Fine Sediment Inputs Research Articles

Tracing the trail of eroded fertile soils during a high intensity rainfall event: A fingerprinting study in war-torn tropical mountains

The depletion of fertile topsoil presents a critical challenge in tropical mountain agroecosystems. Impacts are intensified during heavy storm events that strip unprotected topsoils and pose risks to downstream water ecosystems. To better understand such dynamics, we investigated an agricultural mountainous catchment located on the Democratic Republic of the Congo shore of Lake Kivu. This area is characterised by weak governance systems exacerbated by protracted violent conflicts, which have prevented the application of remediation practices to address extensive land use changes, including deforestation and monoculture proliferation, notably banana plantations. Additionally, the spread of Banana Xanthomonas wilt (BXW) has further augmented land use impacts. Herein the complete diseased mat uprooting (CDMU) method employed to combat the disease has resulted in significant erosion and sediment export rates. Following the recovery of banana plantations from CDMU, sediment export rates continued to increase without a clear responsible party but more as a legacy of past and present land degradation activities and embedded soil erosion and connectivity process pathways. To analyse the impact of high-intensity storm events coupled with the aftermath of conflict instability and the effects of BXW on erosion dynamics, the sediment fingerprinting technique was implemented. We collected 60 sediment source samples across a 42km2 catchment, predominantly cultivated since the mid-20th century. Samples spanned four main land uses/land covers: seasonal crops, banana plantations, bare or degraded soil, and channel banks. Additionally, mixture/target samples were collected at the summit of six fluvial terraces located at different altitudes along the vertical profile of the river after a high-intensity storm event. These terraces serve as natural indicators of sediment accumulation under different flow intensities, with higher terraces representing sediment deposition during higher discharge. Furthermore, sediment samples were collected via a gravity core in delta deposits at the river's inflow into the lake, allowing us to identify the main sources contributing to fine sediment export to the lake. This methodology enabled us to assess how sediment provenance has changed in response to varying discharge levels while elucidating the primary sources responsible for fine sediment inputs to the lake and the subsequent water quality decrease. Our analysis indicates increased contributions occurred from channel banks and degraded areas as discharge increased. Despite steep channel banks and degraded soils being primary sediment sources on the terraces, agricultural and banana sources predominantly contributed to the fine sediments exported to the lake. These findings underscore the hazards of high-intensity storm events in these fragile ecosystems and highlight the role of current banana plantation and cropland management in degrading land and water quality. Understanding the primary factors driving land degradation and sediment export is crucial for preventing further ecosystem degradation and mitigating heightened instability in conflict-affected areas. This research suggests that remediation practices should be incentivised to create buffer structures in upslope agricultural areas and protect the fertile sediments from banana plantations from being exported to the lake if socio-cultural and economic hurdles to implementation can be overcome.

Boulder ramps as a restoration measure: Increasing in the resilience of mountain freshwater ecosystems to environmental changes.

Mountain environments, as biodiversity hotspots, are subject to numerous anthropological pressures. In mountain areas, a common threat to stream biocenoses is the timber industry. Timber industry increases the fine sediment input into the mountain rivers; furthermore, timber transport requires the construction of low-water crossings across streams. Transversal barriers (weirs/fords/pipe culverts) may cause excessive erosion downstream and the accumulation of fine sediments upstream, thereby decreasing habitat heterogeneity. Moreover, mountain stream communities are sensitive to climate change; for e.g., climate change may result in increasing water temperature and decreasing flows. Boulder ramps are considered effective restoration measure for rivers; benthic macroinvertebrate community composition is an effective indicator of stream health. In this study, we selected two mountain streams catchment with forest management. The control was a stream without any objects in the streambed related to timber transport. The other stream contained weirs and pipe culverts. We considered an extensive study period that covered the stages before (2009) and after restoration (2014, 2017-2018, 2019). We present the hypothesis that boulder ramps can restore in-stream habitats, improve biodiversity, and increase the resilience of benthic macroinvertebrate communities to future environmental changes. Our study demonstrates the effectiveness of boulder ramps for mountain streams restoration. We indicated, that the habitat potential of the restored stream-reach for rheophilic and lithophilic invertebrates increased substantially. Moreover, the restored riffles allowed the streambed to be cleared of fine sediments, offering the microrefugia, which were beneficial for mountain stream invertebrates, thereby increasing the diversity and resilience of the benthic communities.

Substratum Raking Can Restore Interstitial Habitat Quality in Swedish Freshwater Pearl Mussel Streams

Functional and oxygenated stream beds provide crucial habitat for multiple endangered stream taxa, including endangered freshwater mussels, fishes, and insect larvae. Stream bed restoration measures such as substrate raking are often applied to mitigate excess fine sediment introductions and stream bed colmation, yet such measures are controversial. In this study, we conducted a systematic experiment in which sites with stream bed raking and removal of macrophytes were monitored over two years and compared with before-treatment conditions and untreated reference sites in the Swedish Brånsån stream, which still contains a population of the endangered freshwater pearl mussel Margaritifera margaritifera. The stream bed restoration resulted in improved habitat quality, as evident from decreased substrate compaction, increased redox potential, and oxygen supply into the stream bed. In contrast to previous studies in Central European catchments with more intensive agricultural catchment uses, the effects of the restoration measure were much longer, extending over two years. Consequently, stream bed raking and macrophyte removal can be considered a useful and more long-lasting restoration measure than currently assumed, especially in streams where excess input of fine sediment has already been mitigated, where catchment land use is rather extensive, and where near-natural flow regimes still prevail.

The effect of increasing fine sediment load and drying duration on the re‐emergence of Gammarus pulex (Amphipoda: Gammaridae) from the subsurface following flow resumption

Abstract Environmental change and growing anthropogenic pressure on water resources is increasing the duration and intensity of drying events in streams in many geographical locations. Favourable sediment characteristics (e.g. high porosity and low fine sediment load within the substrate matrix) may facilitate benthic macroinvertebrate use of subsurface sediments in response to drying. However, the influence of sedimentary characteristics on the use and subsequent recovery of macroinvertebrates from initial vertical migration into, survival during unfavourable conditions within, and subsequent re‐emergence from subsurface sediments have not been directly observed. Transparent mesocosm tanks were used to directly observe the vertical movement and subsequent re‐emergence of Gammarus pulex from subsurface sediments in response to increasing dry period (1, 7, or 21 days) and fine sediment load (0.5–1 mm particle diameter used for light and heavy sediment treatment) and following rehydration and resumption of flowing conditions. Increasing volumes of fine sediment addition limited the ability of G. pulex to access subsurface sediment in response to drying and re‐emerge following rehydration. The longest dry period (21 days) reduced the ability of G. pulex to re‐emerge from the subsurface sediments following rehydration and flow resumption. Increasing fine sediment load negatively affects taxa using subsurface sediments as a refuge. Increased fine sediment deposition has the potential to reduce both access to the sub‐surface and re‐emergence once surface flow resumes. As many rivers are beginning to dry out, or are showing prolonged drying due to global warming, it is increasingly important that river management reduces the input of fine sediment into rivers and increase sediment porosity of riverbeds to facilitate access into the subsurface refuge by benthic fauna.

Impact of fine-grained sediment on mountain stream macroinvertebrate communities: Forestry activities and beaver-induced sediment management

Fine-grained sediments are a natural component of river systems. Human activities generate additional sources of fine sediment. In mountainous areas, the anthropogenic inputs of fine sediments are associated with forestry. The aim of this study was to analyse the differences in the macroinvertebrate communities between the reference and caused by forest harvesting activities increased influx of fine-sediment to mountain streams. The tested hypothesis was that the macroinvertebrate communities will differ depending on the intensity of forest harvesting practices in the stream catchment that causes excessive influx of fine sediment into the stream. The reintroduction of beavers in the study area, and the formation of in-stream beaver dams, contribute the accumulation of sediments in stream sections with slower water. Thus, it was also assumed that by capturing and storing fine sediments, may contribute to the restoration of the natural structure of the benthic communities downstream of the ponds. The study was carried out in a mountain stream catchment area (Carpathians, Poland), in which inflow of fine sediments in the stream sections varied in intensity. The study was conducted over three years (2018–2020). The extensive use of forest roads, timber skidding trails, and timber storage areas produced fine sediments that clogged the interstitial spaces between the stones in the riffles, limiting the presence of rheophilic taxa associated with coarse-grained substrates. The reduction of the number of scrapers and shredders (i.e. primary consumers) associated with the influx of fine sediments may significantly affect the entire food-web structure in stream ecosystems. The capture and deposition of fine sediments in beaver ponds may accelerate the revitalisation of the flowing sections of the stream. Beaver-induced sediment management is strongly recommended as a beneficial practice that could contribute to ecological preservation and the potential of streams, particularly in mountain areas.

Suspended sediment input from crushed-stone ford construction on the Canadian Shield in Quebec

Extensive and aging forest road systems, especially those that include poorly maintained stream crossings, can be significant sources of fine sediment that are detrimental to aquatic ecosystems. With limited resources available for culvert maintenance, alternative low-water crossings such as fords have been designed. Although crushed-stone fords have been used to minimize particle release during stream crossing, few studies have measured the fine sediment input that results from their construction. We used continuous turbidity monitoring paired with grab samples to obtain suspended sediment input from four construction sites with different streamflows, streambed gradients and bed and bank soil textures. Construction took place in the fall of 2018 and the summer of 2019. The results indicate that the suspended sediment load induced by the construction ranged between 72 and 831 kg. Sediment load appeared mainly sensitive to the fine particle content in the streambed and banks. We also observed that suspended sediment concentrations returned to background levels at every site within 2 h after construction. Compared with the failure of unmaintained culverts in which most road fill is washed into the stream, crushed-stone fords construction represent negligible sediment input. Our results suggest that improved fords could be an environmentally beneficial alternative to culverts on seldom-used roads where access is still required but resources for culvert maintenance are lacking.

Impacts of fine sediment input on river macroinvertebrates: the role of the abiotic characteristics at mesohabitat scale

Impacts of fine sediment input on river macroinvertebrates: the role of the abiotic characteristics at mesohabitat scale

Effect of fish pond drainage on turbidity, suspended solids, fine sediment deposition and nutrient concentration in receiving pearl mussel streams.

Extensive fish production in earthen ponds is a common aquaculture practice, which requires draining of the ponds for fish harvesting. Despite their value for biodiversity and water retention, the impact of fish ponds on the receiving streams as regards fine sediment and nutrient pollution remains controversial. This holds particularly true for streams with endangered freshwater pearl mussels, requiring a highly permeable streambed with low fine sediment content for successful juvenile development. This study quantified the amount of fine sediment, suspended solids and nutrients delivered to pearl mussel streams in relation to the pond characteristics, distance to the receiving stream and applications of measures to prevent the input of fines. Comparing fine sediment deposition above and downstream of the pond inlets after 21 pond drainage operations, as well as continuous measurements of the turbidity for 12 operations revealed varying effects of pond fishing on the receiving streams. Average fine sediment deposition was increased by nearly six-fold compared to upstream and maximum turbidity values for single drainage operations exceeded 460 NTU. Draining between 1% and 92% of the water volume of individual ponds resulted in additional loading of 0.07-4.6t suspended particles. Physical mitigation structures that prevent mobilized material from reaching the receiving stream significantly reduced the fine sediment input and deposition rates. Harvesting methods that do not require complete drainage of the pond reduced the turbidity by ten-fold. Without mitigation measures, the impact of pond drainage operations on the fine sediment deposition was comparable to high discharge events. No significant increase in nutrient concentration was observed during most drainage operations. These results reveal remarkable effects of pond drainage on the aquatic environment, as well as the possibility to minimize such impacts by switching to harvest methods that do not require complete pond drainage and installation of sedimentation structures.

Combining Soil Erosion Modeling with Connectivity Analyses to Assess Lateral Fine Sediment Input into Agricultural Streams

Soil erosion causes severe on- and off-site effects, including loss of organic matter, reductions in soil depth, sedimentation of reservoirs, eutrophication of water bodies, and clogging and smothering of spawning habitats. The involved sediment source-mobilization-delivery process is complex in space and time, depending on a multiplicity of factors that determine lateral sediment connectivity in catchment systems. Shortcomings of soil erosion models and connectivity approaches call for methodical improvement when it comes to assess lateral sediment connectivity in agricultural catchments. This study aims to (i) apply and evaluate different approaches, i.e., Index of Connectivity (IC), the Geospatial Interface for Water Erosion Prediction Project (GeoWEPP) soil erosion model, field mapping and (ii) test a connectivity-adapted version of GeoWEPP (i.e., “GeoWEPP-C”) in the context of detecting hot-spots for soil erosion and lateral fine sediment entry points to the drainage network in a medium-sized (138 km2) agricultural catchment in Austria, further discussing their applicability in sediment management in agricultural catchments. The results revealed that (a) GeoWEPP is able to detect sub-catchments with high amount of soil erosion/sediment yield that represent manageable units in the context of soil erosion research and catchment management; (b) the combination of GeoWEPP modeling and field-based connectivity mapping is suitable for the delineation of lateral (i.e., field to stream) fine sediment connectivity hotspots; (c) the IC is a useful tool for a rapid Geographic Information System (GIS)-based assessment of structural connectivity. However, the IC showed significant limitations for agricultural catchments and functional aspects of connectivity; (d) the process-based GeoWEPP-C model can be seen as a methodical improvement when it comes to the assessment of lateral sediment connectivity in agricultural catchments.

The links between entrance geometry, hypsometry and hydrodynamics in shallow tidally dominated basins

AbstractGeomorphological characteristics of tidal basins control hydrodynamics and sediment transport potential within such basins, for example, by adjusting the balance in tidal asymmetry. In this study we examine the effects of entrance geometry on tidal velocity asymmetry, slack water asymmetry, bed shear stress patterns and hypsometric profile shapes by comparison of six shallow meso‐tidal basins of Tauranga Harbour, New Zealand. Numerical model results show how tidal distortion increases with distance from a basin entrance. A simple ratio between basin width and entrance width defines levels of basin dilation. Sub‐basins with a constricted geometry and deep entrance channels are associated with small bed shear stress values and high rates of flood‐directed tidal velocity asymmetry in the sheltered basin centres, indicating a large potential for sediment deposition of larger particles. Moreover, slack water asymmetry within these basins is weakly ebb‐directed, indicating a small potential for transport of fine sediments out of the basins. The constricted depositional basins are characterized by convex hypsometric profiles with elevated intertidal regions. Unconstricted geometries are associated with larger bed shear stress values and more ebb‐directed tidal velocity asymmetry within basin centres, suggesting limited potential for overall sediment deposition. The slack tide duration asymmetry is weakly flood‐dominant indicating that limited input of fine sediment into the basins is possible. The comparatively high‐energy conditions within these exposed basins are associated with a less convex hypsometric intertidal profile. The ability to estimate tidal asymmetries is advantageous when developing management strategies related to ecosystem functioning, navigability or coastal protection in specific geomorphic settings. © 2019 John Wiley & Sons, Ltd.

Combined effects of local habitat, anthropogenic stress, and dispersal on stream ecosystems: a mesocosm experiment.

The effects of anthropogenic stressors on community structure and ecosystem functioning can be strongly influenced by local habitat structure and dispersal from source communities. Catchment land uses increase the input of fine sediments into stream channels, clogging the interstitial spaces of benthic habitats. Aquatic macrophytes enhance habitat heterogeneity and mediate important ecosystem functions, being thus a key component of habitat structure in many streams. Therefore, the recovery of macrophytes following in-stream habitat modification may be prerequisite for successful stream restoration. Restoration success is also affected by dispersal of organisms from the source community, with potentially the strongest responses in relatively isolated headwater sites that receive a limited amount of dispersing individuals. We used a factorial design in a set of stream mesocosms to study the independent and combined effects of an anthropogenic stressor (sand sedimentation), local habitat (macrophytes, i.e., moss transplants), and enhanced dispersal (two levels: high vs. low) on organic matter retention, algal accrual rate, leaf decomposition, and macroinvertebrate community structure. Overall, all responses were simple additive effects with no interactions between treatments. Sand reduced algal accumulation, total invertebrate density, and density of a few individual taxa. Mosses reduced algal accrual rate and algae-grazing invertebrates, but enhanced organic matter retention and the number of detritus and filter feeders. Mosses also reduced macroinvertebrate diversity by increasing the dominance by a few taxa. Mosses reduced leaf mass loss, possibly because the organic matter retained by mosses provided an additional food source for leaf-shredding invertebrates and thus reduced shredder aggregation into leaf packs. The effect of mosses on macroinvertebrate communities and ecosystem functioning was distinct irrespective of the level of dispersal, suggesting strong environmental control of community structure. The strong environmental control of macroinvertebrate community composition even under enhanced dispersal suggests that re-establishing key habitat features, such as natural stream vegetation, could aid ecosystem recovery in boreal streams.

Decoupled water-sediment interactions restrict the phosphorus buffer mechanism in agricultural streams

Our study aimed to explore the effects of agriculture on the phosphorus buffer capacity of 11 headwater streams in Austria. We used phosphorus adsorption curves and re-suspension experiments to determine both, the potential of the sediments to act as phosphorus source or sink and the actual phosphorus exchange between water and sediments. Additionally, we determined the alkaline phosphatase activity (APA) in epilithic and epipsammic biofilms as indicator for the phosphorus demand of the benthic and hyporheic community. We hypothesized that highly polluted streams will show decreased phosphorus buffer capacities, which were either due to saturation or restricted water-sediment interactions.Our results support the second hypothesis. Fine sediment accumulations, organic matter content, and phosphorus concentrations in water and sediments increased with percent cropland in the catchment. Below SRP concentrations of 120μgL−1 in the stream water, sediments showed a high potential for phosphorus release, with zero equilibrium phosphorus concentrations (EPC0) being more than twice as high as SRP concentrations. Above 150μgL−1, EPC0 reached only 20–50% of SRP concentrations, indicating a high potential of the sediments to act as phosphorus sinks. These findings were confirmed by phosphorus uptake of these sediments during re-suspension. While APA in epilithic biofilms decreased with increasing SRP concentrations, APA in epipsammic biofilms showed the reverse pattern, indicating a restricted phosphorus supply of the hyporheic community despite phosphorus surplus in the water column.Our study shows that inputs of fine sediments from agricultural sources may reduce the phosphorus buffering mechanism of stream sediments through restrictions of water-sediment interactions. Consequently, water column and sediment processes are increasingly decoupled and phosphorus-rich stream water will not effectively reach the reactive sites in the sediments responsible for uptake. Therefore, phosphorus mitigation measures in stream ecosystems must comprise sediment management in the catchment as well as in-stream measures for the rehabilitation of the hyporheic zone.

Sensitivity of the early life stages of a mayfly to fine sediment and orthophosphate levels

The ecological effects of interacting stressors within lotic ecosystems have been widely acknowledged. In particular, the ecological effects of elevated fine sediment inputs and phosphate have been identified as key factors influencing faunal community structure and composition. However, while knowledge regarding adult and larval life stage responses to environmental stressors has grown, there has been very limited research on their eggs. In this study, the eggs of the mayfly Serratella ignita (Ephemerellidae: Ephemeroptera) were collected and incubated in laboratory aquaria to hatching under differing concentrations of inert suspended sediment (SS) and orthophosphate (OP), individually and in combination. Results indicate that SS and OP have greater effects on egg hatching in combination than when either were considered in isolation. SS displayed a greater effect on egg survival than OP in isolation or when OP was added to elevated SS treatments. Egg mortality in control treatments was around 6% compared to 45% in treatments with 25 mg l−1 SS and 52% in 0.3 mg l−1 OP treatments. Even relatively modest levels of each stressor (10 mg l−1 SS; 0.1 mg l−1 OP), below national legal thresholds, had significant effects on egg survival to hatching. The results support calls for legal levels of SS to be reassessed and suggest that more research is required to assess the impacts of pollution on invertebrate egg development given their different sensitivity and exposure pathways compared to other life stages.

Responses of fishes and lampreys to the re‐creation of meanders in a small English chalk stream

AbstractRiver rehabilitation initiatives have become commonplace in European water courses as a result of European Union Water Framework Directive requirements. However, the short‐term responses of fishes to such work have thus far been varied, with some river rehabilitation efforts resulting in demonstrable improvements in diversity and size structure, whereas others have resulted in little or no change. Electrofishing and channel character surveys were conducted annually between 2009 and 2014 on a reach of the River Glaven (North Norfolk, UK) before and after rehabilitation work (embankment removal in 2009 and re‐meandering in 2010) as well as on a control reach immediately upstream. To assess the effects of rehabilitation work, before‐after‐control‐impact analysis tested for changes in channel character (geomorphology, substratum composition, and mesohabitat structure) and in fish species richness, relative abundance, population density, and size structure (calculated after fish data entry into the UK Environment Agency's National Fisheries Population Database). Following re‐meandering work (i.e., treatment), habitat heterogeneity and depth variation increased in the treatment reach, but fish responses were not significant except for biomass and density increases of brown trout Salmo trutta and abundance decreases of European eel Anguilla anguilla, in the treatment but not the control reach. These results are consistent with comparable river rehabilitation initiatives elsewhere, and they suggest that larger‐scale rehabilitations are probably needed to produce greater increases in fish density and diversity. It is recommended that future rehabilitation initiatives address catchment‐scale factors that can enhance ecosystem recovery, for example, removal of barriers to colonization, and increases in connectivity and water quality issues linked to eutrophication, elevated fine sediment inputs, and various pollutants.

Infiltration of fine sediment into a coarse mobile bed: a phenomenological study

AbstractExperiments were undertaken to study the nature of granular interaction in running water by examining the influence of fine grain inputs to a coarser sediment bed with a mobile surface. Video recordings of grain sorting by both kinetic sieving and spontaneous percolation are used to diagnose the critical processes controlling the overall bed response. Kinetic sieving takes place in the mobile bed surface, with the finer sediment moving to the bottom of the bedload transport layer at the interface with the underlying quasi‐static coarse bed. We show that the behavior at this interface dictates how a channel responds to a fine sediment input. If, by spontaneous percolation, the fine sediment is able to infiltrate into the underlying quasi‐static bed, the total transport increases and the channel degrades. However, if the fine sediment input rate exceeds the transport capacity or is geometrically unable to infiltrate into the underlying bed, it forms a quasi‐static layer underneath the transport layer that inhibits entrainment from the underlying bed, resulting in aggradation and an increase in bed slope. Copyright © 2016 John Wiley & Sons, Ltd.

A modeling approach for identifying recolonisation source sites in river restoration planning

The colonization of restored river reaches by benthic macroinvertebrates and fish depends strongly on the proximity of source sites. Central European river networks have been fragmented over decades and populations of sensitive species have been eradicated from large parts of the catchments. Identification of remaining source sites (i.e., near-natural river stretches with populations of sensitive organisms) allows to protect them and reconnect them to degraded or restored stretches. We developed an approach to identify source sites of fish and benthic invertebrates and applied it to large parts of Germany. The approach is based on identifying source sites from sampling data (5919 benthic invertebrate and 2584 fish monitoring sites) depending on the occurring number of sensitive species. For river stretches that have not been sampled we conducted statistical modeling with environmental data (e.g. land use, river habitat data) using boosted regression trees to identify source sites characterized by similar environmental conditions. The results are presented as maps on the level of the federal states. Statistical modeling allowed identification of stream type-specific environmental parameters and their thresholds. The maps allow a visual estimation of the recolonisation potential for river sections considered for restoration. The results provide valuable insight into the perspective of restoration in different regions. For restoration planning we suggest application on a catchment level using environmental data with higher resolution and consideration of additional parameters (e.g. fine sediment input) in lowland regions.

Mapping the combined risk of agricultural fine sediment input and accumulation for riverine ecosystems across England and Wales

Fine sediment inputs from agricultural sources are a potential threat to freshwater ecosystems and may impact on the ability of EU members⿿ states to achieve environmental targets under the Water Framework Directive (WFD).An index (the Agricultural Sediment Risk index or ASR) representing the risk of agricultural fine sediment accumulation in rivers was produced using estimates of sediment inputs from the process-based PSYCHIC model and predictions of fine sediment accumulation using River Habitat Survey data. The ASR was mapped across the entire river network of England and Wales.The ASR map and index were combined with a national dataset of fisheries surveys using logistic regression to test its relevance to freshwater biota. The ASR was strongly associated with a group of species sensitive to fine sediment inputs including salmon and trout. Another group of species including roach and perch showed a positive association with low levels of agricultural sediment inputs potentially due to their impacts on predators and competitors.The proposed approach demonstrates how existing national monitoring data and sediment pressure models can be combined to produce an assessment of risk to aquatic ecosystems from agricultural fine sediment sources at a national scale that can be used alongside WFD classification tools to identify potential causative pressures and design remedial actions.

Fine Sediment Input and Benthic Fauna Interactions at the Confluence of Two Large Rivers

Several studies suggest that invertebrate abundance and richness are disrupted and reset at confluences. Thus, junctions contribute disproportionately to the overall aquatic biodiversity of the river. In general terms, authors have reported high abundance and diversity due to the major physical heterogeneity at junctions. However, data are still scarce and uncertainties are plentiful.The impact of a great input of fine sediments on the distribution patterns of benthic invertebrates at a river confluence was quantitatively analyzed herein. The junction of the subtropical Bermejo River (high suspended sediment load) with the large Paraguay River is the selected study area to achieve this aim.While diversity increased slightly downstream the junction (from 0.21 to 0.36), density and richness of the macroinvertebrate assemblage significantly diminished downstream the confluence (from 29050 to 410 ind/m2; p< 0.05) due to the input of fine sediment from the Bermejo River (mean fine sediment increased downstream from 6.3 to 10.2 mg/L), causing a negatively impact on invertebrate assemblage. This study highlights the ecological importance of the sediment input effects on benthic invertebrates, a topic still poorly explored in river ecology.It is speculated that the spatial extent of the impact would be dependent upon the hydrological and sedimentological context, highly unequal between both rivers. New hypotheses should be tested through new studies considering different hydrological stages.

The Influence of Benthic Macrofauna on the Erodibility of Intertidal Sediments with Varying mud Content in Three New Zealand Estuaries

Fine sediment inputs can alter estuarine ecosystem structure and function. However, natural variations in the processes that regulate sediment transport make it difficult to predict their fate. In this study, sediments were sampled at different times (2011–2012) from 45 points across intertidal sandflat transects in three New Zealand estuaries (Whitford, Whangamata, and Kawhia) encompassing a wide range in mud (≤63 μm) content (0–56 %) and macrofaunal community structure. Using a core-based erosion measurement device (EROMES), we calculated three distinct measures of sediment erosion potential: erosion threshold (Ʈ c ; N m−2), erosion rate (ER; g m−2 s−1), and change in erosion rate with increasing bed shear stress (m e ; g N−1 s−1). Collectively, these measures characterized surface (Ʈ c and ER) and sub-surface (m e ) erosion. Benthic macrofauna were grouped by functional traits (size and motility) and data pooled across estuaries to determine relationships between abiotic (mud content, mean grain size) and biotic (benthic macrofauna, microbial biomass) variables and erosion measures. Results indicated that small bioturbating macrofauna (predominantly freely motile species <5 mm in size) destabilized surface sediments, explaining 23 % of the variation in Ʈ c (p ≤ 0.01) and 59 % of the variation in ER (p ≤ 0.01). Alternatively, mud content and mean grain size cumulatively explained 61 % of the variation in m e (p ≤ 0.01), where increasing mud and grain size stabilized sub-surface sediments. These results highlight that the importance of biotic and abiotic predictors vary with erosion stage and that functional group classifications are a useful way to determine the impact of benthic macrofauna on sediment erodibility across communities with different species composition.

Spatial variability of suspended sediment yield in a gravel-bed river across four orders of magnitude of catchment area

The spatial variation in suspended sediment yield in the River Eden, Cumbria, UK, is quantified and examined from a process-based perspective to challenge existing models for scale dependency in sediment yield. Field measurements were used to quantify suspended sediment yields for 14 sub-catchments within a total catchment area of nearly 1400km2, including nine catchments defining a nested basin system starting from 1km2. A novel catchment characterisation system was developed, in which broad-scale landscape characteristics were quantified using existing datasets, types of sediment supply feature were identified in a field survey and their distribution across the catchment was quantified using a survey of air photographs. Specific suspended sediment yield was found not to be related to catchment area. The main fine sediment input was from the riparian zone, where many sediment sources were discrete areas of erosion such as sites with livestock poaching and streamside scars. Connectivity of the rest of the catchment was low. The dominant types of source in an area were related to its topographic, geologic and land use characteristics. Specific suspended sediment yields were higher in lowland sub-catchments where the higher land use intensity and greater extent of superficial sediment deposits resulted in higher erosion susceptibility. Two ‘process domains’ were identified: lowland and upland. It is suggested that these provide a simple concept for explaining spatial variability in sediment yields as a function of easily measurable catchment properties.