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51.
Laboratory experiments were used to study the sensitivity of both male and female Daphnia magna to a toxic cyanobacterium, Microcystis aeruginosa. Male and female D. magna were fed with M. aeruginosa and a green alga (Scenedesmus obliquus) in different mixtures that included 0%, 25%, 50%, 75% and 100% Microcystis. Growth of both males and females declined with increasing proportion of Microcystis in the diet. Males were slightly less sensitive to Microcystis than females with EC50 for growth inhibition at 72% and 63% Microcystis in the diet, respectively. On monospecific Microcystis diet, mortality rates for males (0.16 d–1) and females (0.17 d–1) were similar. The reproductive rate of females was the most sensitive endpoint with an EC50 of 42% Microcystis in the diet, whereas population growth rates were the least sensitive (EC50 of 74% Microcystis). Microcystis in the diet severely depressed growth and reproduction in Daphnia and poses a severe threat to its survival in which only a switch in reproductive strategy might provide Daphnia a refuge to a Microcystis environment that gradually becomes uninhabitable. 相似文献
52.
DEVEX-disdrometer evaluation experiment: Basic results and implications for hydrologic studies 总被引:1,自引:0,他引:1
Witold F. Krajewski Anton Kruger Clelia Caracciolo Peter Golé Laurent Barthes Jean-Dominique Creutin Jean-Yves Delahaye Efthymios I. Nikolopoulos Fred Ogden Jean-Paul Vinson 《Advances in water resources》2006
Increasing our understanding of the small scale variability of drop size distributions (DSD), and therefore of several bulk characteristics of rainfall processes, has major implications for our interpretation of the remote sensing based estimates of precipitation and its uncertainty. During the spring and summer of 2002 the authors conducted the DEVEX experiment (disdrometer evaluation experiment) to compare measurements of natural rain made with three different types of disdrometers collocated at the Iowa City Municipal Airport in Iowa City, Iowa in the Midwestern United States. This paper focuses on the evaluation of the instruments rather than analysis of the hydrometeorological aspects of the observed events. The comparison demonstrates discrepancies between instruments. The authors discuss the systematic and random effects in terms of rainfall quantities, drop size distribution properties, and the observed drop size vs. velocity relationships. Since the instruments were collocated, the effects of the natural variability of rain are reduced some with time integration, isolating the instrumental differences. The authors discuss the status of DSD measurement technologies and the implications for a range of hydrologic applications from remote sensing of rainfall to atmospheric deposition to soil erosion and sediment transport in the environment. The data set collected during the DEVEX experiment is made available to the research community. 相似文献
53.
Fred Worrall Tim P. Burt Gregory R. Hancock Nicholas J.K. Howden John Wainwright 《地球表面变化过程与地形》2020,45(15):3869-3878
This study has hypothesized that for many rivers the trade-off between flow accumulation and the decrease in slope along channel length means that stream power increases downstream and, moreover, that given the low slope angles in headwater and low-order streams, they would have insufficient stream power to erode let alone transport sediment. The study considered the stream power profile, the particle travel distances and the application of the Hjulström curve based on the velocity profile of nine, large UK catchments. The study showed that:
- Some rivers never showed a maximum in their longitudinal stream power profile, implying that some rivers never develop a deposition zone before they discharge at the tidal limit.
- Particle travel distances during a bankfull discharge event showed that for some rivers 91% of the upper main channel would not be cleared of sediment. Furthermore, while some rivers could transport a 2 mm particle their entire length in one bankfull event, for another river it would take 89 such events.
- The Hjulström curve shows that for three of the study rivers the upper 20 km of the river was not capable of eroding a 2 μm particle.
- The study has shown that for all rivers studied, erosion is focused downstream and deposition upstream. Many UK rivers have a dead zone where, on time scales of the order of centuries, no erosion or transport occurs and erosion only occurs in the lower courses of the channel where discharge rather than slope dominates – we propose these as underpowered rivers.
54.
Fred W. Krüger 《GeoJournal》1998,46(2):79-87
Namibia and Botswana differ from other sub-Saharan nations in their record of stable political performance. However, both
countries have to face increasing social problems. In the case of Namibia these are still mainly related to the process of
post-apartheid national reconciliation and identity building, in the case of Botswana they refer to issues of growing tribal
tensions and of a gradual interior restructuring of society. This paper analyzes the various patterns of social transformation
in the two countries and examines in what ways state-society relations are different from other African nations.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
55.
The formation and subsequent reactions of magnesiancalcite overgrowths on calcite were investigated bymeans of closed system seeded precipitationexperiments. These experiments demonstrated that(1) thin overgrowths of magnesian calcite are precipitatedon calcite seeds suspended in seawater;(2) the solubilities of the coatings increase outward from theseed crystals as a linear function of the log ofrelative coating thickness;(3) during the period ofthese experiments (up to 5.5 months), the magnesiancalcite coatings continued to increase in thickness,but became less soluble in composition. Thestabilization reaction, referred to asrecrystallization, can be described by the followingequation:Cax Mg(1-x) CO3 + [z + y(x + z)]Ca2++ 2yHCO3- ]= (1 + y)Ca(x + z)Mg(1 - x -z)CO3 + [z + y(x + z - 1)]Mg2++ y CO2 + yH20;]4) recrystallization rate is dependent on solutionsaturation state, with a reaction order of 3.2 forartificial seawater and 4.0 for natural seawater; and(5) by the cessation of the closed system experiments,overgrowth compositions approached that of the stablecalcite (a few mol % MgCO3).Armoring of suspended carbonate particles in thesurface oceans with magnesian calcite overgrowthswould provide an effective barrier to release of theoceanic supersaturation with respect to calcite. Thicknesses of such coatings would be limited by therecrystallization rate of the magnesian calcite. Estimates based on the recrystallization ratesdetermined in this work indicate coatings on the orderof 0.02 µm in thickness could form on particles asthey sink through the mixed layer. According to thesecalculations, the total amount of carbon precipitatedannually in magnesian calcite overgrowths iscomparable to the riverine flux of dissolved carbon tothe oceans. Field observations of severalinvestigators indicate the likely presence ofmagnesian calcite coatings on planktonic particles,and provide evidence for possible recrystallization ofbiogenic magnesian particles in the marineenvironment. 相似文献
56.
Stress deformation properties of rock and rock discontinuities 总被引:14,自引:0,他引:14
Fred H. Kulhawy 《Engineering Geology》1975,9(4):327-350
The results of an extensive literature survey on the stress deformation properties of rock materials and rock discontinuities are summarized. The results show that: (1) for rock tested under uniaxial conditions, the range of modulus is from about 1 to 100 GN/m2, and the Poisson's ratio range is from 0.02 to 0.73 (0.46 if dilatant values are excluded) with an average value of 0.20; (2) for rock tested under triaxial conditions, the effects of nonlinearity and stress-dependency of the rock modulus are minor with hard, crystalline or homogeneous rock of low porosity, but are significant in porous, clastic or closely jointed rock; (3) anisotropy of the rock modulus is demonstrated mainly by variations in the modulus number or initial stiffness, while the nonlinearity and stress-dependency parameters are fairly constant with sample orientation; (4) nonlinearity and stress-dependency of the rock Poisson's ratio may be significant depending upon the magnitude of stress changes imposed, with the nonlinearity being more significant; and (5) limited data on rock discontinuities indicate that nonlinearity and stress-dependency effects may be quite large. 相似文献
57.
58.
Hoover Richard B. Hoyle Fred Wickramasinghe N. C. Hoover Miriam J. Al-Mufti S. 《Earth, Moon, and Planets》1986,35(1):19-45
There exists a close correspondence between the measured infrared properties of diatoms and the infrared spectrum of interstellar dust as observed in the Trapezium nebula and toward the galactic center source GC-IRS 7. Diatoms and bacteria also exhibit an absorbance peak near 2200 », which is found to agree with the observed ultraviolet absorbance properties of interstellar grains. We review the observational data and consider the known properties of diatoms and bacteria. It is suggested that these characteristics are consistent with the concept of a cosmic microbiological system in which these or similar microorganisms might exist on comets, Europa and in interstellar space. 相似文献
59.
Isotopic fractionation of Cu in tektites 总被引:1,自引:0,他引:1
Tektites are terrestrial natural glasses of up to a few centimeters in size that were produced during hypervelocity impacts on the Earth’s surface. It is well established that the chemical and isotopic composition of tektites is generally identical to that of the upper terrestrial continental crust. Tektites typically have very low water content, which has generally been explained by volatilization at high temperature; however, the exact mechanism is still debated. Because volatilization can fractionate isotopes, comparing the isotopic composition of volatile elements in tektites with those of their source rocks may help to understand the physical conditions during tektite formation.Interestingly, volatile chalcophile elements (e.g., Cd and Zn) seem to be the only elements for which isotopic fractionation is known so far in tektites. Here, we extend this study to Cu, another volatile chalcophile element. We have measured the Cu isotopic composition for 20 tektite samples from the four known different strewn fields. All of the tektites (except the Muong Nong-types) are enriched in the heavy isotopes of Cu (1.98 < δ65Cu < 6.99) in comparison to the terrestrial crust (δ65Cu ≈ 0) with no clear distinction between the different groups. The Muong Nong-type tektites and a Libyan Desert Glass sample are not fractionated (δ65Cu ≈ 0) in comparison to the terrestrial crust. To refine the Cu isotopic composition of the terrestrial crust, we also present data for three geological reference materials (δ65Cu ≈ 0).An increase of δ65Cu with decreasing Cu abundance probably reflects that the isotopic fractionation occurred by evaporation during heating. A simple Rayleigh distillation cannot explain the Cu isotopic data and we suggest that the isotopic fractionation is governed by a diffusion-limited regime. Copper is isotopically more fractionated than the more volatile element Zn (δ66/64Zn up to 2.49‰). This difference of behavior between Cu and Zn is predicted in a diffusion-limited regime, where the magnitude of the isotopic fractionation is regulated by the competition between the evaporative flux and the diffusive flux at the diffusion boundary layer. Due to the difference of ionic charge in silicates (Zn2+ vs. Cu+), Cu has a diffusion coefficient that is larger than that of Zn by at least two orders of magnitude. Therefore, the larger isotopic fractionation in Cu than in Zn in tektites is due to the significant difference in their respective chemical diffusivity. 相似文献
60.