Purpose

The heavily forested Cache la Poudre (CLP) watershed in northern Colorado, USA, was impacted by the High Park wildfire in 2012. The wildfire burned land and vegetation immediately adjacent to the CLP River where blackened, ashy sediment samples were collected from five sites upstream of the City of Fort Collins drinking water intake to evaluate the spatial distribution and characteristics of burned sediments, along with quantifying and characterizing soluble compounds following a leaching experiment.

Materials and methods

At each site, samples were collected from three locations: (1) the edge of the bank adjacent to the water edge (downbank), (2) 1 m upslope of location 1 (midbank), and (3) 2 m upslope of location 1 (upperbank). All solid sediment samples were analyzed for elemental composition, and a subset of solid sediment samples were analyzed with ¹³C solid-state nuclear magnetic resonance spectroscopy. Sediments were mixed with the background CLP River water collected from upstream of the wildfire and allowed to leach for 6 and 24 h to determine the quantity and quality of water-soluble constituents. Filtered samples were analyzed for dissolved organic carbon (DOC), iron, manganese, and inorganic nutrient concentrations, by optical properties, and for disinfection byproduct (DBP) formation.

Results and discussion

Percent carbon and nitrogen content of the solid sediments were good predictors of leachate DOC concentration. The mean fluorescence index was higher for wildfire-impacted sediment leachates (1.50) compared to the background CLP River water (1.37), which may be due to changes in DOM molecular weight and oxidation of organic matter. All sediment leachates showed consistently higher haloacetonitrile and chloropicrin yields (DBP concentration/DOC concentration) compared to background CLP River water, whereas carbonaceous DBPs did not.

Conclusions

The collected sediments showed that burned material accumulated downstream near the river and was composed of inputs from burned soil and biomass along with the mobilization of unburned terrestrial material. The leachates of these sediments have different characteristics compared to the background CLP River water, indicating that DOM leached from sediments following a wildfire may increase aquatic DOC concentrations and N-DBP formation.

Purpose

The low conductivity of sediments for mass and electron transport is the most severe limiting factor in sediment microbial fuel cells (SMFCs), so that sediment ameliorations yielded more remarkable effects than electrode improvements. The objective of this research was to enhance the electricity generation of SMFCs with amendments of biochar to freshwater sediments for conductivity enhancement.

Materials and methods

Laboratory-scale SMFCs were constructed and biochars were produced from coconut shells at different temperatures. Variations in the power output, electrode potential, internal resistance, total organic carbon (TOC) content, and microbial communities were measured.

Results and discussion

Amending with biochar reduced the charge transfer resistances of SMFCs and enriched the Firmicutes (mainly Fusibacter sp.) in the sediment, which improved the SMFC power generation by two- to tenfold and enhanced the TOC removal rate by 1.7- to fourfold relative to those without the amendment.

Conclusions

The results suggested that biochar amendment is a promising strategy to enhance SMFC power production, and the electrical conductivity of biochar should be considered important when interpreting the impact biochar has on the electrical performance of soil or freshwater sediment MFCs.

Purpose

Using Ells River, Alberta, Canada bed sediments, this study aims to determine (1) the erosion, transport, and deposition characteristics of cohesive bottom sediments, and (2) the influence of the microbial community in this regard.

Materials and methods

A 2-m annular flume was used to generate bed shear to assess cohesive sediment dynamics for eroded beds with consolidation/biostabilization periods of 1, 3, and 7 days. Additional optical particle sizing, image analysis, densitometry, and microbial analysis were employed to further the analysis with respect to bed erosion and eroded floc characteristics.

Results and discussion

Sediment dynamics can influence the benthic and planktonic community health within aquatic systems. The critical bed shear stress for erosion increased from 0.05 to 0.19 Pa (for 1- to 7-day runs). Consolidation (dry density) increased with time and depth and eroded biofilm biomass was observed to increase with time. The community structure of the eroded sediment did not change with time suggesting a stable well-established and highly selected community. Hydrocarbon-degrading bacteria were present within the microbial consortium. The sediment was highly hydrophobic (96 %) due to a high natural oil content which likely had a profound effect on sediment dynamics, flocculation, and sediment cohesion. Eroded sediment settled poorly, which will result in the long-range transport of associated contaminants.

Conclusions

The Ells River possesses some unique properties which should be considered when assessing contaminant source, fate, and effect. The most significant of these are small floc size, the hydrophobicity of the sediment, and the biological community as these were found to be influential in both the erosion and flocculation processes. It is important that any management strategies and operational assessments of reclamation strategies that may have implication on river health incorporate the sediment compartments (SS and bed sediment), biology, and the energy dynamics within the system in order to better predict the downstream flux of sediments.

Purpose

We determined the relationship of a planktonic algal bloom with spatial and temporal changes in sediment bacterial communities in a eutrophic urban river in the Taihu Basin, China.

Materials and methods

Surface sediments from different locations on the river, including with and without an algal bloom, were collected monthly for 1 year. Sediment bacterial communities were assessed by sequencing 16S rRNA gene amplicons using an Illumina MiSeq.

Results and discussion

There were distinct spatial and temporal changes in sediment bacterial community structure. Significant decrease in the Shannon diversity corresponded with the peak chlorophyll a (Chl a) concentration. Proteobacteria, Chloroflexi, and Bacteroidetes were the dominant phyla throughout the year; however, their relative abundances changed seasonally. At the time of peak Chl a concentrations, Proteobacteria and Bacteroidetes comprised a greater proportion of the bacterial community. Bacterial community structure also varied spatially at the operational taxonomic unit (OTU) level according to canonical correspondence analysis (CCA), especially in locations with an algal bloom compared with those without an algal bloom. There were positive correlations between multiple bacterial genera and Chl a content, suggesting the potential for facilitative relationships between phytoplankton and bacteria. Based on the CCA, water temperature, NH₄ ⁺-N, TN, NO₃ ^?-N, and TP in the sediment and Chl a contents in the water column were significantly correlated with sediment bacterial community structure (P < 0.05).

Conclusions

Planktonic algal blooms may influence sediment bacterial community structure at all taxonomic levels in urban rivers. This work emphasizes the need for more comprehensive studies regarding the impact of planktonic algal blooms on sediment bacterial communities.

Purpose

Increased sedimentation due to land use intensification is increasingly affecting carbon processing in streams and rivers around the globe. This study describes the design of a laboratory-scale flow-through incubation system as a tool for the rapid estimation of sediment respiration. The measurements were compared with those obtained using an in situ closed chamber respiration method. The influence of sediment size on respiration rates was also investigated.

Materials and methods

Measurements were conducted on a pre-alpine gravel-bed river sediment separated into the following grain size fractions: > 60 mm (14.3%), 60–5 mm (60.2%), 5–2 mm (13.7%), 2–0.063 mm (11.1%) and <0.063 mm (0.6%). Concurrently, in situ and laboratory measurements were carried out on a naturally heterogeneous sediment. In situ respiration was determined in closed chambers as O₂ consumption over time, while in the laboratory, respiration was determined using flow-through respiration chambers. Oxygen concentrations were measured using a fibre-optic oxygen meter positioned at the inflow and outflow from the chamber.

Results and discussion

The mean respiration rates within naturally mixed riverbed sediments were 1.27 ± 0.3 mg O₂ dm^?3 h^?1 (n = 4) and 0.77 ± 0.1 mg O₂ dm^?3 h^?1 (n = 3) for the flow-through chamber system and closed chamber system, respectively. Respiration rates were statistically significantly higher in the flow-through chamber system (t test, p < 0.05), indicating that closed chamber measurements underestimated the oxygen consumption within riverbed sediments. Sediment grain size was found to significantly affect respiration rates in both systems (ANOVA, p < 0.001) with the fine sediment fraction (particle size <0.063 mm) having the highest respiration rate (r_flow-through = 51 ± 23 mg O₂ dm^?3 h^?1). The smallest fractions (2–0.063 and <0.063 mm), which represent approximately 12% of total sediment volume, contributed 60% of total respiration.

Conclusions

The study demonstrated that flow-through respiration chambers more accurately estimate the respiration rate within riverbed sediments than in situ closed chambers, since the former experiment imitates the natural conditions where continuous interstitial flow occurs in the sediments. We also demonstrated that fine sediments (<5 mm) substantially contribute to heterotrophic respiration in the studied gravel-bed river.

Purpose

Rivers feeding into the Loskop Dam, South Africa, pass through coal mining and heavily industrialised areas. Previous studies investigating mass mortalities of crocodile and fish in the river system, revealed the presence of organic compounds, including polycyclic aromatic hydrocarbons (PAHs), in their tissues.

Materials and methods

Samples were collected from nine sites within the dam in winter and summer. Liquid-liquid and microwave-assisted extraction was used for preconcentrating PAHs from water and sediment samples, respectively. Extracts were analysed by gas chromatography-mass spectrometry in the selected ion monitoring mode to determine the presence and levels of the 16 United States Environmental Protection Agency (US EPA)-priority PAHs.

Results and discussion

Significant levels of PAHs were found in both water and sediment samples. Concentrations were higher in sediments than in water, as well as in samples collected in winter. Levels of PAHs in sediments were generally higher than those reported by many researchers for other water bodies in industrialised areas. Zebrafish embryos were used to assess potential risks associated with the water and sediment, and to determine the effects of PAHs on aquatic life. Embryos, exposed to intact water and sediment samples, as well as to diluted sediment extracts, were monitored for 120 h post fertilisation. Sediment proved more toxic to zebrafish embryos than water, causing delayed embryo development and malformations.

Conclusions

These findings are alarming as they indicate that this water system is under stress. These findings can be typical of any water system situated in a coal mining and industrial region.

Purpose

The present study investigated lake and river sediments affected by metals from an acid mine drainage (AMD) from a former uranium mine. The role of bacterial sulfate reduction in the immobilization of contaminants was evaluated, and the analyses of acid volatile sulfide (AVS) and sequential extraction were performed. Consequently, the potential mobility and bioavailability of contaminants were established.

Materials and methods

Sulfur isotopic fractionation (δ³⁴S), AVS, and sequential extraction procedure were used to assess the sulfate bacterial reduction and the availability of contaminants in the environment at six sampling stations.

Results and discussion

The δ³⁴S indicated that bacterial reduction is a key process in the natural attenuation of contamination in the Águas Claras reservoir, precipitating metal sulfides. According to the USEPA criteria, adverse biological effects are expected for sample S1 (inside the reservoir) which is likely to be toxic, while for sediment S4 (in the river), the toxicity is uncertain. The other samples were classified as non-toxic, likely because of the decreased solubility of zinc sulfide. A decrease in the concentration of the contaminants downstream of the reservoir was observed. The predominance of U (0.4 %) in the labile fraction and the elevated concentrations of Zn (0.5 %) and Mn (0.7 %) in the sediments inside the reservoir raises concerns regarding the availability of these contaminants in the environment.

Conclusions

The main environmental impact appears to be concentrated in the Águas Claras reservoir, whereas the Antas creek does not seem to be affected by the AMD process. Although the bacterial sulfate reduction is effective in its production of sulfides capable of immobilizing the contaminants, the presence of Zn and U in the labile and reducible fraction is a matter of concern due to its long-term bioavailability. Thus, continuous monitoring of the redox potential of the waters and sediments, mainly in the reservoir, is recommended in order to assess and possibly prevent later dissolution of sequestered contaminants.

Purpose

The particle-size distribution of runoff sediment is important in understanding, characterizing and modeling the transport behavior of sediment and sediment-associated chemicals. The objective of this study was to investigate the particle-size distribution of sediments eroded from three soils in China under natural rainfall.

Materials and methods

Each of the three soils was packed to a depth of 30 cm in a 20?×?2.1 m runoff plot. Sediments yielded in nine natural rainfall events were analyzed for their particle-size distribution prior to and following dispersion.

Results and discussion

The sediment size measured in the undispersed condition was always larger than the one determined after chemical dispersion, indicating that part of the sediment was eroded in aggregated form. The degree of sediment aggregation depended on the clay content and the organic matter content of the sources. The mean sediment size quantified by mean weight diameter linearly increased with sediment yield for the two soils with relatively high clay content. The rate of increase was greater in the undispersed condition than that in the dispersed condition for these two soils. Comparing sediments to the corresponding source soil, the results of mean weight diameter and enrichment ratio both revealed that aggregate-size distribution was more sensitive to soil erosion than the primary particle-size distribution. Small aggregates, rather than the primary particles, were selectively eroded in the rainfall events.

Conclusions

These findings support the use of both dispersed and undispersed sediment-size distributions for the characterization of sediment transport and the associated sediment-bound nutrients and contaminants.

Purpose

Biochar is a carbon-rich product, able to enhance soil fertility and mitigate CO₂ emissions. While biochar effects on agriculture are becoming known, its impact elsewhere, e.g., on estuarine ecosystems, has yet to be assessed. The main aim of the present study was to determine the effect of biochar on sediment–water retention, CO₂ emissions from sedimentary organic carbon decomposition, sediment pH and electrical conductivity, in aerobic conditions similar to those observed at low tide.

Materials and methods

Sediments from the Mondego Estuary (Portugal) were mixed with pine gasification biochar at different doses (5, 10, 14 %) and immersed in water with different salinity values (15, 25, 30) for 96 h. The influence of biochar on water retention, the residence time of water and CO₂ emissions between ?0.75 and ?1.5 MPa, total organic carbon, pH and electrical conductivity (EC) were determined. Carbon chemical composition and polycyclic aromatic hydrocarbon (PAH) concentrations were determined in sediments and biochar. Differences between biochar treatments after immersion in different water salinities were analysed using the Kruskal–Wallis test.

Results and discussion

Results showed that biochar was able to (a) increase sediment–water content in terms of quantity and residence time, (b) decrease CO₂ emissions, but only with a specific soil–water content and at the highest biochar dose, (c) increase sediment pH at all biochar doses and (d) increase sediment EC at the highest biochar dose. In contrast, the percentage of carbon mineralised was not modified. Biochar carbon was rich in PAHs and less decomposable than sedimentary carbon. The increments observed in sediment pH and EC were unable to change sediment alkaline or saline status according to standard classifications.

Conclusions

Our results suggest that the remarkable water adsorption capacity of biochar–sediment mixtures may be considered the main factor in regulating CO₂ emission rates from sediments, together with high PAH concentrations, which probably restrain the organic matter decomposition process.

Purpose

Particularly in organic viticulture, copper compounds are intentionally released into the environment as fungicide, whereas uranium originates from conventional phosphate fertilization. Both activities contribute to the metal contamination in wine-growing areas. This pilot study aimed to better understand how soil properties influence the presence and environmental fate of copper and uranium with respect to viticultural management.

Materials and methods

We characterized metal binding forms, i.e., their association with different soil constituents, in organically and conventionally cultivated vineyard soils and adjacent upstream and downstream sediments. The available metal fraction and the fractions associated with manganese oxides, organic matter, iron oxides, and total contents were extracted sequentially.

Results and discussion

Total soil copper ranged from 200 to 1600 mg kg^?1 with higher contents in topsoil than subsoil. The majority of copper (42–82%) was bound to soil organic matter. In all fractions, copper contents were up to 2-fold higher in organic than in conventional vineyards, whereas the sediment concentrations were independent of the adjacent viticultural management. A net increase of copper in downstream sediments was found only when water-extractable organic carbon (WEOC) in an adjacent vineyard was elevated. With 11 ± 1 mg kg^?1, total uranium was 25% higher in conventional than in organic vineyard soils. Its affinity to iron or WEOC potentially rendered uranium mobile leading to a substantial discharge to downstream sediments.

Conclusions

Translocation of copper and uranium from vineyards into adjacent stream sediments may rather be attributed to WEOC and iron contents than the viticultural management. Follow-up studies should scrutinize the processes driving metal availability and transport as well as their interaction at the aquatic–terrestrial interface.

Purpose

Wood extractives in sediments originating from the wood industry may interfere with benthic biota in aquatic environments. The research area was the Äänekoski watercourse in Central Finland, which has been affected by the chemical wood industry for over a century. The goal was to determine the dissolution potency of resin acids (RAs) and their derivative, retene, in the sediment, and their current vertical and spatial stratification to assess the load due to potential erosion.

Materials and methods

Sediments were collected from two upstream reference sites and three lake-like basins, located as far as 33 km downstream from the mills. The dissolution potency was studied by two different agitation times and temperatures from sediment-water (1+4 v/v) elutriates. The vertical distribution of extractives was determined from the uppermost 20 cm of sediment. Using spatial interpolation, the distribution of extractives was estimated from two upper sediment layers (0–2 and 2–5 cm) downstream from the source. According to the interpolation, the total amount of dehydroabietic (DHAA) and isopimaric acids (IPA) were calculated as kg/ha in the whole sediment area.

Results and discussion

The total concentration of RAs in the surface sediment reached up to 168 µg/g dw, and they were found to desorb to water up to 77 µg/l. The concentrations of retene were low both in the sediment (<51 µg/g dw) and elutriate (<0.53 µg/l). Spatial interpolation showed that the highest calculated amounts of the most abundant RAs were in Kuhnamo basin, in the sediment layer 2–5 cm; the estimated amount of DHAA and IPA were approximately 440 and 85 kg/ha, respectively.

Conclusions

Disturbances may change the exposure situation, causing desorption of sediment-associated compounds in levels that may be harmful to aquatic animals. The amount of desorption varies depending on the concentration of contaminants in sediment, the nature of disturbance, and the sediment organic carbon content. Low retene concentrations can be explained by oxic conditions and low abundance of RAs in the sediments.

Purpose

Fine sediments are usually collected in situ and air-dried for adsorption experiments, which may lead to particle aggregation and thus have a significant effect on phosphorus (P) adsorption under dynamic conditions. The main purpose of this study was to investigate the changes of aggregates due to drying with shear stress and the effects on the adsorption of P onto air-dried sediments under different shear rates after re-wetting.

Materials and methods

Sediment samples were collected from an alluvial river. Fine wet sediments (<31 μm) were wet-sieved and air-dried, and some air-dried sediments were further sonicated and served as the control. The grain size distribution of the three sediment samples (wet, dried, and sonicated) was measured to evaluate the particle aggregation level. The P sorption capacity of wet and dried sediments was determined by batch equilibrium experiments. The change of aggregate size with shear stress was investigated for dried and sonicated sediments. Sorption equilibrium experiments were performed to investigate the effect of shear stress on the P sorption with and without change of particle aggregation level, respectively.

Results and discussion

Fine particles agglomerated into larger aggregates during the drying process, resulting in a significant increase in the aggregate size. The sorption capacity was lower in aggregated sediment than in original wet sediment. Aggregate size in dried sediment decreased with the increase of shear rate, leading to an increase in the surface area and available adsorption sites, whereas the particle size of sonicated sediment was hardly affected. Accordingly, the P sorption amount of dried sediment increased with increasing shear rate, whereas that of sonicated sediment showed no significant change with shear rate after all sediments were suspended. There was a significant curvilinear correlation between aggregate size and P sorption amount for dried sediments, thus indicating that the P sorption amount increased significantly with decreasing aggregate size.

Conclusions

Sediment aggregation is an important factor affecting P adsorption besides the amount of suspended sediments and the exchange between suspended and bed sediments under dynamic conditions. The P equilibrium adsorption amount increases with shear stress for air-dried sediment. The effect of particle aggregation on the amount of P sorbed onto sediments should not be ignored, and thus, more attention should be paid to the pretreatment of sediment samples in the sorption experiments under dynamic conditions.

Purpose

Erosion processes at the catchment scale control a basin’s morphology and sediment patterns in the river network. Eroded sediments are transported and deposited downstream and may cause environmental problems and relevant effects on water storage and hydropower infrastructures. Quantification of water and sediment yield is complex due to the physical processes involved and their temporal and spatial variability, especially at the light of current global change.

Materials and methods

Numerical models that use spatially distributed information constitute a useful tool for these estimates, when sufficient input data are available. In this study, we applied the hydrological and sedimentological TETIS model to determine the patterns of water and sediment yield in a large mountain catchment. Flow discharge data obtained from two gauged stations were used for calibration and validation of the hydrological sub-model. Data from two reservoir bathymetries at the outlet of the study area were used for calibration of the sedimentological sub-model. After model calibration, several scenarios of climate and land use change were simulated.

Results and discussion

Climate scenarios show a general decrease in average annual precipitation and an increase in temperature, associated with an increase in extreme rainfall events. Global change scenarios lead to a counteracting effect between the increase in sediment transport during extreme events and the decrease in sediment erosion associated with afforestation following the abandonment of agricultural land. In the case of the most extreme climate scenario combined with total catchment deforestation, the model indicates a complete siltation of the reservoir by 2050.

Conclusions

Model performance emphasizes its potential as a tool for evaluating water and sediment yield for large catchments, as well as of its usefulness for water and sediment management in light of future climate and land use change scenarios.

Purpose

Manchester is often heralded as the first industrial city. Large volumes of physical and liquid contaminants were released into its river network throughout the industrial period up to the latter part of the twentieth century. Water quality has improved dramatically in recent decades, but, given their environmental significance, it is important to ascertain the extent to which a legacy of contamination persists in the modern bed sediments.

Materials and methods

Fine-grained bed sediments were sampled at 40 sites in the Mersey and Irwell catchments. Sediments were wet sieved to isolate the <63-μm grain size fraction. Metal concentrations were determined using XRF. Particle size characteristics were also measured. Sediments were subjected to a five-step sequential extraction procedure to ascertain the environmental significance of metal concentrations. Alongside archival research of past industry, enrichment factors, multivariate statistical techniques and conditional inferences trees were used to identify sources of heavy metals.

Results and discussion

Bed sediment-associated heavy metal(loid) concentrations were as follows: As (9.89–110 mg kg^?1), Cr (76.5–413 mg kg^?1), Cu (53.1–383 mg kg^?1), Pb (80.4–442 mg kg^?1) and Zn (282–1020 mg kg^?1). Enrichment factors ranged from moderate to extremely severe, with Pb showing the greatest enrichment across the catchments. Chemical mobility was generally low, but metal(loid) partitioning identified the influence of anthropogenic sources. Statistical analysis highlighted a number of point sources associated with former industrial sites that operated during the industrial period. Conditional inference trees highlighted the role of the textile industry on Cu concentrations in addition to indicating the complexity of sources, fluxes and stores of sediment-associated contamination throughout the system.

Conclusions

Fine-grained sediment-associated metal(loid)s in the Mersey and Irwell catchments are anthropogenically enriched. Concentrations also exceed sediment quality guidelines. A lack of distinct spatial patterning points to a complex network of contaminant inputs across the catchments, even in the headwaters. Whilst potential modern urban sources are likely to be important, spatial patterns and multivariate/data mining techniques also highlighted the importance of releases from former industrial sites as well as the reworking of historically contaminated floodplains and soils.

Purpose

Mining is a common source of metals in aquatic ecosystems. Metal loading in the environment is thought to be a selective pressure that induces compositional and functional changes within the affected microbial community in the sediment. This study aims to explore shifts in the diversity, structure, and functional gene abundance of microbial communities in the sediment of the copper mining-induced contaminated lakes in Finland.

Materials and methods

The sediment microbial community structures and abundance of the functional groups involved in carbon/nitrogen/sulfur cycling in four lakes located downstream from metal mines (Kirkkoselkä (KS), Junttiselkä (JS), Laakajärvi (LJ), and Sysmäjärvi (SJ)) and one reference lake (Parkkimanjärvi (PJ)) in Finland were compared using high throughput sequencing and quantitative PCR.

Results and discussion

Compared to the PJ reference lake sediment, the relative abundances were higher for Bacteroidetes, Gemmatimonadetes, Acidobacteria, and Nitrospirae but lower for Firmicutes and Alphaproteobacteria in the mine-contaminated sediment samples. The number of copies of copper-resistant genes (copA) in the two copper-contaminated sediments (5.34 × 10⁶ and 4.95 × 10⁶ copies ng^?1 DNA for KS and JS, respectively) was significantly higher than that in the PJ sediment (1.33 × 10⁶ copies ng^?1 DNA). Methanogens (mcrA gene) accounted for 5.09–11.5% of the total archaea (16S rRNA) in these lake sediments. In addition, ammonia-oxidizing archaea (amoA gene) in the LJ sediment accounted for 36.0% of the total archaea but only 0.83–1.63% in the sediment of other lakes. The abundance of eight investigated functional groups accounted for 28.8% of the total bacteria in the PJ sediment but less than 1.3% in the metal-contaminated sediments. The canonical correspondence analysis showed that the microbial community structure of Lake LJ was scattered far from the other lakes and was significantly correlated with nitrate; the community structural change in the JS and KS sediments was positively correlated with copper or negatively correlated with nitrate concentration.

Conclusions

These results indicate that the sedimentary indigenous microbial community may shift its composition and structure as well as its function to increase its adaptability and/or resistance to metal-contaminated freshwater sediments.

Purpose

The Three Gorges Dam project is one of the biggest projects in the world. The water level fluctuation zone (WLFZ) was created with impoundment. The purpose of the current study is to investigate the ecotoxicological potential of the sediment extracts from the WLFZ and propose future WLFZ monitoring for early detection of environmental changes brought about by pollution.

Materials and methods

The investigation was performed by using cell-based in vitro bioassays to determine acute cytotoxicity (neutral red retention assay) and mechanism-specific aryl hydrocarbon receptor (AhR)-mediated activity (7-ethoxyresorufin-O-deethylase (EROD) induction assay) of sediment extracts with rainbow trout (Oncorhynchus mykiss) liver cells (RTL-W1).

Results and discussion

Results showed that the cytotoxicity and AhR-mediated toxicity potential of the sediment extracts from the WLFZ in the Three Gorges Reservoir (TGR) were moderate of level compared with the ecotoxicity of sediments from other river systems. However, according to a sediment classification system in Germany, sediments from some individual sites in the WLFZ showed strong toxicity. Compared to the results from the Yangtze River in our previous study, more attention should be paid to the aryl hydrocarbon receptor inducers in the WLFZ in TGR.

Conclusions

The in vitro bioassays used in this study may serve as a valuable tool to predict the potential ecological hazards of the organic pollutants in the WLFZ to the aquatic organisms in some extent.

Purpose

Soils of tidal marshes play an important role in regional carbon (C) cycles as they are able to store considerable amounts of organic carbon (OC). However, the C dynamics of marsh soils of the Elbe estuary have not been investigated so far. Therefore, the aim of this study was to identify the sources and distribution of soil organic carbon (SOC) and the factors influencing the SOC pools of tidal marshes of the study region.

Materials and methods

In this study, SOC pools were determined in different salinity zones and elevation classes of the estuarine marshes. The amount of initial allochthonous OC was derived from the OC content in fresh sediments. The difference to the recent OC content in the soils was interpreted as autochthonous accumulation or mineralization by microorganisms.

Results and discussion

Young, low marshes of the study sites seem to be predominantly influenced by allochthonous OC deposition whereas the older, high marshes show autochthonous OC accumulation in the topsoils (0–30 cm) and mineralization in the subsoils (30–70 cm). SOC pools of the whole profile depth (0–100 cm) did not significantly differ between elevation classes, but decreased significantly with increasing salinity from 28.3 kg m^?2 in the most upstream site of the oligohaline zone to 9.7 kg m^?2 in the most downstream site of the polyhaline zone. Even though the areal extent of the investigated salinity zones was similar, the SOC mass within 100 cm soil depth decreased from 0.62 Tg (1 Tg = 10¹² g) in the oligohaline zone to 0.18 Tg in the polyhaline zone.

Conclusions

Elevation was found to be one factor influencing the SOC pools of tidal marshes. However, salinity seems to be an even stronger influencing factor reducing the above-ground biomass and, accordingly, the autochthonous OC input as well as the allochthonous input by enhanced mineralization of OC along the course of the estuary. An upstream shift of the salinity zones by sea level rise could, therefore, lead to a reduction of the SOC storage of the estuarine marshes.

Purpose

In this paper, a novel modeling approach is applied to assess the unique transport characteristics of hydrophobic (bitumen containing) cohesive sediment for the Ells River, AB, Canada. The modeling offers a new way of treating the transport and fate of fine sediment in rivers and points to the importance of including a sediment entrapment process in the modeling of the Ells River sediment dynamics.

Materials and methods

The modeling approach involves combining two existing models (RIVFLOC and MOBED). Using fine sediment transport parameters derived from laboratory flume experiments (e.g., settling velocity of sediment as a function of floc size and the critical shear stresses for deposition) and the calculated flow field from the MOBED model (using field survey data such as, cross-sectional geometry, river slope, grain size of bed material, and discharge), the RIVFLOC model is used to predict the transport characteristics (including entrapment) of the hydrophobic Ells River sediment.

Results and discussion

The application of the connected RIVFLOC and MOBED models, demonstrated the unique hydrophobic sediment dynamics of the Ells River. The model showed no deposition (in the classical sense) of the hydrophobic sediment as the bed shear stresses, even at base flow, are well above the critical bed shear for deposition (flocculation is shown to occur, but its impact on settling is negligible given the high shear stresses). However, the model showed the possibility of fine sediment ingression into the river bed (interstitial voids) due to the entrapment process which is known to occur at bed shear stresses well above the critical shear stress for deposition.

Conclusions

The salient features of RIVFLOC and MOBED models and their applications for understanding the transport and fate of unique hydrophobic fine sediments are presented. The models are shown to be useful for the understanding and projection of flow characteristics and sediment dynamics (including entrapment), and will be of benefit for the adaptive management of riverine monitoring programs given various flow scenarios including extreme events and climate change.

Purpose

Soil organic carbon (SOC) stock is one of the most important carbon reservoirs on the earth and plays a vital role in the global climate change. However, research on the carbon sequestration and storage of coastal wetland soil is very scarce. Therefore, a study in the coastal wetland was conducted to investigate the SOC distribution, storage, and variation under the influence of human activities.

Materials and methods

Surface soil samples in different seasons and profile soil samples were collected in the Changyi coastal wetland. SOC content, soil physicochemical properties, and sedimentation rate were determined. Organic carbon storage and burial flux were calculated. On the basis of correlation analysis and comparative study, factors affecting the distribution and storage of SOC were investigated.

Results and discussion

The average SOC content of the surface soil in June and November was 4.65 and 6.13 g kg^?1, respectively. The distribution of surface SOC content was consistent with the distribution of vegetation and was affected by the soil particle size. In plant-covered area, the relationship between SOC content and depth could be expressed by the power function y?=?ax ^b . The contribution of plants to SOC was only significant in the shallow layer. As for the deep layer, the SOC content was higher in the mudflat. The organic carbon storage in the upper 1 m soil was estimated at 1.795 kg m^?2 in average and the total organic carbon storage of Changyi wetland was estimated at 6.373?×?10⁷ kg. The sedimentation rate was very low and the average organic carbon burial flux of the whole wetland was 17.5 g m^?2 a^?1.

Conclusions

Low sedimentation rate, weak downward migration, and high decomposition rate of organic matter caused by poor hydrological condition were the reasons why the SOC storage in Changyi wetland was low. Under intensive human activities, the Changyi wetland was drying and the organic carbon storage was reducing. Strategies were proposed to be taken urgently to restore the wetland for the long-term benefit.