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排序方式: 共有341条查询结果,搜索用时 15 毫秒
1.
Abbondanzi F Campisi T Focanti M Guerra R Iacondini A 《Marine environmental research》2005,59(5):419-434
A study was conducted to determine polycyclic aromatic hydrocarbons (PAHs) distribution and microbial population changes in brackish sediments from an Italian lagoon included in the Ramsar List of Wetlands of International Importance. The presence and level of PAH-degrading bacteria were estimated by the most probable number (MPN) enumeration technique, whereas degradation capability towards target compounds was checked against loss of spiked PAHs (Phenanthrene, Anthracene and Fluoranthene) in MPN tubes after incubation in optimal conditions. Chemical analyses and microbiological counts suggested a potential for PAHs biodegradation by natural occurring populations of sediment microorganisms, thus indicating an "optimal range" in sediment PAHs concentrations, outside of which the natural selection of the indigenous microflora did not occur. The MPN procedure here described, provided an effective and reliable way to simultaneously determine microbial population densities and subsequent confirmation of the biodegradation capability of sediment indigenous microflora when exposed to laboratory and environmental concentrations of PAHs. 相似文献
2.
The microhabitat preferences and depth distribution of blenniid species (Blenniidae) in the Gulf of Trieste (North Adriatic Sea) were surveyed using the all‐occurrence sampling method, a non‐destructive visual census method, aided by SCUBA diving. Fourteen species were identified during the entire survey. Four species showed to be indiscriminate in their microhabitat choice while 10 species were classified as infrequent in the surveyed coastal area. A statistically significant correlation was found between the blenniid assemblage and nine microhabitat variables. The blenniid assemblage was divided in two main groups, by the use of canonical correspondence analysis, electivity index, and the depth distribution analysis. The first group comprises species that dwell in surface waters and show a high positive correlation with boulders, the presence of Mytilus galloprovincialis, cirripeds and empty holes bored by Lithophaga lithophaga. The second group includes species that mostly inhabit deeper waters and show a high positive correlation with rocks covered by precoralligenous bioformations. 相似文献
3.
Martina A. Doblin Stephen B. Baines Lynda S. Cutter Gregory A. Cutter 《Estuarine, Coastal and Shelf Science》2006,67(4):681-694
As part of a study of estuarine selenium cycling, we measured the concentration, chemical form (speciation), and distribution of particulate selenium under various river flow conditions in the North San Francisco Bay (from the Golden Gate to the Sacramento and San Joaquin Rivers). We also conducted laboratory studies on the accumulation of selenium by phytoplankton, the critical first step in the transformation of dissolved to particulate selenium. Total particulate selenium concentration in the North SF Bay was relatively constant between high and low flow periods, ranging spatially from 0.05 to 0.35 nmol l−1 and comprising between 5 and 12% of the total water column selenium inventory. Mean concentrations were generally highest in the Carquinez Strait–Suisun Bay region (salinity 0–17) and lowest in Central Bay. However, selenium content of suspended particles varied with river flow, with higher content during low flow (9.76 ± 4.17 nmol g−1; mean ± sd; n = 67) compared to high flow (7.10 ± 4.24 nmol g−1; n = 39). Speciation analyses showed that most particulate selenium is organic selenide (45 ± 27%), with a smaller proportion (typically <30%) of adsorbed selenite + selenate and a varying proportion (35 ± 28%) of elemental selenium. Based on the amount of elemental selenium in the seston (total suspended material), we calculate that resuspension of estuarine sediments could contribute 29–100% of particulate selenium in the water column. While selenium content of SF Bay seston (>0.4 μm) is relatively unenriched compared to phytoplankton (13.6–155 nmol g−1 dry weight) on a mass basis, when normalized to carbon or nitrogen, seston contains a similar selenium concentration to SF Bay sediments or phytoplankton cultures. SF Bay seston is thus comprised of selenium-rich phytoplankton and phyto-detritus, but also inorganic clay mineral particles that effectively “dilute” total particulate selenium. Selenium concentrations in algal cultures (11 species) exposed to 90 nmol l−1 selenite show relatively large differences in selenium accumulation, with the diatoms, chlorophytes and cryptophytes generally having lower selenium cell content (3.8 ± 2.7 × 10−9 nmol selenium cell−1) compared to the dinoflagellates (193 ± 73 × 10−9 nmol selenium cell−1). Because phytoplankton are such a rich (but variable) source of selenium, their dynamics could have a profound effect on the particulate selenium inventory in the North SF Bay. 相似文献
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6.
Modelling nitrogen and oxygen isotope fractionation during denitrification in a lacustrine redox-transition zone 总被引:1,自引:0,他引:1
Moritz F. Lehmann Peter Reichert Alberto Barbieri 《Geochimica et cosmochimica acta》2003,67(14):2529-2542
The stable isotope composition (δ15N and δ18O) of nitrate was measured during Summer 1999 in the anaerobic hypolimnion of eutrophic Lake Lugano (Switzerland). Denitrification was demonstrated by a progressive nitrate depletion coupled to increasing δ15N and δ18O values for residual nitrate. Maximum δ15N and δ18O values amounted to 27.2 and 15.7‰, respectively.15N and 18O enrichment factors for denitrification (ε) were estimated using a closed-system model and a dynamic diffusion-reaction model. Using the Rayleigh equation (closed-system approach), we obtained ε values of −11.2 and −6.6‰ for nitrogen and oxygen, respectively. The average ε values derived using the diffusion-reaction model were determined to be −20.7 ± 3.8 for nitrogen and −11.0 ± 1.7 for oxygen. Both N and O isotope fractionation appeared to be lower when denitrification rates where high, possibly in association with high organic carbon availability. In addition, variations in the isotope effects may be attributed to the variable importance of sedimentary denitrification having only a small isotope effect on the water column. The combined measurement of N and O isotope ratios in nitrate revealed that coupled nitrification-denitrification in the open-water was of minor importance. This is the first study of nitrogen and oxygen isotope effects associated with microbial denitrification in a natural lake. Moreover, this study confirms the high potential of δ18O of nitrate as a valuable biogeochemical tracer in aquatic systems, complementing nitrate δ15N. 相似文献
7.
M. Krämer Ri. Müller H. Bovensmann J. Burrows J. Brinkmann E. P. Röth J.-U. Grooß Ro. Müller Th. Woyke R. Ruhnke G. Günther J. Hendricks E. Lippert K. S. Carslaw Th. Peter A. Zieger Ch. Brühl B. Steil R. Lehmann D. S. McKenna 《Journal of Atmospheric Chemistry》2003,45(1):51-77
Several stratospheric chemistry modules from box, 2-D or 3-D models, have been intercompared. The intercomparison was focused on the ozone loss and associated reactive species under the conditions found in the cold, wintertime Arctic and Antarctic vortices. Comparisons of both gas phase and heterogeneous chemistry modules show excellent agreement between the models under constrained conditions for photolysis and the microphysics of polar stratospheric clouds. While the mean integral ozone loss ranges from 4–80% for different 30–50 days long air parcel trajectories, the mean scatter of model results around these values is only about ±1.5%. In a case study, where the models employed their standard photolysis and microphysical schemes, the variation around the mean percentage ozone loss increases to about ±7%. This increased scatter of model results is mainly due to the different treatment of the PSC microphysics and heterogeneous chemistry in the models, whereby the most unrealistic assumptions about PSC processes consequently lead to the least representative ozone chemistry. Furthermore, for this case study the model results for the ozone mixing ratios at different altitudes were compared with a measured ozone profile to investigate the extent to which models reproduce the stratospheric ozone losses. It was found that mainly in the height range of strong ozone depletion all models underestimate the ozone loss by about a factor of two. This finding corroborates earlier studies and implies a general deficiency in our understanding of the stratospheric ozone loss chemistry rather than a specific problem related to a particular model simulation. 相似文献
8.
R. Lehmann 《Journal of Geodesy》1997,71(9):533-540
Geodetic surface integrals play an important role in the numerical solution of geodetic boundary-value problems. In many
cases they can be evaluated using fast methods in the frequency domain (FFT). However, this is not possible in general, because
the domain of integration may be non-trivial (as is the surface of the Earth), the kernel function may not be of convolution
type, or the data distribution may be heterogeneous. Therefore, fast evaluation strategies are also required in the space
domain. They are more difficult to design because only one property is left where a more or less fast evaluation strategy
can be built upon: the potential type of the kernel function. Consequently, the idea is not to replace well-established frequency
domain techniques, but to supplement them. Our approach to this problem goes in two directions: (1) we use advanced cubature
methods where the integration nodes automatically densify in the vicinity of the evaluation points; (2) we use powerful computer
hardware, namely MIMD computers with distributed memory. This enables us to evaluate geodetic surface integrals of any practical
complexity in reasonable time and accuracy. This is shown in a numerical example.
Received: 7 May 1996 / Accepted:17 March 1997 相似文献
9.
Rayleigh hysteresis, as defined by the well-known Rayleigh relations, has been observed not only when magnetization of pyrrhotite-bearing KTB-samples is measured in parallel to a weak dc magnetic field, but also in experiments where field and measuring directions have been adjusted strictly perpendicularly to each other. Nine-tupels of independent Rayleigh hysteresis loops could thus be compiled. Their characteristic coefficients X ijk of initial susceptibility together with the Rayleigh loss coefficients αjk have been proved to determine completely the samples' weak-field magnetic anisotropy. Interpreting the coefficient matrices ( X ijk ) and (αjk ) as the tensor of initial susceptibility and the Rayleigh tensor, respectively, generalization of the isotropic Rayleigh relations in terms of corresponding tensor relationships has been suggested for the anisotropic case. Application to the KTB samples showed 3-D Rayleigh hysteresis measurements to be an excellent tool for rock magnetic analysis in terms of ore content and crystalline texture. In particular, a magnetocrystalline double texture of the basal planes of pyrrhotite precipitates and their [1120] directions of easy magnetization have been clearly detected. Surprisingly, the welt-known theorem α= const. X 2 I , formulated by Néel (1942) for the isotropic case, has been found to hold true even in tensor generalization (αjk ) = const ( X 2 jk ). To reach sufficient resolution for the measurements performed, a sensitive vibrating coil magnetometer (VCM) has been developed. 相似文献
10.
S. Klimov S. Romanov E. Amata J. Blecki J. Büchner J. Juchniewicz J. Rustenbach P. Triska L. J. C. Woolliscroft S. Savin Yu. Afanas’yev U. de Angelis U. Auster G. Bellucci A. Best F. Farnik V. Formisano P. Gough R. Grard V. Grushin G. Haerendel V. Ivchenko V. Korepanov H. Lehmann B. Nikutowski M. Nozdrachev S. Orsini M. Parrot A. Petrukovich J. L. Rauch K. Sauer A. Skalsky J. Slominski J. G. Trotignon J. Vojta R. Wronowski 《Annales Geophysicae》1997,15(5):514-527
The plasma-wave experiment ASPI (analysis of spectra of plasma waves and instabilities) on board the INTERBALL spacecraft is a combined wave diagnostics experiment. It performs measurements of the DC and AC magnetic field vector by flux-gate and searchcoil sensors, the DC and AC electric field vector by Langmuir double probes and the plasma current by Langmuir split probe. Preliminary data analysis shows the low noise levels of the sensors and the compatibility of new data with the results of previous missions. During several months of in-orbit operation a rich collection of data was acquired, examples of which at the magnetopause and plasma sheet are presented in second part of the paper. 相似文献