Cohesive Strength Meter Research Articles

Biological soil crust elicits microbial community and extracellular polymeric substances restructuring to reduce the soil erosion on tropical island, South China Sea

Soil erosion stands as the preeminent environmental concern globally, attaining heightened significance, particularly within islands where land resources prove notably scarce. Biological soil crusts, referred to as biocrusts, assume a pivotal ecological role in soil conservation. Notably, they augment the horizontal stability of the substrate through the exudation of microbial extracellular polymeric substances (EPS), thereby shielding the soil against shear stress, exemplified in the form of water erosion. While extant research has delved into the anti-erosion mechanisms of biocrusts in arid landscapes, a conspicuous lacuna persists in the exploration of coral island environments. In this study, we collected and assessed 30 samples encompassing dark biocrusts, light biocrusts, and bare soil to scrutinize the potential anti-erosion efficacy of tropical coral island biocrusts within the South China Sea. Employing a cohesive strength meter, we quantified soil shear stress across various stages of biocrust development, revealing a discernible enhancement in soil erosion resistance during the formation of biocrusts. Relative to the exposed bare soil, the soil shear stress exhibited an escalation from 0.33 N m−2 to 0.61 N m−2 and 1.31 N m−2 in the light biocrusts and dark biocrusts, respectively. Mechanistically, we assayed microbial EPS contents, exposing a positive correlation between EPS and soil anti-erodibility, encompassing extracellular protein and polysaccharide. Concurrently, bacterial abundance displayed a significant augmentation commensurate with biocrust formation and development. In pursuit of elucidating the origin of EPS, high-throughput amplicon sequencing was executed to identify microorganisms contributing to biocrust development. Correlation analysis discerned Cyanobacteria, Chloroflexi, Deinococcota, and Patescibacteria as potential microbials fostering EPS production and fortifying erosion resistance. Collectively, our study presents the first evidence that biocrust from tropical coral reef island in the South China Sea promotes resistance to soil erosion, pinpointing key EPS-producing microbials against soil erosion. The findings would provide insights for island environment restoration.

Bacterial carbonic anhydrase-induced carbonates mitigate soil erosion in biological soil crusts

Soil erosion is a significant environmental issue worldwide, particularly in island regions where land resources are exceedingly scarce. Biological soil crusts play a crucial role in mitigating soil erosion, yet the precise effect and mechanism of biological soil crusts against erosion remain ambiguous. In this study, biological soil crusts at various developmental stages from a tropical coral island in the South China Sea were chosen to investigate the role of carbonic anhydrase in mitigating erosion. A cohesive strength meter, real-time quantitative PCR, and 16S rRNA gene high-throughput sequencing were employed to assess variations in soil antiscouribility as well as bacterial abundance and composition during the formation and development of biological soil crusts. Scanning electron microscopy was utilized to detect carbonates induced by bacterial carbonic anhydrase and elucidate their role in the solidification of sand particles. The findings indicate that the formation and development of biological soil crusts significantly enhance anti-scouribility. Comparison to those of bare coral sand, the shear stress increased from 0.35 to 1.11 N/m2 in the dark biocrusts. Moreover, significantly elevated carbonic anhydrase activity was observed in biological soil crusts, demonstrating a positive correlation with antiscouribility. In addition, there was a significant increase in bacterial abundance within the biological soil crusts. The enrichment of Cyanobacteriales and Chloroflexales potentially contributed to the increased carbonic anhydrase activity and antiscouribility. Furthermore, three cyanobacterial strains with carbonic anhydrase activity were isolated from biological soil crusts and subsequently confirmed to enhance sand solidification through microbial carbonate precipitation. This study presents initial evidence for the role of microbial carbonic anhydrase in enhancing the antiscouribility of biological soil crusts during their formation and development. These findings offer novel insights into the functional and mechanistic dimensions underlying the mitigation of soil erosion facilitated by biological soil crusts, which are valuable for implementing sustainable biorestoration and environmental management technologies to prevent soil erosion.

Shear strength and erosion resistance of salt marsh substrates: Which method to use?

Salt marshes provide a variety of ecosystem services, including habitat provision, pollutant storage and attenuation of waves and currents. However, marsh edge, surface and interior erosion is common on many shores due to shear and impact forces applied by waves or currents, as well as gravitational failure. To forecast the future provision of these ecosystem services, the resistance of marsh substrates to hydrodynamic forcing must be determined. At present, a variety of methods exist to measure resistance to failure (e.g. shear vane, torvane and Cohesive Strength Meter; CSM), but what these different methods are measuring, and therefore the differences that result from using different methods, are often poorly specified – or even ignored.While shear vane and torvane devices have, until recently, been used interchangeably in salt marsh studies, the data collected is highly variable within individual datasets, between sites using the same technique, and between techniques at an individual site. In UK marshes, we find that the shear vane apparatus records a higher undrained shear strength than the torvane on the same substrate, possibly due to greater interaction between roots and the larger blades of the shear vane instrument (compared to the torvane).Our results also show that the vane-based and CSM devices measure two different processes, with vanes quantifying the bulk resistance of the substrate to shear and mass movement, while the CSM records the process of grain-by-grain erosion from the substrate surface. These two distinct measurements were not related, so our research challenges the common assumption that substrates with a higher resistance to bulk failure processes also have a high erosion threshold.

A CSM-Based Procedure for Identifying Segments of Agricultural Drainage Ditches to Be Prioritized in Maintenance Work

Highlights Maintenance of agricultural ditches is important to secure food production. To reduce costs, ditch segments in most need of maintenance should be prioritized. CSM measurements can help prioritize maintenance of agricultural ditch segments. Abstract. Productive agricultural land is vital for food production. To help secure the productivity of drained agricultural soils, diches need to be intermittently maintained. Maintenance work is costly, however, so ditch segments in most need of maintenance should be prioritized. Here, we present a procedure for identifying drainage ditch segments likely to need maintenance based on susceptibility to soil erosion by water flowing in the ditch, evaluated using a cohesive strength meter (CSM). An important part of the procedure is to relate the pressure of the CSM jets to pressures acting on the soil. The relationship between CSM jet pressure and pressures at the soil surface was established based on measurements made with a pressure sensor plate and was applied to obtain several values for pressure at the surface, which were used in turn to estimate critical shear stress for erosion. The results showed that the CSM-based method was able to identify differences in critical shear stress for erosion for different field soils. This information can be useful in identifying drainage ditch segments that should be prioritized for maintenance work. Keywords: Drainage data acquisition, Drainage maintenance, Drainage performance evaluation, Soil erosion.

Sediment erodibility in the Changjiang (Yangtze) subaqueous delta: spatial–temporal distribution and sedimentary significance

Delta evolution in the context of no sediment discharge has become a global concern, and an accretion-to-erosion conversion is occurring in the Yangtze estuary. This conversion could threaten Changjiang subaqueous delta development. Sediment erodibility is an important indicator of subaqueous delta vulnerability. However, the present and future erodibility of the Changjiang subaqueous delta remains unclear. In this study, 37 short cores were collected from the Changjiang subaqueous delta, and the critical shear stress of the sediment was measured using a cohesive strength meter (CSM) and compared with estimates based on an empirical Shields diagram. The sediment erodibility was analyzed by comparing the sediment critical shear stress with the bed shear stress simulated using a numerical model (i.e., FVCOM), and sediment activity was introduced to discuss the geomorphological change in the subaqueous delta. The CSM-derived critical shear stress is significantly higher than that derived from the empirical Shields formula, but it better shows the erodibility of the sediment. The annual surface sediment activity ranges from 5% to 30% based on the CSM, indicating low surface erodibility. Moreover, the critical shear stress in this region increases as water depth increases, but the bed shear stress shows the opposite trend. Therefore, the erodibility of the Changjiang subaqueous delta is lower than that of the shallow area, indicating no accretion-erosion conversion or continued vertical erosion under sediment starvation in the coming decades. These findings can provide suggestions for erosion assessment and management in large river deltas under decreasing sediment discharge.

Spatial variability of the erodibility of fine sediments deposited in two alpine gravel-bed rivers: The Isère and Galabre

In mountainous environments, high suspended sediment load during runoff or dam flushing events can lead to important amounts of fine deposits in gravel bed rivers. Fine sediment deposits may contribute to bar elevation, riparian vegetation growth and consequently to bar stabilization. Despite their contribution to the morphodynamic of mountain rivers, the erosion properties of fine sediments in this context is not fully understood.In order to investigate the dynamics of re-suspension of these deposits, field monitoring campaigns were performed to explore both the spatial variability and the controlling factors of the erodibility of fine deposits. A cohesive strength-meter (CSM), along with moisture, grain sizes, geographical position and elevation were used to evaluate both the critical bed shear stress for erosion and erosion rate of fine sediment deposits in two rivers of the French Alps: the Isère and Galabre.The results highlight a large variety of fine sediment deposition areas, which are discontinuous compared to those in estuaries and lowland rivers. A high spatial variability of erodibility was observed on the reach, the bar and the metric scale. While no upstream–downstream trend was observed at the scale of both studied reaches, the locations of the deposits, elevation from the river surface and their moisture were inter-related variables and with the highest correlations to erodibility. Measurements showed that both dry and humid deposits located at the highest and lowest elevation from the river surface respectively, were more easily eroded than intermediate deposits with medium moisture.

Mangrove restoration promotes the anti-scouribility of the sediments by modifying inherent microbial community and extracellular polymeric substance

Coastal erosion will aggravate the loss of shorelines and threaten the safety of coastal engineering facilities. Mangrove is often considered as an efficient coastal guard; however the mechanisms involved in anti-scouribility ascribed to mangrove are still poorly understood. Thus, two artificial mangrove forests (including exotic Sonneratia apetala and native Kandelia obovata) and an unvegetated mudflat control were selected to explore the potential function of microbial extracellular polymeric substance (EPS) on the anti-scouribility of the sediments. A cohesive strength meter was used for the analysis of anti-scouribility, while a sequential extraction and 16S high-throughput sequencing were employed to evaluate the changes in EPS and microbial community driven by mangrove restoration. Principal component, redundancy, and two-matrix correlation heatmap analyses were performed for the analyses of the correlations among shear stress, EPS, microbes, and soil properties. The results showed an obvious enhancement of anti-scouribility after mangrove restoration. Compared to those of unvegetated mudflat, shear stress increased from 1.94 N/m2 to 3.26 and 4.93 N/m2 in the sediments of S. apetala and K. obovata stands, respectively. Mangrove restoration also promoted EPS content in the sediments, irrespective of EPS components and sub-fractions. Both extracellular protein and polysaccharide were found to be positively correlated with anti-scouribility. Coinciding with increased anti-scouribility and EPS, increased bacterial abundances were also detected in the sediments after mangrove restoration (especially K. obovata), whereas Proteobacteria and Bacteroides may be important and influential for EPS secretion and anti-scouribility promotion. Nevertheless, increased total organic carbon, total nitrogen and total phosphorus induced by mangrove restoration may also partially contribute to improvement of anti-scouribility. In conclusion, this is the first study to provide evidence for a link between mangrove restoration and increased EPS which improve resistance to scouring. The present study provides a novel perspective on the revealing of the function of mangrove on erosion mitigation.

Effect of Vegetation Removal on Soil Erosion and Bank Stability in Agricultural Drainage Ditches

Maintenance of agricultural drainage ditches can be difficult to optimize if farmers have no guidelines on where to target their maintenance efforts. A main concern is whether ditch banks will experience soil erosion or mass movement (failure). In order to help identify sites that are more likely to experience soil erosion and/or mass movement, soil susceptibility to detachment was assessed in this study using a cohesive strength meter (CSM) and measurements of shear strength in unsaturated direct shear tests. The results showed that soil roots play an important role in stabilizing ditch banks against mass movement and in reducing the rate of soil detachment. A positive stabilizing effect was detected by CSM and confirmed by shear strength measurements. The conclusion is that native vegetation should be maintained on ditch banks, instead of being removed during maintenance work as is currently the case.

Shorebirds Affect Ecosystem Functioning on an Intertidal Mudflat

Ecosystem functioning and services have provided a rationale for conservation over the past decades. Intertidal muddy sediments, and the microphytobenthic biofilms that inhabit them, perform crucial ecosystem functions including erosion protection, nutrient cycling and carbon sequestration. It has been suggested that predation on sediment macrofauna by shorebirds may impact biofilms, and shorebirds are known to consume biofilm, potentially causing significant top-down effects on mudflat ecosystem functioning. We carried out an exclusion experiment on the Colne Estuary, Essex, to examine whether shorebird presence significantly affects sediment erodibility measured with a Cohesive Strength Meter (CSM) and microphytobenthos biomass measured using PAM fluorescence (Fo) and chlorophyll a content. We also tested for treatment effects on sediment-water nutrient fluxes [nitrate, nitrite, ammonia, phosphate and dissolved organic carbon (DOC)] during periods of both dark and light incubation. Excluding shorebirds caused statistically significant changes in regulating and provisioning ecosystem functions, including mudflat erodibility and nutrient fluxes. The presence of shorebirds lowered the sediment critical erosion threshold τcr, reduced nitrate fluxes into the sediment under illumination, lowered nitrate efflux, and reduced phosphate uptake, compared to sediments where birds were excluded. There were no significant differences in macrofauna community composition within the sediment between treatments after 45 days of bird exclusion, suggesting a direct link between shorebird presence or absence and the significant differences in biofilm-related variables. This study introduces previously unknown effects of shorebird presence on ecosystem functions within this system and highlights an area of shorebird science that could aid joint conservation and human provisioning action.

Seasonal changes in sediment erodibility associated with biostabilization in a subarctic intertidal environment, St. Lawrence Estuary, Canada

The sediment dynamics and the long-term evolution of intertidal environments depend on the erosion threshold, which is often increased by microbial biofilms. Intertidal biostabilization has been well studied in temperate and subtropical environments, but little is known for subarctic conditions with strong seasonal changes in vegetation cover and sea ice presence in winter because of the difficulty in studying the sediments under ice cover. The present project investigated the role of biofilms and their seasonal as well as spatial variations for sediment biostabilization in a subarctic intertidal area near Rimouski (Quebec, Canada). Grain size distribution reflected a seaward increasing trend in sand content and mean grain size. The poor sorting of sediments results from the contribution of sediment transport by ice rafting in addition to the hydrodynamic transport processes in this subarctic intertidal area. A Cohesive Strength Meter (CSM) was used to measure the erosion threshold (τcrit) across the entire intertidal area. Chlorophyll a (Chl a) and extra polymeric substances (EPS) were measured in the top 3 mm. The high marsh had a far higher τcrit than the low marsh, but there was much less variation between low marsh, mudflat and sandflat. τcrit was significantly correlated with Chl a, elevation, and organic matter content. EPS was not a good indicator for τcrit, except for the low marsh/high marsh difference. The results showed a seasonal cycle in intertidal bed erodibility. Lower intertidal areas had highest τcrit in autumn during the storm season, followed by lowest τcrit in winter under sea-ice cover, and no major difference between summer and spring. This study highlights the characteristics of cold climate intertidal areas, where low temperatures in autumn and land-fast ice cover during winter severely affect biofilms and intertidal vegetation, and ultimately their capacity to shield sediments from erosion.

Effect of liming products on soil detachment resistance, measured with a cohesive strength meter

ABSTRACTGood soil structure is important for achieving high productivity of agricultural land and also affects the ability of soil to withstand erosive forces. Given the importance of soil structure, efforts are commonly made to improve it, usually by application of amendments of different kinds (e.g. lime, biochar, compost, manure etc.). However, little is known about the effect of these amendments on the soil resistance to detachment. This study assessed the resistance to detachment of soil cores treated with different liming products, using a cohesive strength meter (CSM) which measures the rate of soil detachment under the action of water jets at different pressures. The amount of soil removed by the water jets was taken as an indirect measure of soil resistance to detachment, under the assumption that more resistant soils will lose less material than more susceptible soils at a given water jet pressure. The results showed that all soil amendments studied reduced detachment of particles under the action of water jets compared with unamended soil (control).

Status Assessment of Agricultural Drainage Ditches

Abstract. Poor maintenance, environmental concerns, land use changes, and adaptation to climate change are creating a growing need for better agricultural drainage. The objectives of this study were to identify ditch properties that can be evaluated visually on-site and related soil erosion processes, and to define parameters requiring more intensive study and estimate these using simplified methods. The study included surveys of ditches in various soils using MADRAS (Minnesota Agricultural Ditch Research Assessment for Stability) to classify ditch status. To explain why some ditch segments were in poor condition, additional field and laboratory studies were carried out. Soil samples were taken for analysis of particle size distribution, unsaturated direct shear strength, and critical stress for erosion. The HEC-RAS data model was used for simulation of hydraulic forces acting at different flow rates. Digital maps of land use in the catchment area in different years were used to estimate changes in runoff conditions over time. MADRAS proved to be a suitable tool for rapid assessment of stability problems in ditches. The HEC-RAS simulations were a good complement to MADRAS in assessing how changes in land use affected the hydraulic load and in highlighting bottlenecks in the system. However, the hydraulic load did not adequately explain the degree of degradation in some ditch segments. Measurements of soil shear strength were a good aid to understanding existing degradation. Thus, assessment of soil erodibility and bank stability is essential in anticipating the risk of future erosion processes in ditches. Keywords: Cohesive strength meter, HEC-RAS, MADRAS, Unsaturated direct shear strength.

Effect of antecedent soil moisture content on soil critical shear stress in agricultural watersheds

Agricultural fields act as major sources of sediment pollution for surface waters. Grassed waterways are often used as a best management practice to prevent gully erosion and control the amount of sediment delivered from the edge of agricultural fields to receiving water bodies. A widely accepted method for estimating erosion of cohesive soils (excess shear stress equation) involves determining the critical shear stress of the soil and comparing it to the shear stress exerted by the flow. Antecedent soil moisture is an important factor influencing runoff and erosion and understanding the relationship between antecedent moisture and critical shear stress for key features within an agricultural watershed (e.g. field and grassed waterway) can improve soil erosion prediction. Critical shear stress was measured in situ using a Cohesive Strength Meter for varying soil moisture conditions in a 3×3m grid in: (1) a field and (2) a grassed waterway, in an agricultural watershed located in Southwestern Wisconsin. Soil properties including bulk density, organic matter content, plastic limit, liquid limit and plasticity index were also measured. Results showed that critical shear stress in the grassed waterway and in the agricultural field increased as soil moisture increased until the soil moisture content reached a breakpoint that was approximately equal to the plastic limit. Above this breakpoint, critical shear stress of the soil decreased making the soil more susceptible to erosion. Exponential relationships between critical shear stress and soil moisture content below the breakpoint indicate that soil moisture explained more of the variability in critical shear stress for the grassed waterway (68%) compared to the agricultural field (27%). These relationships were used in conjunction with the excess shear stress equation and continuous soil moisture measurements to demonstrate changes in soil erosion rate with changes in soil moisture.

Biota and hydrology influence soil stability in constructed wetlands

The impact of hydroperiod and vegetation on soil stability was investigated in wetland swales that treat urban stormwater. Critical shear stress was measured as a proxy for soil stabilization using a Cohesive Strength Meter in three parallel wetlands. Despite efforts to create three replicate wetlands by uniform construction and identical planting, each developed a distinct hydroperiod (low, intermediate, and high water-recession rates) and vegetation (varied biomass of cattails [Typha species]). Critical shear stress (τc) was highest in the high-recession, fast-draining wetland (7.8Pa), followed by the intermediate-recession swale (6.1Pa) and the low-recession, inundated swale (4.1Pa). These values correlated with differential development of moss and algal mats, both of which were highly resistant to erosion (τc of 8.6 and 7.4Pa, respectively). These epibenthic mats were patchy and developed primarily in the wetlands with high and intermediate water-recession rates but were limited in the low-recession wetland, with cattail shade, anaerobic conditions, and substrates consisting of organic matter (τc of 5.6Pa), bare soil (3.0Pa), and muck (1.8Pa). Small inflows sustained ponding and associated cattails, which promoted shade and destabilized the surface soil of the low-recession system, as well as subareas within the better-drained wetlands. Epibenthic mats played an unexpected, disproportionate role in soil stabilization compared to vascular plants.

Spatial and temporal variations in the erosion threshold of fine riverbed sediments

Lowland chalk streams in the UK are experiencing increased deposition of fine sediment due to changes in land-use practices, channel modifications, and groundwater abstraction. The excessive fine sediment deposits have been linked to benthic habitat degradation, the obstruction of surface–groundwater flow, and the storage of contaminants, such as nutrients and pesticides. While research has been conducted on the provenance, transport, deposition, and storage of fine sediment in chalk streams, none has expressly investigated the erosion of fine sediment deposits. A year-long field survey was conducted in two reaches of the Frome-Piddle catchment (Dorset, UK) to quantify spatial and temporal variations in the erosion thresholds of surficial fine sediment deposits. Erosion thresholds were measured at randomly located points within areas of sediment accumulation using a cohesive strength meter (CSM). The threshold measurements were paired with sediment cores for analysis of the physical, chemical, and biological properties of the sediment. Spatial and temporal patterns in the erosion thresholds of fine sediment were analyzed using nonparametric statistical tests and visualized with GIS. The sediment properties underlying the variations in erosion thresholds were examined through correlation and linear regression analyses. Erosion thresholds varied significantly over space and time within the stream reaches. Erosion thresholds were greater for fine sediment deposits found in the center of the channel than in the margins. Thresholds were highest in September 2008 and declined substantially to a minimum in May 2009, with a small peak in March 2009, indicating an annual cycle in erosion thresholds. Effective particle size was identified statistically as the most important sediment property influencing erosion thresholds and was probably underlying much of the spatial variation within the reaches. None of the measured sediment properties adequately characterized the temporal variation in erosion thresholds, however, the results suggest that biological sediment properties and water geochemistry (i.e., cation content) may play a role. By identifying significant spatial and temporal variations in erosion thresholds, this study provides valuable information on the stability of fine sediment deposits, and sediment-bound contaminants, in lowland river systems. This is a crucial step in assessing their local environmental impacts and developing models of fine sediment transport for the effective management of catchment sediment budgets and water resources.

Saltmarsh creek bank stability: Biostabilisation and consolidation with depth

The stability of cohesive sediments of a saltmarsh in Southern England was measured in the field and the laboratory using a Cohesive Strength Meter (CSM) and a shear vane apparatus. Cores and sediment samples were collected from two tidal creek banks, covered by Atriplex portulacoides (Sea Purslane) and Juncus maritimus (Sea Rush), respectively. The objectives of the study were to examine the variation of sediment stability throughout banks with cantilevers present and investigate the influence of roots and downcore consolidation on bank stability. Data on erosion threshold and shear strength were interpreted with reference to bank depth, sediment properties and biological influences. The higher average erosion threshold was from the Sea Purslane bank whilst the Sea Rush bank showed higher average vane shear strength. The vertical variation in core sediment stability was mainly affected by roots and downcore consolidation with depth. The data obtained from the bank faces revealed that vertical variations in both erosion threshold and vane shear strength were affected primarily by roots and algae. A quantitative estimate of the relative contributions of roots and downcore consolidation to bank sediment stability was undertaken using the bank stability data and sediment density data. This showed that roots contributed more to the Sea Purslane bank stability than downcore consolidation, whilst downcore consolidation has more pronounced effects on the Sea Rush bank stability.

Estimation of critical shear stress from cohesive strength meter‐derived erosion thresholds

Measures of cohesive sediment erodibility, such as erosion thresholds and rates, are essential for the development of accurate sediment transport models. In situ devices for erodibility quantification are increasingly being used to capture the high spatial and temporal variability in erodibility found in natural cohesive sediments. The cohesive strength meter (CSM) is a small, hand‐held, commercially available device that has been used extensively in recent years to measure in situ erosion thresholds. However, the device is primarily used for relative measures of erosion thresholds, due to a difficulty in comparing the vertical forces generated by the CSM with horizontal bed shear stress. In this article, we describe the development of a methodology for the estimation of horizontal bed shear stress from CSM‐derived erosion thresholds. The approach used cohesive sediment mixtures with varying clay contents to create sediments with a range of erodibilities and tested them using the CSM and a laboratory annular flume. A calibration is proposed based on a comparison of mean erosion thresholds from each device per clay treatment. This study is the first successful inter‐comparison between CSM‐ and flume‐derived erosion thresholds. Significant differences in erosion thresholds were noted between CSM routines, so it is recommended that the empirical calibration be applied only to erosion thresholds estimated using the Sand 1 routine, and ranging from 40‐90 Pa stagnation pressure. The calibration allows the conversion of CSM‐derived erosion thresholds into critical shear stress, a form that permits incorporation into sediment transport models.

The stabilisation potential of individual and mixed assemblages of natural bacteria and microalgae.

It is recognized that microorganisms inhabiting natural sediments significantly mediate the erosive response of the bed (“ecosystem engineers”) through the secretion of naturally adhesive organic material (EPS: extracellular polymeric substances). However, little is known about the individual engineering capability of the main biofilm components (heterotrophic bacteria and autotrophic microalgae) in terms of their individual contribution to the EPS pool and their relative functional contribution to substratum stabilisation. This paper investigates the engineering effects on a non-cohesive test bed as the surface was colonised by natural benthic assemblages (prokaryotic, eukaryotic and mixed cultures) of bacteria and microalgae. MagPI (Magnetic Particle Induction) and CSM (Cohesive Strength Meter) respectively determined the adhesive capacity and the cohesive strength of the culture surface. Stabilisation was significantly higher for the bacterial assemblages (up to a factor of 2) than for axenic microalgal assemblages. The EPS concentration and the EPS composition (carbohydrates and proteins) were both important in determining stabilisation. The peak of engineering effect was significantly greater in the mixed assemblage as compared to the bacterial (x 1.2) and axenic diatom (x 1.7) cultures. The possibility of synergistic effects between the bacterial and algal cultures in terms of stability was examined and rejected although the concentration of EPS did show a synergistic elevation in mixed culture. The rapid development and overall stabilisation potential of the various assemblages was impressive (x 7.5 and ×9.5, for MagPI and CSM, respectively, as compared to controls). We confirmed the important role of heterotrophic bacteria in “biostabilisation” and highlighted the interactions between autotrophic and heterotrophic biofilm consortia. This information contributes to the conceptual understanding of the microbial sediment engineering that represents an important ecosystem function and service in aquatic habitats.

A physically based model to predict hydraulic erosion of fine‐grained riverbanks: The role of form roughness in limiting erosion

Hydraulic erosion of bank toe materials is the dominant factor controlling the long‐term rate of riverbank retreat. In principle, hydraulic bank erosion can be quantified using an excess shear stress model, but difficulties in estimating input parameters seriously inhibit the predictive accuracy of this approach. Herein a combination of analytical modeling and novel field measurement techniques is employed to improve the parameterization of an excess shear stress model as applied to the Lower Mekong River. Boundary shear stress is estimated using a model (Kean and Smith, 2006a, 2006b) for flow over the irregular bank topography that is characteristic of fine‐grained riverbanks. Bank erodibility parameters were obtained using a cohesive strength meter (Tolhurst et al., 1999). The new model was used to estimate annual bank erosion rates via integration across the Mekong's annual flow regime. Importantly, the simulations represent the first predictions of hydraulic bank erosion that do not require recourse to calibration, thereby providing a stronger physical basis for the simulation of bank erosion. Model predictions, as evaluated by comparing simulated annual rates of bank toe retreat with estimates of bank retreat derived from analysis of aerial photographs and satellite imagery, indicate a tendency to overpredict erosion (root‐mean‐square error equals ±0.53 m/yr). Form roughness induced by bank topographic features is shown to be a major component (61%–85%) of the spatially averaged total shear stress, and as such it can be viewed as an important factor that self‐limits bank erosion.

The engineering potential of natural benthic bacterial assemblages in terms of the erosion resistance of sediments

The secretion of extracellular polymeric substances (EPS) by bacteria has been recognized as important across a wide range of scientific disciplines, but in natural sediments, EPS production by microalgae as a mechanism of sediment stabilization has received much more attention than bacterial products. In the present study, the stabilization potential of a natural benthic bacterial assemblage was tested in cultures growing on noncohesive glass beads. The surface erosion resistance as determined by a cohesive strength meter was significantly enhanced over time compared with controls. Nutrient enrichment of the bacterial assemblages by a general broth (bacteria+) resulted in enhanced stabilization (x 3.6) compared with nutrient-depleted (bacteria) assemblages (x 1.8). This correlated with higher bacterial biomass and EPS concentrations in enriched cultures. Substratum stability was closely related to bacterial cell numbers (R2=0.75/0.78) and EPS protein concentrations (R2=0.96/0.53) (for bacteria/bacteria+ treatments, respectively), but not to EPS carbohydrates. This study implies a greater significance of extracellular proteins in substratum cohesion within the EPS complex than recognized previously. The data show both the importance of bacterial assemblages for microbial sediment stabilization and that a change in abiotic conditions can significantly affect sediment stabilization.