Assessment of river Po sediment quality by micropollutant analysis

Trace metals, PCB congeners and DDT homologues were determined in composite sediment samples collected from 10 representative sites along the river Po in two separate seasons. The aim was to identify the most anthropogenically impacted areas for future monitoring programmes and to aid development of Italian sediment quality criteria. The surface samples were collected during low flow conditions. Trace metal concentrations were assayed by electrothermal (Cd, Co, Cr, Cu, Ni, Pb), flame (Fe, Mn, Zn) or hydride generation (As) atomic absorption spectrometry after microwave assisted acid digestion. Hg was determined on solid samples by automated analyser. Organic microcontaminants were determined by gas-chromatography with 63Ni electron capture detector after Soxhlet extraction. Concentrations of trace metals, total PCB and DDT homologues showed two distinct peaks at the sites immediately downstream of Turin and Milan, respectively, and in each case decreased progressively further downstream. Principal component analysis identified three major factors (from a multi-dimensional space of 35 variables) which explained 85-90% of the total observed variance. The first and second factors corresponded to anthropogenic inputs and geological factors on sediment quality; the third included seasonal processes of minor importance. Sediment quality assessment identified Cd, Cu, Hg, Pb, Zn and organic microcontaminants as posing the most serious threats to river sediment quality. A reference site within the Po basin provided useful background values. Moderate pollution by organochlorine compounds was ascribed both to local sources and to atmospheric deposition.     

Using a battery of bioassays, benthic phytoplankton and the AUSRIVAS method to monitor long-term coal tar contaminated sediment in the Cache la Poudre River, Colorado

This survey provides information on sediment toxicity and structural characteristics of the macroinvertebrates and benthic phytoplankton at 10 locations in the Cache la Poudre River after long-term exposure to coal tar residue. The application of the Australian river bioassessment system (AUSRIVAS) as well as a biotest battery was used to evaluate the river ‘health’ condition. Coal tar is a dense nonaqueous-phase liquid of significant environmental concern due to its toxicity and persistence in the subsurface. Organisms like Selenastrum capricornutum, Daphnia magna and Chironomus tentans, representing different complexities in the biosphere, were selected as test systems for ecotoxicological studies. The results obtained in this study indicate that a biotest battery, macroinvertebrate and benthic phytoplankton communities are in principle suitable biological tools for evaluation of toxic oil and coal-derived substances in long-term contaminated river sediment.     

The accumulation levels of PAHs, PCBs and DDTs are related in an inverse way to the size of a benthic amphipod (Echinogammarus stammeri Karaman) in the River Po

The accumulation of polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and DDTs were investigated in gammarids captured at three sites along the middle River Po; the first was located upstream and the other two were at increasing distances downstream of the confluence of a polluted tributary, the River Lambro. Using a GC-MS technique, the levels of PCBs, PAHs and DDTs were determined separately in large and small gammarids as well as in the fine fraction of sediment samples collected along the sites of capture. Results confirm the River Lambro as a source of these chemicals to the River Po, and show that bioaccumulation differences exist between small and large individuals, the former being more contaminated particularly by PCBs and DDTs. This is likely the result of several interacting factors such as contaminant bioavailability, gammarid-size effects on kinetic parameters and feeding selectivity. The bioaccumulation patterns of PCBs and DDTs, and their higher biota-sediment accumulation factors (BSAF), are consistent with chemical properties and suggest a dietary disequilibrium found only 10 km downstream from the tributary, and in smaller amphipods. Present results show that gammarids may represent an additional source of contaminants, particularly of chlorinated compounds, to the many organisms feeding on them, with a higher risk for those which prey selectively on smaller gammarids.     

First observation of intersex cyprinids in the Po River (Italy)

Barbel (Barbus plebejus, Cyprinidae) were captured in the Po River, upstream and downstream from the confluence of the Lambro River, a polluted tributary of the major Italian watercourse. The gonads of the two groups of barbel have been histologically examined, and only the downstream specimens showed histo-morphological alterations that can be related to the Lambro tributary as a source to the main river of endocrine disrupting chemicals, possibly with estrogenic effects. In fact, 50% of the barbel captured (8 of 16 fish) in the downstream reach showed intersex gonads.     

Modelling sediment-microbial dynamics in the South Nation River, Ontario, Canada: Towards the prediction of aquatic and human health risk

Runoff from agricultural watersheds can carry a number of agricultural pollutants and pathogens; often associated with the sediment fraction. Deposition of this sediment can impact water quality and the ecology of the river, and the re-suspension of such sediment can become sources of contamination for reaches downstream. In this paper a modelling framework to predict sediment and associated microbial erosion, transport and deposition is proposed for the South Nation River, Ontario, Canada. The modelling framework is based on empirical relationships (deposition and re-suspension fluxes), derived from laboratory experiments in a rotating circular flume using sediment collected from the river bed. The bed shear stress governing the deposition and re-suspension processes in the stream was predicted using a one dimensional mobile boundary flow model called MOBED. Counts of live bacteria associated with the suspended and bed sediments were used in conjunction with measured suspended sediment concentration at an upstream section to allow for the estimation of sediment associated microbial erosion, transport and deposition within the modelled river reach. Results suggest that the South Nation River is dominated by deposition periods with erosion only occurring at flows above approximately 250 m³ s⁻¹ (above this threshold, all sediment (suspended and eroded) with associated bacteria are transported through the modelled reach). As microbes are often associated with sediments, and can survive for extended periods of time, the river bed is shown to be a possible source of pathogenic organisms for erosion and transport downstream during large storm events. It is clear that, shear levels, bacteria concentrations and suspended sediment are interrelated requiring that these parameters be studied together in order to understand aquatic microbial dynamics. It is important that any management strategies and operational assessments for the protection of human and aquatic health incorporate the sediment compartments (suspended and bed sediment) and the energy dynamics within the system in order to better predict the concentration of indicator organism.     

Influence of water and sediment quality on benthic biota in an acidified river

Water and sediment quality and benthic biota were investigated in all seasons during three years in the River Akagawa that receives the effluent from a mine drainage treatment plant at its upstream site. The upper reaches kept the low pH, the comparatively high concentrations of metals and a large amount of iron deposited on the riverbed. The predominant macroinvertebrates were Protonemura sp., Capnidae, Nemoura sp. and Chironomidae in the upper and middle reaches. In the lowest reaches, the community structure of the macroinvertebrate changed into Chironomidae, Trichoptera (Hydropsychidae) and Ephemeroptera (Baetis sp.) as the pH was increased. From the results of multivariate analyses, it was found that the restoration of pH and attached algae and the increase in the concentrations of nutrients and organic matter promoted the inhabitation of Chironomidae and Hydropsychidae, whereas the dissolved metals in the river water inhibited the inhabitation of these families. Moreover, the sedimentation of metals would cause a severe damage to the inhabitation of Hydropsychidae compared with that of Chironomidae.     

Interactions of land use and dynamic river conditions on sorption equilibria between benthic sediments and river soluble reactive phosphorus concentrations

Within-river cycling of P is a crucial link between catchment pollution sources and the resulting ecological impacts and integrates the biogeochemistry and hydrodynamics of river systems. This study investigates benthic sediment P sorption in relation to river soluble reactive phosphorus (SRP) concentrations during high- to low-flow changes in a major mixed land use river system in NE Scotland. We hypothesised that sediments comprised P sinks during moderate to higher flows but became P saturated with loss of buffering function during prolonged baseflow. Sediment characteristics were evaluated and equilibrium P concentrations (EPC₀) calculated using a standardised batch adsorption method (EPC₀ values 0.04-1.75 μmol P l⁻¹). Pollution-impacted tributaries (32-69% catchment agricultural land cover) had increased SRP concentrations (0.19-2.62 μmol P l⁻¹) and maintained EPC₀ < SRP values during changing flow conditions. Moorland-dominated tributaries and main stem sites had small SRP concentrations (0.03-0.19 μmol P l⁻¹) but showed EPC₀ > SRP values during summer baseflow so that sediments were indicated as P sources. This deviation from a geochemical sediment-water P equilibrium was attributed to biological accumulation of P from the water column into the sediments. In particular, large stores of sediment P accumulated in main stem reaches below agricultural tributaries and this may be consequential for sensitive downstream ecosystems. Hence, biogeochemical processes at the river bed may strongly influence river SRP cycling between geochemical and biotic pools. The nature of this internal reservoir of river P and its ecosystem interactions needs better understanding to enable best results to be attained from catchment mitigation actions designed to maintain/improve ecological status under the Water Framework Directive.     

Seasonal nutrient dynamics in a chalk stream: the River Frome, Dorset, UK

Chalk streams provide unique, environmentally important habitats, but are particularly susceptible to human activities, such as water abstraction, fish farming and intensive agricultural activity on their fertile flood-meadows, resulting in increased nutrient concentrations. Weekly phosphorus, nitrate, dissolved silicon, chloride and flow measurements were made at nine sites along a 32 km stretch of the River Frome and its tributaries, over a 15 month period. The stretch was divided into two sections (termed the middle and lower reach) and mass balances were calculated for each determinand by totalling the inputs from upstream, tributaries, sewage treatment works and an estimate of groundwater input, and subtracting this from the load exported from each reach. Phosphorus and nitrate were retained within the river channel during the summer months, due to bioaccumulation into river biota and adsorption of phosphorus to bed sediments. During the autumn to spring periods, there was a net export, attributed to increased diffuse inputs from the catchment during storms, decomposition of channel biomass and remobilisation of phosphorus from the bed sediment. This seasonality of retention and remobilisation was higher in the lower reach than the middle reach, which was attributed to downstream changes in land use and fine sediment availability. Silicon showed much less seasonality, but did have periods of rapid retention in spring, due to diatom uptake within the river channel, and a subsequent release from the bed sediments during storm events. Chloride did not produce a seasonal pattern, indicating that the observed phosphorus and nitrate seasonality was a product of annual variation in diffuse inputs and internal riverine processes, rather than an artefact of sampling, flow gauging and analytical errors.     

Geomorphological impacts of Inanda Dam on the Mgeni estuary, north of Durban, South Africa)

 Completed in 1988 at a site some 32 km from the Mgeni river mouth, the Inanda Dam may have influenced bed load transport and deposition. The aims of this study were to ascertain whether the dam had affected fluvial sediment delivery to the lower channel, whether the sediment characteristics and bedform of the estuary had been modified and, within this context, to predict the likely effects of continued sand mining on the Mgeni estuary. Re-examination of estuarine cross sections originally surveyed prior to dam completion, comparison of present estuarine sediment characteristics with those of pre-dam sediments and a bottom survey of the Inanda Dam itself were key elements in the analysis. Mean annual discharge of the Mgeni river has reduced by 4% since dam construction and changes in discharge regime caused by the water release policy resulted in an increase in competent discharge events. Retention of sediment behind the dam wall had led to reduction in available downstream sediment. Sediment volume in the estuary had not changed significantly since 1989, although sediment calibre is now much finer. These changes will probably signal the end of sand mining in the lower river and will ultimately cause the estuary to deepen as residual pre-dam sediment is flushed through and not replaced by new material from upstream. Since Mgeni sediment is a source of material for natural beach nourishment, it is likely that beaches will become narrower. Received: 20 November 1999 · Accepted: 3 April 2000     

The effects of sediment size fraction and associated algal biofilms on the kinetics of phosphorus release

Experiments using flumes containing sediment of three different size fractions, from two sites on the River Tame, investigated the influences of sediment particle size, and an associated biofilm, on sediment-water exchanges in heterogeneous sediment deposits. This is the first study undertaken to understand the kinetics of the release of soluble reactive phosphorus from sediments of natural systems to identify which of the size compartments affected those fluxes most. Samples of fine material (<2 mm), gravel (2-20 mm), and stones (>20 mm) were collected over a period of several weeks and brought to a fluvarium where they were placed in artificial, controlled flow, and flume channels. Synthetic solutions of similar ionic strength to the river were prepared using calcium chloride. Temperature, pH, and dissolved oxygen of the solution overlying the sediment were monitored automatically whilst filtered samples were obtained at 2 h intervals over 48 h. The biomass, expressed as mg m(-2) chlorophyll a, of the algal component of the biofilm from the surface of the sediment was estimated using methanol extraction. Differences in the responses were observed between the sediment size fractions and the two sites, where contaminant concentrations varied. The equilibrium phosphate concentration and a phosphorus transfer index were used to establish that there was a net uptake of phosphorus by all three sediment size fractions, from both sites, at the time of sampling. The kinetic results showed very fast initial reactions of phosphorus release from the larger size fractions with a well-developed filamentous algal growth present implying a different mechanism than diffusion being involved. The stones and associated biofilms also released more phosphorus than the fine fraction, e.g. final release concentrations for the most contaminated site were: fines approximately 2.5 microM, gravel approximately 6.5 microM, and stones approximately 65.0 microM (expressed as soluble reactive phosphorus). Phosphorus fluxes, calculated assuming the concentration of phosphorus in the sediment was less than the equilibrium concentration, were a maximum at the most contaminated site, e.g. fines 6.4 nmol m(-2) s(-1), gravel 27 nmol m(-2) s(-1), and stones 109 nmol m(-2) s(-1) (normalised with respect to the river bed area). These results confirm that sediment having a biofilm and associated particulate material results in a greater flux than fine sediment, which does not support a filamentous biomass. Removal of the fine particulates trapped in the algal growth reduced soluble phosphorus release. These factors demonstrate that both gravel and stone substrates have an important control over the release of soluble reactive phosphorus.     

Quantitative assessment of estrogenic activity in the water environment of Korea by the E-SCREEN assay

In this study, the E-SCREEN assay was optimized and validated for the sensitive quantitative determination of the total estrogenicity in river samples. River water and sediment samples were collected and analyzed with the E-SCREEN. River water (10 l) was extracted using combined solid-phase extraction in static adsorption mode with Soxhlet extraction. Estrogenic pollutants adsorbed to the XAD-4 resin were recovered with 98.24 +/- 5.90% efficiency by elution with ethyl acetate and dichloromethane (1:9). The detection limit by 17beta-estradiol equivalent concentration (EEQ) of the E-SCREEN assay was 8.03 pg EEQ/l. Among the water samples, the estrogenic activity was observed to be higher downstream of the Kumho river (7.43 ng EEQ/l) and upstream of Kum river (2.05 ng EEQ/l) than in other samples. More than 3 mg of equivalent sediment samples from the Kumho river, Kum river and Miho stream showed partial agonistic effects, and the Mankyung river showed a partial agonistic effect with only 1.5 mg of sediment. The highest value of RPE was 83.34 downstream of the Kumho river, and the lowest value of RPE was 6.52 downstream of the Miho stream. Full estrogen agonistic activities were observed downstream of the Kumho river and upstream of the Kum river. The partial agonistic activity was observed in upstream of the Kumho river, downstream of the Mankyung river, and upstream of the Miho stream, and no agonistic action was observed downstream of the Kum river or Miho stream, or upstream of the Mankyung river. The total estrogenic activity in the river water and sediment samples was between 0.50 pg/L and 7.4 ng/L, 3.39 pg/g and 10.70 pg/g.     

Combination of in vitro bioassays encompassing different mechanisms to determine the endocrine-disrupting effects of river water

In this study, the total toxic effects of river water samples were assessed using a series of cell culture bioassays which encompassed different mechanisms, based on specific modes of action. River water samples were collected from three tributaries of the Youngsan River in the western portion of Korea. We confirmed that Youngsan River water was polluted with a complex mixture of estrogenic and dioxin-like compounds. The total toxic effects of the downstream water samples were found to be higher than that of the upstream water samples. In the upstream water samples, total estrogenic activity was measured to be between 0.005 and 0.049 ng-EEQ/l (17beta-estradiol-equivalent concentration) and no CYP1A activity was detected. In the downstream water samples, however, total estrogenic activity was measured to be between 0.021 ng-EEQ/l and 1.918 ng-EEQ/l, and total CYP1A activity was between 0.63 and 3.55 microg-MEQ/l (3-methylcholanthrene-equivalent concentration). When assessed according to a concentration-response curve, downstream water sample extracts exerted dual actions on estrogen receptors, depending on the concentration volume of the samples. The concentration volume range proximal to the original water sample exhibited estrogenic activity, whereas antiestrogenic activity was observed at high concentration volumes (more than 5 times concentration) in the extracts. This study involved a combination of in vitro bioassays, designed to encompass different mechanisms. The bioassays used included the estrogen receptor binding affinity test, E-screen assay, aromatase assay, and EROD assay. These tests provided a great deal of useful information regarding the potency and action modes of estrogenicity and antiestrogenicity inherent in the sampled river water. Although further study is necessary to determine the relationship between toxic responses in in vitro bioassay systems and chronic toxicity in aquatic organisms, our approach is expected to be fairly accurate with regard to the detection of endocrine-disrupting effects in an aquatic environment.     

Effect of watershed parameters on mercury distribution in different environmental compartments in the Mobile Alabama River Basin, USA

Total mercury (THg) and mono-methylmercury (MeHg) levels in water, sediment, and largemouth bass (LMB) (Micropterus salmoides) were investigated at 52 sites draining contrasting land use/land cover and habitat types within the Mobile Alabama River Basin (MARB). Aqueous THg was positively associated with iron-rich suspended particles and highest in catchments impacted by agriculture. Sediment THg was positively associated with sediment organic mater and iron content, with the highest levels observed in smaller catchments influenced by wetlands, followed by those impacted by agriculture or mixed forest, agriculture, and wetlands. The lowest sediment THg levels were observed in main river channels, except for reaches impacted by coal mining. Sediment MeHg levels were a positive function of sediment THg and organic matter and aqueous nutrient levels. The highest levels occurred in agricultural catchments and those impacted by elevated sulfate levels associated with coal mining. Aqueous MeHg concentrations in main river channels were as high as those in smaller catchments impacted by agriculture or wetlands, suggesting these areas were sources to rivers. Elevated Hg levels in some LMB were observed across all types of land use and land cover, but factors such as shallow water depth, larger wetland catchment surface area, low aqueous potassium levels, and higher Chl a concentrations were associated with higher Hg burdens, particularly in the Coastal Plain province. It is suggested that the observed large variability in LMB Hg burdens is linked to fish displacement by anglers, differences in food web structure, and sediment biogeochemistry, with surficial sediment iron oxides buffering the flux of MeHg from sediments to deeper water pelagic food webs.     

Distribution and behavior of trace metals in the sediment and porewater of a tropical coastal wetland

Vertical profiles (0-30 cm below surface) of four trace metals-Cadmium (Cd), Chromium (Cr), Lead (Pb) and Zinc (Zn)-in the sediment and sediment porewater of an ecologically important intertidal mudflat in the Mai Po and Inner Deep Bay Ramsar Site were thoroughly studied over a period of 10 months (from March 1999 to January 2000). Two surveys, one in summer and another in winter, involving a total of eight sampling stations were conducted to study the seasonal variation of the remobilization characteristics of these trace metals in the mudflat sediment. The range of depth averaged concentration of these trace metals in the mudflat sediment was: 0.3-0.8 microg/g (Cd); 9.8-91.0 microg/g (Cr); 7.3-69.1 microg/g (Pb); and 39.5-192.0 microg/g (Zn), while that in the sediment porewater was: 0.3-121.1 microg/l (Cd); 3.0-2704.1 microg/l (Cr); 2.6-105.6 microg/l (Pb); and 32.6-4238.3 microg/l (Zn). In general, levels of dissolved trace metals in the sediment porewater were much higher in the summer than in the winter while their concentrations in the sediment were more or less the same throughout the year. Enrichment of Cd, Pb and Zn in the sediment porewater of the upper oxic layer and that of Cr in the oxic-sub-oxic boundary was generally observed. Regions in the vicinity of the Mai Po mangroves and the river mouths of Shenzhen River and Shan Pui River were found to be hotspots of trace metal pollution. Benthic diffusive fluxes of trace metals from the mudflat sediment were also estimated. Of the four trace metals, cadmium showed the greatest tendency toward remobilization from the sediment phase to the more bio-available porewater phase.     

Eutrophication downstream from small reservoirs in mountain rivers of Central Spain

In this research we examined the hypothesis that upper reaches of rivers and streams can experience eutrophication as a consequence of deep releases from dams. Field studies were conducted in four mountain rivers (Tormes, Riaza, Eresma and Miraflores Rivers) of Central Spain. The watersheds of these rivers are underlain by siliceous rocks. A small deep-release storage reservoir is found in the upper reaches of each river. Two sampling sites, upstream and downstream from the reservoir, were established in stony riffles of each impounded river. Significant (P < 0.01) increases in conductivity and nutrient (NO3-N, NH4-N, PO4-P) concentrations downstream from the reservoirs were measured. Significant (P < 0.01) increases in periphyton chlorophyll a and ash-free dry biomass were also quantified at downstream sites. Significant (P < 0.01) correlation coefficients indicated that phosphate would play a more important role as the limiting nutrient for periphyton. Relative abundances of macroinvertebrate scrapers and collector-gatherers increased downstream from the reservoirs. Furthermore, taxon dominance, total density and total biomass of macroinvertebrates tended to be higher at downstream sites than at upstream sites. In contrast, taxon diversity and relative abundance of macroinvertebrate shredders decreased downstream from the reservoirs. It is concluded that small deep-release storage reservoirs, located in upper reaches of siliceous rivers, can act as nutrient sources, causing eutrophication downstream. Nutrients would ultimately come from land/forest runoff. The fact that terrestrial vegetation was not completely removed before filling reservoirs could also contribute to the eutrophication process.     

Characteristics of humic substances in the Kuji River waters as determined by high-performance size exclusion chromatography with fluorescence detection

Direct measurement by high-performance size exclusion chromatography with fluorescence detection was applied to the characterization of humic substances in river waters from the Kuji River system, which runs through forest hills and an agricultural plain in Japan. The monitoring wavelength of excitation 320 nm and emission 430 nm corresponds to the fluorescence maxima for aquatic fulvic acid. Chromatograms of the river waters showed four peaks; each peak position was in good agreement among these samples. Peak height ratios for the samples from the upstream Kuji River and its tributaries were different from those of the midstream and downstream sections of the Kuji River, which may reflect differences in the characteristics of humic substances and other organic materials supplied from soil to river.     

Effect of river water, sediment and time on the toxicity and bioavailability of molinate to the marine bacterium Vibrio fischeri (Microtox)

The toxicity and bioavailability of molinate to Vibrio fischeri (Microtox((R))) were determined in both laboratory and river water in the absence and presence of sediment after 0, 24, 48, 72 and 96-h exposure. The bioavailability of molinate, expressed as 5min EC50s (bioluminescence) and their fiducial limits calculated using initial measured concentrations, to V. fischeri in laboratory water in the absence and presence of sediment ranged from 1.8 (1.7-2.1) to 3.6 (3.5-3.7) mgL(-1) and 1.3 (1.2-1.4) to 4.2 (3.5-4.5) mgL(-1), respectively. The corresponding values in river water and river water plus sediment were 1.7 (1.6-1.8) to 3.8 (3.6-4.1) and 1.3 (1.3-1.4) to 4.6 (4.2-4.9) mgL(-1), respectively. River water did not significantly (P>0.05) reduce the bioavailability of molinate to V. fischeri compared to that of laboratory water. However, the presence of sediment significantly (P<0.05) reduced the bioavailability of molinate to V. fischeri in both waters. The exposure time also significantly (P<0.05) reduced the bioavailability of molinate to V. fischeri in both waters in the presence and absence of sediment. The type of water did not significantly (P>0.05) affect the loss of molinate during the 96-h exposure period. However, the presence of sediment significantly (P<0.01) increased the loss of molinate from the test solutions, probably by binding to the sediment particles. Exposure period and concentration levels significantly (P<0.05) affected the loss of the herbicides over the 96h.     

The phosphorus budget of the Thame catchment,Oxfordshire: 2. Modelling

The phosphorus budget of the River Thame was modelled at a daily time scale, using estimates of diffuse and point source contributions of discharge. The model simulated suspended sediment (SS), soluble unreactive phosphorus (SUP), soluble reactive phosphorus (SRP) and particulate phosphorus (PP) concentrations within the main river and major tributaries. Diffuse source estimates of phosphorus loads were based on characteristic losses from identified main landscape classes, with hydrology described by a simple conceptual storage model. In-stream flow was modelled using a kinematic wave equation. Transfer of suspended sediment and phosphorus components was approximated by advection. In-stream sources and sinks included uptake and release of soluble reactive phosphorus by bed sediment, instant equilibration between SRP and the PP concentration on suspended sediment, and flow-related entrainment and deposition of suspended sediment. Simulations at sites within the catchment were compared with measurements made in 1998–1999. Results showed the P budget is dominated by mixing of diffuse and point source water, but some within-river processes have been shown to be capable of significantly influencing SRP concentrations. The development of a sediment entrainment and deposition component of the model has proved particularly valuable in emulating the hysteretic relationship between discharge and suspended sediment concentration in the river. It also provides a measure of available bed sediment.     

MoRPh: a citizen science tool for monitoring and appraising physical habitat changes in rivers

The MoRPh survey is designed to enable citizen scientists to monitor physical habitat mosaics and human pressures within short (up to 40 m) river reaches called modules. MoRPh underpins a multiscale Modular River Survey, providing local information, which when collected across 10 contiguous modules, delivers a MultiMoRPh river sub‐reach survey up to 400 m in length. This, in turn, contributes to a HydroMoRPh assessment of reaches extending to tens of kilometres of river length, based on secondary data sources. A 6‐month trial on chalk streams, demonstrates that indices calculated from MoRPh surveys can detect notable differences in hydraulic, sediment, physical and vegetation habitat characteristics across this single river type. Further tests will evaluate applicability to other river types and ability to detect temporal changes. Development of aggregate indices for MultiMoRPh sub‐reaches will aid interpretation of contemporary morphological dynamics, complementing longer term changes extracted at the reach scale by a HydroMoRPh analysis.     

Dynamics of CO2 partial pressure and CO2 outgassing in the lower reaches of the Xijiang River, a subtropical monsoon river in China

The partial pressure of carbon dioxide (pCO(2)) in surface water was surveyed monthly at 6 sampling sites along the entire length of the lower reaches of the Xijiang River, a subtropical monsoon river in China, and at the mouths of its major tributaries, over a whole hydrological year from April 2005 to March 2006, to reveal the seasonal and spatial dynamics of pCO(2). Intensive sampling and measurements were also conducted at Wuzhou gauge station in June and July to investigate the impact of floodwater on pCO(2) and to further explore the relationship between river discharge and pCO(2). The pCO(2) levels were well above atmospheric equilibrium (380 microatm) during the entire survey period with obvious seasonal and spatial variations, ranging from 600 microatm to 7200 microatm for the mainstream and from 700 to 11000 microatm for tributaries, respectively. The pattern of pCO(2) seasonal variation across 6 sites was almost consistent with each other with little difference. The pCO(2) levels in the dry season were relatively low, with relatively slight temporal and spatial fluctuations that were predominantly controlled by in situ biogenic activities. While the pCO(2) in the wet season greatly varied with river discharge, both annual maximum and minimum pCO(2) levels occurring in this period. The much higher pCO(2) in the early wet season were mainly induced by increasing baseflow and interflow that flushed significant soil CO(2) into the streams, whereas the lower pCO(2) observed after floods from July to September, some even lower than pCO(2) levels in the dry season, potentially resulted from in situ plankton blooms. The annual minima pCO(2) levels occurring in this period were caused by the dilution effect of floodwater. There was no obvious downstream trend in pCO(2) variation during the whole survey period, probably a consequence of disturbance from tributaries or spatially distinct channel characteristics and water environments. Based on measurements, we estimate that the water-to-air CO(2) flux in the lower reaches of the Xijiang River is about 8.3-15.6 Mg C ha(-1)y(-1). The role of the Xijiang River as a net source of atmospheric CO(2) is undoubted.