The Interactions Between Sediments and Water

The interactions between sediments and water have received increased emphasis over the last two decades, leading to a greater understanding of physical, chemical and biological processes in a wide range of aquatic systems including river, lake, estuarine and coastal systems. This new understanding has fostered research and assessment projects, and focused awareness on the implications of sediment/water interactions for policy and regulation. The international symposium on “The Interactions Between Sediments and Water“ held in Baveno, Italy from 22–26 September 1996 was the seventh in a series of symposia held since 1976. All focused on issues relating to processes which occur near the sediment/water interface as well as responses to interactions between sediments and water. Previous symposia were held in Amsterdam in 1976, Kingston (Canada) in 1982, Geneva in 1984, Melbourne in 1986, Uppsala in 1990 and Santa Barbara in 1993. Almost 200 oral and poster presentations were made by attendees representing 27 nations, including dozens of participants from developing countries. The scientific sessions were organized around the following themes: sediment/water dynamics, contaminant interactions, sediment/nutrient relationships, sediments as historical records of deposition, sediment/water science in remote areas, sediment/organism interactions, and novel methods and analytical quality control. This paper attempts to summarize the key highlights of the various sessions.

Attempts to study spatial patterns of overbank sedimentation on river floodplains commonly face important operational and sampling problems in documenting deposition rates. Recent advances in the application of fallout radionuclides (137Cs and unsupported 210Pb) to the estimation of medium-term rates of overbank sedimentation offer an essentially unique opportunity to assemble detailed distributed data sets for medium-term deposition rates. Such data can afford a valuable basis for investigating the complex relationship between sedimentation rates and floodplain microtopography and morphology and flow hydraulics. This paper reports the results of an investigation of the spatial pattern of overbank sedimentation rates on the floodplain of the River Culm, Devon, UK. Caesium-137 and unsupported 210Pb measurements have been used to establish the general pattern of deposition rates along an 11 km reach and more intensive measurements have been employed to document the detailed pattern existing within a small area subject to frequent inundation. The resultant data have been used as a basis for interpreting the major controls on the observed patterns.

By means of a recently developed flume, sediment erosion rates as a function of shear stress and with depth in the sediments have previously been determined for relatively undisturbed sediments from several rivers and lakes. These experiments demonstrated that erosion rates depended on at least the following parameters: bulk density (or water content) of the sediments, particle size distribution as well as mean particle size, mineralogy, organic content, and amounts and sizes of gas bubbles. In order to isolate and quantify the effects of one of these parameters, the bulk density, additional experiments have been done with reconstructed sediments and are reported here. These experiments first determined the bulk density as a function of depth in the sediments for three different types of sediments, for three different sediment core lengths, and for compaction times varying from 1 to 60 days. For each of these sediment cores and compaction times, the erosion rate as a function of shear stress and with depth was then measured and related to the local bulk density of the sediment The results demonstrate that, for a particular sediment and shear stress, the erosion rate is a unique function of the bulk density and can be expressed as a product of powers of the shear stress and bulk density.

Grain size is the most basic of classification criteria for sediments. The size distribution of a given sediment records the physical transport processes involved in its formation. By using precise grain size analysis and the model of Kranck et al. (1996a, b), it is possible to break down a sediment into the three major components from which it was formed: material deposited as floes, material deposited as single grains from suspension, and material carried under higher energy conditions. With this method, both the amount of material deposited in a flocculated state and the maximum size, or floc limit, of the particles composing the floc can be determined. Changes in floc limit indicate changes in the aggregation dynamics of the system. As most trace metals and many other contaminants associate closely with the fine particle fraction of sediments, it is important to determine both the areal distribution and reworking history of the floc settled portion of a sediment. This paper discusses the application of the method to coastal inlets in Atlantic Canada and examines the relationship between proportion of floc-settled material and trace metal concentrations. Disaggregated inorganic grain size distributions are also used to illustrate changes in the aggregation dynamics in areas of intense aquaculture.

Flocculated fine-grained sediment is a complex matrix of microbial communities and organic (detritus, cellular debris and extracellular polymers) and inorganic material. Suspended flocs within any aquatic system play a significant ecological role as they can regulate the overall water quality through their physical, chemical and/or biological activity. This paper investigates the complex structural matrix of riverine flocs over a large range of magnifications using correlative microscopic techniques. The significance of floc structural characteristics [(size, shape, porosity, density, inorganic composition, organic composition (bacteria and fibrils)] on the physical (eg. transport and settling), chemical (eg. adsorbing/transforming contaminants and nutrients), and biological (eg. biotransformation and habitat development) behaviour of a floc is investigated. Results suggest that it is the floe’s internal structure that has a significant impact on controlling the above floc behaviours. This internalstructure is complex and is often dominated by the existence of a three-dimensional matrix of fibrillar material secreted by the active microbial community within the floc. This matrix, in conjunction with the inorganic and bioorganic (active and inactive) constituents of a floc, provides an intricate pore structure that may result in water being an important bound component of a floc. These complex interactive structural and functional properties of a floe are considered to influence a floe’s behaviour both physically in how it is transported or settled, chemically in how it adsorbs/transforms contaminants and nutrients, and biologically in how it develops a diverse microhabitat capable of modifying the structural, chemical and biological makeup of the floc.

Flocculation experiments were carried out by using a Couette type of viscometer for applying uniform fluid shears to freshwater suspensions of estuarine fine-grained particles. The suspensions were buffered by a maleic acid/ammonium hydroxide system. The objectives of the study are to investigate the relationship between the steady-state sizes of flocculated particles and applied shear at various pH values. The results showed that the rate of particle flocculation increased with the addition of the buffer. With a particle concentration of 100 mg/1 and pH of 6.0, the floc size decreased with the increasing shear. While with a pH of 7.5, the floc size increased with the shear until it reached a maximum value at the shear of 400 s , and then the size decreased as the shear increased further to 500 s . The maximum size shear was found at 200 s−1 for pH 7.9. By increasing the pH value to 9.0, there was no particle growth with any applied shear. The electrophoretic mobility of the particles was measured. It was found that for any constant shear the floc size is inversely proportional to the absolute value of the mobility. That is, the more repulsive the force between particles, the smaller the floc size. The effect of pH on floc size was not uniform within the pH range tested. The results demonstrated that the higher the shear the more significant the changes of floc size due to the pH effect.

The delivery of suspended sediment from drainage basins has frequently been quantified in mass terms by use of the sediment budget approach, which identifies sources, storage and output of mobilised sediment. An attempt is presented here to define the main components of a generalised suspended sediment budget for a drainage basin in Devon, U.K. in terms of particle size characteristics and grain size selectivity, rather than total amounts of sediment. Samples of sediment mobilised from the hillslopes, fluvial suspended sediment and suspended sediment deposited on the river bed were all collected for particle size characterisation. These samples were then treated to remove organic matter and their chemically dispersed (absolute) particle size composition was measured using a Coulter LS130 laser granulometer. Where possible, measurements of the natural in-situ particle size distribution (effective particle size) were also undertaken. Samples were collected at different times of the year so that temporalvariation of hydrometeorolgical and ground conditions was represented. Comparison of the results for the different components of the delivery process shows that significant particle size selectivity occurs in the mobilisation and transfer of sediment from the hillslopes to the basin outlet. This reflects the particle size selectivity of detachment, transport and deposition processes, which is in turn influenced by the aggregation or flocculation (effective particle size) of the sediment.

An important feature of overbank floodplain deposits is the spatial variability of their particle size composition. Analysis of such spatial variability can assist in developing an improved understanding of the transport and deposition of suspended sediment on river floodplains during overbank flood events, in investigating the fate of sediment-associated contaminants and in calibrating existing floodplain sediment deposition models. The study reported in this paper investigates the spatial variability of the grain size composition of overbank floodplain deposits at different spatial scales, through analysis of surface sediment samples collected from frequently inundated floodplain sites on the Rivers Culm, Stour and Severn in the UK. Significant lateral and downstream variations in the grain size composition of the sediment deposits have been documented at the study sites, and the results obtained have been interpreted in terms of the processes governing overbank floodplain flow and sediment transport and deposition, which are influenced by a number of factors including floodplain geometry and topography.

Variations of sediment size and its gradation of the bed surface layer in a channel bend with nonuniform sediment are investigated experimentally. Four groups of sediment with the same initial median diameter (D _o ) but different initial size gradation (σ _o ) have been used for experiments which were run until the equilibrium bed topography was achieved. Analyses of experimental data have yielded the following results: (1) The time of equilibrium for bed evolution decreases as σ _o increases; (2) the median size of sediment (D) for a given section in the bend increases with increasing distance from the inner bank towards the outer bank, and it also increases with increasing σ _o ; (3) the value of D/D _o along the inner bank decreases with increasing σ _o , and it also shows a gradual decrease in the upper half of the bend and a slight recovery in the lower half; and (5) the transverse variation of σ value exhibits a general trend increasing from the inner bank towards the outer bank.

The use of tile drains for subsurface drainage in agricultural watersheds has created concern for the delivery of sediment to receiving waters and potential undesirable effects on surface and subsurface water quality. In this study, transport characteristics of sediment from tile drains in an agricultural watershed of the Thames River, near Kintore, Ontario, Canada were tested in a 5 m diameter, rotating circular flume located at the National Water Research Institute in Burlington, Ontario. Tile drain sediments were collected and mixed with river water at different speeds in the flume to study transport processes such as deposition, erosion and flocculation as a function of bed shear stress. During deposition and erosion experiments, water samples were collected to determine changes in the concentrations of cations, anions and dissolved organic carbon. The results show that tile drain sediments have a tendency to flocculate when subjected to a range of shear stresses. The median diameter (D₅₀) of the floc size distribution reached a maximum value at a shear stress of 0.169 Nm−2 which can be considered an optimum shear stress for flocculation for this sediment. The critical shear stress at which all of the initially suspended sediment deposited to the flume bed was measured as 0.056 Nm−2. The pH and cation concentrations remained relatively constant during erosion and deposition experiments. Anion concentrations were more variable, most likely due to the presence of bacteria which could have also played a role in the flocculation mechanism of tile drain sediment.

The sediment budget of a reservoir on the Neckar River, Germany, was investigated by means of experimental and numerical methods. Field measurements of channel bathymetry show that sedimentation and erosion occur primarily in the lower backwater-influenced section of the reservoir, which stores approximately 350,000 m3 of fine-grained deposits. Sediment load balances for two major storm events in Dec. 1993 and Apr. 1994 showed net erosion of 32,000±10,000 tonnes and 24,000±5,000 tonnes of sediment, respectively. A balanced sediment budget was found for a minor flood in Jan. 1995. In agreement with the field data, numerical simulation of sediment transport over a period of 45 years demonstrates that the river reservoir served initially as a sediment trap from 1950 to 1978, and since then as a temporary storage basin for sediment.

The exchange of kaolinite clay between a stream and its associated sand bed was investigated in a recirculating flume. Bedforms at the sediment-water interface result in two bed-exchange mechanisms: the bedform shape induces an advective flow through the bed (‘pumping’), and dune propagation causes the trapping and release of pore water (‘turnover’). Chemical and electrostatic interactions then result in filtration of clay by the bed. In order to allow modeling of chemical effects, all flume materials must have defined chemical parameters. This required improvements in the flume water supply, construction of sand-washing equipment, and the use of defined clay and sand preparation procedures. Flume experimental results indicate that clay is extensively trapped by the bed. Advective pumping tends to carry clay to the deep bed where it can be permanently trapped, while turnover tends to continuously mix the upper layers of the bed, hindering penetration to the deep bed.

In warm-monomictic Lake Kinneret, wind-induced internal waves with amplitudes of up to 10 meters are common during April – October. This study was aimed to follow the horizontal and vertical transport of resuspended particles due to internal wave activity using fluorescently-dyed sediment particles (lake sediments and lyophilized algal cells) as tracers. Color-coded (5 colors) tracers were deployed along a transect perpendicular to the shore, and their dispersion was followed by recovery of labeled particles in sediment traps and in bottom sediment samples using epifluorescence microscopy. Wind-induced internal seiches as the driving force for resuspension were followed using a thermistor chain and a current velocity profiler. Examination of sediment trap and bottom sediment samples indicated particle transport from the hypolimnion to the epilimnion that could be linked to the seiche activity. Horizontal transport of particles was most distinct for littoral sediments whereby particles placed at 5 mdepth were exposed to a strong long shore transport.

Sediment trap sampling at an offshore site in southern Lake Michigan has continued for an 18 year period with a sampling frequency ranging from weekly to semi-annually. During the 6 month unstratified period sediment trap mass and tracer profiles are nearly constant and they have been used to describe the extent of sediment resuspension. After stratification, mass flux rapidly declines and particle tracers are removed from the epilimnion at the rate of 0.5–1 m.d−1. Exponential profiles of mass flux clearly show the persistence of a benthic nepheloid layer. High frequency sampling with near-bottom sequencing traps show order of magnitude ranges in mass flux over a few day period.

Velocity and turbidity were measured over a tidal cycle during both spring and neap tides in the Ribble estuary, England. The data was analysed to determine the relative magnitudes of the various components of the residual circulation and sediment flux. The Lagrangian residual circulation was found to be dominated by the influence of freshwater input to the estuary, the landward directed component of the circulation due to the Stokes drift being cancelled by the Eulerian flow induced by set-up. The dominant driving mechanism of the residual flux of suspended sediment was found to vary both spatially and over the spring-neap cycle. During the neap tide the flux in the mid estuary was negligable, however, during the spring tide landward transport of sediment by the mechanism of tidal pumping was found to be the dominant mechanism.

Water-particle interactions often may result in non-conservative chemical behavior when waters from different sources mix with one another. The results presented in this paper address the role of these interactions in freshwater and estuarine mixing and support a larger study to develop a method to help resolve flow distribution and water quality questions in surface waters using a source water “fingerprinting” technique. Inductively coupled plasma-mass spectrometry (ICP-MS) is used to “fingerprint” each water source based upon the concentrations and relative proportions of elements in that source. Estimates can then be made of the fractions of various “fingerprinted” waters in water samples that contain a mixture of source waters. Such estimates depend upon the selection of tracers that behave conservatively during mixing; in this paper, results to establish the maximum particle exchange capacity and conservative mixing behavior are presented for samples collected from the Sacramento River-San Francisco Bay-Delta estuary. Elements likely to behave conservatively include boron, sodium, magnesium, potassium, calcium, strontium, and molybdenum.

We present results from an experimental study of suspended particle (4.5–36.5 urn silicon carbide powder) deposition from surface water to ‘clean’ equi-granular permeable beds in a small 12.5×12.5×15cm box and a re-circulating flume. Enhanced deposition rates of up to 5 times the accepted sediment deposition model (e.g., Einstein, 1968) are explained by filtration of particles in the bed. Compared to this model deposition increases with increasing surface fluid speed, decreasing suspended particle size and increasing bed particle size. These results can be explained by an increased ability of particles to penetrate into the bed with the pore water which increases the effective filter thickness. The predominant deposition mechanism within the bed pores appears to be settling. Enhanced deposition, evident in Einstein’s (1968) experimental data, was previously attributed to flocculation but may be explained better by filtration. These enhanced deposition rates drop off to close to the accepted model predicted rate after a certain volume of sediment has entered the bed, which may be due to the slumping of deposits from the top of bed elements. This reduction in deposition rate occurs long before the bed is filled with fine sediment.

Digital imaging techniques especially geared towards the laboratory characterization of the kinematics of water-sediment interaction are presented. More specifically, the methods proposed apply to the motion of cohesionless spherical particles in transient water flow, with the aim of obtaining both particle velocity and concentration fields from sequences of digital images. A special particle identification algorithm is devised in order to deal with densely packed particles (in contrast to the sparse seedings of tracers used in studies of pure fluid kinematics) and to allow application of digital particle tracking velocimetry (DPTV). A procedure for extracting the concentration field from the knowledge of the discrete particle positions is then detailed. Finally, the various tools are illustrated for the laboratory case of a dambreak wave over a movable bed.

The suitability of the two more extensively used sampling techniques, namely in-situ dialysis and centrifugation followed by filtration were compared. Field measurements derived from Lake Simbirizzi, Sardegna, Italy were used for the comparison. Two groups of substances were selected; the first one consists of the relatively inert in the aquatic environment chlorides and bromides, while the second one consists of substances involved in diagenetic processes (ammonium-nitrogen and soluble reactive phosphorus). The field measurements show that for the applied sampling techniques the measurements are comparable for the first group of substances, while measurements derived by centrifugation followed by filtration give considerably different concentrations using in-situ dialysis for the second group of substances. Since the second group is related to sediment-water interactions (adsorption-desorption and chemical processes), it seems that forced separation by centrifugation influences considerably the derived measurements. As a result in-situ dialysis is expected to give more accurate measurements of porewater concentrations for nutrients and other reactive substances.

We present three examples to show the importance of sediment-water interactions to electric utilities: 1) Selenium (Se), in ash pond effluents, has caused declines in fish populations in North Carolina. A biogeochemistry model appears to explain Se dynamics for several reservoirs. However, further work on sediment water interactions is needed to predict the speed of reservoir Se declines following cessation of inputs; 2) Mercury (Hg), volatilized in stack gases from coal fired power plants, is a public and wildlife health concern. Sediments play a major role in the biogeochemistry of Hg as documented in the Mercury Cycling Model (MCM); As with Se, questions about sediment water interactions limit the confidence in predictions about dynamics and effects of Hg; and 3) One of the recommendations from a recent Pellston Conference was to evaluate the use of a new paradigm as a basis for metals regulations. Under this new paradigm, effects of surface active metals (Ag, Al, Cd, Cu, Ni and Zn) on fish can be viewed as dependent on competition between the gill, a ‘biotic ligand’, and other environmental ligands for metals in discharges. Under this new paradigm, then, the mechanics of toxicity can be viewed as analogous to interactions at the sediment-water interface. It is clear from these three examples that fostering discussion among chemists and toxicologists, through joint participation at meetings and publication in journals used by both fields, is critical for development of accurate assessment capabilities and support of cost effective decision making.

The objective of this study was to verify in which way the ionic composition of the sediment and that of the overlying water column may have an effect on radiocaesium fixation, through possible structural modifications of the frayed edge sites (FES) pools of the sediments. Two experimental protocols have been considered: i) a condition in which sediments were homoionically saturated with either potassium, ammonium, calcium, magnesium or sodium ions, and ii) a mixed potassium-calcium scenario in the liquid phase. Nine freshwater sediments from four different locations were used in this study. For homoionic potassium and ammonium saturated sediments a nearly quantitative radiocaesium desorption (90–100%) was observed, whereas for calcium and magnesium the desorption yields were about 20%. It appears that the action of strongly hydrated ions (Na+, Mg2+, Ca2+) leads to a pronounced enhancement of radiocaesium fixation in the solid phase, whereas poorly hydrated ions (K+, NH4+) have the opposite effect and promote sorption reversibility.Another issue considered in this study concerns the effect of temperature and sediment drying on the radiocaesium fixation. Drying the sediments at 110°C leads to a significant increase in radiocaesium fixation levels, while drying the sediments at room temperature (25°C) has a very limited effect on radiocaesium fixation and appears to put a brake on the aging effects.

Intertidal sediments from a range of depositional environments in the eastern Irish Sea have been analysed with regard to their radionuclide content, particle size distribution and magnetic properties. Concentrations of Am and Cs are highly influenced by the abundance of sediment finer than 32μm in fine-grained sedimentary environments, whilst radionuclide activity in coarser sediments is less dependent on particle size. Investigation of the magnetic properties of these sediments highlights a similar association between this size fraction and magnetic remanence, the latter being shown to offer strong potential as a grain size proxy in monitoring 137Cs concentrations in fine-grained sediments. Analysis of particle size fractions indicates the extent to which 137Cs may be enriched in the clay size fractions of both fine and coarse sediments.

The international reference lake sediment, LKSD-4, was used to compare Hg, organic C and Zn extracted from its’ soluble organic’ phase by two commonly used reagents: 0.1 M Na₄P₂0₇ solution at pH 10 and 0.5 M NaOH solution at pH 12. While recoveries of Hg and Zn by 0.1 M Na₄P₂O₇ are not affected by changes in sample weight to reagent volume ratio (W/V) or contact time, those by NaOH show a marked dependency. In general, the NaOH leach extracts more organic C and Hg from LKSD-4 but less Zn. Over the range of conditions studied, the NaOH-based method extracted 4.7–9.8% C, 27-103 ng g−1 Hg and 19–69 υg g−1 Zn from LKSD-4, compared to 2.3–2.8% C, 17–24 ng g−1 Hg and 64–72 μg g−1 Zn by the Na₄P₂O₇ leach. Clearly, different groups of organic substances are being dissolved by these two reagents and therefore a comparison of data from different laboratories becomes meaningless. This paper suggests that more research is needed into the exact nature of metal-organic associations extracted by selective leaches and into associated artifacts of extraction such as readsorption phenomena.

The adsorption and desorption of hydrophobic organic chemicals to suspended sediments are often quite slow and, because of this, these finite rates of sorption must be included for accurate predictions of the transport of contaminants in surface waters in many cases. A theoretical model of adsorption and desorption which includes finite sorption rates has previously been developed and shown to be in good agreement with experimental results. However, this model generally requires quite large amounts of computer time. In the present study, a simplified but reasonably accurate version of this model is developed which requires much less computer time and is therefore suitable for large-scale calculations of the transport of contaminants in surface waters. Applications of this model are presented in order to illustrate the effects of finite sorption rates on this transport.

Surface sediment samples were collected at six locations of the Lagoon of Venice reflecting potential different contamination sources and representative of different hydrological situations. Analysis of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDFs), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), linear alkylbenzenes (LABs) and coprostanol have been carried out by gas chromatography-mass spectrometry (GC-MS) to assess the influence of various pollution sources: urban, industrial and combustion processes. PAHs, PCBs, coprostanol, and LABs showed the highest levels in the sample collected within the city of Venice (Canal Grande) indicating a very heavy contamination due to combustion sources and to the domestic waste waters directly entering the canals of the city. The highest levels of PCDD/PCDFs were found in samples collected near the industrial area of Porto Marghera. The investigation on the PCDD/PCDFs homologue profiles suggested the presence of two dififeient sources for these pollutants, one due to combustion processes and another one due to the chemical processes of torto Marghera.

PCB congeners and organochlorine pesticides (DDT, lindane and HCB) distribution were studied in Lake Orta sediments. The results indicated a contaminated area in the northern part of the sub-basin. The observed high levels of organochlorine compounds (OCs) may be explained by the focusing phenomenon, ie. the preferential transport of lighter and smaller particles from the emission sources to this area. The PCBs and DDT values were correlated with the organic carbon content and the heavy metal contamination. The toxicity of the sediment samples was related also to PCB content. PCBs and OCs pollution of Lake Orta was of the same order of magnitude as in Lake Como, which is the most contamined lake in Northern Italy.

The space-time distribution of some pollutants (Cu, Pb, Zn, Cd, Fe, Mn, V, Ni, Cr) in the sludge of the canals of Venice was studied. The contamination levels were comparable to, or higher, than those measured in the most polluted sediments of the Lagoon of Venice. Sediments were collected by two different sampling techniques: 1) collection of sediment cores (upper 5 cm) by a syringe-type corer; 2) collection by traps, placed on the bottom of the canal. Traps permitted the sampling of sediments essentially resuspended by overlying water turbulence. This sediment fraction is subjected to variations of its physicochemical parameters (principally change of redox conditions) and therefore to pollutant exchange at the water/sediment interface.The metals principally exchanged during sediment resuspension were Cd, Pb, Zn and Cu. These metals have principally an anthropogenic origin and are bound to the most labile geochemical phases of the sediment (such as sulphides), which can be oxidised during sediment resuspension, releasing metals into the water. Fe, Cr and Ni were only partially exchanged, while Mn and V were generally not exchanged; a significant fraction of these metals is of natural origin and is bound to the most refractory phases of the sediment.

Undisturbed sediment cores from an oligotrophy lake were percolated with artificial porewater to examine the effects of isoetid macrophytes, Littorella uniflora, and benthic microalgae on daily dynamics of sediment retention of phosphorus (P) by either iron (Fe) or manganese (Mn). Retention of Fe and Mn was observed due to oxidation processes mediated by oxygen release from L. uniflora roots and benthic microalgae. Therefore increased retention of P was observed because of P precipitation with oxidized Fe- and Mn-eompounds. During light periods, the ratio between Fe and P precipitation in the sediment was positively correlated with the P uptake by L. uniflora (p < 0.001, r2 = 0.984). The atomic precipitation ratio between Fe and P was between 1 and 2. The ratio between oxidized Fe-compounds and Fe-bound phosphate in the sediment was positively correlated with the root density of L. uniflora (p < 0.001, r2 = 0.995). The ratio between Mn and P precipitation was higher (≥6) than the ratio between Fe and P. The role of benthic primary producers on P retention in freshwater littoral sediments is discussed.

Selenium (Se) is a contaminant of concern in environments affected by discharges from smelting and coal-burning industries. Experiments have been performed to investigate the phase associations of selenium in contaminated sediments under a range of controlled redox conditions. In this study, Se sediment associations were examined using the BCR sequential extraction technique after stabilisation at different redox states. It was shown that although most of the sediment-bound Se is associated with the operationally-defined “organic/sulfide” fraction, as the measured redox potential of the system is increased, more Se moves into the “exchangeable” and “iron/manganese oxyhydroxide” fractions. In these fractions, contaminants can be expected to be more bioavailable. As the mass of Se absorbed to sediments is typically at least an order of magnitude higher than the mass dissolved in porewaters, significant Se exposure may result from oxidative shifts in Se associations.

The sedimentation of organic, amorphous oxides, crystalline oxides and crystalline silicate species of Fe and Al was investigated in humic Lake Örträsket. The covariation with C, N and P sedimentation also was studied. The results showed a strong temporal variation in the occurrence of different Fe species. During high discharge, such as in spring, Fe mostly occurred as crystalline Fe-silicate species, while the amorphous form dominated at other times of the year. Al generally sedimented as crystalline Al-silicate species (95%). Amorphous Fe species showed a strong relation with organic C and N, suggesting an association between organic material and amorphous Fe oxides. P was not as strongly related to amorphous Fe, probably as a result of apatite or clay bound-P species sedimentation.

The behaviour of Co, Fe, Mn, and Ni was investigated in the mixing area of the Po river in six surveys over the period March 1992 to June 1995 during low-to-medium solid load and flow conditions. The concentrations (nM) of dissolved elements for the riverine and marine end-members respectively were in the ranges 0.09 to 0.93 and 0.24 to 0.98 for Co, 10 to 53 and 5 to 20 for Ni, 12 to 87 and 5 to 33 for Fe, and 10 to 155 and 26 to 105 for Mn. Co and Mn behave non-conservativeiy in the Po estuary: an addition to the dissolved phase occurred for these elements that was more marked in the summer period. The concentrations of dissolved and particulate Ni decreased almost linearly with increase in salinity, with more marked variations in dissolved Ni concentration in winter than summer. The concentrations of dissolved and particulate Fe also varied conservatively with salinity. This unusual behaviour for dissolved Fe is attributed to the analytical procedure which excluded kinetically inert colloidal species.

Benthic fluxes of dissolved inorganic N, Si and P nutrients, alkalinity, dissolved inorganic C (DIC), and 0₂ from sediments in the Gulf of Trieste (northern Adriatic, Italy) were measured monthly in the period September 1995 – August 1996 using in situ incubated light benthic chambers. The highest efluxes of DIC, NH₄+, P0₄3−, Si(OH)₄, and N0₃− influxes encountered in late summer — early autumn were the consequence of degradation of benthic microalgae, and in autumn mostly of sedimented phytoplankton. High N0₃− efflux was observed in spring. Only NH₄+ and Si(OH)₄ fluxes were significantly correlated with temperature. This correlation suggests that the rate of downward input and the quality and quantity of sedimentary organic matter (autochthonous and allochthonous) were superimposed on the temperature fluctuations. High DIC, NH₄+ and Si(OH)₄ effluxes observed in July 1996 were due to the late spring — early summer degradation of sedimentary organic matter produced by benthic microalgae, while the autumn phytoplankton bloom was quickly reflected in enhanced benthic fluxes due to higher temperature. Significant correlations between NH₄+, P0₄3− and Si(OH)₄ fluxes suggested their parallel regeneration and utilizationat the sediment-water interface. The nutrient fluxes were linked to 0₂ consumption, suggesting that aerobic oxidation processes were important at the sediment-water interface in the Gulf. The N, P and Si nutrients released from sediment pore waters are probably utilized in benthic microalgal and bottom-water primary production. This indicates that pelagic and benthic communities in the central part of the Gulf of Trieste function relatively independently of each other.

Stable isotope composition of carbon and nitrogen in the sediment and pore water of a eutrophic freshwater lake was studied. Based on changes in the δ13C and δ15N values of dissolved components and sediment fraction, possible processes involved in the decomposition of sedimentary organic matter are outlined. The relative importance of acetate fermentation and CO₂ reduction was estimated using known mathematical models, and ammonia assimilation by methanogenic bacteria is hypothesised to be the main process governing the isotope fractionation of dissolved nitrogen in pore water.

C and N content, C/N (atomic) ratio, and C and N isotopic composition (Δ13C and δ15N) were determined on suspended particulate matter and sediment samples obtained from riverine, estuarine and marine environments in two cruises (September 1995 and March 1996) in the Po estuary (Italy). Isotopic tracers of C and N, reported for the first time for this environment, gave information on sources of organic matter and their distributions. An end-member mixing model based on δ13C values was applied to estimate the relative importance of riverine and marine sources of organic matter in suspended particulate matter and sediments.

The processes regulating the concentration and isotopic composition of dissolved inorganic carbon (DIC) in pore water were investigated in three different parts of Lake Bled. It was found that the isotopic composition of dissolved inorganic carbon (DIC) is strongly influenced by methanogenesis. Simple diagenetic model reproduces the observed carbon-isotope profiles and the DIC concentration data resonably well in investigated parts. This Hidings lead us to conclude that the majority of processes affecting DIC are taken into account in the model.

To gather information on the interactions between the sediment and suspended organic matter pools in the Stagnone di Marsala, water and sediment samples were collected, on a monthly basis, at 11 stations. Water temperature and salinity showed a clear seasonality whilst particulate and sediment organic matter did not show any clear seasonal pattern. Relative abundances of suspended and sediment organic matter, on the other hand, appeared to be site-dependent and controlled mainly by the dynamic balance between resuspension and sedimentation. High quantities of both suspended and sediment total organic matter were present, while very low algal biomasses (in terms of chlorophyll-a concentrations) were observed both in the suspended and sediment pools thus showing the oligotrophy of the site. The contribution of phytoplankton and microphytobenthos to the total organic content of suspended and sediment matter was negligible. The low food availability of organic matter in the Stagnone di Marsala Sound may explain the low abundance of suspension-feeding molluscs, which are substituted by limnovore and detritivore species.

The origin, quality and pathways of particulate organic carbon (POC) were studied from May 1991 to May 1995 in a submarine cave (Grotta Azzurra, Tyrrhenian Sea, Italy) with warm sulphur springs that support dense mats of sulphur-oxidizing bacteria. Multifactorial sampling designs were used to specifically address: (1) differences in the quantity and quality of suspended and sedimenting particles in two distinct regions of Grotta Azzurra, a weakly-illuminated outer region (Central Hall) and an innermost dark region (Snow Hall) characterized by the presence of sulphur waters; (2) the composition and fluxes of particulate material above and below the sulphurous boundary. The water and sediment trap samples were analysed for total particles, POC, particulate organic nitrogen (PON), chlorophyll a (Chl a) and phaeopigments (Phaeo). The microbial mats were assayed for carbon fixation and RubisCo activity. Stable carbon isotope ratios of the bacteria, benthic fauna and sediments were measured. The overallamount and flux of POC did not differ between the two regions of the cave. By contrast photosynthetic pigments decreased significantly from the outer to the inner region. The average POC:PON ratios of suspended and sedimenting material were lower than 10.2, and there was no apparent ageing or degradation from the Central Hall to the Snow Hall. Fluxes of POC above the boundary were on average 23.8% of fluxes measured below the boundary, and no Chi a was detected above the boundary. The mats of sulphur-oxidizing bacteria had a 13C depletion of −30 to −3 l‰, providing a marker for tracing POC from this source. The majority of the benthic animals showed ∂13C from −20 to −24‰. Some polychaete worms and echinoderms were more depleted (−25‰), indicating greater consumption of bacterial carbon, while sponges were not depleted compared with the benthos of the control cave (∂13C −18 to −19‰). It is estimated thatbacteria supply about 31% of the suspended POC in the Snow Hall. A larger input may be available to animals living close to the bacterial mats and to those in the more sulphurous Grotta Sulfurea cave.

The aim of this work was to analyse factors which regulate sulphide mobility in the sediments of a dystrophic lagoon (Sacca di Goro, the southern lagoon of the Po river delta, Italy). In 1995–96, sediment oxygen demand and variations in organic matter content, redox potential, iron availability, inorganic sulphur concentrations and sulphate reduction rates were measured in sediment profiles at three stations (G, 4 and 17) representative of the main areas of the lagoon. Stations differed mainly in their salinity range and primary producer communities. High concentrations of reactive iron (110–275 μmol ml−1) and low sulphate reduction rates (0.8–16.1 mmol m−2 d−) were measured in the sediment. Moreover, high concentrations of reactive ferric iron were detected in winter and spring at the stations closest to the freshwater inputs. Nevertheless, in summer, high concentrations of free sulphides were detected in the porewater, although most of the reactive ferrous iron was not sulphide-bound, indicating that not all of the reactive iron pool was available to buffer against sulphide release and thus measures of reactive iron pools may not be a good measure of the true buffering capacity of the sediment. Furthermore, a considerable production of sulphide may occur in the decaying Ulva biomass in the water column, where its concentration will be independent of the potential buffering capacity of iron in the sediment. Therefore when assessing the vulnerability of coastal lagoons to dystrophic events, both the size and availability of the reactive iron pool as well as the site of sulphide production must be taken into consideration.

It has been observed that the alkalinity concentrations in the hypolimnia of many thermally stratified lakes increase over the duration of the summer. We have quantified the processes, both redox and exchange, that contributed to the alkalinity increases measured in two lakes that are situated on the Precambrian Shield of Ontario, Canada by measuring hypolimnetic mass balances for all substances involved in alkalinity-generating or -consuming reactions. These include nitrate, ammonium, base cations, iron, manganese, sulphate, organic anions, as well as alkalinity itself. In one lake, iron reduction was the dominant source of alkalinity; since this process is probably reversed at fall overturn when hypolimnetic waters mix with oxygenated surface waters, the alkalinity generated by this mechanism is likely temporary in nature. In the second lake, iron reduction and sulphate reduction were both important; the latter should provide more permanent alkalinity.

Sediments were cored, and the sediment-water interface and overlying waters were sampled in 5 lakes from the southwestern Chinese plateau during 1991–95. The geochemistry of HCO₃− at the sediment-water interface was examined by studying detailed profiles of pH, HCO₃− concentrations and δ13C of dissolved inorganic carbon (DIC) in overlying lake water and porewater near the sediment-water interface. Dissolution-precipitation equilibrium of carbonates, diffusion flux, and the extent of the influence of diffusion on the whole lake were calculated. The results show that the HCO₃− near the interface carried isotopic characteristics of decomposition of organic matter during early diagenesis, and that the porewater in surface sediments was unsaturated relative to calcite, and gradually saturated with depth. Furthermore, the interface is a source of HCO₃− to the overlying water. Alkalinity (Alk) diffusion flux from sediments to the overlying water due to concentration gradients ranged from 0.51 to 24.33 × 10−4 mol cm¨ a−1. The calculated contribution of the diffusion of Alk to the overlying water ranged from 0.46% to 49.42%. Diffusion is an important source of Alk in lakes with a long residence time and a relatively shallow depth.

Sulfur (S) conversions were determined during summer stratification in 1995/96 to assess the extent to which benthic release of phosphorus (P) is influenced by the S cycling in eutrophic, dimictic, sulfate-rich (61.33 ± 10.41 mg SO₄2− l−1) freshwater Lake Scharmützelsee. Hypolimnetic SO₄2− reduction (4.56 ± 0.73 g (S) m−2 d−1) fomiing ΣH₂S (44.71 ± 17.57 mg ΣH₂S m−2 d−1), leading to iron sulfide precipitation (5.62 ± 1.72 mg FeS m−2 d−1) and dissolved iron depletion in the hypolimnion has a major influence on benthic P mobilization and release. The most important inorganic S pool is the CRS (FeS₂ + S° + H₂S; 15.1% total S), being 1.3 to 6.6 times higher than the AVS (FeS + H₂S) in the uppermost 0 – 8 cm sediment This diminishes the ability of the sediment to bind P (indicated by 14.6 % loosely bound P (NH₄CI-P) and an exhaustion of the redox-sensitive P (BD-P)), leading to interstitial water P concentrations up to 10.8 mg l− and P release rates of 2.64 ± 0.56 mg P m−2 d−1. As a consequence the P content of the lake increased fourfold within 58 days.

Different factors which interactively control the flux of soluble reactive phosphorus (SRP) at the sediment-water interface (SWI) of Lake Kinneret were studied seasonally. The influence of pH , Eh and microbial activity on SRP flux at the SWI was investigated by manipulating the conditions in the overlying water of intact sediment cores. The calculated diffusive SRP flux out of the sediment was lower in cores sampled during winter and spring than during the period of amixis. Potential SRP release, as measured in the absence of microbial activity, was strongly enhanced upon the transition from oxic to anoxic conditions indicating P release from iron(III)-bound phosphorus. In spring and summer cores, an enhanced SRP flux from sediments at pH 7 in comparison to pH 8 indicated P release from carbonate-bound P which sedimented previously as result of high pH values during the algal spring bloom. Microbial uptake at the SWI was the most important sink for SRP and no net-flux occured under oxic conditions. The higher net-flux of P under anoxic conditions was linked to carbon limitation of the bacteria at the SWI.

Cultural eutrophication from excessive input of nutrients is a major problem for many water bodies around the world. Phosphorus and to a lesser degree nitrogen constitute the limiting elements for growth of plankton cells. Mobility, speciation and partition of nutrients in aquatic ecosystems depend on a number of physicochemical parameters. Experiments have been conducted for quantification of nutrient partition between ambient water and cohesive sediments. The experiments included nitrate and soluble phosphorus (superphosphate — 46% P₂O₅ fertilizer) partitioned between tap water and sediment slurries. The slurries involved kaolinite and bentonite as well as natural organic mud from, Lake Okeechobee, Florida. The nutrient exchange was promoted by sediment resuspension. Resuspension was induced either under homogeneous turbulent conditions in an oscillating-grid tank or by shear flow in a lock-exchange flume. The effects of phosphorus or nitrogen concentration, sediment concentration, water temperature, pH and salinity on nutrient partition were quantified. The results obtained through this study appear to be in agreement with data from other similar laboratory or field studies.

The aim of this investigation was to study the temporal variation in phosphorus release from the sediments and its influence on water quality of stratified lakes. The concentrations of soluble reactive phosphorus (SRP), calcium and sulfate in the interstitial water and the pH in the wet sediments of dimictic lakes were investigated during the spring circulation and at the end of summer stratification. Multiple regression analysis using the calculated diffusive fluxes of SRP out of the sediments and the morphometric characteristics of the lakes (reduced water depth), explained 73 % of the variance of the SRP-accumulation in the hypolimnia during summer stagnation. At the end of summer stratification diffusive fluxes of SRP out of the sediments increased and pH-values and sulfate-concentrations decreased at the sediment surface (0–2 cm) and in the hypolimnia. The maximum diffusive flux of SRP was calculated to be 5.8 mg/m2/d at the end of summer stagnation. Probable reasons for these higher diffusive fluxes of SRP at the end of summer stagnation are higher supply of labile organic matter and thereby higher mineralization rates, lower redox potential and thus higher dissolution of redox sensitive P-binding forms and/or dissolution of phosphorus being bound to Ca-phases at lower pH.

Pore waters, extracted monthly from short cores at two sedimentologically and biologically different locations (AA1 and F) in the Gulf of Trieste (Northern Adriatic), were analyzed for NH₄+, NO₃−, PO₄3−, Si(OH)₄, DIC and periodically for DOC, DON and DOP. Nutrient concentrations were used to model vertical profiles using a diffusion-reaction model which included the macrofaunal influence on sediment-water exchange rates. Winter nutrient profiles showed nearly an exponential increase, or decrease in the case of NO₃−, in nutrient concentrations with depth while the profiles from other seasons exhibited concentration maximum at 3–6 cm, a minimum around 8–10 cm, and then, except for NO₃−, a gradual increase or constant values. This vertical distribution is attributed to seasonal variations in the benthic infauna activity, mostly composed of polychaetes and bivalves and concentrated in the top 4–5 cm, being less active during the winter. The vertical profiles of DOC and DON showed the vertical distribution described above in all periods, while DOP was similar to that of PO₄3−. The comparison of modelled fluxes of nutrients across the sediment-water interface at the location AA1 and those measured using in situ benthic chamber showed quite good agreement for NH₄+ and PO₄3− fluxes but not for NO₃− and Si(OH)₄. Discrepancies could be caused by a topography effect and for Si(OH)₄ by an additional dissolution of the solid phase balancing the diffusive loss into burrows and lessening the effect of bioturbation.

The Schlei is a hypertrophic brackish fjord of the Baltic. In order to assess the optimal method for restorative sediment treatment, an in-situ-experiment was carried out to oxidize the uppermost sediment strata by direct nitrate injection. An area of 3, 5 ha, covered with sapropelic sediment, was treated with 50 t of commercial calcium-salpetre. Half of this area was treated once with a dose of 140 g NO₃-N m−2, whereas the other half was treated twice receiving a dose of 280 g NO₃-N m−2. Two enclosures were installed on these treated areas. The reaction in the interstitial water of the sediment (nitrate, phosphate, sulphate, iron) and in the water bulk (tot-N, tot-P) are documented and compared with an untreated reference area. Moreover, the impact of the sediment treatment on the phyto- and Zooplankton biomass is discussed.

Lake Hosek (north-eastern part of Poland) is a small shallow and without outflow lake which has been limed in 1970. The concentration of Ca was increased from 3–4 mg L−1 to 17 mg L−1 in the water and from 0.2–0.3% dry weight to 0.9–1.7% dry weight in sediments (5 cm upper layer) due to CaCO₃ addition to the lake.In the spring-summer seasons of 1992 and 1993, an experimental study was conducted in Lake Flosek to assess the capacity of bottom sediments to uptake and release mineral phosphorus. The rate of phosphorus exchange between sediments and near-bottom water was experimentally measured under conditions of high (100%), and of reduced (10%) oxygen saturation in near-bottom water.To determine the component of sediments responsible for the uptake of most phosphorus, the proportions of phosphorus forms in sediments were analysed.Sediments of Lake Flosek showed a slight tendency to release phosphates. The rate of this process was similar under high (100%) and low (10%) oxygen saturations ranging from − 0.161 to + 0.200 mg P m−2 d−1 . This is much lower (by 1–2 orders of magnitude) than reported from other harmonic, non-humic lakes.In the total phosphorus pool, the highest content of phosphorus was found in the organic and residual phosphorus fractions (over 70% of the total phosphorus in sediments). The largest part of the readily extractable phosphorus was found in the fraction bound to Al and humic substances (41%). Both these fractions determine a weak ex- change of phosphorus between sediments and water. No difference in P-release related to P-fraction compound was found in the cores taken from three sites in the lake.

Storm event and annual export of suspended sediment (SS) and particulate phosphorus (PP) was measured during three hydrological years (June 1993 to May 1996) in Gelbæk stream, a Danish lowland stream draining a 11.6 km2 arable catchment area. The contribution of subsurface drainage water, surface runoff and stream bank and bed erosion to catchment SS and PP losses was estimated using three different strategies: 1) Simultaneous and comparative monitoring of subsurface water. 2) A mass-balance and budget approach dividing the Gelbæk catchment into two sub-catchments. 3) Application of the fingerprinting technique to single storm events. Subsurface drainage water proved to be a significant SS and PP source. Subsurface drainage water from half of the catchment area accounted for 9.8–15% of the total annual SS loss from the Gelbæk catchment and 9.6–18.2% of the annual PP loss. The mass-balance and budget approach showed stream bank and bed erosion to be the major source of SS and PP in this channelized and highly managed lowland stream. These findings were consistent with the fact that the annual loss of SS and PP from an upper culverted stream sub-catchment was significantly lower than that estimated from a mass-balance for a lower sub-catchment with an open stream channel. Comparison of the tracer content (e.g. 137Cs) of SS collected during four storm events with that of topsoil and subsoil using a simple mixing model revealed subsoil to be a major source of SS.

Dry weight (DW), ignition loss (IL) and concentrations of total nitrogen (TN) and total phosphorus (TP) of the sediment surface layer (0 to 10 cm, 1 cm slices) were analyzed from 20 sites in the eastern Gulf of Finland. The distance of the sampling sites from the mouth of the River Neva explained the nutrient concentrations of the sediments well, while the effect of water depth was negligible. The increase of TN and the decrease of TP along the transect from the river mouth towards the open Gulf were caused by the diminishing share of allocht-honous material supplied from the River Neva. The mean TN concentration of the different accumulation areas was about 40 % higher in the sediment surface than in the deeper layer (9 to 10 cm). The corresponding difference for TP varied from 53 to 56 %. The results suggest considerable netflux of nutrients from sediment to water. The net sediment accumulation of nutrients were estimated as 6.0 g m−2 a−1 of N and 1.7 g m−2 a−1 of P corresponding 22 0001 a−1 of N and 6 1001 a−1 of P for the whole eastern Gulf.

Australian waterbodies have long water residence times, stratification is common, and eutrophication is driven mainly by the internal loads. The 1991 blue-green algal bloom on the Darling-Barwon River was at a time of low river flow (∼100’s ML/day) and hot/still conditions. The sustained low flow allowed significant influx of a sulfate-rich saline groundwater and this caused clay flocculation, water clarification and increased photosynthesis in the surface water, and increased sulfate reduction, pyrite formation and Fe mobilisation in anoxic bottom sediments. Since this time similar optimum bloom periods have not produced blooms, or Fe mobilisation, despite high soluble P concentrations. Algal growth during optimum bloom periods is known to be N-limited and it is possible that, in this case, essential trace elements are limiting N-fixation — clay flocculation is a very efficient process of removing trace elements from the water column. Optimum bloom periods can potentially set up a feed-back involving the bottom sediments and increasing sulfate concentration, to resupply surface water with P and trace metals. It is suggested that this operated in late 1991, but not during later optimum bloom periods as no further Fe mobilisation is recorded. The management implication is to maintain sufficient river flow to prevent any significant groundwater influx.

The spatial variability of several sedimentological. chemical and biological parameters in the uppermost layer of bed sediment (ULBS) in Lake Kinneret, was studied during the development of anoxic conditions in the hypolimnion (May, 1995). ULBS samples were taken along a transect from the littoral to the pelagic zones, during the crash of the Peridinium gatunense bloom, about 2 months after the onset of stratification and when oxygen in the hypolimnion was almost completely depleted. The 2–3mm of theULBS, collected by SCUBA diver, contained relatively fresh material (as shown by high Chl c content) but differed from that of intact Peridinium cells. In the ULBS, the C:N atomic ratio averaged 8 and was similar all over the lake bottom and the average Chlorophyll a:c ratio was 4 (reaching a value of ∼7 at 5m). These ratios in Peridinium cells were about 14.3 and 2, respectively. In addition, δ 13C in the organic matter from the ULBS was lighter than that of Peridinium. This data suggests that substantial degredation of the organic matter already occurs in the water column. Grain size distribution suggests the occurance of intense focussing processes in Lake Kinneret. Chemical and biological parameters in the ULBS exhibited clear depth dependant patterns, suggesting changes in physical and chemical processes occur. Three different zones can be distinguished in Lake Kinneret bottom sediments, a. The littoral oxic photic zone (down to 5–7m), which is influenced by intensive biodegredation and high turbulence, b. Transition zone (7–20m), with an oxic-photic gradient and developed focussing processes, c. Profundal, anoxic zone (>20m), where all fine, settled, organic rich material concentrates and undergoes anaerobic decomposition.

The Tagus estuary, located at the western Iberian coast in front of Lisbon is the largest one in Portugal, where it drains highly populated and industrialised regions. The amount of sediment transported by this river is so great that the submarine delta is one of the largest in the Iberian margin, very well defined and reaching the 70m isobath. The geochemical pattern of the muddy deposit off the Tagus river has been determined by elemental analyses of bulk surficial sediments, collected at the Portuguese margin adjacent to the Tagus estuary. Measurements were carried out by energy-dispersive X-ray fluorescence spectrometry (EDXRF). Sediments were usually composed of fine particles (silts and clays) and were high in organic matter (measured as loss on ignition). Elemental concentrations for Zn and Pb clearly indicate an estuarine contamination probably associated with discharges from urban centers or due to the influence of the industries located downstream. Apparently the depletion on the Cu contents is caused by its release (soluble complexes) into the marine environment. Variations observed in the elemental distribution of the sediments off the Tagus with distance from the estuary seem to be dependent on the grain size distribution related to the dynamics of the sediment transport.

Polluted sediments are periodically subjected to resuspension processes resulting from natural events (e.g. storms, strong waves) as well as from anthropogenically induced activities (e.g. dredging). The main part of the resuspended material is initially in an anoxic state and will be reoxidized more or less quickly in the oxic water column.In laboratory experiments reflecting, as far as possible, natural conditions (e.g. constant pH) the release of Cd, Cu and Zn during this reoxidation phase was investigated. Up to 2% of the particulate bound heavy metals were remobilized from the sediments. In addition the evolution of the concentrations of the anions PO₄, SO₄, NO₃ and NH₄ were measured to examine the influence of microbial processes on the release of trace elements. Cell counts and microbial activity of certain micro-organisms during the release processes were also investigated.The investigations illustrated that biological activity has a significant effect on release. In all sediment samples the release of cadmium was delayed in comparison with the other elements even in sediments from different river systems. The influence of different microbial processes on this divergent behavior was examined. The significance of dredging activities to the remobilization processes during reoxidation of anoxic sediments in the Elbe River is discussed.

In limnological studies the measure of sedimentation fluxes of seston is neglected, in spite of the importance it can have in determining water quality, studying biogeochemical cycles, evaluating the distribution of chemical species, etc. Often sedimentation is obtained only from mass balance models, not taking into account the fact that the uncertainty of determining inputs and outputs makes this evaluation from their difference rather unreliable; other factors of the balance, such as exchanges with the atmosphere, between water and sediments, are equally difficult to define. Though the direct measurement of sedimentation also presents some methodological and logistic difficulties, such as resuspension of material from the bottom, grazing, etc., this does not justify the very scarce attention paid to this kind of determination.This paper reports the sedimentation fluxes of 39 lakes, in different parts of the world, having different limnological and trophic characteristics (0.4 < TP < 369 μg L−1; 0.5 < chlorophyll a <50 mg m−3). The fluxes of PM, C, N and P show a log-log relationship (r ≈ 0.6, p ≤ 0.05) with the common trophic variables (Secchi disk, total phosphorus, chlorophyll and primary production), independently of the morphometric characteristic of lakes. Hence sedimentation seems not to be an intrinsic property of the environment but is related to the trophic state of the system. The results achieved tend to confirm that the nature of sedimenting seston is generally autochthonous, even though the poor correlation between PM and the same trophic variables suggests that PM is more influenced by allochthonous material.

The relationship between macrophyte cover and benthic fluxes of oxygen, nutrients and sulphide has been examined in a shallow fishpond with a nearly homogeneous meadow of Ruppia cirrhosa (Petagna) Grande (Bassin d’Arcachon, western France). In 1993 and 1994, benthic fluxes were measured in early and late summer. These periods were selected to represent the production and decay phases of Ruppia in order to determine the effect on benthic processes. Benthic fluxes of elements were measured by means of multiple dark and light benthic chambers in the presence or absence of community components. In summer 1994, at the end of the incubation period, profiles of acid volatile sulphide (AVS) and chromium reducible sulphur (CRS) were measured also in the 0–5 cm sediment horizon in cores withdrawn from the dark benthic chambers and from the sediment outside the chambers. Oxygen production and consumption were closely related to macrophyte cover, whilst the contributions of plankton and microphytobenthic communities were less significant. In the water column, dissolved inorganic nutrients were almost totally depleted, while dissolved organic nitrogen attained concentrations up to 200 μM. In late summer, Ruppia biomass underwent a significant decay due to the build up of a thick epiphyte layer, mostly around floating leaves. The epiphyte slime was rich in labile organic matter, the decomposition of which led to a significant oxygen uptake as well as to sulphide production. Therefore, we postulate that epiphyte growth can cause disturbance in the aquatic system keeping dissolved sulphide at very high levels. Biogeochemical reactions, such as precipitation of iron sulphide, can exert a control lowering the amplitude of such disturbances.

This paper presents the results of a study on nutrient exchange at the sediment-water interface which is caused by early diagenesis and resuspension of bottom sediments. The research was carried out on anoxic silty-clay sediment cores collected south of the Po river delta (Northern Adriatic Sea, Italy) in late summer.The early diagenetic processes were investigated by means of the integrated study of pore-water chemistry and solid phase composition. Exchange at the sediment-water interface was studied by comparing the fluxes measured in incubated cores with the fluxes calculated by modelling pore-water profiles. Nutrient exchange during resuspension was analysed by simulating a storm event in the laboratory.The high production of nutrients near the sediment-water interface is mainly caused by the anoxic degradation of organic matter and the successive reductions of Mn and Fe-oxyhydroxides and, to a lesser extent, of sulphate. The oxic degradation of organic matter occurs only at the sediment-water interface.In the incubation experiment the increases of phosphate, ammonia, nitrate, silica, and Fe in bottom waters were measured. The comparison between calculated and measured fluxes showed that: a) the fluxes are mainly controlled by molecular diffusion; b) phosphate and Fe sink because of the Fe-oxyhydroxide precipitation and c) nitrification process influences the ammonia and nitrate fluxes.Resuspension caused the release of: a) phosphate through surficial desorption and authigenic apatite dissolution; b) ammonia by means of the oxic degradation of organic matter; and c) dissolved silica generated by biogenic silica dissolution. Resuspension also caused a weak removal of Fe. The more oxic conditions following resuspension favoured the formation of a Fe-oxyhydroxide film at the sediment-water interface which inhibited the phosphate fluxes from sediments to the water column.

Two time-series sediment traps were deployed south-east of the Po River delta (North Adriatic Sea) in 23m of water at 12 meters and 21 meters depths from 12/20/89 to 06/26/90. Temperature, conductivity, salinity, pH, Eh, O₂, transmittance profiles, wave height, speed and direction of currents were measured. Bottom sediments and river suspended matter were periodically collected during the same period. Organic carbon, Al, Fe and Mn concentrations and partitioning of Phosphorous on the solid phase in sediment samples were determined. This made it possible to calculate vertical fluxes at two water levels, and then to quantify and separate veritical fluxes of sinking particles into fluxes of lateral transports (river input), primary production and resuspension. To do so, the approach was based on the identification, comparison and integration of two different label substances (POC and Mn). This led to the finding that sediment fluxes and biogeochemical characters of suspended matter are in good agreement with seasonal differences and short-term events such as river floods, algal blooms and marine storms. A decrease in organic carbon along the water column (5.13% mean value at 12 m, 2.84% mean value at 21 m) with minimal values in the bottom sediment (1.49% mean value) was detected. Resuspension and re-oxygenation increase the mineralization of organic matter (occurring already in water column). Differences in adsorption and desorption in the fine particulates of fundamental elements for primary production such as phosphorous were detected at different levels in the water column and in the bottom sediments.

The fallout radionuclides 210Pb and 137Cs are widely used to date environmental records contained in lake sediments. Since the radionuclide records are themselves the outcome of the transformation of atmospheric fallout by mediating transport processes from the catchment, through the water column and post-depositional migration via pore waters, reliable models of these processes are crucial to accurate dating. The large quantities of data on 210Pb and 137Cs in lake sediments accumulated through their widespread dating applications may be used to study transport models. Their advantages as tracers of transport processes include widespread dispersal through the environment, relatively simple and well known input functions, and ease of measurement.One of the principle factors controlling the transport of any species through the water column is its distribution between aqueous and particulate phases. The relatively solubility of 137Cs in the water column is demonstrated by the reduced 137Cs/210Pb inventory ratios in sediments compared to values expected from direct fallout. Using sediment records from a wide range of Cumbrian lakes, calculations based on simple models indicate that the particulate fraction of weapons fallout 137Cs in the water column ranged from 5–22%, and was proportional to the square root of the sedimentation rate (determined by 210Pb). The K_D value for weapons 137Cs in the water column is estimated to be in the range 1–2×105 L kg−1. This is comparable with K _D values for Chernobyl 137Cs in these lakes (Smith et al. in press) obtained from direct measurements in the water column.

Resuspension in the main basin of Lake Gościąż results approximately a doubling of the total sedimentation rate in the deepest region. The increase pollen flux in bottom traps during overturn was more four times higher than in the upper traps. The most intensive pollen sedimentation was recorded before and just after freeze-up. This could only have been the result of previous sediment resuspension. Mechanisms of sedimentation in the studied lake showed that resuspension and resedimentation can change “the environmental record” of events even in a lake with laminated sediments. Higher sedimentation of pollen during autumn circulation confirmed that in the sediment layers (varves or laminae) particles from previous seasons also could have been found. In the bay pollen sedimentation was lower and depended on plant flowering, there was no resuspension.

A fine resolution study on fossil ostracods in late glacial sediments from Lago di Albano was performed within the framework of the European project PALICLAS. Four cores from two coring sites were analysed. At Site 1, located at 70 m depth, cores PALB94 1E and PALB94 1C were collected, while cores PALB94 6A and PALB94 6B were collected at Site 6 at 30 m depth. The autoecology of the ostracod species, cluster analysis, PCA and CA ordination numerical analyses were performed in order to identify changes in environmental conditions within the sedimentary record. Three assemblages were recognized: (a) dominated by C. neglecta representing sublittoral envirpnments, (b) dominated by C. neglecta accompanied by Potamocypris spp. related to spring influx and (c) dominated by Cyclocypris sp. associated with littoral and relatively warmer conditions.A reconstruction of lake water level, oxygen concentration near the bottom and lake productivity , based on ostracod autoecology and supported by fossil pigment data, was accomplished. Sharp fluctuations in water level and productivity seem to indicate climatic oscillations between ca 30 to ca 17 kyr BP.

This paper reports the results of biological analyses (pigments, diatoms, chrysophyte cysts, cladocerans, chironomids and ostracods) of a ca. 14 m-long sediment core recovered from Lake Albano (Central Italy) in the course of the EU-funded project PALICLAS (PALaeoenvironmental analysis of Italian Crater Lake and Adriatic Sediments).A reconstruction of the environmental evolution and ecosystem response of Lake Albano during the last ca. 30 kyr was possible. Additional information on lake level oscillation is obtained from benthic and planktonic palaeocommunities. Several oscillations in the productivity and the level of the lake were detected in the oldest sediment layers (from ca. 30 kyr BP to ca. 17 kyr BP), followed by a long (ca. 5 kyr BP) period of low productivity in which cold, holomictic conditions prevailed. A period of high biological activity and, probably, meromictic conditions during the early-mid Holocene was detected. A clear impact of human activities in the catchment was found at ca. 4 kyr BP in the form of increased erosion, associated with a decline in the abundance of biological remains. Further signs of human impact on the lake ecosystem are recorded during the Roman period. Although large-scale environmental changes (e.g. regional climate changes) caused many of the observed biological changes, human activities were important during the mid-late Holocene.

Copper, lead, cadmium, zinc, chromium, manganese, iron and nickel were determined in near-shore sediments in the harbour of Trieste (Northern Adriatic), in an area highly exposed to urban and industrial wastes, where severe alteration of benthic population was observed. A typical bivalve of this area, Corbula gibba, was used as bioindicator of sea-bottom pollution. Multivariate statistical analysis of the chemical data interpret concentrations and distributions of heavy metals in these sediments, attesting anthropogenic source for Cu, Pb, Cd and Zn. Using labile fractions of heavy metals in sediments as ‘predictors’, and length or biomass of the bivalve as dependent variables, we obtained, by a multiple regression procedure, a predictive model showing the influence of metals on this benthic organism of this polluted area.

Zebra mussels (Dreissena) have expanded rapidly throughout most of the Laurentian Great Lakes since their inadvertent release in 1986. These exotic molluscs now occur in great numbers on the bottom of western Lake Erie where they are found increasingly in deeper areas of the basin (average depth: 10 m), on soft, muddy substrates. This study is aimed at quantifying the density and the distribution patterns of mussel colonization in the basin as a first step in investigating the effect on sediment properties of such an abrupt change in benthic community structure. Underwater video imagery and diver-collected samples taken from representative offshore areas (seven sites) in western Lake Erie showed colonization levels of up to 20,000 live mussels per m2 in soft sediments (adults with shells >10 mm comprised 47 %). Digital side-scan sonar records confirmed that colonization patterns were not random, but showed distinctive spatial signatures ranging from 30-m-long parallel stripes, to large ovate masses. Broad irregular mats were found in association with hard bottoms (bedrock, boulders, or wrecks and large debris). Mussel densities were averaged from the sites, assuming consistent relationships with substrate type and were combined with digitized percentage of areal coverage of major bottom types in western Lake Erie. This resulted in the first population figure of 1013 in the basin. This figure includes molluscs of all sizes > 0.84 mm.

A large scale study on the western basin of Lake Como (N. Italy) was started in 1995 to examine the effects of the zebra mussel colonization which began in early’ 90. Our results have been related to’ 91–92 data (pre-Dreissena period), before the maximum colonization of zebra mussel. In spring and summer of the post-Dreissena period total phosphorus, P-PO₄,, nitrate and chlorophyll values decreased, while ammonium and transparency increased at every sampling station.Zebra mussel does not modify the trophic state of this sub-basin but it plays an important role in nutrient cycling. The entire population can filter epilimnetic waters 2.1 times per year and can produce 2.9 × 104 t/y of pseudofaeces which are transferred to sediments.

Benthic observations were carried out at 22 stations in the Western Isles region of the Bay of Fundy on the east coast of Canada to evaluate impacts at salmon aquaculture sites. Eleven sites were located under salmon net-pens and 11 sites (reference or control locations) were at distances > 50 m from net-pens. Total S− and redox potential (Eh) in surface sediment and benthic O₂ uptake and CO₂ release were sensitive indicators of benthic organic enrichment. High variability between replicate measurements of sediment gas exchange could reflect spatial patchiness in sedimentation of fecal waste and food pellets under fish pens. Biomass of deposit feeders was significantly increased at cage sites but total macrofauna biomass was similar at cage and reference locations. Surface sediment water content, modal grain size, pore water salinity and sulfate, and total biomass of macrofauna were the least sensitive indicators of enrichment.

In a partly urbanized catchment to the south of Trier, Germany, short term variations in river sediment compounds as well as the bioaccumulation of pollutants on surface associated microbial coatings (biofilms) were investigated weekly during a period of six months. Concentrations of selected heavy metals (Cu, Zn, Pb), polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB) and for microbial characterisation protein, carbohydrate and uronic acid were analyzed. Sorption processes on biofilms were determined by temporal variations in pollutants and microbial parameters and through the comparison of sorbed substances in biofilms and sediments. The results show, that sorption events on biofilms play an important and dynamic role in spring and summer for transport and accumulation of the investigated pollutants in the aquatic environment. The amount of pollutants sorbed on sediment particles is not only dependent on the particulate bound or solved pollutants in the river water, but is strongly controlled by the changing conditions of the biofilms.

An experimental microcosm has been designed for simulating and studying impacts of a wastewater treatment plant (WTP) discharge on a freshwater/sediment ecosystem. The study was focused on the changes in biodiversity of benthic populations, especially bacteria and oligochaetes. Effluents were discharged in the Saône river, near Lyon (France) from a small treatment plant which treated domestic raw water by an activated sludge process. Freshwater and sediments were sampled in the Saône river upstream of the discharge point and placed in microcosms. Following the WTP discharge, physicochemical parameters of the overlying water column and sediments exhibited only a slight change, as compared to a reference.Characterization of the sediment bacterial populations was conducted with the Biolog and API systems. Strain identification and interpretation of data was difficult using thesetwo systems. Bacterial taxa in the sediments increased slightly below the WTP discharge. Gram negative strains dominated in the effluents, but G+ and G− bacteria were balanced in the sediments. Pseudomonas sp. and Bacillus sp., were the dominant strains. Invertebrate populations indicated an effect of the WTP discharge, with increasing of pollution resistant strains (Tubificidae) and disappearance of pollution intolerant strains such as Limnodrilus udekemianus and Quistadrilus multicoetosus.Taken as a whole, biological parameters indicated an environmental changes despite only slight changes in the physicochemistry of water. This experimental microcosm has proven to be a useful tool for studying impact of wastewater discharge on benthic populations.

Arylsulfatase and APase activities were monitored in the upper sediment layer, of Lake Kinneret, Israel, a warm, freshwater, monomictic lake characterized by a heavy spring bloom of the dinoflagellate Peridinium gatunense. Activity of both enzymes varied with depth and season. Highest activity was measured in July and high activities were monitored during the stratified period. Low values were observed in winter, when oxic conditions prevail in the water column and no organic sedimentation occurred. The values for APase ranged from 14–438 nmol PNP g−1h−1 and for arylsulfatase from 103 to 843 nmol PNP g−1h−. Highest APase activity was recorded at 29°C and most of it took place on mud particles and not in the interstitial waters. There were differences in enzyme activity at different stations in the lake, corresponding to differences in nutrient pore water concentrations. Enzymatic activity in Lake Kinneret sediments was related to lake trophic status, water levels, and climate conditions, all of which have an impact on the amount of organic matter reaching the sediments.

The aim of this study is to describe general features of sediment, primary producers and both benthic and planktonic consumers in two shallow saline lakes (Salada de La Muerte and Salada de Piñol) in order to detect main factors influencing food web structure.The lakes are located in Los Monegros district, in the central area of the Ebro River catchment, NE Spain. Both lakes are temporary with salinity well above 30 g L−1. Although they are situated close to each other (distance: 300 m), their communities of primary producers differ dramatically. One lake (La Muerte) is dominated by microbial mats and seems to function through the benthic pathway. The other (Piñol) has some macrophytes but phytoplankton is the main source of primary production. Two cycles (1994/95 and 1995/1996), quite different in their hydrological characteristics, have been studied.PCA demonstrated the major influence of hydrologic features (e.g. water level) over biotic and abiotic parameters. The presence of microbial mats in La Muerte played a key role in stabilizing the sediments. A comparison of food web structure and dynamics in both lakes has been performed and the influence of sediment features is discussed.

The influence of particle size distribution and organic matter on the toxicity of copper was investigated using the nematode Caenorhabditis elegans as testorganism. Sediments taken at various depths from three lakes of different trophic status and artificial sediments were spiked with sublethal concentrations of CuSO₄. After an exposure of 72 h to spiked sediment or liquid medium, body length of the nematodes was determined. Both artificial and natural sediments reduced the effect of copper, with natural sediments being more effective. In natural sediments worms grew normally at concentrations of copper up to 63.5 mg/L, whereas in artificial sediments body length was reduced at concentrations of 11.3 mg Cu/L or higher. Body length was positively correlated with content of fine particles and organic matter, indicating that particle size distribution and organic matter are determinant factors for the ecotoxicology of sediments.

Seven sediments cores collected in the Western Ross Sea (Antarctica) were investigated for heavy metals (Cr, Ni, Pb, Cd, Co, Zn Cu and Fe, Mn), several major elements (Ca, Ba and Al), total and organic carbon, and isotopic composition in order to investigate vertical variability. Samples were analyzed by ET-AAS and ICP-AES for metals and by CF-IRMS for carbon content. Generally, concentrations at the background levels were higher with respect to recent ones excluding in this area any kind of anthropogenic contamination. Cluster and Factor Analysis were applied. Best correlations were obtained for the elements showing geochemical affinities.

The distribution of dissolved and particulate 234Th in the upper 200 m of the water column was obtained for three stations in the Ross Sea off Victoria Land and Terra Nova Bay. At site 24a, close to the retreating ice margin, all the sampled depths showed deficiencies in 234Th relative to the equilibrium with 238U. These are related to uptake of 234Th onto sinking particles. Residence times of 234Th in solution and of particulate 234Th were 130–247 days and 8.1–6.6 days, respectively. A high particle flux (1.23–5.03 g m−2 d−1) was calculated at this station. At the other two sites (11c and 15c), 234Th depth profiles are irregular, probably due to the release of dissolved 234Th by decomposing particles at certain depths, or to the contribution from lateral advection. Bulk mass fluxes measured by floating traps at stations 11c and 15c are very low (66–138 mg m−2 d−1). Also fluxes of organic carbon and nitrogen, and biogenic silica are reported for these two sites. The calculated fluxes are discussed with respect to methodologies and to the dynamics of the ice margin retreat.

PCBs and organochlorine pesticides were determined in water, sediment and Zooplankton of two Himalayan lakes, located at different altitudes and connected to each other in such a way that Superior Lake acts as a sedimentation basin for Inferior Lake. Surficial sediments of both lakes show PCB contamination comparable to lakes of industrialised areas. Biota appear to be the main machanism responsible for micropollutant burial in the sediments of Inferior Lake, whereas inorganic particles are more relevant in Superior Lake. Physical and chemical properties of individual chemicals, particularly Henry’s law constant and K_ow values, seem to regulate distribution in different environmental compartments.