Low Discharge Conditions Research Articles

Organic matter sources, fluxes and greenhouse gas exchange in the Oubangui River (Congo River basin)

Abstract. The Oubangui is a major tributary of the Congo River, draining an area of ~500 000 km2 mainly consisting of wooded savannahs. Here, we report results of a one year long, 2-weekly sampling campaign in Bangui (Central African Republic) since March 2010 for a suite of physico-chemical and biogeochemical characteristics, including total suspended matter (TSM), bulk concentration and stable isotope composition of particulate organic carbon (POC and δ13CPOC), particulate nitrogen (PN and δ15NPN), dissolved organic carbon (DOC and δ13CDOC), dissolved inorganic carbon (DIC and δ13CDIC), dissolved greenhouse gases (CO2, CH4 and N2O), and dissolved lignin composition. δ13C signatures of both POC and DOC showed strong seasonal variations (−30.6 to −25.8‰, and −31.8 to −27.1‰, respectively), but their different timing indicates that the origins of POC and DOC may vary strongly over the hydrograph and are largely uncoupled, differing up to 6‰ in δ13C signatures. Dissolved lignin characteristics (carbon-normalised yields, cinnamyl:vanillyl phenol ratios, and vanillic acid to vanillin ratios) showed marked differences between high and low discharge conditions, consistent with major seasonal variations in the sources of dissolved organic matter. We observed a strong seasonality in pCO2, ranging between 470 ± 203 ppm for Q < 1000 m3 s−1 (n=10) to a maximum of 3750 ppm during the first stage of the rising discharge. The low POC/PN ratios, high %POC and low and variable δ13CPOC signatures during low flow conditions suggest that the majority of the POC pool during this period consists of in situ produced phytoplankton, consistent with concurrent pCO2 (partial pressure of CO2) values only slightly above and, occasionally, below atmospheric equilibrium. Water-atmosphere CO2 fluxes estimated using two independent approaches averaged 105 and 204 g C m−2 yr−1, i.e. more than an order of magnitude lower than current estimates for large tropical rivers globally. Although tropical rivers are often assumed to show much higher CO2 effluxes compared to temperate systems, we show that in situ production may be high enough to dominate the particulate organic carbon pool, and lower pCO2 values to near equilibrium values during low discharge conditions. The total annual flux of TSM, POC, PN, DOC and DIC are 2.33 Tg yr−1, 0.14 Tg C yr−1, 0.014 Tg N yr−1, 0.70 Tg C yr−1, and 0.49 Tg C yr−1, respectively. While our TSM and POC fluxes are similar to previous estimates for the Oubangui, DOC fluxes were ~30% higher and bicarbonate fluxes were ~35% lower than previous reports. DIC represented 58% of the total annual C flux, and under the assumptions that carbonate weathering represents 25% of the DIC flux and that CO2 from respiration drives chemical weathering, this flux is equivalent to ~50% of terrestrial-derived riverine C transport.

FDTD Simulation on Power Absorption of Terahertz Electromagnetic Waves in Dense Plasma

A finite difference time domain (FDTD) method is used to numerically study the power absorption of broadband terahertz (0.1 ∼ 1.5 THz) electromagnetic waves in a partially ionized uniform plasma layer under low pressure and atmosphere discharge conditions. The power absorption spectra are calculated numerically and the numerical results are in accordance with the analytic results. Meanwhile, the effects on the power absorption are calculated with different applied magnetic fields, collision frequencies and electron number densities, which depend strongly on those parameters. Under the dense strongly magnetized plasma conditions, the absorption gaps appear in the range of 0.3 ∼ 0.36 THz, and are enlarged with the increasing electron number density.

An enhanced MODIS remote sensing model for detecting rainfall effects on sediment plume in the coastal waters of Apalachicola Bay

Mapping of total suspended solids (TSS) was conducted to investigate rainstorm-induced characteristic of sediment plume in the coastal waters of Apalachicola Bay. An improved TSS quadratic polynomial regression model (Calibration: R 2 = 0.8586, N = 32; validation: RMSE of 4.76 mg/l, N = 30) for MODIS remote sensing was presented in this study. TSS mapping of MODIS before and after a rainstorm event showed distinct temporal–spatial variability of TSS concentration. Driven by ocean tidal current, a storm plume of width at about 40–50 km was formed flowing towards southwest of study area. The distinct boundary separating the highly turbid (west side) and relatively clear water (east side) was found near Sikes Cut. Further, by taking the TSS mapping under the low river discharge condition due to a local rainstorm as a reference of background TSS, two thresholds of TSS (25 and 45 mg/l respectively) were used to estimate the range of rainstorm plume and the central area of sediment load from surrounding land, and the spatial extent and evolution of the sediment plume during the local rainstorm. Besides, it was found that the storm plume concentration of TSS at east side of Sites Cut was quickly diluted under 25 mg/L, forming a storm plume towards east with width at about 7–8 km. The method developed in this study may be used to support coastal storm water research and management activities.

Associations among pathogenic bacteria, parasites, and environmental and land use factors in multiple mixed-use watersheds

Over a five year period (2004–08), 1171 surface water samples were collected from up to 24 sampling locations representing a wide range of stream orders, in a river basin in eastern Ontario, Canada. Water was analyzed for Cryptosporidium oocysts and Giardia cyst densities, the presence of Salmonella enterica subspecies enterica, Campylobacter spp., Listeria monocytogenes, and Escherichia coli O157:H7. The study objective was to explore associations among pathogen densities/occurrence and objectively defined land use, weather, hydrologic, and water quality variables using CART (Classification and Regression Tree) and binary logistical regression techniques. E. coli O157:H7 detections were infrequent, but detections were related to upstream livestock pasture density; 20% of the detections were located where cattle have access to the watercourses. The ratio of detections:non-detections for Campylobacter spp. was relatively higher (>1) when mean air temperatures were 6% below mean study period temperature values (relatively cooler periods). Cooler water temperatures, which can promote bacteria survival and represent times when land applications of manure typically occur (spring and fall), may have promoted increased frequency of Campylobacter spp. Fifty-nine percent of all Salmonella spp. detections occurred when river discharge on a branch of the river system of Shreve stream order = 9550 was >83 percentile. Hydrological events that promote off farm/off field/in stream transport must manifest themselves in order for detection of Salmonella spp. to occur in surface water in this region. Fifty seven percent of L. monocytogenes detections occurred in spring, relative to other seasons. It was speculated that a combination of winter livestock housing, silage feeding during winter, and spring application of manure that accrued during winter, contributed to elevated occurrences of this pathogen in spring. Cryptosporidium and Giardia oocyst and cyst densities were, overall, positively associated with surface water discharge, and negatively associated with air/water temperature during spring-summer-fall. Yet, some of the highest Cryptosporidium oocyst densities were associated with low discharge conditions on smaller order streams, suggesting wildlife as a contributing fecal source. Fifty six percent of all detections of ≥2 bacteria pathogens (including Campylobacter spp., Salmonella spp., and E. coli O157:H7) in water was associated with lower water temperatures (<∼14 °C; primarily spring and fall) and when total rainfall the week prior to sampling was >∼27 mm (62 percentile). During higher water temperatures (>∼14 °C), a higher amount of weekly rainfall was necessary to promote detection of ≥2 pathogens (primarily summer; weekly rainfall ∼>42 mm (>77 percentile); 15% of all ≥2 detections). Less rainfall may have been necessary to mobilize pathogens from adjacent land, and/or in stream sediments, during cooler water conditions; as these are times when manures are applied to fields in the area, and soil water contents and water table depths are relatively higher. Season, stream order, turbidity, mean daily temperature, surface water discharge, cropland coverage, and nearest upstream distance to a barn and pasture were variables that were relatively strong and recurrent with regard to discriminating pathogen presence and absence, and parasite densities in surface water in the region.

Processes underlying tidal variability of carbon dioxide in the Tagus Estuary (Portugal)

Tidal variation of CO2 system parameters was evaluated in the Tagus Estuary (SW Portugal) based on field observations performed in 2007 in the lower estuary under low river discharge conditions, during spring and autumn. Differences were found in the distribution patterns of environmental variables along a tidal cycle and at neap and spring tides essentially due to tide amplitude effects and tidal asymmetric behavior. A dominant east-westbound current was detected transporting CO2 enriched estuarine water offshore mainly during the ebb, and the reverse tidal current transporting more oxygenated marine water and with high chlorophyll a concentration during the flood. Distinct seasonal features were observed in terms of CO2 water properties and productivity. Regarding the tidal variability of the CO2 system parameters, besides the key processes controlling the variability, such as tidal advection and horizontal mixing of the two water masses present (estuarine and oceanic), specific biogeochemical processes emerged as playing an important role. Data obtained indicate that photosynthesis (in spring), aerobic respiration, and CaCO3 precipitation/dissolution (mainly in autumn) processes co-occur and control short-scale variations of CO2, reflecting the complexity of CO2 chemistry in the Tagus Estuary.

ChemInform Abstract: Electrochemical Lithiation of the Ternary Silver Sulfide Ag4Hf3S8.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A numerical model study of the transport timescale and change of estuarine circulation due to waterway constructions in the Changjiang Estuary, China

A three-dimensional hydrodynamic model was developed for the Changjiang Estuary and adjacent coastal sea to study the transport timescale and change of estuarine circulation due to human activates. The model was calibrated with measured tidal current and salinity forced by observed freshwater discharge and tides. The tracer age was introduced to measure the transport timescale and evaluate the influence of the man-made construction on estuarine circulation through a series of numerical experiments under various hydrodynamic conditions. The results show that it takes about 23 and 35 days, respectively, for dissolved substances to be transported from the Xuliujing to the mouth of the Estuary (122°30′ E) under high and low discharge conditions. The transport time increases significantly in the upper portion of the North Passage and the South Passage due to the diversion of the freshwater discharge from the upstream North Channel and the increase of the friction downstream. However, the rate of increasing transport time along the estuary decreases farther towards the mouth of the Estuary, presumably resulting from the enhancement of gravitational circulation. Two major physical mechanisms that contribute to the transport timescale, tide and river discharge are analyzed and quantified. The results indicate that freshwater discharge is one of the dominant factors controlling the transport timescale in the Changjiang Estuary. The man-made construction has a significant impact on both horizontal estuarine circulation and gravitational circulation, which ultimately influences the estuarine transport processes. The transport time increased about 50% by man-made constructions, especially near the turbidity maximum. The simulation results provide useful information for understanding the change of transport process and circulation caused by the man-made construction in the Changjiang Estuary.

Effects of seasonality and resource limitation on organic matter turnover by Chironomidae (Diptera) in southern Appalachian headwater streams

Despite their high abundance, secondary production, and known reliance on detrital material, the role of chironomids (Diptera) in fine particulate organic matter (FPOM) dynamics has not been well quantified. We conducted field trials using fluorescent pigment markers to estimate seasonal rates of consumption, annual secondary production, assimilation efficiency (AE), and FPOM turnover for non‐Tanypodinae chironomids in two similar, southern Appalachian, headwater streams. An ecosystem‐level experimental manipulation of detrital inputs in one study stream allowed for a unique assessment of turnover in reference, resource‐limited, and resource‐recovery systems. Seasonality had a significant effect on larval gut‐passage time (GPT), with faster GPT at higher temperatures. Mean GPT ranged from 39 min (summer) to 67 min (winter). Mean annual FPOM turnover was reduced in the resource‐limited stream (5.5 kg ash‐free dry weight per year; 12% of mean annual FPOM export) compared to the reference stream (46 kg yr−1; 74% of mean export). Following 4 yr of resource recovery, FPOM turnover increased to 24 kg ash‐free dry wt yr−1 (16% of export) but remained lower than levels measured in the reference stream (53 kg ash‐free dry wt yr−1; 20% of export). Chironomid contribution to FPOM turnover was much higher in low vs. high discharge conditions (74% in low discharge vs. 20% in high discharge). Assimilation efficiency was estimated to be low in all systems (1.7–2.5%). Chironomids turned over a large portion of FPOM available to them, suggesting that turnover by the entire collector community in forested headwater systems may be much higher than expected.

A study on the effect of a dynamic contact force control for improving electrospark coating properties

Abstract The electrospark deposition (ESD) technique was used to apply aluminum-diffusion coatings over a thin stainless steel sheet under low energy discharge conditions to minimize substrate damage. In order to produce uniform coating properties over relatively large areas, automatic deposition treatments were conducted in a multi-scan mode under a dynamic control of the contact force parameter, which was realized by means of a spring-loaded electrode contact. A continuous and uniform Al coating layer was obtained indicating that a dynamic contact force system may be effective in suppressing several coating defects, which are notoriously produced with conventional ESD processes. The benefits of the dynamic force control are mostly related to the concept of a mobile electrode contact, which creates a momentary gap expansion during the spark discharge. The expanded gap is believed to reduce the effect of surface roughness on spark reproducibility and at the same time to stabilize the streamer regime of the spark breakdown, which are both essential for more uniform ESD depositions and lower cathode erosion.

Using chloride and other ions to trace sewage and road salt in the Illinois Waterway

Chloride concentrations in waterways of northern USA are increasing at alarming rates and road salt is commonly assumed to be the cause. However, there are additional sources of Cl− in metropolitan areas, such as treated wastewater (TWW) and water conditioning salts, which may be contributing to Cl− loads entering surface waters. In this study, the potential sources of Cl− and Cl− loads in the Illinois River Basin from the Chicago area to the Illinois River’s confluence with the Mississippi River were investigated using halide data in stream samples and published Cl− and river discharge data. The investigation showed that road salt runoff and TWW from the Chicago region dominate Cl− loads in the Illinois Waterway, defined as the navigable sections of the Illinois River and two major tributaries in the Chicago region. Treated wastewater discharges at a relatively constant rate throughout the year and is the primary source of Cl− and other elements such as F− and B. Chloride loads are highest in the winter and early spring as a result of road salt runoff which can increase Cl− concentrations by up to several hundred mg/L. Chloride concentrations decrease downstream in the Illinois Waterway due to dilution, but are always elevated relative to tributaries downriver from Chicago. The TWW component is especially noticeable downstream under low discharge conditions during summer and early autumn when surface drainage is at a minimum and agricultural drain tiles are not flowing. Increases in population, urban and residential areas, and roadways in the Chicago area have caused an increase in the flux of Cl− from both road salt and TWW. Chloride concentrations have been increasing in the Illinois Waterway since around 1960 at a rate of about 1 mg/L/a. The increase is largest in the winter months due to road salt runoff. Shallow groundwater Cl− concentrations are also increasing, potentially producing higher base flow concentrations. Projected increases in population and urbanization over the next several decades suggest that the trend of increasing Cl− concentrations and loads will continue. Given the susceptibility of aquatic ecosystems to increasing Cl− concentrations, especially short-term spikes following snow melts, deleterious effects on riverine ecosystems would be expected.

Particle release transport in Danshuei River estuarine system and adjacent coastal ocean: a modeling assessment

A three-dimensional hydrodynamic model was created to study the Danshuei River estuarine system and adjacent coastal ocean in Taiwan. The model was verified using measurements of the time-series water surface elevation, tidal current, and salinity from 1999. We conclude that our model is consistent with these observations. Our particle-tracking model was also used to explore the transport of particles released from the Hsin-Hai Bridge, an area that is heavily polluted. The results suggest that it takes a much longer time for the estuary to be flushed out under low freshwater discharge conditions than with high freshwater discharge. We conclude that the northeast and southwest winds minimally impact particle dispersion in the estuary. The particles fail to settle to the bottom in the absence of density-induced circulation. Our model was also used to simulate the ocean outfall at the Bali. Our experimental results suggest that the tidal current dominates the particle trajectories and influences the transport properties in the absence of a wind stress condition. The particles tend to move northeast or southwest along the coast when northeast or southwest winds prevail. Our data suggest that wind-driven currents and tidal currents play important roles in water movement as linked with ocean outfall in the context of the Danshuei River.

Factors Affecting Sea Lamprey Egg Survival

AbstractFactors that affect recruitment of sea lampreys Petromyzon marinus are not well understood; for example, the majority (85%) of sea lamprey eggs are washed out of the nest, and the survival rate of these eggs is unknown. We examined the role of predation and substrate on egg survival in the laboratory and egg predation and dispersion of eggs outside the nest in the field. Substrate type did not influence the numbers of eggs consumed by predators, but the proportion of available eggs consumed was higher for crayfish Orconectes spp. (74%) and creek chub Semotilus atromaculatus (47%) than for white suckers Catostomus commersonii (9%) and logperch Percina caprodes (3%; F = 11.12, df = 3, P &lt; 0.0001). Survival of eggs incubated on silt (69.2%) and sand (50.8%) in the laboratory was significantly higher than survival on gravel (19.1%). Stream sampling indicated that sea lamprey egg density outside of nests on gravel was low (1.9 eggs/m2) compared with egg density in depositional areas (105.7 eggs/m2); eggs sampled outside the nest had near zero viability. In exclosure boxes seeded with sea lamprey eggs and deployed on gravel and silt for 7–10 d, egg retention was significantly greater on spawning gravel (31 ± 3%; mean ± SE) than on silt (6 ± 3%; P ≤ 0.0001); egg retention was affected by the presence of a 0.5‐cm‐mesh cover. Stream discharge appeared to increase egg loss from nests. We conclude that predation rates on sea lamprey eggs outside the nest are high and that substrate type alone has little impact on egg hatching success. Given the low retention of eggs in nests, egg mortality is probably a primary determinant of year‐class strength. Production of sea lampreys outside the nest likely occurs under low stream discharge conditions from eggs deposited on spawning gravel.

Primary and bacterial production processes in the lower Itajaí-Açú estuary, Santa Catarina, Brazil

Primary and bacterial productivity (PP and BP, respectively) were measured in the lower Itajaí-Açu estuary and in the adjacent shelf, under low river discharge conditions. The experiment was carried out in April, 2000, when physical, chemical and biological variables were recorded in five stations: three in the estuary and two in the inner shelf. Salinity, temperature, turbidity and light intensity were determined in vertical profiles; dissolved oxygen, nutrients, chlorophyll-a, primary (PP) and bacterial production (BP) were sampled at surface and near the bottom. The BP ranged from 0.5 to 11.3 mg C.m-3 .h-1 , and the highest values were recorded at the inner stations of the estuary, associated with lower salinity and light intensity and higher dissolved nutrients concentration. On the other hand, PP was higher in the inner shelf, with PP increasing from 2.0 to 30 mg C.m-3 .h-1 . The phytoplankton biomass showed the same trend, increasing from 0.2 to 8.4 µg/L at the surface. The PP was limited by light in the inner estuary and by the PO4 concentration in the inner shelf. The higher productivity zone was formed in front of the estuarine mouth, as expected, since water transparency increases leading the consumption of the rich-nutrient waters from the estuary by autotrophic organisms. The data indicated that the ecosystem metabolism shift from heterotrophic to autotrophic in a very small length scale (few kilometers) during the sampled conditions.

Particle resuspension in the Columbia River plume near field

Measurements of suspended sediment concentration, velocity, salinity, and turbulent microscale shear in the near‐field region of the Columbia River plume are used to investigate the mechanisms of sediment resuspension and entrainment into the plume. An east‐west transect was occupied during spring and neap tide periods in August 2005 and May 2006, corresponding to low and high river discharge conditions, respectively. During the high‐discharge period the plume is decoupled from the bottom, and fine sediment resuspended from the bottom does not leave the benthic boundary layer. The primary modes of sediment transport associated with the plume are advection of sediment from the estuary and removal of sediment from the plume by gravitational settling and turbulent mixing. In contrast, the plume is much less stratified during low‐discharge conditions, and large resuspension events are observed that entrained sediment through the water column and into the plume. Our measurements indicate that two factors control the magnitude and timing of sediment resuspension and entrainment: the supply of fine sediment on the seabed and the relative influence of tidal turbulence compared with buoyancy input from the river. The latter is quantified in terms of the estuary Richardson number RiE. The magnitude of vertical turbulent sediment flux is correlated with RiE during the low‐flow period when there is a sufficient supply of bottom sediment in the near‐field region. Such sediment resuspension may be an important mechanism for the delivery of bioavailable micronutrients to the plume during the summer.

PIC simulations of the separate control of ion flux and energy in CCRF discharges via the electrical asymmetry effect

Recently a novel approach for achieving separate control of ion flux and energy in capacitively coupled radio frequency (CCRF) discharges based on the electrical asymmetry effect (EAE) was proposed (Heil et al 2008 J. Phys. D: Appl. Phys. 41 165202). If the applied, temporally symmetric voltage waveform contains an even harmonic of the fundamental frequency, the sheaths in front of the two electrodes are necessarily asymmetric. A dc self-bias develops and is a function of the phase angle between the driving voltages. By tuning the phase, precise and convenient control of the ion energy can be achieved while the ion flux stays constant. This effect works even in geometrically symmetric discharges and the role of the two electrodes can be reversed electrically. In this work the EAE is verified using a particle in cell simulation of a geometrically symmetric dual-frequency CCRF discharge operated at 13.56 and 27.12 MHz. The self-bias is a nearly linear function of the phase angle. It is shown explicitly that the ion flux stays constant within ±5%, while the self-bias reaches values of up to 80% of the applied voltage amplitude and the maximum ion energy is changed by a factor of 3 for a set of low pressure discharge conditions investigated. The EAE is investigated at different pressures and electrode gaps. As geometrically symmetric discharges can be made electrically asymmetric via the EAE, the plasma series resonance effect is observed for the first time in simulations of a geometrically symmetric discharge.

Nonlinearity and complexity in gravel bed dynamics

The dynamics of river bed evolution are known to be notoriously complex affected by near-bed turbulence, the collective motion of clusters of particles of different sizes, and the formation of bedforms and other large-scale features. In this paper, we present the results of a study aiming to quantify the inherent nonlinearity and complexity in gravel bed dynamics. The data analyzed are bed elevation fluctuations collected via submersible sonar transducers at 0.1 Hz frequency in two different settings of low and high discharge in a controlled laboratory experiment. We employed surrogate series analysis and the transportation distance metric in the phase-space to test for nonlinearity and the finite size Lyapunov exponent (FSLE) methodology to test for complexity. Our analysis documents linearity and underlying dynamics similar to that of deterministic diffusion for bed elevations at low discharge conditions. These dynamics transit to a pronounced nonlinearity and more complexity for high discharge, akin to that of a multiplicative cascading process used to characterize fully developed turbulence. Knowing the degree of nonlinearity and complexity in the temporal dynamics of bed elevation fluctuations can provide insight into model formulation and also into the feedbacks between near-bed turbulence, sediment transport and bedform development.

Cumulative impacts of hydrology and human activities on water quality in the St. Lawrence River (Lake Saint-Pierre, Quebec, Canada)

The cumulative impacts of discharge and human activities on the spatial variations of water quality were assessed from monthly measurements at 80–110 stations in a ~300 km2 widening of the St. Lawrence River (Lake Saint-Pierre, Québec). Water quality was poorest under high discharge conditions and in shallow riparian areas under the influence of small tributaries draining farmlands; spatial variability was amplified by aquatic macrophytes. In 2004, over 40% of the study area exceeded the provincial water quality criterion (total phosphorus = 30 µg P·L–1) to protect aquatic life in rivers. Nutrient retention occurred mostly during summer when macrophytes were abundant and current velocities were low. Erosion of dredged deposits near the navigation channel was observed during summer, when current velocities increased in the deep channel because of flow obstruction by macrophytes growing in shallow areas. Low discharge conditions and high macrophyte abundance sharpened the contrast between the Lake Ontario waters flowing rapidly through the central river channel and the slow-flowing, low-quality tributary waters along the shores. The cumulative effect of high nutrient inflow from urban areas and farmlands, the man-made navigation channel, and seasonal flow obstruction by macrophytes thus induce a marked spatial variability in water quality within Lake Saint-Pierre.

Assessment of Chlorophyll‐a as a Criterion for Establishing Nutrient Standards in the Streams and Rivers of Illinois

Nutrient enrichment is a frequently cited cause for biotic impairment of streams and rivers in the USA. Efforts are underway to develop nutrient standards in many states, but defensible nutrient standards require an empirical relationship between nitrogen (N) or phosphorus (P) concentrations and some criterion that relates nutrient levels to the attainment of designated uses. Algal biomass, measured as chlorophyll-a (chl-a), is a commonly proposed criterion, yet nutrient-chl-a relationships have not been well documented in Illinois at a state-wide scale. We used state-wide surveys of >100 stream and river sites to assess the applicability of chl-a as a criterion for establishing nutrient standards for Illinois. Among all sites, the median total P and total N concentrations were 0.185 and 5.6 mg L(-1), respectively, during high-discharge conditions. During low-discharge conditions, median total P concentration was 0.168 mg L(-1), with 25% of sites having a total P of > or =0.326 mg L(-1). Across the state, 90% of the sites had sestonic chl-a values of < or =35 microg L(-1), and watershed area was the best predictor of sestonic chl-a. During low discharge there was a significant correlation between sestonic chl-a and total P for those sites that had canopy cover < or =25% and total P of < or =0.2 mg L(-1). Results suggest sestonic chl-a may be an appropriate criterion for the larger rivers in Illinois but is inappropriate for small rivers and streams. Coarse substrate to support benthic chl-a occurred in <50% of the sites we examined; a study using artificial substrates did not reveal a relationship between chl-a accrual and N or P concentrations. For many streams and rivers in Illinois, nutrients may not be the limiting factor for algal biomass due to the generally high nutrient concentrations and the effects of other factors, such as substrate conditions and turbidity.

Evaluation and field-scale application of an analytical method to quantify groundwater discharge using mapped streambed temperatures

A method for calculating groundwater discharge through a streambed on a sub-reach to a reach scale has been developed using data from plan-view mapping of streambed temperatures at a uniform depth along a reach of a river or stream. An analytical solution of the one-dimensional steady-state heat-diffusion–advection equation was used to determine fluxes from observed temperature data. The method was applied to point measurements of streambed temperatures used to map a 60 m long reach of a river by Conant Jr. [Conant Jr. B., 2004. Delineating and quantifying ground water discharge zones using streambed temperatures. Ground Water 42(2), 243–257] and relies on the underlying assumption that streambed temperatures are in a quasi-steady-state during the period of mapping. The analytical method was able to match the values and pattern of flux previously obtained using an empirical relationship that related streambed temperatures to fluxes obtained from piezometers and using Darcy’s law. A second independent test of the analytical method using temperature mapping and seepage meter fluxes along a first-order stream confirmed the validity of the approach. The USGS numerical heat transport model VS2DH was also used to evaluate the thermal response of the streambed sediments to transient variations in surface water temperatures and showed that quasi-steady-state conditions occurred for most, but not all, conditions. During mapping events in the winter, quasi-steady-state conditions were typically observed for both high and low groundwater discharge conditions, but during summer mapping events quasi-steady-state conditions were typically not achieved at low flux areas or where measurements were made at shallow depths. Major advantages of using this analytical method include: it can be implemented using a spreadsheet; it does not require the installation or testing of piezometers or seepage meters (although they would help to confirm the results); and it needs only a minimal amount of input data related to water temperatures and the thermal properties of water and the sediments. The field results showed the analytical solution tends to underestimate high fluxes. However, a sensitivity analysis of possible model inputs shows the solution is relatively robust and not particularly sensitive to small uncertainties in input data and can produce reasonable flux estimates without the need for calibration.

Reactivity, interactions and transport of trace elements, organic carbon and particulate material in a mountain range river system (Adour River, France)

The background levels, variability, partitioning and transport of eleven trace elements-Ag, Al, As, Cd, Co, Cr, Cu, Mn, Pb, Zn and U-were investigated in a mountain range river system (Adour River, France). This particular river system displayed a turbulent hydrodynamic regime, characterized by flash-transient discharge conditions leading to fast shifts in suspended particulate matter (SPM) concentrations as high as two orders of magnitude (12 to 600 mg l(-1)). The distribution of SPM was accurately predicted with a "hysteresis" transport model, indicating that about 75% of the annual solids load was exported within 20 to 40 days. Dissolved and particulate concentrations of most trace elements were low compared to their concentrations in other reference river systems expect for Pb and Cr, associated with historical anthropogenic activities. Although dissolved and particulate metal concentrations were steady for most elements during low and average discharge conditions, significant changes were observed with increasing river discharge. The changes in trace element concentrations in the two compartments was found to induce a partitioning anomaly referred to as the particulate concentration effect. This anomaly was significant for Cr, Mn, Pb, Zn, Cu and organic carbon (p < 0.03). The processes driving this anomaly were possibly linked to the modification and/or increase of colloidal organic and inorganic vectors, suggested by the significant increase of DOC (p < 0.001) and dissolved Al concentrations (p < 0.05) during flood conditions. A complementary process linked to the influence of coarse particles of low complexation capacity and transported mainly during high discharge may also effect trace element concentrations. Annual metal fluxes transported by this river system were estimated using the hysteresis SPM model with consideration of these fate processes. Metals in the Adour River system are primarily exported into the Bay of Biscay (Atlantic Ocean).