This article provides a review on the scientific knowledge about the Volga River, including the characteristics of the river basin and an overview of historical and recent limnological research activities. Detailed scientific investigations began about 200 years ago later on that the Volga River was the subject of numerous expeditions and research programmes. The first hydrobiological river research station in Europe was founded in 1900 in Saratov (Biological Volga Station), followed by several research institutions which were established along the Volga River during the last century. This coincided with the major hydromorphological changes on the Volga River. Within a considerable stretch of the river the lotic system was altered into a cascade of large reservoirs, while the head-waters remained as an intact free-flowing section. The intention of this review is to provide a comprehensive overview on hydrobiological data on the Volga River, including recent studies. This knowledge covering the whole river system can build an important basis to formulate guidelines and measures for the improvement of water quality and to provide a sound framework for aquatic conservation issues.

This study develops univariate utilisation- and preference indices and analyses multivariate microhabitat use of spawning nase Chondrostoma nasus (L.) and barbel Barbus barbus (L.) in the Pielach River, a pre-Alpine tributary to the Danube, Austria. During the spawning season we daily surveyed species presence, number of individuals and habitat size. Habitat features, i.e. flow velocity, water depth, shading, cover, flow protection, type of structure, substrate and embeddedness were recorded at ten spawning grounds used by 1900 spawners within one spawning season. Spawning habitat features were compared with available habitat. Nase spawns in fast-flowing water (~1 m/s) that is significantly faster than available habitat. In contrast, barbel constructs redds that differ in water temperature, depth, velocity and cover structure from those of nase. Multivariate analyses (PCA) underlined the importance of shading and, as a consequence, the occurrence of vegetation along river banks for both fish species. This study demonstrates that efficient river restoration requires re-establishing riparian vegetation besides hydromorphological habitat improvements in order to provide adequate spawning grounds for nase and barbel.

Four sites along the downstream part of River Mures, Romania, were investigated in a pilot study aiming to combine the classical chemical-biological monitoring with modem techniques (biomarker analyses) in order to assess the effects of environmental contaminants on aquatic communities. Although significant point and diffuse sources exist, both chemical water analyses (oxygen, nutrients, PAHs, PCBs) and biological investigations (macrozoobenthos, phytoplankton, zooplankton) failed to detect severe pollution. Hence, community response was poor as self purification is strong, which was reflected by beta-mesosaprobic conditions. Only high numbers of faecal coliforms and faecal streptococci, up to 1300 and 490 counts per 100 ml, respectively, surpassing the critical levels indicated contamination by untreated sewage in the upper sites. Moreover, heavy metal contamination (high dissolved fraction of Cr, Cu and Zn in water; Cd and C'u up to 8.7 and 49.2 ppm in sediments, respectively) indicated mining and metallurgical activities in the area. This resulted in sub-lethal effects on fish, proving that current monitoring techniques are not sufficient to assess the ecological effects of environmental pollution. Gradually more sensitive modern methods should be applied.

We compare historical and contemporary data of the flora and vegetation of the Upper Danube river, two reservoirs, and the 20 largest oxbows between the confluence of the headwaters Brigach and Breg and the influx of the Iller (Baden-Wurttemberg, Germany) based on two recent surveys (1989 and 2004) and the evaluation of old herbaria material, regional botanical literature and old floras. In 2004, the Upper Danube River accommodated 28 vascular macrophytes and eight moss species below the low water level. The contemporary aquatic vegetation is characterized by the dominance of Ranunculus fluitans and the sparse occurrence of species sensitive to eutrophication. Nuphar lutea is the dominating species in the oxbows and reservoirs, but also in many slow flowing stretches of the river. The most striking phenomenon observed in all water bodies is a decline or extinction of broad-leaved pondweeds. In the main channel, a loss of Potamogeton-species and Hippuris vulgaris was counterbalanced by the invasion of eutraphent, prevailingly neophytic species (Elodea spp.). A decline in species richness and a change in community composition occurred to a lesser extent after channelization carried out mainly in the early 20 th century and after extensive eutrophication commencing in the 1950s. Historical floristic records enabled a description of reference communities for the Upper Danube as required by the Water Framework Directive (WFD). Oxbows and reservoirs accommodated a more diverse flora than the river channel, but many species were rare. Historical data allowed us to trace back the temporal succession of vegetation in one case. Recent rehabilitation measures led to a rapid increase in species diversity and vegetation cover in several river stretches owing to colonization of abundant species with a high vegetative dispersal capacity. We conclude that the re-assembly of former aquatic communities will fail owing lo the contemporary paucity or extinction of species sensitive to eutrophication and disturbances even when past habitats are restored.

Freshwater ecosystems are situated in depressions in the landscape and, as a consequence, their water quality has been to a great extent dependent on human population density and its whole range of activities. Anthropogenic impacts on freshwater environments can be defined in two dimensions: technological and environmental. For example, emission of pollutants, which'can be controlled by technology, and environmental impacts which are primarily of a physical character, such as modification of hydrological and biogeochemical cycles due to deforestation, urbanization, canalization etc. Ecohydrology is a new paradigm, developed over the lifetime of UNESCO IHP-V, 1997-2001, which attempts to combine these two approaches. It is a new concept which suggests that the sustainable development of water resources is dependent on our ability to maintain the naturally established processes of water and nutrient circulation, and energy flow at the basin scale, by using a dual regulation: biota by hydrology and hydrology by biota (ZALEWSKI 2002). Our profound understanding of a whole range of processes is based on understanding the role of aquatic and terrestrial biota in the dynamics of water over the whole range of scales, from molecular to the landscape basin. The general question in the formulation of the Ecohydrology concept was how to regulate biological processes of freshwater ecosystems using hydrology, and vice versa, how to use biotic ecosystem properties as tools in water management. Both should serve as a reference system for enhancing the capacity of ecosystems to absorb human impacts by using ecosystem properties as management tools. This, in turn, depends on the development, dissemination and implementation of interdisciplinary knowledge based on recent progress in environmental sciences.

Recovery of the fish community of the river Rhine focussed mainly on the return of migratory species, in particular the Atlantic Salmon, Salmo salar, and to a lesser extent on rheophilic fish species. Several limnophilic species that characterize remote parts of the floodplains are, however, also endangered. The status of the fish communities in the floodplains is little known and has been neglected so far. To fill this gap, the present study investigated fish communities in floodplain lakes along the lower River Rhine. To assess how these lakes contribute to fish species richness in the river-floodplain system, the community structure is compared to that in the main channel and to connected backwaters, e.g. side channels, which had been investigated more intensively in recent years. Floodplain lakes with abundant vegetation may have low fish species richness, but they are the only sites with suitable habitats for the reproduction of limnophilic species such as tench Tinea tinca, rudd Scardinius erythrophthalmus and crucian carp Carassius carassius. Limnophilic species in most of these lakes are, however, outnumbered by eurytopic species such as bream Abramis brama, which indicates that the original lateral gradient is truncated with an almost complete loss of 'black fish' habitats in the present active floodplains along the lower Rhine. Floodplain lakes with abundant vegetation should be carefully preserved because they represent original aquatic components that have declined markedly, and they need decades to develop. Preservation of these lakes would enhance the conservation of characteristic limnophilic species. Retention areas outside the present active floodplain - retained as a component of flood risk management - may provide the opportunity to rehabilitate or create floodplain lakes with the suitable spawning and nursery habitats for limnophilic fish. In this way, improved safety may be combined with rehabilitation of floodplain water bodies.

A linked modelling approach using a diffuse model (CASCADE) and a river water quality model (QUESTOR) was implemented to simulate flow and nitrate dynamics in the River Aire catchment in Northern England as a way of exploring the use of performance benchmarking as proposed by the EC 5 th Framework Programme Benchmark Models for the Water Framework Directive project (BMW). A sequence of linked models is presented and discussed in terms of their conceptualization, calibration and performance. Two main conclusions were drawn. Firstly, although it is considered essential that the overall performance criteria for the linked model must be defined and agreed prior to model application, these criteria may not be appropriate or sufficient for the internal data exchange between the two models. In general, when undertaking linked modelling studies, an essential consideration should be that individual models may need to be constrained to produce simulations that do not compromise the performance of the entire model system. Secondly, quantitative assessment of performance should not be used uncritically to determine which model is best. It is important that conceptually flawed model structures should be eliminated from consideration, although such models may still be helpful in indicating an achievable level of performance.

The problems of implementing the Water Framework Directive (WFD) to the naturally eutrophic River Eurajoki, located in Southwest Finland are presented. A review of the methods and data available to classify the ecological status of a river was undertaken and revealed that biological data from the river are sparse. A provisional regression model, which predicts the ecological status (Ecological Quality Ratio, EQR), based on benthic invertebrate fauna, from water quality, and which is built on data from more northern rivers, was found not to be applicable in the region. It was concluded that given the lack of regional reference data, biological elements could not be used for the definition of the ecological status of River Eurajoki. Therefore, chemical elements supporting biological elements (dissolved oxygen, ammonium and total phosphorus) were chosen instead to classify the water quality of the river. The use of a modelling approach to simulate present-day conditions of these chemical elements was explored, and an in-stream water quality model was assessed for its usability in the implementation of the WFD in Finland. The model generated simulations of water quality, which were similar to the observed data over the period 1980-2002, but required a large amount of data to obtain a good simulation. The modelling approach proved to be useful for analysing different scenarios and in particular to estimate the reduction in loading needed to achieve the goals set by the WFD. In conclusion, this model can be recommended for the use by water managers in the implementation of the WFD on river basins with sufficiently monitoring data available.