Habitat loss as the main cause of the slow recovery of fish faunas of regulated large rivers in Europe: the transversal floodplain gradient

In large European rivers the chemical water quality has improved markedly in recent decades, yet the recovery of the fish fauna is not proceeding accordingly. Important causes are the loss of habitats in the main river channels and their floodplains, and the diminished hydrological connectivity between them. In this study we investigate how river regulation has affected fish community structure in floodplain waterbodies of the rivers Rhône (France), Danube (Austria), Rhine and Meuse (The Netherlands). A typology of natural and man‐made aquatic habitats was constructed based on geomorphology, inundation frequency and ecological connectivity, along the transversal river–floodplain gradient, i.e. perpendicular to the main stream of the river. Fish species were classified in ecological guilds based on their flow preference, reproduction ecology and diet, and their status on national red lists was used to analyse the present state of the guilds and habitats. Ecological fish guilds appear to be good indicators of ecological integrity and functioning of river–floodplain systems. A transversal successional gradient in fish community structure that bears some resemblance to the gradient found in natural rivers can still be discerned in heavily regulated rivers. It resembles the longitudinal river gradient; even some predictions of the River Continuum Concept are applicable. Overall, richness and diversity of species and ecological guilds decrease with decreasing hydrological connectivity of floodplain waterbodies. Anthropogenic disturbances have affected fish species unevenly: guilds of specialized species that are highly adapted to specifically riverine conditions have declined far more than generalist species. Fish habitats in the main and secondary channels have suffered most from regulation and contain the highest percentage of threatened species. Rheophilic fishes have become rare because their lotic reproductive habitats are severely degraded, fragmented, absent or unreachable. Limnophilic fishes have become rare too, mainly as a result of eutrophication. Eurytopic fishes have become dominant everywhere. Copyright © 2003 John Wiley & Sons, Ltd.     

Vegetation structure and dynamics of a floodplain wetland along a subtropical regulated river

This paper documents vegetation changes in a floodplain area lying next to a newly constructed reservoir on the River Yamuna (near Delhi), about a kilometre downstream of an older, silted‐up reservoir. The study site was a rectangular depression bounded by dykes on three sides and agricultural fields on the fourth. The composition and abundance of species in the plant community were observed over a ten year period (1986–1996) and changes in water level both at the study site and in the reservoir were followed. Site hydrology was governed by water level changes caused by reservoir operation with effect from 1990, when it was first filled to capacity and water began to seep through the dyke. The study area experienced increasing depth, duration and frequency of flooding. Species richness peaked in 1992, and the plant community developed four distinct zones closely associated with the hydrological gradient. Patchiness also increased though Typha angustata patches merged over time to form a continuous expanse. The microtopography of the study site, and hydrological and plant‐induced changes were largely responsible for community changes. Dyke compaction over time resulted in cessation of seepage and the study site gradually dried up by 1998, with a consequent loss of plant species. The study concludes that the hydrological regime, rather than physical connectivity with the river, may play the dominant role in developing and maintaining plant community structure in floodplain wetlands. Copyright © 2005 John Wiley & Sons, Ltd.     

SHORT‐TERM IMPACTS OF LATERAL HYDROLOGICAL CONNECTIVITY RESTORATION ON AQUATIC MACROINVERTEBRATES

In floodplain ecosystems, the lateral hydrological connectivity between the main river channel and the secondary channels plays a major role in shaping both the habitat conditions and the macroinvertebrate diversity. Among other threats, human activities tend to reduce the lateral connectivity, which increases floodplain terrestrialization and induces a loss of aquatic biodiversity. Consequently, the restoration of lateral connectivity is of growing concern. We studied four secondary channels of the Rhône floodplain that were subjected either to no restoration or to three different restoration measures (river flow increase only, flow increase plus dredging and flow increase plus reconnection to the river). Macroinvertebrate and environmental data were analysed one year before and during a period of five years after restoration. We expected a progressive increase of lateral connectivity according to the type of restoration. Changes in macroinvertebrate assemblages were predicted to be towards more rheophilic communities and proportionally related to the changes in lateral connectivity. In the reconnected channel, lateral connectivity increased and remained high five years after restoration. In the dredged channel, the immediate increase of the lateral connectivity metric induced by sediment removal was followed by a rapid decrease. In the unrestored channel and the channel only influenced by flow increase, the metric remained constant in time. The macroinvertebrate composition and the rarefied EPT richness changes were proportionally related to the changes in lateral connectivity. Alien species richness and densities increased progressively in all channels after restoration. Our results showed that modifications of the lateral connectivity lead to predictable changes in macroinvertebrate diversity. Synergistic interactions between restoration and longer‐term changes (e.g. climatic change, invasion of alien species) encourage long‐term monitoring to assess the durability and trends of restoration measures. Copyright © 2012 John Wiley & Sons, Ltd.     

Lateral distribution of fishes in the main‐channel trough of a large floodplain river: Implications for restoration

Major river channels have been extensively altered worldwide. The development of restoration strategies for those alterations requires fundamental information, including the use of large deep channels by fishes. We trawled within parallel paths distributed across the width of the main‐channel trough of the Mississippi River to identify how the lateral distribution of fishes responds to variations in flow ranging from 50% to 200% of the annual median, water temperature ranging from 9 to 29°C and commercial shipping traffic ranging from 0 to 9 vessels per day. Among the species we encountered, only shovelnose sturgeon (Scaphirhynchus platorynchus) were persistent channel residents that remained concentrated along the main‐channel centreline regardless of flow, temperature and traffic. Other persistent residents showed no distinct pattern in lateral distribution, concentrated along the deep channel margins or varied in lateral distribution with flow. Surprisingly, large adult bluegill (Lepomis macrochirus), which are conventionally viewed as limnophils, were the second‐most abundant species in our samples and became increasingly abundant within the deep channel trough as flow decreased below the annual median. Clearly, those fishes exploit resources contained in the main channel and are, therefore, better viewed as opportunistic limno‐rheophils. Our results imply the existence of poorly understood food resources in the main channel. We conjecture that re‐creation of free‐flowing secondary channels and features that increase the production and transport of invertebrates in channels can help mitigate adverse effects of channel alteration and commercial shipping. Published in 2009 by John Wiley & Sons, Ltd.     

The impact of river regulation on the lotic macroinvertebrate communities of the lower rhǒne,France

Channelization and the construction of 12 dams on the Lower Rhǒne during the last 30 years increased the morphological and biological effects of 19th century embanking along the river. These trends were demonstrated on the main channel, i.e. the central axis of the alluvial floodplain, by the synchronic study of the benthic macrofauna of six lowhead hydroelectric impoundments. The variability of the most significant taxa was investigated by an ANOVA, and the data-samples matrix was processed with a Principal Components Analysis in order to analyse the communities organization and their significance. The benthic macroinvertebrate communities of the channel include some lentic potamic species topographically related with this kind of river (metapotamon), as well as eurytopic and pollution-tolerant species. Thus, the Lower Rhǒne has become a slow potamic river, in the same way as other large mid-European rivers. Flow velocity appears to be the most important structural factor in a large regulated river. The homogeneity of the macroinvertebrate communities and the absence of longitudinal typology show that, in a very intensely regulated river, the River Continuum Concept is too theoretical, and the disturbing effects of dams on longitudinal changes and species response to regulation must be taken into account.     

Model‐based reconstruction of the succession dynamics of a large river floodplain

Most large rivers in Europe and North America suffered flow regulations and channelization in the 19th and 20th century. To study the effects of the altered site conditions on the development of floodplain vegetation and create a benchmark map for their restoration, we calibrated and applied a dynamic floodplain vegetation model that accounts for the processes recruitment as well as morphodynamic disturbance and physiologic stress on vegetation to reconstruct the succession dynamics of the floodplain vegetation of a segment of the Rhine River from shortly after it was channelized (1872) until today (2016). The model calibration was based on historical maps and hydrologic data. Our simulation demonstrated a steady, one‐way progression of the vegetation communities towards mature phases without regression to younger stages. It was possible to attribute this development to a lack of morphodynamic disturbances strong enough to reset succession and to identify physiological stress caused by long inundations periods as the most relevant controlling factor of succession. The resulting vegetation distribution (2016) can be considered an estimation of the potential natural vegetation (PNV) under altered site conditions. The good agreement of the model results with an expert‐based PNV map showed that our approach is a good alternative to create benchmark maps for floodplain conservation and restoration projects. From a research and practitioners' viewpoint, it has the big advantage over the traditional approach that it allows to analyse different points in time as well as to be comprehensive and reproducible.     

Effects of dykes on plant species composition in a large lowland river floodplain

The effects of floodplain fragmentation by dykes on grassland vegetation were evaluated through field studies along the Middle Elbe River (federal states Saxony‐Anhalt and Brandenburg, Germany). Plant species composition was examined in 206 sites between 1996 and 1998 in the entire floodplain, which can be divided into the floodplain types ‘recent floodplain’, ‘older floodplain’ (which is separated from the recent one by dykes) and the ‘margin of the floodplain’ (which is the part of the older floodplain that forms the boundary and is furthest from the river). Dynamics in hydrology were examined weekly between November 1996 and February 1999 with the help of 40 water level wells which were installed near the studied sites. The hydrological parameters ‘average water level’, ‘average groundwater level’, ‘flooding duration’, ‘flooding depth higher 50 cm above soil surface’ and ‘standard deviation of the water level line’ were calculated to characterize the considered floodplain types and to relate species composition to hydrology. Furthermore several parameters of current management of the vegetation were recorded to evaluate the importance of land use versus hydrology for floodplain grasslands. Detrended and canonical correspondence analysis (DCA, CCA) were used to identify major environmental gradients governing the vegetation and to determine if there is a relationship between the different locations within the floodplain, variation in species composition, and gradients of measured environmental variables. The results indicate that the vegetation is closely related to a combination of water level fluctuations, which are different among the floodplain types, and soil moisture, while type and intensity of current management are not important in this context. The results of contingency tables underline the significance of dykes for the occurrence and absence of individual species among the floodplain types. The observed patterns can also be explained by the different hydrological properties of the recent and older floodplain as the results of logistic regression reveal. Furthermore, disturbance and dispersal processes and their alteration by dykes have to be taken into account to explain the pattern of species occurrence. Partial ordination detected residual differences in vegetation among the different floodplain types after accounting for the effects of the measured environmental variables. Grain size distribution is discussed as a further factor that might influence species composition. Copyright © 2004 John Wiley & Sons, Ltd.     

The influence of hydro‐climatic and anthropogenic effects on the long‐term variation of commercial fisheries in a large floodplain river

This study deals with the effects of climate fluctuations and anthropogenic impacts on fisheries of Paraná River over the last 100 years. It is the first attempt to appraise the influence of hydro‐climatic and anthropogenic variables on the population changes of the most important inland fisheries of Argentina. Datasets covering more than eight decades (1935–2016) of a number of frequent and abundant commercial species inhabiting the main channel as well as the large floodplain of Paraná River were used. Our results suggest that fish catches and structure changed over time. Long and short‐term changes and reductions were closely related to fluctuations of 18 hydro‐climatic variables. Positive effects on the ichthyofauna were recorded during humid periods (1930–1940 and 1970–2000), when the frequency of large spring–summer floods increased. An increase in anthropic impacts (accounted for with nine variables) were recorded during the last two decades. We highlight the usefulness of the approach to support the management of the resources, ensuring sustainability of commercial fish assemblages and the long‐term conservation of biodiversity in big rivers.     

Environmental and biological responses of former channels to river incision: A diachronic study on the upper rhǒne river

The impacts of river incision should be a lowering of the river level, which would increase the rate of water flow from the hillslope aquifer to the river and its former channels. If this aquifer is nutrient-poor, it should favour the oligotrophication of the former channels. However, if the lowering of the water-table exceeds the depth of the former channels, then it should lead to the drainage of these channels. These hypotheses were tested on two former channels located in a degrading reach of the Rhǒne River (France); the former channels were influenced by floods, seepage water from the river and hillslope groundwater in 1989. The rate of river incision in this reach has increased since 1980 because of gravel extraction from the river bed (0.5 m between 1989 and 1993). Between 1989 and 1993, the reduction of river infiltration and the increase in the amount of water from the hillslope aquifer in the former channels were demonstrated by the decrease in the phosphate and ammonia content of the water, and its increased alkalinity and conductivity. The responses of aquatic vegetation to river incision depended on the vegetation zone investigated. Two floristic zones that were frequently flooded and species-poor in 1989 remained species-poor in 1993. Two floristic zones, which were species-rich and patchily organized, became dry or very shallow in 1993; as a consequence, aquatic vegetation disappeared, and was replaced by helophytes and terrestrial species. In the last vegetation zone, the increase in the amount of water coming from the hillslope aquifer caused the appearance and development of Chara hispida, an oligotraphent species. Unforeseen impacts of river incision were a decrease in the spatial and temporal heterogeneity of the former channels and lower aquatic macrophyte richness.     

Environmental factors controlling phytoplankton dynamics in a large floodplain river with emphasis on cyanobacteria

Harmful algal blooms are occurring in large river ecosystems and at the mouth of large rivers with increasing frequency. In lentic systems, the chemical and physical conditions that promote harmful algal blooms are somewhat predictable but tracking prevalence and conditions that promote harmful algal blooms in lotic systems is much more difficult. We captured two of the most extreme discharge years within the last 20 years occurring in the Upper Mississippi River, allowing a natural experiment that evaluated how major shifts in discharge drive environmental variation and associated shifts in phytoplankton. Statistical models describing significant environmental covariates for phytoplankton assemblages and specific taxa were developed and used to identify management‐relevant numeric breakpoints at which environmental variables may promote the growth of specific phytoplankton and/or cyanobacteria. Our analyses supported that potentially toxin‐producing cyanobacteria dominate under high phosphorus concentration, low nitrogen concentration, low nitrogen‐to‐phosphorus ratio, low turbulence, low flushing, adequate light and warm temperatures. Cyanobacteria dominated in 2009 when low discharge and low flushing likely led to optimal growth environments for Dolichospermum, Aphanizomenon and Microcystis. Rarely will a single factor lead to the dominance, but multiple positive factors working in concert can lead to cyanobacteria proliferation in large rivers. Certain isolated backwaters with high phosphorus, low nitrogen, warm water temperatures and low potential for flushing could benefit from increased connection to channel inputs to reduce cyanobacterial dominance. Numerous examples of this type of habitat currently exist in the Upper Mississippi River and could benefit from reconnection to channel habitats.     

Aquatic vegetation responses to island construction (habitat restoration) in a large floodplain river

The Upper Mississippi River is maintained in its current navigable state through impoundments, dredging, and other engineering projects. These stressors, along with anthropogenic impacts and natural system processes, led to declines in aquatic vegetation and the loss of fish and wildlife habitat, with a major downturn the late 1980s and early 1990s. Large‐scale restoration projects, such as the one evaluated here, are primarily designed to rehabilitate and enhance fish and wildlife habitat. We determined whether an individual restoration project, construction of an island complex, fulfilled a programmatic goal of re‐establishing diverse and abundant native aquatic vegetation. Eighteen years of aquatic vegetation monitoring data from impact and reference areas were compared to evaluate the anticipated direct effects (within 400 m of the constructed islands) and indirect effects (>400 m downstream of constructed islands) of restoration. Impact areas were also compared with an unrestored negative reference area ~200 km downstream of the project and with a positive reference area in adjacent, relatively natural backwaters. Only indirect effects of restoration were evident. Prevalence and species richness of aquatic vegetation in both of the impact areas and in the negative reference area increased prior to restoration, suggesting large‐scale improvement independent of the project examined here. Indirect effects were demonstrated as further increases in both prevalence and species richness coinciding with restoration in the area >400 m downstream of the restoration. We conclude that increased abundance and diversity of aquatic vegetation was partially achieved, with observed improvements potentially linked to reduced wind fetch.     

Chinook salmon spawning surveys in deep waters of a large,regulated river

In 1986 research divers surveyed and mapped deep-water spawning redds of fall chinook salmon (Oncorhynchus tshawytscha) in selected sites within an impounded segment of the main-stem Columbia River, Washington State, U.S.A. In velocities over 3m s^?1 and depths up to 11 m, two divers riding a manoeuvrable sled made cross-current transects communicating observations of substrate materials and deep-water spawning sites. Surface personnel tracked the position of the sled with a laser locating system that logged the information into data storage. Subsequently, the computerized data were translated into overlaying maps depicting location of redds, substrate materials, and depth contours. Deep-water spawning (>3m) occurred at most survey sites in velocities between 0.6 and 0.8m s^?1. The average depth of spawning was 6.5 m, and the maximum was 9.1 m-deeper than the depth redds can normally be detected by aerial observation (3–4 m). Deep-water spawning ranged from none to substantial in areas of near identical physical characteristics. A method for estimating abundance and density of deep-water redds, based upon the data collected with this mapping technique, is presented. This study combined with current limited information concerning deep-water spawning suggests that up to 80 per cent of the escapement of fall chinook salmon in this reach may spawn in deep water.     

Applicability of the flood‐pulse concept in a temperate floodplain river ecosystem: thermal and temporal components

Annual growth increments were calculated for blue catfish (Ictalurus furcatus) and flathead catfish (Pylodictis olivaris) from the lower Mississippi River (LMR) to assess hypothesized relationships between fish growth and floodplain inundation as predicted by the Flood‐Pulse Concept. Variation in catfish growth increment was high for all age classes of both species, and growth increments were not consistently related to various measures of floodplain inundation. However, relationships became stronger, and usually direct, when water temperature was integrated with area and duration of floodplain inundation. Relationships were significant for four of six age classes for blue catfish, a species known to utilize floodplain habitats. Though similar in direction, relationships were weaker for flathead catfish, which is considered a more riverine species. Our results indicate the Flood‐Pulse Concept applies more strongly to temperate floodplain‐river ecosystems when thermal aspects of flood pulses are considered. We recommend that future management of the LMR should consider ways to ‘recouple’ the annual flood and thermal cycles. An adaptive management approach will allow further determination of important processes affecting fisheries production in the LMR. Copyright © 2006 John Wiley & Sons, Ltd.     

Economic analysis of water temperature reduction practices in a large river floodplain: an exploratory study of the Willamette River,Oregon

This paper examines ecosystem restoration practices that focus on water temperature reductions in the upper mainstem Willamette River, Oregon, for the benefit of endangered salmonids and other native cold‐water species. The analysis integrates hydrologic, natural science and economic models to determine the cost‐effectiveness of alternative water temperature reduction strategies. A temperature model is used to simulate the effects of combinations of upstream riparian shading and flow augmentations on downstream water temperatures. Costs associated with these strategies are estimated and consist of the opportunity costs of lost agricultural production and recreation opportunities due to flow releases from an up‐stream reservoir. Temperature reductions from another strategy, hyporheic flow enhancement, are also examined. Restoration strategies associated with enhanced hyporheic cooling consist of removal/reconnection of current obstacles to the creation of dynamic river channel complexity. The observed reduction of summer water temperatures associated with enhanced channel complexity indicates that restoring hyporheic flow processes is more likely to achieve cost‐effective temperature reductions and meet the total maximum daily load (TMDL) target than conventional approaches that rely on increased riparian shading or/and combinations of flow augmentation. Although the costs associated with the hyporheic flow enhancement approach are substantial, the effects of such a long‐term ecological improvement of the floodplain are expected to assist the recovery of salmonid populations and provide ancillary benefits to society. Copyright © 2008 John Wiley & Sons, Ltd.     

Macroinvertebrate community structure in a regulated river segment with different flow conditions

Two types of modification of the hydrological system are present in the same regulated segment of the Lima River (NW Portugal): (a) a reduced and constant flow from hypolimnetic release; (b) an intense irregular flow (daily and seasonal). Using multivariate techniques it was possible to compare the effects of these two kinds of disturbance on the macroinvertebrate communities. The communities colonizing both sites exhibited a higher variation in composition and diversity when compared to undisturbed sites. However, such variability was even more evident in the first case, in spite of the stability of the environmental conditions. Such temporal replacement of species is linked to the dominance of tolerant taxa with short life cycles. In the regulated segment the poor water quality and the lack of litter input impacted mainly on the shredders group. This work shows the failure of the practice of releasing constant flows as an attempt to mitigate regulation impacts. Copyright © 2002 John Wiley & Sons, Ltd.     

Role of discharge and temperature variation in determining invertebrate community structure in a regulated river

This paper quantifies patterns of discharge and temperature variation in the regulated river Lyon and the adjacent, unregulated river Lochay (Scotland) and assesses the importance of these patterns for benthic invertebrate community structure. Invertebrates were sampled at sites in each catchment in autumn, winter and spring during the 2002–2003 hydrological year. Metrics were used to characterize the discharge and temperature regimes in the period immediately preceding invertebrate sample collection. Metric values were then used in a canonical correspondence analysis (CCA) of the invertebrate sample data, in order to assess the significance of individual metrics and the overall importance of flow and temperature variability for community structure. The variance in the invertebrate data explained by this CCA was compared to that from a CCA using a range of environmental data from the sites (stream‐bed algal cover, channel hydraulic, sedimentary and water quality characteristics). This comparison allowed assessment of the relative importance of environmental variables versus hydrologic and thermal regimes. Invertebrate communities in the Lyon were relatively poor and uneven, with Ephemeroptera, Plecoptera and Coleoptera poorly represented. Distinct site and seasonal clusters were evident in the CCA ordination biplots, with Lyon and Lochay sites separated in dimensions represented by geometric mean sediment size, water temperature and algal cover. The cumulative variance values from ordinations using the discharge and temperature metrics were consistently highest, suggesting that differences in invertebrate communities showed a stronger relation to patterns of discharge and temperature variability than to the broader suite of environmental conditions. Although there were marked thermal differences between sites, temperature metrics appeared no more important than discharge metrics in explaining differences in invertebrate community structure. A number of the temperature and discharge metrics appeared similarly important, suggesting that no one aspect of the hydrothermal regime was any more important than others in helping to understand differences in invertebrate community between the study sites. Copyright © 2007 John Wiley & Sons, Ltd.     

查干湖引松渠道补给水量对年内水质参数的影响

经引松渠道进入查干湖的水量包括前郭灌区回归水和汇集渠道两侧的潜水,对查干湖水体有较大影响.各月渠内来水量与多数水质参数体现出不同程度的相关关系.是造成查干湖TN、TP、DO等参数年内出现特异变化的主要原因之一,应加强对引松渠道的维修和保护工作.     

Spring ecotone and gradient study of interstitial fauna along two floodplain tributaries of the river rhǒne,France

The hyporheic zone of two small tributaries of the upper Rhǒne River where they cross the floodplain (approximately 2 km wide) was studied. The streams originate in karstic areas which directly border the floodplain. The surveys were conducted at 11 sites along the course of the two tributaries at a depth of 50 cm in bed sediments. Ten litre samples were collected six times a year with a Bou-Rouch sampler. The paper examines the distribution of interstitial communities along two transverse (floodplain) profiles, defines the spring ecotone, analyses the heterogeneity, and underlines the importance of the tributaries in the functioning of the floodplain. A comparison between the spatial distributions of fauna and organic matter content along the two profiles underlines the important relationships between the groundwater and surface environment, and makes it possible to differentiate parts of the floodplain where the exchanges are most intense (interstitial spring, karst/floodplain interface). Along the longitudinal course of these streams, we observe the influence of groundwater circulation on community structure/i.e. the interactions between stream channel, alluvial aquifer and adjacent karstic aquifer.     

Effects of levees on soil microbial activity in a large river floodplain

In recent years, large river floodplains have received attention because of their importance in water quality amelioration and biodiversity support. However, few studies have addressed the effects of the extensive anthropogenic modification of floodplains caused by levee construction. To assess the effects of levees on soil properties, we measured microbial activities (electron transport system and extracellular enzyme activities), soil properties (organic matter, pH, ions, soil texture, etc.), coarse woody debris and litter accumulations in a section of the Wisconsin River floodplain in 1999 and 2000, where levees were constructed more than 100 years ago. No significant differences in the amount of organic matter or coarse woody debris were observed outside the levee compared to areas inside the levee. However, significantly higher microbial activities (dehydrogenase, β‐glucosidase, phosphatase) per gram organic matter were found for soils inside the levee. These changes were attributed to (1) modified hydrology, which caused differences in water availability and soil chemistry, and (2) tree species composition (e.g. higher number of oaks outside the levee), which caused differences in substrate quality. These results suggest that modification of the hydrologic regime by levees may alter not only the structure of plant communities but also the organic matter dynamics of floodplain. Copyright © 2005 John Wiley & Sons, Ltd.