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At least 289 records · Page 16Linked to original sources
PubMed2003

A model of microbial activity in lake sediments in response to periodic water-column mixing.

Under stagnant conditions, the mass transport of a soluble substrate from a lake's water column to the sediment/water interface is limited by molecular diffusion. Stagnant conditions coupled with a continuing sediment biological demand create a substrate depletion zone above the sediment/water interface. The frequency at which the substrate depletion zone is destroyed by internal seiches and other intermittent flow phenomena influences the time-averaged substrate concentration at the sediment/water interface. A more frequent mixing results in a greater time-averaged interface concentration and consequently affects the amount of microbial biomass that can be supported in the lake sediments and the flux of the substrate into the sediment. A one-dimensional, two-substrate model is used to examine the impact of mixing frequency on the activity of sulfate-reducing bacteria (SRB) in lake sediments. In the model, sulfate is supplied from the water column, while acetate is generated within the sediments. Mass transport to and within the sediments is by molecular diffusion except for instantaneous mixing events. Between mixing events, sulfate concentration gradients form above the sediment/water interface in the diffusive boundary layer. Sulfate depletion zones can be centimeters thick. When typical biological rate and diffusion coefficients for sulfate and acetate are used as inputs, the model indicates that a more frequent water-column mixing results in greater SRB concentrations. For an assumed bulk water-column sulfate concentration of 4.8 mg x l(-1), the sediment SRB concentrations for the modeled hourly, 6-hourly, daily, and weekly mixing frequencies were 175, 136, 91, and 30 mg x m(-2), respectively. The model also predicts higher time-averaged sulfate flux rates at more frequent water-column mixing. The time-averaged sulfate flux rates for the hourly, 6-hourly, daily, and weekly mixing frequencies were 1.26, 1.13, 0.78, and 0.30 mg x m(-2)h(-1), respectively. Thus, mixing frequency can significantly impact microbial activity in lake sediments.

Diffusion↗
PubMed2003

A review on the application of microbial toxicity tests for deriving sediment quality guidelines.

The results of microbial toxicity tests are needed for the risk assessment of polluted sediments. In comparison with animals the anaerobic microorganisms are more tolerant to natural sediment conditions whereas they are more sensitive for a number of specific pollutants. Microbial toxicity tests from a literature search were classified in seven categories. Category A, B and C use polluted sediments and are applied for sediment monitoring. In category D, a pure chemical is added and the organisms and the test conditions were derived from sediment. Therefore this category can be used for setting sediment quality guidelines which protect sediment functions for the toxic effects of chemicals. In category E, organisms from a polluted site are separated from the sediment and are tested with pure chemicals. Organisms from a more polluted site can be more tolerant to a local pollutant. This is called pollution-induced community tolerance and can be used as evidence for the occurrence of toxic effects in a specific sediment. In category F pure chemicals are tested with a pure culture of microorganisms under sediment conditions. The results of category F tests can be combined with single species tests with animals and plants to obtain sediment quality guidelines sufficient for species protection. This can be compared with the sediment quality guidelines which protect sediment functions. When one of these quality guidelines is exceeded for a compound at a specific location a category E test can be used to determine whether the compound shows toxic effects in that sediment.

Bacteria, Anaerobic↗
PubMed2000

Mercury in dated Greenland marine sediments.

Twenty marine sediment cores from Greenland were analyzed for mercury, and dated by the lead-210 method. In general the cores exhibit a mercury profile with higher mercury concentrations in the upper centimetres of the core. The cores were studied by linear regression of in Hg vs. age of the sediment for the youngest 100 years. As a rule the mercury decreased with depth in the sediment with various degrees of significance. The increase of the mercury flux during the last 100 years is roughly a doubling. The increase may be of anthropogenic origin as it is restricted to the last 100 years. In four cores the concentration of manganese was found also to increase in the top layers indicating diagenesis. In the other cases the higher concentrations were not accompanied by higher manganese concentrations. The mercury flux to the sediment surface was generally proportional to the Pb-210 flux indicating that the mercury mainly originates from atmospheric washout. But the large variability indicates that other processes also influence the mercury flux to Arctic marine sediments.

Environmental Monitoring↗
PubMed2003

Diversion of electron flow from methanogenesis to crystalline Fe(III) oxide reduction in carbon-limited cultures of wetland sediment microorganisms.

Electron flow in acetate-limited cultures of wetland sediment microorganisms was diverted from methane production to Fe(III) reduction in the presence of crystalline Fe(III) oxides at surface area loadings equivalent to that of amorphous Fe(III) oxide. The results indicate that inferences regarding the ability of microbial Fe(III) oxide reduction to compete with other terminal electron-accepting processes in anoxic soils and sediments should be based on estimates of bulk microbially available surface site abundance rather than assumed thermodynamic properties of the dominant oxide phase(s) in the soil or sediment.

Electron Transport↗
PubMed2002

Metal content and particle size distribution of street sediments and street sweeping waste.

During recent years the interest in street sweeping as a pollutant control measure has increased. For planning of street sweeping programs and disposal of street sweeping waste there is a need to investigate the composition of street sweeping waste and the effect of street sweeping on the remaining sediments on the street. Particle size distribution and heavy metal concentration has been measured for street sweeping waste, street sediments before sweeping and remaining sediments after sweeping. The results show that the concentrations of heavy metals are a function of particle diameter and proportional to the inverse of the particle diameter, i.e. the highest concentrations are found in finest fractions. Regression equations for metal concentrations as a function of particle diameter are presented. According to Swedish guidelines for environmental quality of contaminated sites, heavy metal concentration in the sweeping waste can not be considered as a serious problem. The largest amount of metals and sediment is found in the sandy fractions (0.125-0.5 mm). The street sweeper is more effective in removing coarse sediments than fine. This means that the street sediments after sweeping are finer than the sediments before.

Cities↗
PubMed1986

Long-chain carboxylic acids in pyrolysates of Green River kerogen.

Long-chain fatty acids (C10-C32), as well as C14-C21 isoprenoid acids (except for C18), have been identified in anhydrous and hydrous pyrolyses products of Green River kerogen (200-400 degrees C, 2-1000 hr). These kerogen-released fatty acids are characterized by a strong even/odd predominance (CPI: 4.8-10.2) with a maximum at C16 followed by lesser amounts of C18 and C22 acids. This distribution is different from that of unbound and bound geolipids extracted from Green River shale. The unbound fatty acids show a weak even/odd predominance (CPI: 1.64) with a maximum at C14, and bound fatty acids display an even/odd predominance (CPI: 2.8) with maxima at C18 and C30. These results suggest that fatty acids were incorporated into kerogen during sedimentation and early diagenesis and were protected from microbial and chemical changes over geological periods of time. Total quantities of fatty acids produced during heating of the kerogen ranged from 0.71 to 3.2 mg/g kerogen. Highest concentrations were obtained when kerogen was heated with water for 100 hr at 300 degrees C. Generally, their amounts did not decrease under hydrous conditions with increase in temperature or heating time, suggesting that significant decarboxylation did not occur under the pyrolysis conditions used, although hydrocarbons were extensively generated.

Carbon↗
PubMed2004

Determination of metal components in marine sediments using energy-dispersive X-ray fluorescence (ED-XRF) spectrometry.

A rapid energy-dispersive X-ray fluorescence (ED-XRF) spectrometric method for the analysis of metal components of marine sediments has been presented. Calibrations were made using synthetic matrix. The agreement of the results for sediment standard reference materials with reference values is satisfactory. Major advantages of the non-destructive ED-XRF technique over conventional chemical digestion methods include the applicability to analyzing the major oxide components, as well as to trace metals, and the avoidance of hazardous chemicals. The method has been applied to the routine analysis of Hong Kong marine sediment.

Animals↗
PubMed2001

Speciation of organotin compounds in marine sediments by capillary column gas chromatography-atomic absorption spectrometry coupled with hydride generation.

A method for speciation of organotin compounds in marine sediments by solvent extraction combined with hydride generation gas chromatography-atomic absorption spectrometry has been developed. Sediment samples spiked with tributyltin and triphenyltin chlorides were homogenized in hydrochloric acid. The chlorides were extracted twice into toluene. Recoveries of the organotin compounds from the spiked sediment samples were improved by the addition of 8-quinolinol. Tributyltin and triphenyltin chlorides form ion-associates with 8-quinolinol in aqueous hydrochloric acid. The method was optimized with respect to derivatization reactions and extraction conditions. Interferences from Sn(II/IV) and additional 13 ions were investigated. Recoveries of 84-100% for tributyltin and 86-100% for triphenyltin were achieved using this method. The detection limits obtained for tributyltin and triphenyltin chlorides were 95 and 145 pg, respectively, corresponding to a relative detection limit of 95 and 145 ng kg(-1) in the sediment.

Chromatography, Gas↗
PubMed2004

The effect of urbanisation on Laajalahti Bay, Helsinki City, as reflected by sediment geochemistry.

Sediment geochemical techniques were used to assess how the sediment record reflects the loading and land use history of Laajalahti Bay, a semi-enclosed bay adjacent to the city of Helsinki, Finland. A dated sediment core was analysed for concentrations of Al, Ca, K, Mg, Na, Fe, Mn, Cu and Zn and total concentrations of C, N and P together with P fractions and biogenic silica (BSi) at intervals of 1 cm to a depth of 88 cm. Furthermore, five phases of development were identified in the sediment record by using principal components analysis and cluster analysis. The lithology of the core indicates sharp changes in the sedimentation environment in the earlier history of the embayment that are most likely due to isostatic uplift and changes in salinity. The most prominent recent trends were the increasing sedimentation rates and nutrient concentrations towards present. In addition, significant increases were observed in the concentrations of anthropogenic indicators Cu and Zn. In spite of decreased nutrient loading to Laajalahti Bay no significant signs of improvement were observed in the sediment record yet.

Cluster Analysis↗
PubMed2005

Time dependent calibration of a sediment extraction scheme.

Sediment extraction methods to quantify metal concentration in aquatic sediments usually present limitations in accuracy and reproducibility because metal concentration in the supernatant is controlled to a large extent by the physico-chemical properties of the sediment that result in a complex interplay between the solid and the solution phase. It is suggested here that standardization of sediment extraction methods using pure mineral phases or reference material is futile and instead the extraction processes should be calibrated using site-specific sediments before their application. For calibration, time dependent release of metals should be observed for each leachate to ascertain the appropriate time for a given extraction step. Although such an approach is tedious and time consuming, using iron extraction as an example, it is shown here that apart from quantitative data such an approach provides additional information on factors that play an intricate role in metal dynamics in the environment. Single step ascorbate, HCl, oxalate and dithionite extractions were used for targeting specific iron phases from saltmarsh sediments and their response was observed over time in order to calibrate the extraction times for each extractant later to be used in a sequential extraction. For surficial sediments, an extraction time of 24 h, 1 h, 2 h and 3 h was ascertained for ascorbate, HCl, oxalate and dithionite extractions, respectively. Fluctuations in iron concentration in the supernatant over time were ubiquitous. The adsorption-desorption behavior is possibly controlled by the sediment organic matter, formation or consumption of active exchange sites during extraction and the crystallinity of iron mineral phase present in the sediments.

Calibration↗