanation, as does the greater mean genetic distance among pairs of population samples from the Danube (D = 0.0138) than from the Rhine (D = 0.0054). A genetic distance of D = 0.0085 separated Rhenish and Danubian gudgeons.     

Harmonising the bioassessment of large rivers in the absence of near‐natural reference conditions – a case study of the Danube River

1. International river catchments pose challenges for effective water resource management. Catchment‐wide strategies are often complicated by differences in national bioassessment and quality classification. Intercalibration efforts aim to harmonise these differences, but rely on the consistent delineation of near‐natural reference sites that are almost unavailable in today’s landscape, especially for large rivers. 2. We introduce the concept of alternative benchmarking that is based on the notion of aquatic communities at similar (low) levels of impairment associated with least‐disturbed conditions (LDC) as defined by abiotic criteria. Using data acquired during the second Joint Danube Survey, we defined LDC sites based on a multivariate gradient of anthropogenic pressures, mostly related to morphological deterioration, that spans the entire navigable Danube. 3. The river was subdivided into four stretches, each featuring homogeneous biological assemblages. Indirect gradient analysis revealed relationships between the pressure gradient and selected features of the macroinvertebrate and macrophyte community but not for diatoms or phytoplankton. 4. We identified biological metrics suitable for the quality classification of individual stretches or the entire river. Impoundment is the major hydromorphological alteration on the Danube but various metrics still responded significantly to differences in the morphological condition of sites not affected by impoundment. 5. A comparison of macroinvertebrate sampling techniques (airlift versus kick‐and‐sweep) revealed differences in how the acquired data reflect the effects of anthropogenic pressure. Biological metrics based only on kick‐and‐sweep sample data were insensitive to habitat deterioration in the heavily modified Upper Danube. 6. This study exemplifies the empirical approach of alternative benchmarking in intercalibration and offers practical solutions to some of the challenges of large river bioassessment.     

Phases in the development of riverine plankton: Examples from the rivers Rhine and Meuse

Observations on phyto and zooplankton in two hydrographically different rivers were compared in order to discriminate phases in plankton development. Along the longitudinal axis of the River Rhine a gradual increase in the development of phytoplankton was observed, which reached its maximumca. 100 km before the river flows into its artificial sedimentation area. The development of rotifer populations was slightly retarded as compared with that of phytoplankton and highest population densities were only reached in the sedimentation area. Crustaceans developed in significant numbers, not until the river water had entered the sedimentation area. Development of zooplankton coincided here with a strong decrease in the density of phytoplankton. A similar trend in plankton development was observed in the River Meuse, although in this river the highest densities of phyto and zooplankton already occurred in its middle reaches. The differences in the timing of plankton growth in the two rivers are probably caused by differences in flow regime between both rivers. The River Rhine, which is fed by rainwater and melting of glaciers in the Alps, has a relatively constant discharge. On the other hand, the low discharge of the rain-fed River Meuse combined with an increased residence time of the water as a consequence of large numbers of weirs, allows a full cycle of plankton development long before its discharge into the sea. This phenomenon was also reflected in the silicate cycle in the Meuse, where the consumption by planktonic diatoms and the regeneration of silicate of deposits seem to be important. In contrast, in the main branches of the River Rhine only the effects of silicate consumption were detectable.