River Bed Sediments Research Articles

The macroinvertebrate seedbank promotes community persistence in temporary rivers across climate zones

Summary Aquatic macroinvertebrates inhabiting temporary rivers are typically described as having low resistance to riverbed drying. However, little research has examined the ‘seedbank’ within dry riverbed sediments, which comprises aquatic life stages that survive in dewatered sediments and from which active organisms may develop only after surface water returns. We synthesised published and unpublished data from studies that had experimentally rehydrated sediments collected from dry riverbeds, to establish the importance of the seedbank in promoting macroinvertebrate community resistance. Studies from across climate zones were included, to examine seedbank importance in relation to environmental harshness and, in particular, sediment moisture. We also assessed the importance of the seedbank relative to alternative habitats promoting persistence of the flowing river (FR) assemblage. We predicted that the proportion of the FR assemblage present in rehydrated sediments (RS) would decrease with environmental harshness, due to conditions within the sediments becoming less conducive to the survival of biota. A negative relationship between the proportion of FR taxa present in RS and harshness was observed, and this contributed to a reduction in the compositional similarity of FR and RS assemblages as harshness increased. Significant positive correlations were identified between sediment moisture content and macroinvertebrate community metrics (density and taxon richness) in some systems. Habitats external to the dry reach, which contribute to community resilience, were invariably inhabited by a greater number of FR taxa than rehydrated sediments. However, rehydrated sediments included several FR taxa that were not found in any other habitats during the dry phase, including families of Coleoptera and Diptera. Our results indicate the importance of the seedbank as a resistance mechanism for temporary river macroinvertebrates. With climate change scenarios predicting an increase in riverbed drying, maintaining habitats that facilitate the persistence of instream communities during dry phases is an increasing priority. We identified strong relationships between sediment moisture and taxon richness, and river management and rehabilitation activities should therefore aim to retain moisture in drying sediments, by manipulating parameters such as riparian shading.

Spatial Variation of Dissolved Nutrient and Heavy Metal Concentrations in River Bed Sediments as Influenced by Land-Use Patterns in Ogun-Osun River Basin, Nigeria

Evaluation of levels and spatial characteristics of dissolved nutrients and heavy metals in the river bed sediment within a basin are critical to understanding the extent of land-use impact on the river systems. Surface river bed sediments across eight rivers in the Ogun-Osun River Basin in Nigeria were collected and analyzed for Total N, , Total P, Total organic carbon, Cd, Hg, Pb, Zn, Cu, Fe, Mn and Cr. Pollution Load Index (PLI), Accumulation Factor (AF) and Hierarchical Clustering Analysis (HCA) were used to identify the impact of the pollutants and also define the spatial variation across the basin. The pollution load indices of heavy metals were moderately high ranging from 0.41 - 0.60, while AFs were 0.43 - 2.00 and 0.61 - 1.29 for heavy metals and nutrients from upstream to downstream in the rivers systems, respectively. The HCA identified 7 distinct spatial patterns describing pollutant input from the land-use in the basin. Although, heavy metals contents were low in relation to the background values, and the potential for redistribution and secondary pollution was high hence, there was need to impose checks on the activities across agricultural, urban and grazing land-uses that had impact negatively on the river systems in the basin.

Lead Pollution Remanence in an Urban River System: A multi-scale temporal and spatial study

This work aims at studying the fate of sediments contaminated with tetraethyl Pb from leaded gasoline using a two-dimension upscaling approach, from a small urban subcatchment, the Orge River (900 km 2 ) to the whole Seine River basin (64700 km 2 ), in France. In France, the leaded gasoline reduction started in 1986 and leaded gasoline was completely banned after 2000. This work aims at assessing whether the ban of leaded gasoline is related to changes in Pb contamination sources of these river suspended sediment particles (SPM) and bed sediment. Sediment cores and samples collected in the course of previous research projects of the Seine River contamination were used as temporal archives. The study of the isotopic lead ratio showed the fast decrease of the contamination of urban river suspended particulate matter due to the “gasoline” lead source from 2000 to 2011. This source mostly disappeared in the SPM from the Seine River basin that includes urban areas but also agricultural and industrial activities. Nevertheless, it is still present in the small urban catchment of the Orge River. The results on bed sediments showed a different pattern, where the “gasoline” source is still active in densely populated areas, either in the Seine River in the 20 km downstream Paris, or along the Orge River.

Assessment of Geochemical Environment from Study of River Sediments in the Middle Stretch of River Ganga at Ghazipur, Buxar and Ballia area

In order to investigate geochemical environment of the river Ganga, the freshly deposited sediment samples were collected from Ghazipur, Buxar and Ballia urban centers in the pre-monsoon period (May, 2010). The river receives industrial as well as domestic wastes from various drains of above mentioned cities. River Ganga has been one of the major recipients of the industrial effluents in India. The present study assesses the current status of the distribution and concentration of seven heavy metals (Cr, Co, Ni, Cu, Zn, Cd and Pb) in the river sediment at the three urban centers. The total trace metal content in sediment samples were analyzed by atomic absorption spectrophotometer and obtained values were compared with different standard values. The heavy metal concentrations were recorded in the following ranges (mg/kg) Cr (113–230), Co (11–29), Ni (32–75), Cu (39–73), Zn (72–140), Cd (0.45–0.95) and Pb (15–27). Results show that the mean concentration of Cr, Cd, Zn, Cu and Co in sediment at all urban centers exceeded the average worldwide shale concentration. However the mean concentration of Ni and Co was lower than it. Metal enrichment factor (EFc) for all sampling stations was recorded between 0 and 3. The maximum EFc was 1.82 for Cr, 1.18 for Co, 0.87 for Ni, 1.33 for Cu, 1.15 for Zn, 2.43 for Cd and 1.05 for Pb. The exceptionally high EFc value was recorded for Cd at all sampling stations. High EFc value indicates that large amount of pollutants are added through anthropogenic sources. The percentage of anthropogenic and lithogenic values of heavy metal concentration indicates that Cd receives highest value of anthropogenic addition into stream and it is followed by Cr, Cu, Zn and Co. Geoaccumulation indices showed that sediment was uncontaminated to moderately contaminated and may adversely affect the fresh water ecology of the river. Toxicity reference value indicates that the river bed sediment is toxic to benthos organisms for most of the undertaken metals. The data generated may provide useful information to Governmental agencies to control the heavy metal pollution of the river Ganga sediment at these urban centers which may even be worst with the increasing population load in the basin.

Metal concentrations in river water and bed sediments of the Lower Litani River Bassin, Lebanon

The distribution of pollutants in the Litani River is almost severe. It resulted from domestic, sewage, agricultural and industrial sources. In this study, the distribution of metals (Cu, Fe, Cd, Mg, Zn, Pb , Al, Ba, Ni, Mn, Ag and Cr) were measured in river water and the bed sediments of the Lower Litani River Basin (LLRB). The impacts of metals on the water quality were monitored during the rainy, mid rainy and dry season in the year 2012. The objectives of this study were first to identify possible sources of metals (i.e. geological and/or anthropogenic) and then to characterize the chemical behavior of these metals in water and bed sediments. Water and bed load sediments were sampled at six sites along the LLRB main watercourse, The metal concentrations in the river sediments were remarkably high, but varied between different sampling sites, and the concentrations in water were mainly within the permissible limits. The metal contents in bed sediment were highest during closure of summer period. By applying the principal component analysis applied to total and extractable metal contents as a tool for studying metal pollution in the LLRB, it was useful to distinguish between anthropogenic and natural sources.

Microbial composition affects the functioning of estuarine sediments

Although microorganisms largely drive many ecosystem processes, the relationship between microbial composition and their functioning remains unclear. To tease apart the effects of composition and the environment directly, microbial composition must be manipulated and maintained, ideally in a natural ecosystem. In this study, we aimed to test whether variability in microbial composition affects functional processes in a field setting, by reciprocally transplanting riverbed sediments between low- and high-salinity locations along the Nonesuch River (Maine, USA). We placed the sediments into microbial 'cages' to prevent the migration of microorganisms, while allowing the sediments to experience the abiotic conditions of the surroundings. We performed two experiments, short- (1 week) and long-term (7 weeks) reciprocal transplants, after which we assayed a variety of functional processes in the cages. In both experiments, we examined the composition of bacteria generally (targeting the 16S rDNA gene) and sulfate-reducing bacteria (SRB) specifically (targeting the dsrAB gene) using terminal restriction fragment length polymorphism (T-RFLP). In the short-term experiment, sediment processes (CO2 production, CH4 flux, nitrification and enzyme activities) depended on both the sediment's origin (reflecting differences in microbial composition between salt and freshwater sediments) and the surrounding environment. In the long-term experiment, general bacterial composition (but not SRB composition) shifted in response to their new environment, and this composition was significantly correlated with sediment functioning. Further, sediment origin had a diminished effect, relative to the short-term experiment, on sediment processes. Overall, this study provides direct evidence that microbial composition directly affects functional processes in these sediments.

Nutrient dynamics in river bed sediments: effects of hydrological disturbances using experimental flow manipulations

River sediments play a crucial role in the storage and transformation of organic matter (OM). Nutrient dynamics are controlled by the interaction of several key parameters, i.e. river discharge, channel geometry and vertical exchanges of water (upwelling vs. downwelling zones). The main aim of this study was to evaluate the effect of channel forms and discharge variation on nutrient spiralling in the hyporheic zone (HZ) of streams. Four experimental flow manipulations (EFM) were carried out at two reaches with different channel forms (straight vs. sinuous) in an oligotrophic subtropical river in Australia. Flow manipulation consisted of reducing the river width with a temporary dam, diverting and concentrating the main water flux on two different geomorphological units (riffle vs. gravel bar), in order to simulate flooding conditions. Hyporheic waters were analysed for their physicochemical characteristics and nutrient (nitrates + nitrites = NO x and soluble reactive phosphorus [SRP]) and OM contents at two depths (10 and 50 cm) within the bed sediments, both upstream and downstream of the geomorphological units. The physicochemical parameters clearly demonstrated the existence of hyporheic flow paths, characterized by the alternation of downwelling and upwelling areas, with more consistent gradients in gravel bars than in riffles. The HZ acted as source for NO x and SRP, but this role varied between geomorphological units and reaches. The effect of EFM differed between sampling points, irrespective of the type of geomorphological unit. In gravel bars, a flush out during high discharge was observed for NO x , SRP and particulate organic matter (POM) at the sinuous channel, whereas storage and removal were recorded at the straight channel for SRP and NO x , respectively. At the riffle of the sinuous channel, very fine POM accumulated, while removal was noticed for POM. In contrast, at the riffle of the straight channel, SRP accumulated in the HZ and NO x was removed out of the HZ. Nutrient dynamics in the HZ and the response to flow increases were not governed by the geomorphological unit type. Other parameters that determine water residence time in the sediments, such as local heterogeneity in sediment characteristics (grain size, porosity and hydraulic conductivity), channel sinuosity, reach slope and the size and form of the gravel bar, may be more significant explanatory variables for understanding OM and nutrient dynamics in the HZ. This study emphasizes the need for caution in making generalisations about the role of river sediment in nutrient storage and the impact of floods on nutrient dynamics.

Yangtze River sediments from source to sink traced with clay mineralogy

River bed sediments were collected from the main stream and major tributaries of the Yangtze River for clay mineralogy study. Surface sediments from the Yarlung Zangbo River on the Tibetan Plateau were also examined for comparison. The results show that the clay mineral compositions of the Yangtze River display a similar pattern through the whole truck stream, with illite being dominant, kaolinite and chlorite being lesser abundant, and smectite being minor component. Clay mineralogy shows distinct differences in the tributaries, which correspond to the heterogeneous source rocks and weathering intensity of the drainage. The illite crystallity and the illite chemical weathering index (5Å/10Å peak ratio) both increase downstream, indicating a increasing trend of hydrolysis along the river. It also indicates that the upperstream of the drainage is characterized with physical weathering while the middle- and lower reaches are controlled by chemical weathering process. In accordance with the result derived by the illite indexes, sediment input from upperstream including Yalong Jiang, Dadu He, Min Jiang and Jialing Jiang accounts for the major sediment load, whereas Wu Jiang, Xiang Jiang, Gan Jiang and Dongting Lake provide relatively less sediments.

Potential release of selected trace elements (As, Cd, Cu, Mn, Pb and Zn) from sediments in Cam River-mouth (Vietnam) under influence of pH and oxidation

Since contaminated river-bed sediments in the Cam River-mouth (Vietnam) are regularly dredged and disposed on land, an understanding of the influence of time, pH and oxidation on the leaching behavior of heavy metals (Cd, Cu, Mn, Pb and Zn) and arsenic is necessary for the management of these dredged materials. A 96h pHstat-leaching test to examine the leaching behavior of elements at pre-set pH values (2, 4, 6, 8 (natural), 9 and 11) and a BCR 3-step extraction to clarify the element fractionation, were performed on a freshly-collected wet suboxic sediment and a dry oxidized sediment. All heavy metals and arsenic display a V-shaped pH-dependent leaching pattern with important releases at pHs 2 and 11. At the investigated pH values, the release of As, Mn, Pb and Zn from the oxidized sediment is slower and lower if compared with the suboxic sediment while the opposite trend is found for Cd and Cu at pHs 2–8. The transfer from the acid-soluble (exchangeable and carbonate-bound) fraction to the reducible (Fe and Mn hydr/oxide-bound) fraction is consistent with the lower leachability of As, Mn and Zn at pHs 2–8 and Pb at pHs 4–8 after oxidation, while the transfer from the oxidizable (organic matter and sulfide-bound) fraction to the reducible fraction relates to the higher leachability of Cd and Cu at pHs 2–8. The lower leachability of all elements at alkaline pHs 9–11 is due to lower leached concentration of organic matter from the oxidized sediment. Sulfides only play a minor role in controlling the leachability of heavy metals and arsenic.

Organic matter dynamics and stable isotope signature as tracers of the sources of suspended sediment

Abstract. Suspended sediment (SS) and organic matter in rivers can harm brown trout Salmo trutta by affecting the health and fitness of free swimming fish and by causing siltation of the riverbed. The temporal and spatial dynamics of sediment, carbon (C), and nitrogen (N) during the brown trout spawning season in a small river of the Swiss Plateau were assessed and C isotopes as well as the C/N atomic ratio were used to distinguish autochthonous and allochthonous sources of organic matter in SS loads. The visual basic program IsoSource with 13Ctot and 15N as input isotopes was used to quantify the temporal and spatial sources of SS. Organic matter concentrations in the infiltrated and suspended sediment were highest during low flow periods with small sediment loads and lowest during high flow periods with high sediment loads. Peak values in nitrate and dissolved organic C were measured during high flow and high rainfall, probably due to leaching from pasture and arable land. The organic matter was of allochthonous sources as indicated by the C/N atomic ratio and δ13Corg. Organic matter in SS increased from up- to downstream due to an increase of pasture and arable land downstream of the river. The mean fraction of SS originating from upper watershed riverbed sediment decreased from up to downstream and increased during high flow at all measuring sites along the course of the river. During base flow conditions, the major sources of SS are pasture, forest and arable land. The latter increased during rainy and warmer winter periods, most likely because both triggered snow melt and thus erosion. The measured increase in DOC and nitrate concentrations during high flow support these modeling results. Enhanced soil erosion processes on pasture and arable land are expected with increasing heavy rain events and less snow during winter seasons due to climate change. Consequently, SS and organic matter in the river will increase, which will possibly affect brown trout negatively.

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.

Tracing soil organic carbon in the lower Amazon River and its tributaries using GDGT distributions and bulk organic matter properties

In order to trace the transport of soil organic carbon (OC) in the lower Amazon basin, we investigated the distributions of crenarchaeol and branched glycerol dialkyl glycerol tetraethers (GDGTs) by analyzing riverbed sediments and river suspended particulate matter (SPM) collected in the Solimões-Amazon River mainstem and its tributaries. The Branched and Isoprenoid Tetraether (BIT) index, a proxy for river-transported soil OC into the ocean, was determined from the distributions of these GDGTs. The GDGT-derived parameters were compared with other bulk geochemical data (i.e. C:N ratio and stable carbon isotopic composition). The GDGT-derived and bulk geochemical data indicate that riverine SPM and riverbed sediments in the lower Amazon River and its tributaries are a mixture of C3 plant-derived soil OC and aquatic-derived OC. The branched GDGTs in the SPM and riverbed sediments did not predominantly originate from the high Andes soils (>2500m in altitude) as was suggested previously. However, further constraint on the soil source area of branched GDGTs was hampered due to the deficiency of soil data from the lower montane forest areas in the Andes. Our study also revealed seasonal and interannual variation in GDGT composition as well as soil OC discharge, which was closely related to the hydrological cycle. By way of a simple binary mixing model using the flux-weighted BIT values at Óbidos, the last gauging station in the Amazon River, we estimated that 70–80% of the POC pool in the river was derived of soil OC. However, care should be taken to use the BIT index since it showed a non-conservative behaviour along the river continuum due to the aquatic production of crenarchaeol. Further investigation using a continuous sampling strategy following the full hydrological cycle is required to fully understand how soil-derived GDGT signals are transformed in large tropical river systems through their transport pathway to the ocean.

The dependence of meteoric 10Be concentrations on particle size in Amazon River bed sediment and the extraction of reactive 10Be/9Be ratios

Concentrations of meteoric 10Be extracted from detrital Amazon River sediment are strongly particle size-dependent. Such grain size dependency presents a formidable obstacle to routine applications of meteoric cosmogenic 10Be to basin-wide erosion studies. In this study, we explore means to eliminate these grain size effects in bedload from rivers of the Amazon basin by measuring 10Be/9Be ratios after selective chemical extraction of reactive authigenic phases. These phases mainly comprise Fe–Mn-(hydr)oxides that are the main carriers of 9Be and 10Be. We explore the distribution of 10Be and its stable 9Be counterpart between these phases and the partitioning of Be into remaining silicate fractions. This extraction procedure was carried out on bedload samples comprising the three main tectonic units of the Amazon basin, namely the Andes, the Guyana and Brazilian Shields, and the central Amazonian lowlands. For all samples, extracted 10Be concentrations show a strong decrease with increasing particle size, with up to 20 times more 10Be extracted from the finest analyzed (<30μm) grain size when compared to the coarsest (90–125μm) grain size. We attribute this decrease in 10Be concentrations mostly to subsequent dilution by quartz in coarse bedload. However, when normalized to stable 9Be concentrations that are also extracted from reactive phases, grain size effects are effectively removed and the resulting “reactive” 10Be/9Be ratio is uniform for all particle sizes. The reactive 10Be/9Be ratio thus offers important advantages over methods employing single meteoric 10Be concentrations, such that this ratio can now be developed as quantitative tracer for Earth surface processes.

Spatio-temporal patterns of microbial communities in a hydrologically dynamic pristine aquifer

Seasonal patterns of groundwater and sediment microbial communities were explored in a hydrologically dynamic alpine oligotrophic porous aquifer, characterized by pronounced groundwater table fluctuations. Rising of the groundwater level in consequence of snow melting water recharge was accompanied by a dramatic drop of bacterial Shannon diversity in groundwater from H' = 3.22 ± 0.28 in autumn and winter to H' = 1.31 ± 0.35 in spring and summer, evaluated based on T-RFLP community fingerprinting. Elevated numbers of bacteria in groundwater in autumn followed nutrient inputs via recharge from summer rains and correlated well with highest concentrations of assimilable organic carbon. Sterile sediments incubated to groundwater in monitoring wells were readily colonized reaching maximum cell densities within 2 months, followed by a consecutive but delayed increase and leveling-off of bacterial diversity. After 1 year of incubation, the initially sterile sediments exhibited a similar number of bacteria and Shannon diversity when compared to vital sediment from a nearby river incubated in parallel. The river bed sediment microbial communities hardly changed in composition, diversity, and cell numbers during 1 year of exposure to groundwater. Summing up, the seasonal hydrological dynamics were found to induce considerable dynamics of microbial communities suspended in groundwater, while sediment communities seem unaffected and stable in terms of biomass and diversity.

Phosphorus sorption–desorption behaviour of river bed sediments in the Abshineh river, Hamedan, Iran, related to their composition

Phosphorus (P) sorption by sediments may play a vital role in buffering P concentration in the overlying water column. To characterize P sorption-desorption in the river bed sediments, 17 bed sediment samples collected from Abshineh river, in a semi arid region, Hamedan, western Iran were studied through a batch experiment and related to sediment composition. The sorbed fraction ranged from 4.4% to 5.4% and from 38.5% to 86.0% of sorption maxima when 20 and 1,500mgPkg(-1), respectively, was added to the sediment samples. Phosphorus sorption curves were well fitted to the Langmuir model. Zero equilibrium P concentration ranged from 0.10 to 0.51mgPl(-1) and varied with sediment characteristics. Phosphorus desorption differed strongly among the studied bed sediments and ranged from 10.8% to 80.2% when 1,500mgPkg(-1) was added. The results of the geochemical modelling indicated that even under low P addition (2mgl(-1)), the solutions are mainly saturated with respect to hydroxyapatite and ß-tricalcium phosphate minerals and undersaturated with respect to other Ca and Mg minerals, whereas under higher P addition (150mgl(-1)), most Ca-P solid phases, except the most soluble mineral (brushite), will likely precipitate. A Langmuir sorption maximum was positively correlated with carbonate calcium. Estimated P retention capacity of the bed sediments are generally lower and zero equilibrium P concentration values higher in upstream sites than at the downstream sites, suggesting that sediments in upstream and downstream may act as source and sink of P, respectively.

Elemental composition of bottom-sediments from Pangani river basin, Tanzania: lithogenic and anthropogenic sources

River-bed sediments from the Pangani basin, Tanzania, were characterized for elemental compositions, following contamination risks from rapid expansions of human activities in the area. Samples were collected during two individual seasons and analyzed by high-polarizing beam energy dispersive X-ray fluorescence (EDXRF) for eight major and 14 trace elements. Evaluation of enrichment factors (EFs) was used to investigate the elemental flux and assess the contributions of natural and anthropogenic influences. The abundances of the major elements followed the order Si > Al > Fe > Ca > K > Ti > Mn > P, similar to that of the upper earth's crust, and were generally from the weathering of the bed-rock. The high concentrations of typical anthropogenic trace-elements (Cr < 160 mg kg−1, V < 85 mg kg−1, Ni < 60 mg kg−1, Cu < 87 mg kg−1, La < 90 mg kg−1) coupled with high EFs (>2) in some locations indicated contamination associated with agricultural and industrial activities. Factor analysis extracted five principal components that contributed to 96.0% of the total observed variance. The results indicated that river-bed sediments of the Pangani basin were influenced to a larger extent by lithogenic sources than anthropogenic impacts.

Exploring How Organic Matter Controls Structural Transformations in Natural Aquatic Nanocolloidal Dispersions

The response of the dispersion nanostructure of surface river bed sediment to the controlled removal and readdition of natural organic matter (NOM), in the absence and presence of background electrolyte, was examined using the technique of small-angle neutron scattering (SANS). Partial NOM removal induced aggregation of the mineral particles, but more extensive NOM removal restored colloidal stability. When peat humic acid (PHA) was added to a NOM-deficient sediment concentration-related structural transformations were observed: at 255 mg/L PHA aggregation of the nanocolloid was actually enhanced, but at 380 mg/L PHA disaggregation and colloidal stability were promoted. The addition of 2 mM CaCl(2) induced mild aggregation in the native sediment but not in sediments with added PHA, suggesting that the native NOM and the PHA respond differently to changes in ionic strength. A first attempt at using SANS to directly characterize the thickness and coverage of an adsorbed PHA layer in a natural nanocolloid is also presented. The results are discussed in the context of a hierarchical aquatic colloidal nanostructure, and the implications for contemporary studies of the role of dissolved organic carbon (DOC) in sustaining the transport of colloidal iron in upland catchments.

Methane exceeds nitrous oxide in rivers' contribution to warming

Nitrous oxide emissions have been the leading area of concern for scientists investigating the role of streams and rivers in global climate change for the past decade. Nitrous oxide, a potent greenhouse gas, is produced in riverbed sediments through nitrification and denitrification. Efforts to understand the rate at which nitrous oxide diffuses through the water to the atmosphere have dominated the field, yet diffusion is not the only relevant mechanism, nor is nitrous oxide the only relevant gas. Observations by Baulch et al. suggest that the global warming potential of methane gas, which they measured bubbling up from several riverbeds, exceeded that of nitrous oxide.

Characterization of the key pathways of dissimilatory nitrate reduction and their response to complex organic substrates in hyporheic sediments

Laboratory incubations with river‐bed sediment collected from riffles and pools were used to quantify potential pathways of dissimilatory nitrate reduction in the hyporheic zone of a groundwater‐fed river. Sediments collected from between 5‐cm and 86‐cm depth in the bed of the River Leith, Cumbria, United Kingdom, were incubated with a suite of 15N‐labeled substrates (15NO−3, 15NH+4, and 14NO−3) to quantify nitrate reduction via denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and anaerobic ammonium oxidation (anammox). Denitrification was the dominant pathway of dissimilatory nitrate reduction in the hyporheic sediments, although recovery of 15N from the ammonium pool indicated that DNRA was also active. The potential for anammox was confirmed by the production of 29N2 during the 15NH+4 and 14NO−3 incubation, but it was much smaller than denitrification. Potential rates of denitrification were highest in shallow sediments and decayed exponentially with depth thereafter. There were clear differences in denitrification activity between riffle and pool sediments. After the production of 15N‐N2 had stabilized, we added a spike of bacteriological peptone to determine the effect of complex organic substrates on denitrification potential. The potential rate of denitrification increased uniformly at all sediment depths but the total amount of denitrification fueled by the organic substrates decreased markedly with depth, from 90% in the shallow sediments to 30% in the deepest sediments. In addition, a considerable fraction of the 15NO−3 could not be accounted for, which suggested that up to 87% of it had been assimilated in the deepest sediments.

Time and depth scales of fine sediment delivery into gravel stream beds: Constraints from fallout radionuclides on fine sediment residence time and delivery

Particles of fine sediment can clog interstitial pore spaces of coarser grained sediment in river beds and thereby impede the exchange of water, dissolved constituents, and particulate matter with consequent ecological impacts. The extent to which fine sediment reduces connectivity between the stream bed and overlying channel is a function, in part, of sediment residence time. Short residence times imply frequent exchange of matter and reduced impact of fine sediment on stream bed habitat, while long residence times indicate that with respect to fine sediment delivery, the stream bed is more isolated from the overlying channel. Here we present a novel technique to quantify the residence time of river bed sediment at various depths over annual and decadal timescales using the fallout radionuclides (FRNs) 7Be (t1/2=53days) and 210Pbex (t1/2=22.3years). We placed mesh cylinders filled with 7Be-free sediment into a stream bed to quantify the capture of 7Be-tagged particles in the absence of scour and fill. We also took cores to the depth of refusal in alluvial sediment in unregulated and regulated rivers in Vermont and New Hampshire. Sampled watershed areas ranged from 29 to 410km2, and core depths ranged from 19 to 77cm. The 210Pbex activity profiles of cores show that bed sediment is exchanged to the depth of refusal at decadal timescales. In contrast, 7Be activity profiles indicate that fine sediment infiltrating into the bed had residence times ranging from 4 to >300days in unregulated rivers. Cores from a regulated river are notably different—subsurface sediment residence times were always longer than in unregulated rivers at comparable depths, likely owing to restriction of bed mobilization and clogging of bed material by fine sediment. These results suggest that filtration can be an important component of bed material delivery to stream beds, but filtration does not deliver material as deeply into the bed as scour and fill. We find that fallout radionuclides provide direct measurements of time and depth scales of active exchange of particulate matter, which are key controls on hyporheic function.