Dams modify geomorphology, water quantity, quality and timing of stream flows affecting ecosystem functioning and aquatic biota. In this study, we addressed the structural and functional ...macroinvertebrate community alterations in different instream mesohabitats of two Portuguese rivers impaired by dams. We sampled macroinvertebrates in riffles, runs and pools of river sites downstream of the dams (i.e. regulated; n = 24) and in sites without the influence of the dams (i.e. unregulated; n = 7), assessing a total of 64 mesohabitats, following late spring-early summer regular flows. We found a distinct taxonomic structure and trait composition of macroinvertebrate assemblages between regulated and unregulated flow sites, and also between mesohabitats in which the differences were more evident. When analysing each mesohabitat individually, the effect of flow regulation was detected only in run-type mesohabitats for both taxonomic and trait composition, leading us to infer that a selective macroinvertebrate assessment on run mesohabitats would be a valuable contribution to detect regulated flow effects on ecosystems impaired by dams. Additionally, there is evidence that respiration and locomotion traits could be effective tools to identify damming flow alterations. This study supports that the quality assessments of rivers impacted by dams could benefit from a sampling approach focused on run mesohabitats and the detection of some key traits, which would improve assessment accuracy.
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•Macroinvertebrate community structure and function were studied in two dammed rivers.•Bioassesment was done in pools, runs and riffles in regulated and nonregulated sites.•Flow regulation and mesohabitats revealed distinct community structure and traits.•Dams have a detectable impact on invertebrate assemblages in run-type mesohabitats.•Sampling run mesohabitats would improve bioassessment accuracy in regulated rivers.
In the Danube River Basin multiple pressures affect the river system as a consequence of river engineering works, altering both the river hydrodynamics and morphodynamics. The main objective of this ...paper is to identify the effects of hydropower development, flood protection and engineering works for navigation on the Danube and to examine specific impacts of these developments on sediment transport and river morphology. Whereas impoundments are characterised by deposition and an excess of sediment with remobilisation of fine sediments during severe floods, the remaining five free flowing sections of the Danube are experiencing river bed erosion of the order of several centimetres per year. Besides the effect of interruption of the sediment continuum, river bed degradation is caused by an increase in the sediment transport capacity following an increase in slope, a reduction of river bed width due to canalisation, prohibition of bank erosion by riprap or regressive erosion following base level lowering by flood protection measures and sediment dredging. As a consequence, the groundwater table is lowered, side-arms are disconnected, instream structures are lost and habitat quality deteriorates affecting the ecological status of valuable floodplains. The lack of sediments, together with cutting off meanders, leads also to erosion of the bed of main arms in the Danube Delta and coastal erosion. This paper details the causes and effects of river engineering measures and hydromorphological changes for the Danube. It highlights the importance of adopting a basin-wide holistic approach to river management and demonstrates that past management in the basin has been characterised by a lack of integration. To-date insufficient attention has been paid to the wide-ranging impacts of river engineering works throughout the basin: from the basin headwaters to the Danube Delta, on the Black Sea coast. This highlights the importance of new initiatives that seek to advance knowledge exchange and knowledge transfer within the basin to reach the goal of integrated basin management.
(a) Suspended load transport within the Danube River, (b) gravel fractions of the bed load transport within the Danube River. Display omitted
•Hydropower, navigation, and flood protection resulted in a widely engineered Danube River•River engineering affects significantly hydrodynamics and river morphodynamics (hydromorphology) of the Danube River.•Sediment surplus exists in impoundments and lack of sediments in free flowing sections.•River bed erosion causes technical and ecological deficits.•An improved river basin management needs an advanced knowledge exchange and transfer between environmental researchers, key stakeholders and managers.
We reviewed 219 papers and built an inventory of 532 items of ecological evidence on multiple stressor impacts in rivers, lakes, transitional and coastal waters, as well as groundwaters. Our review ...revealed that, despite the existence of a huge conceptual knowledge base in aquatic ecology, few studies actually provide quantitative evidence on multi-stress effects. Nutrient stress was involved in 71% to 98% of multi-stress situations in the three types of surface water environments, and in 42% of those in groundwaters. However, their impact manifested differently along the groundwater–river–lake–transitional–coastal continuum, mainly determined by the different hydro-morphological features of these ecosystems. The reviewed papers addressed two-stressor combinations most frequently (42%), corresponding with the actual status-quo of pressures acting on European surface waters as reported by the Member States in the WISE WFD Database (EEA, 2015). Across all biological groups analysed, higher explanatory power of the stress-effect models was discernible for lakes under multi-stressor compared to single stressor conditions, but generally lower for coastal and transitional waters. Across all aquatic environments, the explanatory power of stress-effect models for fish increased when multi-stressor conditions were taken into account in the analysis, qualifying this organism group as a useful indicator of multi-stress effects. In contrast, the explanatory power of models using benthic flora decreased under conditions of multiple stress.
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•We reviewed 219 papers quantifying effects of multiple stresses on aquatic systems.•Nutrient stress occurred in 71% to 98% of multi-stress situations in surface waters.•Hydromorphological stress alters the nutrient stress sensitivity of water bodies.•R2 of stress-effect models using fish increased under multi-stress conditions.•R2 of benthic flora dropped with multiple stressors involved.
This paper presents an empirical study that uses the movement of RFID tracers to investigate the impacts of low-head dams on solid transport dynamics in coarse-bedded streams. Here we report on the ...influence of two structures located in Ireland's South-East, both of which indicate that particles greater than the reach D90 can be carried through and over low-head dams. This observation suggests that both structures may have reached a state of ‘transient storage’ as hypothesized by previous research. However, when the data were reinterpreted as fractional transport rates using a novel application of existing empirical relations, we observed patterns consistent with supply-limited conditions downstream. Expanding on existing conceptual models and mechanisms, we illustrate how a system may continue to exhibit supply-limited conditions downstream without the need for a net attenuation of sediment to occur indefinitely. We propose that once a transient storage capacity has been reached, the system then enters a state of dynamic disconnectivity where the long-term average sediment flux equals that under reference conditions, but now with the amplitude and wavelength of these sediment fluctuations having increased. We hypothesize that the time-lag associated with the reduced frequency of events competent enough to move bedload over the structure accounts for the time necessary to complete the ‘fill’ phase of the transient storage dynamic; a process that will continue until both the fill and flow thresholds are again met to allow the system to reenter the ‘scour’ phase. This model reconciles how a system may exhibit a sediment deficit for time intervals longer than those experienced under reference conditions. As water and sediment are the drivers of channel morphology and associated habitat units, the impact a structure has on a channel's sediment regime should therefore form part of any assessment regarding the prioritization of barriers for removal or remediation.
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•RFID tracer data was analysed at two sites to assess sediment conveyance.•Tracer fractions >D90 can be carried through and over low-head dams.•Fractional transport rates and particle size distributions also suggest supply-limited conditions persist downstream.•We propose a conceptual model and mechanism to explain how low-head dams may affect downstream conveyance indefinitely.
The majority of central European streams are in poor ecological condition. Pesticide inputs from terrestrial habitats present a key threat to sensitive insects in streams. Both standardized stream ...monitoring data and societal support are needed to conserve and restore freshwater habitats. Citizen science (CS) offers potential to complement international freshwater monitoring while it is often viewed critically due to concerns about data accuracy. Here, we developed a CS program based on the Water Framework Directive that enables citizen scientists to provide data on stream hydromorphology, physicochemical status and benthic macroinvertebrates to apply the trait-based bio-indicator SPEARpesticides for pesticide exposure. We compared CS monitoring data with professional data across 28 central German stream sites and could show that both CS and professional monitoring identified a similar average proportion of pesticide-sensitive macroinvertebrate taxa per stream site (20 %). CS data were highly correlated to the professional data for both stream hydromorphology and SPEARpesticides (r = 0.72 and 0.76). To assess the extent to which CS macroinvertebrate data can indicate pesticide exposure, we tested the relationship of CS generated SPEARpesticides values and measured pesticide concentrations at 21 stream sites, and found a fair correlation similar to professional results. We conclude that given appropriate training and support, citizen scientists can generate valid data on the ecological status and pesticide contamination of streams. By complementing official monitoring, data from well-managed CS programs can advance freshwater science and enhance the implementation of freshwater conservation goals.
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•Analysis of citizen science data accuracy for bio-indicator based pesticide monitoring.•Macroinvertebrate data were used to derive the biological indicator SPEARpesticides.•Citizen science and professional data for SPEAR and hydromorphology agreed well.•Citizen science SPEAR values and measured pesticide concentrations correlated well.•Citizen science stream data is suitable to complement official monitoring programs.
River recovery is defined as the trajectory of change that a reach takes towards an improved condition. In geomorphic terms, this includes improvement in both the physical structure and function of a ...river. While there are numerous case studies that track river recovery at the reach scale, there is a lack of work that characterises the patterns of recovery across diverse river types. Here we use ergodic reasoning to quantitatively analyse changes in the assemblage of geomorphic units (GUs) that occur for rivers that are known to be at different stages of geomorphic recovery. We aim to understand how the physical structure of different river types change as recovery occurs. In this study, we adopted a semi-automated method to map GUs using Open Access LiDAR and Sentinel remote sensing imagery. We analyse the assemblage of GUs for 78 river sections that span eight river styles, three valley settings and two bed material textures – sand and gravel. We find that the patterns of river recovery vary for different river styles. Confined and laterally unconfined rivers exhibit linear and non-linear increases in richness, abundance, evenness, and diversity of GUs during recovery. Partly confined rivers show more variable trends for these measures, and channelised fill rivers show decreased diversity. During recovery, sand bed rivers are more susceptible to adjustment than gravel bed rivers. The presence of benches and islands indicates that recovery is underway across most river types. A statistically significant increase in abundance and area of benches and pools, and a decrease in abundance and area of floodplain steps can also be used to indicate that recovery is underway. Additionally, in early stages of recovery, adjustments take place at the unit level. Then as recovery progresses, adjustments tend to occur at the sub-unit level as evidenced by bank-attached bars becoming more compound in structure. Determining the indicator GUs and changes in assemblages to ‘look out for’ while in the field or on remote sensing images can aid the detection and analysis of river recovery, providing invaluable insight for process-based and recovery-enhancement approaches to river management.
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•Patterns of geomorphic recovery vary across different river styles.•Different mixes of metrics can be used to track geomorphic river recovery.•Recovery is not always linear and morphology does not always become more complex.•Changes in assemblages of units and some individual units can detect recovery.
The time‐based perspective of river‐floodplain processes plays a key role in understanding its evolution until its present situation and in guiding process‐based river restoration. However, despite ...the wide range of approaches to eco‐hydromorphological characterization, the analysis of fluvial dynamics and their drivers under a temporal perspective requires more investigation. For the present study, a historical analysis of river‐floodplain dynamics has been undertaken in a study section of the Duero River upstream of Zamora (Spain), within the framework of the DRAINAGE project for the integral management of flood risk. Aerial imagery for the period 1945–2017 was used to create habitat maps that were subsequently intersected to identify trajectories of habitat change (anthropization, changeless, progression, and regression). In addition, a Natural Trajectory Index (NTI) and an Anthropization Ratio (AR) were proposed as a way to assess the balance of natural trajectories (progression vs. regression) and the weight of anthropization versus the sum of natural trajectories, respectively. Results show a system with a high degree of stability and anthropization, and a current imbalance of natural trajectories with a predominance of progression in the ecological succession. Rejuvenation processes have to be reactivated through nature‐based solutions, allowing for more sustainable water management practices. This perspective aims at promoting a more resilient system. The diachronic analysis presented is widely exportable to other river‐floodplain systems as a basis for process‐based restoration practice.
In the context of the requirements of the Water Framework Directive to reinstate a river’s good ecological status after being severely altered by human pressures, river restoration became a major ...topic in the last decade in the EU and the number of projects implemented especially on hydromorphology increased. However, it is still uncertain whether these restoration measures have positive cumulative effects, i.e. hydromorphological effectiveness, when compared to reference conditions and expectations of stakeholders, because of inconsistent methodologies. Therefore, the goal of our project is to develop a methodology to evaluate the hydromorphological effectiveness of river restoration, based on standardised indicators, at various spatial scales, appropriate for implemented measures and for restored river types, weighting the expectations of actors and adapted to case studies in Romania. This methodological protocol could become also a useful tool to create a strategy and make decisions in the practice of river restoration. Moreover, this kind of analysis could contribute to clarifying the issue of standards in environmental projects.
Worldwide, aquatic biodiversity is severely threatened as a result of anthropogenic pressures such as pollution, habitat destruction and climate change. Widescale legislation resulted in reduced ...nutrient- and pesticide loads, and restoration measures allowed modest recovery of freshwater biodiversity. However, from 2010 onwards, recovery in the otherwise unrestored aquatic habitats stagnated. The aim of the present study was therefore to reveal long-term trends in aquatic biodiversity in an anthropogenic landscape and to explain the observed patterns. To this end, over 40 years of biomonitoring data of the indicative taxa group Trichoptera (caddisflies), with an exceptionally high spatial and temporal resolution, was employed. Periods of recovery, stagnation, and decline were delineated using linear and non-linear modelling approaches. Subsequently, species were grouped based on abundance patterns over time and this grouping was used to ascertain species-specific responses to anthropogenic stressors using a trait-based approach. Richness and abundance of all Trichoptera jointly, as well as of the five most abundant and the remaining 136 species, significantly increased from 1980 to significant breakpoints from 2010 onwards, after which these metrics, except the abundances of the 5 most abundant, declined significantly. Trend-based species groupings were not significantly explained by biological traits or ecological preferences. However, Trichoptera species increasing in abundance were less sensitive to climate change and poor water quality, or concerned sensitive species which benefited from restoration measures. Species with stable or declining abundances showed higher sensitivity to climate change. The Trichoptera declining in abundance indicated that conditions in non-protected or restored habitats did not improve due to climate change on top of the other anthropogenic pressures. These observations reinforce the need for increased efforts to improve the only moderately restored water- and habitat quality in anthropogenic landscapes to halt further aquatic ecosystem degradation and to turn biodiversity losses again into recoveries.
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•Species-specific analyses of Trichoptera over time in an anthropogenic environment.•Richness and abundance increased significantly, but declined from 2010 onwards.•Species grouping revealed positive effects of local restoration measures.•Species-specific sensitivities revealed negative effects of climate change.•Increased efforts to improve water and habitat quality are urgently required.
Non‐native species introductions affect freshwater communities by changing community compositions, functional roles, trait occurrences and ecological niche spaces. Reconstructing such changes over ...long periods is difficult due to limited data availability. We collected information spanning 215 years on fish and selected macroinvertebrate groups (Mollusca and Crustacea) in the inner‐Florentine stretch of the Arno River (Italy) and associated water grid, to investigate temporal changes. We identified an almost complete turnover from native to non‐native fish (1800: 92% native; 2015: 94% non‐native species) and macroinvertebrate species (1800: 100% native; 2015: 70% non‐native species). Non‐native fish species were observed ~50 years earlier compared to macroinvertebrate species, indicating phased invasion processes. In contrast, α‐diversity of both communities increased significantly following a linear pattern. Separate analyses of changes in α‐diversities for native and non‐native species of both fish and macroinvertebrates were nonlinear. Functional richness and divergence of fish and macroinvertebrate communities decreased non‐significantly, as the loss of native species was compensated by non‐native species. Introductions of non‐native fish and macroinvertebrate species occurred outside the niche space of native species. Native and non‐native fish species exhibited greater overlap in niche space over time (62%–68%) and non‐native species eventually replaced native species. Native and non‐native macroinvertebrate niches overlapped to a lesser extent (15%–30%), with non‐natives occupying mostly unoccupied niche space. These temporal changes in niche spaces of both biotic groups are a direct response to the observed changes in α‐diversity and species turnover. These changes are potentially driven by deteriorations in hydromorphology as indicated by alterations in trait modalities. Additionally, we identified that angling played a considerable role for fish introductions. Our results support previous findings that the community turnover from native to non‐native species can be facilitated by, for example, deteriorating environmental conditions and that variations in communities are multifaceted requiring more indicators than single metrics.
We investigated local temporal changes in fish and macroinvertebrates over a period of 215 years. We identified an almost complete turnover from native to non‐native species. Analyses of changes in α‐diversities for native and non‐native fish and macroinvertebrates were nonlinear. Native and non‐native fish species exhibited greater overlap in niche space over time. Temporal changes in niche spaces of both biotic groups are a direct response to the observed changes in α‐diversity and species turnover, potentially driven by deteriorations in hydromorphology. Our results indicate that the turnover from native to non‐native species is multifaceted, requiring more indicators than single metrics.
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