Hydromorphological Degradation Research Articles

Hydromorphological degradation modifies long-term macroinvertebrate responses to water quality and climate changes in lowland rivers

Due to decades of persistent anthropogenic pressures, lowland rivers represent one of the most severely impaired habitats in Europe. Despite improved water quality, novel stressors, particularly climate change, are emerging with most lowland rivers suffering from past hydromorphological degradation. We aim to elucidate how such degradation alters the biological response in multiple-stressor environments, as this has rarely been considered in studies documenting long-term development of anthropogenically impacted rivers. Here, benthic macroinvertebrates, water quality and hydroclimatic variables were monitored over a period of two decades in nine of the largest Czech rivers. Detailed data on hydromorphological degradation allowed us to track distinct patterns in rivers with high and low levels of degradation. Temporal changes in environmental variables showed similar patterns in both site groups, characterised by reduced organic and nutrient pollution but increased hydroclimatic and salinity stress. 150 % increase in total abundance, especially in abundance and richness of sediment-dwelling and non-native taxa was found in both site groups. While the increase in abundance was due to improved water quality and rising water temperature, the longer duration of minimal flows had a negative effect on species richness, hampering species gain particularly at highly degraded sites. Our results provide novel evidence that degree of hydromorphological degradation modifies long-term macroinvertebrate responses to anthropogenic pressures. Less degraded sites displayed several favourable changes, such as 27 % increase in total and 23 % increase in potamal indicator richness, and stabilisation of the assemblages with few functional changes. In contrast, highly degraded sites experienced 9 % reduction in evenness, 235 % increase in proportion of non-native taxa and functional reorganisation, changes congruent with continuous deterioration. While overall water quality at studied sites has improved, consequences of climate change and high degree of hydromorphological degradation limit biotic recovery in multiple-stressor lowland rivers.

River degradation impacts fish assemblages in Kosovo’s Ibër basin

Fish sampling was conducted in 20 sites within the Ibër River and its tributaries in Kosovo (August 2023) collecting 23 taxa (1674 individuals) through a standardized electrofishing protocol. Relevant physico-chemical measurements and anthropogenic pressures were collected and analyzed with respect to the ichthyological data of the surveyed sites. The survey represents a rapid assessment procedure, limitations and prospects of the effort are described. Several attributes of the fish assemblages showed severe degradation, characterized by low population densities, low species diversity, minimal reproductive evidence and the absence of several sentinel species. Multiple anthropogenic pressures have severely modified the fish communities over a wide area; pollution, barriers to movement and hydroelectric dams stand out as important drivers of degradation. Only two fish species showed some level of tolerance to heavily polluted waters (Carassius gibellio, Rutilus rutilus). Low fish population densities were also correlated with various forms of hydromorphological degradation. Only Barbatula barbatula, Phoxinus sp. and Lepomis gibbosus had proportionately higher population densities in areas where impoundments and power-station dam hydropeaking were severe. In this regard, conservation actions are urgently needed.

Assessing the impact of two conventional wastewater treatment plants on small streams with effect-based methods

Sixty percent of discrete surface water bodies in Europe do not meet the requirements for good ecological and chemical status and in Germany, the situation is even worse with over 90% of surface water bodies failing to meet the threshold. In addition to hydromorphological degradation, intensive land use and invasive species, chemical pollution is primarily considered to be responsible for the inadequate ecological status of the water bodies. As a quantitatively important source of micropollutants, wastewater treatment plants (WWTPs) represent an important entry path for chemical stressors. It is therefore important to analyze the effectiveness of the WWTPs in eliminating micropollutants and other chemical stressors to mitigate the negative impacts of the treated wastewater (WW) in aquatic ecosystems. Accordingly, in this study, we evaluated the impacts of two conventional, medium-sized WWTPs on their small receiving water systems in the southwestern region of Hessen in Germany during two sampling campaigns (spring and fall) using effect-based methods (EBM). We hypothesized that due to the insufficient elimination of micropollutants, a broad spectrum of toxic effects would be detected in conventionally treated WW and also in the receiving surface waters downstream the WWTPs. As EBMs a battery of in vitro assays and active biomonitoring using two in vivo assays were applied. The results supported our hypothesis and showed that the untreated WW had a very high baseline toxicity and also high endocrine and mutagenic activities. Conventional WW treatment, consisting of mechanical and biological treatment with nitrification, denitrification and phosphate precipitation, reduced baseline toxicity by more than 90% and endocrine activities by more than 80% in both WWTPs. Despite these high elimination rates, the remaining baseline toxicity, the endocrine, dioxin-like and mutagenic activities of the conventionally treated WW were so high that negative effects on the two receiving waters were to be expected. This was confirmed in the active monitoring with the amphipod Gammarus fossarum and the mudsnail Potamopyrgus antipodarum, as mortality of both species increased downstream of the WWTPs and reproduction in P. antipodarum was also affected. These results indicate that advanced WW treatment is needed to more effectively eliminate chemical stressors to prevent negative impacts of treated WW particularly in small receiving waters.

Direct and indirect effects of salinisation on riverine biota: a case study from river Wipper, Germany

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.

Benthic metabolism and nutrient uptake vary with geomorphology and season in a lowland river

Meandering rivers are characterized by geomorphic units like cut banks, point bars, and thalwegs. These units arise from interactions between hydrological and geomorphological forces. However, the individual contributions of geomorphic units to whole-river metabolism or nutrient processing are unclear because these quantifications are often done at larger spatial scales. We used closed recirculating chambers to measure benthic gross primary production (GPP), respiration (R), N uptake or release, and P uptake or release at bimonthly intervals over 1 y at different geomorphic units in the Mulde River, Germany. We compared GPP, R, and nutrient processing among a cut bank, a point bar, and the thalweg at a natural meander. We also compared the cut bank of this natural meander with a cut bank fixed by riprap at a human-altered meander. In the natural meander, GPP, R, and nutrient processing rates were higher at the point bar than the cut bank or thalweg. These differences are likely related to larger sediment grain sizes that provide a more stable substrate for microbial communities. A strong interaction between geomorphic units and time for GPP and NH4+ fluxes suggested that differences in nutrient processing rates among geomorphic units were restricted to specific times during the year. Specifically, we found that the nutrient processing rates differed among geomorphic units during the summer, but not winter. Furthermore, in June and August 2017, R was 2 to 3× lower at the cut bank stabilized by riprap than at the natural cut bank. Our results demonstrate that rivers are composed of functionally distinct geomorphic units susceptible to human-induced hydromorphological degradation. However, strong interactions between space and time and large within-geomorphic unit variability propose that local drivers influence ecosystem function, suggesting that we need additional research to resolve these drivers at the scales of geomorphic units.

Response of Aquatic Plants to Extreme Alterations in River Morphology

In this study, we aimed to identify the macrophyte pattern and diversity under exposure to substantial hydromorphological degradation in rivers, taking into account the water quality factor. The study was based on 190 small and medium lowland rivers in Poland that had experienced channel alterations. The number of taxa identified (153 species) was consistent with natural/seminatural rivers, and the average species richness for the survey site was 16. Nevertheless, nearly 25% of the survey sites were poor in species for which ten or fewer taxa were noted. The most common species were emergent Phalaris arundinacea; free-floating Lemna minor; heterophyllous Sparganium emersum; filamentous algae Cladophora sp.; and some amphibious species, including Agrostis stolonifera. The surveyed sites represented a wide diversity gradient, from sites poor in species and with low diversity based on relative abundance to highly diverse river sites in less transformed rivers. Our results revealed that macrophyte species were mostly determined by hydromorphological degradation, as well as other distinguished environmental factors, such as water trophy (e.g., Lemna gibba, Bidens tripartita, and Ceratophylum demersum) and channel dimensions (e.g., Nuphar lutea, Sagittaria sagittifolia, and Typha latiflolia).

Analysis of multiple-pressure pattern in rivers and its effects on the structure of macroinvertebrate communities

Natural and anthropogenic factors form stream macroinvertebrate communities depending on their combination, intensity, and spatial pattern. The study aimed to identify macroinvertebrate indicators that respond to land cover, hydromorphology, and wastewater releases individually and to their multiple-pressure pattern. Environmental and macroinvertebrate data from 36 sites were used in the study. Pressure parameters representing hierarchy of their complexity and spatial scale were included in analyses. Correlation analyses were used for evaluation of relationships among pressure characteristics and also pressure–macroinvertebrate relationships. The pressure-based and biological classification of sites was compared and indicator taxa were identified. The arable land in the sub-corridor extending 2–10 km upstream of an investigated site was the main pressure factor influencing the structure of macroinvertebrate communities in the studied streams. The biological effects of small-scale land cover were followed by catchment-scale land cover and hydromorphology. Almost no association of macroinvertebrates with the risk of point source pollution were detected. Classifications based on pressures and community composition corresponded only by the separation of most degraded sites from others. Among the macroinvertebrate indicators characterizing the severe impairment threshold, chironomids and oligochaetes dominated. Different responses of macroinvertebrates to hydromorphological degradation were observed under conditions of high small- and large-scale agricultural pressures (decrease in macroinvertebrate evenness and increase in oligochaete taxa richness, respectively). Linking biological indicators to pressure components and their combinations improves the efficiency of conservation and restoration strategies applied in fluvial ecosystems.

Resource supply and organismal dominance are associated with high secondary production in temperate agricultural streams

Abstract Agricultural land‐use affects the environmental and biological characteristics of stream ecosystems through multiple pathways including nutrient and pesticide contamination, riparian clear‐cutting and hydromorphological degradation. These changes in the abiotic environment can have a direct effect on the productivity of macroinvertebrate communities through environmental filtering and via altered resource conditions encompassing a shift from allochthonous to autochthonous primary production and changes in elemental stoichiometry and food quality. Additionally, macroinvertebrate productivity can be affected indirectly via biological mechanisms, such as changes in species interactions, richness, competition, and predation. We studied the effects of agriculture on structural and functional descriptors of macroinvertebrate communities by assessing environmental characteristics and macroinvertebrate secondary production (MSP), biomass and density in two forested and two agricultural streams and investigated underlying biotic mechanisms. On average, MSP was 1.6–3.6, biomass 2.8–6.2 and density 5–13 times higher in agricultural than in forested streams. This pattern was associated with higher nutrient concentrations, standing crops of riparian herbaceous vegetation, suspended particulate organic carbon, quantity and quality of epilithic biofilms and chlorophyll‐a concentrations in seston and biofilm of the agricultural streams. Species richness and evenness were significantly lower in agricultural than in forested streams. A negative relationship between MSP and species richness and evenness indicated that density compensation and trait dominance were the prevalent mechanisms facilitating higher MSP in agricultural streams. Our findings suggest that the loss of riparian canopy and excess nutrient conditions are the major environmental drivers contributing to homogenization of ecological niches and dominance of highly productive non‐insect generalist species. This study highlights the importance of an ecosystem approach to understanding how complex aggregate stressors affect the regulation of consumer–resource interactions. There is an urgent need to preserve or restore natural riparian vegetation, fostering habitat and resource diversity and limiting nutrient contamination to stream ecosystems. Read the free Plain Language Summary for this article on the Journal blog.

Environmental Flows Assessment for Macroinvertebrates at the River Reach Scale in Different Degrees of Hydromorphological Alteration

One of the most commonly used methods of environmental flows assessment is the approach based on Habitat Suitability Modeling (HSM) and habitat preferences curves (HPCs). Relationships between organisms and physical parameters describing the habitat remain strongly nonlinear and vary depending on several factors. The most common comparisons concern analyses between natural and regulated rivers, where the degree of river channel alterations is undefined and usually refers to all forms of hydromorphological degradation. Additionally, spatial scale and computational range of such a research mostly focus on longer river reaches and statistical analysis of HPC transferability, without detailed estimation of environmental flows values. The main aim of our research was assessment and comparison of environmental flows’ values for macroinvertebrates at two reaches of one river with different degrees of hydromorphological alteration. Two research hypotheses were verified: 1) morphological conditions impact on values of environmental flows and weighted usable area; 2) the mean value of environmental flows at the regulated river reach is higher compared to the natural reach. In order to put the obtained values of environmental flows into a broader context, the analyses of hydrological data and calculation of hydrology-based environmental flows were performed. Additionally, before determination of environmental flows, the transferability of habitat suitability criteria between reaches was verified. The obtained results confirmed research hypotheses, revealing strong dependence of environmental flows values to morphological conditions. Additionally, for both studied sections the lower limits of environmental flows were greater than hydrology-based values.

Do investments in water quality and habitat restoration programs pay off? An analysis of the chemical and biological water quality of a lowland stream in the Zwalm River basin (Belgium)

Early nineties of previous century aquatic biodiversity was low in many West-European countries as a consequence of pollution, hydromorphological degradation and a lack of sound environmental legislation. Since the early 2000s and the implementation of the European Water Framework Directive a big step forward has been made to achieve a good ecological status of surface waters in Europe. However, to date little is known about the response of biological life in small lowland streams on improving conditions for both water quality and habitat diversity. In this study, we assessed possible relationships between chemical water quality and aquatic biodiversity of the Zwalm River basin (Belgium) using a dataset of long-term measurements of the chemical and biological water quality. Specifically, we wanted to know 1) the evolution in chemical and biological water quality over the last three decades and 2) if an improvement in chemical water quality was reflected in the macroinvertebrate and fish diversity. We found that there was indeed a significant improvement in the chemical water quality thanks to the installation of wastewater treatment plants and river basin management plans. Shortly after this improvement in chemical water quality, an increase in the number and abundance of pollution sensitive macroinvertebrate taxa was observed, which was consequently also seen in the biotic index score, based on macroinvertebrates. Despite a limited change in community composition observed for fish, no significant increase in diversity, or in the biotic index score of fish could be found. Currently, a stagnation of the improving water quality is observed. Hydromorphological restoration and free fish migration remain important key factors to improve diversity of macroinvertebrates and fish species. Based on the results of this study, we provide guidelines (for example restoration of spawning beds, decrease diffuse pollution from agriculture) to optimize the investments made in river restoration and to achieve the good ecological status by 2027 as defined by the European Water Framework Directive.

Assessment of the macroinvertebrate community of the Vjosa river through non-destructive DNA metabarcoding of preservative ethanol

Streams and rivers represent hotspots of biodiversity in their natural state. This biodiversity is declining worldwide due to pollution, exploitation and hydromorphological degradation of these systems. One of the last big, natural rivers in Europe is the Vjosa in the Balkan region. The catchment is characterized by natural flow dynamics, resulting in high habitat diversity and turnover, and hosts several sensitive and endemic species (e.g. Isoperla vjosae). DNA metabarcoding represents a promising approach to characterize this biodiversity but methodological drawbacks such as primer bias or incomplete reference databases limit the application, particularly in taxonomically underexplored regions. Here, we assessed stream biodiversity with a focus on macrozoobenthic (MZB) taxa via a non-destructive, voucher-preserving DNA metabarcoding protocol. In this approach, ethanol used for preservation of multi-habitat samples in the field was used as DNA template, allowing to retain the integrity of the original sample and further comparison of molecular and morphological taxa lists. Samples were taken in spring and autumn 2018 at 48 sites allocated over the Vjosa catchment. The preservative ethanol was filtered through 0.43 um nitrocellulose membranes from which DNA was extracted. Subsequently, a 421 bp fragment of the COI gene was amplified with the primer pair BF2/BR2 and Illumina sequenced. After filtering for sequences with similarity to reference entries >85%, 4,123 OTUs were obtained, of which 921 were identified as MZB taxa. Dipterans and ephemeropterans were most abundant, followed by plecopterans. Some taxa (e.g. molluscs) were not identified due to a known primer bias and >7,000 OTUs could not be assigned above 85 % similarity. Using the here presented voucher-preserving approach allowed us to identify the pitfalls of DNA metabarcoding as tool for biodiversity assessment in taxonomically unexplored regions such as the Vjosa catchment. However, the comparison of specimen abundance data and molecular data showed the power of non-destructive fixative metabarcoding for detecting MZB communities with highly increased taxonomic resolution.

Invertebrate functional responses to hydro­morpho­logical degradation in Mediterranean Croatian rivers

Hydromorphological alterations are among the most widespread anthropogenic pressures in rivers andstreams. They comprise channelization, damming, water abstraction etc. resulting in habitats alteration and subsequentchanges in aquatic community composition. Aim of this study was to test to what extent hydromorphologicalalterations influence aquatic macroinvertebrates as one of the best bioindicators to anthropogenic pressures. Our researchtook place in the Mediterranean part of Croatia. We sampled 40 sites which include perennial rivers varyingin catchment size and altitude with similar environmental conditions along river reaches. Large rivers were omittedfrom this research because of their different functional group representation to the rivers covered used in this study.We used the functional approach to hydromorphological alteration by testing the following traits: reproductionstrategy, functional feeding group and microhabitat (substrate type) preference. From the feeding standpoint shredderswere the most affected group by hydrological alteration. Moreover, taxa confined to a specific relatively lesscommon habitat e.g. phytal, pelal and lithal were the most negatively affected by morphological degradation due,mostly, to it leading to a homogenization of the substrate. Finally, the smaller and faster reproductive taxa weremore resilient to hydromorphological degradation. We found that the functional approach to bioassessment complementsthe common biocenotical approach, at the same time it may provide sound ecological knowledge.

Aquatic Worm Assemblages along the Danube: A Homogenization Warning

In this study, we analyzed the impacts of different environmental conditions on aquatic worm communities along the Danube River, based on two longitudinal surveys, the Joint Danube Surveys 2 and 3 (JDS; 2007 and 2013). We identified the most important environmental factors (among analyzed groups) that shape worm communities: hydromorphlogical alterations, flow velocity and substrate (HYMO group), dissolved oxygen, nitrates and nitrites (physico-chemical parameters), zinc and nickel (metals), monobutyltin cation, benzo(b) fluoranthene and benzo(k)fluoranthene, polychlorinated biphenyls PCB 77 and PCB 118 (selected chemical determinants—organotin compounds, Polycyclic aromatic hydrocarbons—PAHs and PCBs). A homogenization of species composition of Oligochaeta assemblages along the Danube was confirmed. As one of main factors related to biotic homogenization, hydromorphological alterations represented by similar changes in flow velocity and substrates along Danube’s course could be singled out. Our results indicate that Oligochaeta could be used for the identification of the level of hydromorphological degradation in large rivers (homogenization), rather than for stressors classified as nutrient and organic pollutants. Our results provide additional evidence in risk assessment of the environment, contributing in water management and monitoring of the ecological status as proposed by the Water Framework Directive.

Hierarchical response of littoral macroinvertebrates to altered hydromorphology and eutrophication

The composition of littoral macroinvertebrate communities in lakes is governed by multiple natural and anthropogenic environmental influences interacting at different spatial scales. Since ecological assessment methods using littoral macroinvertebrates should respond specifically to a single stressor, knowledge on the unique effects of a given stressor is necessary. To effectively disentangle the effects of hydromorphology and trophic state requires analysing macroinvertebrate communities at lake sites with the full range of both stressors. We used a dataset of 98 lakes encompassing the entire gradient of geographical locations, lake types, hydromorphological degradation and trophic states in Central European lakes. We studied the unique and joint effects of hydromorphology and trophic state on macroinvertebrate richness, community composition and the Littoral Invertebrate Multimetric Index based on Composite Sampling (LIMCO). Variation partitioning analyses were conducted to test the importance of hydromorphology relative to trophic state across and within hydromorphological states (natural shorelines, hard and soft shore modifications) and trophic states (oligotrophic to hypertrophic states). At natural, hard and soft modification sites, hydromorphology explained 10, 16 and 19%, respectively, of the average unique variation of diversity, community composition and the LIMCO index, whereas trophic state explained on average 2, 5 and 5%, respectively. Similarly, in low, medium and high trophic state lakes, hydromorphology explained 10, 15 and 7%, respectively, of the average unique variation of diversity, community composition and the LIMCO index, whereas trophic state explained on average 0.3, 3 and 6%, respectively. Our results demonstrate that littoral hydromorphology was a more important driver of macroinvertebrate diversity, community composition and LIMCO than trophic state across hydromorphological states and trophic states. This indicates that multiple stressors in lakes act hierarchically on littoral macroinvertebrate communities and that the hydromorphological degradation of littoral zones is the primary driver for altered communities.

European aquatic ecological assessment methods: A critical review of their sensitivity to key pressures

The European Union has embarked on a policy which aims to achieve good ecological status in all surface waters (i.e. rivers, lakes, transitional and coastal waters). In theory, ecological status assessment methods should address the effects of all relevant human pressures. In this study, we analyze the degree to which methods European countries use to assess ecological status tackle various pressures affecting European waters.Nutrient pollution is by far the best-covered pressure for all four water categories. Out of total of 423 assessment methods, 370 assess eutrophication and pressure-specific relationships have been demonstrated for 212 of these. “General degradation” is addressed by 238 methods, mostly validated by relationships to combined pressure indices.Other major pressures have received significantly less effort: hydromorphological degradation is assessed by 160 methods and pressure-specific relationships have been demonstrated for just 40 of these. Hydromorphological pressures are addressed (at least by one BQE) only by 25% countries for coastal waters and 70–80% for lakes and transitional waters. Specific diagnostic tools (i.e. single-pressure relationships) for hydromorphology have only been developed by a few countries: only 20% countries have such methods for lakes, coastal and transitional waters and less than half for rivers. Toxic contamination is addressed by 90 methods; however, pressure-specific relationships have been demonstrated for just eight of these. Only two countries have demonstrated pressure-specific acidification methods for rivers, and three for lakes.In summary, methods currently in use mostly address eutrophication and/or general degradation, but there is not much evidence that they reliably pick up the effects of other significant pressures such as hydromorphology or toxic contamination. Therefore, we recommend that countries re-examine: (1) those pressures which affect different water categories in the country; (2) relevant assessment methods to tackle those pressures; (3) whether pressure-response relationships have been developed for each of these.

Linking Hydromorphological Degradation with Environmental Status of Riparian Ecosystems: A Case Study in the Stropnice River Basin, Czech Republic

Recently, increasing attention has been paid to the anthropogenic degradation of the riverbed and its relationship to the ecological status of the adjacent river landscape. The key objective of this research was to determine the extent of the disturbance of the selected small streams and their riparian zone in a study area located in a forest and forest-agricultural landscape in the Czech Republic. The next step was to analyze the mutual relationships between the ecological status of the riparian vegetation and the hydromorphological status of the riverbed. The main working hypothesis considered the good hydromorphological status of the river as reflected in the favorable environmental status of the surrounding riparian habitats and vice versa. It was found in more than 90% of the total length of studied watercourses that the character of linkages between channel morphology and the ecological status of riparian vegetation is directly influenced by anthropogenic activities. An interesting finding is that the degraded streams in lowland sites are often encompassed by natural or close-to-natural habitats. On the contrary, the natural status of the riverbed was found in a significantly forested headwater area, but the riparian habitats did not reach even a close-to-natural status. This paper contributes to clarifying the significance of human impact on the river morphology, reflected in the reduction of connectivity between the terrestrial and fluvial parts of the river landscape. It helps to explore the most important disturbances affecting mutual interactions between the river and the riparian habitats.

Organic matter resources fuelling food webs in a human-modified lowland river: importance of habitat and season

The role of hydromorphological degradation and temporal variation for food webs in human-modified rivers is still not fully evaluated. We tested the hypothesis that man-made engineering structures alter macroinvertebrate resource use in the Elbe River (Germany) in relation to seasonal variation. Stable isotopes (δ13C, δ15N) and mixing models revealed that dietary contributions of benthic organic matter (BOM) and phytoplankton were driven by engineering structure. Contributions of biofilm were driven by season, while contributions of terrestrial particulate organic matter (t-POM) were driven by both engineering structure and season. Contributions of t-POM were larger than those of phytoplankton in spring and summer, but not in autumn, which adds to the debate about the sources of organic matter fuelling riverine benthic food webs. Resource availability was not systematically related to resource use, indicating that factors other than resource limitation were responsible for the observed results. By demonstrating that human alterations determine consumer resource use independently from resource availability, our study links hydromorphological modifications to fluxes of matter in riverine food webs. Future studies should quantify organic matter fluxes from various autochthonous and allochthonous pathways in human-modified and natural rivers to allow for a robust synthesis of how hydromorphological modifications alter benthic food webs.

Assessing the context and ecological effects of river restoration – A meta-analysis

A large number of qualitative and semi-quantitative analyses have been performed to summarize the effects of river restoration, while systematic and quantitative assessments of the literature on river restoration are still rare. To fill this gap, we conducted a meta-analysis of 55 peer-reviewed publications describing assessments of river restoration with indicators of biodiversity or ecosystem services (ES). The core of the study was to identify: (i) the overall ecological effects of river restoration; (ii) the restoration effects on different organism groups and types of ES, including the relationship between biodiversity and ES; and (iii) the influence of the restoration context, that is degradation types, restoration techniques and restoration age. The meta-analysis revealed that river restoration significantly enhanced ecological conditions. However, restoration effects varied among organism groups and ES, with plant abundance and regulating services showing higher responses to restoration. Moreover, a trend of a positive correlation was found between biodiversity and ES. Restoration activities showed most significant effects in the context of hydromorphological degradation or land-use changes, and channel reconfiguration and riparian buffer establishment had a particularly favourable outcomes. Only regulating services showed a positive correlation with restoration age; thus, long-time monitoring is necessary to fully understand restoration effects. Overall, our meta-analysis provides evidence-based advice for future projects and assessment of river restoration.

The effects of recreational and commercial navigation on fish assemblages in large rivers

Recreational and commercial navigation is omnipresent, rendering European large rivers highways for cargo vessels, passenger ships and sport boats. Any types of motorized vessels create waves and drawdown eroding shallow shore areas. Consequently, inland navigation alters the living environment of fish with specific habitat requirements on nursing, hatching and spawning along shorelines. We assess the influence of recreational (sport boats) and commercial navigation (passenger ships, cargo vessels) on fish assemblages. Seven fish population metrics (FPM) were analyzed for 396 fish samplings at 88 sites in six large rivers characterized by seven different estimates of navigation intensity to identify FPM sensitive to inland navigation. Navigation intensity was characterized by frequency, total freight transported, total carrying capacity, degree of capacity utilization and by numbers of empty running vessels, aiming to approximate whether frequency, freight or draft of cargo vessels matter most. Densities of lithophilic fish were most sensitive to frequencies of sport boats, passenger ships and cargo vessels and declined as navigation traffic increased. Densities of rheophilic fish declined likewise but were less sensitive than lithophils. Frequency, freight and carrying capacity of cargo vessels had comparable effects on FPM and are equally useful in addition to frequency of sport boats and passenger ships to assess the impacts of recreational and commercial navigation on fish assemblages. Lower species richness indicated a specific influence of vessel draft on fish diversity. Our study shows that both recreational and commercial navigation impair fish assemblages in navigable rivers. Operation-related navigation impacts act on top of river regulation and engineering works to maintain fairways in the main channel. Therefore, impacts from recreational and commercial navigation must be especially addressed in addition to mitigating impacts from river regulation and hydromorphological degradation to achieve environmental objectives such as species conservation, ecological improvements and river rehabilitation.

Integrating river hydromorphology and water quality into ecological status modelling by artificial neural networks

The aim of the study was to develop predictive models of the ecological status of rivers by using artificial neural networks. The relationships between five macrophyte indices and the combined impact of water pollution as well as hydromorphological degradation were examined. The dataset consisted of hydromorphologically modified rivers representing a wide water quality gradient. Three ecological status indices, namely the Macrophyte Index for Rivers (MIR), the Macrophyte Biological Index for Rivers (IBMR) and the River Macrophyte Nutrient Index (RMNI), were tested. Moreover two diversity indices, species richness (N) and the Simpson index (D) were tested. Physico-chemical parameters reflecting both water quality and hydromorphological status were utilized as explanatory variables for the artificial neural networks. The best modelling quality in terms of high values of coefficients of determination and low values of the normalized root mean square error was obtained for the RMNI and the MIR networks. The networks constructed for IBMR, species richness and the Simpson index showed a lower degree of fit. In all cases, modelling quality improved by adding two hydromorphological indices to the pool of explanatory variables. The significant effect of these indices in the models was confirmed by sensitivity analysis. The research showed that ecological assessment of rivers based on macrophyte metrics does not only reflect the water quality but also the hydromorphological status.