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Beavers and their dams, once common in small streams throughout Eurasia and North America, are returning to their original range. Beaver dams are special habitats in the aquatic-terrestrial interface, but their macroinvertebrate fauna is hardly known. This study aims to qualify and quantify the invertebrate fauna of beaver dams, taking into account the maintenance status of the dams. Nine different areas covering the top, middle and base zones of eight maintained and eight abandoned beaver dams were sampled using a specially designed suction device. The invertebrate community of beaver dams proved to be diverse and predominantly rheophilic. Community composition reflects higher flow velocities in streamside habitats; this gradient is more pronounced in maintained than in abandoned dams. Shredders are the most abundant feeding type, but they are less frequent in streamside habitats, in particular in those of maintained dams, where passive filter feeders prevail. Terrestrial/Semiaquatic taxa colonising hygropetric areas were found in streamside habitats especially of abandoned dams. Our results underline that beaver dams provide a wide variety of environmental conditions and habitat types, which enhance biodiversity in small streams and their floodplains. Beaver dams in riverine landscapes should therefore be much more valued in floodplain conservation and management.

Salinisation has direct and detrimental physiological effects on freshwater organisms, yet little is known about its indirect effects. Here, we present a study that investigated the primary and secondary effects of salinisation on aquatic macroinvertebrates. We analysed macroinvertebrate samples from nine sites in river Wipper, Germany against nine stressor variables (water quality, salinisation, hydromorphological degradation). A Principal Component Analysis showed water quality deterioration and physical habitat degradation to constitute the main stressor gradients. Two macroinvertebrate community metrics (index of general degradation and percent Ephemeroptera-Plecoptera-Trichoptera specimens) revealed strong and significant positive correlations with water quality, but only weak and insignificant positive correlations with hydromorphological quality. High-resolution temporal measurements revealed notable oxygen deficits at a salinised and macrophyte-rich site that was dominated by the salt-tolerant macrophyte species Stuckenia [Potamogeton] pectinata (L.) Börner. At the site, oxygen levels frequently dropped below 6 mg/L during nighttime, suggesting an interaction of osmoregulatory stress (through salinisation) and respiratory stress (through plant mitochondrial respiration) that is caused by the mass development of a salt-tolerant macrophyte species. This biologically driven interaction of direct and indirect salinisation effects adds another level of complexity to the influence of multiple stressors in lotic systems and requires consideration in river management and restoration.

The present study provides new insight into suitable microsporidian–host associations. It relates regional and continental-wide host specialization in microsporidians infecting amphipods to degraded and recovering habitats across 2 German river catchments. It provides a unique opportunity to infer the persistence of parasites following anthropogenic disturbance and their establishment in restored rivers. Amphipods were collected in 31 sampling sites with differing degradation and restoration gradients. Specimens were morphologically (hosts) and molecularly identified (host and parasites). Amphipod diversity and abundance, microsporidian diversity, host phylogenetic specificity and continental-wide β-specificity were investigated and related to each other and/or environmental variables. Fourteen microsporidian molecular operational taxonomic units (MOTUs), mainly generalist parasites, infecting 6 amphipod MOTUs were detected, expanding the current knowledge on the host range by 17 interactions. There was no difference in microsporidian diversity and host specificity among restored and near-natural streams (Boye) or between those located in urban and rural areas (Kinzig). Similarly, microsporidian diversity was generally not influenced by water parameters. In the Boye catchment, host densities did not influence microsporidian MOTU richness across restored and near-natural sites. High host turnover across the geographical range suggests that neither environmental conditions nor host diversity plays a significant role in the establishment into restored areas. Host diversity and environmental parameters do not indicate the persistence and dispersal of phylogenetic host generalist microsporidians in environments that experienced anthropogenic disturbance. Instead, these might depend on more complex mechanisms such as the production of resistant spores, host switching and host dispersal acting individually or conjointly.

Aquatic macroinvertebrates are considered effective bioindicators of ecosystem health due to their different sensitivities to habitat alteration and pollution. Macroinvertebrate community structures can be assessed using commonly used diversity-based approaches, however, functional trait-based approaches are increasingly introduced into bioassessments. The Hex River is subjected to intensive mining activities, while urban and industrial effluents, as well as treated and untreated sewage also contributes to pollution in the river. The present study aimed to assess macroinvertebrate responses to different stressors (nutrient and chemical, especially metal pollutants), determine whether diversity-based or trait-based approaches provide a better indication of responses and, to evaluate the relationship between metal bioaccumulation and macroinvertebrate responses to this multi-stressor environment. The mining impacts, in combination with urban, industrial and sewage effluent, altered the macroinvertebrate community structure of the Hex River. The species richness significantly decreased from reference conditions (39 species) to impacted conditions (24 species), while the total number of individuals in tolerant species gradually increased. The diversity-based approach indicated a clear difference between reference and impacted conditions but did not clearly differentiate between different stressors, while the trait-based approach distinguished the urban, industrial and sewage effluent sites from mining impacted sites. The use of both approaches is, therefore, recommended as both complement each other. From the present study, it is evident that macroinvertebrates can be considered as both reliable accumulation-, and response bioindicators to pollution from mining, as well as urban and industrial activities.

Nitrogen and phosphorous concentrations are widely considered to drive macrophyte assemblages in rivers. However, Dissolved Inorganic Carbon (DIC) - available for plants as CO2 and HCO3- - is also of major relevance. Based on literature, we present a conceptual model on the interaction between algae, macrophytes, DIC, pH, light, N, P and the surface water and sedimental compartment. Analysing two separate datasets (i) on river physico-chemistry and chlorophyll-a, and (ii) on river physico-chemistry and macrophytes we quantify three connections within this concept: (1) the correlation of chlorophyll-a versus pH, (2) the correlation of TP versus chlorophyll-a and (3) the occurrence of HCO3-users and CO2-only-users among macrophytes along the DIC gradient. Chlorophyll-a correlated positively with pH (R-squared = 77%, p < .001) due to increased carbon dioxide uptake of phytoplankton. Surface water TP did not linearly correlate with chlorophyll-a concentrations. Obligate and optionally submerged macrophyte species that utilise HCO3- were separated from CO2-only-users by HCO3- concentrations, with an area under the curve (AUC) of 68% and 70% (both p < .001) between groups. Obligate and optionally submerged macrophyte assemblages only composed of HCO3-users and those exclusively composed of CO2-only-users showed an even stronger separation based on the HCO3- concentration, with both an AUC of 82% and 78% (both p < .001). Our results underline that DIC can greatly affect riverine macrophytes. However, absolute concentrations of HCO3- are less relevant, while the connection to pH is more important, reflecting CO2 concentrations. River monitoring and management should consider the interaction between nutrients DIC, surface water and sedimental compartment as important factors affecting macrophyte occurrence, rather than solely focussing on surface water nutrients.

Biotic homogenization is one of the key aspects of the current biodiversity crisis. Here we analyzed the trends of three facets of niche homogenization, i.e. niche specialization, trait α-diversity and spatial β-diversity, over a period of 25 years (1990–2014) using a large dataset of 3782 stream benthic invertebrate samples collected from central European low-mountain streams. We studied a set of traits describing the ecological niche of species and their functions: body size, feeding groups, substrate preferences, flow preferences, stream zonation preferences and saprobity. Trait composition changed significantly during the study period, and we identified an overall increase in niche homogenization. Specifically, community niche specialization significantly decreased by 20.3% over the 25-year period, with declines ranging from −16.0 to −40.9% for zonation-, flow-, substrate-preferences, body size and feeding traits. Trait diversity did not change significantly, although we recorded significant decreases by −14.2% and −10.2% for flow- and substrate-preference and increases by 5.8% and 22.6% for feeding traits and zonation preference over the study period. Trait spatial β-diversity significantly decreased by −53.0%, with substrate-preference, feeding groups and flow-preference traits declining from −61.9% to −75.3% over the study period. This increased niche homogenization is likely driven by the increase of down-stream typical taxa, which are favored by warming temperatures. Further, it is in apparent contradiction with the recorded increase in abundance (+35.9%) and taxonomic richness (+39.2%) over the same period. However, even such increases do not safeguard communities from undergoing niche homogenization indicating that recovery processes may differ with regard to community composition and community traits. Our results emphasize the complexity of community responses to global change and warrant caution when founding conclusions based solely on single community metrics.

Water temperature is a key driver for riverine biota and strongly depends on shading by woody riparian vegetation in summer. While the general effects of shading on daily maximum water temperature Tmₐₓ are well understood, knowledge gaps on the role of the spatial configuration still exist. In this study, the effect of riparian buffer length, width, and canopy cover (percentage of buffer area covered by woody vegetation) on Tmₐₓ was investigated during summer baseflow using data measured in seven small lowland streams in western Germany (wetted width 0.8–3.7 m). The effect of buffer length on Tmₐₓ differed between downstream cooling and heating: Tmₐₓ approached cooler equilibrium conditions after a distance of 0.4 km (~45 min travel-time) downstream of a sharp increase in canopy cover. In contrast, Tmₐₓ continued to rise downstream of a sharp decrease in canopy cover along the whole 1.6 km stream length investigated. The effect of woody vegetation on Tmₐₓ depended on buffer width, with changes in canopy cover in a 10 m wide buffer being a better predictor for changes in Tmₐₓ compared to a 30 m buffer. The effect of woody vegetation on Tmₐₓ was linearly related to canopy cover but also depended on daily temperature range Trₐngₑ, which itself was governed by cloudiness, upstream canopy cover, and season. The derived empirical relationship indicated that Tmₐₓ was reduced by −4.6°C and increased by +2.7°C downstream of a change from unshaded to fully shaded conditions and vice versa. This maximum effect was predicted for a 10 m wide buffer at sunny days in early summer, in streams with large diel fluctuations (large Trₐngₑ). Therefore, even narrow woody riparian buffers may substantially reduce the increase in Tmₐₓ due to climate change, especially in small shallow headwater streams with low baseflow discharge and large daily temperature fluctuations.

Assessments of river restoration outcomes are mostly based on taxonomic identities of species, which may not be optimal because a direct relationship to river functions remains obscure and results are hardly comparable across biogeographic borders. The use of ecological species trait information instead of taxonomic units may help to overcome these challenges. Abundance data for fish communities were gathered from 134 river restoration projects conducted in Switzerland, Germany and Finland, monitored for up to 15 years. These data were related to a dataset of 22 categories of ecological traits describing fish life-history strategies to assess the outcome of the restoration projects. Restoration increased trait functional diversity and evenness in projects that were situated in the potamal zone of rivers. Restoration effect increased with the length of the restored river reaches. In areas with low levels of anthropogenic land use, the peak of the restoration effect was reached already within one to five years after the restoration and effect receded thereafter, while communities responded later in areas with higher levels of anthropogenic land use. In the lower potamal zone, a shift towards opportunistic life-history strategists was observed. In the upper rhithral zone, in contrast, species with an opportunistic life-history strategy increased only in the first five years of restoration, followed by a shift towards equilibrium strategists at restorations older than 5 years. This pattern was more pronounced in rivers with higher level of anthropogenic land use and longer restored river reaches. Restoration reduced the variability in community trait composition between river reaches suggesting that community trait composition within these zones converges when rivers are restored. This study showed how ecological traits are suitable to analyse restoration outcomes and how such an approach can be used for the evaluation and comparison of environmental management actions across geographical regions. peerReviewed

Although experiences with ecological restoration continue to accumulate, the effectiveness of restoration for biota remains debated. We complemented a traditional taxonomic analysis approach with information on 56 species traits to uncover the responses of 3 aquatic (fish, macroinvertebrates, macrophytes) and 2 terrestrial (carabid beetles, floodplain vegetation) biotic groups to 43 hydromorphological river restoration projects in Germany. All taxonomic groups responded positively to restoration, as shown by increased taxonomic richness (10-164%) and trait diversity (habitat, dispersal and mobility, size, form, life history, and feeding groups) (15-120%). Responses, however, were stronger for terrestrial than aquatic biota, and, contrary to our expectation, taxonomic responses were stronger than those of traits. Nevertheless, trait analysis provided mechanistic insights into the drivers of community change following restoration. Trait analysis for terrestrial biota indicated restoration success was likely enhanced by lateral connectivity and reestablishment of dynamic processes in the floodplain. The weaker response of aquatic biota suggests recovery was hindered by the persistence of stressors in the aquatic environment, such as degraded water quality, dispersal constraints, and insufficient hydromorphological change. Therefore, river restoration requires combined local- and regional-scale approaches to maximize the response of both aquatic and terrestrial organisms. Due to the contrasting responses of aquatic and terrestrial biota, the planning and assessment of river restoration outcomes should consider effects on both components of riverine landscapes.

Species reintroductions – the translocation of individuals to areas in which a species has been extirpated with the aim of re-establishing a self-sustaining population – have become a widespread practice in conservation biology. Reintroduction projects have tended to focus on terrestrial vertebrates and, to a lesser extent, fishes. Much less effort has been devoted to the reintroduction of invertebrates into restored freshwater habitats. Yet, reintroductions may improve restoration outcomes in regions where impoverished regional species pools limit the self-recolonisation of restored freshwaters. We review the available literature on macroinvertebrate reintroductions, focusing on identifying the intrinsic and extrinsic factors that determine their success or failure. Our study reveals that freshwater macroinvertebrate reintroductions remain rare, are often published in the grey literature and, of the attempts made, approximately one-third fail. We identify life-cycle complexity and remaining stressors as the two factors most likely to affect reintroduction success, illustrating the unique challenges of freshwater macroinvertebrate reintroductions. Consideration of these factors by managers during the planning process and proper documentation – even if a project fails – may increase the likelihood of successful outcomes in future reintroduction attempts of freshwater macroinvertebrates.

South Africa is the world’s main supplier of Pt. The Bushveld Igneous Complex in South Africa contains 75% of the world’s Pt resources. Mining of this precious metal requires large volumes of water for production and removal of waste products. Most of this wastewater is discharged into river systems. Although the source of contamination with Pt in aquatic systems due to mining activities is known, little to no information is available about the impact of Pt on aquatic organisms. Additionally, other metals are released as byproducts of Pt mining, which might also be discharged into the environment. Therefore, concentrations of Cr, Ni, Cu, Zn, Cd, Pt and Pb were determined in water, sediment and macroinvertebrate samples from a reference site (Site 1), a highly impacted site (Site 2) and a moderately impacted site (Site 3) along the Hex River, South Africa. Aquatic invertebrate families representing different functional feeding groups i.e. scraper-grazers (Lymnaeidae), collector-gatherers (Potamonautidae, Hydropsychidae, Tubificidae and Chironomidae), shredders (Baetidae) and predators (Coenagrionidae and Libellulidae) were studied. In the sediments, the concentrations of Cr and Pt were significantly higher at Site 2 than at Sites 1 and 3, respectively, whereas concentrations of Ni, Cu, Cd, and Pb showed no significant differences between the sites. Depending on the metal, the aquatic invertebrate families showed different grades of bioaccumulation. The results from especially Lymnaeidae, Baetidae, Tubificidae and Chironomidae showed great promise for the use of these taxa for biomonitoring of metal contaminations. The macroinvertebrates accumulated metals associated with Pt mining, with epi-benthic dwelling taxa (Tubificidae) accumulating higher concentrations of Pt and Cr than other families (e.g. Potamonautidae, Coenagrionidae and Lymnaeidae). These results provide valuable information on the behavior of metals related to Pt mining in aquatic ecosystems and therefore can contribute to the risk assessment of these intensive mining activities.

Climate warming often results in species range shifts, biodiversity loss and accumulated climatic debts of biota (i.e. slower changes in biota than in temperature). Here, we analyzed the changes in community composition and temperature signature of stream invertebrate communities over 25 years (1990-2014), based on a large set of samples (n = 3782) over large elevation, latitudinal and longitudinal gradients in central Europe. Although warming was moderate (average 0.5 °C), we found a strong reorganization of stream invertebrate communities. Total abundance (+35.9%) and richness (+39.2%) significantly increased. The share of abundance (TA) and taxonomic richness (TR) of warm-dwelling taxa (TA: +73.2%; TR: +60.2%) and medium-temperature-dwelling taxa (TA: +0.4%; TR: +5.8%) increased too, while cold-dwelling taxa declined (TA: -61.5%; TR: -47.3%). The community temperature index, representing the temperature signature of stream invertebrate communities, increased at a similar pace to physical temperature, indicating a thermophilization of the communities and, for the first time, no climatic debt. The strongest changes occurred along the altitudinal gradient, suggesting that stream invertebrates use the spatial configuration of river networks to track their temperature niche uphill. Yet, this may soon come to an end due to the summit trap effect. Our results indicate an ongoing process of replacement of cold-adapted species by thermophilic species at only 0.5 °C warming, which is particularly alarming in the light of the more drastic climate warming projected for coming decades.

Both ecosystem structure and functioning determine ecosystem status and are important for the provision of goods and services to society. However, there is a paucity of research that couples functional measures with assessments of ecosystem structure. In mid-sized and large rivers, effects of restoration on key ecosystem processes, such as ecosystem metabolism, have rarely been addressed and remain poorly understood. We compared three reaches of the third-order, gravel-bed river Ruhr in Germany: two reaches restored with moderate (R1) and substantial effort (R2) and one upstream degraded reach (D). Hydromorphology, habitat composition, and hydrodynamics were assessed. We estimated gross primary production (GPP) and ecosystem respiration (ER) using the one-station open-channel diel dissolved oxygen change method over a 50-day period at the end of each reach. Moreover, we estimated metabolic rates of the combined restored reaches (R1 + R2) using the two-station open-channel method. Values for hydromorphological variables increased with restoration intensity (D

Increased fine sediment deposition is recognised as one of the major causes of biological impairment of rivers and streams influencing all components of aquatic communities. Notably, stream macroinvertebrates are affected showing changes in abundance and community composition. This makes macroinvertebrates an attractive choice for biomonitoring fine sediment stress. However, there are substantial knowledge gaps regarding the quantification of deposited fine sediment and the identification of taxa sensitive to fine sediment deposition, which could serve as indicators. In this study, we developed a stream type-specific index based on the taxon-specific response of macroinvertebrates to deposited fine sediment in small, coarse substrate-dominated mountain streams. We sampled fine sediment at 73 sampling sites in Western Germany (Europe) in spring 2014 and 2015 using a sediment remobilization technique. Macroinvertebrate taxalists originating from WFD monitoring surveys were available for all sites. We applied Threshold Indicator Taxa ANalysis (TITAN) on the fine sediment mass of the sampling sites and the corresponding macroinvertebrate taxalists to identify indicator taxa, which were then used for index development. In total, TITAN identified 95 reliable indicator taxa, of which some taxa tolerated large amounts of fine sediment (e.g., Gammarus roeselii and Tubificidae Gen. sp.), while others were found to be highly sensitive to increased fine sediment mass (e.g., Elodes sp. and Limnius perrisi). The newly developed index was tested on an independent data set and performed well in detecting fine sediment stress (Spearman's r = 0.63). Furthermore, the index was better related to the deposited fine sediment mass as compared to other fine sediment indices and standard metrics used for monitoring purposes under the Water Framework Directive (WFD). The diagnostic index can be a cost-effective biomonitoring tool for stream managers and can be used as a proxy for the impact of deposited fine sediment on the reach scale.

Considerable uncertainty exists regarding the ability of reach-scale habitat restorations to promote ecological integrity and affect community composition in degraded streams and rivers and the time scales at which these effects take place. Restoration of habitats on the reach scale (hundreds of meters to a few kilometers) is expected to support threatened species because many of them are habitat specialists. In contrast, generalist species are predicted to be replaced in restored reaches. We used a large data set for 62 reach-scale restoration projects in 51 stream systems in Germany, Switzerland, and Liechtenstein and analyzed the changes in fish community composition induced by the restorations in terms of species richness, species turnover, Brillouin diversity index, total fish abundance, and proportion of alien and endangered species. We further analyzed the temporal dynamics of the fish community recovery over a period of 19 y postrestoration. Species richness and Brillouin diversity index increased in most of the restoration projects (66 and 57%, respectively), but recovery to historical reference conditions was not achieved. Total abundance was enhanced by most of the projects. Species composition in restored reaches underwent directed shifts for at least 10 y, with high and variable species turnover in the first years that decreased over time. The effects of restoration on Brillouin diversity index were highly variable in the first few years after restoration, but tended to increase over time. These dynamics must be considered more carefully in future protocols for evaluating restoration results, and final evaluation of restoration outcomes on fish communities should not be made too early. Our results indicate that reach-scale habitat restorations may be a suitable tool for increasing local fish abundance and slightly enhancing species diversity. However, more targeted approaches are needed to support threatened species and repress alien species.

Reintroduction is a common conservation strategy although rarely conducted in freshwater ecosystems. We present a reintroduction approach for freshwater invertebrates. Different natural substrates and exposure times were tested for presence and abundance of stream type specific macroinvertebrates. Wood leaf mixture demonstrated statistically significant highest mean abundances compared to other substrates. Moreover, they displayed comparatively high mean numbers when exposed for six weeks. We thus propose a natural substrate exposure method for reintroduction of aquatic insects.

Species reintroductions - the translocation of individuals to areas in which a species has been extirpated with the aim of re-establishing a self-sustaining population - have become a widespread practice in conservation biology. Reintroduction projects have tended to focus on terrestrial vertebrates and, to a lesser extent, fishes. Much less effort has been devoted to the reintroduction of invertebrates into restored freshwater habitats. Yet, reintroductions may improve restoration outcomes in regions where impoverished regional species pools limit the self-recolonisation of restored freshwaters. We review the available literature on macroinvertebrate reintroductions, focusing on identifying the intrinsic and extrinsic factors that determine their success or failure. Our study reveals that freshwater macroinvertebrate reintroductions remain rare, are often published in the grey literature and, of the attempts made, approximately one-third fail. We identify life-cycle complexity and remaining stressors as the two factors most likely to affect reintroduction success, illustrating the unique challenges of freshwater macroinvertebrate reintroductions. Consideration of these factors by managers during the planning process and proper documentation - even if a project fails - may increase the likelihood of successful outcomes in future reintroduction attempts of freshwater macroinvertebrates.

Studies investigating changes in community composition in response to recent global warming are mostly restricted to one-dimensional (e.g. elevational or latitudinal) gradients, whereas species movements are in reality three dimensional (i.e. elevational, latitudinal and longitudinal). Based on 3,245 benthic invertebrate samples from Central European streams over large elevational, latitudinal and longitudinal gradients during the period from 1986 to 2009, we developed an advanced community temperature index (CTI). This CTI enables the analysis of three-dimensional community range shifts not only in freshwater but also in marine and terrestrial environments. Overall, in contrast to terrestrial communities, benthic invertebrate communities have been able to keep up with recent global warming. However, their ability to track temperature shifts differed grossly between the three spatial dimensions, with the strongest response to elevation. Nevertheless, the price these communities had to pay was high, as total benthic invertebrate abundance and richness in cold-dwelling species have already declined by 21% and 52.5%, respectively. Our approach emphasizes the complex reaction of aquatic communities towards increasing temperatures, allowing a more complete picture of the subtle community shifts in response to global warming.

Hydromorphological restructuring of river sections, i.e. river restoration measures, often has little effects on aquatic biota, even in case of strong habitat alterations. It is often supposed that the biotic response is simply delayed as species require additional time to recolonize the newly generated habitats and to establish populations. To identify and specify the supposed lag time between restoration and biotic response, we investigated 19 restored river reaches twice in a five-year interval. The sites were restored one to ten years prior to the first sampling. We sampled three aquatic (fish, benthic invertebrates, macrophytes) and two riparian organism groups (ground beetles and riparian vegetation) and analyzed changes in assemblage composition and biotic metrics. With the exception of ground beetle assemblages, we observed no significant changes in richness and abundance metrics or metrics used for biological assessment. However, indicator taxa for near-natural habitat conditions in the riparian zone (indicators for regular inundation in plants and river bank specialists in beetles) improved significantly in the five-year interval. Contrary to general expectations in river restoration planning, we neither observed a distinct succession of aquatic communities nor a general trend towards "good ecological status" over time. Furthermore, multiple linear regression models revealed that neither the time since restoration nor the morphological status had a significant effect on the biological metrics and the assessment results. Thus, the stability of aquatic assemblages is strong, slowing down restoration effects in the aquatic zone, while riparian assemblages improve more rapidly. When defining restoration targets, the different timelines for ecological recovery after restoration should be taken into account. Furthermore, restoration measures should not solely focus on local habitat conditions but also target stressors acting on larger spatial scales and take other measures (e.g. species reintroduction) into consideration.

A large number of restoration projects aims to improve the ecological quality of streams and rivers by focusing on the stream structure. However, improved habitat heterogeneity often does not lead to natural recolonization by sensitive freshwater macroinvertebrate communities, particularly when the recolonization potential is low and source populations are absent. In preliminary studies we tested whether natural substrate exposures could be used to sample and transport benthic macroinvertebrates. In this pilot study we used these previously tested natural substrate exposures to sample freshwater invertebrates in a donor stream in order to actively (re-)colonize a recipient stream. In the course of three reintroduction campaigns, we were able to accumulate over 350,000 benthic invertebrates, including 25 indicator taxa of the orders Ephemeroptera, Plecoptera and Trichoptera and 30 taxa scoring positive in the German Fauna Index. In total, 45 taxa, which did not occur in the recipient stream before, were reintroduced. They were transported gently within natural substrate exposures and released on a stream bottom area of 500 m2 in the recipient stream. We intended to study if an increase of benthos fauna in a recipient stream is possible, and if this increase will eventually improve the ecological status. So far, the natural substrate exposure-method demonstrated to be an adequate tool to accumulate and transport benthic macroinvertebrates and, in general, has the potential to increase the biodiversity of streams when used as assisted migration measure.

We investigated temporal effects of restoration on river morphology, on species and functional composition of benthic invertebrates, floodplain vegetation and carabid beetles at three study sites in the mountain river Lahn (Germany). We sampled restored and nearby non-restored sections 3–5 years and 7–9 years after restoration. In the restored sections, instream microhabitat heterogeneity was higher due to the increased presence of finer substrates, while cobbles and coarse gravel were still dominant. Instream habitat composition did not change between the two sampling events. Areas of restored floodplain were characterized by a more diverse habitat mosaic and by unvegetated bars, vegetated islands and secondary channels. In restored sections, floodplain habitat heterogeneity was maintained 7–9 years after restoration, but vegetated areas increased, while unvegetated bars and aquatic areas decreased. The species composition of all studied groups was more variable over time in restored than non-restored sections. In contrast to benthic invertebrates, the immigration rate of floodplain vegetation and carabid beetle species was higher in restored sections. Assemblage composition of all three organism groups changed over time, with the highest change in carabid beetles and smallest in benthic invertebrates. Restoration changed the abundances of functional response groups, mainly for carabid beetles, by supporting species that indicate increased hydrodynamics and early successional stages. Changes of functional response groups in non-restored and restored sections across time indicated decreased hydrodynamics or hydrological connectivity for all organism groups. Although the response of organism groups differed, our results support the conjecture that restored sections accumulate species and enhance the local species pool.

We analyzed the effects of 36 river restoration projects on fish assemblages. We expected increased reproduction in restored river reaches leading to a sustainable increase in the total amount of fishes. The restored reaches are situated in small to medium-sized rivers in lowlands and lower mountainous areas in Germany. Restoration measures considerably changed river morphology and created more natural conditions by improving habitat diversity. For each restored reach, we sampled an upstream unrestored counterpart that resembled the restored reach before the restoration measures were implemented. Restored and unrestored control reaches were sampled by electrofishing and parameters describing river morphology (e.g. width, depth, microhabitats) were recorded along ten equidistant transects. We investigated abundance and age structure of the fish species to reveal effects of hydromorphological changes. On average, restored reaches were wider, shallower and harbored a greater variety of water depths and current velocities. Abundance of adult and of young-of-the-year (YOY) fishes was significantly higher in restored reaches. In particular, small species such as the stone loach (Barbatula barbatula) and gudgeon (Gobio gobio) and species preferring shallow, slow-flowing areas in the juvenile stage increased significantly in abundance. Density of grayling (Thymallus thymallus) and brown trout (Salmo trutta) YOY, which prefer fast-flowing areas, increased less but significantly. Presumably, a higher number of species reproduced in restored reaches, as indicated by the presence of YOY, following the morphological restoration effects. Our results indicate that restoration measures that increase habitat diversity eventually increase richness, diversity and juvenile abundance of riverine fish communities. Consequently, river restoration measures should focus on the generation of a diverse, natural shoreline with shallow, slow-flowing marginal zones, which have particularly positive effects on fish assemblages.

The preferences of aquatic invertebrate species for specific substrata at the river bottom have been subject of many studies. Several authors classified the substratum preferences of species or higher taxonomic units. Most of these compilations, however, are based on literature analyses and expert knowledge as opposed to the analysis of original data. To enhance our knowledge of invertebrate substratum preferences, we applied a ‘Multi-level pattern’ analysis based on almost 1000 substrate-specific invertebrate samples. The samples were taken in 18 streams in Germany, the Netherlands and Austria, comprising a total of 40 sampling sites and equally covering lowland and mountain streams. The main objectives of our analysis were (I) to derive substratum preferences of taxa in lowland and mountain streams, (II) to compare the preferences with existing data and (III) to compare species substratum associations between lowland and mountain streams. Of the 290 taxa analyzed, 188 were associated significantly to specific substrata. Twenty-five taxa in lowland streams and 51 taxa in mountain streams prefer one or two substratum types (of nine substratum types considered in total). In contrast, 112 species (mountain streams n = 84, lowland streams n = 28) are associated significantly with a broader range of substrata. We compared the classifications derived from our data analysis with those provided in the freshwaterecology.info database ( www.freshwaterecology.info ). Our results support the existing classifications of substratum preferences in most cases (70%). For 25 species, substratum preferences for both lowland and mountain streams were derived, many of them indicating different substratum associations in the two stream groups. As substratum preferences differed between closely related species, preferences should always be given at the species level as opposed to coarser taxonomic units.

Numerous river restoration projects have been undertaken to improve the hydromorphology of rivers and their floodplains. Subsequent ecological monitoring is usually restricted to instream quality assessment while riparian areas and floodplains are rarely assessed. A good indicator for riparian assessment could be vegetation as its diversity is dependent on functioning hydromorphological processes. We used a comparative survey to test the effect of newly created river morphological features on riparian plant composition at restored reaches relative to non-restored reaches. We investigated 43 larger river restoration projects in Western Germany, realized between 1987 and 2008. The vegetation surveys were conducted between 2005 and 2013. We hypothesized that a) morphological river restoration leads to the diversification of riparian vegetation, higher reach-scale habitat heterogeneity and optimized habitats. These changes will b) be reflected by ecological preferences indicating hydromorphological functioning. We detected significantly higher species diversity in restored reaches. New habitats and hydrological interactions in the active channel and with the floodplains were fundamental, with the latter indicated by species preferences. In restored reaches we observed higher percentages of plants indicating regular flooding, significantly higher mean moisture tolerances and an extended range of moisture preferences. Conclusions Morphological river restoration can diversify and fundamentally influence riparian vegetation. Ecologically meaningful projects should address the rehabilitation of abiotic processes, focusing on a functioning flooding regime and near-natural riparian habitats. Riparian vegetation will reflect the improvements and consequently could be used as an indicator for restoration efforts. Thus, riparian vegetation assessment is worthy of consideration through legal obligations.

Summary 1. Using an extensive data set from 18 river restoration projects in the lower mountain ranges of Germany and 5462 river reaches in their surroundings, we estimated the spatial extent of the regional fish species pool from which restored river reaches are colonised. 2. Restoration resulted in a marginally significant increase in fish species richness; however, restored reaches still deviated markedly from natural reference conditions. Nearly all (96.6%) species occurring in restored reaches were present in reaches within a distance of 5 km up- or downstream of the restored reach. 3. Species richness in restored reaches was correlated with species richness within a 5-km species pool. This relationship was more pronounced for common than for rare fishes and applied to both the total number of fish species at the restored reach and the number of additional fish species that were not present at unrestored conditions. 4. The richness of the regional species pools was greatly impoverished. On average, only 50% of all species considered to represent natural reference assemblages were present. The limited success in establishing natural fish assemblages in restored reaches was attributed to spatial limitation (e.g. due to fragmentation) and an impoverished regional species pools from which restored reaches recruit. 5. We recommend that integrated river restoration management should consider not only the abiotic prerequisites of successful restorations, but also the structure and quality of the regional species pool.

River restoration is a central issue of present-day River Basin Management. Unfortunately, many studies have shown limited ecological improvements, hypothesizing catchment influences and missing donor populations as main impeding factors. This study evaluates the ecological status after restoration at 46 river reaches in light of catchment influences upstream. Three groups of environmental parameters were investigated: (i) riparian land use and (ii) physical habitat quality in different lengths upstream of the restorations and (iii) land use in the whole catchment upstream. Ecological quality ratios of standardized fish, invertebrate and macrophyte samples were used as response variables. The results imply that sub-catchment variables influence the ecological status more than local habitat improvements. In particular, fish and invertebrate ecological status was positively linked to percent deciduous forest upstream of restored sites, while macrophytes revealed an opposite trend. Furthermore, we found a strong linkage of site-scale ecological status and physical habitat quality up to 5 km upstream of the restorations; the more natural were riparian land use and river habitat quality upstream, the higher was the chance of a good ecological quality in restored reaches. We conclude that site-scale restoration measures are likely to be unsuccessful, if the sub-catchment physical habitat upstream is degraded.

We compared habitat diversity and morphodynamics of ‘actively’ restored reaches (including removal of bank fixation, widening and large wood placement) with ‘passively’ restored reaches (abandonment of channel maintenance) and adjacent non-restored control reaches in medium-sized Central European mountain rivers. Habitat diversity and river morphology were mapped in two consecutive years and changes in habitat composition (channel features, substrates) and morphological changes (active depth and width) were quantified. In both years, habitat diversity was generally higher in the restored reaches compared to their non-restored counterparts, and significantly differed between restoration approaches, with average values in actively restored reaches being about 60% larger than in passively restored reaches. Channel feature composition differed significantly, both between restored and unrestored reaches, and between restoration approaches, whereas substrate composition was similar in all investigated reaches, indicating that restoration had a higher effect on mesohabitat than on microhabitat conditions. Interannual habitat dynamics in respect to channel feature composition were larger in the actively restored compared to the passively restored reaches, while substrate composition remained fairly constant in all reaches. Regarding morphodynamics, changes in depth and width of actively restored reaches differed significantly from changes in passively restored ones in three of the four elements compared. Our findings imply increased habitat richness, diversity and habitat dynamics in the restored reaches, especially in actively restored ones. Analysis of discharge data suggests that flood events exceeding critical shear stress of the bed material, and the time span since restoration determine the potential for morphological changes.

Summary 1. In recent years, river restoration science has been searching for biological indicators of improvement in the physical habitats of streams. To date, research has mainly focused on the use of fish and macroinvertebrates as indicators. Despite their importance in aquatic ecosystems, the response of macrophytes to habitat restoration has been rarely studied. 2. We investigated the macrophyte communities of 40 restored river reaches in the lowland and lower mountainous areas of Germany. Each restored reach was compared to an upstream, unrestored reach using a space-for-time-substitution approach. At each reach, a 100 m stretch was surveyed for submerged and emergent macrophytes, recording the quantity, abundance and growth form of each species. Additionally, microhabitat patterns (substrate, depth, current velocity) and channel parameters (mean and bankfull width, number of channel elements) were recorded. 3. Restored reaches had a significantly higher macrophyte cover, richness, diversity and number of growth forms. Macrophyte diversity and richness were both positively correlated with depth, current and substrate. 4. The analysis of growth forms showed that Lemnids, Helodids, Parvopotamids, Elodids, Peplids and Juncids are all significant indicators of restoration. These species all responded directly to the restoration measures either by highly increasing in abundance or by being present in the restored reaches and absent in the unrestored reaches. While the restored reaches of the lowland rivers were characterized by a high abundance of Peplids and Parvopotamids, the restored reaches of the mountain rivers showed a significantly higher presence and abundance of Lemnids and Helodids. 5. Three macrophyte species (Lemna minor, Persicaria hydropiper, Potamogeton crispus) were regarded as significant indicators of restoration. No species were found to be indicators of unrestored reaches. 6.Synthesis and applications. Macrophyte communities benefit from river restoration by showing increased cover, abundance and diversity. The main drivers of this enhancement are more natural and diverse substrates and an increased floodplain area in the restored reaches, as well as a greater variability of current and depth patterns. Monitoring of macrophytes could thus be an easy and cost-effective means to gauge the success of river restoration measures.

Summary 1. Over the past centuries, European streams have been heavily influenced by humans through pollution and regulation. As a result, the quality and diversity of freshwater riparian habitats have declined strongly, and the diversity of riparian flora and fauna has decreased. Recent restoration measures have resulted in stream habitat improvements, but biodiversity improvements have failed to follow in fragmented streams. It has been suggested that dispersal limitation could play an important role in the lack of biodiversity improvement in restored streams, but to date, there is no conclusive evidence for this assumption. 2. In this study, we investigated whether colonization of restored streams by plants and macroinvertebrates is limited by dispersal. We hypothesized that colonization success increases with increasing availability of (nearby) source populations and with increasing ability of species to disperse over long distances. We related species composition in seven restored stream sections to species’ abundances in the surroundings and to species’ dispersal abilities. 3. For both plants and macroinvertebrates, colonization success is strongly related to the abundance of species in the local and regional species pools. 4. For plants, dispersal strategy has an additional influence on colonization success: short-lived plants with high production of small, well-dispersed seeds colonized best within the 3- to 5-year period after restoration. 5. The existence of dispersal strategy constraints could not be confirmed in macroinvertebrates, possibly because these are limited by a lack of connectivity on larger spatial scales. On the landscape scale, beneficial effects of increased plant diversity might further improve habitat suitability for macroinvertebrates. 6. Synthesis and applications. Dispersal appears to be a limiting factor for successful (re)colonization of restored streams in fragmented landscapes. In plants, this is attributed to limitations in seed dispersal abilities and likely to a lack of nearby source populations as well. In macroinvertebrates, lack of nearby source populations may also be a limiting factor. Hence, we suggest restoring landscape connectivity at larger spatial scales and optimizing the availability of near-natural ‘source’ areas in the vicinity of restoration projects, at least for plants, to improve the success of biodiversity restoration in fragmented habitats.

Summary 1. River restoration has received considerable attention, with much recent focus on restoring river hydromorphology to improve impoverished aquatic communities. However, we still lack a clear understanding of the response of aquatic biota to river restoration. 2. We studied the effects of hydromorphological restoration on benthic invertebrate assemblages in 25 river sites in Germany using standardised methods. Restoration efforts were primarily aimed to restore habitat heterogeneity; correspondingly, habitat diversity increased at most sites. 3. Similarity of benthic invertebrate assemblages between restored and unrestored river sections was low (mean similarity was 0.32 for Jaccard and 0.46 for Sorensen). Community-based metrics, such as the percentage of Ephemeroptera, Plecoptera and Trichoptera taxa, also differed between restored and unrestored sections. 4. Only three of the 25 restored sections were classified as having ‘good ecological quality’ class according to the European Water Framework Directive criteria; hence, poor water quality is probably one factor impeding recolonisation. 5. Our results show that isolated restoration measures do not necessarily result in positive effects on aquatic biota and that better understanding of the interconnectedness within a catchment is required before we can adequately predict biotic responses to structural river restoration.

1. Hydromorphological river restoration usually leads to habitat diversification, but the effects on benthic invertebrates, which are frequently used to assess river ecological status, are minor. We compared the effects of river restoration on morphology and benthic invertebrates by investigating 26 pairs of non-restored and restored sections of rivers in Austria, Czech Republic, Germany, Italy and the Netherlands. 2. Sites were grouped according to (1) region: central Europe vs. southern Europe; (2) river type: mountain vs. lowland rivers; (3) restoration approach: active vs. passive restoration and (4) a combination of these parameters. All sites were sampled according to the same field protocol comprising hydromorphological surveys of river and floodplain mesohabitats, microhabitats at the river bottom and habitat-specific sampling of benthic invertebrates. Restoration effects were compared using Shannon–Wiener Indices (SWIs) of mesohabitats, microhabitats and invertebrate communities. Differences in metric values between non-restored and restored sites were compared for 16 metrics that evaluated hydromorphology and the benthic invertebrate community. 3. Mean SWIs differed for both mesohabitats (1·1 non-restored, 1·7 restored) and microhabitats (1·0 non-restored, 1·3 restored), while SWIs for invertebrate communities were not significantly different (2·4 non-restored, 2·3 restored). Meso- and microhabitat metrics in the restored sections were usually higher compared with the non-restored sections, but the effects on invertebrate metrics were negligible. 4. Measures in southern Europe and mountainous regions yielded larger differences between non-restored and restored sections of rivers. Differences in the meso- and microhabitat metrics were largest for actively restored sections of central European mountain rivers and rivers from southern Europe, followed by passively restored mountain rivers in central Europe. The smallest differences were observed for lowland sites. There was no significant restoration effect on invertebrate metrics in any categories. 5. Synthesis and applications. Restoration measures addressing relatively short river sections (several hundred metres) are successful in terms of improving habitat diversity of the river and its floodplain. Active restoration measures are suitable if short-term changes in hydromorphology are desired. To realize changes in benthic invertebrate community composition, habitat restoration within a small stretch is generally not sufficient. We conclude that restoring habitat on a larger scale, using more comprehensive measures and tackling catchment-wide problems (e.g. water quality, source populations) are required for a recovery of the invertebrate community.

We evaluated 516 species and/or subspecies of European stoneflies for vulnerability to climate change according to autoecological data. The variables considered were stream zonation preference, altitude preference, current preference, temperature range preference, endemism and rare species. Presence in ecoregions was used to analyse the vulnerability of taxa in relation to their distribution. We selected the variables that provided information on vulnerability to change in climate. Thus, we chose strictly crenal taxa, high-altitude taxa, rheobionts, cold stenotherm taxa, micro-endemic taxa and rare taxa. Our analysis showed that at least 324 taxa (62.79%) can be included in one or more categories of vulnerability to climate change. Of these, 43 taxa would be included in three or more vulnerability categories, representing the most threatened taxa. The most threatened species and the main factors affecting their distribution are discussed. Endangered potamal species, with populations that have decreased mainly as a consequence of habitat alteration, also could suffer from the effects of climate change. Thus, the total number of taxa at risk is particularly high. Not only are a great diversity of European stoneflies concentrated in the Alps, Pyrenees and Iberian Peninsula, but so are the most vulnerable taxa. These places are likely to be greatly affected by climate change according to climate models. In general, an impoverishment of European Plecoptera taxa will probably occur as a consequence of climate change, and only taxa with wide tolerance ranges will increase in abundance, resulting in lower overall faunal diversity.

Summary 1 Medium-sized and large mountain rivers are among the most degraded river types in Europe and numerous river restoration projects are currently carried out to achieve ‘good ecological status’. Surprisingly little is known about the effects of river restoration measures on aquatic and terrestrial organisms. We investigated the effects of restoration on hydromorphology, floodplain vegetation, ground beetles and benthic invertebrates of Central European mountain rivers by comparing seven restored, multiple-channel sections with seven nearby non-restored, straight sections. 2 Floodplain mesohabitats and aquatic microhabitats were recorded along 20 transects per river section (200 m long). Samples of floodplain vegetation (444 samples), ground beetles (153) and benthic invertebrates (134) were taken per habitat type and section. Two hydromorphological metrics and 13 biotic metrics were calculated. 3 The number of floodplain mesohabitats was significantly higher in restored sections, but there was no significant effect on the number of aquatic microhabitats. Floodplain vegetation reacted most strongly to restoration, with more vegetation assemblages and higher number of species in restored sections. The number of ground beetle species also increased, but there was no effect on number of species or diversity of benthic invertebrates. 4 Habitat composition and assemblages were compared by cluster analysis. When using mesohabitat data, restored vs. non-restored sections clustered to separate groups, while the use of aquatic microhabitat data produced mixed groups. Floodplain vegetation data clustered in restored and non-restored sections. For benthic invertebrates, the restored and non-restored sections of each individual river were always clustered together. Ground beetle assemblages responded more strongly to restoration than benthic invertebrates but less than floodplain vegetation. 5 Synthesis and applications. River restoration measures which re-created multiple-channel patterns differ in their effect on floodplain vegetation, ground beetles and benthic invertebrates. The strong increase in the number of floodplain vegetation species is due to the creation of additional habitats, while riparian ground beetles react mainly to the increased availability of gravel bars. The lack of response of benthic invertebrates to restoration measures is due to the comparatively small changes in aquatic microhabitat composition. Our results indicate that floodplain habitats react more strongly to re-braiding as a restoration measure compared to in-stream habitats and that floodplain communities might be best suited to judge the immediate effects of restoration.

We examined the effects of stream restoration efforts that re-established multiple-channel sections in otherwise single-channel streams on aquatic habitat diversity and macroinvertebrate assemblages. At seven pairs of sites (single- vs. multiple-channel) we analysed the diversity of aquatic habitat parameters at various spatial scales (e.g. shore length, channel features, substrate diversity, flow variability). We also sampled macroinvertebrates in all available substrates individually and compared alpha- and beta-diversity and nestedness patterns on substrates between single- and multiple-channel sections. Multiple-channel sections showed a considerably more diverse hydromorphology. Taxa number, abundance, and evenness of macroinvertebrate assemblages did not differ significantly. Ten Coleoptera and seven Trichoptera taxa were present exclusively in multiple-channel sections on loam, sand, living parts of terrestrial plants (LPTP), coarse particulate organic matter (CPOM) or large wood. Non-metric Multidimensional Scaling showed that macroinvertebrate assemblages were substrate-specific rather than section-specific. Nestedness did not differ for samples from single- and multiple-channel sections, nor for individual substrates from different sections. We did not observe differences in alpha-diversity from substrates at single- and multiple-channel sections. However, different substrates host different assemblages and the increased substrate diversity in multiple-channel sections might result in higher beta-diversity in these sections. Our results indicate that stream restoration projects aimed at redeveloping near-natural macroinvertebrate diversity should focus on generating several long multiple-channel stretches with large areas of high quality habitats (e.g. large wood), creating stepping stone habitats for re-colonisation, and should allow sufficient time for new assemblages to establish.

Evidence for successful restoration of riverine communities is scarce, particularly for benthic invertebrates. Among the multitude of reasons discussed so far for the lack of observed effects is too short of a time span between implementation and monitoring. Yet, studies that explicitly focus on the importance of restoration age are rare. We present a comprehensive study based on 44 river restoration projects in Germany, focusing on standardized benthic invertebrate sampling. A broad gradient ranging from 1 to 25years in restoration age was available. In contrast to clear improvements in habitat heterogeneity, benthic community responses to restoration were inconsistent when compared to control sections. Taxon richness increased in response to restoration, but abundance, diversity and various assessment metrics did not respond clearly. Restoration age was a poor predictor of community composition and community change, as no significant linear responses could be detected using 34 metrics. Moreover, only 5 out of 34 tested metrics showed non-linear shifts at restoration ages of 2 to 3years. This might be interpreted as an indication of a post-restoration disturbance followed by a re-establishment of pre-restoration conditions. BIO-ENV analysis and fourth-corner modeling underlined the low importance of restoration age, but revealed high importance of catchment-scale characteristics (e.g., ecoregion, catchment size and land use) in controlling community composition and community change. Overall, a lack of time for community development did not appear to be the ultimate reason for impaired benthic invertebrate communities. Instead, catchment-scale characteristics override the effectiveness of restoration. To enhance the ecological success of future river restoration projects, we recommend improving water quality conditions and catchment-scale processes (e.g., connectivity and hydrodynamics) in addition to restoring local habitat structure.

We analysed the development of benthic invertebrate assemblages in restored urban streams in Germany. Prior to restoration these streams were open sewers and not inhabited by benthic invertebrates except Oligochaeta. Therefore, almost all taxa recolonised the streams after restoration. Restoration included the improvement of the water quality and the hydro-morphology. Following stream restoration, benthic invertebrate assemblages are supposed to undergo a distinct succession, as new habitats have been generated. These processes are poorly understood in streams. Our study revealed succession patterns and processes of benthic invertebrate assemblages in restored streams formerly transporting sewage, with pioneer taxa being dominant in the first years and a gradual assemblage maturation. We sampled 13 sites in seven restored streams in the springs of 2012 and 2013. Seven of these sites are connected to near-natural sections, whereas six sites lack this connection. The 13 sites differ in time since restoration and were sampled between one and 20 years after restoration. Additionally, we sampled 21 near-natural sites within the catchment and 11 near-natural sites in neighbouring catchments as potential recolonisation sources. Within 1 year, the restored sites underwent succession, which led to a higher resemblance of their assemblages to those of source sites. Derived from change values and non-metric multidimensional scaling, assemblages of young restored sites changed more than assemblages of old ones. In the first years after restoration we found assemblages with high abundances of pioneer taxa, while 5 years after restoration assemblages were increasingly similar to those of the source sites and mature assemblages were observed a decade after restoration. The succession towards near-natural assemblages is influenced by the instream habitats, catchment conditions and the recolonisation sources in the surroundings. Our findings reveal that monitoring results obtained fewer than 10 years after restoration will still be influenced by ongoing succession.

River restoration enhances not only habitat diversity in the stream channel and riparian zone, but also retention of organic matter, which together are expected to enhance aquatic-terrestrial linkages, and the range of autochthonous and allochthonous resources. Consequently, alterations of food-web structure and trophic relationships can be expected. We applied stable isotope analysis (δ13C, δ15N) to characterize changes in the trophic structure of benthic invertebrate communities between paired restored and unrestored river reaches across 16 European catchments. We sampled dominant taxa of invertebrate assemblages belonging to different functional feeding groups and calculated δ13C range to estimate the diversity of basal resources assimilated, δ15N range as an indicator of the trophic length and standard ellipse area corrected for small samples as a measure of isotopic niche width. We analysed (1) if restoration influences the trophic structure of invertebrates, (2) if effects of restoration depend on the extent of restoration effort, and (3) if effects of restoration depend on restoration measures applied. Our European-scale comparison indicates that river habitat restoration effects trophic structure, primarily by increasing the breadth of resources assimilated by consumers; this effect increases with restoration effort and it depends on restoration measure type.

At the global scale, substantial numbers of stream restoration projects have been carried out in the last decades, utilizing significant investment. Yet comparative studies on the effectiveness of stream restorations are rare, and the few existing studies show inconsistent results. A common flaw in these studies is that the restoration projects investigated often include widely varying sets of restoration measures, which may lead to contradictory findings on restoration outcomes. To overcome this flaw we propose an approach to identify, bundles of restoration measures based on cluster analysis. We applied our approach to a comprehensive dataset of 61 Central European stream restoration projects and compare the restoration effects of these different bundles of restoration measures on fish communities. Restoration projects concentrating on stream bank restoration measures led to improvements in fish diversity. By contrast, complex reconfigurations of entire watercourses led to less diverse fish communities, at least within the first ten years after restoration. In general, changes in species diversity and species turnover depended on the age of a restoration project, and support evidence that the effects of restoration should be monitored more than ten years after restoration. Streams often suffer from recurring syndromes of hydromorphological deficits, relating to different forms of human land use, and analogously, recurring bundles of restoration measures are applied to overcome these deficit syndromes. Thus, our strategy to statistically identify, describe and evaluate bundles of effective restoration measures for similar stream types can help to better inform restoration practice.

Restoration of river hydromorphology often has limited detected effects on river biota. One frequently discussed reason is that the restored river length is insufficient to allow populations to develop and give the room for geomorphological processes to occur. We investigated ten pairs of restored river sections of which one was a large project involving a long, intensively restored river section and one represented a smaller restoration effort. The restoration effect was quantified by comparing each restored river section to an upstream non-restored section. We sampled the following response variables: habitat composition in the river and its floodplain, three aquatic organism groups (aquatic macrophytes, benthic invertebrates and fish), two floodplain-inhabiting organism groups (floodplain vegetation, ground beetles), as well as food web composition and land-water interactions reflected by stable isotopes. For each response variable, we compared the difference in dissimilarity of the restored and nearby non-restored section between the larger and the smaller restoration projects. In a second step, we regrouped the pairs and compared restored sections with large changes in substrate composition to those with small changes. When comparing all restored to all non-restored sections, ground beetles were most strongly responding to restoration, followed by fish, floodplain vegetation, benthic invertebrates and aquatic macrophytes. Aquatic habitats and stable isotope signatures responded less strongly. When grouping the restored sections by project size, there was no difference in the response to restoration between the projects targeting long and short river sections with regard to any of the measured response variables except nitrogen isotopic composition. In contrast, when grouping the restored sections by substrate composition, the responses of fish, benthic invertebrates, aquatic macrophytes, floodplain vegetation and nitrogen isotopic composition were greater in sections with larger changes in substrate composition as compared to those with smaller changes.Synthesis and applications. The effects of hydromorphological restoration measures on aquatic and floodplain biota strongly depend on the creation of habitat for aquatic organisms, which were limited or not present prior to restoration. These positive effects on habitats are not necessarily related to the restored river length. Therefore, we recommend a focus on habitat enhancement in river restoration projects.The effects of hydromorphological restoration measures on aquatic and floodplain biota strongly depend on the creation of habitat for aquatic organisms, which were limited or not present prior to restoration. These positive effects on habitats are not necessarily related to the restored river length. Therefore, we recommend a focus on habitat enhancement in river restoration projects.

We present a multi-trait approach to identify potentially vulnerable species of Ephemeroptera (mayflies), Plecoptera (stoneflies) and Trichoptera (caddisflies), collectively referred to as EPT, to the impacts of climate change (CC). The “climate change vulnerability score” (CCVS) is an aggregation of six autecological traits that are known to be associated with vulnerability to CC: endemism, micro-endemism, temperature preference, altitudinal preference, stream zonation preference, and life history. We assigned a vulnerability score (0 – invulnerable to 6 – highly vulnerable to climate change) to 1940 EPT species and discussed the applicability of the index at three spatial scales: (1) continental (Europe), (2) state (the German Federal State of North Rhine-Westphalia) and (3) a river basin (the Ruhr River). We identified 157 EPT species (ca. 8%) as highly vulnerable to climate change (CCVS ≥ 4), including 95 species of caddisflies, 60 species of stoneflies and two species of mayflies. These are mostly found in France and Italy (52 species each), Spain and Slovenia (36 and 34, respectively), and Austria and Switzerland (30 species each), of which 95 are caddisflies, 60 stoneflies, and 2 mayflies. Using data collected in routine regional sampling we show that although no endemic EPTs were found in the German Federal State of North Rhine-Westphalia, eight species can still be identified as relatively vulnerable to CC (CCVS of 3). Almost all of these species are occurring in the ‘mountainous’ regions of the state (>200 m a.s.l.), the Sauerland and the Eifel. The upper reaches of the Ruhr catchment have been found to be relatively rich in vulnerable species, including several locally rare species. This index can assist conservationists to identify “hotspots” in terms of climate vulnerability and climate change refuge areas that can be considered for protection or the application of restoration measures at a local and regional scale. Nevertheless, not all species have complete autecological information, which hinders our ability to fully recognize the areas of priority. To further stabilize and enhance the applicability of this method, it is essential to fill these knowledge gaps in the future.

Summary The response of benthic invertebrate assemblages to stream restoration is the subject of numerous recent studies, but the process of recolonisation of restored river sections is still poorly understood. Field studies are often flawed by uncertainties about whether the observed species colonised the restored sections after restoration or whether they were present before restoration. We investigated restored streams in the Boye catchment in an urbanised area in Germany. The streams were formerly open sewers and thus were not inhabitable by benthic invertebrates, except for Oligochaeta. Restoration included the construction of underground sewers for wastewater and morphological remodelling of the stream bed. In the spring of 2012, we sampled seven restored former channels connected with near-natural upstream sections, which were never used as sewage channels and are in good status morphologically, and six unconnected restored former channels. Restoration measures had been conducted between one and 19 years before sampling. Additionally, we sampled 21 near-natural sites within the catchment and eleven near-natural sites in neighbouring catchments. Near-natural sites were considered to be potential source sites from which benthic invertebrates might colonise the restored sites. We recorded 128 taxa and categorised them into five dispersal classes that reflect dispersal capabilities and degree of ecological specialisation. Assemblages at restored sites were characterised by lower numbers of taxa and/or high abundance of hololimnic taxa and poorly dispersing winged species and by higher species numbers and abundance of strongly dispersing generalists. A distinct recolonisation sequence was derived from the observed patterns, in which winged, strongly dispersing generalists colonised most rapidly and were followed by hololimnic species, weakly dispersing generalists and habitat specialists. Restored sites connected with near-natural upstream sections were colonised more rapidly than unconnected restored sites, particularly by habitat specialists. Almost 90% of the recolonisation events originated from sources within a distance of 5 km. We observed a succession from pioneer assemblages to more mature communities, which resembles that of the surrounding near-natural sites. In conclusion, assemblages in connected, restored sites needed nine to 19 years to reach maturation, while the settlement of assemblages in unconnected sites is expected to require more time.

It is commonly assumed that the colonization of restored river reaches by fish depends on the regional species pools; however, quantifications of the relationship between the composition of the regional species pool and restoration outcome are lacking. We analyzed data from 18 German river restoration projects and adjacent river reaches constituting the regional species pools of the restored reaches. We found that the ability of statistical models to describe the fish assemblages established in the restored reaches was greater when these models were based on 'biotic' variables relating to the regional species pool and the ecological traits of species rather than on 'abiotic' variables relating to the hydromorphological habitat structure of the restored habitats and descriptors of the restoration projects. For species presence in restored reaches, 'biotic' variables explained 34% of variability, with the occurrence rate of a species in the regional species pool being the most important variable, while 'abiotic' variables explained only the negligible amount of 2% of variability. For fish density in restored reaches, about twice the amount of variability was explained by 'biotic' (38%) compared to 'abiotic' (21%) variables, with species density in the regional species pool being most important. These results indicate that the colonization of restored river reaches by fish is largely determined by the assemblages in the surrounding species pool. Knowledge of species presence and abundance in the regional species pool can be used to estimate the likelihood of fish species becoming established in restored reaches.