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Dikerogammarus villosus, one of the most successful invaders in European river systems, is commonly regarded as a threat to native biodiversity and a main factor structuring the benthic community of invaded systems. The impact of D. villosus has been intensively studied in small-scale experiments and field observations, but its impact on natural communities on a larger scale remains unclear. Here, we investigated the benthic community structure at ten sites covering a broad range of habitats along the River Rhine (Central Europe) and its tributaries, to determine whether D. villosus is one of the main factors structuring the benthic community. Community composition was analysed using non-metric multidimensional scaling, distance-based redundancy analysis, and correlation analyses. D. villosus was one of nine relevant taxa present that altogether reflected a large part of the variation in the benthic samples, but further analyses indicated that the species might be less important for the community structure than other relevant taxa. Moreover, all nine relevant taxa together can explain only a similar amount of variation in our samples than the five relevant non-faunal environmental factors (water temperature, pH, conductivity, percentage of medium-sized gravel and macrophytes). Overall, our results suggested that rather a combination of non-faunal environmental factors than D. villosus mainly structure the benthic community composition on this larger spatial scale.

The characterisation of natural stream conditions is the first important step to analyse ecological quality of streams in the Euphrates basin. We found that the community indices correspond to very good ecological conditions in five natural streams of that region. The macroinvertebrates composition differed significantly between September and May. Number of taxa and Shannon index were significantly higher in autumn than in spring. FPOM and biofilm were the most relevant basal resources of benthic invertebrates.

Eutrophication seriously threatens the ecological quality and biodiversity of running waters. In nutrient-enriched streams and shallow rivers, eutrophication leads to excessive periphyton growth and, in turn, biological clogging, oxygen depletion in the hyporheic zone and finally a reduction in the hyporheic habitat quality. Top-down control of the food-web by manipulating fish stocks, similar to the biomanipulation successfully applied in lakes, offers a promising approach to mitigating the effects of eutrophication in shallow rivers, especially those in which major reductions in nutrient input are not feasible. We conducted a reach-scale experiment over 4 years in a medium-sized eutrophic river to assess whether the top-down effects of two important large European cypriniform fish species, herbivorous common nase (Chondrostoma nasus) and omnivorous European chub (Squalius cephalus), would mitigate the effects of eutrophication. The enhancement of fish stocks was expected to reduce biological clogging, via the top-down control of periphyton by benthic grazing and enhanced bioturbation, thus increasing oxygen availability in the hyporheic zone as well as water exchange between the surface water and the hyporheic zone. As expected, enhancing the stocks of nase and chub increased both oxygen availability and vertical exchange flux of water in the upper layer of the hyporheic zone. However, periphyton biomass (chlorophyll a) was significantly reduced only in deeper pool habitat. Thus, while experimental biomanipulation in a shallow river significantly mitigated the effects of eutrophication in the hyporheic zone, top-down effects on periphyton biomass were rather small. Overall, to our knowledge, our results provide first evidence that the biomanipulation achieved by enhancing herbivorous and omnivorous fish stocks has the potential to mitigate the effects of eutrophication in medium-sized European rivers.

The prey selectivity of fish depends largely on traits of the prey and the predator. Preferable prey traits might be different for visual predators (such as drift-feeding salmonids) and rather non-visual predators (such as benthic feeders). We evaluated the explanatory power of five prey traits and prey long-term abundance for the prey selection of small benthivorous fish by analysing the macroinvertebrate community and the diet of gudgeon (Gobio gobio) and stone loach (Barbatula barbatula) in two small submontane streams. Fuzzy principal component analyses, as well as electivity indices, revealed that the fish fed selectively. Prey size and feeding type were the most descriptive variables for the fish diet, followed by mean abundance, whereas microhabitat preference, locomotion mode and current velocity preference were less important. The fish preferred prey that was both small and consistently abundant, grazers and sediment feeders. Larger prey and shredders were avoided. The selection patterns of both fish species differed from those of visual fish predators but strongly resembled each other. Supporting this, in gudgeon which feeds slightly more visually than the strictly nocturnal stone loach, selectivity concerning prey traits as well as prey mean abundance was slightly more pronounced. We analyzed also selectivity for prey clusters based on the three most important variables. The observed selectivity patterns concerning these clusters were less pronounced but supported the other results. The maximum (neutral) electivity index was that of gudgeon for small, abundant grazers or sediment feeders, including chironomids. We conclude that prey selection of benthivorous fish that forage mainly non-visually can largely be explained by a small number of prey traits which probably work in combination. The prey preferences of these predators seem to be closely connected to their active foraging mode and to depend partly on the ability to detect prey visually.

Flexibility is an important adaptive attribute of the feeding periodicity of grazing mayfly larvae because most natural environments offer a wide variation in local predation risk in terms of space, time or predator species. In this study any changes of diel feeding periodicity and consumption rates of Baetis rhodani (Ephemeroptera) were analysed in response to different densities of benthivorous fish ( Barbatula barbatula , Gobio gobio ) by quantifying gut fullness using the fluorescence of algal pigments. Laboratory experiments with the grazer species B. rhodani were conducted by using different concentrations of chemical fish cues. In order to assess the transferability of the results to a larger scale, the experimental results were compared with field observations in two second order streams using different densities of freely foraging benthivorous fish. During the presence of chemical fish cues in the laboratory experiments the feeding periodicity of the B. rhodani larvae were mostly diurnal while in the absence of fish chemicals nocturnal feeding was observed. The same patterns could be detected in the field during the experiments with the different fish densities. These findings indicate that the larvae were able to assess variations in the predation risk and to alter their feeding habits by making flexible behavioural adjustments. The results from the laboratory experiments further suggested that the behavioural response is controlled by fish density. Behavioural changes were observed for medium and high concentrations of the fish cues but not for a very low concentration. In the field however, the mere presence of fish seemed to be sufficient to induce the observed behavioural shifts. Although the presence of benthivorous fish seemed to cause a lower consumption rate of the B. rhodani larvae in the field, such a reduction could not be found in the laboratory experiments. A conclusion from this study is that the identification of behavioural modifications is an essential component needed for a better understanding of complex trophic interactions in benthic communities. Accurate evaluation and detailed observation of direct and indirect effects cannot be made without the consideration of such behavioural mechanisms.

Whole-ecosystem experiments may reach statistical pertinence when reference and treatment sites are compared. Therefore at least two similar systems must be found. To this aim we tried to answer the question if the hyporheic zones of both streams have similar features. We postulated that both streams would be unsuitable for a comparison in an ecosystem experiment if the structure of their hyporheic invertebrate communities differs significantly. Although pH, electrical conductivity and nitrate concentration differed in the surface water, variability of all chemical criteria studied in the hyporheic zones was much lower between the two streams compared to the variability within each stream. Thus the structure of both hyporheic invertebrate communities did not differ significantly. Because of this uniformity both streams are regarded to be suitable for a comparison in a whole-ecosystem experiment.