Invertebrate bioturbation can reduce the clogging of sediment: an experimental study using infiltration sediment columns

1. Invertebrate bioturbation can strongly affect water‐sediment exchanges in aquatic ecosystems. The objective of this study was to quantify the influence of invertebrates on the physical characteristics of an infiltration system clogged with fine sediment. 2. Two taxa (chironomids and tubificids) with different bioturbation activities were studied in experimental slow infiltration columns filled with sand and gravel and clogged with a 2 cm layer of fine sediment at the surface. We measured the effects of each taxon separately and combined on hydraulic head, water mobility and sediment reworking. 3. The results showed that invertebrates could reduce sediment clogging and this effect was linked to the functional mode of bioturbation of each group. Tubificid worms dug networks of galleries in the fine sediment, creating pathways for water flow, which reduced the clogging of sediment. In contrast, the U‐shaped tubes of chironomids were restricted to the superficial layer of fine sediments and did not modify the hydraulic conductivity of experimental columns. The combination of invertebrates did not show any interactive effects between tubificids and chironomids. The occurrence of 80 tubificids in the combination was enough to maintain the same hydraulic conductivity that 160 worms did in monospecific treatment. 4. The invertebrates like tubificid worms can have a great benefit on functioning of clogged interfaces by maintaining high hydraulic conductivity, which contributes to increased water‐sediment exchanges and stimulates biogeochemical and microbial processes occurring in river sediments.     

Colmation and Depth Filtration within Streambeds: Retention of Particles in Hyporheic Interstices

Colmation refers to the retention processes that can lead to the clogging of the top layer of channel sediments and decolmation refers to the resuspension of deposited fine particles. Internal colmation, clogging of the interstices directly below the armor layer, may form a thin seal that disconnects surface water from hyporheic water by inhibiting exchange processes. The settling of particles under low flow conditions can cause external colmation. Colmated channel sediments are characterized by reduced porosity and hydraulic conductivity as well as by a consolidated texture. The term ‘depth filtration’ refers to the transport and storage of fine matrix sediments in interstitial layers. Depth filtration is of significance for the transport of colloidal and fine particulate inorganic as well as organic matter within the hyporheic interstices and into the alluvial aquifer. The role of depth filtration is assessed for the content (given in mg per liter) of matrix fine particles retained in the coarse framework sediment of a gravel-bed river in Switzerland. Sediment samples were taken by freeze-coring with liquid nitrogen down to 70 cm depth and by piezometers down to 150 cm depth. Seventy-two percent of the mobile matrix fine particles were smaller than 0.1 mm and 50% were smaller than 0.03 mm. The content of fines tended to increase with depth, although higher accumulations were found at intermediate depths in sediments influenced by exfiltrating ground water. Interstitial detrital particles >90 μm showed vertical distribution patterns opposing those of total particles. These relationships revealed a differential significance of import, storage, and transport within three types of hydrological exchange zones (infiltration, horizontal advection, exfiltration) in the cross-section of the stream.     

Hydraulic requirements of stream communities: a case study on invertebrates

1. We relate invertebrate assemblages to direct measurements of near‐bed hydraulic conditions that integrate the complex three‐dimensional structure of flow close to the bottom. 2. We sampled invertebrate taxa from a Mediterranean River along a spatial gradient of increasing shear stress in two seasons (spring and autumn) with different hydrological conditions. We used a recently described ordination technique, Outlying Mean Index (OMI) analysis, to study the response of stream invertebrates to near‐bed hydraulic parameters. 3. The distribution of nearly 70% of the taxa collected was significantly related to the hydraulic parameters assessed. In both seasons, shear stress and Froude number were the most important hydraulic parameters whereas substratum particle size and bed roughness had less influence. Most of the 31 taxa collected in both seasons had a higher OMI (an index showing the deviation between the mean environmental conditions used by a taxon and the mean environmental conditions used by a theoretical taxon uniformly distributed across the studied gradient) in autumn (when flow was greater) and were found in samples with high shear stress and high Froude number. This suggests that benthic invertebrates changed their preferences according to flow conditions. 4. Taxon richness declined with increased shear stress during lower flow in spring. Finally, and agreeing with previous results, the proportion of filter feeders and collector‐gatherers was inversely related to shear stress. 5. Our results are a first step towards better habitat suitability models that could inform management decisions.     

Influence of streambed sediment clogging on microbial processes in the hyporheic zone

1. The hyporheic zone plays a key role in hydrological exchange and biogeochemical processes in streambed sediments. The clogging of sediments caused by the deposition of particles in the bed of streams and rivers can decrease sediment permeability and hence greatly affect hyporheic microbial processes. 2. The main objective of this study was to determine the influence of sediment clogging on hyporheic microbial processes in three French rivers (the Usses, Drôme and Isère). In each river, microbial abundance and activity were studied at three depths (10, 30 and 50 cm) in the sediment at one unclogged (high porosity) and one clogged site (low porosity). 3. The results showed that the sediment clogging had inconsistent effects on microbial processes in the three rivers. Increases (Usses) or decreases (Drôme and Isère) in both aerobic and anaerobic processes were detected at the clogged sites compared to unclogged sites. These results suggest that microbial changes because of the sediment clogging are mainly mediated by the residence time of water within the hyporheic sediments. 4. A single model predicting the effect of clogging on hyporheic microbial processes cannot be applied generally to all rivers because the degree of clogging creates heterogeneous effects on flow rates between surface and interstitial waters. As a consequence, the influence of heterogeneous clogging on surface water–hyporheic exchanges needs to be evaluated by water tracing and hydraulic modelling to determine the links between microbial processes and hydraulic heterogeneity induced by clogging in hyporheic sediments.     

Ecosystem engineering at the sediment–water interface: bioturbation and consumer-substrate interaction

In soft-bottom sediments, consumers may influence ecosystem function more via engineering that alters abiotic resources than through trophic influences. Understanding the influence of bioturbation on physical, chemical, and biological processes of the water–sediment interface requires investigating top-down (consumer) and bottom-up (resource) forces. The objective of the present study was to determine how consumer bioturbation mode and sediment properties interact to dictate the hydrologic function of experimental filtration systems clogged by the deposition of fine sediments. Three fine-grained sediments characterized by different organic matter (OM) and pollutant content were used to assess the influence of resource type: sediment of urban origin highly loaded with OM and pollutants, river sediments rich in OM, and river sediments poor in OM content. The effects of consumer bioturbation (chironomid larvae vs. tubificid worms) on sediment reworking, changes in hydraulic head and hydraulic conductivity, and water fluxes through the water–sediment interface were measured. Invertebrate influences in reducing the clogging process depended not only on the mode of bioturbation (construction of biogenic structures, burrowing and feeding activities, etc.) but also on the interaction between the bioturbation process and the sediments of the clogging layer. We present a conceptual model that highlights the importance of sediment influences on bioturbation and argues for the integration of bottom-up influence on consumer engineering activities. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The influence of sediment mobility and channel geomorphology on periphyton abundance

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The effects of spatial inhomogeneities on flow through the endothelial surface layer

Flow through the endothelial surface layer (the glycocalyx and adsorbed plasma proteins) plays an important but poorly understood role in cell signaling through a process known as mechanotransduction. Characterizing the flow rates and shear stresses throughout this layer is critical for understanding how flow-induced ionic currents, deformations of transmembrane proteins, and the convection of extracellular molecules signal biochemical events within the cell, including cytoskeletal rearrangements, gene activation, and the release of vasodilators. Previous mathematical models of flow through the endothelial surface layer are based upon the assumptions that the layer is of constant hydraulic permeability and constant height. These models also assume that the layer is continuous across the endothelium and that the layer extends into only a small portion of the vessel lumen. Results of these models predict that fluid shear stress is dissipated through the surface layer and is thus negligible near endothelial cell membranes. In this paper, such assumptions are removed, and the resultant flow rates and shear stresses through the layer are described. The endothelial surface layer is modeled as clumps of a Brinkman medium immersed in a Newtonian fluid. The width and spacing of each clump, hydraulic permeability, and fraction of the vessel lumen occupied by the layer are varied. The two-dimensional Navier-Stokes equations with an additional Brinkman resistance term are solved using a projection method. Several fluid shear stress transitions in which the stress at the membrane shifts from low to high values are described. These transitions could be significant to cell signaling since the endothelial surface layer is likely dynamic in its composition, density, and height.     

Linking Sediment Biofilms, Hydrodynamics, and River Bed Clogging: Evidence from a Large River

Abstract We investigated possible effects of the hydrodynamics at the water/sediment interface on river bed biofilms within the reservoir Freudenau (Vienna, Austria) of the Danube River during the period 1996/97. Two study sites (OBB and SSF) that differed in the magnitude of surface/subsurface water exchange were selected and intersite comparisons revealed higher organic matter, bacterial cell numbers, and esterase activity in SSF with lower horizontal outflow. Concentrations of colloidal carbohydrates and uronic acids were unaffected by hydrodynamics. The relative contribution of uronic acids to bulk colloidal carbohydrates was higher in the low-flow site SSF. The distribution patterns of this relative contribution generally matched the subsurface flow pattern. Shortly after impoundment in March 1996 and along with decreased surface flow velocity, maximal biofilm carbohydrate exopolymers concurred with minimal esterase activity in OBB. We hypothesize that this inverse relationship is due to increased diffusional resistance within the exopolymer biofilm matrix that reduces mass transfer and hydrolytic activity. These results, to our knowledge, are the first evidence for microbial participation in the clogging of a large river bed. Biofilm-associated organic carbon increased significantly by a factor of ∼3.3 to 4.4 with progressive clogging as determined by the sediment leakage coefficient, which increased ∼3.8 times. Concomitantly, with ongoing clogging, esterase activity exhibited increasingly higher values at the interface relatively to deeper sediment layers, which translates into steeper depth gradients. Furthermore, minimal inflow from the surface water into the river bed along with steepest esterase gradients concurred with a senescent benthic algal bloom. This suggests an important role for algae in clogging. Either algae obstruct voids mechanically, or their exudates fuel heterotrophic bacteria that in turn are involved in clogging processes. However, our data do not allow unequivocal differentiation between biogenic and physical clogging mechanisms. Received: 14 September 1998; Accepted: 31 December 1998     

The nature of the fluid boundary layer and the selection of parameters for benthic ecology

1. The nature of flow structure close to a river bed is reviewed and suggestions made as to appropriate equations to apply to given hydrodynamic regimes. 2. It is emphasized that in natural rivers the laminar sublayer observed immediately above the bed in some experimental studies is completely disrupted for flows characterized by high turbulence levels. 3. Instead of resorting to direct topographical measurement of bed undulations, the roughness of the river bed also can be quantified using hydraulic data obtained from velocity profiles. 4. Ambuhl's experimental findings of 1959 underpin modern ecological research into the nature of the benthic boundary layer. Common misconceptions concerning Ambuhl's contribution are corrected and it is shown that his results only apply to certain prescribed hydrodynamic conditions. 5. The adoption of a consistent approach to describing the benthic boundary layer is to be preferred, so that diverse studies can be usefully compared.     

When is stream invertebrate drift catastrophic? The role of hydraulics and sediment transport in initiating drift during flood events

1. The term ‘catastrophic drift’ is used to describe the large‐scale displacement of invertebrates that occurs during periods of increased river discharge. However, the physical processes that lead to animals entering the water column at such times remain poorly understood. Specifically, the hypothesis that the movement of bed sediments during floods triggers a large increase in drift has lacked a rigorous field test. 2. Using a portable flume, the hydraulic conditions and rates of bedload transport associated with small, frequent floods were created in situ within a reach of a gravel bed river. Experiments focussed on the patches of fine sediment which are the dominant source of bed material transported during small floods. The flume produced near bed velocities of up to 2 m s^?1 over the patches, increasing shear stress, initiating sediment transport and causing invertebrates to enter the drift. 3. The total number of individuals lost from the bed, as well as the taxonomic composition of the drift, were influenced strongly by shear stress and bedload. The rate of loss from the bed was low at shear stresses <9 dynes cm^?2 (0–4 individuals min^?1 from the 0.5 m² flume bed area). Once shear stress exceeded 9 dynes cm^?2, the threshold that resulted in consistent bedload transport from the patches, the rate of loss of animals increased to a maximum of 56 individuals min^?1. When bedload transport rates were at their highest, the taxonomic composition of the drift was more similar to the benthos than it was to the drift observed when bed material was stable. 4. Absolute rates of bedload transport created by the manipulations were extremely low (<7 g m^?1 s^?1) and typical of those measured during small, frequent floods. Events of this magnitude do not break up the armour layer across the reach as a whole and so exposed patches of fine sediment are the principal source of bedload material. Consequently, discharge events not considered as disturbances in geomorphic terms may initiate frequent episodes of so‐called ‘catastrophic drift’ from patches of stream bed.     

Clogging in subsurface-flow treatment wetlands: Occurrence and contributing factors

Clogging is a major operational and maintenance issue associated with the use of subsurface flow wetlands for wastewater treatment, and can ultimately limit the lifetime of the system. This review considers over two decades of accumulated knowledge regarding clogging in both vertical and horizontal subsurface flow treatment wetlands. The various physical, chemical and biological factors responsible for clogging are identified and discussed. The occurrence of clogging is placed into the context of various design and operational parameters such as wastewater characteristics, upstream treatment processes, intermittent or continuous operation, influent distribution, and media type. This information is then used to describe how clogging develops within, and subsequently impacts, common variants of subsurface flow treatment wetland typically used in the U.S., U.K., France and Germany. Comparison of these systems emphasized that both hydraulic loading rate and solids loading rate need to be considered when designing systems to operate robustly, i.e. hydraulic overloading makes horizontal-flow tertiary treatment systems in the U.K. more susceptible to clogging problems than vertical-flow primary treatment systems in France. Future research should focus on elucidating the underlying mechanisms of clogging as they relate to the design, operation, and maintenance of subsurface flow treatment wetlands.     

Complex hydromorphology of meanders can support benthic invertebrate diversity in rivers

In freshwater environments, high biodiversity is commonly associated with habitat heterogeneity. River bends and meanders are particularly complex morphodynamic elements of watercourses. However, the specific spatio-temporal interactions between hydromorphology and the resident biota have scarcely been studied. This article reviews the relationships between hydraulic processes, and morphological units that are typical for meanders, and analyzes the concomitant spatial and temporal dynamics of habitats suitable for aquatic invertebrates. Flow in river bends is characterized by significant cross-stream velocities, which modify primary flow patterns, and create helical flow trajectories. Consequently, boundary shear stresses at the river-bed are altered, so that complex erosion, transport, and accumulation processes characteristically shape bed and bank morphology. The diversity of substrate types and complex bathymetry in meanders provide a large variety of habitat conditions for benthic invertebrates within a relatively small spatial domain, which are connected via hydraulic pathways. Periodic reversal of hydro-morphological processes between low and high flow, and seasonal growth of aquatic macrophytes creates spatio-temporal dynamics at the meso- and microhabitat scales. Such habitat dynamics increases benthic invertebrate diversity to the extent it is consistent with spatio-temporal scales of invertebrate mobility and life cycle. Furthermore, the presence of flow refugia, and hydraulic dead zones in meanders is essential to sustain species richness. This study concludes that meanders are highly complex morphodynamic elements that exhibit several self-regulating principles supporting invertebrate diversity and resilience in fluvial ecosystems.     

Water-sediment exchanges control microbial processes associated with leaf litter degradation in the hyporheic zone: a microcosm study

The present study aimed to experimentally quantify the influence of a reduction of surface sediment permeability on microbial characteristics and ecological processes (respiration and leaf litter decomposition) occurring in the hyporheic zone (i.e. the sedimentary interface between surface water and groundwater). The physical structure of the water–sediment interface was manipulated by adding a 2-cm layer of coarse sand (unclogged systems) or fine sand (clogged systems) at the sediment surface of slow filtration columns filled with a heterogeneous gravel/sand sedimentary matrix. The influence of clogging was quantified through measurements of hydraulic conductivity, water chemistry, microbial abundances and activities and associated processes (decomposition of alder leaf litter inserted at a depth of 9 cm in sediments, oxygen and nitrate consumption by microorganisms). Fine sand deposits drastically reduced hydraulic conductivity (by around 8-fold in comparison with unclogged systems topped by coarse sand) and associated water flow, leading to a sharp decrease in oxygen (reaching less than 1 mg L⁻¹ at 3 cm depth) and nitrate concentrations with depth in sediments. The shift from aerobic to anaerobic conditions in clogged systems favoured the establishment of denitrifying bacteria living on sediments. Analyses performed on buried leaf litter showed a reduction by 30% of organic matter decomposition in clogged systems in comparison with unclogged systems. This reduction was linked to a negative influence of clogging on the activities and abundances of leaf-associated microorganisms. Finally, our study clearly demonstrated that microbial processes involved in organic matter decomposition were dependent on hydraulic conductivity and oxygen availability in the hyporheic zone.     

Wood anatomy and tree growth covary in riparian ash forests along climatic and ecological gradients

Riparian ash forests subjected to seasonal drought are among the most endangered ecosystems in Europe. They are threatened by climate warming causing aridification and by land-use changes modifying river flow. To assess the impacts of these two stress factors on riparian forests, we studied radial growth and xylem anatomical traits in five narrow-leaved ash (Fraxinus angustifolia) stands across wide climatic and ecological gradients from northern Italy to southern Portugal. Radial growth rates and earlywood hydraulic diameter (Dh) were directly correlated, whilst earlywood vessel density and growth rates were inversely associated. Ash growth positively responded to precipitation. Higher and lower rates of growth increase in response to precipitation were found in dry (annual precipitation 357–750 mm, annual water balance −39 to −48 mm) and wet (annual precipitation 1030 mm, annual water balance 27 mm) sites, respectively. Wet conditions in autumn and winter of the year prior to tree-ring formation lead to larger Dh values, except in the wet site where warmer conditions from prior autumn to current spring were positively associated to wider vessels. Growth was also enhanced by a higher river flow, reflecting higher soil moisture due to elevated groundwater table levels. Peaks in river flow from late winter to early spring increased Dh in dry-continental sites. Growth and potential hydraulic conductivity in drought-prone riparian ash forests are differently impacted by climate variability and river flow depending on site and hydrological conditions. Nevertheless, covariation between radial growth and the earlywood vessel diameter was found, regardless of site specific differences. Wood production and hydraulic conductivity are coordinated through the production of large earlywood vessels which may allow reaching higher growth rates.     

High Glucose Attenuates Shear-Induced Changes in Endothelial Hydraulic Conductivity by Degrading the Glycocalyx

Diabetes mellitus is a risk factor for cardiovascular disease; however, the mechanisms through which diabetes impairs homeostasis of the vasculature have not been completely elucidated. The endothelium interacts with circulating blood through the surface glycocalyx layer, which serves as a mechanosensor/transducer of fluid shear forces leading to biomolecular responses. Atherosclerosis localizes typically in regions of low or disturbed shear stress, but in diabetics, the distribution is more diffuse, suggesting that there is a fundamental difference in the way cells sense shear forces. In the present study, we examined the effect of hyperglycemia on mechanotranduction in bovine aortic endothelial cells (BAEC). After six days in high glucose media, we observed a decrease in heparan sulfate content coincident with a significant attenuation of the shear-induced hydraulic conductivity response, lower activation of eNOS after exposure to shear, and reduced cell alignment with shear stress. These studies are consistent with a diabetes-induced change to the glycocalyx altering endothelial response to shear stress that could affect the distribution of atherosclerotic plaques.     

Internal Flow Effects on Isotropic Confined Sand-Clay Mixtures

Under the effect of internal flows, a liner can undergo a washing out of particles, which modifies the particle size distribution and affects hydraulic, chemical and mechanical characteristics. This paper discusses the effects of internal flows on sand/kaolin mixture, in terms of rate of erosion and modification of the hydraulic conductivity. A parametric study is conducted with a specific device that consists of three modified triaxial cells. These cells allow isotropically consolidating and confining specimens, they prevent a parasitic flow and survey large deformations of specimen. The tests reveal that suffusion of clay is accompanied by a clogging in the specimen that induces a drop in hydraulic conductivity. For high gradients the erosion of clay is accompanied by the backward erosion of sand and finally the specimen collapses. The erosion rate then depends on the values of the different parameters considered (hydraulic gradient, clay content and filter pore opening size).     

Hydrodynamic and Sediment Responses of Open Channels to Exposed Pipe Encasements

The effects of exposed pipe encasements on the local variation of hydrodynamic and sediment conditions in a river channel are examined. Laboratory experiments are performed to assess the response of water level, flow regime and bed deformation to several representative types of concrete encasements. The experimental conditions considered are: three types of exposed pipe encasements exposed on the bed, including trapezoidal shape, circular-arc shape and polygonal shape, and three sets of discharges, including annual discharge, once-in-3-year flood, and once-in-50-year flood. Our experiments show that: (1) the amount of backwater definitely depends on the encasement geometric shape and the background discharge; (2) smaller discharges generally tend to induce local scour of river bed downstream of the encasement, and the order of sensitivity of bed deformation to the encasement geometric shape is trapezoidal > circular-arc > polygonal; (3) comparatively speaking, the polygonal encasement may be considered as a suitable protective structure for pipelines across alluvial rivers, with relatively modest effects on the local hydrodynamic conditions and bed stabilization.     

Dynamic shear stress in parallel-plate flow chambers

An in vitro model using a parallel-plate fluid flow chamber is supposed to simulate in vivo fluid shear stresses on various cell types exposed to dynamic fluid flow in their physiological environment. The metabolic response of cells in vitro is associated with the wall shear stress. However, parallel-plate flow chambers have not been characterized for dynamic fluid flow experiments. We use a dimensionless ratio h / lambda(v), in determining the exact magnitude of the dynamic wall shear stress, with its oscillating components scaled by a shear factor T. It is shown that, in order to expose cells to predictable levels of dynamic fluid shear stress, two conditions have to be met: (1) h / lambda(v) < 2, where h is the distance between the plates and lambda(v) is the viscous penetration depth; and (2) f(0) < f(c) / m, where the critical frequency f(c) is the upper threshold for this flow regime, m is the highest harmonic mode of the flow, and f(0) is the fundamental frequency of fluid flow.     

Regulation of capillary hydraulic conductivity in response to an acute change in shear

The effects of mechanical perturbations (shear stress, pressure) on microvascular permeability primarily have been examined in micropipette-cannulated vessels or in endothelial monolayers in vitro. The objective of this study is to determine whether acute changes in blood flow shear stress might influence measurements of hydraulic conductivity (L(p)) in autoperfused microvessels in vivo. Rat mesenteric microvessels were observed via intravital microscopy. Occlusion of a third-order arteriole with a micropipette was used to divert and increase flow through a nonoccluded capillary or fourth-order arteriolar branch. Transvascular fluid filtration rate in the branching vessel was measured with a Landis technique. Flow (shear)-induced increases in L(p) disappeared within 20-30 s of the removal of the shear and could be eliminated with nitric oxide synthase inhibition. The shear-induced increase in L(p) was greater in capillaries compared with terminal arterioles. An acute change in shear may regulate L(p) by a nitric oxide-dependent mechanism that displays heterogeneity within a microvascular network.