Spatiotemporal heterogeneity of actual and potential respiration in two contrasting floodplains

Floodplains are vital components of river ecosystems and play an important role in carbon cycling and storage at catchment and global scales. For efficient river management and conservation, it is critical to understand the functional role of spatiotemporally complex and dynamic habitat mosaics of river floodplains. Unfortunately, the fundamental understanding of mineralization and carbon flux patterns across complex floodplains is still fragmentary. In this study, respiratory potential (i.e., electron transport system activity [ETSA]) was quantified seasonally across different aquatic and terrestrial habitats (wetted channels, gravel bars, islands, riparian forests, and grasslands) of 2 Alpine floodplains differing in climate, altitude, discharge, flow alteration intensity, and land use (So?a [natural flow regime, 12% grassland area] and Urbach [mean annual discharge reduction by 30% due to water abstraction, 69% grassland area]). In situ respiration (R) was measured, and ETSA–R ratios were calculated to examine differences in exploitation intensity of the overall respiratory capacity among floodplain habitats and seasons. ETSA and R provided potential and actual estimates, respectively, of organic matter mineralization in the different floodplain habitats. Hierarchical linear regression across habitat types showed that organic matter, grain sizes <0.063 mm, and water content were the most important predictors of ETSA in both floodplains, and grain sizes 2–0.063 and >8 mm were also highly important for the So?a floodplain. The combination of ETSA and R measurements conducted in contrasting floodplains increased our understanding of the relationships between floodplain habitat heterogeneity, organic matter mineralization and human impacts, that is, structural–functional linkages in floodplains. These data are integral towards predicting changes in floodplain function in response to environmental alterations from increasing human pressures and environmental change.     

DAM IMPACTS ON AND RESTORATION OF AN ALLUVIAL RIVER-RIO GRANDE, NEW MEXICO

1 INTRODUCTION The construction of more than 75,000 dams and reservoirs on rivers in the United States (Graf, 1999) has resulted in alteration of the hydrology, geometry, and sediment flow in many of the river channels downstream of dams. Additionally, hydrologic and geomorphic impacts lead to changes in the physical habitat affecting both the flora and fauna of the riparian and aquatic environments. Legislation for protection of endangered species as well as heightened interest in ma…     

Hydraulic and geomorphic processes in an overbank flood along a meandering,gravel‐bed river: implications for chute formation

Hydraulic interactions between rivers and floodplains produce off‐channel chutes, the presence of which influences the routing of water and sediment and thus the planform evolution of meandering rivers. Detailed studies of the hydrologic exchanges between channels and floodplains are usually conducted in laboratory facilities, and studies documenting chute development are generally limited to qualitative observations. In this study, we use a reconstructed, gravel‐bedded, meandering river as a field laboratory for studying these mechanisms at a realistic scale. Using an integrated field and modeling approach, we quantified the flow exchanges between the river channel and its floodplain during an overbank flood, and identified locations where flow had the capacity to erode floodplain chutes. Hydraulic measurements and modeling indicated high rates of flow exchange between the channel and floodplain, with flow rapidly decelerating as water was decanted from the channel onto the floodplain due to the frictional drag provided by substrate and vegetation. Peak shear stresses were greatest downstream of the maxima in bend curvature, along the concave bank, where terrestrial LiDAR scans indicate initial floodplain chute formation. A second chute has developed across the convex bank of a meander bend, in a location where sediment accretion, point bar development and plant colonization have created divergent flow paths between the main channel and floodplain. In both cases, the off‐channel chutes are evolving slowly during infrequent floods due to the coarse nature of the floodplain, though rapid chute formation would be more likely in finer‐grained floodplains. The controls on chute formation at these locations include the flood magnitude, river curvature, floodplain gradient, erodibility of the floodplain sediment, and the flow resistance provided by riparian vegetation. Copyright © 2015 John Wiley & Sons, Ltd.     

Terrestrial invertebrates of dry river beds are not simply subsets of riparian assemblages

Dry river beds are common worldwide and are rapidly increasing in extent due to the effects of water management and prolonged drought periods due to climate change. While attention has been given to the responses of aquatic invertebrates to drying rivers, few studies exist on the terrestrial invertebrates colonizing dry river beds. Dry river beds are physically harsh and they often differ substantially in substrate, topography, microclimate and inundation frequency from adjacent riparian zones. Given these differences, we predicted that dry river beds provide a unique habitat for terrestrial invertebrates, and that their assemblage composition differs from that in adjacent riparian zones. Dry river beds and riparian zones in Australia and Italy were sampled for terrestrial invertebrates with pitfall traps. Sites differed in substrate type, climate and flow regime. Dry river beds contained diverse invertebrate assemblages and their composition was consistently different from adjacent riparian zones, irrespective of substrate, climate or hydrology. Although some taxa were shared between dry river beds and riparian zones, 66 of 320 taxa occurred only in dry river beds. Differences were due to species turnover, rather than shifts in abundance, indicating that dry river bed assemblages are not simply subsets of riparian assemblages. Some spatial patterns in invertebrate assemblages were associated with environmental variables (irrespective of habitat type), but these associations were statistically weak. We suggest that dry river beds are unique habitats in their own right. We discuss potential human stressors and management issues regarding dry river beds and provide recommendations for future research.     

Habitat use and preference of adult pike (Esox lucius L.) in an anthropogenically impacted lowland river

To efficiently manage northern pike (Esox lucius), information is needed on its habitat use and preference. However, knowledge gaps still exist, especially on pike habitat use and preference in rivers characterised by artificial environments. We investigated the use of the main river, tributaries and side arms at the macro-scale, and the use and preference of riparian habitats by adult pike at the meso-scale in an anthropogenically impacted river basin. Adult pike were followed in winter and spring by radio telemetry. At the macro-scale pike intensively used the main river in winter and spring, but also frequented specific side arms in winter and specific tributaries in spring, which may indicate the importance of these habitats to adult pike. At the meso-scale, reedy semi-natural banks were used the most, irrespective of any assumption on habitat availability or use. The findings underline the value of protecting the least impacted, (semi)natural habitats for adult pike in an anthropogenically impacted river system. The large behavioural differences in habitat use between individuals at both habitat scales further underline the importance of habitat heterogeneity. The results also provide insight into the impact of riparian habitat restoration on adult pike and may be used to more efficiently manage pike rivers, e.g. by enhancing the lateral connectivity with river side arms or by reconstructing natural riparian habitats.     

MITIGATION OF SEDIMENTATION HAZARDS DOWNSTREAM FROM RESERVOIRS

1 INTRODUCTION Increasing attention is being given to sedimentation hazards downstream from reservoirs as dams built during the past century accumulate progressively greater volumes of sediment. The sediment storage both decreases reservoir capacity and operating efficiency of the dam, and creates a 搒ediment-shadow?downstream where sediment-starved flows commonly erode channel boundaries and create long-term channel instabilities. Numerous studies have documented downstream channel change…     

Invertebrate colonization of leaves and roots within sediments of intermittent Coastal Plain streams across hydrologic phases

Ecological flows between habitats are vital for predicting and understanding structure and function of recipient systems. Ecological flows across riparian areas and headwater intermittent streams are likely to be especially important in many river networks because of the shear extent of these interfaces, their high edge-to-width ratio, and the alternation of wet and dry conditions in intermittent channels. While there has been substantial research supporting the importance of riparian-stream linkages above-ground, comparatively less research has investigated below-ground linkages. We tested the hypothesis that riparian roots are colonized by invertebrates as a food source within stream beds of intermittent headwater streams. We compared benthic invertebrate assemblages colonizing three types of buried substrates (leaves, roots, and plastic roots) among three intermittent Coastal Plain streams, each with a different riparian management treatment (clearcut, thinned, and reference), over a 1-year period. Invertebrate density was significantly lower in root litterbags than in plastic roots litterbags, but neither differed from densities in leaf litterbags. Total invertebrate abundances, however, were significantly higher in leaf and root litterbags compared to abundances in plastic root litterbags. Invertebrate biomass and richness did not vary among substrates, but invertebrate density, abundance, and richness all declined from the wet phase (September–December) through the dry phase (June–August). Meiofauna and aquatic dipterans were the primary colonizing invertebrates during the wet phase. Relative abundance of terrestrial taxa increased during the dry phase, but their absolute abundance remained lower than aquatic taxa during the wet phase. Invertebrate composition did not differ among substrate types, but was significantly different among streams and time periods. Cumulative number of dry days, degree days, and redox depth all strongly correlated with assemblage structure as indicated by ordination scores. Our results suggest that subsurface invertebrates respond to leaves and roots as food sources, but assemblage composition is not substrate specific. Colonization of leaves and roots within stream beds by aquatic and terrestrial taxa supports the idea that headwater intermittent streams are important interfaces for the reciprocal exchange of energy and materials between terrestrial and aquatic ecosystems.     

Environmental heterogeneity affects input, storage, and transformation of coarse particulate organic matter in a floodplain mosaic

Quantifying spatial and temporal dynamics of organic matter (OM) is critical both for understanding ecosystem functioning and for predicting impacts of landscape change. To determine the influence of different habitats and coarse particulate OM (CPOM) types upon floodplain OM dynamics, we quantified aerial input, lateral surface transfer, and surface storage of CPOM over an annual cycle on the near-natural floodplain of the River Tagliamento in NE-Italy. Using these data, we modelled floodplain leaf dynamics, taking account of the spatial distribution and hydrologic connectivity of habitats, and using leaf storage as a response variable. Mean aerial CPOM input to the floodplain was similar from riparian forest and islands, but surface transfer was greater from islands, supporting the suggestion that these habitats act as “islands of fertility” along braided rivers. Leaves were the lateral conveyor of energy to more open parts of the floodplain, whereas CPOM was mainly stored as small wood in vegetated islands and riparian forest. Simulating the loss of habitat diversity (islands, ponds) decreased leaf storage on the whole floodplain, on exposed gravel and in large wood accumulations. In contrast, damming (loss of islands, ponds and floods plus floodplain overgrowth) greatly increased storage on exposed gravel. A random shuffle of habitats led to a storage increase on exposed gravel, while that in large wood accumulations and ponds declined. These results disentangle some of the complexities of CPOM dynamics in floodplain ecosystems, illustrate the value of models in understanding ecosystem functioning at a landscape level, and directly inform river management practice.     

The colonization success of an exotic Poaceae is related to native macrophyte richness, wind disturbance and riparian vegetation

Biotic and abiotic factors are filters that prevent invasions in aquatic and terrestrial ecosystems. In this investigation we tested the hypothesis that the success of a non-native Poaceae (Urochloa subquadripara) is positively correlated with the richness of native macrophytes and negatively correlated with wind disturbance (fetch) and presence of riparian vegetation on coarse spatial scales. Our samplings were carried out in a tropical reservoir (Rosana Reservoir, Brazil). We first compared competing models using the Akaike criterion to find the main combinations of explanatory variables (native macrophyte richness, fetch, and presence of riparian vegetation) associated with the success of U. subquadripara. Then, we applied multiple regressions to assess the coefficient of determination of the best models selected according to the Akaike criterion. The probability of occurrence of U. subquadripara increased significantly with increases in the number of native macrophyte species, but decreased with fetch and the presence of riparian vegetation. Stand width and maximum depth of occurrence (indicators of the success of this Poaceae) were also positively related with native richness and negatively with fetch and riparian vegetation. Our results supported our expectation that wave disturbance is an important variable explaining U. subquadripara success. Because the less exposed sites are also more favorable for colonization by natives, positive relationships between the success of non-native species and native diversity emerge at the coarse scale. Taken together, our results support the theory of “biotic acceptance”; that is, favorable sites are more prone to colonization by both native and non-native species.     

Biomorphodynamic modelling of inner bank advance in migrating meander bends

We propose a bio-morphodynamic model at bend cross-sectional scale for the lateral migration of river meander bends, where the two banks can migrate separately as a result of the mutual interaction between river flow, sediments and riparian vegetation, particularly at the interface between the permanently wet channel and the advancing floodplain. The model combines a non-linear analytical model for the morphodynamic evolution of the channel bed, a quasi-1D model to account for flow unsteadiness, and an ecological model describing riparian vegetation dynamics. Simplified closures are included to estimate the feedbacks among vegetation, hydrodynamics and sediment transport, which affect the morphology of the river-floodplain system. Model tests reveal the fundamental role of riparian plants in generating bio-morphological patterns at the advancing floodplain margin. Importantly, they provide insight into the biophysical controls of the ‘bar push’ mechanism and into its role in the lateral migration of meander bends and in the temporal variations of the active channel width.     

Comparison of two methods for estimating the abundance, diversity and habitat preference of fluvial macroinvertebrates in contrasting habitats

In this research we evaluate the effects of the method used for estimating the potential surface available for benthic macroinvertebrates in macrophyte and unvegetated habitats on several metrics and habitat preference of aquatic macroinvertebrates in the upper catchment of the Henares River (Guadalajara, Central Spain). Three sampling sites were selected: a well-preserved stream (site A), a stream with no wood riparian vegetation (site B), and a straightened and deforested reach (site C). Two habitats were selected in each site: unvegetated habitat (i.e., substrata without macrophytes) and macrophyte habitat (i.e., substrata covered by macrophytes). In each habitat, six macroinvertebrate samples (including all macrophytes or mineral particles) were collected using a Hess sampler. Diversity and density of major families were referred to the surface of the Hess sampler (=Hess surface method) and to the actual surface of either mineral particles or macrophytes (=actual surface method). In general, for the actual surface method, biomass, richness, dominance, and diversity metrics were higher in the mineral habitat than in the macrophyte habitat. This trend was different for the Hess surface method. In general, densities turned out to be higher in the unvegetated habitat than in the macrophyte habitat when using the actual surface method, but the reverse occurred when using the Hess surface method. This fact is relevant for river biomonitoring, especially when reaches with different dominant substrates (macrophytes vs mineral) are compared using just one of the methods. It is concluded that the macrobenthic metrics and density values are influenced by the method used to estimate the potential available surface for aquatic macroinvertebrates.     

洞庭湖洲滩土壤种子库对土壤水分变化的响应

由于三峡大坝及上游水库群的运行,长江中下游水域水文节律随之发生了改变,导致洞庭湖枯水期提前,进而影响洞庭湖洲滩植被及其土壤种子库的分布格局.本研究在洞庭湖4个自然保护区内选取共11个典型洲滩湿地,沿由水到陆方向根据植被类型将洲滩分为泥沙洲滩、泥沙—湖草洲滩过渡带、湖草洲滩、湖草—南荻洲滩过渡带、南荻洲滩5种洲滩类型.通过样带—样方法调查和采样,并结合湿润和水淹两种条件下的土壤种子库萌发实验,分析了土壤水分变化对洲滩种子库萌发特征的影响及土壤种子库与地表植被的关系.结果显示:①土壤含水量沿水到陆方向由泥沙洲滩向南荻洲滩递减;②不同类型洲滩土壤种子库密度没有显著差异;③温室萌发实验中,水淹条件下土壤种子库物种丰富度和种子库密度显著降低,东洞庭湖自然保护区土壤种子库物种丰富度和种子库密度较高;④地表植被物种丰富度高于土壤种子库,泥沙洲滩土壤种子库与地表植被物种组成的Jaccard相似性指数最低.此外,虉草(Phalaris arundinacea)、芦苇(Phragmites communis)、南荻(Miscanthus sacchariflorus)等只在地表植被中存在,而陌上菜(Lindernia procumbens)、通泉草(Mazus japonicus)等只在种子库中存在.结果表明,在进行湿地植被恢复时,不能仅依靠种子库移植技术,还要考虑湖区季节性的水位变化以及个别物种的特异性,配合有针对性的水文调控机制及相关的人工措施恢复其原有植被.     

Effects of human disturbance and riparian conditions on Odonata (Insecta) assemblages in eastern Amazon basin streams

Riparian vegetation is an important determinant of the physical, chemical, and biological condition of streams, and odonates are useful indicators of riparian condition. To identify environmental factors that structure Odonata assemblages in tropical forest streams, we collected adult odonate specimens and habitat data from 50 stream sites located in the Brazilian municipality of Paragominas (Pará state). We collected 1769 specimens representing 11 families, 41 genera, and 97 species. Of these species, 56 were Zygoptera, and 41 were Anisoptera. Improved environmental condition was reflected in increased Zygoptera species richness and reduced Anisoptera species richness. Channel shading was strongly and positively related to Zygoptera richness, and negatively to Anisoptera richness. Zygoptera species richness, but not Anisoptera species richness, was related positively to bank angle, quantity of wood in the stream bed, electrical conductivity, and decreased water temperature. Altered riparian vegetation structure was the principal determinant of odonate assemblage structure. Our results indicate that maintaining intact riparian vegetation is fundamental for conserving or re-establishing aquatic odonate assemblage structure.     

Considering river structure and stability in the light of evolution: feedbacks between riparian vegetation and hydrogeomorphology

River ecological functioning can be conceptualized according to a four‐dimensional framework, based on the responses of aquatic and riparian communities to hydrogeomorphic constraints along the longitudinal, transverse, vertical and temporal dimensions of rivers. Contemporary riparian vegetation responds to river dynamics at ecological timescales, but riparian vegetation, in one form or another, has existed on Earth since at least the Middle Ordovician (c. 450 Ma) and has been a significant controlling factor on river geomorphology since the Late Silurian (c. 420 Ma). On such evolutionary timescales, plant adaptations to the fluvial environment and the subsequent effects of these adaptations on fluvial sediment and landform dynamics resulted in the emergence, from the Silurian to the Carboniferous, of a variety of contrasted fluvial biogeomorphic types where water flow, morphodynamics and vegetation interacted to different degrees. Here we identify several of these types and describe the consequences for biogeomorphic structure and stability (i.e. resistance and resilience), along the four river dimensions, of feedbacks between riparian plants and hydrogeomorphic processes on contrasting ecological and evolutionary timescales. Copyright © 2014 John Wiley & Sons, Ltd.     

Physical and chemical characteristics of a metal-contaminated overbank deposit,west-central South Dakota,U.S.A.

A metal-contaminated overbank deposit in west-central South Dakota resulted from the discharge of a large volume of mine tailings into a river system between the late 1800s and 1977. The deposit along the Belle Fourche River is typically up to 2 m thick and extends about 90 m away from the channel along the insides of meander bends. The sediments contain above-background levels of copper, iron, manganese, zinc, and particularly arsenic, which is commonly two orders of magnitude above background level in the contaminated sediments. Carbonate minerals in the deposit limit the desorption of arsenic by preventing acid formation. Arsenic concentrations provide a measure of the dilution of mine tailings by uncontaminated sediment. The arsenic appears to have been transported and deposited as arsenopyrite, but is now at least partially associated with iron oxides and hydroxides. Within individual samples, arsenic concentration has an inverse relation with grain size that results from the more efficient accumulation of arsenic on the greater surface area of the smaller particles. Arsenic concentration is inversely related to the sample weight percent finer than 16 μm, however, as a consequence of the dilution of the contaminated sediments by uncontaminated sediment with a finer grain-size distribution. Dilution by uncontaminated sediment from tributaries cause arsenic concentrations to decrease by a factor of 3 along 100 km of floodplain. An influx at high streamflow of uncontaminated sediment from terraces and the premining floodplain as well as from tributaries causes arsenic concentrations in parts of the contaminated deposit that are farthest away from the channel to be two to three times less than arsenic concentrations in overbank sediment that is immediately adjacent to the channel.     

Identifying and mitigating dam-induced declines in river health:Three case studies from the western United States

River health can be defined as the degree to which riverine energy source,water quality,flow regime, habitat and biota match the natural conditions.In a healthy river,physical process and form remain actively connected and able to mutually adjust,and biological communities have natural levels of diversity and are resilient to environmental stress.Both physical diversity and biodiversity influence river health.Physical diversity is governed by hydrology,hydraulics,and substrate,as reflected in the geometry of the river channel and adjacent floodplain,which create habitat for aquatic and riparian organisms.Biodiversity is governed by biological processes such as competition and predation,but biodiversity also reflects the diversity,abundance and stability of habitat,as well as connectivity. Connectivity within a river corridor includes longitudinal,lateral,and vertical dimensions.River health declines as any of these interacting components is compromised by human activities.The cumulative effect of dams and other human alterations of rivers has been primarily to directly reduce physical diversity and connectivity,which indirectly reduces biodiversity.Restoration and maintenance of physical diversity and biodiversity on rivers affected by dams requires quantifying relations between the driver variables of flow and sediment supply,and the response variables of habitat,connectivity,and biological communities.These relations can take the form of thresholds(e.g., entrainment of streambed sediment) or response curves(e.g.,fish biomass versus extent and duration of floodplain inundation).I use examples from Wyoming,Colorado,and Arizona in the western United States to illustrate how to quantify relations between driver and response variables on rivers affected by dams.     

In situ measurements of fine sediment infiltration (FSI) in gravel-bed rivers with a hydropeaking flow regime

The overpresence of fine sediment and fine sediment infiltration (FSI) in the aquatic environment of rivers are of increasing importance due to their limiting effects on habitat quality and use. The habitats of both macroinvertebrates and fish, especially spawning sites, can be negatively affected. More recently, hydropeaking has been mentioned as a driving factor in fine sediment dynamics and FSI in gravel-bed rivers. The primary aim of the present study was to quantify FSI in the vertical stratigraphy of alpine rivers with hydropeaking flow regimes in order to identify possible differences in FSI between the permanently wetted area (during base and peak flows) and the so-called dewatering areas, which are only inundated during peak flows. Moreover, we assessed whether the discharge ratio between base and peak flow is able to explain the magnitude of FSI. To address these aims, freeze-core samples were taken in eight different alpine river catchments. The results showed significant differences in the vertical stratification of FSI between the permanently wetted area during base flow and the dewatering sites. Surface clogging occurred only in the dewatering areas, with decreasing percentages of fine sediments associated with increasing core depths. In contrast, permanently wetted areas contained little or no fine sediment concentrations on the surface of the river bed. Furthermore, no statistical relationship was observed between the magnitude of hydropeaking and the sampled FSI rate. A repeated survey of FSI in the gravel matrix revealed the importance of de-clogging caused by flooding and the importance of FSI in the aquatic environment, especially in the initial stages of riparian vegetation establishment. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.